变压吸附分离co-co2-N2混合气制取高纯co

Separation of CO-CO,-N, gas mixture for high-purity CO by pressure swing adsorption Li-qun Zhu*, Jin-ling Tu and Ya-jun Shi Chemical Engineering Research Centre, East China University of Chemical Technology, Shanghai 200237, China A pressure swing adsorption (PSA) process for separating CO from a CO-C02-N2 mixture is proposed. The adsorbent used in this process is active carbon supported copper, which has been developed by this laboratory. By cycling the pressure of a bed of this adsorbent between ambient pressure and 20-30 Torr at room temperature, high purity CO can be obtained from the CO-C02-N2 gas mixture with a high recovery. The CO product purity depends crucially on the step of CO cocurrent purge after adsorption in the cycle and the regeneration of sorbent. Keywords: CO separation; PSA; active carbon; adsorption Introduction Efficient methods for separating carbon monoxide (CO) from gas mixtures are important in industry, since almost all CO sources are gas mixtures with nitrogen, hydrogen, methane and carbon dioxide’. Many liquid adsorbents for CO have been proposedza. However, the energy require- ments of absorption are high and the solvents are easily deactivated by trace amounts of polar gases. As the new solid adsorbents for CO were developed successfull$.6, separation of CO from gas mixtures by pressure swing adsorption (PSA) processes became possible. Gas separation by PSA technology has become an important unit operation in the chemical industry in recent years. The major applications of the technology involve air drying, air separation, octane improvement and H, purification 7-9 CO recovery from blast furnace . gases by PSA technology was first developed jointly by the Kawasaki Steel Corporation and Osaka Oxygen Ind. Ltd. The process consists of two stages of separation. The first is PSA for carbon dioxide removal by active carbon and the second is for CO recovery where sodium-type mordenite is used”. Mitsubishi Kakoki Kaishi Ltd. also developed a PSA process by using a CO-selective adsorbent developed by Hirai er al.’ for producing CO”. In this paper, a PSA cycle for separating CO from a CO- C02-Nz mixture by using active carbon supported copper, the preparation of which6 was developed in this labora- tory, is proposed. This cycle does not require a pretreat- ment bed and can give a high-purity CO product from CO-CO*-N, with a high recovery. *Present address: No. 35 Hua Yan Bei Road, West Building, Beijing, 100083, China Process description The CO-selective adsorbent used was prepared by supporting cupric chloride on active carbon and then heated in nitrogen at 300°C. This preparation method was developed in this laboratory. This active carbon supported copper obtained by this method has a high CO adsorption capacity and a satisfactory selectivity. It can separate CO from CO-C02-N, gas mixture efficiently. The proposed PSA cycle involves four steps: (1) pressurization, (2) adsorption, (3) CO cocurrent purge and (4) countercurrent evacuation. A detailed description of each step follows. Adsorption The feed gas flows through the adsorption bed at feed pressure. The bed, which contains active carbon supported copper, has been previously pressurized to feed pressure with a nitrogen-rich gas or feed gas. The feed pressure in this process is ambient pressure. The effluent from this step constitutes a N,-rich gas, a part of which is taken as the N, product and the remainder used for the bed pressurization step described later. Adsorption is con- tinued until the CO* breaks through completely. CO cocurrent purge To increase the CO product purity, which is a more select- ively adsorbed component, a CO cocurrent purge step after adsorption has been introduced. This step not only enriches CO in the void spaces of the bed, but also dis- places the CO? and Nz which have been adsorbed on 0950-4214/91/030173-04 0 1991 Butterworth-Heinemann Ltd Gas Separation 8 Purification 1991 Vol 5 September 173 PSA process for separating CO from CO-CO,-N, gas mixture: Li-qun Zhu et al. sorbent by CO. The effluents in this step are recycled to the feed mixture. Countercurrent evacuation When the CO cocurrent purge step finished, the bed was evacuated countercurrently by a mechanical vacuum pump. A vacuum desorption step must be used because CO is a strong adsorbate. The evacuation step serves to obtain CO product and to clean the bed. So effecting separation of the CO-C02-N2 mixture was possible. Some of the gas obtained from this step is collected as CO product and the rest used as the purge gas in step 3. Pressurization At the end of the evacuation step, the pressurization step begins. The bed is pressurized to the feed pressure level by