Abstract:
Ceramic materials that are highly resistant to strong acids such as concentrated sulfuric acid and halides such as hydrogen iodide are employed to make block elements through which a large number of circular ingress channels extend in perpendicular directions and which are joined and piled in the heat exchanging medium section to provide a compact heat exchanger that excels not only in corrosion resistance but also in high-temperature strength.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     This invention relates to heat exchangers that have the heat exchanging section composed of ceramic blocks and which are applicable to wide areas including the atomic industry, aerospace, industries in general, and consumers use.  
         [0002]     No corrosion-resistant materials have heretofore been available that enable concentrated sulfuric acid solutions to be vaporized and hydrogen iodide solutions to be vaporized and decomposed under high-temperature (&gt;1000° C.) and high-pressure (&gt;6 MPa) conditions; heat exchangers for such purposes have also been unavailable. To date, several ceramics manufacturers have made attempts to fabricate heat exchangers for high-temperature operation by using ceramic blocks but all failed to make large enough equipment on account of inadequacy in the strength of the blocks.  
       SUMMARY OF THE INVENTION  
       [0003]     An object, therefore, of the present invention is to provide a heat exchanger that withstands heat exchange in large capacities ranging from several tens to a hundred megawatts in high-temperature (&gt;1000° C.) and high-pressure (&gt;6 MPa) environments of strong acids and halides in a solution as well as a gaseous phase and which yet can be fabricated in a compact configuration.  
         [0004]     According to the present invention, ceramic materials that are highly resistant to strong acids such as concentrated sulfuric acid and halides such as hydrogen iodide are employed to make block elements through which a large number of circular ingress channels extend in perpendicular directions; by joining such block elements and piling them in the heat exchanging medium section, the invention provides a compact heat exchanger that excels not only in corrosion resistance but also in high-temperature strength.  
         [0005]     The compact heat exchanger of the invention which withstands high temperature (−1000° C.) and high pressure as well as exhibiting high corrosion resistance can also be used as an intermediate heat exchanger in hot gas furnaces. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0006]      FIG. 1  shows the concept of a nuclear thermochemical IS plant;  
         [0007]      FIG. 2  shows the design concept of a concentrated sulfuric acid vaporizer in actual operation;  
         [0008]      FIG. 3  shows the shapes of ceramic blocks and experimentally fabricated ceramic pillars;  
         [0009]      FIG. 4  shows a method of fabricating a ceramic pillar;  
         [0010]      FIG. 5  shows individual ceramic blocks which are joined in a plurality of pillars and then bundled together to form a heat exchanging section;  
         [0011]      FIG. 6  shows how ceramic pillars are eventually bundled together and how they are combined with section plates and partition plates to establish helium passageways;  
         [0012]      FIG. 7  shows how section plates and partition plates are assembled;  
         [0013]      FIG. 8  shows ceramic flow rate regulating plates as attached to the top and bottom of the fabricated heat exchanging section;  
         [0014]      FIG. 9  shows reinforcing rings as subsequently attached to the fabricated heat exchanging section;  
         [0015]      FIG. 10  shows the heat exchanging section as it is tightened by means of tie rods;  
         [0016]      FIG. 11  shows the installation of inner tubes;  
         [0017]      FIG. 12  shows how a pressure vessel for accommodating the heat exchanging section is assembled;  
         [0018]      FIG. 13  shows how the heat exchanging section is installed within the pressure vessel;  
         [0019]      FIG. 14  shows earthquake-resistant structures as they are fitted between the pressure vessel and the heat exchanging section;  
         [0020]      FIG. 15  shows how a top reflector and helium inlet bellows are attached;  
         [0021]      FIG. 16  shows a top cover as it is fitted on the pressure vessel;  
         [0022]      FIG. 17  shows a mechanical seal as it is fitted on the pressure vessel;  
         [0023]      FIG. 18  shows the autoclave employed in a high-temperature, high-pressure corrosion test; and  
         [0024]      FIG. 19  shows the results of the high-temperature, high-pressure corrosion test conducted on various ceramics and refractory alloys.  
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0025]     The invention provides a heat exchanger essential for realizing commercialization of a nuclear thermochemical IS plant that can produce large quantities of hydrogen and oxygen from the water feed using nuclear heat with 950° C.  FIG. 1  shows the concept of a nuclear thermochemical IS plant. Among the various components shown, those which are operated under the most rigorous conditions are the sulfuric acid vaporizer and the hydrogen iodide decomposer.  
         [0026]      FIG. 1  shows the concept of a nuclear thermochemical IS plant; the reaction involved is such that using the hot thermal energy of 850° C. as supplied from the hot gas furnace, water as the feed is decomposed into hydrogen and oxygen primarily through the combination of a sulfuric acid decomposing and regenerating cycle with a hydrogen iodide decomposing and synthesizing cycle.  
