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Simulation and energy analysis of CO2 capture from CO2-EOR extraction gas using cryogenic fractionation
Journal of the Taiwan Institute of Chemical Engineers 103 (2019) 67–74
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Simulation and energy analysis of CO2 capture from CO2 -EOR extraction gas using cryogenic fractionation Bingcheng Liu∗, Mengmeng Zhang, Xuan Yang, Ting Wang Qingdao University of Science and Technology, Qingdao 266061, China
a r t i c l e
i n f o
Article history: Received 13 April 2019 Revised 12 July 2019 Accepted 15 July 2019 Available online 19 July 2019 Keywords: CO2 capture Cryogenic fractionation Process simulation Energy analysis
a b s t r a c t CO2 -EOR is a recognized effective technology to enhance oil recovery by injecting CO2 into the reservoir to increase layer pressure. With the exploitation of oil, the overflow of large amount of CO2 has caused great difficulties to gas treatment and transportation, even affects the climate if it is discharged directly. CO2 capture from extraction gas plays an increasingly important role in oilfield due to these problems. Compared with other methods, cryogenic fractionation is an ideal method for large extraction gas volume and high CO2 content. In the presented work, Aspen HYSYS was used to simulate the cryogenic fractionation separation process with 10 0,0 0 0 Nm3 /d extraction gas and 81.4% CO2 content. Meanwhile, the main factors including pressure of fractionating tower, condensation temperature, reboiling temperature, and CO2 content in raw gas which effect system cooling consumption are also analyzed. The simulation results show that the CO2 purity and recovery can reach over 95% and 90% respectively for high CO2 concentration. The effect of these parameters on system overall cooling consumption is significant and the reasonable parameters are obtained in this work. The results will provide some improvements for scale-up application of cryogenic fractionation in extraction gas CO2 capture. © 2019 Taiwan Institute of Chemical Engineers. Published by Elsevier B.V. All rights reserved.
1. Introduction Recently, more and more concern has been paid to the global climate change caused by the greenhouse effect [1,2]. The report made by Intergovernmental Panel on Climate Change shows that in 2100,the CO2 content in atmosphere will cause the mean global temperature rising about 1.9 °C and the mean sea level will increase 3.8 m. In the last decades, CO2 emission in developing countries, especially in China, has rapidly increased, CO2 capture, utilization and storage (CCUS) is an effective way to solve greenhouse effect [3–5]. CO2 -EOR is a mature technology which injects CO2 into oil field to enhanced oil recovery and it can not only increase the crude oil production but also storing and utilizing CO2 , so it can be a optimal choice for CCUS [6,7]. There are about 80 business and research-scale CO2 -EOR projects around word in 20 0 0 and until 2009, there have been 183 projects related to CO2 -EOR in Shengli Oilfield [8,9]. But the main problem of CO2 -EOR is that CO2 gas injected into the oil field will overflow the ground with the extraction gas. Fig. 1 shows that the CO2 content in extraction gas increase year by year in Shengli Oilfied and eventually, the CO2 content is around 85%−95%. Without correct treatment of extraction gas, a large amount of CO2 will be discharged directly into ∗
Corresponding author. E-mail address: [email protected] (B. Liu).
the atmosphere which will lead to more serious climate problems. Therefore, CO2 capture from extraction gas is also an important issue. There are several choices for CO2 capture from extraction gas (i.e. chemical absorption, membrane, cryogenic fractionation, etc.), and the selection of different methods highly depends on the CO2 concentration and the scale of extraction gas [10,11]. Monoethanolamine (MEA) absorption which widely used in CO2 capture from extraction gas is a kind of chemical absorption [12]. It can achieve a high level of CO2 capture rate (about 90%) via strong chemical reaction. However, there are several drawbacks in MEA absorption including large volume occupancy, solvent regeneration efficiency and corrosion [13,14]. The work of Rao and Rubin reveals that the cost of solvent regeneration accounts for about 10% of the total CO2 capture cost [15]. Moreover, chemical absorption can only be applied to small extraction gas volume and low CO2 concentration (approximately 20%) [16]. But, with the gradual exploitation of oil at present, the CO2 concentration in extraction gas increase sharply. So the chemical absorption is not suitable for CO2 capture from extraction gas at present. Membranes separation is also a relatively novel method for CO2 capture from extraction gas, and it is widely used in recent years due to its easy to installation, low energy consumption and eco-friendliness [17–19]. Nevertheless, there are also some shortcomings in the use of membranes. First, membranes are extremely strict with temperature. When the
https://doi.org/10.1016/j.jtice.2019.07.008 1876-1070/© 2019 Taiwan Institute of Chemical Engineers. Published by Elsevier B.V. All rights reserved.
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B. Liu, M. Zhang and X. Yang et al. / Journal of the Taiwan Institute of Chemical Engineers 103 (2019) 67–74 Table. 1 Component of extraction gas from a single well. Component
Content (mol%)
CO2 CH4 C2 H6 C3 H6 i-C4 n-C4 i-C5 n-C5 C 6+ N2
80.14 13.14 2.3 2.64 0.25 0.78 0.13 0.16 0.1 0.12
Table. 2 Characteristic parameters of extraction gas. Material parameters
Numerical value
Pressure (MPa) Temperature (K) Flow rate (Nm3 /d)
0.30–0.60 293.15 100,000
Fig. 1. CO2 content in extraction gas of Shengli Oilfied.
temperature exceeds 100 °C, the membranes structure will be destroyed rapidly [20]. Moreover, membranes are also sensitive to impurities (e.g. heavy hydrocarbon, water vapor), therefore, pretreatment is for membranes separation. So membranes separation is difficult apply to long-term, large-scale extraction gas operation in industry conditions [21]. Compared with membrane and chemical separation, there have been paid more and more attention to the cryogenic-based approaches in recent years [22]. Cryogenic fractionation is to separate CO2 from extraction gas by using different condensation and desublimation properties of each component in the extraction gas [23]. Compared to other separation methods, cryogenic fractionation can achieve higher CO2 purity (99.99%) and recovery(99.99%) and it can handle large-scale gas volume which operated under moderate pressure, meanwhile, it also avoid the heavy cost of regenerating the solvent [24]. There are some different cryogenic fractionation process using in industry. Operated packed beds using in cryogenic fractionation for CO2 capture was proposed by Tuinier et al. [25]. The biggest advantage in this method is H2 O and CO2 can automatically separate due to their different dew and sublimation points [23]. But, the high energy consumption is also the main problem in this process. Cryogenic distillation is a kind of commonly used separation method [23] Holmes and Ryan [26] are first proposed this method for purifying natural gas, which can effectively solve the problem of CO2 solidification in distillation tower [27]. The heavy hydrocarbon(C2 –C5 ) added to the condenser in the distillation column to prevent CO2 solidification was applied by Holmes et al. [27,28]. Although cryogenic distillation is a widespread method for industrial and has a lot of benefits, high energy consumption has always been one of the major drawbacks of cryogenic distillation [29,30]. Essentially, the main limitation of cryogenic method is high operating cost for cold production [23,24]. In recent years, there are various cryogenic method have been proposed in order to reduce the high cooling consumption of cryogenic method effectively. Song et al. [31] proposed a novel cryogenic CO2 capture process based on Stirling coolers with heat integration from flue gas. The result shows that the proposed system can save about 45% energy requirement in a typical 600MW coal power plant. Knapik et al. [24] used N2 stream which came from nitrogen removal unit (NRU) as cold source to capture CO2 from flue gas. This method can significantly reduce high cooling cost in normal cryogenic CO2 capture method and the energy
consumption for per kilogram CO2 product of this process reduce to 0.125 KWh. Despite these methods can effectively decrease cooling consumption of cryogenic method, the energy consumption of cryogenic fractionation for CO2 capture still largely depends on the operating pressure, and the concentration of CO2 in the extraction gas. In this paper, we use cryogenic distillation model proposed by Holmes and Ryan [26], and the real industrial situation of extraction gas is simulated by Aspen HYSYS V8.6. Meanwhile, the effects of liquefaction temperature, condensation temperature, re-boiling temperature and fractionation tower operating pressure on energy consumption, CO2 recovery and CO2 product concentration were analyzed in detail. Based on this simulation results, we can conclude main factors and laws that affect the energy consumption of the system. That will be of guiding significance to capture CO2 from extraction gas by cryogenic fractionation technology.
