Process for producing unsaturated carboxylic acids

Acrylic acid or other unsaturated carboxylic acid is made by hydrolysing the corresponding nitrile in the presence of excess sulphuric acid to form amide sulphate, adding amide in an amount such that the total amount of amide and nitrile fed to the process is greater than 1 mole per mole sulphuric acid, hydrolysing the amide sulphate and amide to the desired acid and separating the acid from the sulphate by-products.

This invention relates to a process for producing an unsaturated carboxylic 
acid, by hydrolysis of a corresponding nitrile. For instance (meth) 
acrylonitrile can be hydrolysed by the process to (meth) acrylic acid. 
It is well known that unsaturated nitriles and unsaturated amides can be 
hydrolysed by reaction with water in the presence of a hydrolysis reagent 
to form unsaturated acids. 
In Chemical Abstracts 72:89793T a nitrile is hydrolysed in an acid salt 
melt containing water, the melt being formed in an example of 24 parts 
potassium bisulphate, 28 parts sodium bisulphate and 1 part ammonium 
sulphate. Handling the melt is inconvenient and normal processes are 
conducted in aqueous solution. Thus in Chemical Abstracts 86:89183H an 
amide or nitrile is hydrolysed in an aqueous solution of an ammonium salt 
as the hydrolysis reagent. In particular, the use of ammonium sulphate 
gave 32% conversion of acrylamide and other ammonium salts that were used 
were ammonium chloride, bromide, bisulphate and nitrate. In Chemical 
Abstracts 92:93905K tin IV compounds are used as the hydrolysis reagent. 
In Chemical Abstracts 86:71925A a Be salt is used. 
Despite these various proposals, sulphuric acid has become accepted as 
being particularly suitable for use as the hydrolysis reagent. For 
instance in Chemical Abstracts 55:2485G 1 mole acrylonitrile is reacted 
with 1 mole sulphuric acid and 2 moles water. The conversion of a nitrile 
to the acid by this reaction goes in two stages, with the formation in the 
first stage of an amide sulphate and the hydrolysis of this in the second 
stage to the carboxylic acid, with ammonium bisulphate as a by-product. 
It has been well accepted that it is desirable to use excess sulphuric 
acid, in order that the reaction proceeds at a satisfactory rate and 
yield. For instance Chemical Abstracts 87:185002Q reports an investigation 
on the kinetics and recommends 1.17 moles sulphuric acid per mole 
methacrylamide, and 1.5 to 2.5 moles sulphuric acid per mole acrylamide 
sulphate are used in Chemical Abstracts 99:71293C. 
It is necessary to separate the (meth) acrylic acid or other desired 
product from the reaction mixture containing ammonium bisulphate and it 
can be difficult to achieve perfect separation. In Chemical Abstracts 
83:79933B and Japanese Unexamined Application 49-135917the reaction 
mixture is subjected to phase separation to separate a methacrylic 
acid-rich layer from a layer containing 40 to 50% sulphuric acid, 10 to 
20% ammonium bisulphate, 30 to 40% water, 1 to 2% methacrylic acid and 0.4 
to 0.6% methacrylamide. This layer is distilled to recover the small 
amount of methacrylic acid and methacrylamide, which is recycled back to 
the hydrolysis stage. This recycled acrylamide will thus constitute a very 
small amount of the total charge in that stage. 
In Chemical Abstracts 91:158321U acetone cyanohydrin is reacted in aqueous 
solution with more than 1 mole sulphuric acid to form acrylamide sulphate 
in the presence of unreacted sulphuric acid, and further water is added 
and the mixture hydrolysed to form methacrylic acid. In Chemical Abstracts 
83:44010W a similar process is described starting from methacrylonitrile, 
with the final product being subjected to phase separation and recycling 
of the small amount of distillate. 
Although processes that start from nitrile and excess sulphuric acid can be 
operated conveniently to give a good yield of acrylic acid or other 
unsaturated acid end product they suffer from a number of disadvantages. 
In particular, they produce a large volume of ammonium bisulphate as a 
by-product. The inconvenience of producing these large quantities is 
increased by the fact that it is contaminated with a significant amount of 
sulphuric acid. It is therefore necessary either to dump large volumes of 
this by-product (which is environmentally difficult) or to convert it to a 
usable material, for instance by reaction with aqueous ammonia to form 
ammonium sulphate, which can then be used as a fertiliser. However the 
amount of ammonia that is required is very high, due partly to the 
presence of the sulphuric acid. 
Another difficulty is that the presence of free sulphuric acid tends to 
result in significant levels of sulphur dioxide in the final acrylic acid 
or other carboxylic acid stream, and this is undesirable. Another 
difficulty is that it is, in conventional manner, necessary to include a 
polymerisation inhibitor throughout the reaction but the excess sulphuric 
acid tends to interact with the inhibitor and so large amounts of the 
inhibitor, which is generally rather expensive, have to be added in order 
to maintain adequate levels of inhibition. 
