US 2686819 A
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Description (OCR text may contain errors)
Filed Sept. l, 1949 Patented Aug. 17, `195.4
UNITE STATES PAT T" FF ICE, 2,686,819
SYNTHESIS 0F METHANE William B. Johnson, Far Hills,- N. J., assigner to The M. W. Kellogg Company, Jersey City, N. JL,- a `corporation `of Delaware Application September 1, 1949, Serial No. 113,50?,VY
16A Claims. 1,.
This" invention relates to a' synthesis of methane from carbon monoxideand steam; more particularl y it is vconcerned with' a synthesis in which producer gas is the source o'f the carbon monoxide. In this manner a gas' of lowheating value is converted into one oihigh'B'; t. u. content'.
Producer gas, although easily made from any available type of coal or coke, is of comparatively limited utility because of its extremely loiv gross*` heating. value of approximately 136B.r t. u. per cubic foot. While this `gas is widely used in industry, it is not distributed Without enrichment to the public", In addition, larger distribution lir'resare` required as` the use of a greater volume of gas is necessary to furnish any given quantity of heat.` Consequently, there is a need for an economical process for obtaining a gasof high heating value from the product of a sim.-
ple and inexpensive producer gas system. This richer gas should desirably be suitable for mixing with any heating gas in order that it may serve as an enriching agent for' manufactured gases andas anextender for limited supplies of natural gas. Methane is ideal for the purpose in view of its high heating Value, low toxicity and theiclose similarity of its combustion characteristics to those of" common natural gases. Moreover,.the` proper utilization of methane is already Well understoodby public utility companies',` servicei'nen and gas appliance manufacturers.
Other processes for the synthesis of hydrocarbons from carbon monoxide have involved the preliminary removaly of inert-nitrogen from air and the burning` of coal or coke in relatively purev thesis for methane in which the only reactantsv consumed, are inexpensive and readily available. Ai secondobject ofA the invention is to provide an improved" method` for manufacturing a gas of high heating value.
A third object ofthe invention is to provide a method for synthesizing methane from producer gas `which does'not require-a preliminary removal `A fourth object` offthe invention is to provide a synthesis oifl methane which does not involve The process of the present invention 2V burning coal orcoke in substantially pure oxygen.
A nfth object of the invention is to provide a nevv' process for making methane in Which substantially'pure hydrogen is also produced.
A sixth object of the invention is to provide an economical, continuous, cyclic process for pro'- ducing methane andhydrogen from inexpensive and readily available rawmaterials;
Other objects ofthe invention will in part be obvious and Willin partappear'inthedetailed description hereinafter.
The present invention concerns a cyclic process for synthesizingAv methane from carbon monoxide; preierably'furnishedby pr'oducer'gas, and steam in which a carbide-forming metal' is re'- acted with steam at a sufficiently high temperature to producev hydrogen and an oxide of the metal. The metal oxide is then c'arbided with carbon monoxide at al suitably elevated temperature' and the metal carbide is reduced with the aforementioned hydrogen at a temperat'ure'high enough to prodnceinethaneand regenerate the metal. The invention accordingly comprises the several steps and the relation of one or more of such steps with respect to each of the others thereof, which vvilly be exemplified in the method hereinafter disclosed; and the scope` of thel invention will be indicated in the claims.
The process of the present invention consumes only air, Water and either coal or coke in producing methane and hydrogen,` for theY metal in powdered forni is regeneratedl and recycled, while the hydrogen required in one step is pro` duced in excess in ano-ther reaction- Moreover,
no extreme temperatures are encountered, sincethe' coal or eolie may be burnedin air rather than substantially pure oxygen. Nitrogen and' other inerts, as Well as carbon dioxide, areeliminated, inexpensively and" Without difficulty byI merely venting allgaseons productsof the carbiding reactionto the atmosphere; alternatively, some or all of the nitrogen may be recovered in rela tively pure forni".` All of the chemical changes described hereinafter are exothermic'and the Water employed as a coolant in a variety of heat' exchangersprovidcs'- a more than ample supply of steam for the `oxidation stepA In the preferred process #the oxidation; carbiding andlreducticnreactions are carried out simultaneously and continuously in different reaction zones` by circulting the fluidized solids through the system.