2. Process description The extraction gas from a single well of Shengli Oilfied station is showed in Table 1 and 2. Compared with other oil field, the extraction gas in Shengli Oilfied exhibits these characteristics: (a) High carbon dioxide concentration; (b) High heavy hydrocarbon concentration; (c) Containing saturated water. In order to avoid the sour corrosion of equipment and pipe, prevent the formation of hydrate in the gas treatment process and ensure the safe and reliable operation of the device, the extraction gas needs to be dehydrated in pretreatment, getting dry raw gas for CO2 separation in the cryogenic fractionation device. However, the pretreatment process is not included in the scope of this paper. Based on cryogenic distillation proposed by Holmes and Ryan [23,26], the cryogenic fractionation process was shown in Fig. 2, which has been improved to suit the extraction gas conditions in this paper. The raw gas is initially compressed by compressor system, then cooled by the precooler (heat exchanger) and liquefied by liquefier. The liquefied raw gas is then sent to the distillation tower, in where the component steam is dived into two parts: top and bottom products [23]. The former is CH4 stream, which is condensed by the condenser at the top of the tower and then flows out after the cooling capacity was released through the pre-cooler. The latter is CO2 products, which flows out after being boiled by the reboiler at the bottom of the tower.
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Fig. 2. The flow diagram of cryogenic fractionation process.
Table 3 Main equipment parameter.
3. Simulation basis 3.1. Peng–Robinson equation The simulation in this paper based on Aspen HYSYS, which is widely used in petroleum and chemical industries and contains more complex perception calculation packages and unit operations. Acid Gas is a thermodynamic property package which built in Aspen HYSYS and it is applied for rigorous equilibrium calculation and kinetic reactions [32]. The Peng–Robinson (PR) equation which belongs to Acid Gas is ideal for calculating the gas-liquid equilibrium of the system and the liquid density of the hydrocarbon system. The extraction gas is considered as Acid Gas, hence, the PR equation is selected as the calculation model in the simulation in this paper. 3.2. Capture performance In order to evaluate the CO2 capture performance of whole cryogenic fractionation process, two main parameters are introduced: CO2 purity, CO2 recovery and cooling load. The calculation of CO2 purity and system cooling load is summarized as follows:
C O2 purity
mC O2 out = mtotal out
(1)
where mCO2 out is the mass flow of feed gas the outlet of the process and mtotal out is the mass flow of gas mixture from the outlet of process.
C O2
recovery
=
mout × wC O2 out min × wC O2 in
(2)
where min and m out are the mass flow of raw gas and outlet product. wCO2 out and wCO2 in are the mass fraction of CO2 in raw gas and outlet product.
Etotal cooling = Ecompressor system + Eliqu f ier + Econdenser
(3)
Ecompressor system = Ecooler
(4)
Eliqu f ier = mCO2 × h
(5)
Main equipment
Temperature(°C)
Pressure increasing (kPa)
1-COMP 2-COMP
156.9 139.4 Temperature(°C) 40 40 40 −23 −27 −4 Operating pressure 3MPa
900 2040 Pressure drop (kPa) 50 50 20 40 20 20 Number of stages 8
COOLER-1 COOLER-2 PRECOOLR LIQUEFIER Condenser Reboiler FRA-TOWER
where Ecompressor system , Eliqufier and Econdenser are the cooling load of compressor system, liqufier and condenser separately. Ecooler is the cooling load of two cooler in compressor system. For the cooling load of liqufier, mCO2 is the mass of liquefaction CO2 , h is the cold energy for liquefying CO2 . 3.3. Simulation assumption The simulation of overall cryogenic fractionation is based on Aspen HYSYS. The cryogenic fractionation HYSYS model is established based on Fig. 2. Meanwhile, the main parameters are shown in Table 3. In order to simplify the calculation, the following simulation assumption are used: (1) The thermodynamic calculation is based on Peng–Robinson (PR) equation [32] (2) The adiabatic efficiency of compressor is set 85% [11] (3) The heat loss of whole process is neglected (4) the extraction gas through pretreament process contains no high heavy hydrocarbon and saturated water. 4. Results and discussion 4.1. Effect of the operating pressure of fractionating tower The purity of the CO2 product and the recovery rate of CO2 are set as 92.5% and 90%, respectively. The effect of fractionating tower pressure on condensation temperature, liquefaction
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Fig. 3. Effect of fractionating tower operating pressure on condensation temperature and liquefaction temperature. Simulation condition are as follows: the flow rate of raw gas is 10 0,0 0 0 Nm3 /d, feed pressure is 0.3 MPa, CO2 content in raw gas is 80.14%.
Fig. 4. Effect of operating pressure of fractionating tower on cooling load of per unit CO2 and compressor power. Simulation condition are as follows: the flow rate of raw gas is 10 0,0 0 0 Nm3 /d, feed pressure is 0.3 MPa, CO2 content in raw gas is 80.14%.
temperature, cooling load of the system and the power of the compressor is analyzed. The variation of condensation temperature and liquefaction temperature with operating pressure of fractionating tower is shown in Fig. 3. As the increase of operating pressure of fractionating tower, the dew point temperature of CO2 increases, the liquefaction temperature and the condensation temperature at the top of the tower also increase gradually. In order to ensure the recovery rate of CO2 and reduce the cooling load at the top of the tower, liquefaction temperature should be lowered appropriately and the condensation temperature should be increased. As shown in Fig. 4, when the fractionating tower pressure is 2.5 MPa, the liquefaction temperature is −33 °C, the condensation temperature is −36.36 °C, and when the pressure is 4.0 MPa, the liquefaction temperature is −20 °C and the condensation temperature is −23.89 °C. As shown in Fig. 4. the cooling consumption of per unit CO2 and compressor power varies with operating pressure of fractionating tower. As pressure rises from 2.5 MPa to 4.0 MPa, the per unit product cooling load decreases initially and then increases. When the pressure is 3.1 MPa, the lowest per unit product cooling load is 419.72 MJ/t. When the pressure rises from 2.5 MPa to 3.1 MPa, the cooling load of the liquefier decreases and the cooling load of
Fig. 5. The effect of operating pressure of fractionating tower on total energy consumption. Simulation condition are as follows: the flow rate of raw gas is 10 0,0 0 0 Nm3 /d, feed pressure is 0.3 MPa, CO2 content in raw gas is 80.14%.
