Abstract:
A heat exchanger formed of two plates or stages with one being on top of the other and with a plurality of such heat exchangers stacked on each other. The first stage has an inlet opening, an inlet passage and passages opened to the end of the stage. The second stage has an inlet opening leading to passages which lead to an outlet passage and an outlet opening. The second stage also has an inlet opening leading to passages which have an outlet at the end. A combustible gas enters the passage region of the first stage where the mixture is preheated and then passes to a passage region of the first stage. Portions of the gas passages in the passage region contain a combustion promoting catalyst so that the gas burns in the passage region. The products of combustion leave through the outlet end and are conveyed to an inlet of the passage region of the second stage.

Description:
This invention relates to a method of providing heat to promote an endothermic reaction or be extracted as sensible heat. 
     SUMMARY OF THE INVENTION 
     BACKGROUND OF THE INVENTION 
     Field of the Invention 
     According to the invention there is provided a method of providing heat to promote an endothermic reaction or be extracted as sensible heat, the method comprising providing first passage means having first inlet means thereto and first outlet means therefrom, when considering fluid flow along said first passage means towards the first outlet means from the first inlet means said first passage means having an upstream portion leading towards a downstream portion of said passage means, second passage means having second inlet means thereto and second outlet means therefrom, third passage means having third inlet means thereto and third outlet means therefrom, heat conducting wall means provided between said upstream portion of the first passage means and the second passage means and between said downstream portion of the first passage means and the third passage means, introducing combustible gas and oxidant into the upstream portion of the first passage means wherein said fuel gas and oxidant are heated by heat conducted through said wall means from the second passage means, burning the combustible gas with said oxidant in said downstream portion of the first passage means, supplying the resultant products of combustion to the second inlet means to the second passage means for said products of combustion to flow along the second passage means to said second outlet means wherein the products of combustion give up heat which is conducted through said wall means to said upstream portion of the first passage means to heat further introduced said combustible gas and oxidant, and either (i) introducing one or more fluid substances into the third passage means through the third inlet means to undergo an endothermic chemical reaction using heat conducted through said wall means from said downstream portion of the first passage means such that products of the reaction leave through said third outlet means or (ii) introducing an heatable fluid into the third passage means to receive heat conducted through said wall means from said downstream portion of the first passage means such that the heatable fluid becomes heated and thereafter leaves through the third passage means bearing sensible heat. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will now be further described, by way of example, with reference to the accompanying drawings in which: 
     FIG. 1 is a diagramatic side elevation of an heat exchanger stage comprising two plates, for use in carrying out the method according to the invention, 
     FIG. 2 is a diagramatic elevation from the opposite side of the heat exchanger stage in FIG. 1, 
     FIG. 3 is a diagramatic section on line III—III in FIG. 1 showing one of the plates in section, 
     FIG. 4 is a diagramatic section on line IV—IV in FIG. 1 showing the other plate in section, 
     FIG. 5 shows diagramatically and partly in cross-section, apparatus for carrying out the method according to the invention comprising a plurality of the heat exchangers in FIG. 1, the part section in FIG. 5 being on line V—V in FIG. 6, 
     FIG. 6 is a diagramatic section on line VI—VI in FIG. 5, and 
     FIG. 7 is a diagramatic section on line VII—VII in FIG.  5 . 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     With reference to the accompanying drawings a heat exchanger  2  is formed by a pair of plate-like stages  4  and  6  which may be of rectangular form as shown. Each stage  4  and  6  is initially of open top form and has a respective base  8  or  10 . The stage  4  has a peripheral wall  12  around three of its sides with an inlet opening  14  leading to an inlet passage  16 , in part defined by a wall  18 , leading to a manifold region  20  from which passages  22 , defined at least in part by walls  24 , lead to an open side  26  of stage  4 . A flow of fluid through stage  4  can follow the path(s) indicated by arrows  28  from the inlet opening to an outlet at the open side  26 . For the purpose of the explanation below, the passages in the stage  4  may be considered as being formed by a passage portion  30  and a passage portion  32  each generally demarcated or indicated by phantom lines. With respect to the direction of fluid flow  28 , the passage portion  30  is an upstream portion and the passage portion  32  is a downstream portion. The stage  6  has a peripheral wall  34  around three of the sides of the base  10 , the wall having inlet openings  36  and  38  and an outlet opening  40 . The stage  6  is divided substantially in half by a wall  42  to one side of which is a passage portion  44  demarcated or indicated by phantom lines and to the other side of which is a passage portion  46  demarcated or indicated by further phantom lines. The passage portion  44  comprises manifold regions  48  and  50  between which extend passages  52  defined at least in part by walls  54 , and from the manifold  50  an outlet passage  58  extends to the outlet opening  40 . A flow of fluid through the passage portion  44  can follow the path(s) indicated by arrows  60 . The passage portion  46  comprises an inlet manifold region  62  from which lead passage  64 , defined at least in part by walls  66 , to an open side  68  of the stage  6 . A flow of fluid through the passage portion  46  can follow the path indicated by arrows  70 . 