         [0027]     To be more specific, H 2 O as supplied into the Bunsen reactor is decomposed under high-temperature, high-pressure conditions in the presence of both H 2 SO 4  and HI. After the reaction, the liquid portion containing H 2 SO 4  and HI is supplied into the acid separator where it is separated into two layers of H 2 SO 4  and HI. The HI containing solution passes through the purifier to be supplied into the distillation column; the resulting HI vapor is decomposed in the HI decomposer and the product H 2  is recovered from the condenser. The distillation residue in the distillation column and the condensate in the condenser are returned to the reactor.  
         [0028]     The H 2 SO 4  containing solution coming from the acid separator passes through the purifier to be supplied into the concentrator and the concentrated H 2 SO 4  solution is subjected to vaporization in the H 2 SO 4  vaporizer; the resulting vapor is fed into the H 2 SO 4  decomposer, where it is decomposed into S02, H 2 O and O 2 , which then pass through the condenser to return to the Bunsen reactor.  
         [0029]      FIG. 2  shows the design concept of a concentrated sulfuric acid vaporizer in actual operation. A concentrated sulfuric acid solution is supplied from the furnace bottom of the vaporizer toward the upper arm, whereas helium gas with 689° C. is introduced laterally through the upper arm of the vaporizer; the two feeds are respectively guided to the perpendicular channels through each of the ceramic blocks in the vaporizer, where they undergo heat exchange until the concentrated sulfuric acid is completely gasified.  
         [0030]      FIG. 3  shows the shapes of ceramic blocks and experimentally fabricated ceramic pillars. Individual blocks are piled up along the four sides of the cross-shaped perforated section plate provided through the center of the sulfuric acid vaporizer shown in  FIG. 2  and they are held in position as the sulfuric acid feed is flowed upward through six or nine channels (holes) opened in two sides of each block. The hot helium gas feed is flowed laterally through four channels (holes) opened in a side of each block, whereby the sulfuric acid is heated via each block. The two groups of channels are formed in the block in such a way that they do not communicate with each other.  
         [0031]      FIG. 4  shows a method of fabricating a ceramic pillar by stacking a plurality of ceramic blocks. As shown, a sufficient number of blocks to form a pillar are vacuum sealed into a metal vacuum chamber and heated from the outside, so that the blocks are joined one on top of another by means of brazing sheets to form a single pillar.  
         [0032]      FIG. 5  shows individual ceramic blocks which are joined in a plurality of pillars and then bundled together to form a heat exchanging section.  
         [0033]      FIG. 6  shows how ceramic pillars are eventually bundled together and how they are combined with section plates and partition plates to establish helium passageways.  
         [0034]      FIG. 7  shows how section plates and partition plates are assembled, with four ceramic blocks being inserted and fixed in the center between adjacent partition plates.  
         [0035]      FIG. 8  shows ceramic flow rate regulating plates as attached to the top and bottom of the fabricated heat exchanging section and  FIG. 9  shows reinforcing rings as subsequently attached to the fabricated heat exchanging section.  
         [0036]      FIG. 10  shows the individual constituent elements of the heat exchanging section as they are tightened by means of tie rods.  
         [0037]      FIG. 11  shows the installation of inner tubes on side walls of the heat exchanging section that has been tightened by the tie rods.  
         [0038]      FIG. 12  shows that a pressure vessel for accommodating the heat exchanging section is assembled as shown.  
         [0039]      FIG. 13  shows how the heat exchanging section is installed within the pressure vessel after it has been assembled as shown in  FIG. 12 .  
         [0040]      FIG. 14  shows earthquake-resistant structures as they are fitted between the pressure vessel and the heat exchanging section.  
         [0041]      FIG. 15  shows how a top reflector and helium inlet bellows are attached to the heat exchanging section as it has been mounted in the pressure vessel with the aid of the earthquake-resistant structures.  
         [0042]      FIGS. 16 and 17  shows a top cover and a mechanical seal, respectively, as they are fitted on the pressure vessel to complete a heat exchanger for sulfuric acid.  
       EXAMPLE  
       [0000]     (A) Design Concept of a Ceramic Compact Concentrated Sulfuric Acid Vaporizer and Experimental Fabrication of Individual Elements  
         [0043]     Table 1 shows the design specifications of a concentrated sulfuric acid vaporizer for use in a nuclear thermochemical IS plant in actual operation that can be connected to a hot gas furnace of 200 MW.  FIG. 2  shows the design concept of the concentrated sulfuric acid vaporizer.  
                                                                                             TABLE 1                       Specifications of Sulfuric Acid Vaporizer in       Actual Operation                                Hydrogen production rate   25,514 N 3 /h       Heat load on vaporizer   63 MV            Heating helium gas   In/out temperature   689° C./486° C.           Flow rate   1.2 × 10 8  Nm 3 /h       Process   In/out temperature   455° C./486° C.       Inlet   H 2 O/(L/G)   363/816 kmol/h           H 2 SO 4  (L/G)   1552/408 kmol/h           Total   3139 kmol/h       Outlet   H 2 O/(L/G)   0/1178 kmol/h           H 2 SO 4  (L/G)   0/1949 kmol/h           Total   70,045 Nm 3 /h            Heat exchange   Δt1   203° C.   Δt2   31° C.   LMTD   92° C.                Heat transfer coefficient           400 kcal/m 2  ° C. (as assumed)            Pressure   Helium inlet/H 2 SO 4  inlet   3 MPa/2 MPa                  
 
