US2376668A - Process for production of formaldehyde - Google Patents

Description

May 22, 1945.

PROCESS FOR PRODUCTION OF FORMALDEHYDE Filed April 16. 1942 INVE NTOR WALL ENE/Q .DERBY BY 6W AT ORNEY Patented May 22, 1945 PROCESS FOR P RODUCTION OF FORMALDEHYDE Wallene R. Derby, Oakwood, Ohio, assignor to Monsanto Chemical Company, a corporation o! Delaware Application April 16, 1942, Serial No. 439,223

11 Claims.

This process relates to the production of formaldehyde by the oxidation of low molecular weight aliphatic hydrocarbons.

Previously known processes for producing formaldehydeby the oxidation of hydrocarbons have employed as catalysts either nitric oxide, nitrogen peroxide or a great variety of solid catalysts such as the oxides of the various metals. While a great deal of work has been done on these processes the yields of aldehyde per cubic foot of hydrocarbon have been uniformly low and for this reason have not been commercially developed to any great extent except in those instances Where a plentiful supply of suitable hydrocarbons was available.

In general it has been found that the use of solid catalysts is unsatisfactory for this reaction. I prefer to employ the linown gaseous catalysts such as those derived from nitric acid, e. g., NO, N203 and NO2. For the purpose of supplying these catalysts I may utilize nitric acid or substances capable of supplying oxides of nitrogen to the reacting gases. In addition to nitric acid, other usable substances are ethyl nitrite, isopropyl nitrite or other alkyl nitrites.

I have also found, that in order to effectively oxidize hydrocarbons to formaldehyde, the oxidation reactions must be carried out in a chamber or tubeiwhich has been suitably treated so as to prevent objectionable catalysis of the hydrocarbon oxidation reaction and also the further oxidation of the formaldehyde produced thereby. By objectionable catalysis of the hydrocarbon oxidation reaction I means the undesired conversion o'f the hydrocarbons to carbon dioxide and water. that is complete oxidation, of the hydrocarbons. A suitable material for the construction of the oxidation chamber is difcult to'iind due to the complexity of the oxidation reaction.

I have now found that excellent yields of formaldehyde are obtained if I employ a reaction chamber constructed of a nickel-molybdenumchromium alloy, in which the proportions may be within the following limits:

Fe, W, Si, Mn Balance to make 100 A specific alloy which has been found to be suitable for this purpose has the following composition:

Percent Nickel 55 Molybdenum 1'7 Chromium l5 Tungsten 4 Fe, Si, Mn Balance the major portion of the balance being iron.

In comparable tests, in which tubes constructed of steel, stainless steel. nickel and the Ni, Mo, Cr alloy were employed as the reaction chamber, and when oxidizing propane-air mixtures, the folowing yields of HCHO per cubic foot of hydrocarbon gas charged were obtained:

Steel 3 Grams per cu. it. of propane Stainless steel 5-6 Grams per cu. ft. of propane Nickel 3 Grams per cu. ft. of propane Ni, Mo, Cr alloy- 12.2 Grams per cu. ft. of propane While my discovery may be applied generally Where hydrocarbon oxidation reactions are carried out, it is of particular value where such oxidation reactions are being carried out under substantially adiabatic conditions. An adiabatic reactor and the application thereof for the production of formaldehyde is described and claimed in a copending application of C. A. Hochwalt, et al., Serial No. 433,648, illed March 6, 1942. My invention may be employed therein, by constructing theoxidation chamber, whether tubular or other- `wise of my preferred alloy composition, as herein disclosed.

