CHAMBAL FERTILIZERS & CHEMICALS LTD.PRESENTATION

ON“STUDY OF AMMONIA PLANT-2

&PERFORMANCE OF HEAT EXCHANGER”

DEPARTMENT OF PETROLEUM ENGINEERINGUNIVERSITY COLLEGE OF ENGINEERINGRAJASTHAN TECHNICAL UNIVERSITYKOTA

VISHESH BHADARIYAB.TECH 4TH YEAR PETROLEUM ENGINEERING

The Company dealers provide Urea and other agro-inputs like DAP (Di-Ammonium Phosphate), MOP (Murate of Potash), SSP (Single Super Phosphate), pesticides and seeds. Most of these products are sourced from reputed suppliers and sold under the “Uttam” umbrella brand

Ammonia I plant is based on Holder Topsoe Technology and reformer is of side fired type, the other plant (Ammonia II) is based on M W Kellogg’s technology and reformer in use is of top fired type. There are some design and operating conditions differences in both reformers

Making AMMONIA

H

N HH

NITROGEN from the fractional distillation of air

HYDROGEN from the reaction of steam with hydrocarbons

N2 + 3H2 <=> 2 NH3 - 92.4 kJ mol-1 A reversible reaction.It is exothermic.At dynamic equilibrium nitrogen, hydrogen and ammonia.

This plant produces 1700 metric tons per stream day of anhydrous liquid ammonia and a proportionate production of carbon dioxide by-product. The unit design is based on a high pressure catalytic reforming method to produce the above from primary raw materials comprising Naptha or natural gas, steam and air.

Ammonia is produced from a mixture of hydrogen and nitrogen where the ratio of Hydrogen to Nitrogen should be 3:1. For this ammonia plant the source of hydrogen is steam and hydrocarbons in the form of natural gas. The source of nitrogen is atmospheric air as in all other ammonia plants. Under the normal condition, liquid ammonia is produced at 37°C for direct use as feed to the Urea Plant. If Urea plant is not operating, the total ammonia production will be sent as liquid at -33°c to the storage facility. By-product carbon dioxide is delivered to urea plant or vented to atmosphere.

FEED DESULFURISATION

RSH + H2 RH + 2H2S

R(1)SR(2) + 2H2 R(1)H + R(2)H + H2S

The feed stock passes through a knockout drum to removes liquids/oil.The NG feed

contains organic sulfur compounds. These can best remove by catalytic conversion

to hydrogen sulfide, H2S, in presence of excess hydrogen and subsequent removal of

the H2S by absorption by Zinc Oxide. Feed flows through the Desulfurizer furnace. The

natural gas is joined by a hydrogen rich stream. Feed flow down through the bed, where

the organic sulfur compounds are decomposed and the sulfur is hydrogenated to

hydrogen sulfide. H2S is absorbed by Zinc Oxide. Maximum efficiency of Zinc Oxide

desulphurization is favored by increasing temperature, the temperature between 350-

400OC.

ZnO + H2S ZnS + H2O

Raw synthesis gas is produced from the feed in series of three major steps .

First, is the partial reforming of the desulphurized feed in the primary reformer

at 810°C.

secondary reformer where the methane is reduced to a low level by

introducing sufficient air to supply the required amount of nitrogen for

ammonia synthesis . Gases at the exit of secondary reformer are at 9790C and contains

about 0.3% CH4 along with N2, H2, CO & CO2. nickel based catalyst used.

CH4 + H2O CO + 3H2 ∆H0 = +206.3 KJ/g-mole

CH4 + 2 H2O CO2 + 4H2 ∆H0 = +165.1 KJ/g-mole

REFORMING

and contain about 10% CH4 along with H2, CO & CO2

GAS PURIFICATION SECTION:

 In this section, the shifting of carbon-monoxide to carbon-dioxide is done in two steps using a High Temperature Shift Converter and a Low Temperature Shift Converter. Subsequently carbon-dioxide is removed from rest of the gases by absorbing it into hot Potassium carbonate solution

Gases are further cooled down to 3710C before entering to HTS by way of superheating the HP steam

HIGH TEMPERATURE SHIFT CONVERSION OPERATION:

 

The process gas first enters the top of high temperature shift converter at a temperature

of 371°C through a distributor and flows down through the iron-based catalyst to exit

from the bottom of the converter. The catalyst converts a large volume of CO without

excessive temperature increase despite the exothermic reaction. The reaction is as

follows:-

 

CO + H2O CO2 + H2 -41.2 kj/g-mole

 

This reaction is reversible, with the shifting of CO favoured by the low temperature.

However, the rate of reaction is favoured by the high temperature. The process gas then

enters the shell side of HTS Effluent BFW heater to give up heat to boiler feed water

going to steam drum. Process gas at the exit of HTS contains ~ 3% CO.

