Simulation of NG Processing Plant

8/10/2019 Simulation of NG Processing Plant

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SIMULATION OF NATURAL GAS PROCE SSI NG PLAN T

FOR BUMPLESS SHIFT

Ramzan, N*, Naveed, S, Muneeb, R.*, Tahir, F. M .^Department of Chemical Engineering, University of Engineering & Technology, Lahore 54890, Pakistan

Email: [email protected], [email protected]: +92 42 -9029159, Mobile: +92 342- 5002555

Correspondence Author: [email protected]. pk

Abstract: A large fraction of energy supplies in the world are fulfilled through natural gas. The un-even variations in the supply anddemand often affect the production process. Modeling and simulation is an effective tool to predict, plan and handle such variations.Often it is desired that a bumpless shift takes place in these situations. In this paper, a LPG/NGL recovery plant, which employsTurbo-expander technology for LPG/NGL recovery, is simulated using a steady state flow sheeting simulator (Aspen H YSYS). All

major units such as Turbo-expander, separators, heat exchangers, distillation column, reboiler and condenser in the base case have been simulated. The operating conditions are taken to match the field conditions of a plant in Pakistan PengRobinson (PR) equationof state (EOS) has been employed. The simulation was made offline with different inlet gas composition as i f changing wells linedup on the plant. The responses were recorded forvarious changes in the inlet gas composition and pressure. The performance of the

plant was studied to ensure that the steady state operation of the plant is not significantly affected (bumpless shift). This shall lead tomaximum productionusingcustomercontractual limits.

KeyWords: Turboexpander, Process Simulation, AspenHYSYS, LPG/NGL recovery, Steady State Operation

1. Introduction

Gas plants have several distinctive features that

cause operation complexity. One unique

characteristic is the fact that inlet feed stream

conditions do not remain constant. This is due to the

combination of feed streams from different well

formations, variation of pressure and composition of

inlet stream with passage of time and depletion of

reservoirs. As a result economic optimum set o f plantoperating process parameters such as temperature

and pressure change. To obtain optimum production

and meet required quality standards, mathematical

modeling and optimization of gas processing plant is

mandatory.

Computer Simulation is an important tool for

analysis and design of chemical processes. Oil andGas processing is believed to be an area where

Simulation could be used very advantageously

because hydrocarbons and other organic compounds

do not cause such troubles at calculations as strong

polar and ionized compounds. This research is to

counter the above mentioned problems of a Natural

gas processing plant.

At this processing facility, during wells shifting (forany maintenance purpose or to maintain wellhead

pressure), fluctuations in operating parameters take

place. These fluctuations result

(LPG), Natural Gas and NGL. The obj ective of thisresearch is to obtain optimum and on spec

production of Natural gas, LPG and NGL by

minimizing the fluctuation during wells shifting.

2. Case Study

Figure 1 describes the objectives and strategy for the

production dynamics of the plant.The disturbances

arrive in the process from feed in the form of

variation in inlet temperature, pressure, or

composition due to shift in the natural gas source

well. These disturbances mainly effect the operation

of Turbo Expander, De-Ethanizer Column and

LPG/NGL Splitter Column units in the NG

processing plant which lead to off spec product. In

this situation, the product specifications (see figure

1) are met by controlling the discharge temperatureof Turbo Expander, Top and Bottom stream

temperature of De-Ethanizer column and the

LPG/NGL Splitter Column. Now the expander

PvPM, De-Ethanizer column refrigeration and

reboiler loads and reflux ratio, reboiler and

condenser loads for the LPG/NGL splitter column

can be manipulated in order to control these

temperatures to eventually achieve the control

objectives.

mailto:[email protected]








Simulation of Natural Gas Processing Plant for Bumpless Shift

DISTURBANCES

Feed Composition variations

Inlet Pressure variations

Inlet Temperature variations

Feed From Wells

Manipulating Variables

Thru

Expander RPM Discharge Temperture

Refrigeration Load T h m

Reboi ler Load

v-207 Bottom Temperature

V-207 Top Temperature

Re*URa;e

Condenser; Load

Reboiler Load

Production and Quality Control Parameters

Sales Gas

Qual i ty:

Dew Point* -1 "C

Temp<49 5C

Water &ntent<11.2 g/IDO SCM

Production:

