MANUFACTURING OF LINEAR LOW DENSITY POLYETHYLENE

(LLDPE)(960 TPD)

PROJECT REPORT

SESSION 2012-2013

Submitted to Panjab University, Chandigarh

In Partial fulfillment of the requirement For the degree of

BACHELOR OF ENGINEERING (CHEMICAL)2013

PROJECT GUIDE:Dr. SUKHMEHAR SINGH

SUBMITTED BY:BHAGWAT BHARDWAJ

ROLL NO. CH-9217

DR. S. S. BHATNAGAR UNIVERSITY INSTITUTE OF CHEMICAL ENGINEERING & TECHNOLOGY

PANJAB UNIVERSITY, CHANDIGARH

MANUFACTURING OF LINEAR LOW DENSITY POLYETHYLENE (LLDPE)(960 TPD)

Linear low-density polyethylene (LLDPE) is a substantially linear polyethylene, with significant numbers of short branches, commonly made by copolymerization of ethylene with longer-chain olefins. Linear low-density polyethylene differs structurally from conventional low-density polyethylene because of the absence of long chain branching. The linearity of LLDPE results from the different manufacturing processes of LLDPE and LDPE. In general, LLDPE is produced at lower temperatures and pressures by copolymerization of ethylene and such higher alpha olefins as butene, hexene, or octene. The copolymerization process produces an LLDPE polymer that has a narrower molecular weight distribution than conventional LDPE and in combination with the linear structure, significantly different rheological properties.

Production and properties

The production of LLDPE is initiated by transition metal catalysts, particularly Ziegler or Philips type of catalyst. The actual polymerization process can be done in either solution phase or gas phase reactors. Usually, octene is the copolymer in solution phase while butene and hexene are copolymerized with ethylene in a gas phase reactor. The LLDPE resin produced in a gas phase reactor is in granular form and may be sold as granules or processed into pellets. LLDPE has higher tensile strength and higher impact and puncture resistance than LDPE. It is very flexible and elongates under stress. It can be used to make thinner films, with better environmental stress cracking resistance. It has good resistance to chemicals and to ultraviolet radiation. It has good electrical properties. However it is not as easy to process as LDPE, has lower gloss, and narrower range for heat sealing.

Processing

LDPE and LLDPE have unique theoretical or melt flow properties. LLDPE is less shear sensitive because of its narrower molecular weight distribution and shorter chain branching. During a shearing process, such as extrusion, LLDPE remains more viscous and, therefore, harder to process than an LDPE of equivalent melt index. The lower shear sensitivity of LLDPE allows for a faster

stress relaxation of the polymer chains during extrusion, and, therefore, the physical properties are susceptible to changes in blow-up ratios. In melt extension, LLDPE has lower viscosity at all strain rates. This means it will not strain harden the way LDPE does when elongated. As the deformation rate of the polyethylene increases, LDPE demonstrates a dramatic rise in viscosity because of chain entanglement. This phenomena is not observed with LLDPE because of the lack of long-chain branching in LLDPE allows the chains to slide by one another upon elongation without becoming entangled. This characteristic is important for film applications because LLDPE films can be down gauged easily while maintaining high strength and toughness. The rheological properties of LLDPE are summarized as "stiff in shear" and "soft in extension". It is not taken in most curbside pickups in communities. LLDPE can be recycled though into other things like trash can liners, lumber, landscaping ties, floor tiles, compost bins, and shipping envelopes.

Process types :-

1) Slurry process: - it is also called basell’s hastalen slurry process. It consists of two continuous stirred tank reactors operated in parallel or in series according to the grade of polymer required. The process is designed to produce uni or bimodal polymer using Ziegler Natta catalyst. The polymer is produced at relatively low temperature ( 70 - 110 oC ) and low pressure ( 1 - 5 MPa ) in a saturated hydrocarbon medium. The polymer form suspension or mobile slurry. The reaction medium is removed and polymer is separated from the hydrocarbon inert diluents. The obtained powder is mixed with stabilizers and generally extruded into pellets.

2) Gas phase process: - In this process the catalyst and the co-monomer are fed to a slurry stirred reactor in which pre polymerization occur. Pre polymerization under mild conditions help to prevent hot spots or it will hinder the process. The polymer is transferred to a dryer where hot nitrogen evaporates the solvent. Then the polymer powder acts as a catalyst for the main polymerization reactor and is fed continuously to the fluidized bed reactor.

The fluidization reactor comprises of two main parts - Cylindrical part - Disengagement partThe cylindrical part is equipped with gas distribution in order to fluidize the content of the bed.

The disengagement reduces the velocity of flowing gas and constantly disengages the polymer particle from output gas.

Finally, the gases are compressed and returned to bottom of the reactor ethylene, butene, or hexane, H2 as a chain terminator and N2 as an inert gas is introduced at different points to assure perfect mixing and prevent condensation which could damage the blades of the compressor.

The circulating gas fluidizes the bed and removes the heat of the reaction.

Comparison of Gas phase process and slurry process :-

In slurry process due to use of solvent, same additional equipments is required like solvent stores , purifiers etc. therefore gas phase processes are

- More compact - simpler - have lower cost - environmental impact is less

In gas process, it creates no wall sheeting or fouling due to the ployethene’s solubilities in solvent medium which is the main problem in slurry process.

In slurry process there are some useful points like - Mild operating conditions - Ease of heat removal - Ease of processing - high monomer conversions

In gas phase process, production of more off spec polymer during grade change especially changing from 1 catalyst to another catalyst type.

In gas phase process, agglomeration and lump due to poor heat removal from growing polymer practices leading to the formation of hot spots followed by sintering of the polymer.

In gas phase process, disintegration of growing polymer particle due to undesirable stress leading to the formation of fines. Increasing the fines content in the gas phase reactor is catastrophic for all gas phase plants, leading a loss of homogeneity of fluidization and eventually leading to the blockage of subsequent process unit.

In gas phase process, electrostatic charge leading to agglomerate formation are wall sheeting especially near the inclined part of the disengagement zone of gas phase reactor.

Slurry reactors have a very efficient heat removal and wide co monomer range but the product range is limited due to solubility of the produced polymer.

Application

LLDPE has penetrated almost all traditional markets for polyethylene, it is used for plastic bags and sheets (where it allows using lower thickness than comparable LDPE), plastic wrap, stretch wrap, pouches, toys, covers, lids, pipes, buckets and containers, covering of cables, geomembranes, and mainly flexible tubing. In 2009 the world market for LLDPE reached a volume of almost 24 billion US-Dollars (17 billion Euro).[1]LLDPE manufactured using metallocene catalysts is labeled mLLDPE.

Physical Properties

Table-1

Property Value

Density 0.92 g/cm³

Surface hardness SD48

Tensile strength 20 MPa

Flexural modulus 0.35 GPa

Notched izod 1.06+ kJ/m

Linear expansion 20×10−5/°C

Elongation at break 500%

Strain at yield 20%

Max. operating temp. 50 °C

Water absorption 0.01%

Oxygen index 17%

Flammability UL94 HB

Volume resistivity 1016 Ω·cm

Dielectric strength 25 MV/m

Dissipation factor 1 kHz

909090

Dielectric constant 1 kHz

2.3

HDT @ 0.45 MPa 45 °C

HDT @ 1.80 MPa 37 °C

Material drying NA

Melting Temp. Range 120 to 160 °C

Mould Shrinkage 3%

Mould temp. range

20 to 60 °C

GRADEPROPERTIES

FEATURES APPLICATIONM.I. DENSITY (gm/cm)

STRESS EXPONENT

COLOUR

W50A009 0.7 - 1.0 0.950 - 0.954 1.45 - 1.55 >50

Excellent processability with optimum balance of tape strength and elongation.

Stretched tape for woven fabric, jumbo bag, tarpaulin.

I60A080 7.5 - 9.0 0.957 - 0.960 1.2 - 1.25 >60

Excellent processability with optimum balance of mechanical properties & low degree of warpage.

General purpose, multi purpose, vegetable crates etc.

I60U080 7.5 - 9.0 0.957 - 0.960 1.2 - 1.25 >60

UV stabilized, excellent processability with optimum balance of mechanical properties & low degree of warpage.

Soft drink crates, Vegetable crates, milk crates etc.

I50A180 19 - 23 0.950 - 0.954 1.2 - 1.3 >55

Excellent processability with optimum balance of mechanical properties & low degree of warpage.

Household containers like bucket, mug etc.

S56A010 0.7 - 1.0 0.950 - 0.954 1.45 - 1.55 >50

Excellent extrudability, and good balance between liner strength and knot strength.

Low denier application like fishing net, mosquito net etc.

E45A003 0.5 -0.7 0.943 - 0.946 2.0 - 2.1 >50conform to DOT specifications for JFC.

Telecommunication for sheathing.

P41A004 0.39 - 0.43 0.938 - 0.941 1.70 - 1.90 >45

Excellent processability, ESCR, carbon black dispersion, abrasion resistance. tape strength and elongation.

Coating on steel pipes used for gas transportation.

C43D006 0.64-0.75 0.945 - 0.948 2.0 - 2.1 >55conform to DOT specifications for JFC.

Base resin for cable insulation applications.

RESIN TYPE

GRADE M.I.DENSIT

Y (gm/cm)

STRESS EXPONEN

T

COLOUR

FEATURES APPLICATION

FILM

F20S0090.8 -1.1

0.918 - 0.921

1.25 - 1.35 <0Excellent processability, with optimum balance between mechanical & optical properties. Low gel count, resistance to leakage & pinholes, excellent sealing characteristics.

General purpose film, liquid packaging film, lamination film.F20S020

1.8 - 2.1

0.918 - 0.921

1.25 - 1.35 <0

E20AN009

0.8 - 1.1

0.918 - 0.921

1.25 - 1.35 <0 Non slip film.Lamination, cling film

FILM-OCTENE

O20S0090.8 - 1.1

0.918 - 0.921

1.25 - 1.35 <0 Octene film grade.

INJECTION MOULDING

I24A53050 -60

0.922 - 0.927

1.25 - 1.35 >50Excellent gloss & high flow.

Caps and closures, shopping baskets, master batch.

EXTRUSION COATING

E36A0606.0 - 8.0

0.920 - 0.923

1.2 - 1.3 >45Excellent processability.

Extrusion coating on HDPE woven fabrics.

ROTO MOULDING

R35A0424.0 - 5.0

0.933 - 0.939

1.2 - 1.32 >40Excellent processability& impact strength.

Outdoor storage tanks, toys.

RAW MATERIALS AND SUPPLY

Monomer: Ethylene (C2H4 ) from natural gas after recovery of C3 and C4 gases.

Comonomer: butane- I

Solvent : cyclohexane C6H12 – ethylene is highly soluble in cyclohexane most of C6H12 is recovered from operation but make up is required.