 [How to Assemble the Concentrated Sulfuric Vaporizer]
 
         [0044]     (i) Fabricate a plurality of ceramic blocks (see  FIG. 3 ) in each of which helium channels cross concentrated sulfuric acid solution channels at right angles.  
         [0045]     (ii) Fabricate a ceramic block pillar as shown in  FIG. 4  by vacuum sealing into a metallic vacuum chamber a sufficient number of ceramic blocks to form a pillar and heating the blocks from the outside.  
         [0046]     (iii) Join individual ceramic blocks in a plurality of pillars and bundle them together as shown in  FIG. 5  to form a heat exchanging section.  
         [0047]     (iv) Eventually bundle ceramic pillars together and combine them with section plates and partition plates to establish helium passageways as shown in  FIG. 6 .  
         [0048]     (v) Attach the ceramic heat exchanging section to the assembled section plates and partition plates as shown in  FIG. 7 .  
         [0049]     (vi) Attach ceramic flow rate regulating plates to the top and bottom of the fabricated heat exchanging section as shown in  FIG. 8 ; subsequently attach reinforcing rings to the fabricated heat exchanging section as shown in  FIG. 9 .  
         [0050]     (vii) Tighten the heat exchanging section by means of tie rods as shown in  FIG. 10 .  
         [0051]     (viii) Install inner tubes as shown in  FIG. 11 .  
         [0052]     (ix) In a separate step, assemble a pressure vessel for accommodating the heat exchanging section as shown in  FIG. 12 .  
         [0053]     (x) Install the heat exchanging section within the pressure vessel as shown in  FIG. 13 .  
         [0054]     (xi) Further, fit earthquake-resistant structures between the pressure vessel and the heat exchanging section as shown in  FIG. 14 .  
         [0055]     (xii) Attach a top reflector and helium inlet bellows as shown in  FIG. 15 .  
         [0056]     (xiii) In the last step, fit a top cover and a mechanical seal on the pressure vessel as shown in  FIGS. 16 and 17 , respectively.  
         [0000]     (B) Concentrated Sulfuric Acid Corrosion Test  
         [0057]     The various ceramics and refractory alloys shown in Table 2 were filled into glass ampules together with concentrated sulfuric acid and subjected to a high-temperature, high-pressure corrosion test in an autoclave (see  FIG. 18 ) under high-temperature (460° C.) high-pressure (2 MPa) conditions for 100 and 1000 hours. Test results are shown in Tables 3 and 4 and in  FIG. 19 . The results for the 1000-h test are summarized in Table 5. Silicon carbide and silicon nitride were found to have satisfactory corrosion resistance.  
                                         TABLE 2                           Test Sections for High-Pressure Boiling H 2 SO 4  Corrosion Test (×100 h and 1000 h)            Description   Ampule No.   Designation   Symbol   Classification   Remarks                100 h test   1   SiC   SiC-1   ceramic   atmospheric pressure sintering of 97 wt %                           SiC, 1 wt % B and 2 wt % C           2   Si—SiC   Si—SiC—N-1       atmospheric pressure sintering of 80 wt %                           SiC and 20 wt % Si (as silicon impregnated)           3   Si 3 N 4     Si 3 N 4 -1       atmospheric pressure sintering of 1 wt %                           SrO, 4 wt % MgO and 5 wt % CeO 2             4   Sx   SX-2   H 2 SO 4  resistant steel   preliminarily oxidized at 800° C. × 90 h           5   FeSi   FS-1   high-Si ferrous alloy   14.8 Si—Fe           6       FS-2       19.7 Si—Fe       1000 h test   1   SX   SX-2/half   H 2 SO 4  resistant steel   oxidized with the atmosphere at 800° C. × 90 h                           in half size           2       SX-2/small       oxidized with the atmosphere at 800° C. × 90 h                           in small size           3   SX   SX-4/RT-1   H 2 SO 4  resistant steel   oxidized with nitric acid in small size                   SX-4/70.1       oxidized with nitric acid in small size           4   SiC   SiC   ceramic           5   Si—SiC   Si—SiC—N-3   Si-impregnated silicon                       carbide ceramic           6   Si 3 N 4     Si 3 N 4     ceramic           7   FeSi   FS-2/untreated   high-Si ferrous alloy   19.7 Si—Fe                   FS-2/stress       19.7 Si—Fe, vacuum annealed at 1100° C. × 100 h                   relieved                  
 