 

 

LOW TEMPERATURE SHIFT CONVERSION OPERATION:

 

After HTS Effluent BFW heater, process gas enters Low Temperature Shift Converter Guard

at 217°C and passes through the copper based catalyst bed and flows to Low Temperature

Shift Converter. The catalyst in LTS Converter is also copper based because copper based catalyst

favours the shift reaction at a lower temperature which allows a closer approach to equilibrium.

The hot shifted gases from the outlet of LTS Converter containing ~ 0.3% CO are cooled down to

62°C before entering the CO2 Absorber.

CO2 REMOVAL (BENFIELD UNIT)

 The raw synthesis gas enters directly to the CO2 absorber through an internal distributor located

near the bottom of the tower. The up flowing gas passes through three packed beds of rings

The lower section of the CO2 absorber contains two packed beds of rings. The solution enters the

absorber above the top bed in the lower section. The upper section contains one packed bed of

rings. The lean solution enters the tower above this last bed.

The lean Benfield Solution used for CO2 removal has the following composition :-

Potassium Carbonate (K2CO3 ) : 30%

ACT-I : 0.5-0.8%

Vanadium Pentaoxide : 0.5%

Catalysed solution of potassium carbonate absorbs CO2 by chemical reaction. The CO2 beccomes

hydrated, forming carbonic acid, which in turn reacts with a carbonate ion to form bicarbonate ion,

as follows:-

CO2 + 2H2 H2CO3

H2CO3 + K2CO3 2KHCO3

METHANATION

In this reaction carbon monoxide and carbon dioxide present along with synthesis gas are converted back to methane as these acts as poisons for the ammonia synthesis reaction. The main constituent of the catalyst used for methanation reaction is nickel oxide and it is available in the form of spheres. The reaction involved in the process is

CO + 3H2 = CH4 + H2O + heat CO2 + 4H2 = CH4+ 2H2O + heat The catalyst operates in the temperature range of about 300°C to 340°C.

AMMONIA SYNTHESIS AND REFRIGERATION SECTION:

 

In this section, the purified synthesis gas is compressed and then

converted into ammonia. The production of ammonia from synthesis

gas includes the three processes.

1. Synthesis Gas Compression,

2. Ammonia Conversion

3. Anhydrous Liquid Ammonia Separation.

 

SYNTHESIS GAS COMPRESSION:

After cooling and condensate separation, methanator effluent synthesis gas containing

a volumetric ratio of hydrogen to nitrogen of approximately three to one is delivered to

the suction of the synthesis gas compressor.

The synthesis gas is compressed the gas is cooled up to 4.80C and condensate separated

and removed. The gas then flows through the molecular sieve drier to ensure all the

moisture & carbon oxides are removed prior to sending the synthesis gas to the ammonia

converter. Final discharge pressure of synthesis gas compressor is 153 kgf/sq. cm.

AMMONIA CONVERSION:

The ammonia synthesis takes place by the following

reaction:

3H2 +N2 2NH3 + Heat

Favorable conditions:

1. High Pressure 2. High Temperature

The ammonia conversion is done by two converters:

1. Horizontal converter 2. Vertical converter

 

An IRON catalyst lowers activation energy so bonds between nitrogen and hydrogen are broken more easily

A high temperature increases the energy of reactants to overcome activation energy and increase rate

Conditions which affect the RATE of reaction

Increasing pressure INCREASES % yield of ammonia

Equilibrium shifts to the RIGHT WHY?

Effect of pressure

The equation shows:4 molecules of gas on the LEFT2 molecules of gas on the RIGHT

Increasing pressure causes the equilibrium to adjust and reduce the effect by having FEWER gas molecules so shifts to the right

Why temperature affects the equilibrium

The reaction is EXOTHERMICIf the temperature increases, the equilibrium shifts to the LEFT to compensate for the extra enthalpy inputN2 + 3H2 2 NH3 - 92.4 kJ mol-1

This means less ammonia forms

An increase in temperature will increase rate but REDUCES % yield of ammoniaEquilibrium shifts to the left.WHY?

SEPARATION OF ANHYDROUS LIQUID AMMONIA:

 

Condensing and sub cooling the ammonia in the synthesis gas loop up to -17.8°C at 153Kgf/cm² reduces the ammonia

in the recycle stream from 20.92 volume% to 2.4 %. The final chilling to -17.8°C of the gas stream is done in Ammonia

Unitized Chiller by counter current cold gas from Ammonia Separator and the cold ammonia from the refrigeration

system. The liquid ammonia product separates in Ammonia Separator which is let down in Ammonia Letdown vessel

from where it goes to the Unitized chiller to be used as refrigerant. Synthesis gas from the Ammonia Separator goes to

Synthesis Gas Compressor last wheel as recycle gas through Ammonia Unitized Chiller at 31.8°C. Here it is joined

by make up and flows back to Ammonia Converter as feed.