the top condenser rises slowly, so the cooling load of per unit CO2 decreases. When the pressure rises from 3.1 MPa to 4.0 MPa, the increasing speed of cooling load on top of tower is higher than the decreasing speed of cooling load on liquefier. Hence, the cooling load of per unit CO2 tends to increase. When the operating pressure of fractionating tower rises, the power of compressor rises accordingly, and compressor power and fractionating tower operating pressure are linearly and positively correlated. In the process of fractionating tower pressure rising from 2.5 MPa to 4.0 MPa, compressor power increases from 301 kW to 382 kW. Figs. 3 and 4 show the change of fractionating tower operating pressure on liquefaction temperature, condensation temperature, compressor power and cooling load. When operating pressure of fractionating tower is 3.1 MPa, the sum of per unit product cooling load and compressor power is the smallest. According to Eqs. (3)–(5), Fig. 5 shows the effect of operating pressure of fractionating tower on total energy consumption. When operating pressure of fractionating tower is 3.1 MPa, total energy consumption reaches the smallest(1362.9 MJ/t). Therefore, 3.1 MPa is a reasonable operating pressure for fractionating tower. 4.2. Effect of the condensation temperature The operating pressure of fractionating tower is set as 3.1 MPa, the purity of CO2 product is set as 92.5%. The effect of condensation temperature on CO2 recovery rate and system energy consumption are analyzed. Fig. 6 shows the effect of condensation temperature on CO2 recovery rate at different liquefaction temperature. When the liquefaction temperature changes from −21 °C to −26 °C, the different curve of the relationship between CO2 recovery rate and condensation temperature basically coincides, indicating that the CO2 recovery rate is controlled by the condensation temperature and is not affected by liquefaction temperature. For a certain liquefaction temperature, the recovery of CO2 and the output of CO2 decrease, with the increase of condensation temperature. The relationship between the condensation temperature and cooling load of condenser at different liquefaction temperature is shown in Fig. 7. For a certain liquefaction temperature, the cooling load of condenser first decreases and then increases with the increase of the condensation temperature. For different liquefaction temperature curves, when the condensation temperature is
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Fig. 6. Effect of condensation temperature on CO2 recovery rate. Simulation conditions are as follows: the flow rate of raw gas is 10 0,0 0 0 Nm3 /d, feed pressure is 0.3 MPa, CO2 content in raw gas is 80.14%, the operating pressure of fractionating tower is 3.1 MPa.
Fig. 7. Effect of condensation temperature on condenser cooling load. Simulation conditions are as follows: the flow rate of raw gas is 10 0,0 0 0 Nm3 /d, feed pressure is 0.3 MPa, CO2 content in raw gas is 80.14%, the operating pressure of fractionating tower is 3.1 MPa.
−27 °C, the cooling load is the lowest. Comparing with different liquefaction temperature curves, it shows that with the decrease of liquefaction temperature, the cooling load of condenser decreases. When the condensation temperature is −27 °C and the liquefaction temperature is −26 °C, the cooling load of condenser is 19.5 kW. When condensation temperature is −27 °C and the liquefaction temperature is −21 °C, the cooling load of condenser is 63.14 kW. Fig. 8. shows the variation curves of cooling load of per unit product with condensation temperature at different liquefaction temperature. For a certain liquefaction temperature, with the increase of condensation temperature, the cooling load of the system increases and the cooling load of per unit product also increases. When the condensation temperature increases from −34 °C to −27 °C, the growth trend of per unit product cooling load is relatively flat. However, when the condensation temperature increases from −27 °C to −25 °C, the cooling load of per unit product rises sharply. For a certain condensation temperature, with the increase of liquefaction temperature, the per unit product cooling load without obvious increases.
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Fig. 8. Effect of condensation temperature on per unit product cooling load. Simulation conditions are as follows: the flow rate of raw gas is 10 0 0 0 0Nm3 /d, feed pressure is 0.3 MPa, CO2 content in raw gas is 80.14%, the operating pressure of fractionating tower is 3.1 MPa.
Fig. 9. Effect of the reboiling temperature on CO2 product purity. Simulation condition are as follows:the flow rate of raw gas is 10 0,0 0 0 Nm3/d, feed pressure is 0.3 MPa, CO2 content in raw gas is 80.14%, the operating pressure of fractionating tower is 3.1 MPa, the condensation temperature is −27 °C.
Figs. 6– 8 show that condensation temperature control not only the CO2 recovery rate, but also affect the cooling load of the system. When the condensation temperature is −27 °C, the system has low cooling load and high CO2 recovery rate. So, optimum condensation temperature is chosen as −27 °C. 4.3. Effect of the reboiling temperature The operating pressure of fractionating tower is set as 3.1 MPa, CO2 content in raw gas is 80.14% and the condensation temperature is −27 °C. Then the influence of different reboiling temperature on CO2 product purity and per unit product cooling load is analyzed. As shown in Fig. 9, as the reboiling temperature rises from −17 °C to −3 °C, the purity of CO2 product rises. When the reboiling temperature changes from −17 °C to −4 °C, the CO2 product purity increases slowly, but when the reboiling temperature higher than −4 °C, the purity of CO2 product rises sharply. The highest CO2 product purity can reach 94.1% when the reboiling temperature is −3 °C.
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Fig. 10. Effect of purity reboiling temperature on per unit product cooling consumption. Simulation condition are as follows: the flow rate of raw gas is 10 0 0 0 0Nm3/d, feed pressure is 0.3 MPa, CO2 content in raw gas is 80.14%, the operating pressure of fractionating tower is 3.1 MPa, the condensation temperature is −27 °C.
Fig. 11. Effect of CO2 content in raw gas on CO2 product purity. Simulation condition are as follows: the flow rate of raw gas is 10 0,0 0 0 Nm3/d, feed pressure is 0.3 MPa, CO2 content in raw gas is 80.14%, the operating pressure of fractionating tower is 3.1 MPa, the condensation temperature is −27 °C, the reboiling temperature is −4 °C.
It can be seen from Fig. 10 that as the reboiling temperature decreases, the consumption of CO2 cooling capacity decreases accordingly. When the reboiling temperature is higher than −4 °C, the energy consumption of the system increases rapidly. Combining the Figs. 9 and 10, with the increase of reboiling temperature, CO2 product purity and cooling load of per unit product increase accordingly. When the purity of CO2 product continues to improve, a large amount of CO2 begins to evaporate in reboiler and condense in top condenser, so the heat load of bottom rises rapidly, which leads to the rapid increase of top cooling load and the increase of per unit product cooling consumption per unit product. When the reboiling temperature is −4 °C, CO2 product purity and cooling load of per unit product make a sudden increasing. Therefore, the optimum reboiling temperature is −4 °C. 4.4. Effect of different CO2 content in raw gas The operating pressure of fractionating pressure is set as 3.1 MPa, the condensation temperature and reboiling temperature are −27 °C and −4 °C separately. The effect of different CO2 content in raw gas on CO2 product purity and energy consumption is analyzed. The relationship between CO2 content in raw gas and CO2 product purity shows in Fig. 11. When the CO2 content in raw gas is gradually increased above 80%, the purity of the CO2 product gradually increases due to the gradual decrease of the hydrocarbon content in raw gas. As the CO2 content is higher than 91%, the hydrocarbon content in raw gas decreases sharply, and the purity of CO2 products increases rapidly. Fig. 12. shows the effect of different CO2 content in raw gas on compressor power and cooling load of system. With the CO2 content in raw gas increasing from 80% to 94%, the compressor power decreases. When CO2 content in raw gas exceeds 91%, the compressor power decreases sharply. Meanwhile, the cooling load of the system first decreases and then increases with the increase of CO2 content in raw gas, and reaches the minimum when the CO2 content in raw gas is 91%. When the CO2 content in raw gas is 92.5%, the two curves intersect at one point which is the minimum point of the system energy consumption. Fig 13 also verify this viewpoint, when CO2 content in raw gas is 92.5%, according to Eqs. (3)–(5),total energy consumption reaches the minimum (1100 MJ/t).
Fig. 12. Effect of CO2 content in raw gas on compressor power and cooling load of system. Simulation condition are as follows: the flow rate of raw gas is 10 0,0 0 0 Nm3 /d, feed pressure is 0.3 MPa, CO2 content in raw gas is 80.14%, the operating pressure of fractionating tower is 3.1 MPa, the condensation temperature is −27 °C, the reboiling temperature is −4 °C.
