Optimum operating conditions for improving natural gas dew point and condensate throughput

Accepted Manuscript Optimum operating conditions for improving natural gas dew point and condensate throughput Abeer M. Shoaib, Ahmed A. Bhran, Mostafa E. Awad, Nadia A. El-Sayed, Tamer Fathy PII:

S1875-5100(17)30416-X

DOI:

10.1016/j.jngse.2017.11.008

Reference:

JNGSE 2340

To appear in:

Journal of Natural Gas Science and Engineering

Received Date: 30 May 2017 Revised Date:

6 September 2017

Accepted Date: 11 November 2017

Please cite this article as: Shoaib, A.M., Bhran, A.A., Awad, M.E., El-Sayed, N.A., Fathy, T., Optimum operating conditions for improving natural gas dew point and condensate throughput, Journal of Natural Gas Science & Engineering (2017), doi: 10.1016/j.jngse.2017.11.008. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

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Optimum operating conditions for improving natural gas dew point and condensate throughput Abeer M.Shoaiba, Ahmed A. Bhrana,b, Mostafa E.Awada, Nadia A. El-Sayeda, and Tamer Fathyc a

Department of Petroleum Refining and Petrochemical Engineering, Faculty of Petroleum and Mining Engineering, Suez University, Suez, Egypt Chemical Engineering Department, College of Engineering, Al Imam Mohammad Ibn Saud Islamic University, Al Riyadh, Kingdom of Saudi Arabia c

South Dabaa Petroleum Company, Egypt

Abstract

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Natural gas dew point temperature is a vital quality parameter. The effective control of this specification is important if the natural gas integrity and quality are to be maintained. The present work focuses on improving the dew point and condensation production rate of south Dabaa field dew point control unit (DPCU) located in the Egyptian western desert and owned to the South Dabaa Petroleum Company. Influence of the operational variables on outlet gas dew point and produced condensate were investigated. The simulation results illustrated that feed gas inlet temperature, composition and flow rate, Joule Thomson (JT) valve downstream, and upstream pressure and hot bypass flow rate have a great effect on the sales gas dew point as well as the condensate throughput. A field experiments were conducted to validate the simulation results. It is noticed that there is a good agreement between simulation and experimental results, considering the outlet gas dew point at different operating conditions. Lingo optimization software was used to find the plant optimum conditions. Two quadratic equations were developed based on regression analysis for calculating the dew point and plant condensate rate at any operational variables. The impact of replacing the existing JT valve by a turbo expander was studied. The simulation results indicate that the turbo expander is more effective in comparison with JT valve refrigeration system in decreasing the sales gas dew point and increasing the condensate production rate. Keywords: Dew point control, sales gas, gas processing, JT valve, turbo expander

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1. Introduction

Hydrocarbon dew point has always been a vital operational parameter; it is becoming a critical tariff parameter for the natural gas industry. At the beginning of the natural gas processing chain, it is highly important to control the gas dew point. The dew point controlling process aims to ensure that liquids (either hydrocarbons or water) are not formed in the pipelines and consequently, a safe and reliable transportation can be achieved. The byproduct liquid which is recovered or produced by this process could be used as a valuable fuel or alternatively stabilized and marketed as condensate (Herring, 2011; Foglietta, 2004).

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The dew point is dependent on pressure, temperature and the composition of the gas (Herring, 2011). Dew point is defined as the highest temperature (at a given pressure) at which hydrocarbons in natural gas can condense. Natural gas pipelines are designed for single-phase transportation and, therefore, hydrocarbon condensation could have severe consequences for the safe transportation of the gas. Thus, accurate measurement and prediction of hydrocarbon dew points are of great importance to obtain safe and effective utilization of the natural gas pipelines. In addition, the economics of processing for both gas producers and shippers depend on dew point temperature ((Brown et al, 2008 ; Skylogianni, 2013; Grygorcewicz, 2010; Rusten et al, 2008).

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Recovery of natural gas liquids (NGL) components is required for hydrocarbon dew point control of natural gas as well as a source of revenue (Mokhatab et al, 2006). In field operations, hydrocarbon recovery is necessary for fuel conditioning or dew point control. Liquid hydrocarbons can be extracted from natural gas by many processes such as refrigeration, lean oil absorption, solid bed adsorption, membrane separation, and twister device new technology. The refrigeration processes include mechanical refrigeration; Joule-Thomson (JT) valve refrigeration, and cryogenic refrigeration by turboexpander (Kidnay and Parrish, 2006).

