Control of cryogenic extractive distillation process for separating CO2–C2H6 azeotrope

Control of cryogenic extractive distillation process for separating CO2–C2H6 azeotrope

Computers and Chemical Engineering 128 (2019) 384–391 Contents lists available at ScienceDirect Computers and Chemical Engineering journal homepage:...

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Computers and Chemical Engineering 128 (2019) 384–391

Contents lists available at ScienceDirect

Computers and Chemical Engineering journal homepage: www.elsevier.com/locate/compchemeng

Control of cryogenic extractive distillation process for separating CO2 –C2 H6 azeotrope Haiqin Wang a,b, Minglong Fan a,∗, Zubin Zhang a,b, Jingyu Hao a, Ce Wang a a b

College of Pipeline and Civil Engineering, China University of Petroleum (East China), Qingdao, Shandong, China Shandong Key Laboratory of Oil & Gas Storage and Transportation Safety, Qingdao, Shandong, China

a r t i c l e

i n f o

Article history: Received 31 March 2019 Revised 9 June 2019 Accepted 20 June 2019 Available online 24 June 2019 Keywords: CO2 -Ethane azeotrope Extractive distillation Dynamic Controllability

a b s t r a c t The existence of heterogeneous azeotrope of CO2 –C2 H6 makes it difficult to separate CO2 from the hydrocarbons in the associated gas by cryogenic extractive distillation process. A four-column extractive distillation system using natural gas liquid as heavy solvent is used to separate CO2 –C2 H6 azeotrope. Carbon dioxide, ethane, propane and butane are recovered from the top of the extractive distillation column, deethanizer, depropanizer and debutanizer respectively. The C3+ hydrocarbon mixture called natural gas liquid is recovered at the bottom of the debutanizer, one part of which is transported as NGL product and the other part is pumped back near the top of the extractive distillation column as solvent. The purpose of this paper is to develop an effective plant wide control structure for this four-column extractive distillation system. Fixed reflux ratio and reflux/feed ratio control structure are used to investigate the controllability of four-column extractive distillation. Results show that both control structures have successfully realize the dynamic simulation of the extractive distillation process. However, under the disturbance of feed flow rate and composition, the controllability of reflux/feed ratio control structure on purity of top distillation products is better than that of fixed reflux ratio control structure. The reflux/feed ratio controller of four columns can adjust the reflux ratio in real time according to the disturbance of feed flow and composition, which greatly improves the stability of the top distillation products purity. © 2019 Elsevier Ltd. All rights reserved.

1. Introduction CO2 -EOR technology (Algharaib and Al-Soof, 2010) has the advantages of both enhancing crude oil recovery (economic benefits) and achieving effective CO2 storage (environmental benefits). However, the current natural gas low-temperature cryogenic process has the phenomenon of CO2 –C2 H6 azeotrope generation, which hinders the further separation and purification of natural gas. Most papers only proposed the steady-state separation design of CO2 – C2 H6 azeotrope, but the operating process parameters of the device were constantly changing in the actual production. In order to control the process parameters within a reasonable working range, continuous control of operating variables (flow rate, temperature and pressure) was required, namely dynamic simulation. Many dynamic control structures for distillation column have been reported, but due to the influence of feed composition, relative volatility between components, product purity, and energy consumption, no single control structure is suitable for all distillation columns. According to our knowledge, Gorak and



Corresponding author. E-mail address: [email protected] (M. Fan).

https://doi.org/10.1016/j.compchemeng.2019.06.017 0098-1354/© 2019 Elsevier Ltd. All rights reserved.

Schoenmakers (2014) first proposed two basic control structures: single-ended control structure and double-ended control structure. When selecting control structure, Gorak & Schoenmakers proposed the method of feed composition sensitivity analysis to select the control structure reasonably. If the disturbance is only the change of feed flow, the single-ended control structure can well control the quality requirements of the product. If the disturbance is not only the change of feed flow but also the change of feed composition, only the control flow rate and the tray temperature are constant, the composition of the product will deviate from the specified value, so the double-ended control structure is needed. After determining the control structure, it is necessary to select the controller (proportional controller, proportional integral controller and proportional integral differential controller) and tune the controller constant. Luyben (2010) carried out dynamic simulation of isopropanol dehydration process with fixed reflux ratio control structure. Results showed that when the disturbance of the feed flow was ±20%, the purity of the water at the bottom of the distillation column and the isopropanol at the top of the extractive column would fluctuate greatly due to the solvent flow could not correspondingly increase or decrease in time. According to the combustion control structure of the furnace (there is always excess