The base metal employed in the process is a reactant' and` not a catalyst in the true sense of the term. Nevertheless, catalysts may beiadvantageously used along with the carbide-forming metal in practicing the present invention, for they permit the carbiding operation to be conducted at lower temperatures. Examples of such catalysts include manganous oxide, manganese dioxide and copper, among others; and they are used in small quantities, as for instance 1% or 2% by weight of the metal reactant.
Promoters in small amounts ranging up to 1.5 of the weight of the base metal may also be added to the carbide-forming metal or to the mixture of metal and catalyst. These substances increase the carbon content of the metal carbide, thereby permitting a reduction in the quantity of primary metal employed in the process. Non-volatile alkali and alkaline earth oxides, as for instance the oxides, hydroxides and carbonates of potassium, sodium, calcium, strontium and barium are suitable promoters. In general, it may be said that any catalyst or promoter which has been successfully used in the Fischer- Tropsch synthesis will produce equivalent effects in the present process.
Any metal capable of forming a carbide may be utilized in this synthesis. Iron, cobalt, nickel,
zinc, manganese, chromium, tin, and molybdenum constitute only a few of the metals available for the purpose. The first three named, which make up the fourth period of groupv VIII of the periodic table of elements, appear superior to the others, and iron is the preferred metal by reason of low cost and high activity. Best results are secured when the metal is in fluidized form in order to obtain the advantages inherent in a continuous, cyclic, luidized process in which the solid reactants are rapidly circulated while suspended in streams of gaseous reactants, and iiow under the influence of gravity like pseudoliquids developing pseudo-hydrostatic uistatic pressures after separation from the gas streams. Fluidized operations not only provide superior control of reaction temperatures but also afford maximum surface Contact between the reacting gases and solids. Moreover, due to the increased surface area of the fluidized solids, their chemical activity is enhanced and all of the reactions involving them are carried out at lower temperatures than is the case with larger particles.
For the application of the fiuid technique described herein, the powder should all pass through a Ai-mesh screen. But to reduce the minimum transport velocity in the reaction coolers and to minimize bridging or blocking during gravity flow down the hoppers and standpipes, it is desirable to have a range of particle sizes averaging about 200-mesh or finer.
The temperatures of each of the three reactions involving a metal or compound thereof vary with the different carbide-forming metals; those required for iron, cobalt and nickel are set forth below. Pressures ranging from atmospheric to 50 pounds per square inch gage (p. s. i. g.) are recommended for all three reactions, but higher pressures, up to say 500 p. s. i. g., may be justified in order to increase reaction rates or permit the use of smaller and less expensive equipment.
As the source of carbon monoxide, producer gas is preferably employed in the instant process; its production is exemplified in the following equation:
I. Air (7O2-}-26N2)|14C 14CO+26N2 Water gas may be substituted but is less satisfactory inasmuch as the relatively large quantity of hydrogen present reduces the carbon efciency in the carbiding reaction. For the same reason,
coke is a better fuel for the producer than coal inasmuch as coal producer gas contains a minor amount of hydrogen.
Carbon monoxide from any other source may be used provided there are no excessive quantities of impurities which will inhibit or interfere with the carbiding reaction described below unless such impurities can be removed in a commercially feasible manner.
In another step a carbide-forming metal at an elevated tempera-ture is oxidized with steam to produce a metal oxide and hydrogen.
the temperature be maintained between 950 and 1050 degrees Fahrenheit.
The oxide, such as iiuidized ferrosic oxide, is then reacted with the producer gasv to form a metal carbide.
This reaction proceeds very rapidly and a more active carbide results when pressures ranging from atmospheric up to 30 p. s. i. g. are employed. It is thought that a variety of carbides of the selected metal are actually produced, especially when ferrosic oxide is involved. However, the carbon content of the mixture of iron carbides appears to approximate that of ferrous carbide and the product may be regarded as ferrous carbide for all practical purposes. yIhe nitrogen in the producer gas is inactive in this reaction and the only effects due to its presence arise from its reduction of the carbon monoxide partial pressure. In this instance only the solid resultant is essential in the present process, so the gaseous products as well as inert gases are usually vented to the atmosphere. However, a portion of the exhaust gases may be scrubbed free of carbon dioxide in any suitable manner to provide substantially pure nitrogen which may be added to the excess of almost pure hydrogen produced in reaction II to form a satisfactory feed for an ammonia synthesis plant. In addition, the carbon dioxide may be recovered from the absorption liquid and recycled back to the gas producer where it will be reduced by the incandescent coke to carbon monoxide thereby increasing the carbon efciency of the process.
