US2800504A - Production of lower aliphatic acids - Google Patents

Description

July 23, 1957 A. ELCE ET AL PRODUCTION OF LOWER ALIPHATIC ACIDS Filed Oct. 9, 1953 2,800,504 PRODUCTION OF LOWER ALIPHATIC Actus Alec Elce, Banstead, Ian Kenneth Miles Robson, Clapham Common, London, and Donald Peter Young, Sanderstead, England, assignors to The Distillers Company Limited, Edinburgh, Scotland, a British company Application October 9, 1953, Serial No. 385,182

Claims priority, application Great Britain October 15, 1952 7 Claims. (Cl. 26d- 533) The present invention relates tothe production of aliphatic acids, particularly acetic acid.

It has been found that when oxidising parafiinic hydrocarbons containing 4-8 carbon atoms at temperatures above about 130 C. and subsequently distilling the product that there are obtained as successive'fractions: (l) volatile non-acidic Vorganic compounds boiling in the presence of water in the range about C. or lower up to about 99 C. (called herein light ends`); (2) aliphatic acids principally of one to four carbon atoms; and (3) high boiling residues. It has further been found that the light ends fraction contains a substantial proportion of water together with traces of aldehydes andvarying proportions of ketones, alcohols, esters and other compounds including unreacted hydrocarbons. It is known that certain of these compounds, such as aldehydes and some ketones, may be oxidised to yield acids, while others such as alcohols and esters are not easily Oxidised, particularly when water is present.

It is an object of our invention to oxidise in liquid phase paraimic hydrocarbons of about four to eight carbon atoms, with gases containing molecular oxygen, to produce aliphatic acids of one to four carbon atoms. It is a further object of our invention to oxidise volatile nonacidic oxidation products in such a way that further yields of acids of one to four carbon atoms may be obtained therefrom Vwithout the necessity for effecting prior separation of said volatile non-acidic oxidation products into their constituents.

lt has now been found that by oxidation of these light ends in the liquid phase, under conditions similar vto those used for the oxidation of the hydrocarbon,A there lare obtained further useful amounts of the desired acidproducts ,of which acetic acid is the main constituent.

It has now further been found that although the light cnrs when oxidised separately yield a substantial amount periods without build up of unoxidisable material in the reaction system and without any evidence of a decline in the rate of production of acids expressed on a reactor volume basis. This is surprising as it was expected that in any process for the oxidation of parains to yield acids it would be necessary to remove the unoxidisable constituents of the light ends by one means or another from the process, in order to prevent accumulation of unoxidisable constituents in thesystem and a consequent lowering of the output of the desired acidic products.

The invention accordingly comprises the novel processes and steps of processes, specific rembodiments of which-are described hereinafter by way of example and in accordance with which `we now preferto practice the invention. y`Accordingly, the present invention is for a process for the production of acetic acid and other aliphatic acids which comprises oxidising in the liquid phase a -parain hydrocarbon lof four to eight carbon atoms' with a gas the liquid phase. The present invention also comprises the above process including the step of adding fresh paraffin hydrocarbon to the light ends and thereafter oxidising said mixture.

The oxidation of the parafn hydrocarbons of four to eight carbon atoms is suitably carried out for example as described in copending application No. 385,272, Habeshaw et al., filed October 9, 1953. According to application No. 385,272 a paraihn hydrocarbon fraction containing hydrocarbons of 4 to 8 carbon atoms, at least 40% of said fraction consisting of hydrocarbons of 6-8 carbon atoms, wherein at least 40% of the'parans of 6-8 ca rbon atoms of said fraction consist of branched chain parafns having methyl substituents, and wherein said fraction is of boiling range not exceeding 95 C., is oxidised in the liquid phase with a gas consisting of or comprising molecular oxygen.

The n parain hydrocarbon of four to eight carbon atoms employed for the oxidation is desirably one which does not exert too high a vapour pressure, specically those which are normally liquid at room temperatures of about 30 C. and atmospheric pressure. p

y lt has been found that the reaction mixture is normally a homogeneous liquid at operating temperature,'but that on cooling two liquid layers separate. Of these the upper layer is mainly unchanged hydrocarbon, while the lower layer contains the bulk of the acidic products and water. It is, therefore, a feature of the process of the present invention to operate by removing continuously or intermittently a part of the reaction mixture as a homogeneous liquid, cooling this material to a temperaturebelow about C., and preferably to as near room temperature as may be convenient, separating the liquid layers, returning all the upperlayer to the reactor and removing at least part of this lower layer as the product.