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The expansion across JT valve method is a relatively simple hydrocarbon dew point control system used in gas producing plants where pressure drop is available and very low temperatures are not required. The pressure drop across the valve causes the gas to expand and subsequently cools it. The produced liquids primarily pentane components and heavier, C5+, and water are separated in a low temperature separator. Thus the process can accomplish dew point control for both water and hydrocarbons in a single unit (Grygorcewicz, 2010). The use of JT to recover liquids is an attractive alternative in many applications. The JT process has some advantages over the turboexpander and other refrigeration processes. In JT process, Low gas rates can be applied with modest ethane recovery. Besides, the process can be designed with no rotating equipment in addition to simplicity of design and operation. Furthermore, this process has the least capital cost, compared to other processes. However, the least amount of NGLs can be recovered by JT system (Natural Gas Council Plus working group, 2005; Gas Processors Suppliers Association, 2004). Liquefied gases with very low boiling temperatures are called cryogenic fluids and are increasingly utilized in industrial, domestic and scientific applications. Cryogenic process plants in recent years are almost exclusively based on the low-pressure cycles which use a radial inflow turbo expander to generate refrigeration. The use of radial inflow turbo expanders brings forth attractive features like better efficiency and reliability in the operating range of the cryogenic process plants. It considerably affects the economical parameters of the process plants (Ghosh et al, 2010; Rahul et al, 2015). The main difference between the J-T design and turbo expanders is that the gas expansion is adiabatic across the valve, (Gas Processors Suppliers Association, 2004). 2

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There are many factors that should be taken into account when choosing the best hydrocarbon dew point controlling technology. The main factors include inlet conditions (gas pressure, richness and contaminants), downstream conditions (i.e. residue gas pressure), and overall conditions (i.e. utility costs and plant location). In addition to feed gas composition and operation mode, the most decisive technical characteristics of any process are the feed gas pressure and permissible unit pressure drop (Mokhatab et al, 2006). Process optimization is necessary to improve the performance of dew point process plants due to feed changes, equipment limitations and poor operation, operating conditions, chemical contaminants and environmental considerations (Rahimpour et al, 2013). Thus, this work is directed to improve the performance of the dew point unit through studying and optimization of the operating conditions of the dew point process.

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The DPCU of south Dabaa Egyptian field was chosen for accomplishing the present study. JT valve is the refrigeration technique applied in this plant. The main objectives of this work are: 1) To study the influence of the operating conditions on outlet gas dew point. This objective was achieved by using Aspen HYSYS simulation program of version 7.3.

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2) To identify the optimum operating conditions of the investigated DPCU. The considered operating conditions are gas inlet temperature, inlet gas flow rate, JT valve upstream pressure, JT valve downstream pressure, hot bypass flow rate, and gas composition (C6+ fraction in inlet flow). The optimization in this work has been carried out using Lingo software version 14; where Lingo is a comprehensive tool designed to build and solve linear and nonlinear mathematical optimization models efficiently. It is a powerful language capable of solving most classes of optimization models.

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3) To develop two correlations for relating gas dew point and condensate throughput to the DPCU operational variables.

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4) To investigate the influence of replacement JT valve by a turbo expander refrigeration technique on the PDCU operation. 2. Case study description

The case study on which this research was applied is the DPCU of south Dabaa field, located in the western desert of Egypt and owned to a South Dabaa Petroleum Company. The DPCU under consideration operates at an acceptable limit for each operating variable. The inlet gas flow rate, inlet gas temperature, JT valve upstream pressure, JT valve downstream pressure, hot bypass stream flow rate, and concentration of C6+ hydrocarbons in inlet gas are varied in the range of 15-35 MMSCFD, 24-42 oC, 1080-1245 Psig, 730-820 Psig, 0-4 MMSCFD, and 0.165-0.735 mol% respectively. The inlet and outlet gas compositions of the plant at average values of operating conditions are presented in Table 1. 3

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Methane Nitrogen CO2 Ethane Propane i-Butane n-Butane i-Pentane n-Pentane n-Hexane n-Heptane

Concentration, mole fraction Inlet gas Outlet gas 0.8377 0.8544 0.0055 0.0057 0.0207 0.0207 0.0708 0.0694 0.0354 0.0314 0.0079 0.0061 0.0116 0.0081 0.0041 0.0021 0.0033 0.0014 0.0022 0.0005 0.0008 0.0002

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Table 1: Inlet and outlet gas composition of the DPCU at average values of the operating conditions

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Figure 1 shows the process flow diagram of the DPCU of the south Dabaa field. A portion of the inlet gas is cooled in a gas/gas exchanger and the remaining portion of gas is cooled in gas/liquid exchanger in parallel arrangement. Inlet gas is pre-cooled by cross-exchange with the cold residue gas from the second stage cold separator, while the hydrocarbon liquids are pre-cooled from the low pressure (LP) separator. These cooled gases from gas/gas and gas/liquid exchangers are then blended together.

