GB2087420A - Methanator Employing Finned Heat Pipes - Google Patents

Abstract

The invention relates to the manufacture of substitute natural gas having a high heating value, and to a methanator for use therein. Such manufacture normally uses large quantities of carbon monoxide, typically obtained from a carbonaceous feedstock, which gives rise to a temperature control problem when hydrogen and carbon monoxide react in the presence of a catalyst to produce the substitute gas product. According to the present invention, this latter reaction takes place in a first chamber (32) where the catalyst material is supported on heat pipes (34) for example as fins (35) the pipes (34) extending to at least one second chamber (33) in which heat is transferred from the pipes (34). The heat generated by the reaction in the first chamber (32) can thus be continuously withdrawn by transfer along and from the heat pipes (34) enabling the reaction to be correspondingly cooled.

Description

SPECIFICATION Methanator Employing Finned Heat Pipes Background of the Invention This invention relates to catalytic reactors employed for conversion of large quantities of hydrogen, and carbon monoxide gases into methane.

Devices used for catalytic methanation are commonly known as methanators. Conventional methanators associated with processes for producing synthesis gas are relatively small units intended to convert small amounts of carbon monoxide contained in the synthesis gas product into methane. Because only small amounts of carbon monoxide are reacted, these methanators are virtually self-regulating, as far as temperature is concerned, even though the methanation reaction is highly exothermic. The synthesis gas product associated with these processes would usually have a heating value of about 300 BTU/SCF. When large quantities of CO are being methanated, the final product would be expected to have a heating value of about 9001000 BTU/SCF, indicating a high degree of methanation.

Because of present day concern over the availability of energy, great interest has developed in processes for production of substitute natural gas, the principal component of which is methane; such substitute natural gas would have a heating value of about 900-1000 BTU/SCF.

Methanation processes are beingconsidered in which relatively large quantities of hydrogen, and carbon monoxide, are reacted. Foremost concerns to a designer of a methanator for use in such processes will by controlling the temperature within the reaction chamber of the methanator, and removing heat from the catalyst used to promote the reaction.

Several different methods have been suggested for temperature control and heat removal from the catalyst, with some employing recirculation of product gas, others employing pelietized catalysts in heat transfer tubes surrounded by heat transfer fluid, and yet others using heat pipes having a catalyst coating on their outer walls.

The present invention provides a method and apparatus for reacting large quantities of hydrogen, and carbon monoxide in the presence of a catalyst to yield methane under controlled temperature conditions. The apparatus aspect of the invention comprises at least two chambers, heat pipes extending between the chambers and fins comprising catalyst material attached to sections of the heat pipes disposed within one of the chambers. In accordance with the method aspect of the present invention a stream of gas including large quantities of hydrogen, and carbon monoxide is introduced to a reaction chamber in which a plurality of fins comprising catalyst material are disposed. The fins are attached to heat pipes which communicate with a second chamber through which a heat transfer fluid is passed.Heat given off in the reaction chamber is absorbed by the fins and is transferred through the heat pipes to the heat transfer medium flowing through the second chamber.

Summary of the Invention In accordance with an illustrative embodiment demonstrating features and advantages of the apparatus aspect of the present invention, a methanator is provided which includes first and second chambers, heat pipes extending from within said first chamber into said second chamber, and fins comprised of catalyst material disposed within said first chamber and attached to said heat pipes. In accordance with an illustrative embodiment demonstrating features and advantages of the method aspect of the present invention, a method for producing methane from a gas stream including large quantities of hydrogen, and carbon monoxide gases is provided. The gas stream is introduced into a first chamber within which a plurality of fins comprised of a catalyst material are disposed.The fins are attached to heat pipes extending from within the first chamber into a second chamber through which a heat transfer fluid is passed. In the presence of the catalyst the hydrogen, carbon monoxide and/or carbon dioxide react, giving off heat. The heat of reaction is absorbed by the fins and transferred through the heat pipes to the heat transfer fluid flowing through the second chamber.

Brief Description of the Drawings The above brief description, as well as further objects, features and advantages of the present invention will be more fully appreciated by referring to the following detailed description of a presently preferred but nonetheless illustrative embodiment in accordance with the present invention, when taken in connection with the accompanying drawings wherein: Fig. 1 is a schematic flow diagram of a typical process for making a substitute natural gas, employing the method aspect of the present invention; Fig. 2 is a schematic arrangement of a first embodiment of the methanator of the apparatus aspect of the present invention; and Fig. 3 is a schematic arrangement of another embodiment of the methanator of the apparatus aspect of the present invention.

Description of the Preferred Embodiment Referring to Fig. 1, there is shown a flow diagram of a process for manufacturing a substitute natural gas from a coal feedstock which incorporates the method aspect of the present invention. The process 10 includes gasification zone 12, purification zone 14, and methanation zone 1 6. It should be understood that although coal is shown in this embodiment, other feedstocks, such as liquid hydrocarbons, can be used to make the substitute natural gas.

