Simulation and energy performance assessment of CO2 removal from crude synthetic natural gas via physical absorption process

Journal of Natural Gas Chemistry 21(2012)633–638

Simulation and energy performance assessment of CO2 removal from crude synthetic natural gas via physical absorption process Wanjun Guo1 , Fei Feng1,2 ,

Guohui Song1 , Jun Xiao1 , Laihong Shen1∗

1. Key Laboratory of Energy Thermal Conversion and Control of Ministry of Education, School of Energy and Environment, Southeast University, Nanjing 210096, Jiangsu, China; Department of Mechanical Technology, Nanjing College of Chemical Technology, Nanjing 210048, Jiangsu, China [ Manuscript received April 5, 2012; revised June 18, 2012 ]

Abstract The paper presents an energy performance assessment of CO2 removal for crude synthetic natural gas (SNG) upgrade by Selexol absorption process. A simplified process simulation of the Selexol process concerning power requirement and separation performance was developed. The assessment indicates that less pressure difference between crude SNG and absorption pressure favors the energy performance of CO2 removal process. When both crude SNG and absorption pressures are 20 bar, CO2 removal process has the best energy performance. The optimal specific power consumption of the CO2 removal process is 566 kJ/kgCO2 . The sensitivity analysis shows that the CO2 removal efficiency would significantly influence the total power consumption of the removal process, as well as higher heating value (HHV) and CO2 content in SNG. However, the specific power consumption excluding crude SNG and SNG compressions changes little with the variance of CO2 removal efficiency. If by-product CO2 is compressed for CO2 capture, the process would turn into a CO2 -sink for the atmosphere. Correspondingly, an increase of 281 kJ/kgCO2 in specific power consumption is required for compressing the separated CO2 . Key words CO2 removal; physical absorption; synthetic natural gas; power consumption

1. Introduction Synthetic natural gas (SNG) production from biomass by thermochemical process is considered as an attractive technology currently [1]. A sustainable alternative route for natural gas production is pressingly needed as the natural gas resources are finite in the world and the depletion will come in near future. Among the candidates for liquid and gaseous synthetic fuels by biomass thermochemical processes, methane is one of the most promising options, since the synthesis reaction approaches chemical equilibrium and its conversion efficiency is high [2]. If the CO2 capture and storage (CCS) is applied to the CO2 removal from crude SNG, the process of SNG production from biomass would be a CO2 -sink for the atmosphere [2]. Recently several research groups have investigated suitable technologies and processes for SNG production from biomass [1−7]. However, the investigation on CO2 removal for crude SNG upgrade has received little attention so far. Based on different chemical and physical processes, CO2 re∗

moval processes can be grouped into the following types: absorption in alkaline solution, physical absorption, membrane permeation, adsorption, and methanation [8−11]. However, suitable techniques for CO2 removal should be carefully chosen by taking several factors into account, especially partial pressure of CO2 and plant size [11]. For CO2 removal from biogas for cogeneration combined heat and power plant in Europe, the power consumption with pressure swing adsorption (PSA) technique is about 459 kJ/kgCO2 , while that with monoethanol amine washing (MEA) and diethanol amine washing (DEA) is less than 275 kJ/kgCO2 [10]. Considering the composition of crude SNG and the plant size of biomassbased SNG production factory, physical absorption with Selexol solvent, pressure swing absorption and polymeric membranes have been regarded as suitable techniques for CO2 removal from crude SNG [2]. Gassner et al. further investigated different design strategies and process layouts for CO2 removal from crude SNG with membrane system, and the optimal power consumption of the system was determined to be about 620 kJ/kgCO2 [4].

Corresponding author. Tel: +86-25-83795598; Fax: +86-25-83793452; E-mail: [email protected] This work was supported by the Special Fund for Major State Basic Research Projects of China (2010CB732206).

Copyright©2012, Dalian Institute of Chemical Physics, Chinese Academy of Sciences. All rights reserved. doi:10.1016/S1003-9953(11)60412-X

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Alternately, physical absorption with Selexol solvent was considered as an attractive option for crude SNG upgrade [2,5,7]. In the case of raw gas containing more than 10 vol% of CO2 [12], CO2 removal based on physical absorption processes allows to keep the energy demand on a reasonable level, with high potential for its application in large industrial scale [12]. This study focused on power requirement of the CO2 removal from crude SNG via Selexol absorption process. A simplified performance model of the CO2 removal process was developed and integrated in the commercial process simulation software Aspen Plus. Based on the process simulation, an energy performance assessment was carried out for a case study. 2. Process simulation 2.1. Properties of Selexol solvent The Selexol solvent is a mixture of dimethyl ethers polyethylene glycol with the formulation of CH3 (CH2 CH2 O)n CH3 , where the subscript n ranges from 3 to 9 [14]. The average mole weight and density of the Selexol solvent are 280 g/mol and 1030 kg/m3, respectively. The performance of a physical solvent can be predicted by its solubility. The relative solubilities of common components in crude SNG are listed in Table 1 [14]. Table 1. Relative solubilities of some gases in Selexol solvent Gas Relative solubility (k)