Catalyst: a mixture of Titanium tetrachloride ( TiCl4) and Vanadium Oxytrichloride ( VOCl3)

Chain transfer and terminating agent: hydrogen ( H2)

Catalyst deactivators: Pentadione Pelargonic acid ( C8H17COOH )Purification medium: molecular sieves

Silica gelActivated alumina

Polymer additives: antioxidants and UV stabilizers.

STRIPPING & BLENDING BAGGING

FINISHED PRODUCT

OFF GASES & GREASE TO VAPOURIZER

EXTRUSION & PELLETIZATION

ADDITIVES

POLYMER SEPERATION

SOLUTION ADSORBER

POLYMERIZATIONCATALYST PREPERATION

ALUMINA

SPENT ALUMINA

PURIFICATIONMONOMER

SOLVENT

SOLVENT RECOVERY

MONOMERSOLVENT

MAKEUP SOLVENT

ETHYLENE

Process block diagram

PROCESS DESCRIPTION

REACTION AREAThe process solvent, cyclohexane through purifiers (35 oC & 4 kg/cm2) is combined with co-monomers Butene-1 / Octene-1, & pumped to absorber cooler, ethylene from Outside Battery Limits after passing through purifiers is dissolved at absorber cooler in cyclohexane + butene-1 stream to form reactor feed solution.

This solution stream is pumped by reactor feed pump (45 oC & 170 kg/cm2) is tempered to achieve reaction feed temperature & then solution is catalytically reacted to produce poly-ethylene in solution.

The reaction area includes two main reactors : a pipe reactor & a stirred autoclave reactor (250 oC & 155 kg/cm2) & finally a trimmer reactor (270-300 oC). The combination of reactor will be used as per grade requirement. The catalyst causes ethylene & co-monomers to polymerize exothermically (93-95% conversion). Hydrogen is used as chain terminator injected in solution before entering the reactor.

The liquid stream leaving the reactor contains polyethylene (approx.20%) dissolved in cyclohexane together with un-reacted ethylene and butene-1. Deactivator is then added to terminate the reaction & solution is passed through solution pre-heater (295-310 oC), then second deactivator is added to promote efficient catalyst removal in solution adsorber (295-310 oC/115 kg/cm2).

After catalyst removal in solution adsorber, the solution is then depressurized in stages intermediate pressure separation (IPS), low pressure separation (LPS) causing cyclohexane, ethylene and butene to flash off & sent flashed vapours to recycle area. The molten polymer containing residual amounts of hydrocarbon is sent to finishing area.

RECYCLE AREAThe area consists of intermediate & low pressure vapor recovery system & five distillation column systems (Low Boiler column, High Boiler column, RB (grease) column, FE (ethylene) column & CM (butene) column).

Vapor Recovery:

(a) Intermediate pressure vapor recovery: The vapor stream from the IPS contains cyclohexane, the untreated ethylene, co-monomers, grease & small quantities of reaction by-products.This overhead vapor & the intermittent hot solvent flow used for flushing the solution adsorber are combined and sent to the recycle area for purification.

(b)Low pressure vapor recovery: Cyclohexane vapors from two stages of LPS passes through knock out pot is condensed and collected in LPS hold up tank. From LPS hold up tank it is pumped to LB column through LB feed heaters.

DISTILLATION AREAThe distillation section of the recycle area separates all of the light & heavy components from the reaction area and facilitates purging of impurities. It consists of five distillation column systems:(a) Low boiler (LB) column:

To separate low boilers (ethylene and butene-1/butene-2). The overhead stream contain ethylene, butene-1 & butene-2 are condensed & sent to FE column via FE column feed dryers. Bottom is sent to HB column.

(b)High boiler (HB) column:To separate cyclohexane & high boilers. The overhead vapor (99.9% cyclohexane) is condensed & this is recycled to reaction area. HB bottom contains high boiler (grease) is fed to RB column.

(c) Grease (RB) column:To concentrate high boiler (grease). Overhead vapor is condensed & returned back to HB column bottom sump. Bottom contains high boilers is purged to Dowtherm vaporizer where it is used as fuel. RB column acts as an extended stripping section of the HB column.

(d)Ethylene (FE) column:To separate unreacted ethylene & butene-1/butene-2. Overhead vapor condensed using propylene as refrigerant in overhead condenser. Ethylene recovered from overhead is recycled to outside battery limits. Bottom is fed to CM column.

(e) Butene (CM) column:To concentrate butene-1 in the overhead for recycle to reaction area. Column concentrates butene-2 in the bottom which is sent to outside battery limits.

FINISHING AREAThe polymer separated from the solvent & other components in the reaction area is further purified & processed into finished product.

Main extruder:Molten polymer from LPS-2 is fed to main extruder, which is driven by (variable frequency drive) motor. Polymer enters the rotating extruder screw. The screw forces the molten polymer through the die plate of under-water pelletizer.

In NOVA Sclairtech polyethylene technology extruder feed is melt polymer so power consumption in comparison to other technologies is lower.

Satellite extruder:To add solid additive in main extruder.

Under-water pelletizer:Consists of die plate & a melt cutter. The molten polymer comes out through the die plate of under-water pelletizer. The melt cutter then chops the extrudate into pellets. The water/pellet slurry from the pelletizer flows into the delumper.

Delumper/Dewatering Classifier:The water/pellet slurry from the pelletizer flows into the delumper. Any oversize lumps are rejected & any fines & undersize pellets are separated.

Stripper:Resin product leaving the pelletizer contains approx. 2.5% (wt.) volatile hydrocarbons, which must be reduced to 500ppm (max.). Volatile hydrocarbons trapped within the product pellets removed in stripper, where they are stripped by counter-current flow of desuperheater steam.

Spin Dryer:The stripped resin pellets flow out of stripper mixed with transport water & conveyed to spin dryer to dry down the product pellets. From spin dryer the dried product pellets are conveyed through pneumatic conveying system to bagging silos.

Liquid & solid additive:Additives enhance the quality & range of applications for which polyethylene can be used. Additives used are as follows:-

Additive Type Feed-in

Antioxidants Liquid IPS tail by dosing pump

Slipping agent Liquid Main extruder by pump

UVS Liquid IPS tail by pump

Polymer processing aid Dry Main extruder by satellite extruder

Anti -block Dry Main extruder by satellite extruder

MATERIALBALANCE

MATERIAL BALANCE

AIM: To design a polyethylene plant of capacity 960 TPD(tonnes per day).

1.Estimation of feed to LLDPE plant Capacity per hour = 960/24 tonnes per hour = 40 tonnes per hour (TPH) LLDPE produced =40 TPH

Assuming losses : Polymer loss in bagging area = 0.24 TPH(0.60% of LLDPE produced) Polymer loss in stripper = 0.04 TPH(0.10%) Polymer loss in extruder = 0.27 TPH(0.675%) Polymer loss as grease = 8.84 TPH(22.10%)

Total loss = 9.39 TPH Amount of ethylene reacted (according to losses) =9.39 TPH

Assuming 95 % conversion of ethylene to polythene: Therefore the amount of ethylene fed to LLDPE plant

= (40 + 9.39)/.95 = 51.98 TPH

Amount of ethylene unreacted = 51.98-(40+9.39) =2.59 TPH

2.Absorber:

In absorber 21% by weight ethylene is dissolved in cyclohexane. Ethylene feed, (B) = 38.98 TPH Solution (cyclohexane + ethylene), (C) = 51.98/0.21 = 247.52 TPH

Solvent (cyclohexane) feed , (A) = 247.52 * 0.79 = 195.54 TPH Material balance across absorber :

Figure-1

A+B = C195.54 + 51.98 = 247.52 247.52 = 247.52 Input = Output

3.Reactor: (Assuming 95 % conversion of ethylene to polyethylene)

ReactionnC2H4 (C2H4)n

ethylene polyethylene

cAB

D,E,F,G

Figure-2

Input : Ethylene, (A) =51.98 TPH Cyclohexane, (B) =195.54 TPH

Output: Product (LLDPE + Grease) = 0.95 * 51.98 = 49.38 TPH Unreacted ethylene (D) = 51.98 - 49.38 TPH

= 2.60 TPH Cyclohexane , (E) = 195.54 TPH Grease, (F) = 8.84 TPH LLDPE, (G) = Product – Grease = 49.38 - 8.84 TPH = 40.54 TPH

Material balance: Input = Output A+B = D+E+F+G

51.98 + 195.54 = 2.60 + 195.54 + 8.84 + 40.54 247.52 = 247.52

Input = Output

4.Intermediate Pressure Separator : IPS gives out 50:50 of polyethylene & cyclohexane.

Input A:LLDPE = 40.54 TPHUnreacted ethylene = 2.60 TPHGrease = 8.84 TPHCyclohexane = 195.54 TPH

A B

C

IPS

Figure-3

Output B:LLDPE = 40.54 TPH

Cyclohexane = 40.54 TPH Grease = 3.47 TPH {Assumption: Grease removed in

Overheads is 61%approx.)

Overheads C:Cyclohexane = 155 TPHUnreacted ethylene = 2.60 TPHGrease = 5.37 TPH { 8.84 - 3.47 }Material balance:

A = B+C195.54 + 40.54 + 2.60 + 8.84 = 40.54 +40.54 + 2.60 + 155 + 3.47 + 5.37

247.52 = 247.52

Input = Output

5.Low Pressure System - IIt gives Output B in which cyclohexane is 5% of (cyclohexane + LLDPE) in Product stream B.Input A:It is output of IPS.LLDPE = 40.54 TPHCyclohexane = 40.54 TPHGrease = 3.47 TPH

A B

C

LPS-I

Figure-4Output B: Cyclohexane = 40.54/0.95 * 0.05

= 2.13 TPH Grease = 1.09 TPH(Assumption: Grease removed in Overheads(C) =69% approx..) LLDPE = 40.54 TPH

Overheads (C):Cyclohexane = 40.54 - 2.13

= 38.41 TPHGrease = 3.47-1.09=2.38 TPH

Material balance: A = B+C40.54 + 40.54 + 3.47 = 2.13 + 1.09 + 40.54+ 38.41 + 2.38 84.55 = 84.55 Input = Output

6.Low Pressure System – IIIt gives Output B in which cyclohexane is 2% of (cyclohexane + LLDPE) in product stream B.Input A:It is the output of LPS-I.LLDPE = 40.54 TPHCyclohexane = 2.13 TPHGrease = 1.09 TPH

A B

C

LPS-II

Figure-5

Output B:Cyclohexane = 2.13/0.98 * 0.02

= 0.04 TPHLLDPE = 40.54 TPH

Overheads C:Cyclohexane = 2.13 – 0.04

= 2.09 TPHGrease = 1.09 TPHMaterial balance:A=B+C40.54 + 2.13 + 1.09 = 0.04 + 40.54+ 2.09 + 1.09 43.76 = 43.76 Input = Output

7.Extruder Output B of LPS-II is the input of Extruder.