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air in the furnace), a ratio control method of the solvent and the feed flow was proposed, which could well control the product purity under the disturbance of large feed flow. Arifin and Chien (2008) similarly carried out dynamic simulation of isopropanol dehydration process, and found that fixed reflux ratio control structure could not well control the product purity of extractive distillation column and solvent recovery column. The reflux/feed ratio control structure of two columns was proposed, which greatly improved the control range of feed flow and composition. Luyben (2016) proposed the control structure of isopropanol dehydration feed splitting process. Compared with the traditional control structure, the control structure was complex and the dynamic simulation was not robust. Based on the extractive distillation separation process of maximum-boiling azeotropic aetone/chloroform, Luyben (2008) developed a dual-ended temperature control structure, which could simultaneously control the both overhead and bottom products purity of the columns, and greatly improved the control range of feed flow and composition. Luyben (2013) carried out the dynamic simulation of CO2 – C2 H6 azeotrope extractive distillation process. The simulation results showed that the traditional single-ended control structure could not well control the purity of CO2 product on the top of the extractive distillation column under the condition of feed composition disturbance. Therefore, the composition control structure of extractive distillation column top distillate was proposed, which showed good controllability and high purity of carbon dioxide products under the condition of feed flow and composition disturbance. Wang et al. (2016) carried out dynamic simulation of divided-wall column process and pressure-swing distillation of acetonitrile/n-propanol azeotrope respectively, and proposed that temperature-composition cascade control structure and pressure compensation-temperature control structure could achieve effective control of large-scale feed flow and composition disturbance. Wang et al. (2018) studied the control structure controllability of extractive distillation process for toluene-methanol-water ternary azeotropic mixture by using fixed reflux ratio control structure, reflux/feed ratio control structure and reboiler/feed ratio control structure. None of the aforementioned control structures could handle composition disturbances well. In order to improve the controllability and economy of the control structure, a proportional control structure for the bottom temperature of the extractive column was proposed. In this paper, based on the CO2 –C2 H6 azeotrope cryogenic extractive distillation process designed in our earlier stage, fixed reflux ratio control structure and reflux/feed ratio control structure were established, and the dynamic response characteristics of feed flow and composition disturbance of two control structures are analyzed.

2. Steady-state studied The flowsheet is shown in Fig. 1 with detailed information provided by our previous research (Wang et al., 2019) with some modifications. four-column extractive distillation process of CO2 – C2 H6 azeotrope is introduced as follows: the solvent NGL and the azeotropic mixture with composition of 10.2 mol% CO2 , 35.7 mol% C2 H6 , 26.65 mol% C3 H8 and remaining composition range from butane (C4) down to heptane (C7) are fed into extractive distillation column, and the key composition of CO2 is distillated from the system first. The bottom distillate of extractive distillation column passes through deethanizer, depropanizer and debutanizer successively to recovery the ethane, propane and butane products. The bottom distillate NGL of the debutanizer is divided into two parts: one is returned to the extractive distillation column as solvent to break the CO2 –C2 H6 azeotrope; the other is transported as NGL

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product. Aspen Hysys simulations are used in the steady-state and dynamic studied with Peng–Robinson physical properties. Fig. 2 gives the composition profiles of four columns in extractive distillation process. Qualified products satisfying the oil field specifications of liquid ethane (General Administration of Quality Supervision 2015) and liquefied petroleum gas (General Administration of Quality Supervision 2011) can be obtained at the top of extractive distillation column, de-ethane column, de-propane column and de-butane column, with the corresponding purity of carbon dioxide 95%, ethane 96%, propane 98% and butane 95% respectively. The results show that the cryogenic extractive distillation process designed for processing CO2 –C2 H6 azeotrope is effective and efficient. 3. Dynamic control Dynamic simulation can directly reflect the change of process parameters (flow, temperature, pressure, etc.) at a certain time and its influence on products (purity, temperature, flow, etc.). Therefore, dynamic simulation analysis is carried out for the extractive distillation process shown in Fig. 1. Before dynamic simulation, first define equipment size, then select appropriate control structure, and finally start dynamic simulation. 3.1. Equipment size calculation Before starting dynamic simulation, the size of all relevant equipment should be determined (at least approximate estimation). If the equipment size is not appropriate, unrealistic damping effect will appear in the simulation process. Equipment size calculation is based on the residence time. That is, the sizes of condensers and reboilers are designed to provide a 5 min holdup if half-full. Pumps and valves are set suitable pressure drops to meet the dynamic operation (Wang et al., 2015). According to the steady-state simulation process parameters, Table 1 gives the calculation results of the volume of related equipment in dynamic simulation. In steady-state simulation, the column pressure is no restriction. However, in dynamic simulation, the pressure in the distillation column should be checked according to its geometric size (diameter, height, and spacing, etc.). If the pressure distribution in the column is not reasonable, the flow in the column will keep fluctuating until equilibrium, which is easy to cause instability of the column. Therefore, all stages sizes in the dynamic simulation process are calculated with Aspen Hysys Tray Sizing Utility (see Table 2). 3.2. Sensitivity tray regulations In the dynamic control structure of distillation column, the purity of distillate product is controlled by using top reflux ratio and the bottom reboiler energy consumption respectively. When the feed flow or composition changes, the temperature will be unevenly distributed along the direction of column height. Temperature-sensitive stage is selected as the control stage of reboiler energy consumption, which can prevent light composition flowing to the bottom of the column and the recombination composition channeling into the top of the column. Therefore, products meeting the purity requirements can be obtained. The commonly methods for determining sensitivity stages are slope criterion (Luyben, 2006) and constant temperature criterion (Hori and Skogestad, 2007). Common slope criterion is used to select the position of the sensitivity stage for the designed extractive distillation process. Based on the temperature profile of four columns obtained by steady-state simulation, Fig. 3 gives the temperature difference