CO2 1

H2 0.013

CH4 0.0667

CO 0.028

H2 O 733

N2 0

The solubility of a gas in Selexol solvent depends on its partial pressure and temperature. Either higher partial pressure or lower temperature leads to higher solubility. The actual solubility of CO2 as a function of temperature is as follows [14]: χCO2 (SCF · gal−1 · psia−1 ) = 0.1164 − 0.00144 · t

(1)

where, t is the solvent temperature in the range of −1 ∼ 25 ◦ C. 2.2. Process description Figure 1 presents a general flow sheet of Selexol absorption process [13]. Crude gas is compressed by a compressor (CMCG ) and then cooled in a water cooler (CLCG ) to keep the temperature in the absorber at a low level. Before entering the absorber, crude gas is mixed with the recycle gas that comes from the first stage flash tank (FT1). The mixed crude gas enters the absorber and flows counter currently with Selexol solvent. After absorption in the absorber, the CO2 -rich solvent leaves from the bottom of the absorber and enters FT1, which is operated at 6 bar. The Selexol solvent leaving the absorber also contains a tiny amount of CO, CH4 and H2 . In FT1 practically the whole amount of CO, H2 and CH4 absorbed by Selexol solvent is released, together with part of CO2 [13]. Subsequently, the recycle gas is compressed by a gas compressor (CMRG ) and cooled by a water cooler (CLRG ) and then mixed with the crude gas. The solvent leaving FT1 is further depressurized and flows through the following flash tanks (FT2-FT4) step by step, which are operated at 4.2 bar, 2.3 bar and 1 bar, respectively. Part of CO2 retained in the solvent is released at every stage flash tank during the stripping process, and Selexol solvent is regenerated simultaneously. The CO2 -lean solvent leaving FT4 flows to a dehydrator (DH), where partial or all water is removed. After dehydration, the Selexol solvent pressure is raised by a solvent pump to the value adequate to absorption pressure. Before entering the absorber, the CO2 -lean solvent is cooled down by an industrial refrigeration system (Refrig). The CO2 stream released in FT2-FT4 is delivered for final compression by a series of compressors (CM1-CM5) and coolers (CL1-CL5), respectively. Alternatively, it is exhausted directly to the atmosphere as a waste by-product. Purified gas (i.e. SNG) that exits from the top of the absorber is compressed to 65 bar by a single stage compressor (CMPG ) and then cooled to 25 ◦ C by a heat exchanger (CLPG ).

Figure 1. A general flow sheet of Selexol process for CO2 removal

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There is no heat duty for the Selexol process [14], but only power consumption of the Selexol process should be considered. Normally gas compression and heat transfer can be directly simulated by relevant models in Aspen Plus. The isentropic efficiency of a compressor was set as 0.8. However, the simulation of the Selexol process based on Aspen Plus still suffers from some barriers. The flow rate of the Selexol solvent should be estimated firstly, which is needed for calculation of the power requirements of the pump and refrigeration system. The separation performances of absorber and flash tanks should also be determined, which influence the power requirements of compressing the recycle gas and separated CO2 .

where, pA is the absorption pressure in bar. Then the flow rate of Selexol solvent for capturing the required amount of CO2 is estimated as follows: γ(fCO2 ,A + fCO2 ,resd ) fSel (kg/s) = (5) χCO2 · pA · xCO2 ,CG By solving Equations 3 to 5, the correlation for estimating the Selexol solvent flow rate is expressed as follows: fSel (kg/s) =

2.3. Process modeling 2.3.1. Selexol solvent f low rate

fCG · xCO2 ,CG · α χCO2 · (pA · xCO2 ,CG − γ · pCO2 ,FT4 )

(6)