The coal is introduced through line 18 into gasification zone 12 for reaction in a conventional manner with steam and/or oxygen which are introduced through lines 20 and 21 from sources 22, 23 respectively. A stream of product gas comprising hydrogen, carbon monoxide, carbon dioxide, methane, water vapor and impurities is removed through line 24 and is then passed to purification zone 14. Within purification zone 14 steam is condensed and some of the impurities, such as sulfur compounds, are removed.

Thereafter a product stream containing approximately 95% by weight hydrogen and carbon monoxide is passed through line 26 into methanation zone 1 6. Within methanation zone 16 the 'hydrogen and carbon monoxide contained in the product stream are reacted in the presence of a catalyst, such as nickel, to yield methane. It should be noted that when fins are formed form catalyst material they should be made from a material which lends itself to being formed into fins. Likewise, when the catalyst is to be sprayed onto fins, it should by selected from a class of materials which lend themselves to this practice.

Fig. 2 is a schematic arrangement of the apparatus associated with methanation zone 16.

The methanator, generally referred to by reference number 30, includes a first chamber 32 is adapted to receive the product gas stream from line 26. A second chamber 33 is disposed adjacent reaction chamber 32 and is adapted for passage of a heat transfer fluid therethrough. The heat transfer fluid could comprise any suitable fluid, but preferably would be water or oxygencontaining gas, which after absorbing heat while flowing through chamber 33 could be introduced to gasification zone 12 through line 20 or 21. In the embodiment of Fig. 2, an oxygen-containing gas is used as the heat transfer fluid.

A plurality of heat pipes 34 extend from within chamber 32 into chamber 33. At the locations of penetration of the walls of the chambers 32, 33, gas tight seals are provided to prevent leakage into and/or out of the chambers 32, 33. Fins 35 are attached to heat pipes 34 within chamber 32.

These fins 35 are made from a catalyst material, such as nickel, which serves to promote the reaction of hydrogen with carbon monoxide. Fins 35 also act as extended heat transfer surface within chamber 32, providing for transfer of heat from within chamber 32 into the heat pipes 34.

The heat liberated during the methanation reaction in chamber 32 is absorbed by fins as and conducted through heat pipes 34 to the heat transfer fluid flowing through chamber 33.

A plurality of temperature sensing elements 36 are disposed within chamber 32, and are connected through electrical circuit 37 to a temperature gauge 38. Various design and operating techniques can be employed to regulate the temperature within chamber 32, such as by varying the number of heat pipes or fins disposed within chamber 32, or by varying the flow rates of the fluids passing through lines 21, 26.

In Fig. 3 an alternative embodiment of the methanator apparatus 30 is shown. In this embodiment a plurality of secondary chambers 33a and 33b are used. Water is passed from a source 22, such as a boiler, through chamber 33a, and after being heated to steam is sent through line 20 to gasification zone 12. Similarly an oxygen-containing gas is passed from a source 23, such as an oxygen plant through secondary chamber 33b, and after being heated is introduced through line 21 to gasification zones 12. In this embodiment additional fins 39 are attached to sections of the heat pipes 34 within the secondary chamber 33a. These additional fins need not be made from catalyst material, since they are serving only as extended heat transfer surfaces, as contrasted with fins disposed within primary reaction chamber 32 which function both as extended heat transfer surface and a catalysts.

If desired, additional fins 39 can also be attached to heat pipes 34 within chamber 33b.

Returning to Fig. 2, the incoming gas stream flowing through line 26 is preheated in a heat exchanger apparatus 40. Methanation product removed from chamber 32 is passed through line 41, into heat exchange apparatus 40. Within this apparatus the relatively hot product gas comes in indirect heat exchange contact with the stream of gases flowing through line 26, serving to preheat the later stream prior to its introduction to chamber 32. Alternatively, heat exchange apparatus 40 could be supplied with heat from an external source, if the amount of heat removed from the product stream through the finned heat pipes 34 is so great as to preclude preheating of incoming feed.

Returning to Fig. 1, a substitute natural gas product stream comprising methane, steam and a small amount of impurities is removed from methanation zone 16 through line 41 and is passed through desuperheater 42 in order to reduce the product to its dewpoint. Thereafter the product stream flows through condenser 43.

While passing through condenser 43 a portion of the steam included in the product stream is condensed out; the product stream is then passed through separator 44 within which the condensate is separated out of the product stream. The product stream is then sent through a dehydrator 46 for further removal of water. Dried product gas is removed from product dehydrator 46 through line 48.

