Abstract:
A method of desorbing a volatile component from a spent adsorbent is disclosed, which includes flowing a stream of supercritical carbon dioxide through a rotating packed bed of the spent adsorbent; and collecting an effluent from the rotating packed bed, which contains supercritical carbon dioxide and the volatile component entrained therein. In one embodiment of the present invention a spent activated carbon from a process for recovering 2,2,3,3-tetrafluro-1-propanol from a gas stream containing 2,2,3,3-tetrafluro-1-propanol is regenerated by flowing a stream of supercritical carbon dioxide through a rotating packed bed of the spent activated carbon.

Description:
FIELD OF THE INVENTION 
     The present invention relates generally to a method of desorbing a volatile component from a spent adsorbent with rotating packed bed, also to a method of recovering 2,2,3,3-tetrafluro-1-propanol from a gas stream by adsorption. 
     BACKGROUND OF THE INVENTION 
     2,2,3,3-tetrafluro-1-propanol (abbreviated as TFP hereinafter) has been used as the solvent in the DVD production process because of its high solubility towards cyanine dyes and its high volatility. TFP is expensive and causes a seriously environmental problem after its emission, it therefore is essential to recover it from the exhausted gases. 
     There are some existing techniques to remove volatile organic compounds from gases such as condensation, adsorption, catalytic oxidation, and thermal oxidation [1]. But to deal with a ppm concentration level, adsorption onto porous adsorbents is believed to be the most appropriate technique. One of the keys for a successful adsorption process is the adsorbent that generally possesses porous structure and a high adsorptive capacity. Among the adsorbents, activated carbon is the most commonly used due to its high surface area and low cost. Because a huge gas flow rate is needed to treat for the recovery of TFP with a ppm concentration in a DVD plant, the adsorption by activated carbon is therefore chosen in the present invention. 
     When the adsorption approaches saturation, desorption is needed for the subsequent use. The desorption of activated carbon loaded with different adsorbates including benzene, toluene, ethyl acetate, etc, by supercritical carbon dioxide has been proved to be an effective means. The advantages of using supercritical CO 2  as the desorbent over the conventional steam desorption technique include a better subsequent adsorption efficiency due to the same temperature in adsorption and desorption, an absence of the condensed water in activated carbon pores, and a safer operation due to its low desorption temperature. Another important aspect to use supercritical fluid desorption for the recovery of TFP is that there is in general no steam in the display and semiconductor industries. 
     Because the gas containing TFP is exhausted from a clean room, the adsorption of TFP and desorption of activated carbon should be completed in a limited space. Recently, the high-gravity operation has been proved to be more efficient over the packed-bed tower in different unit operations resulting from a significant increase in mass transfer rate [T. Kelleher and J. R. Fair, “Distillation Studies in a High-Gravity Contactor,” Ind. Eng. Chem. Res., 35, pp. 4646-4655, 1996; M. Keyvani and N. C. Gardner, “Operating Characteristics of Rotating Beds,” Chem. Eng. Prog., 85, pp 48-52, 1989; C. C. Lin and H. S. Liu, “Adsorption in Centrifugal Fields Basic Dye Adsorption by Activated Carbon,” Ind. Eng. Chem. Res., 39, pp. 161-167, 2000; and C. C. Lin; W. T. Liu and C. S. Tan, “Removal of Carbon Dioxide by Absorption in a Rotating Packed Bed,” Ind. Eng. Chem. Res., 42, pp. 2381-2386, 2003]. High gravity (Higee) first proposed by Ranshaw and Mallinson [C. Ramshaw and R. H. Mallinson, Mass Transfer Process, U.S. Pat. No. 4,283,255, 1981] is an operation in a rotating packed bed (abbreviated as RPB hereinafter) in which the centrifugal force replaces the gravitational force. Due to the presence of a centrifugal force, 20 to 100 gravities depending on rotating speed and radius of the rotating bed can be generated. As a result, the equipment size can be significantly reduced. This would definitely be beneficial for the recovery of TFP in a plant with limited space. 
