Patent Publication Number: US-2006002845-A1

Title: Sulfuric acid process

Description:
REFERENCE TO RELATED APPLICATIONS  
      This application claims priority of U.S. Provisional Patent Application No. 60/584,100 filed Jun. 30, 2004 under 35 U.S.C. §119. 
    
    
     FIELD OF THE INVENTION  
      This invention describes improvements that can be incorporated into sulfuric acid based processes that use high temperature heat for all or a portion of the energy needs for a hydrogen production facility. These improvements include the use of a directly heated sulfuric acid decomposition reactor or the use of very high pressures in this reactor to reduce the need for cooling the process stream in order to capture most of the SO 2 . These improvements dramatically increase the process energy efficiency of the sulfuric acid processes, avoid many of the materials engineering issues, and reduce the capital costs associated with the design and construction of the processes.  
     BACKGROUND OF THE INVENTION  
      Sulfur cycles are a group of thermochemical processes that can make hydrogen, mainly using high temperature thermal energy from a high temperature heat source. Two sulfur cycles are the so called “Westinghouse Sulfur Process” and the “General Atomic Iodine/Sulfur Process”. The Westinghouse Sulfur Process (“WSP”), is described in  Proceedings of ICONE  12, “Optimization of the Westinghouse Sulfur Process for Hydrogen Generation and Interface with an HTGR” by E. J. Lahoda et al.  12   th  Intl. Conference on Nuclear Engineering, Arlington, Va., dated Apr. 25-29, 2004, pp 1-3; and in  AICHE Reports  “Interfacing the Westinghouse Sulfur Cycle with the PBMR for the Production of Hydrogen” by R. Matzie et al., New Orleans, dated Feb. 27, 2004 pp 1-10. The Iodine/Sulfur (“S/I”) Process is described in  Proceedings of ICONE- 11, “Nuclear Energy in Non-Electric Power Applications” by E. J. Lahoda et al.,  11   th  International Conference on Nuclear Engineering, Tokyo, Japan, Apr. 20-23, 2003, pp 1-9. Both processes are compared in  American Nuclear Society  Global Paper #88017, “Improvements in the Westinghouse Process for Hydrogen Production” by J. E. Goossen et al., American Nuclear Society Annual Winter Meeting, New Orleans, La. Nov. 2003, pp 1-5. At 1000° C. only the WSP system reaches 50% efficiency. Both cycles require temperatures in excess of 760° C. to have at least 40% efficiency.  
      The high temperature heat sources are any that produce heat available for use above 760° C., such as an HTGR (High Temperature Gas Cooled Reactor), a high temperature solar concentrator, a natural gas fired combustor or any combination of these heat sources. The portion of the process where sulfuric acid is decomposed into sulfur dioxide (SO 2 ), water vapor and oxygen typically takes place at these high temperatures. The first issue with these cycles is to have an efficient method for capturing the SO 2 . This is due to the relatively low solubility of the SO 2  in water. The Westinghouse Sulfur Process (“WSP”) generates hydrogen using high temperature process heat and electricity. The energy to drive the WSP as well as other sulfur cycle based processes such as the Sulfur-Iodine process is pulled from the power generation loop of a HTGR such as a Pebble Bed Modular Reactor (“PBMR”).  
      The Westinghouse Sulfur Process produces hydrogen in a low-temperature electrochemical step, in which sulfuric acid and hydrogen are produced from sulfurous acid. This reaction can be run at between 0.17 and 0.6 volts with a current density of 200 ma/sq.cm at about 60° C. The second step in the cycle is the high temperature decomposition of sulfuric acid at 760° C. or above. Previous work by Westinghouse has identified catalysts and process designs to carry out this reaction in concert with an HTGR such as the PBMR. The final step in this process is absorption of the SO 2  in water at room temperature to form sulfurous acid and a SO 2  free stream of O 2 .  
      This is a well known process which is hereby defined as “standard WSP”= 
      (1) H 2 SO 4 ⇄SO 2 +H 2 O+O.5O 2  (&gt;760° C. heat required)];  
      (2) SO 2 +2H 2 O+0.50 2 ⇄H 2 SO 3 +H 2 O+O.5O 2  (T&lt;100° C.); and  
      (3) H 2 O+H 2 SO 3 →H 2 +H 2 SO 4  (electrolyzer at about 100° C. or less).  
      The Iodine/Sulfur Process also starts with a reversible reaction where sulfuric acid is decomposed at over 760° C. to form sulfur dioxide as above, followed by reaction of the sulfur dioxide with Iodine to form HI.  
      This is a well known process which is hereby defined as “standard S/I”= 
      (1) H 2 SO 4 ⇄SO 2 +H 2 O+O.5O 2  (greater than 760° C. heat required)  
      (2) I 2 +SO 2 +2H 2 O+O.5O 2 +excess H 2 O⇄2HI+H 2 SO 4 +O.5O 2 +excess H 2 O (about 100° C. to 200° C. heat generated); and  
      (3) 2HI⇄H 2 +I 2  (greater than 400° C. heat required)  
      The common step is: 
 