It would be desirable to minimise these problems whilst maintaining the 
good reaction rates and yields associated with the use of excess sulphuric 
acid as the hydrolysis reagent. 
In the invention, an ethylenically unsaturated acid of the formula R.sup.1 
C.dbd.CR.sup.2 COOH, in which R.sup.1 is H, alkyl or aryl and R.sup.2 is H 
or CH.sub.3, is made from the corresponding nitrile by a process 
comprising feeding the nitrile to a reaction vessel and reacting it in 
aqueous solution in that vessel with water in the presence of an amount of 
sulphuric acid that is above 1 mole per mole nitrile and thereby forming a 
first reaction product containing the corresponding amide sulphate, 
feeding the corresponding amide to the first reaction product in an amount 
such that the total amount of amide and nitrile fed to the process is 
above 1 mole per mole sulphuric acid, hydrolysing the amide and amide 
sulphate by reaction in aqueous solution with water and thereby forming 
the corresponding acid and a sulphate by-product, and separating the 
corresponding acid from the sulphate by-product. 
In the final acid, R.sup.1 may be hydrogen, alkyl (usually C1-8 and 
preferably methyl) or aryl, preferably phenyl. R.sup.2 may be hydrogen or 
methyl. By referring to the "corresponding" amide and nitrile we mean that 
the values of R.sup.1 and R.sup.2 in the nitrile, amide and amide sulphate 
are such that the desired values of R.sup.1 and R.sup.2 are obtained in 
the final acid. Generally the values of R.sup.1 and R.sup.2 in the 
nitrile, amide, amide sulphate and acid are identical. However it is also 
possible to start with, for instance, a nitrile that is saturated but 
which will form the desired unsaturated acid as a result of desaturation 
occurring during the process. For instance acetone cyanohydrin is a 
saturated nitrile but will behave during the process as methacrylonitrile 
due to the end group being desaturated, to reveal the methacrylic group, 
during the process. The most preferred process uses acrylonitrile and 
acrylamide or methyacrylonitrile and methacrylamide as the starting 
materials for the production of acrylic or methacrylic acid. The preferred 
process uses acrylonitrile and acrylamide to make acrylic acid. 
The amount of sulphuric acid in the first stage must be an excess over the 
amount of acrylonitrile in order to force the reaction to a satisfactory 
yield. Generally it is at least 1.05 moles sulphuric acid (per mole 
nitrile). There is no critical upper limit on the amount of sulphuric acid 
but increasing the amount does increase the amount of sulphate by-product, 
and so it is desirable to avoid too large an excess. Generally there is no 
advantage in using more than 1.5 moles and best results are usually 
obtained with around 1.1 to 1.3 moles, preferably about 1.2 moles, per 
mole nitrile. 
The amount of amide that is added to the reaction product must be such that 
the sulphate by-product is substantially free of sulphuric acid and thus 
the total amount of nitrile and amide fed into the process is preferably 
at least 1.01 moles per mole sulphuric acid that is fed into the process. 
If too much amide is added then again this increases the amount of 
by-product, increases the amount of ammonium sulphate and the 
consequential risk of crystallisation, and it also has the disadvantage of 
tending to slow down the reaction and of making it more difficult to 
separate the carboxylic acid from the amide and other by-products. The 
total amount of nitrile and amide is therefore usually not more than 1.7 
moles, preferably not more than 1.5 moles, per mole sulphuric acid. 
Generally it is in the range 1.1 to 1.5 moles. It is generally preferred 
that the amount of amide should be from about 0.05 to 0.5, preferably 
about 0.1 to 0.4, moles above the difference between the molar proportions 
of sulphuric acid and nitrile. Thus when, as is often preferred, the 
amount of sulphuric acid is 1.2 moles per mole nitrile, the difference 
between the molar proportions of sulphuric acid and nitrile is about 0.2 
and the preferred amount of amide is then about 0.3 to 0.6 moles, most 
preferably about 0.4 moles. 
It is very surprising that, contrary to all the suggestions in the 
literature, it is possible to obtain very good conversion of nitrile in 
aqueous solution to acid, through the amide sulphate, even though the 
overall process is deficient in sulphuric acid. 
The process has a number of major advantages. One is that the total amount 
of by-product is less, per mole of carboxylic acid produced, than is 
obtained in the absence of amide addition. Another is that the by-product 
is not only produced in a smaller amount but is also more convenient to 
handle and use. In particular, in the conventional process the by-product 
consisted almost entirely of ammonium bisulphate and sulphuric acid 
whereas now it consists substantially of ammonium bisulphate and sulphate, 
and is substantially free of sulphuric acid. It is therefore more 
convenient to handle, is environmentally less sensitive, and has a lower 
ammonia demand if, as is often preferred, it is desired to convert it 
entirely to ammonium sulphate by reaction with aqueous ammonia. 