The reaction chamber within which the principal oxidation reaction is carried out, should be heated, which is done by the external application of heat at a uniform temperature level. The attainment of a uniform temperature of the reaction tube precludes the use of direct heat by fuels as such heat has ordinarily been applied. In place of such direct application of heat, it has been found that the temperature may be maintained more uniformly by the use of a liquid heating medium surrounding the chamber or tubes. By this means a uniform temperature of the tube or chamber is attained Without the use of an excessive temperature gradient through the tube walls. The liquid heating medium surrounding the reaction tubes moreover provides not only a large reservoir of heat at constant temperature but also provides means whereby heat may flow freely through the walls thereof with a greatly decreased lm temperature of the tube walls. The provision of a metallic material of construction for the reaction chamber or tubes greatly facilitates the transfer of heat, which factor resultsin a considerably increased yield of aldehyde.

In general, various aliphatic hydrocarbons may be used in my process, including methane, natural gas, ethane, ethylene, propane, propylene, butane, etc. Such hydrocarbons are normally gaseous at ordinary temperatures and pressures, however other volatile hydrocarbons, such as the natural gasoline hydrocarbons may also be employed.

In carrying out the reaction for the production of formaldehyde by oxidation of low molecular weight hydrocarbons I mix together the gaseous hydrocarbon and air in the proportions of approximately by volume of a hydrocarbon such as propane, together with about 85% by volume of air. The proportions of air and hydrocarbon may be varied somewhat, in any event it is desirable to keep the proportion of hydrocarbon above the upper explosive limit. Other hydrocarbons may ofcourse be used in my process.

Natural gas, having for example the following compositions in percent by volume:

Per cent Methane 83.5 Fithane 9,5 Propane y 4.4 N2, H2 and CO 2.6

is a cheap and readily available hydrocarbon for the present purpose. When employing this gas. a mixture thereof with air in the proportions to 35% natural gas to 75% to 65% of air preferably say gas to 70% air is made up and constitute the charge gas of the present process. The composition of this gas-air mixture is above the upper explosive limit.

Such a hydrocarbon-air mixture is passed into a preheater which serves to preheat the gas mixture to within the range of 250 C.,to 350 C. with 310 C.`being about the preferredrange when oxidizing natural gas.

To a separate preheater I supply the residual y gas mixture which has already passed through the reactor and from which the oxidized hydrocarbons, i. e., formaldehyde have been largely removed by scrubbing. This gas is for convenience termed recycle gas and as stated above is supplied to the preheater wherein it is raised to a temperature within the range of 350 C. to 550 C., a temperature of about 480 C. being preferred when oxidizing natural gas. The preheated hydrocarbon-air mixture and the preheated recycle gas are mixed together in the approximate ratio of 100 volumes of the hydrocarbon-air mixture together with from 200 to 300 volumes of preheated recycle gas mixture. A preferred ratio is 100 volumes of hydrocarbon-air to 260 volumes of recycle gas. The mixing of these two gases is conveniently done in such a way as to conserve the heat in the gases and to produce a gas mixture at a temperature in the neighborhood of 430 C. To this gas mixture is now added a small amount of aqueous nitric acid vapor (also superheated to approximately 430 C.) in such a proportion that the final mixture contains about 1.2 volumes of 100% HNO.: vapor. Nitric acid may be employed within the range of from 0.2

volume to 2.0 volumes of HNOa vapor per 200 volumes of total gas mixture.

After the nitric acid has been vaporized into the gas mixture the oxidation reaction starts. At this point the gases enter the reactor chamber. Some exothermic heat is liberated from the oxidizing hydrocarbon gases which heat serves to raise the temperature of the gases in the reactor. The gas temperature which is desirably finally obtained is a temperature in the neighborhood of 750 C. A complete oxidation reaction where the reactants are oxidized completely to carbon dioxide and water is undesirable and is prevented by control of two factors; namely, (1) the time of sojourn in the reactor chamber, and (2) the maintenance of the Walls of the reactor chamber uniformly at a temperature not over about 575 C.

The first necessary condition mentioned above is readily obtained by controlling the velocity of gases passing through the reactor chamber. The second condition is obtained by surrounding the walls of the reactor chamber with a fluid transfer medium such as molten salts or molten lead.