The CO2 content in raw gas is set as 80.14%, 86.0%, 91.0% and 93.5% respectively, and the relationship of the CO2 content in raw gas, the CO2 product purity and cooling load of per unit product is shown in Fig. 14. Comparing the four curves, it shows that as the CO2 content in raw gas reduce, the cooling consumption of per unit product required to capture the same purity CO2 is significantly increased. When the CO2 content in raw gas is 80.14%, 86%, 91% and 93.5%, the cooling consumption of per unit product required for purity of CO2 products from the lowest to the highest is 79 MJ/t, 54.2 MJ/t, 51.8 MJ/t and 60.5 MJ/t respectively. Similarly, the highest purity of CO2 product can reach 92.5%, 95.0%, 95.8% and 99.0% respectively. In the same way, cooling load of the system increases more and more rapidly as the CO2 product purity rise and when CO2 product purity reaches a certain value, the cooling load of system increases sharply. Figs. 11– 14. shows that with the CO2 content in raw gas increases, the CO2 product purity increases accordingly, the compressor power decreases and the cooling load of system and cooling load of per unit product decrease first and then increase. It can be
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concluded that for the separation of CO2 by cryogenic fractionation, the reasonable content of CO2 in raw gas is 86%−93%. 4.5. Technical evaluation
Fig. 13. The effect of CO2 content in raw gas on total energy consumption. Simulation condition are as follows: the flow rate of raw gas is 10 0,0 0 0 Nm3 /d, feed pressure is 0.3 MPa, CO2 content in raw gas is 80.14%, the operating pressure of fractionating tower is 3.1 MPa, the condensation temperature is −27 °C, the reboiling temperature is −4 °C.
The work of Yousef et al. [33] for low-temperature CO2 removal from biogas is used to evaluate the technical feasibility of this work. The simulation data of two work are enumerated in the Table 4. The performance of distillation pressure, condensation temperature and reboiling temperature in cryogenic fractionation are different with the low-temperature process from the work of Yousef et al. [33]. That is because the CO2 concentration in biogas is far less than in extraction gas. Meantime, according to the bubble point and dew point of CO2 from Maqsood et al. [34], the operating pressure of fractionating tower, condensation temperature and reboiling temperature is also reasonable. Apart these three parameters, other parameters have a good agreement with the work of Yousef et al. [33]. The comparison of the work of Yousef et al. [33] and Maqsood et al. [34] proves that the simulation data in this work are reasonable and reliable. 5. Conclusion The study simulates the whole cryogenic fractionation process for CO2 separation from EOR extraction gas using Aspen HYSYS. The model is simulated based on Holmes and Ryan [26,28]. The PR equation is used as thermodynamic calculation. Meanwhile, a series assumptions are adopted in process simulation. According to this model, the effect of operating pressure of fractionating tower, reboiling temperature, condensation temperature and CO2 content in raw gas on CO2 product purity and cold consumption are analyzed and through change these operational parameters, a set of reasonable operational parameters are obtained with the scale of raw gas is 10 0,0 0 0 Nm3 /d. The main findings of proved cryogenic fractionation are as follows:
Fig. 14. Effect of purity change of CO2 products on per unit product cold consumption. Simulation condition are as follows: the flow rate of raw gas is 10 0,0 0 0 Nm3 /d, feed pressure is 0.3 MPa, CO2 content in raw gas is 80.14%, the operating pressure of fractionating tower is 3.1 MPa, the condensation temperature is −27 °C, the reboiling temperature is −4 °C.
(a) With the increase of the operating pressure, the liquefaction temperature, the condensation temperature, the CO2 product purity and the compressor power increase accordingly. However, the cooling load decreases first and then rises. When the operating pressure is 3.1 MPa, the per unit cooling consumption reaches the lowest which is 419.72 MJ/t. (b) With the increase of condensation and reboiling temperature increase, the CO2 recovery rate and CO2 product purity increase, while the system cooling load also increase. When the condensation temperature and reboiling temperature reach at −27 °C and −4 °C respectively, the CO2 recovery rate and CO2 purity is 90% and 92.5% separately. (c) With the CO2 content in raw gas increase, the CO2 product purity increases, however, the system cooling consumption
Table 4 The main parameters of cryogenic fractionation compared to low-temperature process [33].
Mole flow (kmol/h) Composition (mol%) CO2 CH4 Gas inlet temperature (°C) Gas inlet pressure (MPa) Distillation pressure (MPa) Condenser temperature (°C) Reboiling temperature Per unit energy consumption of CO2 captured (MJ/ton CO2 ) CO2 purity (%)
Cryogenic fractionation
Low-temperature process [33]
4460
1000
80 20 25 0.3 3.1 −27 −4 1290 91.3
40 60 35 0.12 4.9 −75.7 13.6 1377 92.5
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decreases. Therefore, cryogenic fractionation method is suitable to separate high CO2 content extraction gas with the scale of 86%−93%. The energy analysis in this work will provide positive advice for cryogenic fractionation using in scale-up application and a series reasonable parameters are also obtained. However, there are also several challenges in cryogenic fractionation, such as the effect of the liquefication temperature and the treatment of noncondensable gas. Therefore, these challenges will be discussed and optimized in future works. Acknowledgments This work was supported by Department of Science & Technology of Shandong Province (No. ZR2018LB025). References [1] Olajire AA. CO2 capture and separation technologies for end-of-pipe applications–a review. Energy 2010;35:2610–28. [2] Safdarnejad SM, Hedengren JD, Baxter LL. Dynamic optimization of a hybrid system of energy-storing cryogenic carbon capture and a baseline power generation unit. Appl Energy 2016;172:66–79. [3] Zhang L, Shen Q, Wang M, Sun N, Wei W, Lei Y, Wang Y. Driving factors and predictions of CO2 emission in China’s coal chemical industry. J Clean Prod 2019;210:1131–40. [4] Li L, Zhao N, Wei W, Sun Y. A review of research progress on CO2 capture, storage, and utilization in Chinese academy of sciences. Fuel 2013;108:112–30. [5] Yu S, Horing J, Liu Q, Dahowski R, Davidson C, Edmonds J, et al. CCUS in China’s mitigation strategy: insights from integrated assessment modeling. Int J Greenh Gas Control 2019;84:204–18. [6] DTapia JF, Lee JY, Ooi REH, Foo DCY, Tan RR. Optimal CO2 allocation and scheduling in enhanced oil recovery (EOR) operations. Appl Energy 2016;184:337–45. [7] Ampomah W, Balch RS, Cather M, Will R, Gunda D, Dai Z, et al. Soltanian optimum design of CO2 storage and oil recovery under geological uncertainty. Appl Energy 2017;195:80–92. [8] Mikkelsen M, Jørgensen M, Krebs FC. The teraton challenge. A review of fixation and transformation of carbon dioxide. Energy Environ Sci 2010;3(1):43–81. [9] Zhang L, Wang S, Zhang L, Ren SR, Guo Q. Assessment of CO2 EOR and its geostorage potential in mature oil reservoirs, Shengli Oilfield, China. Pet Explor Dev 2009;36(6):737–42. [10] Porter RTJ, Fairweather M, Pourkashanian M, Woolley RM. The range and level of impurities in CO2 streams from different carbon capture sources. Int J Greenh Gas Control 2015;36:161–74. [11] Song C, Liu Q, Ji N, Deng S, Zhao J, Li Y, et al. Reducing the energy consumption of membrane-cryogenic hybrid CO2 capture by process optimization. Energy 2017;124:29–39. [12] Yeo ZT, Chew TL, Zhu PW, Mohamed AR, Chai SP. Conventional processes and membrane technology for carbon dioxide removal from natural gas: a review. J Nat Gas Chem 2012;21(3):282–98. [13] Zhang X, Liu H, Liang Z, Idem R, Tontiwachwuthikul P, Al-Marin MJ, et al. Reducing energy consumption of CO2 desorption in CO2 -loaded aqueous amine solution using Al2O3/HZSM-5 bifunctional catalysts. Appl Energy 2018;229:562–76.