Input A:LLDPE = 40.54 TPHCyclohexane = 0.04 TPH

EXTRUDERA B

Figure-6

Polymer loss(Accumulation) = 0.27 TPHOutput B:LLDPE = 40.27 TPHCyclohexane =0.04 TPHMaterial balance:A= B + Accumulation40.54 + 0.04 = 40.27+0.04+0.27 40.58 = 40.58 Input = Output

8.Stripper

In the stripper, the stream D contains 99.9% of the polymer entering the stripper.Input A:It is output of Extruder.LLDPE = 40.27 TPHCyclohexane = 0.04 TPH Input C:Steam = 3.46 TPH(Assumption)

A B

C D

STRIPPER

Figure-7Output D:LLDPE = 0.999*40.27=40.23 TPH

Output B:Cyclohexane = 0.04 TPHLLDPE = 0.04 TPHSteam = 3.46 TPHMaterial balance:

A + C = B + D40.27 + 0.04 + 3.46 = 0.04 + 3.46 + 0.04 + 40.23 43.77 = 43.77 Input = Output

9.Bagging Stripper stream D is going to bagging area.Polymer loss in bagging area = 0.24 TPHLLDPE input to bagging area = 40.23 TPH

LLDPE produced per hour = 40.23-0.24=40 Tonnes

10.Distillation:

F

D

W

DISTILLATION

Figure-8

Feed F:Feed has cyclohexane ,grease,& ethylene.Cyclohexane = from ( IPS overheads+ LPS-I overheads+ LPS-II

overheads+ Stripper Stream B)= 155 + 38.41 + 2.09 + .04 = 195.54 TPH

Ethylene = 2.60 TPH (from IPS overheads)Grease = 8.84 TPH (from IPS overheads ,LPS-I overheads,&

LPS-II overheads)Total = 206.98 TPHEthylene = 2.60 TPHCyclohexane + Grease = 204.38 TPHWt fraction of ethylene = .013Wt fraction of cyclohexane + grease = .987

DistillateWt fraction of ethylene in distillate = .99Wt fraction of cyclohexane + grease in distillate = .01

ResidueWt fraction of ethylene in residue = .005Wt fraction of cyclohexane + grease in residue = .995

Overall material balance:F = D + W206.98 = D + W

Ethylene balance:XFF = XDD + XWW,Here, XF, XD, & Xw are the mole fractions of ethylene in feed, distillate & residue respectively.2.60 = ((206.98 - W)*.99) + (W*.005)W = 205.39 TPHD = 1.59 TPHDistillateCyclohexane + grease in distillate = D*(1-XD) TPH

= 1.59 * .01 TPH = .0159 TPH

Ethylene in distillate = D*(XD) TPH = 1.59 * .99 TPH

= 1.5741 TPH

ResidueCyclohexane + grease in residue = W*(1-XW) TPH

= 205.39 * .995 TPH = 204.363 TPH

Ethylene in residue = W*(XW) TPH = 205.39 * .005 TPH

= 1.027 TPH

Overall Material balance:F = D + W206.98 = 1.59 + 205.39206.98 = 206.98

ENERGYBALANCE

ENERGY BALANCE

Figure-91. Energy balance across Reactor feed heater

HX

B1500c

A300C

Figure-10

Input A:

Mass flow rate of cyclohexane = 195.54 * 103 kg/hr

Mass flow rate of ethylene = 51.98 * 103 kg/hr

Average Specific heat capacity of cyclohexane(90 oC) = 2.235

kJkg K

Average Specific heat capacity of ethylene(90 oC) = 0.74

kJkg K

150oC

270oC

30oC295oC

50oC

50oC

80.7oC

237oC

285oC

170oC

Energy in + Qfrom steam = Energy out

Qsteam = Energy out – Energy in

= Mass flow ratecyclohexane * Cp cyclohexane* (∆T) + Mass flow rateethylene*Cp ethylene*(∆T)

= (195.54*2.2325*1000*(150-30)) +

(51.98*0.74*1000*(150-30))

= 5,70,59,652

KJhr

2. Energy balance across Reactor

Figure-11

Energy Balance

Enthalpy Inlet Stream + Heat Generation due to polymerization(Exothermic Reaction) = Enthalpy outlet stream

Heat of Reaction = 22380 kcal of heat per kmol of monomer reacted.

Taking 25 OC as reference temperature for enthalpy calculations and also assuming temperature of reactor between 250-280 OC for Cp calculations.

HX

B2950c

A2700C

Average value of Cp cyclohexane (210 OC) = 2.89 kJ/kg K

Cp ethylene (input stream at 90 OC) = .74 kJ/kg K

Cp ethylene (output stream at 150 OC) = .78 kJ/kg K

Cp LLDPE (270 OC) = 5.60 kJ/kg K

Neglecting grease in the output stream of reactor, we have:-

(m*CP cyclo*(T-150)) - (methyl.*CP ethyl.*(150-25))input+ (methylCP ethyl(T-25))output + mLLDPECP LLDPE(T-25) = Heat Generation

→ { (195.54*2.89*(T-150))cy - (51.98*.74*(150-25))ethyl input + 40.54*5.60*(T-25) + (2.60*.78*(T-25))output ethylene } = 136092.780

→ 565.12*(T-150) – 4808.15 + 227.02*(T-25) + 2.028(T-25) = 136092.780

→ T = 270 OC

3. Energy balance across Solution Adsorber Pre-heater

Figure-12

Energy in + Qfrom steam = Energy out

Mass flow rate of cyclohexane = 195.54 * 103 kg/hr

Mass flow rate of ethylene = 2.60 * 103 kg/hr

Mass flow rate of LLDPE = 40.54 * 103 kg/hr

Cp cyclohexane (280 OC Mean temperature) = 2.90 kJ/kg K

Cp LLDPE (280 OC) = 5.60 kJ/kg K

EXTRUDERA B

500C2850C

Cp ethylene (280 OC) = .79 kJ/kg K

Qsteam = (mEthyleneCP Ethylene + mCyclohexaneCP Cyclohexane + mLLDPECLLDPE)*(∆T)

= (2.60*0.79 + 40.54*5.60 + 195.54*2.90)*103*25

= 19903.60 * 103

kJhr

4.Energy balance across Extruder

Figure-13

Inlet A:

Mass flow rate of LLDPE = 40.54 * 103 kg/hr

Mass flow rate of Cyclohexane = 0.04 * 103 kg/hr

Energy in – Energy out = Qlost

Cp LLDPE (170 OC) = 5.60 kJ/kg K

Cp cyclohexane (170 OC) = 2.81 kJ/kg K

Qlost = mLLDPE*Cp LLDPE*(235) + mcyclohexane*Cp cyclohexane*(235)

= (40.54 * 1000 * 5.60 + 0.04* 1000 * 2.81) * (235)

= 53377.05 * 103

kJhr

5.Energy balance across Distillation Column

F

170oC

D80.70C

W

237 oC

1.8MPa

100oC

Figure-14

Feed has Cyclohexane, grease and ethylene and entering at 170 oC Cyclohexane = 195.54 TPH Ethylene = 2.60 TPH Grease = 8.84 TPH

Cp of cyclohexane at 170 oC = 2.81

kJ

kg o C

Cp of ethylene at 170 oC(liquid state, absorbed in cyclohexane)

= 0.78

kJ

kg o C

Weight fraction of ethylene in feed = .013 Weight fraction of cyclohexane + grease in feed = .987 Cp of feed = (2.81*.987) + (.78×0.013) = 2.78

Distillate is leaving at 80.7 o C Weight fraction of ethylene in distillate = .99 Weight fraction of cyclohexane + grease in distillate = .01 Cp cyclohexane at distillate temperature = 2.0 kJ/kg K Cp ethylene at distillate temperature = 1.5 kJ/kg K Cp of distillate = (2.0×0.01) + (1.50×0.99)

= 1.505

kJkg K

Residue is leaving at 237 o C Weight fraction of ethylene in residue = .005 Weight fraction of cyclohexane + grease in residue = .995

Cp cyclohexane at residue temperature = 2.90 kJ/kg K Cp ethylene at residue temperature = 0.80 kJ/kg K Cp of residue = (.995*2.90)+(0.80*.005)

= 2.89

kJkg K

Neglecting Grease and assuming binary distillation,we have:-Referring T-x-y diagrams of cyclohexane & ethylene at 1.8 MPa, we infer that :

Feed is in liquid state at 170 oC & 1.8 MPa. Residue is also in liquid state at the given residue conditions. While distillate after partial condensing at very high pressure is in vapour

state. Partial condensed liquid is sent as Reflux in the Distillation column.

Enthalpy of distillate: Reference temperature for enthalpy calculations = 25 oC

= mdistillateCpdistillateΔT + mcyclohexaneLcyclohexane

Here ,mdistillate = mass flow rate of distillate Cpdistillate = specific heat capacity of distillateΔT = temperature differenceLcyclohexane = latent heat of vaporization of cyclohexane

Latent heat of vaporization of cyclohexane = 440

kJkg

Enthalpy of distillate = { (1.59*1.505*(80.7-25)) + (.0159*440)} * 103

= 140.28 * 103 kJ/hrEnthalpy of residue:

Taking Tr = 25 oC, = mresidueCpresidueΔT + mcyclohexane * Lcyclohexane

= { (204.363*440) + (205.39*2.89*(237-25)) } * 1000

kJhr

= 215758.065 * 103

kJhr

Enthalpy of feed: Taking reference temperature for enthalpy calculations 25 oC, we have :-

= mfeedCpfeedΔT + mcyclohexaneLcyclohexane

= { (206.98*2.78*(170-25)) + (195.54*440) } * 103

kJhr

= 169471.238 * 103

kJhr

Distillate = 1.59 TPH Assuming reflux ratio = 2.5 Mass flow rate into condenser = D(1 + R)

= 1.59*3.5 = 5.565 TPH

Now Qc ( Condenser load ) is equal to heat required to decrease the temperature of distillate from 100 0C to 80 0C and the latent heat of condensation of cyclohexane vapours.