The yield of metal carbide is substantially that of theory when an excess of ferrosic oxide is employed. Where the carbon monoxide is in excess, an undesirable deposition of carbon on the ferrous carbide occurs. Not only does this reduce the carbon eiciency of the process, but it also retards the subsequent reduction of the metal carbide to methane and iron. An excess of ferrosic oxide on the other hand increases the carbiding rate and this excess of solids permits better control of heat transfer throughout the system without interfering with the other reactions of the process cycle. Thus, a deficiency of the oxide is disadvantageous and it should be present in at least stoichiometric proportions or an exassetato emrbly sthe iexcessfof lferrosicuoxid'e aamounts` to very fastreactionrate .notslowed down` as itlis in the rcasewol? a dense :phase 1 xed 1' or circulating bed refierrosicbxide where `the lrateis reduced by the .introduction zof carbon .monoxide into solidsor'sub-normal:activitydue to their substantial fferrous carbide `content and also by'the increasing concentration oftcarbonldioxide as Ithe gase'szrise throughcthetbed. l i
iSuitable temperature ranges ifor the .carbiding operation aref450 to S800 degrees Fahrenheit in theicase Iof iron, 550 to 650::1degrees Fahrenheit being preferred, and .300 hto `e500 degrees Fahrenheit rior seither fcobalt or nickel. AI owering the temperature favors `the reaction equilibrium while `raising it increases ythe .reaction rate.
`.cN-ext the metal carbide is reacted I'with all or a portion of `.the hydrogen ,produ-ced iin reaction II. Thefhydrogen reduces the .carbide Lto the metal and `'combines with the carbon liberated to form methane. l i
lBle'zC#SH2-1'le"-I-SCI-Ifl` Moderately elevated `pressures `of a few atmospheres favor the reaction, while higher pressures tend :to yield higher hydrocarbons than methane. With `stoichiometricl .proportions `of reactants this fairly rapid1reaction `proceeds to over 95% of completion, and where hydrogen i-nfa `clesirable `excess of about is used, the yields aare very close to "that oi' theory.` It will be noted that the hydrogen ,produced in .reaction II amounts `to one-third :more than is `requiredfor reaction IV,and all of this excess may p be employed in the latter reaction where `a `prod-- uctggas of somewhatlower heating value islacceptable. The `operative temperature range `for iron extends `from 650 to 950degrees Fahren? heit" and the latter tgure shouldnot `be exceeded inasmuch as iron beginsto soften and become somewhat tacky at 1950.1degrees. :For :iron `the recommended temperature range isfrom I'i50to 8D01degrees Fahrenheit. Considerably less heat is Arequired for cobalt.or nickelas the temperaturecmay .range from `400 to `600 degrees Fahrenheit. 1
The invention is best understood .byreference to `the accompanying drawing "which is `a .flow sheet of the-novel process in itsjpreferredform` iig-ure is, purely schematicinnature and is notlintendedto illustrate the optimum locations orldirnensions of any of theapparatus depicted.
.Air .from the blower 1| issupplied` through valved Yline 2 to-gasproducer 3` Whichis preferably -`f theslagging type. The coke or coal feed for the .producer `enters throughhopper li and the producer gas passes through line E to a waste yheat boiler `l whereit gives `up `most ofits heat. Leaving through line l., the gas passes toa cycloneseparator 8 in which `ily ash is removed trom the gas stream. `Pipe!) carries the `gas to a scrubber 'I0 Where it is scrubbed with isrinsuni'cient to iinterfere with :the subsequent reaction with ferrosic oxide.