2.3.2. Separation ratios

The increase in solvent temperature during absorption process influences the required Selexol solvent flow rate. Generally, the temperature increase is caused by two reasons: (1) the heat transfer between the crude gas and the Selexol solvent, and (2) the absorption heat of the captured gases. As cooling all streams entering the absorber is recommended [13], there can be no heat transfer between the solvent and crude gas. Furthermore, this also simplifies the estimation of Selexol solvent flow rate by the method presented in literature [14]. In this study, the absorber was designed to be operated at around 25 ◦ C. In order to estimate the Selexol solvent flow rate expediently, it was assumed that the performance of the absorber could be approximately represented and predicted by the solubility at 25 ◦ C. In this modeling, the error on mass flow rate of Selexol solvent only influences the power consumptions of solvent pump and refrigeration system, which account a relatively small proportion of the total power consumption. It could be reckoned that this assumption has little influence on the total power consumption of the system. According to the required CO2 removal efficiency, the amount of CO2 that needs to be captured in the absorber by Selexol solvent (fCO2 ,A ) is calculated as follows: fCO2 ,A (kmol/s) = fCG · xCO2 ,CG · α

efficiency, the ratio of the actual flow rate to the equilibrium flow rate of the Selexol solvent is estimated as follows [13]: 1.26 − 0.0000138pA γ= (4) (1 − α)0.07

(2)

In this study, separation ratio of a species means the ratio of the amount of a species captured by or retained in the Selexol solvent to its total amount at the inlet of absorber or flash tank. For CO, CH4 and H2 , the separation ratios of a species j can be estimated as follows (see SI 1): rj,A = kj · rCO2 ,A

(7)

where, rCO2 ,A is the separation ratio of CO2 in the absorber. The separation ratio of H2 O in absorber is estimated as follows (see SI 1): rH2 O,A =

1 fCG · (1 − α · xCO2 ,CG ) 1+ kH2 O · χCO2 · (fSel /γ) · pA

(8)

Lampert and Ziebik considered that the whole amount of CO, H2 and CH4 absorbed by the Selexol solvent is released in the first flash tank practically [13]. The small amount of CO, H2 and CH4 will not contribute much to the total power consumption of the CO2 removal process. Then only separation ratio of CO2 in FT1 needs to be determined. According to mass conservation, the total amount of CO2 captured in the absorber equals the amount released in FT1 plus the amount retained in the solvent, which can be expressed as follows:

where, fCG is the mole flow rate of crude gas in kmol/s, xCO2 ,CG is the mole fraction of CO2 in crude gas, and α is the required CO2 removal efficiency in %/%. In the flash tanks, the residual time is long enough to assume that equilibrium can be achieved in these tanks. Hence the amount of the residual CO2 in the lean solvent (fCO2 ,resd ) is calculated as follows [14]:

Then the mole fraction of CO2 in the recycle gas can be expressed as follows: fCO2 ,RG xCO2 ,FT1 = (10) fj,A

fCO2 ,resd (kmol/s) = χCO2 · fSel · pCO2 ,FT4

Subsequently, the amount of CO2 retained in the solvent is calculated as follows:

(3)

where, χCO2 is 1.3731×10−4 kmol/(kg·bar) at 25 ◦ C; fSel is the mass flow rate of Selexol solvent in kg/s; pCO2 ,FT4 is the pressure in the last stage flash tank. In the absorber, the equilibrium can not be achieved due to the limited residual time [14]. For a given CO2 removal

fCO2 ,A (kmol/s) = fCO2 ,RG + fCO2 ,FT1

fCO2 ,FT1 (kmol/s) = χCO2 · fSel · pFT1 · xCO2 ,FT1

(9)

(11)

Assuming an initial value of xCO2 ,FT1 then iteratively calculating Equations 9 to 11 until xCO2 ,FT1 is converged, the separation ratio of CO2 in FT1 can be determined finally.

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For the other stages of flash tanks, the separation ratio of CO2 is equal to the ratio of the flash pressure in the former stage flash tank to that in the current stage flash tank. 2.3.3. Power consumptions of pump and refrigeration system With knowing the Selexol solvent flow rate, the power consumption of the solvent pump (pPump ) is estimated as follows [14]: pPump (kW) =

VSel · pPump 36ηPump

(12)

where, VSel is the volume flow rate of the solvent in m3 /h; pPump is the pressure difference over the pump in bar; and pump efficiency ηPump was assumed to be 0.85 including the efficiencies of transmission and electric motor. The solution heat of the other gases is negligible because the amount of the other gases captured by Selexol solvent is much less than that of CO2 [14]. In addition, the temperature of Selexol solvent increases about 2 ◦ C due to compression [14]. Before entering the absorber, it should also be cooled down. Then the power consumption of the industrial refrigeration system (pRefrig ) can be estimated as follows: cCO2 · fCG · xCO2 ,CG · α + cSel · fCG · t COP (13) where, cCO2 is the solution heat of CO2 , i.e., 102.345 kJ/kmol. cSel is the heat capacity of the Selexol solvent, i.e. 2.05 kJ/kg. The coefficient of performance (COP) was assumed to be 3.5. t is the solvent temperature increase due to compression. pRefrig (kW) =