introducing some of the N,-rich gas produced during step 2 countercurrently or feed gas cocurrently. The bed is now ready to undergo a new cycle starting from step 2. Feed gas PU Pressurization u Figure 2 Schematic diagram of apparatus for PSA for CO-C02-N, separation: I, adsorption bed; 2, pressure gauge; 3, manometer; 4. vacuum pump; 5. flowmeter; 6, gas volumometer; SV, solenoid valve; TC, thermocouple: SP, sampling port One mode that contains three adsorbent beds has been proposed for operating this four-step process. Figure I shows a schematic flow diagram for this arrangement. This configuration allows continuous feed gas flow and product gas withdrawal. The performance of a PSA process is determined by three separation results: product purity, product recovery and throughput, all at cyclic steady-state. The product purity is defined as the volume- averaged concentration over the entire product. The throughput is defined as the total feed volume (l(STP)) treated per unit time per unit weight of sorbent (l(STP) h-’ kg-‘). The product recovery is defined as: Percentage CO = (CO from step 4) - (CO used in step 3) recovery (CO in feed) - (CO effluent from step 3) Experimental In order to simplify the experiment in the laboratory, a single-bed apparatus for simulating the multibed process was designed. A schematic diagram of the apparatus is shown in Figure 2. The adsorption column was packed with 20-40 mesh active carbon supported copper, which was made by the method developed in this laboratory. Table 1 shows the sorbent and bed characteristics. Five solenoid valves located at the feed, purge, vent, evacuation Table 1 Characteristics of adsorption column Column height, L 27.7 cm Column inside radius, r 2.632 cm Particle size, dp 0.0675 cm Bulk density, pe 0.5858 g cmm3 Interparticle void fraction S 0.494 Total void fraction 6, 0.693 Heat capacity of particle C, 0.2 cal g-’ K-’ and pressurization line were used alternately to direct the flow into and out of the column. The pressure and tem- perature history for each experiment were recorded. Gas samples were taken from the sampling ports by syringes and analysed by gas chromatography (GC). A mechanical vacuum pump was used for the evacuation step. The premixed feed mixtures contained 66.4% CO, 25.7% Nz and 7.9% CO,. N,, CO?, CO A B C A typical PSA run proceeded as follows: the bed was cleaned before each run by heating and degassing. Step 1 was initiated by opening the solenoid valve (SV) connected to the pressurization gas. When the bed pressure achieved ordinary pressure, the SV connected to the feed line and vent line were opened simultaneously and step 2 started. Step 3, CO cocurrent purge, was effected by simultaneously closing the feed valve and opening the purge valve. The purge valve and vent valve were closed and the counter- current valve was opened simultaneously to achieve step 4. The flow rate in cycle was adjusted by a line needle valve. Acyclic steady-state was generally reached after live cycles from start-up with a clean bed. After the steady- state was achieved, a complete set of flow rates and product samples were taken. The feed pressure was ordinary pressure and the end pressure of step 4 was 20-30 Torr. Results and discussion Typical results for separation of CO-CO,-N, Figure 1 The proposed PSA process for separation of CO-CO,-N1 Changes of typical outlet gas compositions with time are shown in Figure 3. Only N, came out for the first few minutes, and then CO, came out abruptly. This sudden increase of CO, content, which was several times more than that of the inlet gas, was due to the result of substitute 174 Gas Separation 8 Purification 1991 Vol 5 September 4 3 0” t 2 I 0 PSA process for separating CO from CO-C02-N, gas mixture: Li-qun Zhu et al. 2 4 6 0 IO 12 14 Time (min) Figure 3 Changes of typical outlet gas compositions with time adsorption of CO onto sites first occupied by COz. This CO* breakthrough was followed by a gradual CO break- through within a few minutes. This figure indicated that CO can be separated efficiently from the CO-COz-N, mixture by using this active carbon supported copper. For the PSA cycle, the detailed experimental results and the effluent concentration of a typical run are presented in Table 2 and Figure 4. These are the cyclic steady-state results, which are accomplished approxi- mately after five cycles from start-up. The results showed that a high-purity CO product with a high recovery could be achieved in PSA by using this sorbent. The variation of effluent flow rate and concentration during step 2 and step 3 indicated the displacement of CO, and N, by CO. It was found that step 3 is necessary for a high-purity CO product. Effects of ambient