1CNext the .gaseousmixture consisting :chiefly of nitrogen and carbon 4.monoxide nproceels throu'g-hpipe I I 3 .into transport uline JM f where the hot uidized .ferrosi'c .1oxide, usually 1in "stoichiometric excess, is injected 'Jfrom standpipe I5 attached to a sealed vessel |16 Sand controlled by-1a slide `valve ll. If necessary r4alsmall `quantity Vof a .suitable .aerat'ing iagent, "such `assteam, 4may be introduced into vpipe I 5 `and Salso yat the bottomfo'f hopper LI-Ieatl onecor more points to maintain th'elinely divided `ox-ide -in fl-uidizeol Jcond-ition. .As -the up owdered 1. oxide lsweeps l up through in'e fill ias tra Tdilute .turbulentsuspension -in -the carrier stream of gases` an Ivexothermic [reaction eoinmencesmin which the lcarbide `.of ithe metal i'sciiiormed. `Theudensity ofthe 'suspensioninthis carrier `line is a function-pf ith'e static .pressure and the preferredrangeis `from 6:3 to 5.0 lbs/cu. it. `"The i cptimumuvelocity of Athe-umixture `is-about 20lto `30 Lft/sec. Suchconditionsarereadily obtained by `known ldesign principles in selecting the proper pressure drop :through `.pipe E4 `and choosing aniinternaldiameter for this -lineadei `quatewfor.theliiow ratesoffreactants. flhefh'eat Fahrenheit. for the .producer gas is notfar` above atmospheric temperature. `A `water-cooled `re action cooler `Itis .provided Ato.Imaintaina @reaction `ten'lperaturefof-550m 650 degrees Fahrenheit...n this vessel, the reaction zone -is Ylof greater crossesectional area 'than line `-lf1l,\thereby reducing the .velocitycof fthe-suspension `to Labout 3 i to i0 `iii/sec. andi'ncreasingits density to approximately 20 to fiOlbs/icu. it. Thel'ength of thisreaction zone is. of course determined lby the requirements .of adequate ytime for the lreaction and sufficient heat transfer area to `maintain the reaction temperature `withir-i'the limits "indicated. `Line. i5? whichcarries `thel'mixture lintothe closed vessel `2li fextends welldown into the fin terior but not fbelow the surface =of thedense bed (approximately fZ0-fto 100 lbsf/cuQt.) of iron carbide 1 and l excess vierrosic i oxide stored therein. At `this `point thewgases 1in `the mixture, chiefly carbon dioXideand Lnitrogen under a `pressure of A20 to 30 p. s. i. fg., separate Lfrom the finely divided solids, and the settling `or separation efciency lis i enhanced greatly `by `the `downward discharge oi' the suspensioninto-settlerf. The diameter-:of `this vessel -is `selected to provide an upwardvelocity of gases therein of from 1-'to 2 it./sec. 'and lits length 'is designed to furnish storagespacei'or-an amplevsupply lof the metal compounds plus `suicient disengaging space thereabove toattain the optimum gravity lseparationoi powder from thegases. The gases are exhausted to the `atmosphere through a filtering device -2`i `and 'exhaust pipe "22. Any entrained iine particles are removed by the "filter, whichis preferably o'f porous metal or ceramic:construe--` tion, and consists oi several sections. n lvalve arrangement operated by an automatic time cycle controller (not shown) is.provided=to1clear the filter of adhering powder by blowing back the exit .gases through each `sectionof the-filter in succession.` `This mechanism Lis so 4Iadusted as to always `be clearing ione section of the-filter while the other sections -arelltering the exhaust gases. lIrffa cycloneseparator'isusedin place of be passed through a Water scrubber to avoid the loss of fines.
' In the event that either the nitrogen or carbon dioxide in the exhaust gases are to be recovered, line 22 is connected to an absorber or scrubber (not shown) where the carbon dioxide is dissolved by passing the gases through any suitable absorption liquid from which the carbon dioxide may later be recovered. The absorber eiiluent consists of relatively pure nitrogen contaminated with the rare gases of the atmosphere in small amounts along with lesser quantities of unreacted carbon monoxide and perhaps hydrogen.