Gas Flow~7QMM5CFD

Com po in ts Recovery:

Propane < 0 .8 \'i

LPG

Production:

LPG Production-3600 Kgrtir

Quality:

4.&5<Vapor Pressure* 11.0 barg

95% Boiling Point < 2 °C

Specific Gravity .53-.57

Pentane Content < 2%

V-2D8 Bottom Tempera ture

V-2Q8 Top Temperature

NGL

Production:

NGL product flow rale ~ 4QDQ BPD

Quality:

Reid vapor pressure

(max.) 0.69 Bar

(avgj 0.56 Bar

(min.) 0.41 Bar

Sediment (ASTM D96)*0.Q5%

Fig. 1 Simplified block diagram representation of problem

under study

In this work, a simulation model of Natural gas

process plant is developed in order to study

disturbances, such as variation in inlet gas

composition, pressure and temperature due to well

shifting. The optimum operational parameters shall be found out to ensure the on specs production of

Natural gas, LPG and NGL.




3. Process Description:

The Gas field under consideration utilizes three oil &gas reservoirs from which condensate, associated

gas and crude oil are produced. As shown in Figure 2,

hydrocarbon (HC) fluid from different wells is

collected in Production and test manifold,

transmitted to production separator and the

Production separator splits this feed into HC gas, HC

liquid and water. HC gas and liquid both contain

saturation water which is removed in Dehydration

beds. Raw natural gas consists primarily of methane(CH4) as well as various amounts of heavier

hydrocarbon gases. The moisture free gas passes

through Brazed Aluminum Heat Exchanger

(B AHX) and its temperature is lowered to almost -45

°C. From the BAHX, HCs condensed are separated

in turbo expander suction drum and the separated gas

at high pressure is expanded into lower pressure

causing temperature to drop to less than -65 °C

liquid streams from low temperature separator,

suction drum and moisture free hydrocarbons liquid

are routed to de-ethanizer column where ethane andmethane are stripped off. These lighter HCs after

passing through BAHX are compressed to line

pressure and injected into domestic transmission

Line. Vapor from the top tray of the de-ethanizer

column is routed through the condenser where it is

cooled to -35 °C causing all heavier HCs to condense

and are returned back to the distillation column as

reflux. The overhead gas product from the de-

ethanizeris sent through the cold box where it iswarmed by the inlet gas. It is then compressed in the

gas compressor driven by an electrical motor before

entering the distribution pipeline.The hot (165 °C)

LPG/NGL mixture feed from de-ethanizer column is

transmitted to LPG/NGL splitter column at the 13th

plate.The splitter reboiler temperature is 200 °C and

top temperature is maintained around 68 °C. These

temperatures can be used to manipulate LPG

W M s l 1 i . i 3 . 1 0 r i 2 > M 18.16

M i x ed P h a u s

N a t u r a l G a s

H «Bv i s f s L q u d

Fig. 2 Process diagram

causing further condensation. Condensed HCs are

separated in low temperature separator, from where

both HC liquid streams and the gas stream are mixedand exchanged in a BAHX causing their temperature

to rise.The gas from low temperature separator is

transmitted to turbo compressor and then routed to

transmission line for domestic supplier The HC

purity. The LPG recovered at the top of the column is

further cooled in a cooler through chilled water and

moved to LPG storage bullets.The NGL from the

column is cooled in the de-ethanizer preheater and

its temperature is further lowered in fined-fan cooler

and then sub-cooled in a chilled water cooler and

transmitted toNGL storage tanks




Aspen HYSYS Simulation model:

HYSYS is an important simulation tool with

wide

application. For determination of different operating

parameters a base case is developed. First of all

simulation study of base case is carried out. The NG

Processing plant under consideration is designed to

process nominal 29500 SmVh of well fluids. The

well fluid is being processed to obtain NGL, LPG

and Residual Gas products. Well fluid conditions are

given in Table 1 and composition is given in Table 2.

For the development of the case the Peng-Robinson

equation of state has been selected in the Basis

Environment as it is the most widely

used property

package for natural gas processing due to its

directness and accuracy. The De-ethanizer column

separates the feed into ethane as the distillate

product, whereas the heavier componentsC3-C5 and

above are fed to the splitter. The following

constraints are considered in de-ethanizer column

simulation for quality control and maximum

throughput.