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Dew point depression of the inlet gas is achieved through the use of a single Joule Thomson (JT) expansion. As the high pressure stream is expanded across a JT valve to a lower pressure, its temperature will decrease. This reduction in temperature causes heavy hydrocarbons to condense out of the gas phase to form liquids, resulting in a residue gas with a lower dew point.

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A hot gas bypass on the JT skid is the second piping line and valve controlling the flow of the inlet gas stream within the JT skid. This hot gas bypass allows “trim” temperature control of the cold separator by bypassing inlet gas around the gas/gas exchangers. This gas stream allows a portion of the inlet gas to heat up the cold separator. Due to this action, either the gas flow or inlet gas temperature or pressure will be changed. The condensed hydrocarbon liquid and wet ethylene glycol are separated within the LP separator with an internal liquid-liquid plate pack coalesce, which assists in separating the fairly viscous glycol from the liquid hydrocarbon. Liquid hydrocarbon overflows an internal weir to a surge section. The wet glycol exits out through a glycol boot which is further regenerated and pumped to reinjection points within the plant. About 80 wt% ethylene glycol is injected into the 4

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inlet of the gas/gas exchangers, the gas/liquid exchanger and JT services to inhibit gas hydrates formation while the gas is cooled.

Figure 1: Process flow diagram of Dabaa field dew point control unit

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Prior to the JT expansion, any liquids that may have formed due to precooling are removed in the pre-separator. The pre-separator is a two phase vertical vessel which allows for two phase separation. The liquids discharge under level control to the LP separator and the gas is routed to the JT valve. Gas is expanded over the JT valve and the liquids that have formed are removed in the cold separator. The gas from the cold separator is sent back to the gas/gas exchanger, where it exchanges heat with the inlet gas before being routed to the sales gas compressors. The sales gas compressors boost the residue gas up to a sales gas pressure of 1350 psig. The LP separator receives the liquid discharges from the pre-separator and cold separator. The LP separator is a horizontal vessel which facilitates the degassing of the hydrocarbon liquids prior to the stabilizer tower. The gas leaving the LP separator is boosted in pressure through the flash gas compressor and routed to the suction of the sales gas compressors. The liquid hydrocarbons from the LP separator are sent back to the gas/liquid exchanger to exchange heat with the inlet gas and discharged to the stabilizer tower. In the stabilizer tower, the liquids are reboiled and stabilized to meet the required specification by separating the butanes and lighter components.

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3 Results and discussions

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The present work which applied to the hydrocarbon DPCU of the south Dabaa Egyptian field aims to study the natural gas dew point control process to improve the quality of outlet gas to meet pipeline specification requirements as well as to increase the plant profit. In addition, the optimum operating conditions were identified, using LINGO optimization software, version 14. Thus, the first part of results was directed to investigate the effect of the plant operational conditions on sales gas dew point and condensate production rate. The second part of this section introduces two correlations for calculating dew point and condensate rate in relation to the plant operational conditions. In the third part, the optimum conditions for obtaining the maximum condensate rate when keeping the outlet gas dew point at constant value of -5 oC at 700 Psig were discussed. The last part discusses the influence of replacing the JT valve with a turboexpander on the sales gas dew point and the condensate production of the considered DPCU. 3.1 Effect of the plant operational conditions on sales gas dew point and condensate production rate

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The effect of increasing the JT valve upstream pressure on dew point and condensate production rate was studied. Figure 2 shows the effect of JT upstream pressure (1080, 1100, 1120, 1140, 1160,1180,1200,1220 and 1240 psig) on the gas dew point at different inlet temperatures of the feed gas to the considered DPCU. The hot bypass flow, JT downstream pressure, feed gas flow rate, and C6+ hydrocarbons concentration in feed gas were kept constant at 0 MMSCFD, 730 psig, 15 MMSCFD, and 0.165 mol% respectively.

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It is obvious that as the JT valve upstream pressure is increased, the gas dew point is decreased. This can be attributed to the increase of the pressure drop through JT valve with increasing the upstream pressure and this consequently resulted in temperature drop of the gas. This reduction in temperature causes more condensation of heavier hydrocarbons that leads to obtain a residue gas with a lower dew point. It is also shown in Figure 2 that increasing the inlet gas temperature at constant JT valve upstream pressure increases the gas dew point. Increasing of inlet gas temperature increases the cold separator temperature and hence increases gas dew point temperature. This study also considered the influence of JT valve downstream pressure on the sales gas dew point at different hot bypass flow rates. The hot bypass stream is a quantity of gas that bypassed gas/gas exchanger and gas/liquid exchanger and goes directly to cold separator. Feed gas flow rate, C6+ hydrocarbons concentration in feed gas, inlet feed temperature and JT valve upstream pressure were maintained constant at 35 MMSCFD, 0.248 mol%, 26 oC, and 1160 psig respectively.