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Fig. 1. Extractive distillation process of CO2 –C2 H6 azeotrope with NGL solvent.

Fig. 2. Composition profiles of four columns in extractive distillation process.

Table 1 Calculation results of vessels volume. Items

3

Q/(m /h) V/(m3 )

Extractive distillation column

Deethanizer

condenser

reboiler

condenser

reboiler

Depropanizer condenser

reboiler

Debutanizer condenser

reboiler

3.32 1.11

24.57 8.19

16.19 5.4

38.0 12.67

13.38 4.46

18.07 6.02

3.61 1.2

8.69 2.89

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Table 2 Calculation results of columns size. Items

Extractive distillation column

Deethanizer

Depropanizer

Debutanizer

Type Diameter/(m) Spacing/(m) Height/(m) P/(kPa)

Packed 0.6096 / 18.9 0.8619

Sieve 1.067 0.6096 12.19 3.999

Sieve 0.9144 0.6096 24.38 8.929

Packed 0.6096 / 24.93 5.603

Fig. 3. Temperature difference curve between stages.

curves between stages of extractive distillation column, deethanizer, depropanizer and debutanizer, which can be used to determine the position of sensitivity stages. Fig. 3 shows that the temperature difference curve slope of extractive distillation column, de-ethane column, de-propane column and de-butane column is the largest at the positions of 45th stage, 10th stage, 20th stage and 30th stage, respectively. Therefore, the aforementioned stages position are recommended as the sensitivity control stages for dynamic simulation.

3.3. Comparison of dynamic control structures Based on the steady-state simulation operating parameters of extractive distillation process, the Aspen HYSYS dynamic analysis is carried out with pressure-driven simulations. According to the inherent characteristics of distillation column, six variables including feed flow rate, condenser energy consumption, reflux ratio, top distillation flow, reboiler energy consumption and bottom distillation flow are controlled during dynamic simulation. Typical control

structures are fixed reflux ratio control structure and reflux/feed ratio control structure. Fig. 4 shows the fixed reflux ratio control structure. Where, F1 is feed flow controller (negative feedback). FC is fixed reflux ratio controller. P1, P2, P3 and P4 are respectively column top pressure controllers (positive feedback) controlled by condenser energy consumption. T1, T2, T3 and T4 are sensitivity stages temperature controllers (negative feedback) controlled by reboiler energy consumption. L1, L3, L5, L7 and L2, L4, L6 L8 are respectively liquid level controllers of reflux tank and reboiler (positive feedback). R1 is the feed flow/solvent flow proportional controller (negative feedback). T5 is solvent temperature controller (negative feedback). Fig. 5 shows the reflux/feed ratio control structure. Where, R2, R3, R4 and R5 are respectively the reflux/feed flow proportional controllers (negative feedback) of column top condenser, and the other controllers settings are consistent with the reflux ratio control structure. After the control structure is determined, the appropriate controller constants need to be given. Flow controllers are set at Kc = 0.1 and Ti = 0.2 min. Liquid level controllers are set at

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Fig. 4. Control structure of extractive distillation process with fixed reflux ratio.