     The stages  4  and  6  may be formed from any suitable heat conducting material for example metal, which may be stainless steel, or ceramic material. Ceramic material may be moulded to form the stages  4  and  6  as may be metal. Or metal block may be etched, ground or otherwise machined or cut to form the stages  4  and  6 . Or the stages  4  and  6  may be formed from component parts of sheet or strip material adhered together in a substantially fluid light manner, for example by brazing or welding in the case of sheet or strip metal. 
     As indicated in FIGS. 1 and 2, a heat exchanger  2  is formed by mounting a said stage  4  on top of said stage  6  in a fluid tight manner whereby the upper wall of the passages  52 ,  58 ,  64  and regions  48 ,  50  and  62  and regions  48 ,  50  and  62  is formed by the base  8  of the stage  4 . 
     Preferably a plurality of heat exchangers  2  are stacked one on top of another, the uppermost stage  4  in the stack being provided with a suitable cover to form an upper wall of the passages  16 ,  22  and region  20  in that uppermost stage  4 ; each lower stage  4  in the stack being covered by a respective base  10  of an aforesaid stage  6  directly above that stage  4 . 
     A stack of heat exchangers  2  is shown in FIGS. 5 to  7  inside a containment shell or pressure vessel  70  having flat top and bottom walls  74  and  76 , curved side walls  78  and  80 , and hemispherical end walls  82  and  84 . The top wall  74  of the pressure vessel  72  forms the upper wall for the passages and region in the uppermost stage  4 , whilst the base  10  (FIGS. 1 and 4) of the lowermost stage  6  in the stack sits on the bottom wall  76  of the vessel. Fluid tight, spaced partitions  86 ,  88 ,  90  and  92 ,  94 ,  96  extend down both sides of the stack. A region  98  between the partitions  88  and  90  serves as inlet manifold to the inlet openings  14  in the stages  4 ; the inlet manifold  98  having an inlet  100  thereto. A region  102  between partitions  86  and  88  serves as an outlet manifold for the outlet openings  40  in the stages  6 ; the outlet manifold  102  having an outlet  104  therefrom. A region  106  between the partitions  92  and  94  serves as an inlet manifold for the inlet openings  36  in the stages  6 ; the inlet manifold  106  having an inlet  108  thereto. A region  110  between the partitions  94  and  96  serves as an inlet manifold for the inlet openings  38  in the stages  6 ; the inlet manifold  110  having an inlet  112 . Between the partitions  86  and  92  and the end wall  84  of the pressure vessel  70  is a region  114  serving an outlet manifold for the outlet openings at the open sides  26  (FIGS. 1,  2  and  3 ) of the stages  4 ; the outlet manifold  114  having an outlet  116 . Between the partitions  90  and  96  and the end wall  82  of the pressure vessel  70  is a region  118  serving as an outlet manifold for the outlet openings at the open sides  68  (FIGS. 1,  3 ,  4  and  5 ) of the stages  6 ; the outlet manifold having an outlet  120 . 
     A fuel or combustible gas, for example natural gas or methane, from a suitable supply  122  and oxidant, for example air or oxygen, from a suitable supply  123  are fed to the inlet  100  at a temperature below the ignition temperature of the combustible gas/oxidant mixture for the mixture to be heated (in a manner to be described below) in the passage portion  30  of each stage  4 . The passages  16 ,  20 ,  22  in the stages  4  may be of sufficiently small dimensions to prevent propagation of flames. Alternatively or additionally the gas velocity may be maintained sufficiently high to prevent backward propagation of combustion. From the passage portions  30  in stages  4  the heated combustible gas/oxidant mixture continues along the passages  22  in the passage portions  32  of the stages  4  where combustion of the mixture is promoted by suitable catalyst means in those parts of the passages  22  in the passage portion  32 . The catalyst means may be provided in particle or granular form or in the form of a coating on walls of the passages  22 . The combustion products enter outlet manifold  116  from whence they are carried via ducting  124  to inlet  112  from which they pass via manifold  110  into the passages  64  (FIG. 4) of the stages  6  and then leave through the outlets  68  (FIGS. 4 and 5) to the manifold  118  for exit through the outlet  120 . Heat from the products of combustion in the passage portions  46  of the stages  6  transfers across the bases  8  to the incoming mixture of combustible gas and oxidant to heat the mixture. Heat from the gases leaving the outlet  120  may be recovered by means of suitable heat exchange means. 
