Upload
ogenioja
View
254
Download
0
Embed Size (px)
Citation preview
8/10/2019 Simulation of NG Processing Plant
http://slidepdf.com/reader/full/simulation-of-ng-processing-plant 1/6
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.
8/10/2019 Simulation of NG Processing Plant
http://slidepdf.com/reader/full/simulation-of-ng-processing-plant 2/6
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.
8/10/2019 Simulation of NG Processing Plant
http://slidepdf.com/reader/full/simulation-of-ng-processing-plant 3/6
Simulation of Natural Gas Processing Plant for Bumpless Shift
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
8/10/2019 Simulation of NG Processing Plant
http://slidepdf.com/reader/full/simulation-of-ng-processing-plant 4/6
Simulation of Natural Gas Processing Plant for Bumpless Shift
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
8/10/2019 Simulation of NG Processing Plant
http://slidepdf.com/reader/full/simulation-of-ng-processing-plant 5/6
Simulation of Natural Gas Processing Plant for Bumpless Shift
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
M o l e F r a c t i o n
\
5
M o l e F r a c t i o n
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
8/10/2019 Simulation of NG Processing Plant
http://slidepdf.com/reader/full/simulation-of-ng-processing-plant 6/6
Simulation of Natural Gas Processing Plant for Bumpless Shift
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