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24 C 32 C

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Figure 2: Effect of JT valve upstream pressure on outlet gas dew point at different feed gas inlet temperatures

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Figure 3: Effect of JT valve downstream pressure on outlet gas dew point at different hot bypass flow rates 7

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The simulation results presented in Figure 3 show that by increasing the JT valve downstream pressure, the sales gas dew point is increased. Increasing of downstream pressure decreases the pressure difference across JT valve and hence decreases the temperature drop through this valve and consequently leads to an increase in gas dew point. It is noticed also that the gas dew point is increased by increasing the hot bypass flow rate at all investigated JT valve downstream pressures. Increasing of hot bypass flow rate raises cold separator temperature and consequently increases gas dew point. 4.5 4 3.5

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Concentration of C6+ hydrocarbons in plant feed gas, mole % Figure 4: Effect of feed gas composition on sales gas dew point at different plant feed gas flow rates

The influence of increasing C6+ hydrocarbons concentration, in plant feed gas, on the outlet gas dew point at different values of feed gas flow rate was investigated (Figure 4). Inlet feed temperature, JT valve upstream pressure, JT valve downstream pressure, and hot bypass flow rate were maintained constant at 36 oC, 1100 Psig, 760 psig, and 3 MMSCFD respectively. It is clear from Figure 4 that increasing C6+ hydrocarbons concentrations from 0.165 up to 0.543 mol% has no significant influence on increasing the gas dew point. But at concentrations higher than 0.543 mol%, there is a great influence in increasing the dew point with increasing these 8

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heavier hydrocarbons concentrations. Additionally, it is noticed that the dew point is decreased with increasing the plant feed flow rate at all investigated C6+ hydrocarbons concentrations. For the first glance, it is not acceptable that the dew point is decreased with increasing the plant feed flow rate. However, if it is known that the studied flow rates are lower than the designed value of 35 MMSCFD, it will be acceptable that increasing gas feed rates (lower than the designed value) can decrease the outlet gas dew point. On the other hand, for feed rates higher than the design value, increasing the feed flow rate will increase the load on the DPCU and hence some noncondensable hydrocarbons will be raised in the outlet gas. This increases consequently the sales gas dew point.

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Concentartion of C5+ hydrocarbons in DPCU feed gas,mol %

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Figure 5: Effect of increasing C5+ hydrocarbons in the PDCU feed gas on condensate production rate at different feed gas inlet temperatures.

It is obvious from the last discussion that the effecting gas hydrocarbon composition on dew point temperature was taken as C6+; however, while studying the effect of gas hydrocarbon composition on the condensate production rate, it is found that C5+ has a greater effect than C6+ since the condensate formed from JT valve condensate method has a high percentage of C5+. The influence of increasing C5+ hydrocarbons concentration on condensate production at different values of DPCU gas inlet temperature was studied with keeping other variables constants. The hot bypass flow, JT upstream pressure, JT downstream pressure, and feed gas of the plant were kept constant at 0 MMSCFD, 1120 psig, 760 psig, and 25 MMSCFD respectively. 9

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The C5+ hydrocarbons were increased in the entering gas to the considered plant taking the values of 0.75, 0.95, 1.15 and 1.35 mol%. The experimental results presented in Figures 5 show that by increasing the percentage of C5+ hydrocarbons in the DPCU gas feed, the condensate production rate is increased. It is also noted that the condensate production is decreased with increasing the feed gas inlet temperature. This is because the increasing of gas inlet temperature decreases the gas dew point as well as the recovery percentage of heavier hydrocarbons which consequently reduces the condensate production rate.