Qc = { (5.565*1.505*(100-80.7)) + (0.05565*440) } * 103

kJhr

= 186.130 * 103

kJhr

Now Overall Energy balance: QB = WHW + DHD + QC - FHF

Here,HW, HD, & HF are the enthalpies of Residue, Distillate & Feed respectively. = ((205.39*215758.065 +1.59*140.28 + 186.130) – (206.98*169471.238))

* 103 kJ/hr

= 9237801.304 * 103

kJhr

= 2566.056 * 103 kJ/sec6.Steam requirement in Reactor feed heater

Steam condenses at 201.40C & 16 bar. Latent heat of condensation of steam at the process conditions

= 1933.20 kJ/kgQsteam = mass flow rate of steam(m) * L (Steam is condensed to preheat the feed in heater )

m*L = 57059652→ m * 1933.2 = 57059652→ m = 29515.65 kg/hr

7.Steam requirement in Solution Adsorber Pre-heater

Cp of steam at 473 K = 2.10

kJkgK

Cp = C1 + C2T+ C3T2+ C4T3+ C5 T4

Where

C1 = .336 * 105

C2 = -.2699 * 105

C3 = 2.61* 103

C4 = -.0890 * 105

C5 = 1169And T = temp in KSuppose steam inlet and outlet temperature as 3000C and 2200C

Average Cp of steam = 2.10

kJkgK

Qsteam = 2.10 * msteam * (300-220) m = 19903.60*1000/(2.10*80)

Mass flow rate of steam = 118473.81 kg/hr

EQUIPMENT DESIGN

1. Reactor design

c

VoCA0

CA

Figure-15

Inlet Stream of Reactor:

It consists of solution of 21%(by wt.)ethylene absorbed in cyclohexane.

Volume of solution = Volume of Solvent(because a gas is dissolved in liquid)

Solvent mass flow rate= 195.54 TPH=195.54*10^3 kg/hr

Density of solvent(cyclohexane) at temp 270 0C = 779 kg / m3

Volumetric flow rate of feed = 195.54* 103 / 779

= 251.01 m3/hr

Reaction of Process

n(C2H4) (C2H4)n

C2H4 (C2H4)n

According to literature, under conditions 270 0C and 15.20MPa in an autoclave Mixed Flow Reactor and polymerization in solution phase, polymerization kinetics comes out to be of first order with respect to monomer and catalyst concentration appears in the rate constant.

Taking 1st order reaction we have:

Design equation for MFR (Mix flow reactor) / CSTR:

ζ = Hold up time

CA0= Initial conc. of ethylene in inlet

CA=Final conc. of ethylene in outlet

V= Volume of reactor

V0=Volumetric flow rate of feed

-rA=Rate of reaction

k=Rate constant

= Expansion coefficient

-rA = KCA (1st order reaction)

k= 25.62 hr-1 ( from manual )

(for liquid phase reaction)

For XA = 0.95,

V=

.9525.62∗(1−.95) *

195 .54*103

779

= 186.15 m3

Calculation for Agitator power & Agitator dimension.

Figure-16

Taking turbine as agitator for the system

Now Correlation between agitator & tank dimensions for turbine are :-

, , = , ,

, h=Dt , E=Da

Industrial paddle agitators turn at speeds between 20 and 150 rpm.

Assuming the speed of 210 rpm= 3.5 rps.

Volume of MFR = 186.15m3

ΠDt2 H

4 = 186.15 m3

ΠDt2 * 4 .0Dt

4 = 186.15 m3

Dt = 3.89 m

Da =

Dt

3 = 1.30 m

L = .25 * Da = .325 m

h = Dt = 3.89 m

H = 4.0 * Dt = 15.56 m

E = Da = 1.30 m

J =

Dt

12 = .320 m

W =

Dt

5 = .260 m

N = 3.50 rps

Where,

Dt = Diameter of tank

Da = Diameter of shaft / impeller

L = Length of blades

h = Height of baffles

H = Height of tank

E = Height of impeller

J = Clearance

W = Width of baffle

N = Speed of impeller

µcyc (viscosity of cyclohexane at 270 0C) = 6.926 * 10-3 Poise

µethyl (viscosity of ethylene at 270 0C) = 1.472 * 10-4 Poise

µ (avg viscosity of solution at 270 0C) = .4 * 10-4 Poise

ρcyc (viscosity of cyclohexane at 270 0C) = 779 Kg/m3

ρethyl (viscosity of ethylene at 270 0C) = .802 Kg/m3

ρ (avg viscosity of solution at 270 0C) = (ρaMa + ρbMb )/ Ma + Mb

= [(779*78) + (.802*28)]/(78+28)

= 573.40 Kg/m3

Reynolds Number of solution = (Da2 *N*ρ)/µ

= 1.302 * 3.50 * 573.40 / (.4 * 10-3)

= 84.80 * 106

Power number = 5.00 (for disc mounted flat blade turbine from Coulson &

Richardson vol-6, fig.10.59)

P = 5.00 * D5 * N3 * ρ

= 5.00* (1.30)5 * (3.50)3 * 573.40

= 456.40 kW

Paddle = 5 (Using 5 paddles at equal lengths of the MFR for effective mixing)

Total shaft power of the agitator = 5 × 456.40 = 2282 kW

Mechanical design of reactor :-

A CSTR is preferred usually under following circumstances

- Liquid phase reaction- Low pressure reaction- Intense agitation is required - Large residence time- Better temperature control required - Less cost

Reason to choose CSTR :- Mixing of two liquid phase reactant High mass and heat transfer efficiency required Reactant are in liquid phase and homogenous.

Mechanical design includes - Thickness of shell- Impeller design - Diameter of impeller - Material of construction - Insulation of selection

Dished bottom reactor requires less power than flat one hence dished bottom is chosen.

Impeller selected is a pitched blade turbine 45 0 because - The weighted viscosity of reaction mixture is .4 * 10-4 poise which lies

in the range of turbine.- Efficient turbulent flow impeller for blending such liquids.- Combined axial and radial flows are achieved- Low cost - Wide application range.

N = 3.50 rps(speed of agitator),

Da =

Dt

3 = 1.27 m

L = .25 * Da = .317 m

h = Dt = 4.09 m

H = 4.0 * Dt = 9.914 m

E = Da = 1.273 m

J =

Dt

12 = .317 m

W =

Dt

5 = .2543 m

Where,

Dt = Diameter of tank

Da = Diameter of shaft / impeller

L = Length of blades

h = Height of baffles

H = Height of tank

E = Height of impeller

J = Clearance

W = Width of baffle

N = Speed of impeller

Shell thickness :-Diameter of shell = 3.89 mOperating pressure = 15.20 MPaDesign pressure = 1.2 * 15.20 MPa(factor of safety=1.2)

= 18.24 MPa

Working or operating temperature = 270 0CDesign temperature =1.2* 270 0C

= 324 0C

Material selection

For reactor :-

Carbon Steel Type 310

It’s composition is:-

Cr = 24-26 % , Ni = 19-22.9 % , C = 0.25 %

Advantages:-

High Strength & Resistant to scaling at high temperatures

This alloy shows increased resistance to high temperature corrosion

For blades :-

Carbon Steel Type 410

It’s compostion is:-

Cr= 11.5-13.5 % , C= 0.15 %

Advantages:-

Lowest cost general purpose Stainless Steel

Widely used where corrosion is not severe

For baffles

Carbon Steel Type 405

It’s composition is:-

Cr= 11.5-14.5 % , C = 0.08 % , Al = 0.1 -0.3 %

Advantages:-

Good Weld ability and cladding properties

Version of type 410 with limited hardenability but improved weldability

Baflle spacing

Baffle Spacing is calculated from the following formula

Baffle spacing = π x

Dt

4 = 3.14 x

3. 894

= 3.054m

Baffle spacing = 3.054 m

Width of Baffle ==

Dt

12 = 0.324 m

Width of Baffle = 0.324 m

Distance from Bottom =

Dt

2 =

3. 892

= 1.945 m

Minimum practical wall thickness

There will be wall minimum wall thickness required to ensure that any vessel is sufficiently rigid to withstand its own weight and any incidental loads. As a general guide , the wall thickness of any vessel should not be less than the values given below ; the values include a corrosion allowance of 2 mm

For cylindrical Tanks , wall thickness is given by –

t =

P* ri

S* E j - 0 . 6 p + Cc

Where,

P= Internal pressure

Ri = internal radius

Ej = Efficiency of Joint = 0.85

S = Maximum allowable working stress = 360000 Kpa

Cc = 4 x 10-3

t= wall thickness (m)

t =

18 .24 x 106 x 1.945360000 x 103 x 0 .85 - 0 .6 x18 .24 x 106

+ 4 x 10-3

= .124 m

= 12.40 cm

Then outer Diameter of Shell:-

Do = Di + 2t

= 3.89 + 2 x .124

Do = 4.138 m

Heads & closures

The ends of cylindrical vessel are closed by Heads of various shapes. The principal types used are –

1) Flat plates & formed Flat Heads

2) Hemispherical Heads

3) Ellipsoidal Heads

4) Torrispherical Heads

Flat plates are used as covers for Man ways and as the channel covers of Heat Exchangers. Formed Flat End, known as “ Flange – only “ ends , are manufactured by turning over a flange with a small radius on flat plate. The corner radius reduces the abrupt change of shape, at the junction with the cylindrical section; which reduces the local stresses to some extent .

Flange only heads are the cheapest type of formed head to manufacture, but their use is limited to low pressure & small diameter vessels.

Standard Torrispherical Heads (Dished Heads) are the most commonly used end closure for vessels up to operating pressure of 15 bars. They can be used for higher pressures, but above 10 bars, their cost should be compared with that of an equivalent ellipsoidal head.

Above 15 bars, an ellipsoidal Head will usually prove to be the most economical enclosure t o use.

A hemispherical head has the strongest shape; capable of resisting about twice the pressure of a torrispherical head of same thickness. The cost of forming a hemispherical head will, however, be higher than that for a shallow, torispherical head. Hemispherical heads are used for high pressures.

So, according to our requirement, hemispherical head is the best choice.

Hemispherical head design

D = 2H

D = Diameter of reactor

D = 3.89 m

H = 1.945 m

Material and conditions:-

SA – 240/340 steel

S = 360000 kPa allowable stress

E = 1 (head longitudinal joint efficiency)

Internal pressure(design) = 18.24 MPa

Corrosion correction factor = .201 mm

treq = P∗L

(2∗S∗E−.2∗P) + corrosion factor

= 49.721 mm

= 4.9 cm

Where,

L=(Do-2*t)/2

Vessel supports

The Method used to support a vessel will depend on the size, location, shape , weight of Vessel , temperature , pressure , interval & external fittings .

Skirt supports are used for tall, Vertical Columns.

Brackets or Legs are used for all types of vessel. Supports must be designed to carry the weight of the vessel and contents and any superimposed loads, such as Wind loads.

Supports will impose Localized Loads on the vessel wall and the design must be checked to ensure that the resulting stress concentrations are below the maximum allowable design stress.

The Bracket construction permits support of the cylinder without fixing the supports to the shell.

So, selected support is bracket support , supported from steel work.