     A stream of fluid, from a suitable supply  126 , capable absorbing heat, preferably a large proportion of heat, generated in the passage portions  32  of the stages  4  is supplied via ducting  128  into the passage portions  44  of the stages  6  via the manifold  106  and leaves via the manifold  102  and outlet  104  for collection and/or further processing. The fluid supplied by supply  126  may be a substance or mixture of substances, and may be liquid or gaseous, which can undergo an endothermic chemical reaction, or the fluid may remove the heat conducted across the bases  8  of the passage portions  32  of the stages  4  as sensible heat. Examples of suitable endothermic reactions are dehydrogenation of at least one hydrocarbon (for example, ethane or propane or butane or a mixture comprising at least two of those) to at least one olefine or diene, dehydrogenation of one or more paraffins, conversion of hydrocarbons to aromatics, and steam reforming of hydrocarbons, for example substantially methane, to produce hydrogen and oxides of carbon. To promote these reactions suitable catalyst means may be provided in the passage portions  44  of the stages  6 ; this catalyst means may be provided in particle or granular form or in the form of a coating on the walls of the passages  48 ,  52 ,  58 ,  60  (FIG.  4 ). An alternative endothermic reaction may be, in the absence of catalyst means, thermal cracking of hydrocarbons to produce olefines, for example conversion of ethane, propane or other paraffins to ethylene and other products. An example of removing the heat as sensible heat is the raising of steam in the passage portions  44  in the stages  6  from water supplied to inlet manifold  106 , though other fluids besides water may be used and need not be vapourised. 
     The passages  52 ,  58  and region  48 ,  62  in the stages  6  (FIG. 4) may be deeper or shallower than passages  16 ,  22  and regions  20  in the stages  4  (FIG. 3) to provide more or less residence time for conducting the desired reactions. 
     The products of combustion conveyed by the ducting  124  may be subject to treatment in external treatment means  127  which may be an adiabatic catalyst zone to complete the combustion of the combustible gas and/or an heat exchanger to vary the temperature of the gas entering inlet  112 . Means  130  and  132  may be provided for adding gas to the ducting  124  or removing gas therefrom. By one or more of these arrangments the gas in the ducting  124  supplied to the inlet  112  may be at the appropriate temperature to provide the desired amount of preheat to the combustible gas and oxidant supplied through inlet  100 . In particular adding hot gas to the duct  124  by means  130  or  132  and, if desired, varying or reducing the flow of combustible gas and oxidant through the passages  22  can provide a convenient mode of heating the passage portions  30  in the stages  4  at start up. 
     In the arrangement described with reference to FIGS. 3 and 4 the fluid in passages  52  flows in co-current with the flow in passages  22 . However by using the opening  104  as an inlet and the opening  108  as an outlet, the fluid flow in the passages  52  can be opposite to that of the arrows  60  and thus in counter-current to the flow in the passages  22 . 
     The skilled addressee will easily understand that the passages  52 ,  58  can be disposed transversely or cross-wise to the direction of the passages  22 . 
     The apparatus disclosed has the following advantages: 
     (A) The stages  4  and  6  may be constructed of materials which do not have the mechanical strength to fully withstand the pressure difference between the atmosphere and the pressure inside the stages  4  and  6 . The material inside the vessel  70  merely has to withstand pressure diferences between the passage portions  44  and those prevailing in passage portions  30 ,  32 ,  46  and those pressure differences may be kept small. 
     (B) Combustion products in the manifolds  114  and  110  can be outside flamable limits so leaks to atmosphere will not be dangerous. 
     (C) The mixing of the combustible gas and oxidant can be carried out at relatively low temperature and then the mixture is conveyed along the passages  22  to the combustion regions formed by the passage portions  32  where the combustion temperatures are reached; the passages  22  in the passage portions  30  may be narrow to form a flame trap. 
     (D) The endothermic reaction carried out the passage portions  44  may remove heat rapidly from the combustion regions in the passage portions  30  so that high combustion temperatures which promote formation of NO x  need not be reached. 
     If desired the pressure vessel  70  may be omitted. In this case the stages  4  and  6  will have to be formed of suitable pressure resistant material, and appropriate folds provided instead of the regions  98 ,  102 ,  106 ,  110 ,  114  and  118 .