With respect to the method aspect of the present invention, an example of a process employing the method would be as follows: Coal is introduced through line 1 8 into gasification zone 1 2. Steam at a temperature of approximately 75O0F. is introduced to zone 12 through line 20. An oxygen-containing gas preheated to a temperature of approximately 3000 F. or higher to maximize gasifier efficiency is introduced to zone 12 through line 21. Within zone 12 the coal, steam and oxygen react in a conventional manner to yield a synthesis gas stream comprised of hydrogen, carbon monoxide, carbon dioxide, methane and impurities. This product stream flows through line 24 into purification zone 14.Certain impurities, such as sulfur compounds, are removed from the synthesis gas stream within zone 14. The purified synthesis gas stream is then introduced into chamber 32. In chamber 32 hydrogen and carbon monoxide react in the presence of nickel catalyst of which fins 35 are made. It should be understood that these fins 35 could be made entirely from nickel, or other catalyst, or could be catalyst coated heat transfer members. The temperature within chamber 32 will likely be within a range of from 7000F. to 10000F, and the pressure within chamber 32 will likely be within a range of from 200 psig to 1 500 psig.

A typical heat pipe working fluid contemplated for the apparatus of the present invention is mercury. Other materials, including water and potassium, may be used depending upon the intended operating temperature within chamber 32. The pressure of the heat transfer fluid flowing through chamber 32 will depend upon the particular fluid employed. By varying the number of fins, heat pipes, or both, and/or by varying the temperature or flow rate of the synthesis gas or the heat transfer fluid passed through chamber 33, the operating temperature within chamber 32 can be regulated.

The substitute natural gas product stream removed fron chamber 32 through line 41 will likely be at a temperature of from 4000F. to 10000 F. When the temperature of the product stream in line 41 is between 6000F and 10000F, it should possess sufficient heat for preheating incoming feed within heat exchange apparatus 4th. If the temperature of the product stream passing in line 41 is between 4000 F. and 6000F, additional heat from an external source should be introduced to heat exchange apparatus 40, in order to sufficiently preheat the feed flowing through line 26 prior to the introduction of this feed into chamber 32. The substitute natural gas product stream removed from heat exchange apparatus 40 will likely be at a temperature of approximately 4000F. The product stream is thereafter passed through desuperheater 42; the product stream temperature is reduced to approximately 2500F. The desuperheated product stream flows through.condenser 43 wherein its temperature is reduced to approximately 1 100F.

Condensate is next removed from the product stream at separator 44, and then additional moisture is removed at dehydrator 46. Dried product gas is removed through line 48.

Claims (10)

Claims

1. A method of manufacturing substitute natural gas comprising the steps of: (a) reacting a carbonaceous feedstock with steam and oxygen to yield a product gas including hydrogen and carbon monoxide, (b) passing said product gas into a first chamber, (c) disposing a plurality of heat transfer fins comprised of catalyst material within said first chamber, (d) attaching said fins to heat pipes extending between said first chamber and a second chamber, (e) flowing said product gas over said fins, said hydrogen and carbon monoxide reacting in the presence of said catalyst material to yield said substitute natural gas and heat, said heat being absorbed by said fins and said heat pipes, (f) passing a heat transfer fluid through said second chamber, said fluid absorbing heat from said heat pipes, said fluid absorbing heat from said heat pipes, and (g) removing said substitute natural gas from said first chamber.

2. The method of claim 1, wherein said step of disposing a plurality of heat transfer fins within said first chamber comprises disposed fins including nickel catalyst within said chamber.

3. The method of claim 2 further comprising the step of preheating said product gas stream before passing said product gas stream into said first chamber.

4. The method of claim 3 further comprising the step of dehydrating said substitute natural gas after said step of removing said gas from said first chamber.

5. A method of making substitute natural gas from hydrogen and carbon monoxide comprising the steps of (a) passing a stream including hydrogen and carbon monoxide gases into a first chamber, (b) disposing a plurality of heat transfer fins within said first chamber, said fins comprised of a catalyst material, (c) attaching said fins to a plurality of heat pipes extending between said first chamber and a second chamber, '(d) flowing said stream over said fins, said hydrogen reacting with said carbon monoxide in the presence of said catalyst to yield said substitute natural gas and heat, said heat being absorbed by said fins and said heat pipes, (e) passing a heat transfer fluid through said second chamber, said fluid absorbing heat from said heat pipes, and (f) removing said substitute natural gas from said first chamber.

6. A methanator comprising a first chamber adapted to receive a stream of gas including hydrogen and carbon monoxide, means for introducing said stream of gas into said first chamber, a second chamber adjacent said first chamber and adapted to receive a heat exchange fluid, means for introducing said heat exchange fluid to said second chamber, a plurality of heat pipes extending from within said first chamber into said second chamber, a plurality of fins comprising catalyst material attached to said heat pipes within said first chamber, means for removing substitute natural gas from said first chamber, and means for removing said heat transfer fluid from said second chamber.

7. The methanator of claim 6 wherein said catalyst material comprises nickel.

8. The methanator of claim 7 further comprising a third chamber adjacent said first chamber, and a plurality of additional heat pipes extending from within said first chamber into said third chamber.

9. The methanator of claim 8 further comprising additional fins comprised of catalyst material attached to said additional heat pipes within said first chamber.

10. The methanator of claim 7 further comprising additional fins attached to said heat pipes within said second chamber.