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Figure 6: Effect of increasing C5+ hydrocarbons concentration in the PDCU feed gas on condensate production rate at different values of upstream pressure. The influence of C5+ hydrocarbons concentrations in the DPCU inlet gas on condensate production at different values of JT valve upstream pressure was investigated and presented in Figures 6. The other operation variables were not changed where hot bypass flow, inlet temperature, downstream pressure, and feed gas of the plant were kept constant at 0 MMSCFD, 36 OC, 820 psig, and 35 MMSCFD respectively. It is clear that increasing the C5+ hydrocarbons in the inlet gas leads to an increase of the condensate production rate at all investigated JT valve upstream pressures. In addition, the condensate production rate is increased by increasing the JT valve upstream pressure. This can be attributed, as stated before, to the increase of the pressure drop through JT valve with increasing the upstream pressure and consequently resulted in more temperature drop of the gas. This reduction in temperature causes more condensation of heavier hydrocarbons that leads to obtain a higher condensation production rate. 10

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The effect of gas composition on condensate production rate at different values of JT valve downstream pressure was studied with keeping other variables constants. The hot bypass flow, inlet temperature, upstream pressure, and feed gas flow of the plant were maintained at 2 MMSCFD, 28 oC, 1240 psig, and 30 MMSCFD respectively. From the results shown in Figure 7, it is obvious that the condensation rate is increased when the C5+ hydrocarbons mol% in the plant feed gas is increased at all the investigated downstream pressures. The increasing of heavier hydrocarbons (C5+) in the DPCU feed gas up to 1 mol% leads to a significant increase in the condensate rate at all the JT valve downstream pressures. Nevertheless, the increasing of these heavier hydrocarbons by values higher than 1 mol% has insignificant effect on the condensate production rate. It is also noticed that condensation rate is reduced with increasing the JT valve downstream pressure.

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Concentartion of C5+ hydrocarbons in DPCU feed gas, mole %

Figure 7: Effect of increasing C5+ hydrocarbons concentration in the PDCU feed gas on condensate production rate at different values of downstream pressure.

The present work considered also the influence of gas composition on condensate production rate at different values of feed gas inlet flow rates. The hot bypass, inlet temperature, upstream pressure, and downstream pressure were maintained at 0 MMSCFD, 32 oC, 1160 psig, and 730 psig respectively. According to Figure 8, the condensate production rate was increased by increasing C5+ hydrocarbons concentration in the feed gas until it reach 1 mol%. Higher concentrations (more than 1 mol%) of these heavier hydrocarbons have a minor effect on

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increasing the condensate rate. It is also clear from Figure 8 that increasing plant inlet gas flow rate increases the condensate production rate as it is expected.

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Concentartion of C5+ hydrocarbons in DPCU feed gas, mol %

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Figure 8: Effect of increasing C5+ hydrocarbons concentration in the PDCU feed gas on condensate production rate at different values of inlet flow rate.

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Figure 9: Effect of increasing C5+ hydrocarbons concentration in the PDCU feed gas on condensate production rate at different bypass hot stream flow rates. At last, the effect of gas composition on condensate production rate at different flow rates of hot bypass stream was studied. The other operational variables were maintained at constant values. The inlet flow rate, inlet temperature, upstream pressure, and downstream pressure were maintained at 20 MMSCFD, 24 oC, 1200 psig, and 790 psig respectively. The experimental results presented in Figure 9 show that increasing of heavier hydrocarbons (C5+) concentration in the plant feed gas leads to an increase in the condensate production rate and this influence is meaningful at lower concentrations (0.75 – 1 mol%). It is also noticed that the flow rate of bypass hot stream has insignificant influence on the condensate rate at lower C5+ hydrocarbons concentrations (0.75 - 0.95 mol%) in the feed gas stream. However, at higher concentration (>0.95 mol%), a moderate increase of the condensate rate was achieved by increasing hot bypass flow rate as illustrated in Figure 9.

3.1.1 Validation of simulation results To validate the obtained simulation results of the considered case study, it is important to compare between the simulation and experimental results. These results provide an investigation of the effect of operational conditions on the outlet gas dew point and condensate production. The influence of each operating variable was studied keeping the other variables constant. The inlet gas flow rate, inlet gas temperature, JT valve upstream pressure, JT valve downstream pressure, hot bypass stream flow rate, and concentration of C6+ hydrocarbons in inlet gas were varied in the range of 15-35 MMSCFD, 24-42 oC, 1080-1245 Psig, 730-820 Psig, 0-4 MMSCFD, and 0.165-0.735 mol% respectively. The experimental results compared with the simulation results of sales gas dew points calculated within the above mentioned operating conditions 13

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ranges are illustrated in Figure 10. According to this Figure, it is obvious that the simulation results of gas dew points are in good agreement with the experimental data. This agreement can be proven by the excellent fitting between the experimental and simulated dew point temperatures. The fitting line equation (x nearly equal y) has an excellent R-squared value of 0.997. This in turn indicates the effectiveness of Aspen HYSYS 7.3 simulation software in estimating the dew points and consequently the condensation rates of the investigated plant in the acceptable operating conditions limits.