H 2 SO 4 ⇄SO 2 +H 2 O+O.5O 2 
 
      What is needed is an improvement to the WSP and S/I systems to improve efficiency and solve materials corrosion issues as well as reduce capital costs. It is a main object of this invention to provide such an efficient, corrosion reduced, cost effective system.  
     SUMMARY OF THE INVENTION  
      The above needs are met and issues solved by providing a method of operating the standard Westinghouse Sulfur Process (standard WSP) or the standard Iodine Sulfur Process (standard S/I) at a pressure greater than 1100 psi (7.58 MPa) to allow recovery of SO 2  from a H 2 SO 4  decomposition step at temperatures above 4.4° C. with lower H 2 O to SO 2  ratios. Preferably the pressure will be greater than 1200 psi (8.27 MPa) and most efficiently greater than 1450 psi (10.0 MPa) up to 1700 psi (11.7 MPa). Preferably the SO 2  will be recoverable at from 20° C. to 75° C. The use of the above pressures will allow the SO 2  absorption reaction: SO 2 +H 2 O⇄H 2 SO 3  (&gt;120° C.) for both standard WSC and standard S/I systems to run without refrigeration. This also allows both systems as noted above to reduce the number of moles H 2 O required in the reactions by up to 50%, preferably by 15% to 50%: 
 