The primary oxidation product may be distilled to separate as distillate the light ends, for example the fraction which under batch distillation conditions will boil over the range of about 30 C. to about 99 C. During the latter part of the distillation under these conditions some of the water separates asa lower layer in the distillate, and this may'be separated and removed if so desired. Separation of the light ends may also,however, be carried out in a continuous manner, employing a continuous still operated with a head temperature of about 65-70 C., and a base temperature of about 105 C. In this case no separation of an aqueous layer from the distillate normally occurs.

The products of oxidation may be worked up to vyield further light ends, which may conveniently be termed secondary light ends, acids of -one to four carbon atoms and higher boiling material in the same way as the products from the primary oxidation of the hydrocarbon. The secondary light vends so recovered may again be oxidised, although it will be found that these yield a lower proportion of acids than the primary light ends, and that a higher proportion of the charge is recovered as unoxidised light ends. While therefore the process may be repeated as often as desired, it will probably be found uneconomic to repeat the process more than two or three times. Alternatively the secondary light ends recovered from the secondary oxidation may be combined with a fresh batch of primary light ends and the mixture oxidisred or they may be combined with the feed of primary light ends in a continuous oxidation process for oxidation of light ends.

According to a preferred embodiment of the invention light ends are mixed with fresh hydrocarbon, suitably with an amount of the latter equal to the balance of the first product after removal of the light ends therefrom and the mixture again oxidised. VVery suitably on mixing the light ends with fresh hydrocarbon, the aqueous phase which forms is separated and discarded, and the mixture of light ends and fresh hydrocarbon of reduced water content oxidised. This second oxidation product may again be distilled to recover light ends, and the process can be repeated indefinitely. It will be realised that the only liquid products resulting from the overall process are those remaining as still-base product after distillation to separate the light ends, and these mainly comprise the desired acids of one to four carbon atoms. It is preferred, however, to operate continuously by introducing the parafnic hydrocarbon feed together with air or oxygen into an oxidation reactor, withdrawing oxidation products from the reactor, subjecting the product to distillation whereby light ends are separated as an overhead fraction and returning these to the oxidation reactor in conjunction with the feed of fresh hydrocarbon.

The oxidation process of the present invention, mainly the oxidation of the parain hydrocarbon, of the the light ends, or of the mixture of light ends and parain hydrocarbon may each be carried out under similar conditions.

rllhese oxidations may be effected with any gas containing molecular oxygen, whether in the form of air or of mixtures poorer or richer in oxygen than is air; part of the molecular oxygen may, if desired, be in the form of ozone. The use of superatmospheric pressures in the oxidation will generally be necessary in order to maintain a major part of the reactants in the liquid phase.

The temperature of oxidation should be suiciently Vhigh to provide an economically attractive rate of oxida- `tion and a low level of peroxidic compounds.

Thus, the temperature should not be so low that insutiicient oxidation to acids occurs or alternatively be so high that excessive oxidation occurs of the hydrocarbons, or of their primary oxidation products, to oxides of carbon and water. Temperatures in the range about 130 C. to 200 C. have been found suitable, although higher or lower temperatures may be used if so desired.

These oxidations may be carried out in a variety of types of apparatus, provided only that the reaction mixture is maintained substantially in the liquid phase, and that adequate contact is maintained between the liquid reaction mixture and the oxidising gas. Thus, the oxidation may be carried out in a pressure reactor fitted with a mechanical stirrer for agitation and dispersion of the gas throughout the liquid. Alternatively, the reactor may be in the form of a vertical tower with the feed of oxidising gas at the base, or at a number of points up the tower, in which case the necessary agitation is effected by the gas itself; in this case it is advantageous to subdivide the gas feed by mechanical means to obtain a line dispersion at the point or points of entry and throughout the reaction zone. Alternatively, the oxidising gas may be fed into a stream of liquid moving with high velocity in a circulatory system, such as a coil reactor with forced circulation.

These oxidations may be carried out in the absence o f any added catalyst, or alternatively if so desired, the oxidation may be carried out in the presence of a suitable oxidation catalyst. Such catalysts include compounds of those metals that are capable of existing in more than one valency` state. Examples of suitable metals include manganese, cobalt, nickel, vanadium and copper. These catalysts may be conveniently added 'n the form of their oil soluble salts with organic acids, or alternatively the catalyst metal may be added in the form of an anion, whether as the free acid or a salt thereof, for example as a vanadate.