Types of flange & selection

Several different types of flange are used for various applications. The principal types used in process industries are –

1) Welding Neck Flanges

2) Slip-on Flanges Hub & Plate types

3) Lap – Joint Flanges

4) Screwed Flanges

5) Blank or Blind Flanges

Welding Neck Flanges are suitable for extreme service conditions; where the flange is likely to be subjected to temperature, shear & vibration loads.

So, the selected flange is welding neck Flange

Gaskets

Gaskets are used to make a leak- tight joint between two surfaces. They are made from semi- plastic materials.

The gasket factor ‘m’ is the ratio of the pressure under the operating conditions to the internal pressure in vessel or pipe.

The internal pressure will force the flanges apart, so the pressure on the gasket under operating conditions will be lower than the initial tightening up pressure.

The gasket factor gives the minimum pressure that must be maintained on the gasket to ensure a satisfactory seal.

The following factors must be considered when selecting a gasket material –

1) The process conditions, pressure, temperature, corrosive nature of process fluid.

2) Whether repeated assembly & disassembly of joit is required

3) The type of Flange & Flange face.

2. Reactor feed heater design

Steam in(201.4°C,16bar) Steam out(16bar, 201.4°C)

P.S.out(150°C,17.42MPa) Process stream in(30°C,17.6MPa)

Reactor feed heater is a condenser which is exchanging heat by condensing saturated steam on shell side to heat up the process stream.

From Energy Balance,

Q (heat duty) = 57059652 kJ/hr

Process stream is taken on tube side and steam is at shell side as steam will condense on outer surface of tubes.

Steam inlet temp = 201.4 0C

Steam outlet temp = 201.4 0C

Tc1 = Process stream Inlet temp = 30 0CTc2 = Process stream Outlet temp = 150 0C

Now, QC = 57059652

kJhr

To find LMTD:

CONDENSER

LMTD =

(201 .4−30)−(201 .4−150)

ln (201. 4−30201. 4−150 ) = 99.63°C

For Condenser, No correction factor (FT) is required. FT = 1.

To calculate heat transfer area: Since cold fluid is majorly cyclohexane(organic solvent) + ethylene and

hot fluid is steam. From Coulson & Richardson Vol.6, Overall heat transfer coefficients table, we have the range – 500-1000 W/ m2K

Assuming U = 800 W/m2K Q = UA (LMTD) ⇒ A=198.86 m2

To calculate number of tubes:

Nt =

Aπ d o L

Assume O.D. of tube = 20 mm, I.D. = 16 mm and length, L = 4.88 mMaterial of tubes – Carbon Steel,Allowing for tube sheet thickness and taking L = 4.83m.

∴Nt =

198 . 86

π (201000 )(4 . 83 )

= 656.30 = 656 tubes approx. As the shell-side fluid is steam which is relatively clean:-

Using the triangular pitch, Pt = 1.25do = 0.025 m

Using exchanger with one shell side and two tube side passes.K1 = 0.249 ; n1= 2.207

Now, bundle diameter, Db = do( N t

K 1 )1

n 1

= .71 mUsing split ring floating head, from fig. 12.10 (RC-vol 6), clearance = 64 mm⇒DS – Db = 0.064m ⇒ DS = 0.773 m

TUBE-SIDE HEAT TRANSFER CO-EFFICIENT:

Mean Process Stream temperature =

30+1502 = 90 0C

Cross sectional Area of single tube, At =

π4 (di)2 = 2.01 × 10-4 m2

No. of tubes per pass, n =

6562 = 328

Tube flow area = n * At = 328 * 2.01 * 10-4 = 0.066 m2

Process stream mass velocity = m.

tube flow area =

247 . 52∗10003600* .066

= 1041.75 kg

.

m2 s

Density of process stream at 90 0C = 729.93

kg.

m3

µ(solution) at process stream mean temperature = 540*10-6 Pa.s

Cp = 2.119kJ/kg.K ; k = 0.112 W/m.K

Solution linear velocity = 1041 .75729 .93 = 1.43 m/sec.

Reynolds no. (Re) = ρνD

µ =

729 . 93∗1 . 43∗.016

540∗10−6 = 30927.40

Prandtl no. (Pr) = Cpµ/k = 540∗10−6∗2 .119∗103

. 112 = 10.22Re implies turbulent flow and for turbulent flow,we have

Dhi

k= 0 . 023* Re0. 8 * Pr . 33(μ / μw )0 .14

Neglecting µ/µw term as assuming viscosity is a weak function of temperature.

hi=k * 0 . 023* Re0 .8 * Pr. 33

D

hi=. 112* 0 . 023 * 30927 . 400.8 * 10 . 22.33

. 016 = 1355.87 W/m2 °C

SHELL-SIDE HEAT TRANSFER CO-EFFICIENT:

ho = 0.95 kL ¿¿(Nr)(-1/6)

where,ho = mean condensation film coefficient W/m2 °CkL = condensate thermal conductivity W/m °Cρl = condensate density Kg/m3

ρv = vapor density Kg/m3

µL = condensate viscosity Ns/m2

Ʈh = tube loading = condensate flow per unit length kg/m.s

Amount of condensate: Q =m.

∗L

m.

=QL

m.

=57059652*103

3600*1933 . 2 = 8.20 Kg/sL = latent heat of vaporization of water at 201.4°C & 16 bar = 1933.20kJ/kgkL = 0.58 W/m°Cρl = 862.813 kg/m3

ρV = 7.42 kg/m3

µL = .452 mPas

Ʈh =

m.

nt∗l =

8 .20656*4 . 83 = .00259 kg/m.s

Average no.of tubes in vertical tube row, Nr =2/3*

D b

P t =2/3*

709 .5225 = 18.91

ho = 0.95 kL ¿¿(Nr)(-1/6) = 2645.96 W/m2 °C

OVER-ALL HEAT TRANSFER CO-EFFICIENT:

Outside and inside Fouling/dirt co-efficients are 5000 W/m2°C and 3000 W/m2°C and thermal conductivity of steel = 50 W/m°C.

1

U o

= 1h o

+ 1h Od

+d o ln(d o

d i)2 k W

+ d o

d i

.1

h id

+ d o

d i

.1h i

= 1

2645 .96+ 1

5000+

0 .02 ln (2016)

2 x 50+20

16 ( 11355 . 87

+ 13000 ) .

= .00123⇒ Uo = 809.78 W/m2 °C

This is close to the assumed value.TUBE-SIDE PRESSURE DROP:

Δ P = Np∗(8*jf∗LDi

+ 2 .5 )∗ρν2

2

= 2*(8*3 .7*10-3∗4 . 83. 016

+ 2 .5 ) * 729. 93*1 . 432

2 = 17 . 07 kPa This value of pressure drop is permissible.Here, Np = no. of tube passes ; jf is evaluated from the graph between jf and Re (Coulson & Richardson Vol.6)

SHELL-SIDE PRESSURE DROP:Mass flow rate of steam = 8.20kg/s

AS =

( pt−d o ) DS lb

pt = 0.023 m2

Mass flow rate =

8 .20. 023 = 356.52

kg

m2 sec

Equivalent diameter, de =

1. 10do

( pt2−0 . 917 d

o2 )

= 0.0144 mRe = 356.52*14.4*10-3/.452*10-3 = 113580From fig. 12.24 (RC vol 6), jf = 2.2 * 10-2

ΔPS =

12 [8 j F L

de

ρv

v 2

2 ]

=

12

[8∗. 022∗ 4 .83. 0144

∗862 .813∗. 412

2 ]

= 2.14 kPa(Quite negligible)

CHOOSING BAFFLE SPACING:25% baffle cut is chosen

So baffle spacing =

Ds

5 = .155 m

COST ESTIMATION

FIXED AND WORKING CAPITAL:

Fixed capital is the total cost of the plant ready for start-up. It is the cost paid to the contractors.

It includes the cost of :

1. Design, and other engineering and construction supervision.

2. All items of equipment and their installation.

3. All piping, instrumentation and control systems.

4. Buildings and structures.

5. Auxiliary facilities, such as utilities, land and civil engineering work.

It is a once-only cost that is not recovered at the end of the project life, other than the scrap value.

WORKING CAPITAL:

Working capital is the additional investment needed, over and above the fixed capital, to start the plant up and operate it to the point when income is earned.

It includes the cost of :

1. Start-up.

2. Initial catalyst charges.

3. Raw materials and intermediates in the process.

4. Finished product inventories.

5. Funds to cover outstanding accounts from customers.

Most of the working capital is recovered at the end of the project. The total investment needed for a project is the sum of the fixed and working capital.

For a petrochemical plant working capital can be assumed safely to 15% of the fixed capital.

ESTIMATION OF PURCHASED EQUIPMENT COSTS:-

For Shell & tube heat exchangers –

Purchased cost = (Bare cost) * Type factor * Pressure factor

For horizontal & Vertical pressure vessels –

Purchased cost = (Bare cost) * Material factor * Pressure factor

For Distillation Column –

Installed cost = Cost * Material factor

For purchase cost of miscellaneous equipments –

Ce= CSn

Where,

Ce=Purchasing equipment cost ,$ (from table 6.2, RC-6,page no. 259)

C =Cost constant (from table 6.2, RC-6,page no. 259)

S =Characteristic size parameter (from table 6.2, RC-6,page no. 259)

n= Index for that type of equipment (from table 6.2, RC-6,page no. 259)

1. Compressors

Power = 1500 kW(power consumption of 5 compressors)

Ce= 91000 (1500)0.8 = $ 31616544.29

2. Reactors

Capacity, MFR=186.15 m3

Ce = 115000 (186.15)0.45

Ce =$ 1208192.703

3. Storage Tanks

Horizontal floating roof type tanks(Stainless Steel) – 2 tanks

Capacity = 1600 m3

Ce = 13050 (1600)0.55

= $ 754877.50

4. Absorption column

Bare vessel cost = $ 210,000

Material factor = 1

Pressure factor =1.1

Vessel cost = 1.1 * 1 * 210000 = $ 231000

Packed cost ( stainless steel) = $ 7500 / m3

Vol. of packing = 3.14 /4 * 212 = 346.185 m3

Cost of column packing = 7500 * 346.185=$ 2596388

Total cost of column = $ 2827388

5. Reboilers

Reboilers - 2

Bare cost = $ 600,000

Type factor = 0.8

Pressure factor = 1

Purchased cost = $ 600,000 * 0.8 * 1

= $480,000

6. Condenser cost

Condenser Units - 4

Bare cost = $ 447,058

Type factor = 0.85

Pressure factor =1.1

Purchased cost = 447,058 * 0.85 * 1.1

= $418,000

7. IPS cost

Material factor =1, carbon steel

Pressure factor = 1.4

Diameter = 2.50 m

Height = 8 m

Bare cost =$ 640,000

Purchased cost =$ 640,000 * 1 * 1.4

= $ 896,000

8. LPS

Diameter = 1.50 m

Height = 6 m

Material factor = 1

Pressure factor = 1.2

Bare cost = $ 720,000

Purchased cost = 1.2 * 1 *720,000 =$ 864,000

9. Extruder

Barrel diameter = 2 m

Length = 10 m

Material factor =1

Pressure factor =2.2

Purchased cost = 2.2 * 1 * 560,000

= $ 1,232,000

10.Stripper

diameter = 4.00 m

height = 10 m

Material factor = 1

Pressure factor = 1

Bare cost = $ 560,000

Purchased cost = 1 * 1 * 560,000

= $ 560,000

11.Distillation column cost

Total Cost = $550,000

12.Other equipments

drier = $ 88208.536

filter = $ 310676.32

PCE = cost of (compressor + reboiler + absorber + condenser + extruder + IPS + LPS + stripper + storage tanks + distillation column + other miscellaneous equipments)

PCE = $ 41805887

Physical plant cost (PPC):PPC is calculated from table 6.1 ,RC-6,page no. 252 .