WSP: H 2 O+SO 2  →H 2 SO 3  
 
SI: I 2 +SO 2 +H 2 O⇄2HI+SO 3 
 
      In these methods, a plurality of direct contact reactors can be used for the decomposition of sulfuric acid or SO 3 , where the use of a plurality of direct contact reactors allows the use of ceramic materials as heat transfer media and/or catalyst supports in the reactors. Also, a plurality of direct contact reactors can be operated in alternating sequence in conjunction with a nuclear reactor using He as a coolant, and He or a molten salt can be used as a heat transfer medium between a high temperature heat source such as a high temperature reactor and a decomposition reactor.  
      In these methods, zeolite or other absorbent beds can be used to remove sulfur compounds, radioactive materials or other transfer compounds or decomposition products from intermediate heat transfer loops or from the gas stream, back to the high temperature heat source. These zeolite or other absorbent beds provide a thermal capacitance and a means of leveling out temperature variation due to process upsets in either the high or low temperature processes. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
      A full understanding of the invention can be appreciated from the following detailed description of the invention when read with reference to the accompanying drawings wherein:  
       FIG. 1  is a schematic diagram of one embodiment of the so called Westinghouse Sulfur Process Cycle (“WSP”);  
       FIG. 2  is a schematic diagram of the Sakuri embodiment of the so called Iodine/Sulfur Process (“S/I”);  
       FIG. 3  is a graph of the change in efficiency vs. wt % sulfuric acid in a WSP system operating at 1,450 psi vs. 1,000 psi;  
       FIG. 4 , which best describes the invention, is a block diagram of a directly heated reactor system; and  
       FIG. 5  is a diagram of an indirectly heated reactor.  
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT  
      Referring to  FIG. 1 , a standard WSP process  2  is shown operating at less than 900 psi. In  FIG. 2 , the standard S/I process  4  is shown operating at less than 900 psi. In  FIG. 1 , thermal energy  10  at about 760° C. to 1000° C. is passed into oxygen generator  12  to provide the reaction shown, passing H 2 O, SO 2  and O 2  to an oxygen recovery unit  14  where H 2 O and SO 2  are passed to an electrolyzer  16  energized with D.C. electricity  18  to provide H 2  shown as  6  and H 2 SO 4 , where the latter is vaporized in vaporizer  20  by thermal energy  22  to feed vaporized H 2 SO 4  to the oxygen generator/sulfuric acid decomposition reactor  12 , as shown.  
      As shown in  FIG. 2 , a Sakuri 2000 process schematic of the reactions for S/I vs. temperature, sulfuric acid  26  is vaporized in vaporizer  28  and then passed to decomposition reactor  30  at about 760° C. to 810° C. to generate O 2  and pass O 2 , H 2 O and SO 2  to iodine reactor  32  which generates HI which is decomposed in second decomposition reactor  34  to provide H 2  shown as  6  after passing through the excess I 2  separator  36 . The decomposition reactor  34  provides the main source of I 2 , shown as  38  for the iodine reactor  32 , also known as a Bunsen reactor.  
      By operating the entire WSP or SI cycle at a high pressure of roughly 1450 psi (10.0 MPa-mega pascals), we have found that SO 2  can be removed from the system at temperatures above 20° C., preferably at 20° C. to 75° C., without the use of energetically inefficient refrigeration systems or excess water. In addition, operation of the cycle at a higher pressure would allow for the removal of SO 2  in one consolidated step, due to higher removal efficiency of the SO 2  by water. High system pressure operation has other advantages. The use of high pressure can also increase overall process efficiency by allowing for the direct gas phase conversion of sulfurous acid (H 2 SO 3 ) to sulfur trioxide and hydrogen in the electrolyzer, the hydrogen generation portion of the Westinghouse Sulfur Process (“WSP”). Consequently, for every mole of H 2  produced, only one mole of H 2 O and one mole of SO 2  is required.  
      The hydrogen generation reaction under this scheme is: H 2 O+SO 2 →H 2 SO 3 →H 2 +SO 3 . If a lower pressure is used (&lt;1000 psi), more water must be used to separate out the SO 2  from the O 2 . This results in a lower concentration of sulfurous acid (H 2 SO 3 ). During the vaporization step, additional energy must be used to vaporize the excess water. The hydrogen generation reaction at lower pressures is: 2H 2 O+SO 2 →H 2 +H 2 SO 4 .  
      Thus the steps of WSP previously defined in the background are changed to:  
      (1′) SO 3 ⇄SO 2 +O.5O 2  (greater than 1100 psi and &gt;760° C. heat required)  
      (2′) H 2 O+SO 2 +O.5O 2 ⇄H 2 SO 3 +O.5O 2    
      (3′) H 2 SO 3 ⇄H 2 +SO 3    
      (electrolyzer at 100° C. and 1450 psi).  
      In the S/I process, the overall reactions are not changed, but the resulting sulfuric acid ( 26  in  FIG. 2 ) is a higher concentration and as in the case of the WSP, the energy required for vaporization is reduced, providing:  
      (1′) SO 3 ⇄SO 2 +O.5O 2    
      (2′) I 2 +SO 2 +H 2 O⇄2HI+SO 3    
      (3′) 2HI⇄H 2 +I 2    