Table 1: Well Fluid Conditions

Total Feed Rate to

Processing Plant29500 Sm 3 /h

Battery L i m i t PressureOperating (min)

62.5 Barg

Battery L i m i t Temperature(min) 5 C

(max) 45 C

Battery L i m i t PressureDesign

94 Barg

Table 2: Base case inlet conditions at T = 45°C

Component Composition

CI 0.69

C2 0.08C3 0.04

i-C4 0.01

n-C4 0.02

i-C5 0.01

n-C5 0.01

C6 0.01

C7+ 0.12

1. Reflux condenser outlet temperature: The

outlet temperature constraint of -35 °C is used

in order to maintain top temperature up to -10

°C.

2. Propane Recovery: Propane recovery is

the percentage of propane product

recovered from the feed. This is the initial

estimate.

3. Ethane Content in Bottoms: Percentage of

ethane in total LPG/NGL mixture should be

minimized in order to control Reid Vapor

Pressure of LPG.

4. Propane Content in column Top: Propane

content is the Percentage of propane in Ethane

stripped from the column. This propane

should be less than 1% in order to recover

maximum of propane in LPG.

For the case of LPG/NGL splitter column simulation

the LPG/NGL feed mixture comes from the de-

ethanizerreboiler. The column employs eighteen

stages with the feed entering at the thirteenth plate.

The parameters used as constraints or initial

estimates in Splitter column simulation for quality

control and maximum throughput are propane

recovery, ethane content in LPQ pentane content in

LPG, Specific gravity of LPQ vapor pressure of LPG

and Reid Vapor Pressure of NGL.

During Turbo expander simulation the effect of inlet

Table 3. Base case simulation results

Case K-201 out.Temp (°C)

V207

bottomTemp.

(°C)

V-207 TopTemp. (°C)

V-208

BottomTemp

CQ

V-208

TopTemp

CQ

Natural

Gas(MMscfd)

LPG

Prod(Kg/hr)

NGL

Prod(Kg/hr)

Base-68 63 173 99 -4 45 169.62 59 60 19.52 3287 18931




temperature, pressure and outlet pressure variations

has been studied. Directly this affects outlet

temperature of Turbo-expander. With decrease intemperature lighter components are condensed

causing more LPG recovery.

For inlet conditions discussed in Table 2, the main

parameters to be controlled to obtain optimum and

on specs Natural gas, LPG and NGL are listed in

Table 3.

4. Results and Discussions:

Figures 3 to 6 represent the dynamics of the splitter

and de-ethanizer column. These profiles are typical

for plate columns with sharp indentations appearing

at the plate location where feed is introduced. After

the development of the base case, the introduction of

disturbances is carried out in a systematic manner as

explained under. Four different cases are thendeveloped.

Case 1: For Bumpless shift study we consider any

pseudo case in which the first well is shut down and

feed is increased from the third and fourth well

equally. As the first well is rich in heavier

components, so its shut down will change inlet gas

compositions.

Case 2: The first well is taken in service with

maximum Load and feed reduced from the third and

fourth well equally to maintain their Well head

pressure.

Case 3: The third well is shut down and feed

increased from the second and fifth well equally.

Case 4: The second well is shut down to maintain

wellhead and feed adjusted from the third and fifth

well equally.

Table 4. Simulation of results of various cases

Case#

K-201

out.

Temp

(°C)

V207

bottom

Temp. (°C)

V-207

Top

Temp.

(°C)

V-208

Bottom

Temp

(°C)

V-208

Top

Temp

(°C)

Natural Gas

(MMscfd)

LPG

Prod

(Kg/hr)

NGL

Prod

(Kg/hr)

Base

case-68.63 173.99 -4.45 169.62 59.60 19.52 3287 1 8931

Case-1 -67.85 165.47 -5.32 166.69 60.02 21.33 3652 17000

Case-2 -69.10 178.35 -4.00 171.27 59.48 18.61 3111 19888

Case-3 -68 80 180 73 -3 90 171 97 59 69 18 34 3013 20235

Summary of simulation results (Throughput &

parameters to be adjusted to obtain these results) of

base case and all other cases are given in Table4.