Cf = fL * Ce

Cf = fixed capital cost fL = the “lang factor” which depends on the type of process. Ce = the total delivered cost of all the equipment items.

Process plant cost = PCE * 3.40= $ 41805887 * 3.4= $ 142,140,015

Fixed capital = 1.45 * PPC= 1.45 * 142,140,015= $ 206, 103,022

Working Capital Cost = 15% of fixed capital cost = .15 * 206103022 = $ 30915453Total Investment Cost = Fixed capital cost + Working capital cost = $ 237018475.70ESTIMATION OF OPERATING COSTSThe cost of producing a chemical product will include the items listed below. They are divided into two groups.1. Fixed operating costs: costs that do not vary with production rate. These are the bills that have to be paid whatever the quantity produced.

2. Variable operating costs: costs that are dependent on the amount of product produced.Fixed costs1. Maintenance (labour and materials).2. Operating labour.3. Laboratory costs.4. Supervision.5. Plant overheads.6. Capital charges.7. Rates (and any other local taxes).8. Insurance.9. Licence fees and royalty payments.Variable costs1. Raw materials.2. Miscellaneous operating materials.3. Utilities (Services).4. Shipping and packaging.

Variable cost estimation

Operating cost Assumed %Raw material cost 25% of TPC = 0.25yOperating labour 15% of TPC = 0.15ySupervisory/Clerical labour

20% of operating labour

=0 .20*0.15y

Utilities 15% of TPC = 0.15yMaintenance and repair 10% of FCI = $ 20610302Operating supplies 15% of M & R = $ 3091545Lab charges 23% of operating

labour= 0.23*0.15y

Royalties & Insurance 2% of FCI = $ 4122060.44Fixed charges 11% of TPC = .11yPlant overhead charges

Miscellaneous chargesAdministrative Expenses

Distribution & marketing expenses

60% of Operating labour10% of M & R25% of operatinglabour5% 0f TPC

= 0.60*0.15y

= $ 2061030.20= 0.25*.15y

= .05y

Total = 29884937.64 + 0 .902y

Total production cost, y = 0.902y + 29884937.64 TPC(y) = $ 304,948,343.30

Now, these costs are of year 2004, Using chemical engineering plant cost index (CEPCI) 2012, we have costs in 2012 :-

Index in 2004 = 444.2 Index in 2012 = 575.4

Total production cost in 2012 = TPC in 2004 * Index∈2012Index ∈2004

= 304948343.30 * 575.4444.2

= $ 393383362.90 = Rs. 2.08 * 1010

= Rs. 2080 crores Total Investment cost in 2012 = TIC in 2004 * 575.4/444.2

= $ 305753833.70 = Rs. 1.62 * 1010

= Rs. 1620 crores

PROFIT ESTIMATION

Current market selling price of LLDPE = Rs. 78.97/kg

Annual Production of LLDPE = 350 kT

Total income(on annual basis) = 350*106*78.97

= Rs. 2760 crores

Gross Earning = Income - TPC

= Rs. 680 crores

Tax payments = 40% of gross earning

= .40 * .68 * 1010 = Rs. 272 crores

Net profit= Rs. 408 crores

Depreciation = 10% of fixed capital cost = Rs. 1092346017

Payout period =

Total Investment CostNet profit after tax + depreciation

=

1 .62*1010

408∗107⊕1092346017 = 3.13 years

Rate of return =

Net profit after tax Total Investment Cost

* 100

=

4080000000

1. 62*1010 * 100

= 25.18%

UTILITIES

The word "Utilities" is now generally used for the ancillary services needed in the operation of any production process. These services will normally be supplied from a central site facility; and will include:

1. Electricity.

2. Steam, for process heating.

3. Cooling water.

4. Water for general use.

5. Demineralised water.

6. Compressed air.

7. Inert-gas supplies.

8. Refrigeration.

9. Effluent disposal facilities

ELECTRICITY.

Power is required for chemical process, motor, lightening, pumps, compressor, & other general & mechanical purposes. It may be purchased from local supply authority or generated at plant by steam turbine, generator.

STEAM

The steam for process heating is usually generated in water tube boilers; using the most economical fuel available. The process temperatures required can usually be obtained with low-pressure steam, typically 2.5 bar (25 psig), and steam is distributed at a relatively low mains pressure, typically around 8 bar (100 psig).

COMBINED HEAT AND POWER (CO-GENERATION)

The energy costs on a large site can be reduced if the electrical power required is generated on site and the exhaust steam from the turbines used for process heating. The overall thermal efficiency of such systems can be in the range 70 to 80 per cent; compared with the 30 to 40 per cent obtained from a conventional power station, where the heat in the exhaust steam is wasted in the condenser. Whether a combined heat and power system scheme is worth considering for a particular site will depend on the size of the site, the cost of fuel, the balance between the power and heating demands; and particularly on the availability of, and cost of, standby supplies and the price paid for any surplus power electricity generated. On any site it is always worth while considering driving large compressors or pumps with steam turbines and using the exhaust steam for local process heating.

COOLING WATER

Natural and forced-draft cooling towers are generally used to provide the cooling water required on a site; unless water can be drawn from a convenient river or lake in sufficient quantity. Sea water, or brackish water, can be used at coastal sites, but if used directly will necessitate the use of more expensive materials of construction for heat exchangers.

WATER FOR GENERAL USE

The water required for general purposes on a site will usually be taken from the local mains supply, unless a cheaper source of suitable quality water is available from a river, lake or well.

COMPRESSED AIR

Compressed air will be needed for general use, and for the pneumatic controllers that are usually used for chemical process plant control. Air is normally distributed at a mains pressure of 6 bar (100 psig). Rotary and reciprocating single-stage or two-stage compressors are used. Instrument air must be dry and clean (free from oil).

INERT GASES

Where large quantities of inert gas are required for the inert blanketing of tanks and for purging this will usually be supplied from a central facility. Nitrogen is normally used, and is manufactured on site in an air liquefaction plant, or purchased as liquid in tankers.

EFFLUENT DISPOSAL

Facilities will be required at all sites for the disposal of waste materials without creating a public nuisance.

INSTRUMENT AND PROCESS CONTROL

Instrumentation is the most important factor in ensuring safety and smooth working of the plant. A separate control room is provided in modern plants where on panels indicators and recorders are present.

Instruments are used in the industry to measure process variables such as temperature, pressure, density, level specific heat, conductivity, humidity, flow rate, chemical composition etc.

The primary objectives of the designer when specifying instrumentation and control schemes are:

1. Safe plant operation:

(a) To keep the process variables within known safe operating limits.

(b) To detect dangerous situations as they develop and to provide alarms and automatic shut-down systems.

(c) To provide interlocks and alarms to prevent dangerous operating procedures.

2. Production rate:

To achieve the design product output.

3. Product quality:

To maintain the product composition within the specified quality standards.

4. Cost:

To operate at the lowest production cost, commensurate with the other objectives.

These are not separate objectives and must be considered together.

TEMPERATURE CONTROL / MEASUREMENTS:

Various instruments are used for eg.

1. Thermocouple

2. Resistance Thermometers

3. Thermostats

4. Mercury in Glass Thermometers.

Controllers are used to maintain temperature within specified limits. Every temperature control problems is essential one. The temperature lay involved in the measurement of this variable is an important factor. Thermal element is usually placed in a well to protect it and allow servicing of element without interrupting the process. The location of the temperature element often has as much to do with the efficiency element often as other parts of the control loops. Temperature bulb should always be located at point where the coefficient of heat transfer will be as large as possible e.g. if vapors and liquid are at same temperature, then bulb should be kept in liquid because of high heat transfer coefficients.

PRESSURE CONTROL / MEASUREMENTS

It is quite necessary for most system handling vapours of gas. It can be measured by using pressure gauges.

Self operated pressure regulation is often used in pressure control. It is installed directly in the line the control sensing apparatus is paced about 10 diameters from the unit. This location eliminates erroneous pressure caused by turbulence. Sudden change in velocity, shock and vibrations difficulties often occur when self operated regulation are used with liquids.

LEVEL CONTROL / MEASUREMENT

The measurement of level can be defined as the determination of location of interface with respect to a fixed plane. The main objective of a level control system is to maintain the level of the liquid in a tank at the Act point value. The different is pressure transmitter senses the pressure difference / a function of liquid level in the tank) and gives out an electrical signal, which after signal conditioning is given to the P.I.D. controller. The controller compares the measured variables with the set point and depending upon the error, gives an output to the control valve. The control value, in turn controls the flow. Due to the control of flow, the level is controlled to its set point.

FLOW CONTROL / MEASUREMENTS

It is defined as volume per unit time at specific temperature and pressure conditions. It is generally measured by positive displacement of rate meters.

CONDENSER CONTROL

Temperature control is unlikely to be effective for condensers, unless the liquid stream is sub-cooled. Pressure control is often used, or control can be based on the outlet coolant temperature.

DISTILLATION COLUMN CONTROL

The primary objective of distillation column control is to maintain the specified composition of the top and bottom products, and any side streams; correcting for the effects of disturbances in:

1. Feed flow-rate, composition and temperature.

2. Steam supply pressure.

3. Cooling water pressure and header temperature.

4. Ambient conditions, which cause changes in internal reflux.

The compositions are controlled by regulating reflux flow and boil-up. The column overall material balance must also be controlled; distillation columns have little surge capacity (hold-up) and the flow of distillate and bottom product (and side-streams) must match the feed flows. The feed flow-rate is often set by the level controller on a preceding column. It can be independently controlled if the column is fed from a storage or surge tank. Feed temperature is not normally controlled, unless a feed pre heater is used. Temperature is often used as an indication of composition. The temperature sensor should be located at the position in the column where the rate of change of temperature with change in composition of the key component is a maximum.

