The present invention pertains to a process for the preparation of monochloroacetic acid and an apparatus suitable for use in this process.

Chloroacetic acids can be obtained by reacting acetic acid with chlorine in a chlorination step, resulting in the fornnation of a chloroacetic acid stream and HCI as side product. Acetic anhydride is often added as catalyst. The resulting chloroacetic acid stream generally comprises a mixture of the desired product monochloroacetic acid (MCA) with "overchlorinated" products like dichloroacetic acid (DCA) and trichloroacetic acid (TCA). In particular, DCA may be present in the reaction product mixture in an amount of up to 6 wt%. To convert these "overchlorinated" products to monochloroacetic acid, the product of the chlorination reaction is often subjected to a hydrogenation step, in which the dichloroacetic acid and other overchlorinated compounds are converted to monochloroacetic acid. In the prior art this hydrogenation step is sometimes also referred to as dechlorination step. This results in the formation of a product stream comprising monochloroacetic acid and a further side product stream comprising HCI.

However, it was found that when subjecting the resulting chloroacetic acid stream to a dechlorination step, the presence of acid chlorides and/or acid anhydrides may interfere with the dechlorination step, creating a dark-colored product due to the formation of aldehydes, which give rise to the formation of condensation products. Further, excessive formation of aldehydes may cause fouling in the dechlorination reactor and downstream equipment. It also adds to the emission to the environment of the production site. Furthermore, it is desirable to reduce the use of acetic anhydride and acetyl chloride, which is used as a catalyst for the chlorination step. Therefore, prior to feeding the chloroacetic stream to the dechlorination step, a purification step has to be conducted wherein the acid chlorides and anhydrides, such as acid chlorides and anhydrides of monochloroacetic acid and acetic acid, are removed from the liquid chloroacetic acid stream.

CN104058947 describes a method for the production of monochloroacetic acid wherein the chloroacetic acid stream is subjected to distillation, followed by catalytic hydrogenation. In the distillation all the HCI leaves at the top. The bottom of the distillation does not contain water and HCI. Therefore, acid anhydrides are formed in the bottom of the distillation. The presence of these acid anhydrides interferes with the dechlorination step, creating a dark-colored product due to the formation of aldehydes, which give rise to the formation of condensation products. Further, excessive formation of aldehydes may cause fouling in the dechlorination reactor and downstream equipment. It also adds to the emission to the environment of the production site. Furthermore, these formed anhydrides are not recovered.

CN104649887 also describes a process for the production of monochloroacetic acid wherein the chloroacetic acid stream is subjected to vacuum distillation followed by a three stage condensation. This process has the same drawbacks as the process of CN104058947.

In WO 2013/057125 a process is disclosed for the purification of a liquid feed comprising monochloroacetic acid and dichloroacetic acid wherein a small amount of water is added to the liquid feed prior to the dechlorination. The liquid feed is subjected to a stripping operation using HCI gas to remove anhydrides and/or acid chlorides in the feed, prior to subjecting it to the water addition and subsequent dechlorination. In GB 928179 the hot off-gas from the chlorination reactor is fed to an absorber. The HCI gas from the chlorination is fed to a three stage absorption process. The first step comprises cooling the gas, the second step comprises a washing step with the acetic acid feed and a third step comprises a saturation of the acetic acid feed with HCI. This document describes a batch process.

In NL 109769 the HCI gas leaving the chlorination is fed directly to a stripper, where an atmospheric stripping process is conducted at 1 10°C. The HCI gas leaving the stripper is led directly to an absorber.

In US 4,281 ,184 a continuous chlorination process is described, with a stripping and an absorption section. The HCI gas from the chlorination is fed to the acetic acid absorber after a single step condensation step. The HCI gas from this acetic acid absorber is cooled to 15°C and fed to the stripper. After the stripper the HCI gas is also cooled to below 35°C and fed to the acetyl chloride absorber.