Furthermore the absence of free sulphuric acid in the second reaction 
mixture reduces the likelihood of sulphur dioxide being formed by reaction 
of sulphuric acid with other components in the process. There is less 
tendency for sulphur dioxide to be driven off with and contaminate the 
acid product. 
Free sulphuric acid also tends to react with the preferred polymerisation 
inhibitors, (paramethoxyphenol or methylene blue) during the reaction, so 
that in the process of the invention less of the inhibitor needs to be 
added (as compared to the conventional process) to produce the same amount 
of inhibitor in the final product (in the case of paramethoxyphenol) and 
throughout the reaction. This saving on inhibitor further increases the 
efficiency of the process. 
In the first step of the process, excess sulphuric acid is used in order to 
drive the reaction to the intermediate amide sulphate as fast as possible. 
Amide is then added in an amount which exceeds the excess of sulphuric 
acid from the first step, the resultant mixture is then hydrolysed and the 
product acid removed. Since free sulphuric acid is not present in this 
second stage of the reaction, it seems that the hydrolysis is being 
promoted by the by-product of the reaction, probably ammonium bisulphate. 
The by-product of the reaction comprises a mixture of ammonium sulphate and 
ammonium bisulphate, the proportions of which depend upon the relative 
quantities of nitrile, amide and sulphuric acid used in the reaction. 
The by-product is preferably removed from the reactor as an aqueous 
solution of its components. Ammonium bisulphate is relatively soluble in 
water but the other by-product ammonium sulphate is not highly soluble at 
low temperatures. Since it is preferred to store the by-products for 
further use or disposal as solutions, it may therefore be necessary to 
store them at raised temperatures and/or to dilute them with additional 
water. In general it is preferred that the solids content of the 
by-product solution is diluted to a concentration of less than 90% by 
weight, more preferably less than 80 or 70% by weight of the total 
solution. Since ammonium sulphate is much less soluble than ammonium 
bisulphate it is preferable that the amount of ammonium sulphate, based on 
the total of ammonium bisulphate and ammonium sulphate, is less than 50% 
by weight, although advantageously at least 5 or 10% by weight. 
The temperature of reaction of sulphuric acid, water and nitrile in the 
first step, for instance when the starting material is acrylonitrile, is 
preferably maintained in the range 90.degree. C. to 100.degree. C., more 
preferably 95.degree. C. to 98.degree. C., when the pressure is 
approximately atmospheric. Sufficient water should be present to allow the 
hydrolysis to proceed. There can be excess but it is usually satisfactory 
to have an amount of water that is substantially equimolar to the amount 
of nitrile. 
In the second stage, the water for the hydrolysis reaction may all be 
provided in the form of solvent for the reaction product from the first 
stage and/or solvent for the amide that is added to the reaction product, 
or may be added as liquid water. Preferably however some at least of the 
water is generally supplied as steam to the reactor vessel, suitably below 
the surface of the liquid in the reactor vessel. The steam may also serve 
to help drive the acrylic acid product from the reaction mixture. Thus 
excess steam may be blown through the first reaction product in an amount 
such that some of the steam passes through the product and strips the 
corresponding acid from the mixture. 
It may be advantageous to aid removal of the acrylic acid by the 
application of further steam into the vapour space above the liquid in the 
reactor vessel. 
For optimum reaction and removal of acrylic or other acid as the product, 
the temperature in the reactor vessel should be kept within the range 
160.degree. C. to 190.degree. C., preferably in the range 165.degree. C. 
to 175.degree. C. when the pressure is approximately atmospheric. At lower 
temperatures acrylic acid as the product is not distilled completely from 
the mixture and is removed with the liquid by-products. At higher 
temperatures it becomes difficult to prevent polymerisation of the 
ethylenically unsaturated compounds present in the reaction mixture. 
Although the reaction steps may be carried out at pressures other than 
atmospheric pressure, it is in general found to be satisfactory to operate 
around atmospheric pressure. Sometimes it may be useful to increase the 
pressure with a consequential rise in temperature, e.g. in the first step, 
to speed up the reaction. Sometimes it may be advantageous to operate the 
hydrolysis reaction at a reduced pressure in order to aid removal of the 
acid and/or to allow the acid to be removed at a temperature of which 
polymerisation is minimised. 
Both steps of the reaction process are exothermic. In order to keep the 
reaction temperature in the second step down in the conventional process 
it was often necessary to add water to the reactor. A further advantage of 
the present invention is that that any such water has effectively been 
replaced by amide solution. 