The hot metal carbide and oxide, at a temperature only a few degrees below that in reactor IB, descend from tank 20 through standpipe 23 controlled by slide valve 24 into a rapid currentof hydrogen in line 25. Any aeration necessary for tank 20 or pipe 23 can be supplied by flue gases or by compressing and recycling the effluent from line 22. The hydrogen employed in this step is produced in excess in the oxidation of the metal as described hereinafter. Since this gas is comparatively cool, the hot solids furnish all or most of the heat required to start the rapid exothermic reaction. In pipe 25 and enlarged reaction cooler 2G, which maintains the reaction temperature between 750 and S degrees Fahrenheit, the metal carbide in the suspension is reduced while in concurrent flow to iron and methane by the hydrogen which is preferably present in stoichiometric excess. This powdered iron and ferrosic oxide suspension is then discharged by line 2l just above the bed of solids in hopper 2t where the pressure is in the to 30 p. s. i. g. range. The methane and any unreacted hydrogen separate from the powder and leave through filtering device 29, similar to filter 2l. Thereafter the product gas is passed through pipe 30, cooler 3i and then to suitable storage facilities.
From the bottom of vessel 28, where the density is of the order of 70 to 100 lbs/cu. ft. and its temperature only slightly below that of reaction cooler 2t, the hot iiuidized metal and excess oxide iiow dov/n standpipe 32 controlled by slide valve 33 to carrier line 34 and are there picked up by a swift, valve-controlled stream of steam from the boiler t or reaction coolers. The turbulent suspension of reactants passes through enlarged` reaction lcooler 35 maintained at 950 to 1050 degrees Fahrenheit and the powder is deposited by pipe 3B in tank I6 on top of the layer of metal oxide which has a density of about 50 to 80 lbs/cu. it. The steam enters transport line 34 at a pressure equal to the 20 to 30 p. s. i. g. maintained in hopper 9&5 plus the designed pressure drop between the bottom of standpipe 32 and hopper N. Its temperature is adjusted to the yminimum required to initiate reaction with the iron in the powdered mixture to form hydrogen and ferrosic oxide while in transport to the receiving hopper.
The hydrogen and excess steam leave tank I6 through the cyclone or iilter 3T which separates the nes. Since the steam constituent of the eii'luent in pipe 38 will inhibit the reduction of iron carbide by hydrogen in the following step of the cycle, as much moisture as possible iS condensed out of the mixture in a water-cooled condenser from which the condensate leaves by trap line 40 while the hydrogen passes up pipe 4l. By suitable adjustment of valve 42 some or all of the excess hydrogen may be drawn off, if desired, through `line 43 as a substantially rates.
pure product while the 'remainderis recycled through pipe 44 and compressor 45 totransport line 25 for the reductionstage of the process. Any excess hydrogen'recycled through the reduction step is not lost and will be recovered along with Vthe methane from cooler 3l.
Where it is necessary, to aerate the bottom of tank 28 and/or line 32, this may be accomplished with either some of the excess hydrogen from pipe 43 or 44 or product gas from cooler 3l.
In maintaining the desired concurrent flow in all three reactions involving fluidized solids, the maintenance of approximately equal velocities in carrier lines I4, I9, 25,21, 34 and 36 of from 20 to 30 ft./sec. is recommended and the densities in these lines are from 0.3 to 5 lbs/cu. ft. depending` on reaction pressure. .The velocities in reaction coolers IS, 26 and 35 are preferably 3 to 10 ft./sec. and the suspensions there have densitiesk of 20 to 40 lbs./cu. ft. In settlers or hoppers I6, 2li and 2S, upward gas velocities of from 1 to 2 ft./sec..are preferred. These velocities are obtained by designing this equip-V ment along the linesv indicated previously.
The proper lengths of standpipes l5, 23 and 32 are determined in conventional manner to produce sufficient fluistatic pressure to inject the solid reactant into the carrier lines at suitable Since none of the i hoppers may collect fiuidized solids at a greater rate than the other settlers overan extended period without throwing the` system out of operating balance, it is apparent that `the flow rates of solids, calculated as Fe, down each of the three standpipes `must be equal in the long run. Therefore, the operating adjustments on the plant to attain the desired velocities and stoichiometry will be made on the iiuid reactants by means of the valves inV lines 2, 44 and the steam line.
In the event that it proves desirable to achieve better control of reactant temperatures, this may be readily accomplished by the installation of heat exchangers in one or more of the reactant supply lines (the standpipes and pipes I3 and 44) to heat or cool the reactant to the desired ligure.