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Figure 10: Comparison between experimental and simulated sales gas dew point temperatures obtained at different operating conditions of the considered DPCU

3.2 Correlations for calculating dew point and condensate rate

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This work is interested also in introducing two correlations; these two correlations are used to represent the effect of the independent variables (feed gas temperature, feed gas flow rate, hot gas bypass flow rate, JT valve upstream pressure, JT valve downstream pressure, and the hydrocarbons concentration) on both the gas dew point temperature and the condensation production rate, respectively. Regression analysis is used to extract the introduced correlations, depending on the experimental data. . The significance of these conditions is identified from the analysis of variance (ANOVA) method. The two resulted correlations to estimate the gas dew point and condensate production rate are as below:

= −1.9307 − 0.6792 0.0392 + 0.355

− 0.0632 + 0.4552 − 0.0262 + " " " + 0.4209 ! ! + 0.0192 + 0.0006

(1) 14

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= −1062.132 + 12.23 1032.188 %! − 0.404 #

+ 18.095 + 9.414 + 0.683 " − 226.252 "%! − 0.01361997

− 0.714 (2)

+

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"

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Where DPgas is the sales gas dew point in oC at 700 Psig, Fcond is the condensate rate in bbl/day, Tin is the gas inlet temperature in oC, Fin is the inlet gas flow rate in MMSFD, Fhs is the hot bypass flow rate in MMSFD, Pup is the JT valve upstream pressure in Psig, Pdown is the JT valve downstream pressure in Psig, C' ! is the mol% of C6+ hydrocarbons in the feed gas and C'%! is the mol% of C5+ hydrocarbons in the feed gas. The R2 statistical test was used to evaluate how well the experimental data were represented by the correlations. R2 is a value that always falls between 0 and 1. It is the relative predictive power of a model, Lazic 2004. The closer R2 up to 1 gives the better model representing the experimental data, Mapiour et al, 2010. R2 for the first and second correlations are 0.96 and 0.95 respectively, so that there is good agreement between experimental results and correlation results. The equations are valid within the operating conditions studied. Equations (1) and (2) have been normalized to be more efficient depending on the pseudo critical temperature and pressure of the natural gas stream, and assuming that the maximum flowrate of the natural gas is 35 MMSCFD. Equations (3) and (4) represent the normalized correlations: = ‫ ـــ‬2.48 + 39.171 (

+ 0.886

= ‫ ـــ‬1056.2431 ‫ ـ‬774.863 425.742 ( ‫ ـ‬455.133 218.619 "%! (4)

!

+ 65.015

+ 627.049 ( + 1025.756 (

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+ 1.781 "

(

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+ 0.675

− 16.947 " ) ‫ ـ‬0.781

( "

+ 41.32 ) + " %! − 1417.096 ( − 15.918

+ ! (3)

)

"

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−

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− 2.039

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25.708

(

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Where the subscript R denotes the reduced value of the corresponding parameter; for example, TinR represents the reduced temperature of the inlet gas stream, calculated by dividing the inlet gas temperature to the pseudo critical temperature of inlet gas stream. The subscript N denotes the normalized or dimensionless value of the corresponding parameter; for example, FinN represents the normalized input flow rate, calculated by dividing the input flow rate to the maximum allowable flow rate. 3.3 Optimal operating conditions As discussed above, it is found that there is a great influence of the DPCU operating conditions on the dew point of outlet sales gas as well as on the condensate production rate. Thus, the present research work aims to identify the plant optimal conditions. The optimization of this process can be obtained by a new method, namely the Adomian decomposition method (Abbasbandy, 2003; Fatoorehchi and Abolghasemi, 2012). In the present work, the operating 15

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parameters are optimized in LINGO optimization software version 14 to achieve a maximum condensate rate when keeping the dew point at a constant value of -5 oC at 700 Psig. By applying this optimization software, the optimum operating condition of feed gas temperature, feed gas flow rate, hot gas bypass flow rate, JT valve upstream pressure, JT valve downstream pressure, and the C6+ hydrocarbons concentration in feed gas could be predicted and estimated. The solution of that program for the DPCU of south Dabaa field located in the western desert of Egypt introduces the optimal operating conditions to produce a maximum condensate rate at a desired sales gas dew point of -5 oC. The problem formulation for the optimization problem could be summarized in the following equations and their constraints; where the objective function is to maximize the condensate rate; Maximize Fcond;

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(3)

subject to

(4)

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= -5;

Constraints for upper and lower bounds of each variable are formulated as follows: ≤ 1-./

,2./ ≤

≤ 12./

,234 ≤

≤ 1234

,567 ≤

≤ 1567

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, -./ ≤

,589:/ ≤ ,

;<=

≤

≤ 1589:/

!