 

Ammonia vapors from unitized chiller are again converted into liquid Ammonia by compressing in Refrigerant

Ammonia Compressor and condensing. This liquid Ammonia is sent to Urea plant for producing Urea. If Urea plant is

not consuming the full production, then it can be sent to Ammonia storage tanks from the last stage of unitized chiller

at –330C.

PROCESS CATALYST SUPPLIER VOLUME (m3)

Hydrotreating Cobalt- Molybednum HTAS 30.3

Final Desulfurization Cobalt- Molybednum HTAS 13.0

Final Desulfurization Zinc Oxide HTAS 28.0

Primary Reforming Nickel Based Johnson Matthey 35.6

Secondary Reforming Nickel Based Johnson Matthey 38.7

HTS Converter Iron Based Johnson Matthey 73.0

LTS Guard Copper Based Sud - Chemie 25.0

L T S Converter Copper Based Sud - Chemie 75.0

Methanation Nickel Based Sud - Chemie 30.0

Horizontal Type Converter Iron Based HTAS 85.6

Vertical Type Converter Iron Based HTAS 73.49

CATALYSTS USED FOR AMMONIA PRODUCTION IN PHASE-II

Fig1.1: Ammonia conversion profile normal case

Fig1.2: Ammonia conversion equilibrium condition

Theory of heat exchanger: Introduction : Heat exchangers are equipment that transfer heat from one medium to another. The proper design, operation and maintenance of heat exchangers will make the process energy efficient and minimize energy losses. Heat exchanger performance can deteriorate with time, off design operations and other interferences such as fouling, scaling etc. It is necessary to assess periodically the heat exchanger performance in order to maintain them at a high efficiency level. This section comprises certain proven techniques of monitoring the performance of heat exchangers, coolers and condensers from observed operating data of the equipment.

Purpose of the Performance Test :To determine the overall heat transfer coefficient for assessing the performance of the heat exchanger. Any deviation from the design heat transfer coefficient will indicate occurrence of fouling. Performance Terms and Definitions:Overall heat transfer coefficient, U Heat exchanger performance is normally evaluated by the overall heat transfer coefficient U that is defined by the equationQ=U x A x LMTD  Where Q = Heat transferred in kCal/hrA = Heat transfer surface area in m2

LMTD = Log Mean Temperature Difference in 0C U = Overall heat transfer coefficient

 When the hot and cold stream flows and inlet temperatures are constant, the heat transfer coefficient may be evaluated using the above formula. It may be observed that the heat pick up by the cold fluid starts reducing with time.

Heat duty of the exchanger can be calculated

either on the hot side fluid or cold side fluid

as given below.

Heat Duty for Hot fluid, Qh = W x Cphx (Ti-To)

………..Eqn-1,

Heat Duty for Cold fluid, Qc = w x Cpc x ( to-ti)

………...Eqn-2

If the operating heat duty is less than design heat duty, it may be due to heat losses, fouling in tubes, reduced flow rate (hot or cold) etc. Hence, for simple performance monitoring of exchanger, efficiency may be considered as factor of performance irrespective of other parameter. However, in industrial practice, fouling factor method is more predominantly used.

Sr. no.

Physical quantities

Shell side fluid (hot)(methanator effluent)

Tube side fluid (cold)(cooling water)

1 Mass flow rate (kg/hr)

75691 1007000

2 Temperature (°c)

Tin Tout Tin Tout

106.4 41 36 413 Specific heat

(kcal/kg °c).803 .859 1

4 Design pressure (kgf/cm2)

29.8 4

Effectiveness of heat exchanger:

CALCULATION: Design parameter

Qmax= MhCh (Th1-Tc1)

MhCh = 0.831 × 75691

=62899 kcal/hr °c

Qmax= 62899 × (106.4-36)

=4428089.6 kcal/hr

Sr. no.

Physical quantities

Shell side fluid (hot)(methanator effluent)

Tube side fluid (cold)(cooling water)

1 Mass flow rate (kg/hr)

83462 1180000

2 Temperature (°c)

Tin Tout Tin Tout

80 37 34 42

3 Specific heat (kcal/kg °c)

.803 .859 1

Operating parameters:

Qactual=MhCh(Th1-Th2)

= 83462 × 0.831 × (80-37)

=2982347 kcal/hr

Effectiveness =Qactual÷ Qmax

=MhCh(Th1-Th2)÷ Cmin(Th1-Tc1)

=2982347 ÷ 4428089.6

Effectiveness =0.6735

=67.35%

THANK YOU