2.4. Process simulation using Aspen Plus Based on the mentioned correlations, estimations of Selexol solvent flow rate, separation ratios in the absorber and flash tanks and power consumptions of the solvent pump and refrigeration system were integrated in Aspen Plus by userspecified Fortran subroutines. Two tear streams were defined to simulate the recycle gas and the recycle solvent. With taking the recycle gas into account, the separation ratio of CO2 (rCO2 ,A ) in the absorber model is larger than the required CO2 removal efficiency (α). In this simulation, a Design-Spec mode was developed to guarantee the amount of CO2 separated meets that required. The Design-Spec mode was coupled with the user-specified FORTRAN subroutines. The simulation of the CO2 removal by Selexol process was based on the Design-Spec mode and the user-specified Fortran subroutines. The validation of the simulation has indicated that the proposed mode is reliable for predicting the power consumption of the CO2 removal process (see SI 2). 3. A case study on crude SNG upgrade Considering the scale of a plant for biofuel production [16], the flow rate of crude SNG is assumed to be

50 kmol·h−1, i.e., about 1120 Nm3 ·h−1 . A typical composition of crude SNG is shown in Table 2 [17]. The CO2 removal efficiency of 95% was set as basic scenario. Considering the pressure range of methanation via either fixed or fluidized bed technologies [6,7], the crude SNG pressure (pcSNG ) range of 5−30 bar and absorption pressure (pA ) range of 15−30 bar were considered in this study. Total power consumption (TPC) and specific power consumption (SPC) were used to evaluate the energy performance of the CO2 removal process. Table 2. Parameters of crude SNG and SNG with various CO2 removal efficiencies Parameter CO2 removal efficiency Flow rate (kmol/h) H2 (%) N2 (%) CO (%) CO2 (%) CH4 (%) H2 O (%) HHV (MJ/Nm3 )

Crude SNG 50 1.2 4.9 0.3 45.9 47.6 0.1 17.52

90% 29.29 2.05 8.36 0.51 7.83 81.24 0 30.20

SNG 95% 28.15 2.13 8.70 0.53 4.08 84.55 0 31.43

98% 27.46 2.19 8.92 0.55 1.67 86.68 0 32.22

3.1. Total power consumption Total power consumption (TPC) of the CO2 removal from crude SNG includes the power consumptions for compressing crude SNG (CMRG ), separated CO2 (CM1-CM5), recycle gas (CMRC ) and recycle solvent (Pump), as well as that of the refrigeration system (Refrig). Figure 2 shows that TPC ranges from 151 kW to 272 kW with CO2 removal efficiency of 95% within the scope of this study. At a given pA , TPC decreases with the increase of pcSNG , due to the decrease of pressure difference over compressor CMCG . For the same reason, at a given pcSNG , TPC decreases with the decrease of pA . Under the conditions that pcSNG is equivalent to pA (as shown by the dash line in Figure 2), i.e., compressor CMCG has zero load, TPC seemingly increases with the increase of the pressure. However, in the dash line TPC has a minimum value at 20 bar. This can be explained as follows. When both pA and pcSNG increase from 15 bar to 30 bar, the power requirement of CMPG decreases linearly with the increase of the pressure. However, the separation ratio of CO2 in FT1 first decreases sharply and then gradually. Correspondingly, the power requirement of CMRG first increases significantly and then gradually. On the whole, TPC has the minimum value at 20 bar in the dash line. However, TPC at 15 bar in the boundary line is 154 kW, which is very close to the minimum value. It seems that higher pcSNG should be chosen in the design of such a CO2 removal process. However, it should be stated that the higher pcSNG might cause more power consumptions in biomass gasification unit or methanation unit. The optimal crude SNG pressure should be determined by an integrated assessment of the whole process of SNG production.

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to that of TPC. Under the conditions that pcSNG is equivalent to pA (as shown in the dash line in Figure 3), SPC also has the minimum value at 20 bar. TPC at 15 bar in the dash line is 577 kW, which is also very close to the minimum value. Gassner et al. have performed an integrated design of a membrane separation system for the crude SNG upgrade [4]. The optimal specific power consumption of the membrane separation system is about 620 kJ/kgCO2 , which is slightly larger than the optimal specific power consumption of the Selexol process. This confirms that the Selexol absorption process has an advantage in CO2 removal from crude SNG. 3.3. Sensitivity analysis Figure 2. Total power consumption (TPC) of the CO2 removal process at various crude SNG pressures (pcSNG ) and absorber pressures (pA )