temperature Generally, low temperatures are suitable for adsorption and high temperatures for desorption. In the PSA cycle, Table 2 Steady-state results of typical run of CO-NZ-CO2 separa- tion by PSA Step Time (s) yco (W Ycoz (%) Flow rate (ml) (STP) 60 2.94 2 120 3.70 180 5.32 00.: 0:04 137.8 167.4 177.2 240 9.04 35.54 285.5 300 57.47 9.74 137.8 3 360 55.02 9.59 68.9 420 75.67 20.15 118.1 480 80.68 14.57 147.7 490 97.84 0.0 196.9 510 98.01 0.0 1004.2 4 540 98.71 0.01 354.4 600 98.98 0.86 256.6 720 99.17 0.83 206.7 2 avg. CO cont. 5.91 6.62 Total amount 767.9 3 avg. CO cont. 68.92 13.83 Total amount 472.6 4 avg. CO cont. 98.58 0.12 Total amount 2010.2 Percentage CO recovery 95.46 I l * l < I Adsorption ’ Purge l 4 Evacuation 00.6 d I- c U * l \ 0 0.4 0.2 t b h A 4 . l g , IAA A I 2 3 4 5 6 7 6 9 IO II 12 Time (min) Figure 4 Effluent concentration of gases in steady-state PSA: 0, co; a, co2 Table 3 Effects of ambient temperature 1 Pressure (MPa) 0.13 i Temperature (K) 303.2 CO feed concentration (%) 66.4 CO2 feed concentration (%) 7.9 Cycle time (min) 12 P/F, 0.791 Throughput (I(STP) (h-’ kg-‘))\ 107.59 Product gas CO concentra- tion (%) 97.26 CO1 concentration (%) 1.02 CO recovery (%) 95.53 0.13 0.13 313.2 323.2 66.4 66.4 7.9 7.9 12 12 0.807 0.796 111.46 109.83 98.58 98.82 0.13 0.00 95.46 87.95 the net adsorption capacity of adsorbent can be affected strongly by temperature. It has been found that the effect of temperature on isotherms of CO, CO, and N2 on active carbon supported copper is contrary12 to the norm. So, increasing temperature appropriately, the selectivity of sorbent can be improved. The results in Table 3 show that CO product purity can obviously be improved by raising ambient temperature, while the net adsorption capacity of sorbent is kept about constant. However, if the ambient temperature is raised sufficiently, the separation results become poor. Effects of cycle time Table 4 shows the effects of cycle time when the amount of feed gas treated remains constant. For this condition, a shorter cycle time corresponds to a higher throughput and faster flow rate, which can improve the separation. How- ever, for CO-C02-N2 mixture, the separation results strongly depend on the regeneration of sorbent. A shorter cycle time is not suitable for CO-C02-N, separation Table 4 Effects of cycle time Pressure (MPa) 0.13 0.13 Temperature (K) 313.2 313.2 CO feed concentration (%) 66.4 66.4 CO2 feed concentration (%) 7.9 7.9 Cycle time (min) 9 12 P/F, 0.785 0.758 Throughput (I(STP) (h-’ kg-‘)) 143.95 111.46 Product CO concentration gas (%) 98.58 98.58 CO2 concentration (%) 0.23 0.13 CO (%) recovery 85.81 95.46 Gas Separation Et Purification 1991 Vol 5 September 175 PSA process for separating CO from CO-C02-N2 gas mixture: ii-gun Zhu et al. Table 5 Effects of bed pressurization by N, against pressurization by feed Bed pressurization By feed 8~ Nz Pressure (MPa) 0.13 0.13 Temperature (K) 313.2 313.2 CO feed concentration (%) 66.4 66.4 CO* feed concentration (%) 7.9 7.9 Cycle time (min) 12 12 P/F,, 0.959 0.907 Throughput (I(STP) (h-’ kg-‘)) 122.23 1 12.73 Product CO concentration gas (%) 99.38 98.98 COz concentration (%) 0.12 0.00 CO recovery (%) 58.81 96.96 because it can make the regeneration of sorbent become poor, which is controlled by the evacuation capacity of the vacuum pump. Effects of bed pressurization by N2 against pressurization by feed A comparison of pressurization by N2 and by feed is given in Table 5, where the other conditions have been kept the same. The data shown indicates that bed pressurization by N2 is more suitable for CO separation from CO-CO*-N, mixture. The main function of using N, in pressurization is to sharpen the concentration wavefront in the bed. If pressurization by feed gas was used, the concentration wavefront was diffuse after the pressurization step. It caused CO breakthrough earlier and decreased CO recovery. Conclusion Separation and recovery of CO from a CO-CO*-N, gas mixture can be achieved by PSA with active carbon supported copper as sorbent. This adsorbent was made by the method developed in our laboratory. The proposed four-step PSA cycle can separate a CO-C02-N, mixture into high-purity CO with high recovery. The CO product purity strongly depends on the regeneration of sorbent and on the CO cocurrent purge step after adsorption in the cycle. References 1 2 3 4 5 6 7 8 9 10 11 12 Kohl, A.L. and Riesenfeld, F.C. GasPun~carion 3rd Edn. Gulf Publishing Co., Houston, USA (1979) Deringer, H. Chimia (1947) 1 125 Haase, D.J. Chem Eng News (1975) (August 4) 52 Hirai, H., Wada, K. and Komiyama, M. Bull Chem Sot Jpn (1986) 59 109-117 Hirai, H., Wada, K. and Komiyama, M. 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