      In effect, operating the entire system at high pressure allows sulfurous acid to be converted in the electrolyzer to hydrogen and sulfur trioxide, which in turn increases overall efficiency by reducing the water requirement of the entire cycle up to 50%, leading to reduced energy requirements for the vaporizers of both the WSP and S/I processes. An added benefit of high system pressure is a compressed hydrogen product and a compressed oxygen product that do not require further compression.  
      In order to test the increase in the efficiency of these processes, chemical process models of the WSP were executed at a variety of pressures. At a pressure of 1,000 psi (6.9 MPa), the required temperature to dissolve sulfur dioxide into the water feed stream while achieving a low residual SO 2  level in the O 2  product was found to be roughly 40° F. (4.4° C.). Cooling the water inlet stream and the oxygen generation products (H 2 O, SO 2  and O 2 ) to 40° F. (4.4° C.) requires the use of inefficient refrigeration equipment. For this analysis, it was assumed that the cooling efficiency of the refrigeration equipment was 50%. An electrical to thermal energy conversion rate of 42% was used to convert electrical energy into reactor thermal energy. These conversion percentages mean that for every unit of electrical energy used in the cooling of these streams, the thermal equivalent is roughly five times higher. It was found that by operating the system at 1,450 psi (10.0 MPa), the SO 2  dissolved in water at a temperature of 70° F. (21.1° C.), requiring 30° F. less cooling. The result is that the thermal efficiency of the WSP is increased by 15-20%.  
      The overall thermal efficiency (calculated using the lower heating value of H 2 ) of the WSP as a function of sulfuric acid weight percentage is shown in  FIG. 3 . The use of higher pressures results in dramatic improvement in efficiency, line  42  vs. low pressure results, line  44 . In addition to higher process efficiencies, higher operation pressures are desirable to avoid the need for compression of the hydrogen product which incurs a large energy penalty on the system. There is also an added benefit of minimizing process equipment size and capital cost. As modeled, the removal of sulfur dioxide from the oxygen can be accomplished in a single unit operation. Similar results would occur for the SI system.  
      Another major issue regarding these cycles is the materials of construction and the operation of the high temperature decomposition reactor, in which the reaction of the H 2 SO 4 ⇄SO 2 +H 2 O+O.5O 2  takes place. Corrosion is an issue due to the high temperature (&gt;760° C.) and the chemically aggressive character of the components (H 2 SO 4 , SO 2 , SO 3 , H 2 O and O 2 ). Since this vessel will operate at high temperature and pressure, a process approach that would utilize relatively low cost materials construction while still being corrosion resistant and have good heat transfer capabilities with low maintenance requirements is required for the economic feasibility of this family of processes.  
      In order to solve the sulfuric acid decomposition reactor issues, the use of a plurality of directly heated, direct contact sulfuric acid decomposition reactors  46 ,  48  as shown in  FIG. 4  is proposed. In this approach, a hot heat transfer medium  50  (for instance helium) from a nuclear reactor  52 , or intermediate heat exchanger that is heated by a reactor or other energy source is sent through one reactor to heat a bed  54  of alumina or zirconia or other types of material suitable to this environment. The bed has a catalytic surface that increases the rate of sulfuric acid decomposition reaction. Sulfuric acid is received from electrolyzer  16  and vaporizer  20 . Once the bed in the first reactor  46  has reached the appropriate temperature (for instance 760° C. to 925° C.), the hot heat transfer fluid  50  is diverted to the second, now cold, reactor  48  to begin heating it. In the meantime, sulfuric acid vapor is then sent through the first (now hot) reactor  46  where the decomposition reaction takes place and gradually cools the reactor due to the endothermicity of the decomposition process.  
      Once the first reactor  46  has cooled to below a minimum operating temperature and the second reactor  48  has reached the desired temperature, hot heat transfer  50  fluid is diverted back to the first reactor  46  that has now cooled. The sulfuric acid flow is then diverted from the first reactor  46  to the second, now hot, reactor  48 . Of course, more than two reactors can be used so as to optimize the cycle time. In addition, the cycling can be timed such that some initial heat transfer medium flow is first put through the cold reactor and then through a zeolite bed to remove any residual sulfuric acid vapor before the full heat transfer medium flow is re-initiated.  
      A circulator  56  is also shown as well as oxygen recovery unit  14  and various valves  66 . The application of such technology is also applicable to the other “sulfur family” of hydrogen generating thermochemical cycles like the S/I system. Variations of their features and attached processes may occur depending on specific design requirements and adjacent processes.  
      The use of a plurality of directly heated reactors allows a much closer approach of the reactor bed to the temperature of the hot heat transfer medium. This higher temperature in turn increases the conversion of the sulfuric acid vapors. These higher conversion rates reduce the total flow rate in the process and the attendant parasitic loses for cooling, reheating and pumping. Other benefits of using the directly heated reactor approach of  FIG. 4  are the ability to use much lower grade materials.  