3.2. Specif ic power consumption Figure 3 shows that the specific power consumption (SPC) of the CO2 removal process ranges from about 566 kJ/kgCO2 to 1020 kJ/kgCO2 in the case of this study. With CO2 removal efficiency of 95%, the variance of SPC is similar

Figure 3. Specific power consumption (SPC) of the CO2 removal process at various crude SNG pressures (pcSNG ) and absorber pressures (pA )

Mainly depending on the methanation pressure, the crude SNG pressure may vary greatly. The main product SNG can be either compressed further for delivery, expanded or burned for power generation [4,13]. Thus, with disregard for the power consumptions of both crude SNG and SNG compressions, the power consumption of the Selexol process either with or without CO2 compression is more useful to guide an integrated design of SNG production process. In this section, the sensitivity analysis was carried out to study the effect of CO2 removal efficiency on the total and specific power consumptions excluding crude SNG and SNG compressions (TPCE and SPCE), as well as the HHV of SNG. Figure 4 (a) and (b) shows that at a given pA , TPCE increases approximately linearly with the increase of CO2 removal efficiency, while SPCE varies very slightly. TPCE with CO2 efficiencies of 95% and 98% are about 5.4% and 8.4% respectively, larger than that with CO2 efficiency of 90%. At a given CO2 removal efficiency, both TPCE and SPCE increase with the increase of pA . Table 2 shows HHV of crude SNG is 17.52 MJ/Nm3 . By the separation process, HHV of SNG increases significantly to 30.02, 31.43 and 33.22 (in MJ/Nm3 ) with the CO2 removal efficiency of 90%, 95% and 98%, respectively. Chinese national technology standard requires that HHV of natural gas should be greater than or equal to 31.4 MJ/Nm3 [18]. For the

Figure 4. Effect of CO2 removal efficiency on (a) total power consumption (TPCE) and (b) specific power consumption (SPCE) excluding crude SNG and SNG compressions

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composition of the crude SNG in this study, CO2 removal efficiency should not be less than 95%. Additionally, there are also requirements on CO2 content in natural gas for different utilization purposes. For example, if used as domestic fuel, CO2 content in natural gas should not be larger than 3 vol%. Thus, CO2 removal efficiency should be determined firstly according to relevant technical requirements with respect to HHV and CO2 content in natural gas. The by-product CO2 stream released from the flash tanks can be considered as a waste. Alternatively, the separated

CO2 can be compressed further for CCS. If by-product CO2 stream is captured, the process would turn into a CO2 -sink for the atmosphere with a cost of power consumption for compressing the separated CO2 . Within the scope of this study, SPCE with CO2 compression at various pA and CO2 removal efficiencies is about 281 kJ/kgCO2 , larger than that without CO2 compression. That is the energy cost for CO2 capture by the Selexol process. Finally, Table 3 summarizes the ranges and influences of the main variables investigated in this study.

Table 3. The ranges and influences of the main variables Variable pcSNG (bar) pA (bar) CO2 removal efficiency (%)

Range 5−30 15−30 90−98

Influences power consumption of CMCG , TPC, SPC power consumptions of other power devices except CMCG , TPC, SPC composition and HHV of SNG, TPC

4. Conclusions

References

A simplified process simulation of the Selexol process concerning power requirement and separation performance was developed based on commercial simulation software Aspen Plus. The process modeling can be further integrated in a whole process simulation of SNG production from biomass, or applied to CO2 removal from other fuel gases. The results of a case study indicates that less difference between the crude SNG pressure and the absorption pressure favors the energy performance of the CO2 removal process. When the crude SNG and absorption pressures are both 20 bar, total and specific power consumptions reach their minimum values, i.e. 151 kW and 566 kJ/kgCO2 , respectively. The optimal specific power consumption of the CO2 removal process is slightly less than that of a membrane separation system. As both the gasification pressure and methanation pressure may influence the crude SNG pressure, the optimal crude SNG pressure should be determined by an integrated assessment of the whole process of SNG production. The sensitivity analysis indicates that the CO2 removal efficiency significantly influences the total power consumption excluding crude SNG and SNG compressions of the CO2 removal process, as well as HHV of SNG, however, it has little impact on the specific power consumption excluding crude SNG and SNG compressions. CO2 removal efficiency should be determined firstly according to relevant technical requirements with respect to HHV and CO2 content in natural gas. If by-product CO2 stream is compressed for CCS, the process would turn into a CO2 -sink for the atmosphere. Correspondingly, the specific power consumption increases to 281 kJ/kgCO2 compared with that without CO2 compression.

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