GB 727,074 is directed to the production of monochloroacetic acid, wherein gaseous chlorine is passed in to a mixture of glacial acetic acid and acetyl chloride or acetic anhydride. In order to inhibit formation of dichloroactic acid an ionizing agent is added to the reaction mixture. The acetyl chloride formed is distilled off using hydrogen chloride as a sweep at a temperature of between 80 and 120°C. Subsequently the acid chloride is recovered by absorption in glacial acetic acid. The residue of the reaction, containing ionizing agent and MCA, is subjected to fractional distillation.

There is need in the art for a process for the production of monochloroacetic acid which allows efficient dechlorination and which process is attractive from a technical, environmental and economic point of view. The present invention provides such a method.

The present invention pertains to a process for the chlorination of acetic acid to form monochloroacetic acid comprising the following steps

a. reacting acetic acid with chlorine using acetic anhydride and/or acetyl chloride as catalyst to form a liquid phase and a gaseous phase, - the liquid phase comprising at least monochloroacetic acid, acid chlorides, acid anhydrides and optionally un reacted acetic acid, and

- the gaseous phase comprising HCI and acetic acid, monochloroacetic acid, acid anhydrides and acid chlorides, b. stripping said liquid phase at a temperature of between 120 and 180°C and a pressure between 1 and 7 bara with gaseous HCI to form a gaseous HCI stream comprising acetic acid, acid chlorides and optionally acid anhydrides and monochloroacetic acid, and a liquid stream comprising monochloroacetic acid and dichloroacetic acid;

c. cooling the gaseous HCI stream comprising acetic acid, acid chlorides and optionally acid anhydrides and monochloroacetic acid to a temperature of between 10 and 60°C (preferably around 35°C), and d. feeding the cooled gaseous HCI to an absorber with acetic acid as absorbent absorbing the acetic acid, acid chlorides and optionally monochloroacetic acid and acid anhydrides, thus forming a stream comprising acetic acid and acid chlorides.

It was found that with the process according to the invention a monochloroacetic acid stream is formed comprising a very low amount of anhydrides. Furthermore, the acid chlorides are removed more effectively from the liquid phase than with other known methods such as distillation or with the method according to GB 727,074. Furthermore, the process according to the invention can be operated continuously.

In a preferred embodiment of the process the acetic acid and acid chlorides- comprising stream obtained in step d is re-directed to step a. Thus, with this process the catalyst is removed both from the liquid phase and the gaseous phase of the chlorination reaction and re-directed into the process. When redirecting the acetic acid and acid chlorides to the chlorination process a very economical consumption of catalyst is obtained.

It is noted that the term "acid chlorides" as used throughout the specification denotes acetyl chloride or a mixture thereof with chloroacetyl chloride and/or dichloroacetyl chloride. The term "(acid) anhydrides" as used throughout the specification denotes acetic acid anhydride, optionally mixed with one or more anhydrides selected from the group consisting of acetic acid anhydride, DCA anhydride, MCA anhydride, DCA-MCA anhydride, acetic acid - MCA anhydride and acetic acid - DCA anhydride.

In the process according to the invention the chlorination process of step a is preferably conducted in a chlorination reactor. Chlorination reactors are known in the art and need no further elucidation here.

Besides HCI, the gaseous phase of the chlorination comprises acetic acid, monochloroacetic acid, acid chlorides and acid anhydrides. Typically, the amounts of the components of the gaseous phase comprise, besides HCI, 5-15 wt% acetic acid, 1 -10 wt% monochloroacetic acid, 5-20 wt% acetyl chloride, 5- 15 wt% chloroacetyl chloride and 0.1 -0.2 wt% acid anhydrides.