It is preferred to employ materials for the construction of the reactor and oxidation equipment which, pos- Example 1 The oxidation apparatus consisted of a stainless steel reactor fitted with a stirrer, a bottom air inlet, a line for removing waste gas, connections for the addition and withdrawal of liquid and means for the measuring and controlling of the reaction temperature. Provision was made to withdraw continuously a part of the liquid products through a cooler to a separator, from which the lower layer could be withdrawn as product and the upper liquid layer returned to the reactor.

The reactor was charged with 1.5 litres of n-heptane containing 0.01% W./v. of manganese dissolved in the form ofthe naphthenate. The temperature was raised to 160 C. and the air feed was started and maintained at a rate of about 7 litres/minute throughout the experiment.

Circulation of the reactor contents through the cooler `and separator was controlledA at about 0.3 litre/hour. The lower liquid layer appearing in the separator was removed as product and replaced with fresh n-heptane, sufficient to keep constant the total volume in the system, at an average rate of 0.18 litre/hour.

The Voxidation was continued for a total of 50 hours under the above conditions. The product was distilled batchwise to recover as first fraction light ends boiling from 21 to 95 C., followed by a mixture of acids of one to four carbon atoms and water. 'I'he latter fraction was dehydrated and distilled to yield the individual acids in successive fractions.

Overall yields from the oxidation, expressed as grams from grams of n-heptane converted, were as follows:

Grams/100 grams n-heptane consumed Light ends 25.9

Formic acid v 3.3 Acetic acid 27.5 Propionic acid 9.9 Butyric acid 6.8

Total C1C4 aliphatic acids 47.5

Distillation residues 31.2

1283 grams of light ends thus obtained were oxidised in the apparatus used above, except that the reaction mixture was not circulated through the cooler and separator. These light ends contained by analysis 0.34 equiv/100 g. of saponiable compounds and 0.51 equiv./ 100 g. of carbonyl compounds. The charge contained 0.01% w./v. manganese dissolved as the naphthenate.

The reactor pressure was increased to 200 lbs/sq. in. and the air feed started. On raising the temperature to 60 C. absorption of oxygen started but it was soon necessary to increase the temperature to about 168 C. to maintain the rate of oxidation. After 7.5 hours total duration the presence of substantial amounts of oxygen in the waste gas indicated that oxidation was complete, and the operation was concluded.

The liquid reaction products were distilled as in the rst step, to separate first a secondary light ends fraction, followed by a mixture of aliphatic acids of one to four carbon atoms and water. Further distillation of the wet ac ids fraction yielded the following overall results, ex-

the oxidation.

Y pressed .grams per 10o .artigli-tends Acharged' fo Example A straight run gasoline fraction v`b.`:p. --.95 C. from Middle East petroleum'iwas oxidisedhnde'r continuous conditions according totheme'thod described in VExample 1. The hydrocarbon chargd'and 'fed contained 0.01% manganese dissolved a's the naphtheate. .'Ihe `temperature and pressure of the oxidationwerel'SOlO" Cfand 300 lbs/sq'. in. respectively,'the rate of vcirculation'ofthe reactor contents through the `eoolerand separator being about 2.1 litres/hour, and the 'rat'e'of fair 'feed tothe oxidation about 1 1'2 litres/minutes. lThe :feed rate `of fresh hydrocarbon to the system 'was about "0.43V litre/ hour, being the .amount necessary to fbalancethe'frate of withdrawal of the lower aqueous-'acid layer 'o'fthe cooled product. R .1.1.

The oxidation was continued fora total of 102 hours, and the product was subjected to" distillation in a continuousstill, consisting of a `72 `x"1.5 inch Yeoluniii packed with glass helices, the point "of feed being halfway 'up "the column. The 'still' was `operated at atmospheric pressure with temperatures Vnr85-87" C. a'tfthe heaeffnu 10e-10s C. at the base. The liquid feed 'rate 110 `litres/hour and the reflux ratioab'outZal, the liglitfehds' btain'edas overhead distillate being 45.5% byw'eig'htf the vfeed. The base product consisted of'a mixture of l'vjt'at'er,fali phatic Vacids of one to `four carbon atoms and residues, and was separated into its various Acons tituents `by further distillation.