         [0058]    
       
         
               
             
               
               
               
               
             
               
               
               
               
               
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
               
               
               
               
               
             
           
               
                 TABLE 3 
               
             
             
               
                   
               
               
                   
               
               
                 Results of Size Measurements in High-Pressure Boiling H 2 SO 4  Corrosion Test (×100 h) 
               
             
          
           
               
                   
                 Length (mm) 
                 Width (mm) 
                 Thickness (mm) 
               
             
          
           
               
                 Ampule 
                   
                   
                 Before 
                 After 
                 Change 
                 Before 
                 After 
                 Change 
                 Before 
                 After 
                 Change 
               
               
                 No. 
                 Designation 
                 Symbol 
                 test 
                 test 
                 (%) 
                 test 
                 test 
                 (%) 
                 test 
                 test 
                 (%) 
               
               
                   
               
             
          
           
               
                 1 
                 SX-2 
                 SX-2/half 
                 26.824 
                 26.71 
                 −0.42% 
                 3.949 
                 3.944 
                 −0.13% 
                 1.516 
                 1.358 
                 −10.42% 
               
               
                 2 
                   
                 SX-2/small 
                 1.798 
                 1.789 
                 −0.50% 
                 3.988 
                 4.1 
                 2.81% 
                 1.545 
                 1.589 
                 2.85% 
               
               
                 3 
                 SX-4 
                 SX-4/RT-1 
                 15.493 
                 15.453 
                 −0.26% 
                 3.943 
                 3.878 
                 −1.65% 
                 1.635 
                 1.624 
                 −0.67% 
               
               
                   
                   
                 SX-4/70.1 
                 15.071 
                 15.063 
                 −0.05% 
                 3.937 
                 3.903 
                 −0.86% 
                 1.627 
                 1.744 
                 7.19% 
               
               
                 4 
                 SiC 
                 SiC 
                 39.727 
                 39.71 
                 −0.04% 
                 4.035 
                 4.034 
                 −0.02% 
                 2.993 
                 2.991 
                 −0.07% 
               
               
                 5 
                 Si—SiC 
                 Si—SiC 
                 40.029 
                 40.04 
                 0.03% 
                 4.061 
                 4.06 
                 −0.02% 
                 3.077 
                 3.080 
                 0.10% 
               
               
                 6 
                 Si 3 N 4   
                 Si 3 N 4   
                 39.826 
                 39.8 
                 −0.07% 
                 4.065 
                 4.068 
                 0.07% 
                 3.013 
                 3.021 
                 0.27% 
               
               
                 7 
                 FeSi 
                 FS-2/untreated 
                 19.083 
                 19.101 
                 0.09% 
                 3.638 
                 3.7 
                 1.70% 
                 3.595 
                 3.638 
                 1.20% 
               
               
                   
                   
                 FS-2/stress 
                 19.585 
                 20.055 
                 2.40% 
                 5.700 
                 3.705 
                 −35.00% 
                 5.557 
                 3.578 
                 −35.61% 
               
               
                   
                   
                 relieved 
               
               
                   
               
             
          
         
       
     
         [0059]    
       
         
               
             
               
               
               
               
               
               
             
               
               
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
               
               
               
             
           
               
                 TABLE 4 
               
             
             
               
                   
               
               
                   
               