Although the temperature of gases entering the reactor may be somewhat below the temperature of the reactor and the temperature of the gases leaving the reactor are generally somewhat above the temperature of the reactor, the over-all effect of the reactor chamber is to conserve the heat of the reacting gases without adding material amounts of heat to the entering gases and also without withdrawing material amounts of heat from the exit gases. 'I'he reactor thus conforms to a more or less adiabatic chamber maintained at the preferred temperature as stated above and providing an optimum time of sojourn of the gases without a material net transfer of heat from the gases to the reactor.

It has been found that the optimum time of sojourn of the gases in the reactor ranges from 0.1 second to 0.2 -second and may be as high as 0.4 second.

Because of the complete submergence of the tubes in the molten liquid bath at a constant temperature, some transfer of heat desirably occurs from end to end of the reactor tubes through the bath. As pointed out above, the

gases at the exit end of the tube are at a higher I temperature than the bath temperature, and accordingly some heat ilow from the hotter gas through the tube walls into the liquid bath, which thereby becomes heated in the zone adjacent said hotter tubes. At the same time the incoming gases are at a somewhat lower temperature than the bath and accordingly some heat is transferred from the bath through the tube walls and to the gases. However, the bath being fluid will tend to equalize itself as to temperature either by natural convection, by mechanical agitation or both. I may therefore so adjust the temperature level of the reactor (the bath temperature) that the oxidation of the hydrocarbons is carried out to the desired extent without a material net transfer of heat between the gases undergoing oxidation and the bath surrounding the chamber or tubes.

The attainment of the above conditions is not independent of the conditions of preheat of the gases. It is accordingly necessary to preheat the gases to a degree such that the oxidation reaction may be carried out under substantially adiabatic conditions. It has been found that such a degree of preheating may be carried out without loss of yield, by preheating the hydrocarbonair mixture separately from the preheating of the "recycle gas. Preferably the hydrocarbonair mixture is heated to a point short of incipient g oxidation. The permissible temperature will vary Per cent CH4-- C2 and higher hydrocarbons NO Such gas contains less oxygen than the fresh hydrocarbon-air mixture, and consequently may be preheated to a somewhat higher temperature, as above mentioned. However, due to the reactive nature of the gases when mixed it is undesirable in the present process to rst mix the hydrocarbon-air portion with the recycle" portion and then preheat the resulting mixture.

The gases upon leaving the reactor at a temperature of about 750 C. enter a primary cooler which may take the form of a steam boiler in which the gases contact boiler tubes which are maintained at a temperature of about 200 C. and which in turn cool the gases to a temperature of about 300 C. Upon leaving the cooler or boiler the gases are passed to a secondary cooler where they are contacted with the condensate obtained by cooling the gases and consisting mainly of an aqueous solution of formaldehyde. The temperature herein is further decreased to in the neighborhood of from 30 C. to 40 C.

Upon leaving the secondary cooler the gases are passed to a scrubber where they are additionally washed with a small amount of pure water in order to remove additional formaldehyde and then from the scrubber they pass to a blower which forces the gases, except for dischargeor exit gases, back to the preheater mentioned above. The gases leaving the blower when oxidizing methane have a volume of approximately 350 volumes and since the above preferred mix,-

ing proportions call for approximately only 260 volumes of "recycle gas the excess gas amounting to approximately 90 volumes is discharged to the atmosphere.

My process may be understood by reference to the accompanying diagrammatic ow sheet comprising the single figure of the drawing. The temperatures and gas volumes thereon indicated are those preferred when oxidizing methane, or-

obtained from the'system by means of pipe I 8,

conveyed to preheater I9 wherein the temperature is raised to in the neighborhood of 480 C.

