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Ind. Eng. Chem. Res. 2003, 42, 6166-6170
SEPARATIONS Activated-Carbon-Supported NaOH for Removal of HCl from Reformer Process Streams Maw-Tien Lee,† Zhen-Qin Wang,‡ and Jen-Ray Chang*,‡ Department of Chemical Engineering, National Chung Cheng University, Chia-Yi, Taiwan, Republic of China, and Department of Applied Chemistry, National Chiayi University, Chia-Yi, Taiwan, Republic of China
An environmentally friendly method was developed to remove HCl from industrial waste gases, specifically the gas produced from a catalytic reforming process. The method uses activatedcarbon-supported NaOH (NaOH/C) to treat the HCl-containing gas. A continuous-upflow fixedbed reactor was used to test the performance of the prepared NaOH/C. Because chlorides contained in the off-gas stream of a reformer are normally less than 100 ppm, the cycle length of the NaOH/C will be expected to reach 1 year, and an accelerated aging test with a HCl concentration of 223 000 ppm was developed to shorten the test time. The accelerated aging test was confirmed to be reliable because the HCl treatment capacity of the NaOH/C for the accelerated aging test is almost the same as (within 15% deviation) that for standard tests with a gas stream containing 1000 ppm HCl. Effects of the water content on the performance of the NaOH/C were investigated by the accelerated aging test, and the results indicated that the HCl treatment capacity of the NaOH/C increases with increasing water content of the NaOH/C up to 27% and then slightly decreases. NaCl formed by the neutralization of HCl in the gas with NaOH on NaOH/C was removed by flowing low-pressure steam, and the activity of NaOH/C was rejuvenated by resoaking the activated carbon with a NaOH solution. Thus, the activated carbon of NaOH/C can be reused repeatedly. In addition, the HCl treatment capacity of the NaOH/C is about 6 times that of commercial alumina, normally used in the reformer. The developed method lends itself to industrial application. Introduction The reforming process, one of the most important processes in the petroleum industry, is used to convert paraffins and naphthenes into a high-octane-number blending component for gasoline.1-3 Monometallic Pt on alumina catalysts and bimetallic Pt-Re or Pt-Ir on alumina catalysts are normally used in this process. The bimetallic catalyst is more stable than the Pt catalyst, allowing operation at lower pressure, which thermodynamically favors high-octane-number products; the bimetallic catalysts are preferred in commercial naphtha reforming today.2,3 Platinum is thought to serve as a catalytic site for hydrogenation and dehydrogenation reactions. The alumina support has to be chlorinated to provide acid sites for isomerization, cyclization, and hydrocracking reactions. Some refiners add chlorine compounds such as hydrogen chloride or dichloroethane continuously to the process to maintain the chlorine level on the catalyst, thereby maintaining the acidity of the γ-Al2O3 support.2,3 The presence of chlorine is beneficial for the reforming process, whereas chlorine in the reforming process also causes many problems. Salt such as NH4Cl formed from chloride plugs up the downstream unit, resulting in an increase of the pressure drop.4,5 † ‡
National Chiayi University. National Chung Cheng University.
Wet scrubbing has been widely used to remove acid sulfur and halide compounds from vapors. Wet scrubbing is effective and inexpensive, but utilization of the process has to suffer from scaling inside the tower, equipment plugging, and corrosion.6 Moreover, the wet scrubbing process removes HCl in the recycle gas of the reformer concomitantly with the addition of water to the gas stream that reduces the catalyst activity of the reforming process and causes corrosion in downstream units. A fixed bed of γ-Al2O3 is generally used in the refinery to remove HCl from the gas stream. The main disadvantages of the process are the side reactions involved during HCl removal and the problem of solid waste disposal. During the removal of HCl, aluminum chloride, which is formed by the reaction of γ-Al2O3 with HCl, catalytically converts olefins into a much higher molecular weight hydrocarbon. Because of this, costly steam stripping of the deposited hydrocarbon on the γ-Al2O3 adsorbent is necessary before dumping the used adsorbent into a landfill. Recently, some inventions related to the removal of HCl patented by Yan disclosed that HCl can be removed from a dry gas stream by contact with nonporous solid caustic particles.7 The caustic particles can be filled in the reactor as a fixed bed, moving bed, or fluidized bed. Among them, the fixed bed is the most preferred one. Nonporous caustic is relatively higher in mechanical strength than porous material; hence, it is less likely
10.1021/ie0207055 CCC: $25.00 © 2003 American Chemical Society Published on Web 10/29/2003
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to crumble or collapse. However, the reaction bed should have a large void volume to lessen the building up of the pressure drop caused by the accumulation of salt crystal formed during sour gas treatment. Therefore, solid caustic should mix with activated carbon, porous resin, and the like inert support to increase the void volume. Hunson et al.,8 Jonas,9 Tsutsui and Tanada,10 Brown,11 Turk et al.,12 and Nevskaia et al.13 showed that the impregnation of activated carbon with an acid or base solution will improve the adsorption properties of the activated carbon. Such a modified product is called impregnated activated carbon (IAC). Similar to porous alumina, activated carbon of a high surface area benefits the removal of HCl from the gas stream. However, unlike alumina, which is not inherently inert, activated carbon is relatively inactive, and the formation of a high molecular weight hydrocarbon can be minimized. Because of these two merits, NaOH IAC was developed to remove HCl from the gas stream. An accelerated adsorbent aging test was developed to investigate the effects of preparation conditions on absorbent performances. Because the HCl removal capacity for the accelerated adsorbent aging test is the same as that for the normal performance test, the accelerated aging test can provide valuable time-effective information for process development in a short time. To reduce the solid waste and thus to alleviate waste management problems, thereby minimizing the cost of the HCl removal process, a regeneration method has also been developed. In the process, low-pressure steam is used to wash the NaCl formed on the activated carbon.