≤1

;<=

(5) (6) (7) (8) (9) (10)

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where the scripts L and U correspond to lower and upper limits of the corresponding variables, respectively. The problem is a non-linear program (NLP). The overall mathematical formulation entails a number of 7 variables. Three of them are non-linear. 15 constraints are involved; two of these constraints are non-linear. The global optimum solution suggests that the maximum condensation rate could be achieved at 24 oC, 35 MMSCFD, 4 MMSCFD, 1156 Psig, 820 Psig, and 0.0735 mo% for feed gas temperature, feed gas flow rate, hot gas bypass flow rate, JT valve upstream pressure, JT valve downstream pressure, and C6+ hydrocarbons concentration in feed gas respectively. 3.4 Replacing JT valve by a turboexpander This research work investigated also the effect of replacing JT valve by a turbo expander in the considered DPCU at the predetermined optimum operating conditions. To achieve this objective, Aspen HYSYS version 7.3 with Peng Robinson equation of state “PR EOS” was used as a simulation tool. The replaced JT valve specifications are: line size is 6 inches; valve size is 3 inches, body is globe style, flange rating and size 600 Psig and 3 inches respectively, actuator spring and diaphragm type is fail close. However, the investigated turbo expander has the 16

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following specifications: delta T across it is -17.4 oC, delta P is 335 Psig, normal molar flow is 33.72 MMSCFD, expander duty is 185 KW, and nozzle diameter is 0.164 ft. The simulation results show that the replacement of JT valve by a turbo expander decreased the hydrocarbon dew point from -4.6 oC To -10.3 oC and condensate production increased by 150 bbl/day (from 925 bbl /day to 1075 bbl. /day).

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From the economic view point, replacing JT valve by turboexpander requires buying a turboexpander and a second stage compressor; as the sales gas line pressure is high. The sum of the equipment and installation costs for turboexpnder and the compressor, according to Aspen HYSYS, version9, will be $458,300 and $1,731,400 respectively. The corresponding extra operating costs including the utility costs are assumed to be 30% of the equipment and installation cost. This means that a total of $2,846,610 is required. On the other hand, the increased amount of condensate, with a price 45 $/bbl, would result in a revenue of $2,430,000/year. This means that the Return On Investment (ROI) will be about 85.5% , which is a reasonable impact. These results indicate that it is preferable to replace the JT valve by a turbo expander in the studied plant. Conclusion

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The first objective of the present work aims to study the influence of the operational conditions on the sales gas dew point and the condensate production of south Dabaa field. The simulation results showed that the sales gas dew point was increased with increasing inlet gas temperature, JT valve downstream pressure, and hot bypass flow rate. However, increasing of JT valve upstream pressure, and plant feed gas flow rate decreased the gas dew point. It is also noticed that increasing C6+ hydrocarbons concentrations in feed gas from 0.165 up to 0.543 mol% has no significant influence in increasing the gas dew point. But at concentrations higher that 0.543 mol%, the effect of increasing these heavier hydrocarbons concentration on increasing the gas dew point was noticeable. Results also illustrated that the condensate production rate was increased with increasing feed gas flow rate, C5+ hydrocarbons concentration in feed gas, and hot bypass flow rate. On the other hand, condensate rate was decreased with increasing feed gas inlet temperature and JT valve downstream pressure. It is important to note that the flow rate of hot bypass affects the condensate rate only at higher concentrations (>0.95 mol%) of C5+ hydrocarbons in feed gas where a moderate increase of the condensate rate was achieved by increasing bypass hot flow rate. The present study aims also to identify the DPCU optimum conditions for obtaining the maximum condensate rate when keeping the outlet gas dew point at its desirable value of -5 oC (measured at 700 Psig). The optimum operational conditions and response value for each variable could be predicted and estimated by using Lingo software version 14. The solution of that program indicates that the maximum condensation rate could be achieved at 24 oC, 35 MMSCFD, 4 MMSCFD, 1156 Psig, 820 Psig, and 0.0735 mo% for feed gas temperature, feed gas flow rate, hot bypass flow rate, JT valve upstream pressure, JT valve downstream pressure, and C6+ hydrocarbons concentration in in feed gas respectively. By using regression analysis, two correlations for predicting dew point and condensate rate in relation to the plant operational conditions were introduced in this work. The last part of the 17

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present study was directed to investigate the impact of replacing the JT valve with a turboexpander on the sales gas dew point and condensate production of the considered DPCU. The simulation results obtained at the predetermined optimum conditions indicates that the replacement of JT valve by a turboexpander decreased the hydrocarbon dew point from -4.581 o C to -10.32 oC and increased condensate production by 150 bbl/day. This shows the effectiveness of the turboexpander in improving the plant dew point temperature and condensate production rate. References

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1. Abbasbandy, S., Improving Newton-Raphson method for nonlinear equations by modified Adomian decomposition method , Appl. Math. Comput. , 145 , 2003, 887 – 893.