The hydrolysis and steam stripping of the amide sulphate and amide mixture 
is preferably conducted in a first hydrolysis reactor and the liquid 
residue from that may be taken to a second hydrolysis reactor in which 
amide and amide sulphate residues in that liquid may be subjected to 
hydrolysis. In particular, excess steam may be blown through the liquid in 
this second reactor in an amount such that some of the steam passes 
through the reactor and strips carboxylic acid from the reactor. This 
stream of steam from the second reactor advantageously is then recycled as 
all or part of the supply of steam to the first reactor. 
Generally the reaction between the nitrile and water in the presence of 
sulphuric acid will have been conducted initially in a separate reactor 
with the reaction product from that being fed to the primary hydrolysis 
reactor. 
It will be appreciated that although the process can be carried out as a 
batch process it is particularly suitable for carrying out as a continuous 
process. Thus the reactants are continuously fed to the reaction vessels 
and product acid and by-products are continuously removed from the 
reaction mixture. 
When the acid product is driven off from the liquid reaction mixture and 
distilled over as a mixture with steam, the vapour may be cooled and 
condensed after which it may be further purified or concentrated or 
diluted, depending upon the desired end use. 
In order to prevent polymerisation during the process a polymerisation 
inhibitor needs to be included in the mixture at all stages of the 
reaction. Conventional polymerisation inhibitors may be used. Different 
inhibitors may be used in different stages of the process. The preferred 
inhibitors include methylene blue and paramethoxyphenol. 
The following examples illustrate the invention:

COMATIVE EXAMPLE 
In a pilot trial of a continuous process, acrylonitrile (10k mole/hr), 
inhibitor, and 87% aqueous sulphuric acid (12 k mole/hr H.sub.2 SO.sub.4 
+10k mole/hr H.sub.2 O ) are reacted together in a stirred reactor, the 
temperature in the reaction mixture being maintained in the range 
95.degree.-98.degree. C. The product mixture, comprising acrylamide 
sulphate (10k mole/hr) plus excess sulphuric acid (2k mole/hr), is then 
fed into a stirred primary hydrolysis reactor where the liquid is 
maintained at about 175.degree. C. by the addition of water (approx. 50 
kg/hr). 
Steam at temperature of 165.degree. C. for hydrolysis and stripping is 
supplied to the liquid in the primary hydrolysis reactor by introduction 
under the surface at a rate of about 370 kg/hr. Acrylic acid/steam product 
mixture (about 60% by weight acid) is driven from the reactor at a rate of 
about 8.7k mole/hr of acid. It is contaminated with from 100 to 300 ppm 
sulphur dioxide. 
The reactor contents comprising mainly ammonium bisulphate and sulphuric 
acid are fed to a stirred secondary hydrolysis reactor which is maintained 
at a temperature of 165.degree. C. Steam is added to this reactor below 
the surface of the liquid at a rate of about 370 kg/hr. A mixture of steam 
with further acrylic acid (approx. 34 kg/hr) is distilled off the mixture 
in the secondary reactor and passed back below the liquid surface of the 
primary reactor as the source of steam for the hydrolysis and stripping. 
The liquid by-product mixture comprising about 10k mole/hr ammonium 
bisulphate and 2k mole/hr sulphuric acid plus small amounts of acrylic 
acid and polymer is then diluted in-line with about 550 kg/hr water and 
stored for disposal. The ratio of the total dry weight of sulphate 
by-product (ammonium bisulphate and sulphuric acid) to dry weight acrylic 
acid is 2.15:1. 
EXAMPLE 
The process of the comparative example is repeated with the same feeds of 
acrylonitrile, sulphuric acid and water. As the characterising step of the 
reaction, the water feed to the primary hydrolysis reactor is replaced by 
a 50% aqueous solution of acrylamide at a rate of 2.9k moles/hr. 
Acrylic acid is recovered at a rate of about 11.2k moles/hr as a 60% 
mixture with water, and is contaminated with only about 10-15 ppm sulphur 
dioxide. 
The liquid by-product mixture contains 11.1k mole/hr ammonium bisulphate 
and 0.9k mole/hr ammonium sulphate plus small amounts acrylic acid and 
polymer. The ratio of the total dry weight of sulphate by-product 
(ammonium bisulphate and ammonium sulphate) to dry weight of acrylic acid 
product is 1.73:1. 
Thus the yield of by-product, per unit weight acrylic acid, is 
substantially less and the by-product is less acidic and the amount of 
sulphur dioxide in the acrylic acid product is much less. A further 
advantage is that the amount of p-methoxyphenol that had to be added to 
the primary hydrolysis reaction to give a concentration in the product 
acid in the range 200-300 ppm was reduced in the Example of the process 
according to the invention by 60% as compared to the Comparative Example.