Near the top and bottom of the column the change is usually small. With multi component systems, temperature is not a unique function of composition. Top temperatures are usually controlled by varying the reflux ratio, and bottom temperatures by varying the boil-up rate. If reliable on-line analyzers are available they can be incorporated in the control loop, but more complex control equipment will be needed.

PID LEGENDS:

Instrument locally mounted Process connecting lines

Instrument at control center Orifice for flow meas.

Instrument transmitting Electric lines

Control valve pneumatic Pneumetic lines

Controller self contained Capillary lines

FIRST LETTER SECOND LETTER THIRD LETTERD- Density A-Alarm A-AlarmF-Flow C-Control C-ControlH-Hand activated E-Element V-ValveL-Level G-GlassM-Moisture I-IndicatingP-pressure R-RecorderT-Temperature S-Safety weir

PID OF DISTILLATION COLUMN

Temperature Control Composition Control (R controlled and bottom product as fixed ratio of feed flow)

Composition Control (Top product controlled by feed)

PID OF REACTOR

INSTRUCTIONS FOR OPERATION

Before starting, always make sure that the followings:

No visible damage is evident in the system.

All electrical switches are turned off.

All water valves are closed.

All valves are tightly closed.

Start up procedure.

Check the level in the tank.

Switch on the main supply.

Turn on the main pump.

Open valve for pump.

Start mechanical pump.

Open the steam valve.

Check the flow, temperature& pressure during the operation.

System shut down.

Reduce the flow of the feed & steam.

Reduce all flow rates & wait for pasture to drop.

Turn off the pump.

When the flow slow down close the valve tightly to avoid cavitations.

Disconnect the packing line.

GENERAL SITE CONSIDERATIONS

PLANT LOCATION AND SITE SELECTION

The location of the plant can have a crucial effect on the profitability of a project, and the scope for future expansion. Many factors must be considered when selecting a suitable site. The principal factors to consider are:

1. Location, with respect to the marketing area.

2. Raw material supply.

3. Transport facilities.

4. Availability of labor.

5. Availability of utilities: water, fuel, power.

6. Availability of suitable land.

7. Environmental impact, and effluent disposal.

8. Local community considerations.

9. Climate.

10. Political and strategic considerations.

Marketing area

For materials that are produced in bulk quantities; such as cement, mineral acids, and fertilizers, where the cost of the product per tonne is relatively low and the cost of transport a significant fraction of the sales price, the plant should be located close to the primary market. This consideration will be less important for low volume production, high-priced products; such as Pharmaceuticals. In an international market, there may be an advantage to be gained by locating the plant within an area with preferential tariff agreements; such as the European Community (EC).

Raw materials

The availability and price of suitable raw materials will often determine the site location. Plants producing bulk chemicals are best located close to the source of the major raw material; where this is also close to the marketing area.

Transport

The transport of materials and products to and from the plant will be an overriding consideration in site selection. If practicable, a site should be selected that is close to at least two major forms of transport: road, rail, waterway (canal or river), or a sea port. Road transport is being increasingly used, and is suitable for local distribution from a central warehouse. Rail transport will be cheaper for the long-distance transport of bulk chemicals. Air transport is convenient and efficient for the movement of personnel and essential equipment and supplies, and the proximity of the site to a major airport should be considered.

Availability of labour

Labour will be needed for construction of the plant and its operation. Skilled construction workers will usually be brought in from outside the site area, but there should be an adequate pool of unskilled labour available locally; and labour suitable for training to operate the plant. Skilled tradesmen will be needed for plant maintenance. Local trade union customs and restrictive practices will have to be considered when assessing the availability and suitability of the local labour for recruitment and training.

Utilities (services)

Chemical processes invariably require large quantities of water for cooling and general process use, and the plant must be located near a source of water of suitable quality. Process water may be drawn from a river, from wells, or purchased from a local authority. At some sites, the cooling water required can be taken from a river or lake, or from the sea; at other locations cooling towers

will be needed. Electrical power will be needed at all sites. A competitively priced fuel must be available on site for steam and power generation.

Environmental impact, and effluent disposal

All industrial processes produce waste products, and full consideration must be given to the difficulties and cost of their disposal. The disposal of toxic and harmful effluents will be covered by local regulations, and the appropriate authorities must be consulted during the initial site survey to determine the standards that must be met. An environmental impact assessment should be made for each new project, or major modification or addition to an existing process.

Local community considerations

The proposed plant must fit in with and be acceptable to the local community. Full consideration must be given to the safe location of the plant so that it does not impose a significant additional risk to the community.On a new site, the local community must be able to provide adequate facilities for the plant personnel: schools, banks, housing, and recreational and cultural facilities.

Land (site considerations)

Sufficient suitable land must be available for the proposed plant and for future expansion. The land should ideally be flat, well drained and have suitable load-bearing characteristics. A full site evaluation should be made to determine the need for piling or other special foundations.

Climate

Adverse climatic conditions at a site will increase costs. Abnormally low temperatures will require the provision of additional insulation and special heating for equipment and pipe runs. Stronger structures will be needed at locations subject to high winds (cyclone/hurricane areas) or earthquakes.

Political and strategic considerations

Capital grants, tax concessions, and other inducements are often given by governments to direct new investment to preferred locations; such as areas of high unemployment. The availability of such grants can be the overriding consideration in site selection.

SITE LAYOUT

The process units and ancillary buildings should be laid out to give the most economical flow of materials and personnel around the site. Hazardous processes must be located at a safe distance from other buildings. Consideration must also be given to the future expansion of the site. The ancillary buildings and services required on a site, in addition to the main processing units (buildings), will include:

1. Storages for raw materials and products: tank farms and warehouses.

2. Maintenance workshops.

3. Stores, for maintenance and operating supplies.

4. Laboratories for process control.

5. Fire stations and other emergency services.

6. Utilities: steam boilers, compressed air, power generation, refrigeration, transformer stations.

7. Effluent disposal plant.

8. Offices for general administration.

9. Canteens and other amenity buildings, such as medical centres.

10. Car parks.

When roughing out the preliminary site layout, the process units will normally be sited first and arranged to give a smooth flow of materials through the various processing steps, from raw material to final product storage. Process units are normally spaced at least 30 m apart; greater spacing may be needed for hazardous processes. The location of the principal ancillary buildings should then be decided. They should be arranged so as to minimise the time spent by personnel in travelling between buildings. Administration offices and laboratories, in which a relatively large number of people will be working, should be located well away from potentially hazardous processes. Control rooms will normally be located adjacent to the processing units, but with potentially hazardous processes may have to be sited at a safer distance. The sitting of the main process units will determine the layout of the plant roads,

pipe alleys and drains. Access roads will be needed to each building for construction, and for operation and maintenance.

Utility buildings should be sited to give the most economical run of pipes to and from the process units. Cooling towers should be sited so that under the prevailing wind the plume of condensate spray drifts away from the plant area and adjacent properties. The main storage areas should be placed between the loading and unloading facilities and the process units they serve. Storage tanks containing hazardous materials should be sited at least 70 m (200 ft) from the site boundary.

A TYPICAL PLANT LAYOUT:-

COMMISSIONING, START-UP AND SHUT DOWNCOLUMN COMMISSIONING:It refers to the process of preparing the column for operation. The main objectives are:

1) Clear the system of undesirable materials.2) Test the column and rectify potential problems.3) Take preventive actions against performance deterioration.

Most commissioning operations are performed using readily available liquids and gases, such as air, nitrogen, steam, water or oil. An important thing to note is that the column and associated equipments are seldom designed to cater specifically to commissioning operations. Therefore, they must be tailored to suit the limitations of the available system.START-UP:The following steps are observed:

1. Commissioning 2. Pressure-up3. Column heating (and/or cooling)4. Introduction to feed5. Introducing heating and cooling sources6. Bring to desired operating conditions

SHUT DOWN:1. Reducing column/unit rates.2. Shutting down heating/cooling surfaces.3. Stopping feed4. Draining liquid5. Cooling/heating the column/unit6. Bringing the unit to atmospheric pressure7. Eliminating undesirable materials8. Preparing to opening to atmosphere

VARIOUS COMMISSIONING ACTIVITIES

1. LINE BLOWING: A common pre-start up practice is to pressurize the column with air or nitrogen. The unit then serves as “vapour reservoir” for blowing lines connected to the unit to remove construction debris.

2. PRESSURIZING OR DEPRESSURIZING: Pressurizing or depressurizing is performed during commissioning, start-ups and shut-downs. They are used to check the columns for pressure retention and leaks. Remove air or inert gas prior to start-up, free the column of gas

(e.g. hydrocarbon) at shut-downs or prepare the column for entry by personnel for inspection of the column internals. Location and set pressure of relief valves and/or bursting disks as well as major vents should be checked.

3. PURGING: A column needs to be purged with an inert gas prior to start-up to remove air if it is used for separating combustible materials. The inert gas can then be purged with process gas. The reverse steps are performed at shut-down. Drain and vent valves must be opened intermittently so that no dead pockets are left unpurged.

4 . STEAMING:A unit may be steamed during commissioning to drive the air out heat the column up, clear blockages or leak test the unit. Problem associated with it could be creation of vacuum, water hammer, overheating etc. Steaming is done before purging.

5. LEAK TESTING: After purging and before introducing the process gas column is leak tested. The most common technique is pressurizing the unit up with inert gas (liquid N2) with all vents and drain closed.

6. WASHING: Washing is done during start up and shut down for one or more of the following reasons - To remove scale, mud etc.

- To coo, the column

- To dissolve or carry away sticky deposits. Water should be drained out after washing.

7. DRY OUT : Water should be drained out after washing. To remove water from the column dry out can be done using hot water or liquid.

8. BLINDING AND UNBLINDING: Blinds and /or slip plates (‘spades’) are usually installed in all lines which leave or enter a unit in order to positively eliminate leakage of materials into the unit when air is introduced. During start up, the plates are removed. The sequence of blinding and unblinding must be carefully prepared / planned prior to start up or shut down. A checklist of all blinds should be made. Blinds be properly tagged, and tag ID must be reflected on checklist.

MAINTENANCE AND SAFTEY MEASURES:-Safety and Loss Prevention

Any organization has a legal and moral obligation to safeguard the health and welfare of its employees and the general public. Safety is also good business; the good management practices needed to ensure safe operation will also ensure efficient operation. The term "loss prevention" is an insurance term, the loss being the financial loss caused by an accident. This loss will not only be the cost of replacing damaged plant and third party claims, but also the loss of earnings from lost production and lost sales opportunity. All manufacturing processes are to some extent hazardous, but in chemical processes there are additional, special, hazards associated with the chemicals used and the process conditions. The designer must be aware of these hazards, and ensure, through the application of sound engineering practice, that the risks are reduced to acceptable levels.