      While both the intermediate heat exchanger (shown in  FIG. 5 ) and the directly heated reactor designs (shown in  FIG. 4 ) can use low cost carbon steel or stainless steel outer vessels  62 , lined with ceramics or other suitable materials; the tubes  72  that contain the catalyst  60  and are the heat transfer surfaces in the indirect heat exchanger design, will have to be of very expensive alloy (if one can be identified) in order to withstand the temperature and pressure while providing corrosion resistance.  
      The directly heated reactor shown in  FIG. 4  can use non-structural catalyst support material  60  such as alumina, zirconia, or other appropriate materials with or without a catalytic surface as the heat transfer media. Suitable seals  64  that maintain the boundary between the sulfuric acid vapor and the hot helium must also be identified for intermediate heat exchangers.  
      A final consideration is the efficient transfer of heat across the tubes  72  and into the catalyst bed on the inside of the tubes for the intermediate heat exchanger  70 . Since the decomposition reaction is very endothermic, this may be a significant design issue that may require the use of extremely high surface area to volume ratios for the heat transfer area. Again, this will increase both the capital cost and the likelihood that significant maintenance costs will be required during operation for intermediate heat exchangers  70 , an example of which is shown in  FIG. 5 . The complete separation of the heat transfer medium and sulfur process streams may also reduce regulatory issues due to leakage of sulfuric acid into the reactor or intermediate heat exchanger circuit if an HTGR is used as a heat source.  
      The large thermal mass of the reactor beds in the direct contact reactors of  FIG. 4  will minimize the effect of process upsets. This concept will add operating stability to the system to allow either the heat source or the hydrogen process to coast through an instability caused by the other process. Elimination of material expansion/contraction issues in the tubesheet/tube interface of the intermediate heat exchanger of  FIG. 5  and eliminate the attendant sealing issues. Finally, a lower pressure drop due to the ability to use larger catalytic materials for the bed  54  of the reactor(s)  46 ,  48  of the directly heated reactors will result.  
      Obstacles do exist in the use of the directly heated reactors preferred in this invention and shown in  FIG. 4 . As mentioned above, the use of an auxiliary process to maintain a clean heat transfer medium will likely be required to eliminate the potential for corrosion issues in the intermediate heat exchanger or reactor and to eliminate the production of activated species of sulfur.  
      Some direct advantages of the above highly pressurized, directly heated reactor system of this invention ( FIG. 4 ) include: 
      The use of higher system pressure allows for the consolidation of the SO 2  recovery process to a single unit operation.     The use of higher system pressures allows for gas phase conversion of sulfurous acid (H 2 SO 3 ) to SO 3  and H 2  in the electrolyzer, thereby reducing the power needs of the electrolyzer by minimizing the use of water.     The use of higher pressure system allows for increased efficiency due to the high temperature decomposition of SO 3  rather than more complex H 2 SO 4 . Again, this is due to the reduced water requirement of the system.     A plurality of direct contact reactors for sulfuric acid and SO 3  decomposition can be used in hydrogen generating sulfur cycles such as the Westinghouse Sulfur Process and the Sulfur Iodine Process; two or more reactors in alternating sequence as direct contact reactors can be used.     Ceramic materials as the heat transfer media and or catalyst support can be used in the direct contact reactors instead of expensive materials for the heat transfer surfaces.     Inexpensive ceramic may be used instead of expensive, pressure bearing ceramic and or metal as the boundary between the hot and cold portions of the decomposition reactor.     No seals that are difficult to fabricate and maintain are needed between the cold and hot portion of the decomposition reactor.     No seals are required between the hot, clean gas from the reactor and the decomposing SO 3  and H 2 SO 4 .     Thermal capacitance is supplied to minimize the effects of process variations either in the chemical or nuclear processes.    

      Inexpensive auxiliary processes to clean up residual contamination in the heat transfer medium can be used to mitigate any SO 2 ; SO 3  or S species carryover to the clean hot gas system. The advantages of this approach include: 
      A thermal capacitance is further added to the clean gas stream to help level out variations in the cold chemical or hot gas supply processes.     Besides trapping sulfur based compounds, these beds will insure minimal radioactive contamination of the chemical process stream, during equipment failure or accidental in the nuclear heat generation process    

      Having described the presently preferred embodiments, it is to be understood that the invention may be otherwise embodied within the scope of the appended claims.