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Journal of the Taiwan Institute of Chemical Engineers journal homepage: www.elsevier.com/locate/jtice
Simulation and energy analysis of CO2 capture from CO2 -EOR extraction gas using cryogenic fractionation Bingcheng Liu∗, Mengmeng Zhang, Xuan Yang, Ting Wang Qingdao University of Science and Technology, Qingdao 266061, China
a r t i c l e
i n f o
Article history: Received 13 April 2019 Revised 12 July 2019 Accepted 15 July 2019 Available online 19 July 2019 Keywords: CO2 capture Cryogenic fractionation Process simulation Energy analysis
a b s t r a c t CO2 -EOR is a recognized effective technology to enhance oil recovery by injecting CO2 into the reservoir to increase layer pressure. With the exploitation of oil, the overflow of large amount of CO2 has caused great difficulties to gas treatment and transportation, even affects the climate if it is discharged directly. CO2 capture from extraction gas plays an increasingly important role in oilfield due to these problems. Compared with other methods, cryogenic fractionation is an ideal method for large extraction gas volume and high CO2 content. In the presented work, Aspen HYSYS was used to simulate the cryogenic fractionation separation process with 10 0,0 0 0 Nm3 /d extraction gas and 81.4% CO2 content. Meanwhile, the main factors including pressure of fractionating tower, condensation temperature, reboiling temperature, and CO2 content in raw gas which effect system cooling consumption are also analyzed. The simulation results show that the CO2 purity and recovery can reach over 95% and 90% respectively for high CO2 concentration. The effect of these parameters on system overall cooling consumption is significant and the reasonable parameters are obtained in this work. The results will provide some improvements for scale-up application of cryogenic fractionation in extraction gas CO2 capture. © 2019 Taiwan Institute of Chemical Engineers. Published by Elsevier B.V. All rights reserved.
1. Introduction Recently, more and more concern has been paid to the global climate change caused by the greenhouse effect [1,2]. The report made by Intergovernmental Panel on Climate Change shows that in 2100,the CO2 content in atmosphere will cause the mean global temperature rising about 1.9 °C and the mean sea level will increase 3.8 m. In the last decades, CO2 emission in developing countries, especially in China, has rapidly increased, CO2 capture, utilization and storage (CCUS) is an effective way to solve greenhouse effect [3–5]. CO2 -EOR is a mature technology which injects CO2 into oil field to enhanced oil recovery and it can not only increase the crude oil production but also storing and utilizing CO2 , so it can be a optimal choice for CCUS [6,7]. There are about 80 business and research-scale CO2 -EOR projects around word in 20 0 0 and until 2009, there have been 183 projects related to CO2 -EOR in Shengli Oilfield [8,9]. But the main problem of CO2 -EOR is that CO2 gas injected into the oil field will overflow the ground with the extraction gas. Fig. 1 shows that the CO2 content in extraction gas increase year by year in Shengli Oilfied and eventually, the CO2 content is around 85%−95%. Without correct treatment of extraction gas, a large amount of CO2 will be discharged directly into ∗
Corresponding author. E-mail address: [email protected] (B. Liu).
the atmosphere which will lead to more serious climate problems. Therefore, CO2 capture from extraction gas is also an important issue. There are several choices for CO2 capture from extraction gas (i.e. chemical absorption, membrane, cryogenic fractionation, etc.), and the selection of different methods highly depends on the CO2 concentration and the scale of extraction gas [10,11]. Monoethanolamine (MEA) absorption which widely used in CO2 capture from extraction gas is a kind of chemical absorption [12]. It can achieve a high level of CO2 capture rate (about 90%) via strong chemical reaction. However, there are several drawbacks in MEA absorption including large volume occupancy, solvent regeneration efficiency and corrosion [13,14]. The work of Rao and Rubin reveals that the cost of solvent regeneration accounts for about 10% of the total CO2 capture cost [15]. Moreover, chemical absorption can only be applied to small extraction gas volume and low CO2 concentration (approximately 20%) [16]. But, with the gradual exploitation of oil at present, the CO2 concentration in extraction gas increase sharply. So the chemical absorption is not suitable for CO2 capture from extraction gas at present. Membranes separation is also a relatively novel method for CO2 capture from extraction gas, and it is widely used in recent years due to its easy to installation, low energy consumption and eco-friendliness [17–19]. Nevertheless, there are also some shortcomings in the use of membranes. First, membranes are extremely strict with temperature. When the
https://doi.org/10.1016/j.jtice.2019.07.008 1876-1070/© 2019 Taiwan Institute of Chemical Engineers. Published by Elsevier B.V. All rights reserved.
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B. Liu, M. Zhang and X. Yang et al. / Journal of the Taiwan Institute of Chemical Engineers 103 (2019) 67–74 Table. 1 Component of extraction gas from a single well. Component
Content (mol%)
CO2 CH4 C2 H6 C3 H6 i-C4 n-C4 i-C5 n-C5 C 6+ N2
80.14 13.14 2.3 2.64 0.25 0.78 0.13 0.16 0.1 0.12
Table. 2 Characteristic parameters of extraction gas. Material parameters
Numerical value
Pressure (MPa) Temperature (K) Flow rate (Nm3 /d)
0.30–0.60 293.15 100,000
Fig. 1. CO2 content in extraction gas of Shengli Oilfied.
temperature exceeds 100 °C, the membranes structure will be destroyed rapidly [20]. Moreover, membranes are also sensitive to impurities (e.g. heavy hydrocarbon, water vapor), therefore, pretreatment is for membranes separation. So membranes separation is difficult apply to long-term, large-scale extraction gas operation in industry conditions [21]. Compared with membrane and chemical separation, there have been paid more and more attention to the cryogenic-based approaches in recent years [22]. Cryogenic fractionation is to separate CO2 from extraction gas by using different condensation and desublimation properties of each component in the extraction gas [23]. Compared to other separation methods, cryogenic fractionation can achieve higher CO2 purity (99.99%) and recovery(99.99%) and it can handle large-scale gas volume which operated under moderate pressure, meanwhile, it also avoid the heavy cost of regenerating the solvent [24]. There are some different cryogenic fractionation process using in industry. Operated packed beds using in cryogenic fractionation for CO2 capture was proposed by Tuinier et al. [25]. The biggest advantage in this method is H2 O and CO2 can automatically separate due to their different dew and sublimation points [23]. But, the high energy consumption is also the main problem in this process. Cryogenic distillation is a kind of commonly used separation method [23] Holmes and Ryan [26] are first proposed this method for purifying natural gas, which can effectively solve the problem of CO2 solidification in distillation tower [27]. The heavy hydrocarbon(C2 –C5 ) added to the condenser in the distillation column to prevent CO2 solidification was applied by Holmes et al. [27,28]. Although cryogenic distillation is a widespread method for industrial and has a lot of benefits, high energy consumption has always been one of the major drawbacks of cryogenic distillation [29,30]. Essentially, the main limitation of cryogenic method is high operating cost for cold production [23,24]. In recent years, there are various cryogenic method have been proposed in order to reduce the high cooling consumption of cryogenic method effectively. Song et al. [31] proposed a novel cryogenic CO2 capture process based on Stirling coolers with heat integration from flue gas. The result shows that the proposed system can save about 45% energy requirement in a typical 600MW coal power plant. Knapik et al. [24] used N2 stream which came from nitrogen removal unit (NRU) as cold source to capture CO2 from flue gas. This method can significantly reduce high cooling cost in normal cryogenic CO2 capture method and the energy
consumption for per kilogram CO2 product of this process reduce to 0.125 KWh. Despite these methods can effectively decrease cooling consumption of cryogenic method, the energy consumption of cryogenic fractionation for CO2 capture still largely depends on the operating pressure, and the concentration of CO2 in the extraction gas. In this paper, we use cryogenic distillation model proposed by Holmes and Ryan [26], and the real industrial situation of extraction gas is simulated by Aspen HYSYS V8.6. Meanwhile, the effects of liquefaction temperature, condensation temperature, re-boiling temperature and fractionation tower operating pressure on energy consumption, CO2 recovery and CO2 product concentration were analyzed in detail. Based on this simulation results, we can conclude main factors and laws that affect the energy consumption of the system. That will be of guiding significance to capture CO2 from extraction gas by cryogenic fractionation technology.