Experimental Section Material and Adsorbent Preparation. The carbonsupported alkali adsorbents were prepared by soaking activated carbon in a NaOH solution. The activated carbon (GAC 830, in granular form with a surface area of 1050 m2/g, a pore volume of 0.85 mL/g, an iodine no. of 75 mg/g minimum, and an apparent density of 0.54 g/mL) was purchased from Norit Americas Inc., Atlanta, GA. The activated carbon was first dried at 120 °C to remove physically adsorbed water. The dried activated carbons were brought into contact with an equal volume of a NaOH solution with a concentration ranging from 0 to 12 N (equiv/L) and a soaking time ranging from 0 to 240 min, followed by drying at 80 °C. The resulting material was noted as NaOH/C. The water content on the NaOH/C was controlled by the drying time. The water evaporated during drying was collected in a liquid-nitrogen trap. The NaOH content on the NaOH/C was determined by means of a volumetric procedure employing 6 N HCl as the titrant, and the H2O content was calculated from the weight of dry NaOH/C minus the sum of the weight of activated carbon and NaOH determined by titration. Performance of HCl Removal. The performance tests were carried out in a continuous-upflow fixed-bed reactor. The reactor was a poly(vinylethylene) tube with an inside diameter of 2.5 cm and a length of 40 cm. For an accelerated aging test, the reactor was packed with 10 g of NaOH/C diluted with an inert ceramic in a ratio of 1:2. For a standard test, only 5 g of NaOH/C was packed. The test gas stream was prepared by passing 20 mL/min of dry air through a reservoir filled with an
Figure 1. Weight percent of NaOH deposited on activated carbon vs impregnation time in the preparation of NaOH/C.
aqueous HCl solution to pick up chlorides and moisture. The chloride concentration in the gas could be varied by varying the amount and concentration of acid in the reservoir. Typical gas compositions were 223 000 and 1000 ppm for accelerated and standard tests, respectively. To determine the adsorption capacity (g of HCl adsorbed/g of adsorbent) of NaOH/C, the effluent gas was scrubbed with a NaOH solution and an online pH meter with a precision of 0.01 pH was used to determine the pH value of the scrubbed solution. The deviation of the adsorption capacity caused by the sensitivity of the pH meter is about 0.5% in the case where 99% HCl in the waste gas was treated. UV spectroscopy and electric conductivity titration were used to measure the amount of HCl reacted with NaOH/C. The total amount of HCl removal was estimated from a breakthrough curve and verified by the use of waste NaOH/C characterization. Regeneration of Used NaOH/C. The regeneration procedure consists of (1) washing away the NaCl formed on the adsorbent, the unreacted NaOH, or the HCl adsorbed on activated carbon, (2) drying the activated carbon support, and (3) resoaking the carbon support with a NaOH solution. In the washing step, either water or low-pressure steam can be used; however, if lowpressure steam is used, the top of the reactor is controlled at a temperature lower than 98 °C. After no chloride was detected in the effluent water, the activated carbon support was purged with low-pressure steam (103-105 °C) for about 2 h, followed by upflowing of 12 N NaOH solutions to the reactor for about 2 h for soaking. The NaOH solution soaked carbon was purged with low-pressure steam for about 30 min for drying, followed by the HCl removal cycle. The HCl removal and regeneration cycle was repeated five times to assess the reusability of NaOH/C. The surface area loss of the activated carbon support during the regeneration process was determined with an Omnisop 360 analyzer. Results and Discussion Preparation of NaOH/C. The preparation of the absorbents was conducted by soaking the activated carbons in sodium hydroxide aqueous solutions with various concentrations. To maximize the HCl treatment capacity, the effects of the soaking time and NaOH concentration on the amount of sodium hydroxide adsorbed by the activated carbon support were investigated. As shown in Figure 1, the loading of NaOH on
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Figure 2. Weight percent of NaOH deposited on activated carbon vs NaOH concentration in the preparation of NaOH/C. Figure 4. Effects of the water content on the breakthrough curve of NaOH/C at P ) 1 atm, T ) 28 °C, and HCl concentration ) 223 000 ppm. Water content: (A) 29 wt %; (B) 27 wt %; (C) 25 wt %; (D) 22 wt %; (E) 17 wt %; (F) 9 wt %.