The preheated natural gas-air Vmixture and the preheated recycle gas mixture are conducted by pipes to mixing device in such amount as to consist of approximately 100 volumes of hydrocarbon-air mixture and 260 volumes of recycle gas. The resulting mixture will therefore consist of a total of 360 volumes of mixed gas at a temperature of approximately 430 C., that is, at an intermediate temperature between that of the constituent gas mixtures. The gases, after leaving mixing device 20, pass by means of pipe 2i to anothery mixing device 22, at. which point nitric acid vapor is introduced by means of pipe 48 connecting with the source of nitric acid i2. A vaporizer for vaporizing the nitric acid and preheating the vapors to the temperature of the gas with which it is to be mixed is indicated at The nitric acid containing gas mixture thereafter passes by means of pipe 28 into the nickel-molybdenum-chromium metal tubes forming part of reactor 2t. Reactor 24.

consists of the alloy metal tubes which are preferably immersed in a liquid heating medium such as molten salts or molten lead, the temperature of the liquid surrounding the tubes being maintained at approximately 575 C., and preferably between 550 C. and 650 C. The time of sojourn maybe between 0.1 second and 0.2 second but should not extend to as much as 0.4 second. The time of sojourn of the gases in the reactor is controlled by proportioning the volume of gases passed through the tubes to the volume of the reactor tubes.

The gases leave the reactor tubes by means of pipe 20 and pass directly into a primary cooler 26 which may take the form of a steam boiler. In primary cooler 30, the gases are cooled down to in the neighborhood of about 300 C. and thereafter leave by pipe 2l, passing directly into a secondary cooler 28. In secondary cooler 20 the gases are cooled by contact with cooled condensate circulated through the secondary cooler. The condensate is removed from a lower point of the secondary cooler by means of pump t@ and pumped into cooler 30 wherein the temperature is lowered approximately to that of the available cooling water temperature. The circulated condensate, together with the new condensate, leaves cooler 30 and flows by pipe 3i back into the secondary cooler 20. The gases leave secondary cooler 2B by means of pipe 32 and enter scrubber 33 winch may conveniently take the form of a platecolumn. A small quantity of water is supplied by means of pipe 34 to scrubber 33 and is discharged from the scrubber by pipe t6. The product consisting of the condensate is withdrawn from pipe 30 which connects pump 29 to cooler 30 by means of pipe 31, and is then combined with the scrubber liquid flowing in pipe 35 forming the product of the process in pipe 38. The product of the process consists of an aqueous solution of formaldehyde and aldehydes and acids which may thereupon be collected in tank 39.

When oxidizing \a natural gas consisting largely of methane, as described above, the aqueous condensate will have the following approximate composition:

Per cent Formaldehyde 16.1

Methanol c l 2.3 Water plus small amounts of formic and nitric acids Balance The gases leaving scrubber 33 are drawn through pipe 40 into blower Il and then enter pipe 42 which returns recycle gas to pipe Il. At the same time exit gas is withdrawn by means of pipe from the system in such an amount as to maintain a constant pressure in the system.

Some latitude is possible in respect to the method and temperature of preheating the gases. Thus, for example, while the preheat temperature oi' the natural gas-air mixture may be employed with a contact time of less than about 0.4 second in the reactor it is possible to increase the amount of preheat to a higherflgure than that givenabove vby suitably decreasing the contact time. Since the point of incipient oxidation is dependent upon both temperature and contact time I may, however, increase the temperature to a higher degree by decreasing the contact time in the preheater without encountering serious oxidation therein. Similarly, lower temperatures' may, if desired, be employed.

The same general considerations govern the temperature for preheating the recycle" gas. Thus higher temperatures such as 500 C. or even 550 C. may be obtained by appropriately adjusting the contact time. Lower temperatures may likewise be employed. Moreover the proportions of recycle" gas used may be changed from that given above.

The general consideration governing the choice of preheat of the gases is to assure the substantially adiabatic condition of the reactor during operation. The adiabatic state may, of course, be achieved at various temperature levels, within the limits stated, hence it is not desired to limit the operation of the reactor to any particular temperature level. By appropriate choice oi preheater temperature and by adiusting the other variables in known manner, it is possible to arrive at a reaction temperaturer which temperature will be generally within the limits ranging from 725 C. to 775 C.,l at which the reactor will, from a heat standpoint, be substantially adiabatic.