               
                 Results of Weight Measurements and Corrosion Rate 
               
               
                 in High-Pressure Boiling H 2 SO 4  Corrosion Test (×100 h) 
               
             
          
           
               
                   
                 Weight (g) 
                   
                   
                 Corrosion 
                   
               
             
          
           
               
                 Ampule 
                   
                   
                 Before 
                 After 
                 Weight change 
                 Area 
                 rate 
                   
               
             
          
           
               
                 No. 
                 Designation 
                 Symbol 
                 test 
                 test 
                 (%) 
                 (mg) 
                 (cm 2 ) 
                 (g/m 2  h) 
                 Remarks 
               
               
                   
               
             
          
           
               
                 1 
                 SX-2 
                 SX-2/half 
                 1.2162 
                 0.9816 
                 19.29% 
                 −234.6 
                 0.03052 
                 0.961 
                 Ampule broke in 800 h 
               
               
                 2 
                   
                 SX-2/small 
                 0.0772 
                 0.0656 
                 15.03% 
                 −11.6 
                 0.00322 
                 0.360 
               
               
                 3 
                 SX-4 
                 SX-4/RT-1 
                 0.7570 
                 0.6738 
                 10.99% 
                 −83.2 
                 0.01857 
                 1.244 
                 Ampule broke in 360 h 
               
               
                   
                   
                 SX-4/70.1 
                 0.7967 
                 0.7198 
                 9.65% 
                 −76.9 
                 0.01805 
                 1.183 
                 Ampule broke in 360 h 
               
               
                 4 
                 SiC 
                 SiC 
                 1.4476 
                 1.4487 
                 −0.08% 
                 1.1 
                 0.05826 
                 −0.002 
               
               
                 5 
                 Si—SiC 
                 Si—SiC 
                 1.4823 
                 1.4856 
                 −0.22% 
                 3.3 
                 0.05964 
                 −0.006 
               
               
                 6 
                 Si 3 N 4   
                 Si 3 N 4   
                 1.5611 
                 1.5653 
                 −0.27% 
                 4.2 
                 0.05883 
                 −0.007 
               
               
                 7 
                 FeSi 
                 FS-2/untreated 
                 1.6720 
                 1.6330 
                 2.33% 
                 −39.0 
                 0.03022 
                 0.129 
               
               
                   
                   
                 FS-2/stress 
                 1.7425 
                 1.7097 
                 1.88% 
                 −32.8 
                 0.05043 
                 0.065 
               
               
                   
                   
                 relieved 
               
               
                   
               
             
          
         
       
     
         [0060]    
       
         
               
             
               
               
               
               
               
               
               
               
             
           
               
                 TABLE 5 
               
             
             
               
                   
               
               
                   
               
               
                 Summary of 1000 h Test 
               
             
          
           
               
                   
                   
                   
                   
                   
                 Cross section 
                   
                   
               
               
                   
                   
                 Dimensional 
                 Corrosion 
                   
                 observed at 
                   
                 Overall 
               
               
                 Designation 
                 Symbol 
                 change 
                 rate 
                 Appearance 
                 magnification 
                 Other 
                 rating 
               
               
                   
               
               
                 SX-2 
                 SX-2/half 
                 X 
                 X 
                 ⊚ 
                 ⊚ 
                 — 
                 X 
               
               
                   
                 SX-2/small 
                 ◯ 
                 Δ 
                 ⊚ 
                 ⊚ 
                 — 
                 Δ 
               
               
                 SX-4 
                 SX-4/RT-1 
                 Δ 
                 X 
                 ⊚ 
                 ⊚ 
                 — 
                 X 
               
               
                   
                 SX-4/70.1 
                 Δ 
                 X 
                 ⊚ 
                 ⊚ 
                 — 
                 X 
               
               
                 SiC 
                 SiC 
                 ⊚ 
                 ⊚ 
                 ⊚ 
                 ⊚ 
                 ◯ 
                 ◯ 
               
               
                 Si—SiC 
                 Si—SiC 
                 ⊚ 
                 ⊚ 
                 ⊚ 
                 ⊚ 
                 ◯ 
                 ◯ 
               
               
                 Si 3 N 4   
                 Si 3 N 4   
                 ⊚ 
                 ⊚ 
                 ⊚ 
                 ⊚ 
                 ◯ 
                 ◯ 
               
               
                 FeSi 
                 FS-2/untreated 
                 ⊚ 
                 Δ 
                 X 
                 X 
                 — 
                 X 
               
               
                   
                 FS-2/stress relieved 
                 X 
                 Δ 
                 X 
                 X 
                 — 
                 X