2. Process description The extraction gas from a single well of Shengli Oilfied station is showed in Table 1 and 2. Compared with other oil field, the extraction gas in Shengli Oilfied exhibits these characteristics: (a) High carbon dioxide concentration; (b) High heavy hydrocarbon concentration; (c) Containing saturated water. In order to avoid the sour corrosion of equipment and pipe, prevent the formation of hydrate in the gas treatment process and ensure the safe and reliable operation of the device, the extraction gas needs to be dehydrated in pretreatment, getting dry raw gas for CO2 separation in the cryogenic fractionation device. However, the pretreatment process is not included in the scope of this paper. Based on cryogenic distillation proposed by Holmes and Ryan [23,26], the cryogenic fractionation process was shown in Fig. 2, which has been improved to suit the extraction gas conditions in this paper. The raw gas is initially compressed by compressor system, then cooled by the precooler (heat exchanger) and liquefied by liquefier. The liquefied raw gas is then sent to the distillation tower, in where the component steam is dived into two parts: top and bottom products [23]. The former is CH4 stream, which is condensed by the condenser at the top of the tower and then flows out after the cooling capacity was released through the pre-cooler. The latter is CO2 products, which flows out after being boiled by the reboiler at the bottom of the tower.
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Fig. 2. The flow diagram of cryogenic fractionation process.
Table 3 Main equipment parameter.
3. Simulation basis 3.1. Peng–Robinson equation The simulation in this paper based on Aspen HYSYS, which is widely used in petroleum and chemical industries and contains more complex perception calculation packages and unit operations. Acid Gas is a thermodynamic property package which built in Aspen HYSYS and it is applied for rigorous equilibrium calculation and kinetic reactions [32]. The Peng–Robinson (PR) equation which belongs to Acid Gas is ideal for calculating the gas-liquid equilibrium of the system and the liquid density of the hydrocarbon system. The extraction gas is considered as Acid Gas, hence, the PR equation is selected as the calculation model in the simulation in this paper. 3.2. Capture performance In order to evaluate the CO2 capture performance of whole cryogenic fractionation process, two main parameters are introduced: CO2 purity, CO2 recovery and cooling load. The calculation of CO2 purity and system cooling load is summarized as follows:
C O2 purity
mC O2 out = mtotal out
(1)
where mCO2 out is the mass flow of feed gas the outlet of the process and mtotal out is the mass flow of gas mixture from the outlet of process.
C O2
recovery
=
mout × wC O2 out min × wC O2 in
(2)
where min and m out are the mass flow of raw gas and outlet product. wCO2 out and wCO2 in are the mass fraction of CO2 in raw gas and outlet product.
Etotal cooling = Ecompressor system + Eliqu f ier + Econdenser
(3)
Ecompressor system = Ecooler
(4)
Eliqu f ier = mCO2 × h
(5)
Main equipment
Temperature(°C)
Pressure increasing (kPa)
1-COMP 2-COMP
156.9 139.4 Temperature(°C) 40 40 40 −23 −27 −4 Operating pressure 3MPa
900 2040 Pressure drop (kPa) 50 50 20 40 20 20 Number of stages 8
COOLER-1 COOLER-2 PRECOOLR LIQUEFIER Condenser Reboiler FRA-TOWER
where Ecompressor system , Eliqufier and Econdenser are the cooling load of compressor system, liqufier and condenser separately. Ecooler is the cooling load of two cooler in compressor system. For the cooling load of liqufier, mCO2 is the mass of liquefaction CO2 , h is the cold energy for liquefying CO2 . 3.3. Simulation assumption The simulation of overall cryogenic fractionation is based on Aspen HYSYS. The cryogenic fractionation HYSYS model is established based on Fig. 2. Meanwhile, the main parameters are shown in Table 3. In order to simplify the calculation, the following simulation assumption are used: (1) The thermodynamic calculation is based on Peng–Robinson (PR) equation [32] (2) The adiabatic efficiency of compressor is set 85% [11] (3) The heat loss of whole process is neglected (4) the extraction gas through pretreament process contains no high heavy hydrocarbon and saturated water. 4. Results and discussion 4.1. Effect of the operating pressure of fractionating tower The purity of the CO2 product and the recovery rate of CO2 are set as 92.5% and 90%, respectively. The effect of fractionating tower pressure on condensation temperature, liquefaction
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Fig. 3. Effect of fractionating tower operating pressure on condensation temperature and liquefaction temperature. Simulation condition are as follows: the flow rate of raw gas is 10 0,0 0 0 Nm3 /d, feed pressure is 0.3 MPa, CO2 content in raw gas is 80.14%.
Fig. 4. Effect of operating pressure of fractionating tower on cooling load of per unit CO2 and compressor power. Simulation condition are as follows: the flow rate of raw gas is 10 0,0 0 0 Nm3 /d, feed pressure is 0.3 MPa, CO2 content in raw gas is 80.14%.
temperature, cooling load of the system and the power of the compressor is analyzed. The variation of condensation temperature and liquefaction temperature with operating pressure of fractionating tower is shown in Fig. 3. As the increase of operating pressure of fractionating tower, the dew point temperature of CO2 increases, the liquefaction temperature and the condensation temperature at the top of the tower also increase gradually. In order to ensure the recovery rate of CO2 and reduce the cooling load at the top of the tower, liquefaction temperature should be lowered appropriately and the condensation temperature should be increased. As shown in Fig. 4, when the fractionating tower pressure is 2.5 MPa, the liquefaction temperature is −33 °C, the condensation temperature is −36.36 °C, and when the pressure is 4.0 MPa, the liquefaction temperature is −20 °C and the condensation temperature is −23.89 °C. As shown in Fig. 4. the cooling consumption of per unit CO2 and compressor power varies with operating pressure of fractionating tower. As pressure rises from 2.5 MPa to 4.0 MPa, the per unit product cooling load decreases initially and then increases. When the pressure is 3.1 MPa, the lowest per unit product cooling load is 419.72 MJ/t. When the pressure rises from 2.5 MPa to 3.1 MPa, the cooling load of the liquefier decreases and the cooling load of
Fig. 5. The effect of operating pressure of fractionating tower on total energy consumption. Simulation condition are as follows: the flow rate of raw gas is 10 0,0 0 0 Nm3 /d, feed pressure is 0.3 MPa, CO2 content in raw gas is 80.14%.