In a preferred embodiment the gaseous phase from step a comprising HCI and acetic acid, monochloroacetic acid, acid chlorides and acid anhydrides is subjected to a condensation step, wherein at least part of the acetic acid, acid chlorides, acid anhydrides and monochloroacetic acid is removed from the HCI gas, resulting in purified HCI gas. Said condensation step may be a single condensation step, but also multi-condensation steps may be used. The condensation may take place in one or more heat exchangers. The acid chlorides, acetic acid, monochloroacetic acid and acid anhydrides will condense upon lowering of the temperature and may be re-directed to the chlorination step. This also contributes to the overall yield of the entire process. The HCI gas is at the same time purified by the removal of the acid chlorides, acetic acid, monochloroacetic acid and acid anhydrides and may be used in the stripping step. In the purified HCI gas less than 10 wt%, preferably less than 2 wt% or even less than 1 wt% of acetyl chloride is present and less than 0.5 wt%, preferably less than 0.1 wt%, in some embodiments less than 0.05 wt% of acetic acid, monochloroacetic acid, chloroacetyl chloride and acid anhydrides is present.

The stripping of the liquid phase is conducted by guiding HCI gas through the liquid phase. The acid chlorides, acid anhydrides and optionally un reacted acetic acid present in the liquid phase are removed from the liquid phase by the HCI gas. In fact, the concentration of acid anhydrides is lowered per se by the presence of the hydrogen chloride, because of the change of equilibrium. Said stripping step may be accomplished by simply bubbling the HCI gas through the liquid product stream, but preferably the product stream is fed through a gas stripper.

Stripping is a physical separation process where one or more components are removed from a liquid stream by a vapor stream. In industrial applications the liquid and vapor streams can have co-current or countercurrent flows. Stripping is usually carried out in either a packed or a trayed column. Stripping is mainly conducted in trayed towers (plate columns) and packed columns, collectively referred to as stripping towers, and less often in spray towers, bubble columns and centrifugal contactors.

Trayed towers consist of a vertical column with liquid flowing in at the top and out at the bottom. The vapor phase enters in the bottom of the column and exits out of the top. Inside the column there are trays or plates. These trays force the liquid to flow back and forth horizontally while the vapor bubbles up through holes in the trays. The purpose of these trays is to increase the amount of contact area between the liquid and vapor phases.

Packed columns are similar to trayed columns in that the liquid and vapor flows enter and exit in the same manner. The difference is that in packed towers there are no trays. Instead, packing is used to increase the contact area between the liquid and vapor phases. Many different types of packing are used and each one has advantages and disadvantages.

As mentioned previously, stripping towers can be trayed or packed. Packed columns, and particularly when random packing is used, are usually favored for smaller columns with a diameter less than 0.6 m and a packed height of not more than 6 m. Packed columns can also be advantageous for corrosive fluids, high foaming fluids, when fluid velocity is high, and when particularly low pressure drop is desired. Trayed strippers are advantageous because of ease of design and scale up. Structured packing can be used similar to trays despite possibly being the same material as dumped (random) packing. Using structured packing is a common method to increase the capacity for separation or to replace damaged trays.

Trayed strippers can have sieve, valve, or bubble cap trays while packed strippers can have either structured packing or random packing. Trays and packing are used to increase the contact area over which mass transfer can occur as mass transfer theory dictates. Packing can have varying material, surface area, flow area, and associated pressure drop. Older generation packing include ceramic Raschig rings and Berl saddles. More common packing materials are plastic Pall rings and ceramic Intalox saddles. Each packing material of this newer generation improves the surface area, the flow area and/or the associated pressure drop across the packing. Also important is the ability of the packing material to not stack on top of itself. If such stacking occurs, it drastically reduces the surface area of the material.

Conventional stripping towers may be used in the process according to the invention. They are known in the art and need no further elucidation here.

In conventional processes where a stripping step is applied in order to remove acid chlorides, such a stripping step is typically conducted at temperatures of between 80 and 120°C, at atmospheric pressure. The temperature range of between 80 and 120°C is a logical choice for a skilled person as the boiling point of acetyl chloride is 52°C.

However, we have found that it has an advantage to conduct the stripping at a temperature of between 120 and 180°C and a pressure between 1 and 7 bara so as to form a gaseous HCi stream comprising acid chlorides and optionally acetic acid. By conducting the stripping step at a temperature of at least 120 °C, preferably, at least 122°C, more preferably at least 125°C, and most preferably at least 130°C, it is ensured that any chloroacetyl chloride, if present, is also removed from the liquid phase.