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2. Brown, A. Milton, M., Vargha, G., Mounce, R., Cowper, C., Stokes, A., Benton, A., Lander, D., Ridge, A., Laughton, A.,”Measurement of the hydrocarbon dew point of real and synthetic natural gas mixtures by direct and indirect methods”, Analytical Science Team, National Physical Laboratory, Teddington, UK, 2008. 3. Fatoorehchi, H., Abolghasemi, H., Investigation of nonlinear problems of heat conduction in tapered cooling fins via symbolic programming, Applications and Applied Mathematics,7, 2012, 717 – 734.

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4. Foglietta, J.H., “Dew point turbo expander process: A solution for high pressure fields”, Proceedings, JAPG Gas Conditioning Conference, Neuquen, Argentina, October 18, 2004. 5. Gas Processors Suppliers Association, Engineering data book, SI version, 12th ed., Volume I, 2004. 6. Ghosh, P., Nandi, B. R., Sarangi, S., 2010. Prediction on performance of cryogenic turboexpander using meanline approach. Proceedings of ICEC 23 – ICMC 2010, Wroclaw, Poland, 2010: 977 – 984. 7. Grygorcewicz, J-P, “Improvement of Hydrocarbon Dew Point Determination via Gas Chromatography”, A dissertation submitted in fulfillment of the requirements of Courses ENG4111 and ENG4112 Research Project towards the degree of Bachelor of Engineering, October 2010. 8. Herring, J. “Finanical impact of accuarately measuring hydrocarbon dew point,” in 90th Annual Convention of the Gas Processors Association, 2011. 9. Kidnay, A.J., Parrish, W.R.,”Fundamentals of natural gas processing”,CRC Press, Boca Raton, 2006. 10. Lazic Z R. Design of experiments in chemical engineering. 1st edition; Willey-CH Verlag GmbH: Weinlheim, Germany, 2004. 11. Mapiour, M., Sundaramurthy, V., Dalai, A. K. and Adjaye, J. Effects of Hydrogen Partial Pressure on Hydrotreating of Heavy Gas Oil Derived from Oil-Sands Bitumen: Experimental and Kinetics. Energy Fuels, 24, 2010, 772–784.

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12. Mokhatab, S., Poe, W.A., Speight, J.G., “Handbook of natural gas transmission and processing”, Elsevier Inc. USA, 2006. 13. Natural Gas Council Plus Liquid Hydrocarbon Drop Out Task Group, White Paper on Liquid Hydrocarbon Drop Out in Natural Gas Infrastructure, February, 2005. 14. Rahimpour M.R., Saidi M., Seifi M., Improvement of natural gas dehydration performance by optimization of operating conditions: A case study in Sarkhun gas processing plant, Journal of Natural Gas Science and Engineering, 15, 2013, 118-126. 15. Rahul Verma , Ashish Alex Sam, Parthasarathi Ghosh,”CFD analysis of turbo expander for cryogenic refrigeration and liquefaction cycleset”, Physics Procedia, 67, 2015: 373 – 378

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16. Rusten B. H., Gjertsen L. H., Solbraa E., Kirkerød T., Haugum T. and Puntervold S.,”Determination of the phase envelope – Crucial for process design and problem solving, GPA Annual Conference, , Grapevine, Texas, USA, 2008.

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17. Skylogianni, E.,” Measurements and Modelling of hydrocarbon dew points for natural gases.” Master thesis, Department of Energy and Process Engineering, School of Chemical Engineering, Norwegian University of Science and Technology (NTUA), 2013.

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Highlights

The improvement of dew point and condensation production rate of south Dabaa field dew point control unit (DPCU) – Egyptian western desert - is studied; A simulation for that unit has been developed using HYSYS

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The influence of operating conditions on natural gas dew point and condensation rate is studied.

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Two correlations for relating gas dew point and condensate throughput to the DPCU operational variables have been developed.

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An Non-Linear Program (NLP) algorithm is developed to find the optimum operating conditions of the investigated DPCU. The considered operating conditions are gas inlet temperature, inlet gas flow rate, Joule Thomson (JT) valve upstream pressure, JT valve downstream pressure, hot bypass flow rate, and gas composition (C6+ fraction in inlet flow).

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The influence of replacement JT valve by a turbo expander refrigeration technique on the PDCU operation has been investigated.

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