Fig. 5. Control structure of extractive distillation process with reflux/feed ratio. Table 3 Controller tuning parameters. Structures Parameters

Controlled variable Manipulated variable Transmitter range/(°C) Controller range/(kW) Gain Kc Integral time Ti /(min)

Fixed reflux ratio

Reflux/feed ratio

T1

T2

T3

T4

T1

T2

T3

T4

T45 QR1 30–60 0–900 2.12 0.176

T10 QR2 40–80 0–1400 4.02 0.302

T20 QR3 0–140 0–800 20 0.532

T30 QR4 70–100 0–200 2.11 7.64

T45 QR1 30–60 0–900 1.28 0.242

T10 QR2 40–80 0–1400 3.42 0.218

T20 QR3 0–140 0–800 6 0.513

T30 QR4 70–100 0–200 3.16 3.47

Kc = 2 and Ti = 10 min. Pressure controllers are set at Kc = 2 and Ti = 2 min. Temperature controllers are tuned by running relayfeedback tests and applying Tyreus–Luyben tuning rules. Controller parameters are given in Table 3. Table 3 shows that controller output values of the two control structures are stable, and the controller opening is within the adjustable range, indicating that both reflux ratio control structure and reflux/feed ratio control structure have successfully realized dynamic control of extractive distillation process. However, the fluctuation of feed flow and composition is inevitable in the actual production process. Therefore, the dynamic response characteristics of feed flow and composition disturbance are analyzed.

3.3.1. Dynamic response of feed flow Under the condition of constant feed composition, the dynamic characteristics of fixed reflux ratio control structure and reflux/feed ratio control structure by the same feed flow disturbances are analyzed. Fig. 6 shows the dynamic response curves of the distillation products purity (Fig. 6A) and distillation products flow rate (Fig. 6B) of the extractive distillation column, deethanizer, depropanizer and debutanizer in the extractive distillation process. Black represents the dynamic response curves of the fixed reflux ratio control structure, while red represents the dynamic response curves of the reflux/feed ratio control structure. The solid lines represent that feed flow is decreased from 140 to 135 kgmol/h, and

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Fig. 6. Dynamic response curves of two control structures: (A) Distillate product purity; (B) Distillation product flow.

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Fig. 7. Dynamic response curves of two control structures: (A) Distillate product purity; (B) Distillation product flow.

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the dashed lines represent that feed composition is increased from 140 to 145 kgmol/h. Fig. 6(A) shows that when the feed flow fluctuates, the distillate product purity of each column in the extractive distillation process changes and reaches a new stable state after a period of time. For the extractive distillation column, deethanizer and depropanizer, the distillate products purity are negatively correlated with the feed flow disturbance, and the dynamic responses of reflux/feed ratio control structure are more stable than that of fixed reflux ratio control structure. That is to say, when the feed flow disturbance occurs the product purity deviation degree of reflux/feed ratio control structure is smaller. For the debutanizer, the distillate product purity is negatively correlated with the feed flow disturbance when the fixed reflux ratio control structure is adopted, whereas the purity of the top product is positively correlated with the feed flow disturbance when the reflux/reflux ratio control structure is adopted. Likewise, the dynamic response characteristics of the reflux/reflux ratio control structure are more stable. Fig. 6(B) shows that when the feed flow fluctuates, the distillate product flow of each column in the extractive distillation process changes and reaches a new stable state after a period of time, and the dynamic response characteristics of the two control structures are almost the same. For the extractive distillation column, deethanizer and depropanizer, the distillate products flow are positively correlated with the feed flow disturbance. For the debutanizer, the distillate product flow is negatively correlated with the feed flow disturbance. 3.3.2. Dynamic response of feed composition Under the condition of constant feed flow rate, the dynamic characteristics of fixed reflux ratio control structure and reflux/feed ratio control structure by the same feed composition disturbances are analyzed. Fig. 7 shows the dynamic response curves of the distillation products purity (Fig. 7A) and distillation products flow rate (Fig. 7B) of the extractive distillation column, deethanizer, depropanizer and debutanizer in the extractive distillation process. Black represents the dynamic response curves of the fixed reflux ratio control structure, while red represents the dynamic response curves of the reflux/feed ratio control structure. The solid lines represent that the molar fraction of CO2 in feed composition is decreased from 10% to 8%, and the dashed lines represent that the molar fraction of CO2 in feed composition is increased from 10% to 12%. Fig. 7(A) shows that when the CO2 composition in feed fluctuates, the distillate product purity of each column in the extractive distillation process changes and reaches a new stable state after a period of time. For the extractive distillation column, the distillate product purity is negatively correlated with the feed composition disturbance, and the dynamic response characteristics of the two control structures are almost the same. For the deethanizer, depropanizer and debutanizer, the distillate products purity are positively correlated with the feed composition disturbance, and the dynamic responses of reflux/feed ratio control structure are more stable than that of fixed reflux ratio control structure. That is to say, when the feed composition is disturbed by 20% (CO2 mole fraction) the product purity deviation degree of reflux/feed ratio control structure is smaller. Fig. 7(B) shows that when the CO2 composition in feed fluctuates, the distillate product flow of each column in the extractive distillation process changes and reaches a new stable state after a period of time, and the dynamic response characteristics of the two control structures are almost the same. For the extractive distillation column and debutanizer, the distillate product flow is positively correlated with the feed composition disturbance. For the deethanizer and depropanizer, the distillate products flow are negatively correlated with the feed composition disturbance.