O 150 J

TTJ

JZIJ

r

Fig. 3: De-ethanizer column temperature profile

0.700¬

0.600 -

s - Methar

ErhanE

e (Light)

5

0.700¬

0.600 -PnHjar

i-Epar

e (Light)

e (LightV

5

M o l e F r a c t i o n

5


\

5


I X

5

l.D0e-001 - t3 • V.

5

l.D0e-001 -

5

l.D0e-001 -

50.000 -

1 D 15 20 25 3 ) 35

Fig. 4: De-ethanizer column composition profile

<

f

I 1

Fig. 5: Splitter temperature profile

- H - M

i - a - Et

:hai e /Lig

isns (Lgh

ht)

)

\S- Pr

-4a- i-E

•pane (Lig

utane (Lig

ht)

ht)

r

Fig. 6: Splitter compositionprofile




Figure 7 gives the comparison of the entire variables

which have direct response on the specifications of

products. As it can be seen they

remain somewhat constant throughout the changes

to the process indicating that process parameters'

change has been employed successfully.

200

150

100

50

-50

-100

rr

• K -201 out

• V - 2 0 7 b o t t o m

• V - 2 0 7 t o p

• V - 2 0 8 B o t t o m

• V - 208 - T op

Base Case 1 Case 2 Case 3 Case 4

CaseSimulation Case

Fig. 7: Desiredproduct stream temperatures

Conclusions:

The change in inlet conditions (Composition,

pressure and temperature) effect the throughput and

quality o f products. To counter the above mentioned

problem, plant is simulated on HYSYS. From this

simulation work about 80 % of bumps have been

reduced. Different cases of inlet composition

variations verify the possibility of controlling

product quality and

maintaining it at a constant Based on the resultsobtained from the cases study, it should be noted that

the optimum operating parameters change. So,

during wells shifting, to obtain optimum and

References

[1] Bullin, Keith: A. "Economic Optimization of Natural Gas Processing Plants IncludingBusiness Aspects." PhD. Texas A & M University.1999.

[2] K e i t h A. B u l l i n , P.E., Jason Chipps ,"Optimization of natura l gas gathe ringsystems and gas plants" Bryan Research andEngineering, Inc., 1999

[3] Jogei rMykle bust, Opti mal operation anddesign of natural gas processing networks,G as T e c h n o l o g y C e n t r e a t

NTNU/SINTEF,Norway 2004

[4] E l v i r a Mar ie B. Ask St i g Stra nd,

SigurdSkogestad, "Implementation of MPCon a deethanizer at kar sto gas pl an t"University of Science and Technology, N-7491 Trondheim, Norway, 2005

[5] John c. Polasek, Stephen t. Donnelly, jerry a. bu ll in , "Process Simulation and Optimizationof Cryogenic Operations Using Multi-StreamBrazed Aluminum Exchangers" Brya nResearch and Engineering, Inc, 2001

[6] JaroslavPozivil "Use of Expansion Turbinesin Natural Gas Pressure Reduction Stations"Dept. of Informatics and Control Engineering,Technickal995

[7] "Fundamenta l Data and ThermodynamicModeling for Cryogenic LNG Fluids toImprove Process Design, Simulation andOperation" ARC / Chevron, Austraila, 2001

[8] J. K. ab del -la l and Moham ed aggour"Petroleum and gas field processing"

[9] Marshal l, W. R., and R. L. Pigford. TheApplication of Differential Equations toChemical Engineering Problems.Universityof Delaware, Newark, Delaware (1947).

[10] Tiller, F. M. , and R. S. Tour, "StagewiseOperationsApplications of the Calculus ofFinite Differences to Chemical Engineering,"Trans. AIChE, 40,317-332 (1944).

on specs production of the desired products the plant

may be tuned to operating parameters determined

through this simulation.The results obtained through

this simulation will be very useful for

Fluctuation control of plant operating

parameters, resulting in quality improvement

and production enhancement during wells

shifting

Simulation for changing inlet gas composition

and pressure with life of gas field, causing up

to mark recovery of hydrocarbon reserves and

for line up of future wells.

NFC-IEF R Journal of Engineering & Scientific Research

Simulation of NG Processing Plant

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