The process described above is cyclic and involves the rapid circulation of iiuidized solids with the various reactions occurring simultaneously and continuously in different reaction zones. This represents the greatly preferred form; Nevertheless, it is readily apparent that the process can be performed in an intermittent manner with all reactions occurring in sequence in a single reaction zone,or alternatively in fixed beds of solids in three reactors using known cycle control devices. In such cases, the particle size of the solid reactant may range from the fine powders discussed hereinbefore up to granules of approximately diameter. However, it is to be understood that the eiiiciency of the intermittent cyclic process is considerably below that or" the continuous circulatory one because the reactor or reactors would necessarily be kept at a single average temperature rather than in the optimum range for each of the reactions and the temperature control in a wide fixed bed is inferior to that obtainable in a turbulent circulating suspension of the solids in a relatively narrow stream of gas. Where the fixed bed consists of granular rather than fluidized solids, the variations in temperature in various parts of the bed are far greater and higher reaction temperatures are required. In addition the operating costs of the intermittent process would obviously be much greater than the circulating continuous process. Y
thesis of methane from carbon monoxide and steam which comprises oxidizing a suspension of a carbide-forming fluidized metal obtained from the reduction step mentioned hereinafter with a carrier stream of steam at a temperature sufficiently elevated to produce principally hydrogen and an oxide of the metal, separating the hydrogen and said metal oxide, reacting a suspension of said iiuidized metal oxide with a carrier stream of carbon monoxide at a temperature sufficiently elevated to produce a carbide of the metal, separating said metal carbide from the gases in the reaction products, reducing a suspension of said fluidized metal carbide with a carrier stream comprising at least a portion of said hydrogen at a temperature suiciently elevated to produce the metal and a gasiform mixture comprising chiey methane, and separating the iiuidized metal and the methane.
15. A continuous cyclic process for the synthesis of methane from carbon monoxide and steam which comprises oxidizing a suspension of fiuidized iron obtained from the reduction step mentioned hereinafter with a carrier stream of steam at a temperature of from 700 to 1200 degrees Fahrenheit to produce principally hydrogen and ferrosic oxide, separating the hydrogen and ferrosic oxide, reacting a suspension of the fluidized ferrosic oxide with `a carrier stream of carbon monoxide at a temperature f from A450 to 800 degrees Fahrenheit to produce a carbide of iron, separating said carbide of iron from the gases in the reaction products, reducing a suspension of said fluidized carbide of iron with a carrier stream comprising at least a portion of said hydrogen at a temperature of from 650 to 950 degrees Fahrenheit to produce iron and a gasiform mixture comprising chiefly methane, and separating the fluidized iron and the methane.
16. A continuous cyclic process for the synthesis of agas of high heating value Vwhich comprises burning carbonaceous matter in a restricted quantity of air to form producer gas, oxidizing a suspension of yluidized iron obtained from the reduction step mentioned hereinafter with a carrier stream of steam at a temperature of from '700 to 1200 degrees Fahrenheit to produce principally hydrogen and ferrosic oxide, separating the ferrosic oxide and the hydrogen, reacting a suspension of the uidized ferrosic oxide with a carrier stream of producer gas at a temperature of from 450 to 800 degrees Fahrenheit to produce a carbide of iron, separating said carbide of iron from the gases in the reaction products, reducing a suspension of said fluidized carbide of iron with a carrier stream comprising at least a portion of said hydrogen at a temperature of from 650 to 950 degrees Fahrenheit to produce iron and a gasiform mixture comprising chiey methane, and separating the fluidized iron and the methane.
References Cited-in the le of this patent UNITED STATES PATENTS Number Name Date 2,130,163 Tiddy et al Sept. 13, 1938 2,364,123 Benner Dec. 5, 1944. 2,369,548 Elian Feb. 13, 1945 2,409,235 Atwell Oct. 15, 1946 2,449,635 Barr Sept. 2l, 1948 2,537,496 Watson Jan. 9, 1951 2,544,574 Walker et al. Mar. 6, 1951 FOREIGN PATENTS Number Country Date 13,861 Great Britain June 15, 1907 OTHER REFERENCES Bahr et al., Berichte (July-Dec. 1933), pages 1238 to 1241.