Overall yields from the oxidation, vexpressed -as grams of recovered materials from 100 grams of hydrocarbon consumed, were as follows:

f .Grams/IOO-granis hydrocarbon consumed The light ends thus obtained by distillation were oxidised batchwise according to the method of Example 1 for the oxidation of light ends, at a temperature of 150- 170 C. and a pressure of 300 lbs/sq. in. The duration of a number of batch experiments, up to the point where absorption of oxygen from the air feed had virtually ceased, was 7-13 hours according to the rate at which the temperature was increased.

The oxidation products were combined and distilled in a continuous still to remove light ends and the still base product subsequently distilled to separate the acids as described above. Overall yields of materials recovered expressed as grams per 100 grams of light ends charged to the oxidation were as follows:

Grams/ grams light ends c arged ASecondary light ends 21.7

.Formic acid f 8.4 Acetic acid 40.4 l VPropion'ic acid 6.5 Blityric acid 1.6

.Total C1C4 aliphatic acids 56.9

Distillation residues 4.6

' Example 3 The secondary light ends recovered in Example 2, which contained by analysis 0.30 equivalent/ 100 grams of 'saponiliable compounds and 0.91 equivalent/ 100 gram-s of carbonyl compounds, were further oxidised 'batchwise according to the method of Example 1 for the oxidation of light ends, at a temperature of 174 C. and 300 lbs/sq. in. pressure. The oxidation proceeded for v3.5 lhours 'after which the absorption of oxygen had vir- 'tually'ceased 'Ihe Aoxidation product was distilled, and the 'yields were as follows:

Grams/ 100 rams light The oxidation reactor is shown diagrammatically in lthe accompanying ligure and consists of a Vertical stainless steel tube 1 '2.6 inches in diameter and 6 feet high,

having air feed points 2 at the base and about the midpoint. Fresh `hydrocarbon is introduced continuously by lin'e 3 through aipreheater 13. Both gaseous and liquid reaction products are removed from the reactor by line w4, and'after coolingV in cooler 5 are fed to gas/liquid separator 6, from which the waste gases are withdrawn Vby line 7. The liquid products pass to liquid/liquid separator 8, where the lower aqueous acid product phaseis allowed to settleas a lower layer, part of this layer being withdraw'as product'throu'gh line 9. The upper hydrocarbon layer is returned to the reactor through preheater 10, with part of the lower separated layer. The whole aqueous acid product is fed by line 9 through a preheater 10A to the mid-point of a continuous distillation column 11 which is packed with JA inch rings and has the equivalent of 5 theoretical plates above and below the feed point, provided with a reboiler 11A and cooler 11B as shown. The column is operated with a reflux ratio of 4: 1, the temperatures being approximately 66 C. at the head, approximately 82 C. at the feed point, and 104- C. in the reboiler. The total distillate is returned by line 12 to the oxidation reactor after admixture with the fresh hydrocarbon feed. The base product from the still is withdrawn by line 14, and submitted to further distillation to recover the acids formed as main product.

During the steady oxidation period of hours, at a temperature of -175 C., and a pressure of 600 lbs./ sq. in. with a waste gas rate of 1800 litres/hour (as free gas containing 0-1% oxygen), a hydrocarbon feed consisting of a straight run gasoline fraction, B. P. 20-95 C., from Middle East petroleum, was fed into the reactor at the rate of 510 grams/hour. Light ends, recovered as distillate from the continuous still at the rate of 432 grams/hour, were fed back completely to the reactor and thus constituted 46% of the total feed to the oxidation reactor. No decrease in the rate of oxidation was noted during the oxidation, the production of acids of one to four carbon atoms remaining constant at S6 grams/hour! litre of reactor space. The base product from the continuous still was drawn off at a rate of 565 grams/hour, and containedabout 60% of acids of one`to four carbon atoms, practically all the remainder consisting of water and substances boiling above butyric acid. At the end of this experiment the aqueous product in theV reactor contained 36% w./w. of light ends, showing that no accumulation of light ends had occurred. The yield of acids of one to four carbon atoms, isolated by fractional distillation of the base product from the continuous still, was 66 parts per 100 parts of hydrocarbon consumed (including hydrocarbon removed in the waste gas stream). Of the acids of one to four carbon atoms recovered, about 75% by weight consisted of acetic acid.