This condition is'independent of the pressure of the gases within the reactor, hence the present process may be operated at atmospheric pressure, as well as at superatmospheric pressures. Pressures generally may be, say 25, 50 or even 100 lbs. or more per square inch pressure. However, at pressures of 200 lbs. and over the course of the oxidation reaction is influenced unfavorabiy.

As stated the reactor is preferably constructed of nickel, molybdenum, chromium alloys, and immersed in a liquid heat transfer bath. The tubes should be of at least l/L inch internal diameter and not ,substantially larger than 1 inch internal diameter. Within this range of sizes the adiabatic condition is readily obtained and maintained for satisfactory operation.

What I claim is:

1. In the process for producing formaldehyde by the partial oxidation of normally gaseous aliphatic hydrocarbons in which a gas mixture containing said hydrocarbon and air is oxidized and formaldehyde produced, the step of reacting said gases during said oxidation in a reaction zone, the surfaces in contact with said oxidizing gases being formed of a nickel-molybdenum-chromium alloy.

'2. In the process for producing formaldehyde by the partial oxidation of normally gaseous ali phatic hydrocarbons in which a gas mixture containing said hydrocarbon and air is oxidized and formaldehyde produced, the step of reacting said sob . gases in a reaction zone formed by a metallic alloy having a composition substantially as follows:

Percent Nickel 40-60 Molybdenum 25-10 Chromium 25-10 the balance of said alloy being composed of metal selected from the class consisting of iron, tungsten, silicon and manganese.

by the partial oxidation oi' a normally gaseous aliphatic hydrocarbon, in which a. preheated gas mixture containing said hydrocarbon is oxidized and formaldehyde produced, the step of reacting said gases after preheating in a reaction zone defined by surfaces composed of an alloy having the balance of said alloy being composed of metals chosen from the class consisting of iron, tungsten. .I

silicon and manganese.

5. In the process for producing formaldehyde y the partial oxidation of normally gaseous aliphatic hydrocarbons,'in which a heated gas mixture containing said hydrocarbons and oxygen is reacted while flowing through a heated tubular reactionv zone forming formaldehyde, the step of 5 maintaining said reaction in contact with surfaces composed of an alloy having the approximate composition:

Per cent Nickel 40-60 Molybdenum 25-10 Chromium 25-10 the balance of said alloy being substantially iron.

6. The process defined in claim 5 in which the y alloy has a composition approximately as follows:

the balance being substantially al1 iron.

7. In the process for producing formaldehyde' Y by the partial oxidation of normally ygaseous ali- 65 phatic hydrocarbons in whichal heated gas mixture containing said hydrocarbons and oxygen together with catalytic amounts of nitric axide is oxidized to formaldehyde while flowing through a heated tubular reaction zone, free of solid catalytic material, the said tubular reaction zone being maintained at an oxidizing temperature by immersion in a liquid heatytransfer medium, the step of maintaining said gases in said zone by confining the same by means of surfaces composed of an alloy composed essentially of nickel,

molybdenum and chromium.

8. The process defined in claim 7 in which the tubular reactor is maintained within the temperature range of from 550 C. to 650 C.

9. The process defined in claim 7 in which the tubular reactor is maintained within the temperature range of from 550 C. to 650 C. and the oxidizing gases are permitted to attain a temperature within the range of 725 C. to 775 C.

prior to cooling.

the composition:

' Per cent Nickel 40-60 Molybdenum 25-10 25 Chromium 25-10 10. In the process for producing formaldehydev by the partial oxidation of normally gaseous hydrocarbons. in which process a heated gaseous mixture containing said hydrocarbons and oxygen is caused to react, and in which reaction, formaldehyde together with complete' oxidation products of said hydrocarbons are formed, the improvement which comprises carrying out said oxidation reaction in contact with an alloy comwhereby the amount of said complete oxidation products is decreased and the amount of form- 5 the reaction is carried out in contact with an alloy

Images