Figure 3. Breakthrough curve for (A) blank test and (B) activated carbon at P ) 1 atm, T ) 28 °C, and HCl concentration ) 223 000 ppm (accelerated aging test).
the activated carbon increases with the soaking time and becomes insignificant after 3 h. On the basis of these results, all the NaOH/C for performance tests was prepared with a soaking time of 4 h. The effects of the NaOH aqueous concentration on the NaOH loading were studied by varying the NaOH concentration. Because the amount of NaOH loading increases with the NaOH concentration (Figure 2), a 12 N NaOH aqueous solution was used to prepare NaOH/C in order to maximize the NaOH loading. Mechanism of NaOH/C for HCl Removal and Accelerated Aging Test. (a) Chemical Principle: Neutralization of HCl with NaOH. Prior to the study of water effects, a blank test was conducted with an empty reactor column. The experimental results (Figure 3) showed that the materials of the experimental apparatus would not adsorb hydrogen chloride gas, suggesting that the performance test would not be influenced by the adsorption of HCl on the reaction system. Unlike physical adsorption of HCl on activated carbon, we speculated that most of the acid in waste gas was removed by acid/base naturalization. To confirm our speculation, activated carbon and the NaOH/C were tested concomitantly. The test results indicated that the NaOH/C sample could subsist for about 530 min in the accelerated aging test, which is 5 times that of the activated carbon (Figures 3 and 4). Moreover, stoichiometric calculation indicated that essentially all the NaOH was reacted with HCl. X-ray diffraction (XRD) spectroscopy characterizing the species on the used NaOH/C further confirmed the formation of NaCl
Figure 5. XRD spectra characterizing the species formed on the NaOH/C after the performance test.
(Figure 5). Those results not only proved that NaOH/C is superior to activated carbon in HCl removal but also confirmed our speculation of HCl removal by base neutralization. (b) Performance Test Method: Accelerated Aging Test. For a standard fixed bed or a continuous catalytic reformer, the HCl concentration in the gas stream is less than 100 ppm. If HCl removal is mostly due to acid/base neutralization and all the NaOH on NaOH/C is used for the neutralization, the life cycle of the prepared NaOH/C will be more than 1 year. Because the life tests are the most crucial and time-consuming in assessing the performance of NaOH/C, an accelerated aging test method should be developed for saving the test time. Intuitively, the time for the life test will be decreased by increasing the HCl concentration in waste gas. Moreover, if the HCl removal process is dominated by a rapid acid/base neutralization reaction, we can predict the life cycle by the accelerated aging test. To justify the accelerated aging test method, two feeds of different HCl concentrations, one of 1000 ppm and the other of 223 000 ppm, were tested for comparison. As shown in Figures 4 and 6, the cycle life for the accelerated aging test is about 530 min, which is equivalent to an adsorption capacity of 0.39 g of HCl adsorbed/g of adsorbent, while that for the standard test is about 36 days, which is equivalent to 0.34 g of HCl adsorbed/g of adsorbent. Based on accelerated aging test results, the estimated life cycle for the standard test was about 41 days. Because the deviation between experimental and
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Figure 6. Breakthrough curve for NaOH/C of 27 wt % water content at P ) 1 atm, T ) 28 °C, and HCl concentration ) 1000 ppm (standard test).
Figure 7. Weight percent of NaCl deposited on activated carbon vs the amount of steam used for regeneration.