By way of comparison, in oxidations carried out under similar conditions in the same oxidation vessel using the same hydrocarbon feed, but without recycle of the light ends to the oxidation, the corresponding yield of acids of one to four carbon atoms (of which about 70% by weight was acetic acid) was about 40 parts per 100 parts of hydrocarbon consumed.

We claim:

l. A process for the production of lower aliphatic acids vwhich comprises oxidising in the liquid phase a parafiin hydrocarbon of four to eight carbon atomswith molecular oxygento produce lower aliphatic acids and light ends, distilling the oxidation product to separate as light endsthe materials ,boiling below 99 C. in thev presence of water, admixing said light ends with a parafin hydrocarbon of four to eight carbon latoms, thereafter' oxidising in the liquid phase this mixture with molecular oxygen without substantial accumulation of light ends to produce lower aliphatic Vacids at a greater ratio of said acids to hydrocarbon consumed than in such oxidation of the hydrocarbon alone.

2. A process for the production of lower aliphatic acids as claimed in claim 1, wherein the oxidation product of the said mixture is distilled to separate the materials boiling below 99 C. in the presence of water, and thereafter admixing said materials with a paraffin hydrocarbon of four to eight carbon atoms and oxidising in the liquid phase with molecular oxygen to produce lower aliphatic acids.

3. A process for the production of lower aliphatic acids which comprises oxidising in the liquid phase a parain hydrocarbon of 4 to 8 carbon atoms with molecular oxygen to produce an oxidation product containing lower aliphatic acidsgand light ends, cooling the oxidation product to a temperature below C., to allow it to separateinto .two layers, returning the upper layer contain- 4ingf'unreaet'ed hydrocarbon to the oxidation zone, distilling the lower layer vto separate as light ends the materials boiling below 99 C. in the presence of water, mixing said, light ends with a parain hydrocarbon of 4 to 8 carbon atornsand thereafter oxidising in the Vacid which comprises oxidising in the liquid phase a paraffin hydrocarbon of four to eight carbon atoms with molecular oxygen to produce lower aliphatic acids, separating at least part of the oxidation product, distilling said separated oxidation product to separate as light ends the material boiling below 99 C., in the presence of Water, returning said light ends to the parain hydrocarbon supplied to the oxidation zone, thereafter oxidising in theliquid phase this mixture with molecular oxygen without substantial accumulation of light ends to produce acetic acid at a greater ratio of acid to hydrocarbon consumed than in such oxidation of the hydrocarbon alone, and continuously recovering the acetic acid from-the separated part of the oxidation product boiling above 99 C. in the presence of water.

5. A continuous process for the production of acetic acid as claimedin vclaim 4, wherein the separated oxidation product is cooled to a temperature below 80 C. and allowed to separate into two layers, the upper layer returned to the oxidation zone and at least part of the lower layer subjected to the said distillation process.

6. A continuouSprocess for the production of acetic acid as claimed in claim 4 wherein the light ends being returned to the oxidation zone are mixed with the hydrocarbon being employed, the aqueous phase formed, separated and discarded and the mixture of light ends and hydrocarbon passed to the oxidation zone.

7. A processV for fthe production of acetic acid as claimed in claim 4 wherein the oxidation is carried out at a temperature in the range 13D-200 C.

References Cited in the le of this patent UNITED STATES PATENTS

Claims (1)

1. A PROCESS FOR THE PRODUCTION OF LOWER ALIPHATIC ACIDS WHICH COMPRISES OXIDISING IN THE LIQUID PHASE A PARAFFIN HYDROCARBON OF FOUR TO EIGHT CARBON ATOMS WITH MOLECULAR OXYGEN TO PRODUCE LOWER ALIPHATIC ACIDS AND LIGHT ENDS, DISTILLING THE OXIDATION PRODUCT TO SEPARATE AS LIGHT ENDS THE MATERIALS BOILING BELOW 99*C. IN THE PRESENCE OF WATER, ADMIXING SAID LIGHT ENDS WITH A PARAFFIN HYDROCARBON OF FOUR TO EIGHT CARBON ATOMS, THEREAFTER OXIDISING IN THE LIQUID PHASE THIS MIXTURE WITH MOLECULAR OXYGEN WITHOUT SUBSTANTIAL ACCUMULATION OF LIGHT ENDS TO PRODUCE LOWER ALIPHATIC ACIDS AT A GREATER RATIO OF SAID ACIDS TO HYDROCARBON CONSUMED THAN IN SUCH OXIDATION OF THE HYDROCARBON ALONE.

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