SIMULATION OF NATURAL GAS PROCESSING PLANT FOR BUMPLESS SHIFTnijesr.iefr.edu.pk/journalFolder/6/2fa68.pdf · SIMULATION OF NATURAL GAS PROCESSING PLANT FOR BUMPLESS SHIFT Ramzan,

SIMULATION OF NATURAL GAS PROCESSING PLANT 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] Phone: +92 42 -9029159, Mobile: +92 342- 5002555

Correspondence Author: [email protected]

Abstract: A large fraction of energy supplies in the world are fulfilled through natural gas. The un-even variations in the supply and demand 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 employs Turbo-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) equation of state (EOS) has been employed. The simulation was made offline with different inlet gas composition as if changing wells lined up 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 to maximum 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 of plant operating 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 and Gas 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 (for any 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 this research 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 temperature of 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.

in production loss of Liquefied Petroleum

NFC-IEFR Journal of Engineering & Scientific Research

Gas

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

Reboiler 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 5 C

Water &ntent<11.2 g/IDO SCM

Production:

Gas Flow~7QMM5CFD

Com points 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 Temperature

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 and methane 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 is warmed 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 Mixed P h a u s

Natural 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 mixed and 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 to NGL 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 Plant 29500 Sm 3 /h

Battery L i m i t Pressure Operating (min) 62.5 Barg

Battery L i m i t Temperature (min) 5 C (max) 45 C

Battery L i m i t Pressure Design 94 Barg

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

Component Composition CI 0.69 C2 0.08 C3 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 bottom Temp. (°C)

V-207 Top Temp. (°C)

V-208 Bottom Temp CQ

V-208 Top

Temp CQ

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 18931



temperature, pressure and outlet pressure variations has been studied. Directly this affects outlet temperature of Turbo-expander. With decrease in temperature 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 then developed.

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

Case-4 -70.14 175.33 -3.95 169.82 59.92 19.51 3227 19004

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 T T

J JZI J

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

Mol

e Fr

actio

n

5

Mol

e Fr

actio

n \

5

Mol

e Fr

actio

n

I X

5

l.D0e-001 - t3 • V. 5

l.D0e-001 -

5

l.D0e-001 -

5 0.000 -

1 D 1 5 20 2 5 3 ) 3 5

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 composition profile



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-207 bottom

• V-207 top

• V-208 Bottom

• V-208-Top

Base Case 1 Case 2 Case 3 Case 4

Case Simulation Case

Fig. 7: Desiredproduct stream temperatures

Conclusions:

The change in inlet conditions (Composition, pressure and temperature) effect the throughput and quality of 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 results obtained 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 Including Business Aspects." PhD. Texas A & M University. 1999.

[2] K e i t h A. B u l l i n , P.E., Jason Chipps , "Opt imizat ion of natural gas gathering systems and gas plants" Bryan Research and Engineering, Inc., 1999

[3] JogeirMyklebust, Optimal operation and design of natural gas processing networks, G a s 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 M a r i e B . A s k S t i g S t r a n d , SigurdSkogestad, "Implementation of MPC on a deethanizer at karsto gas plant" University of Science and Technology, N -7491 Trondheim, Norway, 2005

[5] John c. Polasek, Stephen t. Donnelly, jerry a. bullin, "Process Simulation and Optimization of Cryogenic Operations Using Multi-Stream Brazed A l u m i n u m Exchangers" Bryan Research and Engineering, Inc, 2001

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

[7] "Fundamental Data and Thermodynamic Modeling for Cryogenic LNG Fluids to Improve Process Design, Simulation and Operation" ARC / Chevron, Austraila, 2001

[8] J. K. abdel - la l and Mohamed aggour "Petroleum and gas field processing"

[9] Marshall, W. R., and R. L. Pigford. The Application of Differential Equations to Chemical Engineering Problems.University of Delaware, Newark, Delaware (1947).

[10] Tiller, F. M. , and R. S. Tour, "Stagewise OperationsApplications of the Calculus of Finite 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.


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SIMULATION OF NATURAL GAS PROCESSING PLANT FOR BUMPLESS SHIFTnijesr.iefr.edu.pk/journalFolder/6/2fa68.pdf · SIMULATION OF NATURAL GAS PROCESSING PLANT FOR BUMPLESS SHIFT Ramzan,