     SUMMARY OF THE INVENTION 
     A primary objective of the present invention is to provide a novel method of desorbing a spent adsorbent. The method involves subjecting a supercritical carbon dioxide in contact with a rotating packed bed of the spent adsorbent. 
     Another objective of this invention is to recover TFP from a gas stream with a high flow rate and a ppm level of TFP. The adsorption by activated carbon is used to achieve this purpose. The activated carbon particles are packed in a rotating bed though the adsorption is carried out without rotating the bed. The desorption is carried out in a RPB using supercritical carbon dioxide as the desorbent. The synergistic effect of supercritical fluid extraction and centrifugal force on desorption will be examined. In the desorption operation, the effects of pressure, temperature, supercritical CO 2  flow rate, and rotating speed on the desorption efficiency are studied. 
     The present invention provides a method of desorbing a volatile component from a spent adsorbent, wherein the volatile component is adsorbed on the spent adsorbent, the method comprising flowing a stream of supercritical carbon dioxide through a rotating packed bed of the spent adsorbent; and collecting an effluent from the rotating packed bed, which contains supercritical carbon dioxide and the volatile component entrained therein. 
     Preferably, the adsorbent is activated carbon, alumina, silica or zeolite. More preferably, the adsorbent is activated carbon. 
     Preferably, the volatile component is a hydrocarbon, an alcohol, an ether, an ester, a halogen-substituted hydrocarbon, a halogen-substituted alcohol, a halogen-substituted ether, a halogen-substituted ester or a mixture thereof. More preferably, the volatile component is a halogen-substituted alcohol. In one of the preferred embodiments, the volatile component is 2,2,3,3-tetrafluro-1-propanol. 
     Preferably, the desorption method of the present invention further comprises expanding the effluent at a reduced pressure to form a gaseous stream of carbon dioxide with the volatile component entrained therein; and passing the gaseous stream through a cold trap, so that the volatile component is collected in the cold trap and a clean carbon dioxide gaseous stream is discharged from the cold trap. 
     Preferably, the rotating packed bed of the spent adsorbent is mounted rotatably on an axis in a housing; the stream of supercritical carbon dioxide is fed into an axial area of the rotating packed bed so as to enable the stream of supercritical carbon dioxide to flow radially through the rotating packed bed; and the effluent is discharged from the housing. More preferably, the housing is at a pressure of 7.2-20 MPa, and a temperature of 298-353 K. 
     Preferably, the rotating packed bed has a speed of 500-2000 rpm. 
     The present invention also provides a method of recovering 2,2,3,3-tetrafluro-1-propanol from a gas stream, comprising the following steps: 
     a) flowing a gas stream containing 2,2,3,3-tetrafluro-1-propanol through a packed bed of activated carbon to adsorb 2,2,3,3-tetrafluro-1-propanol onto the activated carbon; 
     b) flowing a stream of supercritical carbon dioxide through a rotating packed bed of the spent activated carbon resulting from step a) so as to desorb 2,2,3,3-tetrafluro-1-propanol from the spent activated carbon; 
     c) collecting an effluent from the rotating packed bed, which contains supercritical carbon dioxide and 2,2,3,3-tetrafluro-1-propanol entrained therein; 
     d) expanding the effluent at a reduced pressure to form a gaseous stream of carbon dioxide with 2,2,3,3-tetrafluro-1-propanol entrained therein; and 
     e) passing the gaseous stream through a cold trap, so that 2,2,3,3-tetrafluro-1-propanol is collected in the cold trap and a clean carbon dioxide gaseous stream is discharged from the cold trap. 