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In this section, two control structures have been explored, and both of which exhibit better controllability for feed flow rate and feed composition disturbance. However, the fixed reflux ratio control structure has the defect of large fluctuation range of distillation product purity. The reflux/feed ratio control structure is proposed, which can adjust the reflux ratio in real time with the fluctuation of feed flow and feed composition, greatly improving the stability of distillation product purity. 4. Conclusions The fixed reflux ratio and reflux/feed ratio control structures of the CO2 –C2 H6 azeotrope extractive distillation process are studied. Both control structures have successfully realize the dynamic simulation of the extractive distillation process. Taking the distillation products purity and flow rate as evaluation criteria, simulation results show that the reflux/feed ratio control structure has good controllability. Declaration of Competing Interest The authors declare no competing financial interest. Acknowledgments We are thankful for support from the National Natural Science Foundation for young scholars of China (grant no. 51306210) and assistance from the staff at the research group of Natural Gas Treatment Technology (China University of Petroleum). References Algharaib, M., Al-Soof, N.A., 2010. Economical evaluation of CO2 -EOR projects in the middle east. Pet. Sci. Technol. 28 (2), 198–217. Arifin, S., Chien, I.L., 2008. Design and control of an isopropyl alcohol dehydration process via extractive distillation using dimethyl sulfoxide as an entrainer. Ind. Eng. Chem. Res. 47 (3), 790–803. General Administration of Quality Supervision, 2015. Inspection and quarantine of the People’s Republic of China, standardization administration of the People’s Republic of China. Oil and Gas Field Liquid Ethane: Q/SH1025 0968-2015. Standards Press of China, Beijing. General Administration of Quality Supervision, 2011. Inspection and quarantine of the People’s Republic of China, standardization administration of the People’s Republic of China. Liquefied Petroleum Gases: GB 11174-2011. Standards Press of China, Beijing. Gorak, A., Schoenmakers, H., 2014. Distillation: Operation and Applications. Academic Press, USA. Hori, E.S., Skogestad, S., 2007. Selection of control structure and temperature location for two-product distillation columns. Chem. Eng. Res. Des. 85 (3), 293–306. Luyben, W.L., 2010. Plantwide control of an isopropyl alcohol dehydration process. Aiche J. 52 (6), 2290–2296. Luyben, W.L., 2016. Control comparison of conventional extractive distillation with a new split-feed configuration. Chem. Eng. Process. 107, 29–41. Luyben, W.L., 2008. Control of the maximum-boiling acetone/chloroform azeotropic distillation system. Ind. Eng. Chem. Res. 47 (16), 6140–6149. Luyben, W.L., 2013. Control of an extractive distillation system for the separation of CO2 and ethane in enhanced oil recovery processes. Ind. Eng. Chem. Res. 52 (31), 10780–10787. Luyben, W.L., 2006. Evaluation of criteria for selecting temperature control trays in distillation columns. J. Process. Control 16 (2), 115–134. Wang, X., Xie, L., Tian, P., et al., 2016. Design and control of extractive dividing wall column and pressure-swing distillation for separating azeotropic mixture of acetonitrile/N-propanol. Chem. Eng. Process. 110, 172–187. Wang, Y., Zhang, X., Liu, X., et al., 2018. Control of extractive distillation process for separating heterogenerous ternary azeotropic mixture via adjusting the solvent content. Sep. Purif. Technol. 191, 8–26. Wang, H.Q., Fan, M.L., Zhang, Z.B., 2019. Design and optimization of alternative processes for the separation of the CO2 –C2 H6 azeotrope to enhance product quality. Chem. Eng. Process. 136, 201–210. Wang, Y., Liang, S., Bu, G., et al., 2015. Effect of solvent flow rates on controllability of extractive distillation for separating binary azeotropic mixture. Ind. Eng. Chem. Res. 54 (51), 12908–12919.