the top condenser rises slowly, so the cooling load of per unit CO2 decreases. When the pressure rises from 3.1 MPa to 4.0 MPa, the increasing speed of cooling load on top of tower is higher than the decreasing speed of cooling load on liquefier. Hence, the cooling load of per unit CO2 tends to increase. When the operating pressure of fractionating tower rises, the power of compressor rises accordingly, and compressor power and fractionating tower operating pressure are linearly and positively correlated. In the process of fractionating tower pressure rising from 2.5 MPa to 4.0 MPa, compressor power increases from 301 kW to 382 kW. Figs. 3 and 4 show the change of fractionating tower operating pressure on liquefaction temperature, condensation temperature, compressor power and cooling load. When operating pressure of fractionating tower is 3.1 MPa, the sum of per unit product cooling load and compressor power is the smallest. According to Eqs. (3)–(5), Fig. 5 shows the effect of operating pressure of fractionating tower on total energy consumption. When operating pressure of fractionating tower is 3.1 MPa, total energy consumption reaches the smallest(1362.9 MJ/t). Therefore, 3.1 MPa is a reasonable operating pressure for fractionating tower. 4.2. Effect of the condensation temperature The operating pressure of fractionating tower is set as 3.1 MPa, the purity of CO2 product is set as 92.5%. The effect of condensation temperature on CO2 recovery rate and system energy consumption are analyzed. Fig. 6 shows the effect of condensation temperature on CO2 recovery rate at different liquefaction temperature. When the liquefaction temperature changes from −21 °C to −26 °C, the different curve of the relationship between CO2 recovery rate and condensation temperature basically coincides, indicating that the CO2 recovery rate is controlled by the condensation temperature and is not affected by liquefaction temperature. For a certain liquefaction temperature, the recovery of CO2 and the output of CO2 decrease, with the increase of condensation temperature. The relationship between the condensation temperature and cooling load of condenser at different liquefaction temperature is shown in Fig. 7. For a certain liquefaction temperature, the cooling load of condenser first decreases and then increases with the increase of the condensation temperature. For different liquefaction temperature curves, when the condensation temperature is
B. Liu, M. Zhang and X. Yang et al. / Journal of the Taiwan Institute of Chemical Engineers 103 (2019) 67–74
Fig. 6. Effect of condensation temperature on CO2 recovery rate. Simulation conditions are as follows: the flow rate of raw gas is 10 0,0 0 0 Nm3 /d, feed pressure is 0.3 MPa, CO2 content in raw gas is 80.14%, the operating pressure of fractionating tower is 3.1 MPa.
Fig. 7. Effect of condensation temperature on condenser cooling load. Simulation conditions are as follows: the flow rate of raw gas is 10 0,0 0 0 Nm3 /d, feed pressure is 0.3 MPa, CO2 content in raw gas is 80.14%, the operating pressure of fractionating tower is 3.1 MPa.
−27 °C, the cooling load is the lowest. Comparing with different liquefaction temperature curves, it shows that with the decrease of liquefaction temperature, the cooling load of condenser decreases. When the condensation temperature is −27 °C and the liquefaction temperature is −26 °C, the cooling load of condenser is 19.5 kW. When condensation temperature is −27 °C and the liquefaction temperature is −21 °C, the cooling load of condenser is 63.14 kW. Fig. 8. shows the variation curves of cooling load of per unit product with condensation temperature at different liquefaction temperature. For a certain liquefaction temperature, with the increase of condensation temperature, the cooling load of the system increases and the cooling load of per unit product also increases. When the condensation temperature increases from −34 °C to −27 °C, the growth trend of per unit product cooling load is relatively flat. However, when the condensation temperature increases from −27 °C to −25 °C, the cooling load of per unit product rises sharply. For a certain condensation temperature, with the increase of liquefaction temperature, the per unit product cooling load without obvious increases.
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Fig. 8. Effect of condensation temperature on per unit product cooling load. Simulation conditions are as follows: the flow rate of raw gas is 10 0 0 0 0Nm3 /d, feed pressure is 0.3 MPa, CO2 content in raw gas is 80.14%, the operating pressure of fractionating tower is 3.1 MPa.
Fig. 9. Effect of the reboiling temperature on CO2 product purity. Simulation condition are as follows:the flow rate of raw gas is 10 0,0 0 0 Nm3/d, feed pressure is 0.3 MPa, CO2 content in raw gas is 80.14%, the operating pressure of fractionating tower is 3.1 MPa, the condensation temperature is −27 °C.
Figs. 6– 8 show that condensation temperature control not only the CO2 recovery rate, but also affect the cooling load of the system. When the condensation temperature is −27 °C, the system has low cooling load and high CO2 recovery rate. So, optimum condensation temperature is chosen as −27 °C. 4.3. Effect of the reboiling temperature The operating pressure of fractionating tower is set as 3.1 MPa, CO2 content in raw gas is 80.14% and the condensation temperature is −27 °C. Then the influence of different reboiling temperature on CO2 product purity and per unit product cooling load is analyzed. As shown in Fig. 9, as the reboiling temperature rises from −17 °C to −3 °C, the purity of CO2 product rises. When the reboiling temperature changes from −17 °C to −4 °C, the CO2 product purity increases slowly, but when the reboiling temperature higher than −4 °C, the purity of CO2 product rises sharply. The highest CO2 product purity can reach 94.1% when the reboiling temperature is −3 °C.
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Fig. 10. Effect of purity reboiling temperature on per unit product cooling consumption. Simulation condition are as follows: the flow rate of raw gas is 10 0 0 0 0Nm3/d, feed pressure is 0.3 MPa, CO2 content in raw gas is 80.14%, the operating pressure of fractionating tower is 3.1 MPa, the condensation temperature is −27 °C.
Fig. 11. Effect of CO2 content in raw gas on CO2 product purity. Simulation condition are as follows: the flow rate of raw gas is 10 0,0 0 0 Nm3/d, feed pressure is 0.3 MPa, CO2 content in raw gas is 80.14%, the operating pressure of fractionating tower is 3.1 MPa, the condensation temperature is −27 °C, the reboiling temperature is −4 °C.
It can be seen from Fig. 10 that as the reboiling temperature decreases, the consumption of CO2 cooling capacity decreases accordingly. When the reboiling temperature is higher than −4 °C, the energy consumption of the system increases rapidly. Combining the Figs. 9 and 10, with the increase of reboiling temperature, CO2 product purity and cooling load of per unit product increase accordingly. When the purity of CO2 product continues to improve, a large amount of CO2 begins to evaporate in reboiler and condense in top condenser, so the heat load of bottom rises rapidly, which leads to the rapid increase of top cooling load and the increase of per unit product cooling consumption per unit product. When the reboiling temperature is −4 °C, CO2 product purity and cooling load of per unit product make a sudden increasing. Therefore, the optimum reboiling temperature is −4 °C. 4.4. Effect of different CO2 content in raw gas The operating pressure of fractionating pressure is set as 3.1 MPa, the condensation temperature and reboiling temperature are −27 °C and −4 °C separately. The effect of different CO2 content in raw gas on CO2 product purity and energy consumption is analyzed. The relationship between CO2 content in raw gas and CO2 product purity shows in Fig. 11. When the CO2 content in raw gas is gradually increased above 80%, the purity of the CO2 product gradually increases due to the gradual decrease of the hydrocarbon content in raw gas. As the CO2 content is higher than 91%, the hydrocarbon content in raw gas decreases sharply, and the purity of CO2 products increases rapidly. Fig. 12. shows the effect of different CO2 content in raw gas on compressor power and cooling load of system. With the CO2 content in raw gas increasing from 80% to 94%, the compressor power decreases. When CO2 content in raw gas exceeds 91%, the compressor power decreases sharply. Meanwhile, the cooling load of the system first decreases and then increases with the increase of CO2 content in raw gas, and reaches the minimum when the CO2 content in raw gas is 91%. When the CO2 content in raw gas is 92.5%, the two curves intersect at one point which is the minimum point of the system energy consumption. Fig 13 also verify this viewpoint, when CO2 content in raw gas is 92.5%, according to Eqs. (3)–(5),total energy consumption reaches the minimum (1100 MJ/t).
Fig. 12. Effect of CO2 content in raw gas on compressor power and cooling load of system. Simulation condition are as follows: the flow rate of raw gas is 10 0,0 0 0 Nm3 /d, feed pressure is 0.3 MPa, CO2 content in raw gas is 80.14%, the operating pressure of fractionating tower is 3.1 MPa, the condensation temperature is −27 °C, the reboiling temperature is −4 °C.