     Preferably, the rotating packed bed of the spent activated carbon in step b) is mounted rotatably on an axis in a housing; the stream of supercritical carbon dioxide is fed into an axial area of the rotating packed bed so as to enable the stream of supercritical carbon dioxide to flow radially through the rotating packed bed; and the effluent in step c) is discharged from the housing. More preferably, the packed bed of the activated carbon in step a) is mounted rotatably on the axis in the housing but is not rotating; the gas stream containing 2,2,3,3-tetrafluro-1-propanol is fed into an axial area of the packed bed so as to enable the gas stream containing 2,2,3,3-tetrafluro-1-propanol to flow radially through the packed bed; and the resulting gas stream containing a reduced amount of 2,2,3,3-tetrafluro-1-propanol is discharged from the housing. Preferably, the housing during step b) is at a pressure of 7.2-20 MPa, and a temperature of 298-353 K. 
     Preferably, the packed bed of the activated carbon in step a) and the rotating packed bed of the activated carbon in step b) are kept at substantially the same temperature. 
     Preferably, the rotating packed bed in step b) has a speed of 500-2000 rpm. 
     Preferably, the cold trap in step e) comprises a liquid methanol. 
     Preferably, the method of recovering 2,2,3,3-tetrafluro-1-propanol further comprises repeating steps a) to e), wherein the desorbed activated carbon resulting from step b) is used as the activated carbon in the preceding step a), and the clean carbon dioxide gaseous stream from step e) is used as a source of the stream of supercritical carbon dioxide in step b). 
     The features and the advantages of the method of the present invention will be more readily understood upon a thoughtful deliberation of the following detailed description of the nonrestrictive embodiments with reference to the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows a schematic diagram of an experimental rotating packed bed system suitable for adsorption and desorption according to the present invention. 
         FIG. 2  is a plot showing an adsorption breakthrough curve using activated carbon as the adsorbent for recovering 2,2,3,3-tetrafluro-1-propanol from a gas stream according to the present invention. 
         FIG. 3  is a plot showing the effect of rotating speed on desorption at a temperature of 315 K, a pressure of 11.72 MPa, and a CO 2  flow rate of 300 cm 3   N /min for desorbing 2,2,3,3-tetrafluro-1-propanol from the spent activated carbon according to the present invention. 
         FIG. 4  is a plot showing the effect of pressure on desorption at a temperature of 315 K, a rotating speed of 1000 rpm, and a CO 2  flow rate of 200 cm 3   N /min for desorbing 2,2,3,3-tetrafluro-1-propanol from the spent activated carbon according to the present invention. 
         FIG. 5  is a plot showing the influence of flow rate on desorption at a temperature of 315 K, a pressure of 11.72 MPa, and a rotating speed of 1000 rpm for desorbing 2,2,3,3-tetrafluro-1-propanol from the spent activated carbon according to the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     2,2,3,3-Tetrafluro-1-propanol (TFP) is effectively recovered from a gas stream by adsorption using activated carbon as the adsorbent at 315 K. The desorption of the loaded activated carbon for the subsequent adsorption use can be successfully achieved by supercritical CO 2  in a rotating packed bed. A synergistic effect of supercritical CO 2  extraction and centrifugal force on desorption is observed. The higher pressure and rotating speed are, the higher desorption efficiency occurs. Due to a compacted volume of a rotating packed bed and a significant enhancement in desorption efficiency at high centrifugal forces, a rotating packed bed substituted for a fixed packed bed is therefore suggested for the regeneration of the activated carbon loaded with TFP. 
     The experimental apparatus for the adsorption and desorption is illustrated in  FIG. 1 , and the parts and elements used therein are described as follows with designated numerals: 
     