calculated results is less than 15%, the accelerated aging test was thought to be reliable and provide time-effective information in developing the HCl removal process. (c) Mechanism of HCl Removal. In the process of HCl removal by NaOH/C, the reactions involving HCl and NaOH are formulated as follows:
HCl(g) + H2O(l) f HCl(aq)
(1)
HCl(aq) + NaOH(aq) f NaCl(aq) + H2O(l) (2) The mechanism by which the HCl is removed from the gas consists of three steps that occur in series: (1) HCl transport from the waste gas stream to the pore mouth of NaOH/C; (2) the HCl molecule diffuses through the water film and dissolves in the aqueous NaOH layer (eq 1); (3) aqueous HCl reacts with NaOH to form NaCl. Because the neutralization reaction is irreversible and spontaneous, in the second step, the mass transfer is greatly enhanced; besides, using an activated carbon support also minimizes the mass-transfer resistance. The activated carbon provides a very high surface area to facilitate dispersion of aqueous NaOH and thus decreases the diffusion path of HCl for reacting with NaOH. Effects of Water on the Performance of NaOH/ C. When HCl diffuses through the water film and dissolves in it (eq 2), the mass-transfer rate of HCl is increased by the naturalization reaction. In the literature, an enhancement factor is used to account for the increase in the mass-transfer rate due to a chemical reaction and the factor normally increases with increasing NaOH concentration.14,15 In addition, the resistance of mass transfer is also affected by the accumulation of NaCl crystal. Operating with too little water, NaCl formed in the reaction would be deposited in the pore of the adsorbents. Pore-mouth plugging caused by NaCl buildup inhibits the diffusion of hydrogen chloride into the inner pore to react with NaOH. The unreacted NaOH not only reduces the HCl treatment capacity but also causes disposal problems of exhausted NaOH/C. Operating with too much water, on the other hand, would decrease both the enhancement factor and the HCl treatment capacity. Moreover, when water is much more than optimal, a brine film will be formed on an activated carbon pellet. After vaporization of water, accumulation of NaCl crystal in the interstitial volume will increase the pressure drop associated with gas flow through the bed.
Figure 8. Breakthrough curve (]) for fresh NaOH/C of 27 wt % water content, ([) after the first regeneration, (O) after the second regeneration, (0) after the third regeneration, (b) and after the fourth regeneration at P ) 1 atm, T ) 28 °C, and HCl concentration ) 223 000 ppm (accelerated aging test).
The effects of the water content of NaOH/C on the performance of HCl removal are shown in Figure 4. The HCl adsorption capacity increases with increasing water content of NaOH/C up to 27% and then decreases. As expected, for the samples with water content less than 22%, the deposited NaOH was not naturalized completely. However, surprisingly, the HCl removal capacity for the sample of 27% water content is higher than that for NaOH naturalization. This excess HCl removal was caused by the adsorption of HCl on an activated carbon support as evidenced by the pH value of 5-6 of the effluent water stream after steam regeneration. Steam Regeneration of Used NaOH/C. The used absorbents can be regenerated with low-pressure steam. As shown in Figure 7, about 1500 g of steam was needed to treat 100 g of absorbents. The salt dissolved in water was characterized by XRD and was identified as sodium chloride. Further performance tests for the steamregenerated NaOH/C are shown in Figure 8. The experimental results indicated that the NaOH/C could be regenerated. After the first regeneration, the HCl removal capacity decreases slightly by about 5%. However, the HCl removal capacity has no significant decrease with subsequent HCl removal and regeneration cycle. The loss of the HCl treatment capacity may be caused by the loss of the surface area during steam regeneration. After five times of regeneration, the NaOH/C was unloaded and the Brunauer-Emmett-
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Taiwan, National Chung Cheng University, and Refining & Manufacturing Research Center of Chinese Petroleum Corp. (RMRC) is acknowledged. Literature Cited
Figure 9. Breakthrough curve for ([) NaOH/C of 27 wt % water content and (0) commercial alumina at P ) 1 atm, T ) 28 °C, and HCl concentration ) 223 000 ppm (accelerated aging test).
Teller surface area measurement indicated that the surface area of the activated carbon support is about 95% of that of the fresh activated carbon. These results suggest that the NaOH/C is reusable and no further waste treatment is necessary after the regeneration. Moreover, as shown in Figure 9, the HCl removal capacity of NaOH/C is about 5 times that of alumina. Because the experimental results prove that NaOH/C is effective with a high HCl removal capacity and the activated carbon support can be reused repeatedly, the performance of this material is thought to be much superior to alumina for HCl removal. Conclusions HCl in a reforming process can plug up the downstream unit and cause serious corrosion problems. In this paper, we presented a new method for the removal HCl in a gas stream by using NaOH/C. The experimental results indicated that the NaOH/C has a higher HCl removal capacity than alumina, which is generally used in refinery to remove HCl from the gas stream, and can be regenerated without significant loss of activity. Moreover, because HCl is removed by acid/base neutralization and the NaCl formed on the activated carbon can be removed by a steam wash, no further waste treatment is necessary. Because of these advantages, we believe that NaOH/C is a worthy substitute for alumina. Acknowledgment The support of the National Science Council (Contract No. NSC 88-EPA-Z-194-001), Environmental Protection Administration Government of the Republic of China,
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Received for review September 5, 2002 Revised manuscript received September 5, 2003 Accepted September 5, 2003 IE0207055