Safety and loss prevention in process design can be considered under the following broad headings:

1. Identification and assessment of the hazards.

2. Control of the hazards: for example, by containment of flammable and toxic materials.

3. Control of the process: Prevention of hazardous deviations in process variables (pressure, temperature, flow), by provision of automatic control systems, interlocks, alarms, trips; together with good operating practices and management.

4. Limitation of the loss. The damage and injury caused if an incident occurs: pressure relief, plant layout, provision of fire-fighting equipment. In this chapter the discussion of safety in process design will of necessity be limited.

INTRINSIC AND EXTRINSIC SAFETY

Processes can be divided into those that are intrinsically safe, and those for which the safety has to be engineered in. An intrinsically safe process is one in which safe operation is inherent in the nature of the process; a process which causes no danger, or negligible danger, under all foreseeable circumstances (all possible deviations from the design operating conditions). Clearly, the designer should always select a process that is inherently safe whenever it is practical, and economic, to do so. However, most chemical manufacturing processes are, to a greater or lesser extent, inherently unsafe, and dangerous situations can develop if the process conditions deviate from the design values. The safe

operation of such processes depends on the design and provision of engineered safety devices, and on good operating practices, to prevent a dangerous situation developing, and to minimize the consequences of any incident that arises from the failure of these safeguards.

The term "engineered safety" covers the provision in the design of control systems, alarms, trips, pressure-relief devices, automatic shut-down systems, duplication of key equipment services; and fire-fighting equipment, sprinkler systems and blast walls, to contain any fire or explosion.

THE HAZARDS

Toxicity

Most of the materials used in the manufacture of chemicals are poisonous, to some extent. The potential hazard will depend on the inherent toxicity of the material and the frequency and duration of any exposure. It is usual to distinguish between the short-term effects (acute) and the long-term effects (chronic). The permissible limits and the precautions to be taken to ensure the limits are met will be very different for these two classes of toxic materials. Industrial hygiene is as much a matter of good operating practice and control as of good design.

Control of substances hazardous to health

The employer is required to carry out an assessment to evaluate the risk to health, and establish what precautions are needed to protect employees. A written record of the assessment would be kept, and details made available to employees. The designer will be concerned more with the preventative aspects of the use of hazardous substances. Points to consider are:

1. Substitution: of the processing route with one using less hazardous material, Or substitution of toxic process materials with non-toxic, or less toxic materials.

2. Containment: sound design of equipment and piping, to avoid leaks. For example, specifying welded joints in preference to gasketed flanged joints (liable to leak).

3. Ventilation: use open structures, or provide adequate ventilation systems.

4. Disposal: provision of effective vent stacks to disperse material vented from pressure relief devices; or use vent scrubbers.

5. Emergency equipment: escape routes, rescue equipment, respirators, safety showers, eye baths.

In addition, good plant operating practice would include:

1. Written instruction in the use of the hazardous substances and the risks involved,

2. Adequate training of personnel.

3. Provision of protective clothing.

4. Good housekeeping and personal hygiene.

5. Monitoring of the environment to check exposure levels. Consider the installation of permanent instruments fitted with alarms.

6. Regular medical check-ups on employees, to check for the chronic effects of toxic materials.

Explosions

An explosion is the sudden, catastrophic, release of energy, causing a pressure wave (blast wave). An explosion can occur without fire, such as the failure through over-pressure of a steam boiler or an air receiver. When discussing the explosion of a flammable mixture it is necessary to distinguish between detonation and deflagration. If a mixture detonates the reaction zone propagates at supersonic velocity (approximately 300 m/s) and the principal heating mechanism in the mixture is shock compression. In a deflagration the combustion process is the same as in the normal burning of a gas mixture; the combustion zone propagates at subsonic velocity, and the pressure build-up is slow. Whether detonation or deflagration occurs in a gas-air mixture depends on a number of factors; including the concentration of the mixture and the source of ignition. Unless confined or ignited by a high-intensity source (a detonator) most materials will not detonate. However, the pressure wave (blast wave) caused by a deflagration can still cause considerable damage.

Sources of ignition

Though precautions are normally taken to eliminate sources of ignition on chemical plants, it is best to work on the principle that a leak of flammable material will ultimately find an ignition source.

Electrical equipment

The sparking of electrical equipment, such as motors, is a major potential source of ignition, and flame proof equipment is normally specified. Electrically operated instruments, controllers and computer systems are also potential sources of ignition of flammable mixtures.

Static electricity

The movement of any non-conducting material, powder, liquid or gas, can generate static electricity, producing sparks. Precautions must be taken to ensure that all piping is properly earthed (grounded) and that electrical continuity is maintained around flanges. Escaping steam, or other vapours and gases, can generate a static charge. Gases (CO2) escaping from a ruptured vessel can self-ignite from a static spark.

Process flames

Open flames from process furnaces and incinerators are obvious sources of ignition and must be sited well away from plant containing flammable materials.

Miscellaneous sources

It is the usual practice on plants handling flammable materials to control the entry on to the site of obvious sources of ignition; such as matches, cigarette lighters and battery-operated equipment. The use of portable electrical equipment, welding, spark-producing tools and the movement of petrol-driven vehicles would also be subject to strict control Exhaust gases from diesel engines are also a potential source of ignition.

Pressure

Over-pressure, a pressure exceeding the system design pressure, is one of the most serious hazards in chemical plant operation. Failure of a vessel, or the associated piping, can precipitate a sequence of events that culminate in a disaster. Pressure vessels are invariably fitted with some form of pressure-relief device, set at the design pressure, so that (in theory) potential over-pressure is relieved in a controlled manner.

Temperature deviations

Excessively high temperature, over and above that for which the equipment was designed, can cause structural failure and initiate a disaster. High temperatures can arise from loss of control of reactors and heaters; and, externally, from open fires. In the design of processes where high temperatures are a hazard, protection against high temperatures is provided by:

1. Provision of high-temperature alarms and interlocks to shut down reactor feeds, or heating systems, if the temperature exceeds critical limits.

2. Provision of emergency cooling systems for reactors, where heat continues to be

generated after shut-down; for instance, in some polymerization systems,

3. Structural design of equipment to withstand the worst possible temperature excursion.

4. The selection of intrinsically safe heating systems for hazardous materials.

Steam, and other vapour heating systems, is intrinsically safe; as the temperature cannot exceed the saturation temperature at the supply pressure. Other heating systems rely on control of the heating rate to limit the maximum process temperature. Electrical heating systems can be particularly hazardous.

Fire protection

To protect against structural failure, water-deluge systems are usually installed to keep vessels and structural steelwork cool in a fire. The lower section of structural steel columns are also often lagged with concrete or other suitable materials.

Noise

Excessive noise is a hazard to health and safety. Long exposure to high noise levels can cause permanent damage to hearing. At lower levels, noise is a distraction and causes fatigue.

The basic safety and fire protective measures that should be included in all chemical process designs are listed below:-

1. Adequate, and secure, water supplies for fire fighting.

2. Correct structural design of vessels, piping, steel work.

3. Pressure-relief devices.

4. Corrosion-resistant materials, and/or adequate corrosion allowances.

5. Segregation of reactive materials.

6. Earthing of electrical equipment.

7. Safe location of auxiliary electrical equipment, transformers, switches gear.

8. Provision of back-up utility supplies and services.

9. Compliance with national codes and standards.

10. Fail-safe instrumentation.

11. Provision for access of emergency vehicles and the evacuation of personnel.

12. Adequate drainage for spills and fire-fighting water.

13. Insulation of hot surfaces.

14. No glass equipment used for flammable or hazardous materials, unless no suitable alternative is available.

15. Adequate separation of hazardous equipment.

16. Protection of pipe racks and cable trays from fire.

17. Provision of block valves on lines to main processing areas,

18. Protection of fired equipment (heaters, furnaces) against accidental explosion and fire.

19. Safe design and location of control rooms.

SAFETY CHECK LISTS

Check lists are useful aids to memory. A check list that has been drawn up by experienced engineers can be a useful guide for the less experienced. A short safety check list, covering the main items which should be considered in process design, is given below.

Design safety check list

Materials

(a) Flash-point

(c) Auto ignition temperature

(d) Composition

(e) Stability (shock sensitive?)

(g) Corrosion

(h) Physical properties (unusual?)

(i) heat of combustion/reaction

Process

1. Fermenter:

(a) Exothermic—heat of reaction

(b) Temperature control—emergency systems

(d) Effect of contamination

(e) Effect of unusual concentrations (including catalyst)

(f) Corrosion

2. Pressure systems

(a) Need?

(b) Design to current codes (BS 5500)

(c) Materials of construction—adequate?

(d) Pressure relief—adequate?

(e) Safe venting systems

(f) Flame arresters

SAFETY AND LOSS PREVENTION

Control systems

(a) Fail safe

(b) Back-up power supplies

(c) High/low alarms and trips on critical variables

(i) Temperature

(ii) Pressure

(iii) Flow

(iv) Level

(v) Composition

(d) Back-up/duplicate systems on critical variables

(e) Remote operation of valves

(f) Block valves on critical lines

(g) Excess-flow valves

(h) Interlock systems to prevent mis-operation

(i) Automatic shut-down systems

Storages

(a) Limit quantity

(b) Inert purging/blanketing

(c) Floating roof tanks

(f) Earthing

(g) Ignition sources—vehicles

General

(a) Compliance with electrical codes

(b) Adequate lighting

(c) Lightning protection

(d) Sewers and drains adequate, flame traps

(e) Dust-explosion hazards

(f) Build-up of dangerous impurities—purges

(g) Plant layout

(h) Separation of units

(ii) Access

(iii) Sitting of control rooms and offices

(iv) Services

(i) Safety showers, eye baths

Fire protection

(a) Emergency water supplies

(b) Fire mains and hydrants

(c) Foam systems

(d) Sprinklers and deluge systems

(e) Insulation and protection of structures

(f) Access to buildings

(g) Fire-fighting equipment

REFERENCES:- Sinnot, R.K, “Coulson and Richardson’s Chemical Engineering”, vol-6,

Fourth edition, 2005

Trrybal, Robert E., “Mass transfer Operations”, Third edition, 1981

Perry, Robert H, Green, Don W., “Chemical Engineers’ Handbook”, 1999

Chattopadhyay, P “Unit Opeartions of Chemical Engineering”, vol-1, 2003

McCabe, Warren L, Smith, Julian C, Harriott Peter “Unit Operations Of Chemical Engineering”, Sixth edition, 2001

Process biotechnology fundamentals

Eckman, Donald P., “Industrial Instrumentation”, 1991

Hesse, Herman C., Rushton, J.Henry, “Process Equipment Design”,

Peters, Max S.,Timmerhaus,Klaus D. “Process Design and Economics for Chemical Engineers”, Fourth edition , 1958

Loh, H.P, Lyons, Jennifer, “Process Equipment Cost Estimation”, Jan 2002