       
         
               
               
             
           
               
                   
                   
               
             
             
               
                   
                  1. CO 2  Cylinder 
               
               
                   
                  2. N 2  Cylinder 
               
               
                   
                  3. Filter 
               
               
                   
                  4. Pump 
               
               
                   
                  5. Surge Tank 
               
               
                   
                  6. Regulator 
               
               
                   
                  7. Flow Controller 
               
               
                   
                  8. Constant Temperature Oven 
               
               
                   
                  9. Saturator 
               
               
                   
                 10. Ball Valve 
               
               
                   
                 11. Needle Valve 
               
               
                   
                 12-1, 12-2. Three Way Valve 
               
               
                   
                 13. Metering Valve 
               
               
                   
                 14. Rotary Autoclave 
               
               
                   
                 15. Cold Trap 
               
               
                   
                 16. Wet Gas Meter 
               
               
                   
                 17. Pressure Indicator 
               
               
                   
                 18-1, 18-2, 18-3. Thermal Couple 
               
               
                   
                 19. Heating Tape 
               
               
                   
                 20. Packed bed of activated carbon 
               
               
                   
                 21. Housing 
               
               
                   
                   
               
             
          
         
       
     
     The virgin activated carbon particles (Calgon, WSIV) with an average pore size of 3.81 nm and a BET surface area of 932 m 2 /g were grounded and screened to obtain the particles with an average diameter of 0.1 cm. The activated carbon particles were boiled in deionized water to remove fines and dried in an oven at 393 K. The treated activated carbon particles mixed with the same size of quartz were then packed in a rotating bed  20  with a diameter of 3.2 cm and a height of 3 cm, which was mounted rotatably on an axis in a housing  21  of the rotary autoclave  14 . 
     In adsorption, 160 cm 3  of TFP with a purity of 99.7% (Seedchem) was loaded into a saturator  9  first. The content of TFP in nitrogen (San Fu Gas Company, Taiwan, 99.99%) was prepared by passing the nitrogen stream from the N 2  cylinder  2  with a flow rate of 36 cm 3 /min through the saturator  9 . The gas bypassed the packed bed  20  first until the concentration of TFP became steady, and then it flew into the packed bed  20  by switching the three-way valve  12 - 1 . About 1.0 g of the activated carbon particles was packed in the bed. The adsorption was performed at a temperature of 315 K. Because the rotating speed was set as zero, the operation can be regarded as a radial flow in a fixed packed bed  20 . TFP in the effluent stream was condensed and collected in a cold trap  15 . The collected TFP was analyzed with a flame ionization detector (abbreviated as FID hereinafter) gas chromatographer (China Chromatography, 8900). From the measured concentration of TFP, a breakthrough curve was obtained. 
     After a certain period of operation, the adsorption was stopped and the subsequent desorption was performed. In the supercritical carbon dioxide desorption stage, carbon dioxide with a purity of 99.5% (San Fu Gas Company, Taiwan) from the CO 2  cylinder  1  was compressed by a pump  4  (Milton Roy, NSI-33R) first and then was delivered to and stored in a surge tank  5 . The CO 2  stream with the desired flow rate and temperature then flew into the rotating packed bed  20  with a preset rotating speed. The pressure was controlled within 1.0% of the desired value. The effluent supercritical CO 2  stream was expanded via a metering valve  13 . The total volume of the expanded CO 2    22  was measured by a wet gas meter  16  (Shinagawa Co., W-NK-1B) as is used as a source of the stream of supercritical carbon dioxide in step b). TFP in the expanded CO 2  was collected in a cold trap  15  in which a 50 mL of methanol was present. A sample in the cold trap was taken out frequently and sent to a FID gas chromatographer for analysis. 