The CO2 content in raw gas is set as 80.14%, 86.0%, 91.0% and 93.5% respectively, and the relationship of the CO2 content in raw gas, the CO2 product purity and cooling load of per unit product is shown in Fig. 14. Comparing the four curves, it shows that as the CO2 content in raw gas reduce, the cooling consumption of per unit product required to capture the same purity CO2 is significantly increased. When the CO2 content in raw gas is 80.14%, 86%, 91% and 93.5%, the cooling consumption of per unit product required for purity of CO2 products from the lowest to the highest is 79 MJ/t, 54.2 MJ/t, 51.8 MJ/t and 60.5 MJ/t respectively. Similarly, the highest purity of CO2 product can reach 92.5%, 95.0%, 95.8% and 99.0% respectively. In the same way, cooling load of the system increases more and more rapidly as the CO2 product purity rise and when CO2 product purity reaches a certain value, the cooling load of system increases sharply. Figs. 11– 14. shows that with the CO2 content in raw gas increases, the CO2 product purity increases accordingly, the compressor power decreases and the cooling load of system and cooling load of per unit product decrease first and then increase. It can be
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concluded that for the separation of CO2 by cryogenic fractionation, the reasonable content of CO2 in raw gas is 86%−93%. 4.5. Technical evaluation
Fig. 13. The effect of CO2 content in raw gas on total energy consumption. Simulation condition are as follows: the flow rate of raw gas is 10 0,0 0 0 Nm3 /d, feed pressure is 0.3 MPa, CO2 content in raw gas is 80.14%, the operating pressure of fractionating tower is 3.1 MPa, the condensation temperature is −27 °C, the reboiling temperature is −4 °C.
The work of Yousef et al. [33] for low-temperature CO2 removal from biogas is used to evaluate the technical feasibility of this work. The simulation data of two work are enumerated in the Table 4. The performance of distillation pressure, condensation temperature and reboiling temperature in cryogenic fractionation are different with the low-temperature process from the work of Yousef et al. [33]. That is because the CO2 concentration in biogas is far less than in extraction gas. Meantime, according to the bubble point and dew point of CO2 from Maqsood et al. [34], the operating pressure of fractionating tower, condensation temperature and reboiling temperature is also reasonable. Apart these three parameters, other parameters have a good agreement with the work of Yousef et al. [33]. The comparison of the work of Yousef et al. [33] and Maqsood et al. [34] proves that the simulation data in this work are reasonable and reliable. 5. Conclusion The study simulates the whole cryogenic fractionation process for CO2 separation from EOR extraction gas using Aspen HYSYS. The model is simulated based on Holmes and Ryan [26,28]. The PR equation is used as thermodynamic calculation. Meanwhile, a series assumptions are adopted in process simulation. According to this model, the effect of operating pressure of fractionating tower, reboiling temperature, condensation temperature and CO2 content in raw gas on CO2 product purity and cold consumption are analyzed and through change these operational parameters, a set of reasonable operational parameters are obtained with the scale of raw gas is 10 0,0 0 0 Nm3 /d. The main findings of proved cryogenic fractionation are as follows:
Fig. 14. Effect of purity change of CO2 products on per unit product cold consumption. Simulation condition are as follows: the flow rate of raw gas is 10 0,0 0 0 Nm3 /d, feed pressure is 0.3 MPa, CO2 content in raw gas is 80.14%, the operating pressure of fractionating tower is 3.1 MPa, the condensation temperature is −27 °C, the reboiling temperature is −4 °C.
(a) With the increase of the operating pressure, the liquefaction temperature, the condensation temperature, the CO2 product purity and the compressor power increase accordingly. However, the cooling load decreases first and then rises. When the operating pressure is 3.1 MPa, the per unit cooling consumption reaches the lowest which is 419.72 MJ/t. (b) With the increase of condensation and reboiling temperature increase, the CO2 recovery rate and CO2 product purity increase, while the system cooling load also increase. When the condensation temperature and reboiling temperature reach at −27 °C and −4 °C respectively, the CO2 recovery rate and CO2 purity is 90% and 92.5% separately. (c) With the CO2 content in raw gas increase, the CO2 product purity increases, however, the system cooling consumption
Table 4 The main parameters of cryogenic fractionation compared to low-temperature process [33].
Mole flow (kmol/h) Composition (mol%) CO2 CH4 Gas inlet temperature (°C) Gas inlet pressure (MPa) Distillation pressure (MPa) Condenser temperature (°C) Reboiling temperature Per unit energy consumption of CO2 captured (MJ/ton CO2 ) CO2 purity (%)
Cryogenic fractionation
Low-temperature process [33]
4460
1000
80 20 25 0.3 3.1 −27 −4 1290 91.3
40 60 35 0.12 4.9 −75.7 13.6 1377 92.5
74
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decreases. Therefore, cryogenic fractionation method is suitable to separate high CO2 content extraction gas with the scale of 86%−93%. The energy analysis in this work will provide positive advice for cryogenic fractionation using in scale-up application and a series reasonable parameters are also obtained. However, there are also several challenges in cryogenic fractionation, such as the effect of the liquefication temperature and the treatment of noncondensable gas. Therefore, these challenges will be discussed and optimized in future works. Acknowledgments This work was supported by Department of Science & Technology of Shandong Province (No. ZR2018LB025). References [1] Olajire AA. CO2 capture and separation technologies for end-of-pipe applications–a review. Energy 2010;35:2610–28. [2] Safdarnejad SM, Hedengren JD, Baxter LL. Dynamic optimization of a hybrid system of energy-storing cryogenic carbon capture and a baseline power generation unit. Appl Energy 2016;172:66–79. [3] Zhang L, Shen Q, Wang M, Sun N, Wei W, Lei Y, Wang Y. Driving factors and predictions of CO2 emission in China’s coal chemical industry. J Clean Prod 2019;210:1131–40. [4] Li L, Zhao N, Wei W, Sun Y. A review of research progress on CO2 capture, storage, and utilization in Chinese academy of sciences. Fuel 2013;108:112–30. [5] Yu S, Horing J, Liu Q, Dahowski R, Davidson C, Edmonds J, et al. CCUS in China’s mitigation strategy: insights from integrated assessment modeling. Int J Greenh Gas Control 2019;84:204–18. [6] DTapia JF, Lee JY, Ooi REH, Foo DCY, Tan RR. Optimal CO2 allocation and scheduling in enhanced oil recovery (EOR) operations. Appl Energy 2016;184:337–45. [7] Ampomah W, Balch RS, Cather M, Will R, Gunda D, Dai Z, et al. Soltanian optimum design of CO2 storage and oil recovery under geological uncertainty. Appl Energy 2017;195:80–92. [8] Mikkelsen M, Jørgensen M, Krebs FC. The teraton challenge. A review of fixation and transformation of carbon dioxide. Energy Environ Sci 2010;3(1):43–81. [9] Zhang L, Wang S, Zhang L, Ren SR, Guo Q. Assessment of CO2 EOR and its geostorage potential in mature oil reservoirs, Shengli Oilfield, China. Pet Explor Dev 2009;36(6):737–42. [10] Porter RTJ, Fairweather M, Pourkashanian M, Woolley RM. The range and level of impurities in CO2 streams from different carbon capture sources. Int J Greenh Gas Control 2015;36:161–74. [11] Song C, Liu Q, Ji N, Deng S, Zhao J, Li Y, et al. Reducing the energy consumption of membrane-cryogenic hybrid CO2 capture by process optimization. Energy 2017;124:29–39. [12] Yeo ZT, Chew TL, Zhu PW, Mohamed AR, Chai SP. Conventional processes and membrane technology for carbon dioxide removal from natural gas: a review. J Nat Gas Chem 2012;21(3):282–98. [13] Zhang X, Liu H, Liang Z, Idem R, Tontiwachwuthikul P, Al-Marin MJ, et al. Reducing energy consumption of CO2 desorption in CO2 -loaded aqueous amine solution using Al2O3/HZSM-5 bifunctional catalysts. Appl Energy 2018;229:562–76.
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