     The adsorption breakthrough curve at a temperature of 315 K, a gas flow rate of 36 cm 3 /min, and an inlet concentration of TFP of 78 ppm is shown in  FIG. 2 . The breakthrough curve in each adsorption and desorption cycle was observed to reproduce with an average deviation of less than 2.0%. In the adsorption, the rotating speed was set as zero. The gas flow was in radial position in the packed bed. It is seen from  FIG. 2  that the chosen activated carbon is effective for the removal of TFP, the adsorption capacity of the activated carbon at 200 min is found to be about (0.3 g of TFP)/(g of activated carbon). The equilibrium capacity of the activated carbon towards TFP is definitely higher than that, as seen from  FIG. 2 . The accumulated amounts of TFP in the cold trap in both the adsorption and desorption stages were compared with the inlet amount of TFP in the adsorption stage, the difference was found to be always less than 5%, indicating a satisfactory mass balance in each cycle. 
       FIG. 3  shows the desorption efficiency at different rotating speeds for a temperature of 315 K, a pressure of 11.72 MPa, and a supercritical CO 2  flow rate of 300 cm 3   N /min, wherein “N” in the flow rate unit denotes volume at standard conditions. When the rotating speed is zero, the desorption occurs simply by the extraction of TFP from the loaded activated carbon by supercritical CO 2 . As the rotating speed is increased, it can be seen from  FIG. 3  that the desorption efficiency in a RPB is much enhanced over that in a conventional packed bed, indicating that the centrifugal force plays a positive role in desorption. When the rotating speed is higher than 1400 rpm, a complete desorption can be achieved within 30 min, while only 58% of desorption is achieved in a conventional packed bed under supercritical operation. The enhancement is mainly attributed to a decrease in the interphase mass transfer resistance and possibly to a decrease in the intraparticle diffusion resistance at very high rotating speed. 
     When the desorption temperature, rotating speed, and CO 2  flow rate were maintained at 315 K, 1000 rpm, and 200 cm 3   N /min, respectively,  FIG. 4  shows an increase in desorption efficiency with an increase in pressure. It is known that the solubility of a substance in a supercritical fluid is generally increased with increasing pressure. As a result, more adsorbed TFP onto activated carbon can be extracted by supercritical CO 2  at higher pressures, leading to an increase of desorption efficiency as indicated in  FIG. 4 . 
     Since interphase mass transfer coefficient is a hydrodynamic property, desorption efficiency is therefore expected to be affected by the flow rate of supercritical CO 2 . It can also be seen from  FIGS. 3 and 4  that CO 2  flow rate plays an important role. At a temperature of 315 K, a pressure of 11.72 MPa, and a rotating speed of 1000 rpm, a large discrepancy in desorption efficiency for different CO 2  flow rates is observed. A complete desorption can be achieved within 40 min for a CO 2  flow rate of 300 cm 3   N /min, but less than 80% of desorption is achieved for a CO 2  flow rate of 200 cm 3   N /min.  FIG. 5  shows the dependence of desorption efficiency on CO 2  flow rate. A higher CO 2  flow rate obviously leads to a higher desorption efficiency. However, the improvement becomes less for a CO 2  flow rate higher than 300 cm 3   N /min at the given operating conditions. When the other activated carbon and operating conditions were used for the desorption purpose, the same trend was observed. For a use of 0.5 g of the Merck activated carbon (No. 2514), the desorption rate was found to be doubled within a 10 min operation if the CO 2  flow rate was increased from 200 to 1000 cm 3   N /min at a temperature of 315 K, a pressure of 11.72 MPa, and a rotating speed of 730 rpm. 
     The temperature effect on the desorption efficiency were also examined by the inventors. The results show that the optimal desorption temperature depends on the desorption pressure, which falls within 305-315 K for the desorption pressure of 8.96-15.86 MPa. 
     The recovery of TFP from a gas stream containing ppm level of TFP is found to be achieved by the adsorption with an radial flow in a packed bed. For reuse of the activated carbon, the regeneration of the TFP-loaded activated carbon is successfully done by using supercritical CO 2  as the desorbent in a rotating packed bed. Because a synergistic effect of supercritical CO 2  extraction and centrifugal force on desorption is observed, the proposed regeneration technique is superior to the supercritical CO 2  regeneration technique resulting from the presence of the additional centrifugal forces. The higher pressure and rotating speed are observed to result in the higher desorption efficiency. Due to a compacted volume of a rotating packed bed and a significant enhancement in desorption efficiency at high centrifugal forces, a rotating packed bed substitute for a fixed packed bed is therefore suggested.