Preferably, the stripping step is conducted at a temperature of at most 180°C, more preferably 170°C, and most preferably 160°C. The temperature required for the stripping step may be obtained by heating the HCI with steam. In a preferred embodiment, in step b, the liquid phase is stripped at a pressure between 1 and 7 bara with gaseous HCI having a temperature of between 120 and 180°C to form a gaseous HCI stream comprising acetic acid, acid chlorides and optionally acid anhydrides and monochloroacetic acid, and a liquid stream comprising monochloroacetic acid and dichloroacetic acid. It is also possible to recycle the bottom product stream over a steam heater in order to obtain the required temperature in the stripping step. The liquid stream formed in the stripper comprising monochloroacetic acid and dichloroacetic acid may be fed directly to the dechlorination step.

In a preferred embodiment the stripping step is performed at a pressure of at least 1 .6 bara, preferably at least 3 bara, more preferably at least 4 bara and most preferably at least 5 bara. Preferably, the stripping step is performed at a pressure not higher than 10 bara, more preferably not higher than 8 bara. The advantage of performing this stripping step under elevated pressure is that due to the higher pressure in the stripper column, more HCI will dissolve in the bottom product of the stripper column, thus preventing the formation of anhydrides from acid chlorides. As a result, the liquid stream formed in the stripper comprising monochloroacetic acid and dichloroacetic acid comprises less anhydride or even no anhydride at all. This is an advantage because, as already described, said liquid stream is now suitable for being fed directly to the dechlorination step. Subsequently, the gaseous stream coming from the stripper is cooled to a temperature of between 10 and 60°C, preferably around 35°C, and fed to an absorber using acetic acid as the absorbent. In the absorber, acid chlorides are absorbed in the acetic acid. The HCI gas leaves the absorber at the top and may be fed to a HCI purification and absorption section.

For the absorption step an absorption tower may be used. Absorption towers are also packed or plate columns as described above and are known in the art. Absorption towers and need no further elucidation here.

At least part of the liquid stream leaving the bottom of the absorber, which comprises acid chlorides and chloroacetic acid, is preferably fed to the chlorination step. The optimal removal of this catalyst from both the liquid phase and the gaseous phase of the chlorination reactor creates a process with a very economical consumption of catalyst, a high yield of monochloroacetic acid and a monochloroacetic stream stripped of components that may interfere with the dechlorination step.

The present invention is further directed to an apparatus which is suitable for the process according to the invention. The apparatus according to the invention comprises:

- a chlorination reactor with at least two inlets, an upper outlet and a lower outlet,

- a stripping tower with an upper inlet and a lower inlet, an upper outlet and a lower outlet, which upper inlet is directly or indirectly connected with the lower outlet of the chlorination reactor,

- at least one condenser with an upper inlet, a lower outlet for gas and a lower outlet for liquid, which lower outlet for liquid is connected with the upper inlet of the chlorination reactor and which lower outlet for gas is directly or indirectly connected with the lower inlet of the stripping tower,

- a cooler with an upper inlet and a lower outlet

- an absorption tower with a lower inlet and an upper outlet, an upper inlet and lower outlet, which lower inlet is directly or indirectly connected with the upper outlet of the stripping tower and which lower outlet is directly or indirectly connected with the inlet of the chlorination reactor wherein the condenser and absorption tower are disposed above the chlorination reactor and the stripping tower is located below the chlorination reactor or at approximately the same height as the chlorination reactor.

The present invention will be further elucidated by Figure 1 , which gives a schematic view of an apparatus which is used to perform one of the embodiments of the process according to the invention. Figure 1

This figure gives a schematic view of an apparatus which is used in the process according to the invention. To a chlorination reactor (1 ) with an inlet for chlorine (2) and an inlet for the catalyst and acetic acid (3), the reactants chlorine, acetic acid and acetyl chloride and/or acetic anhydride are led. The resulting gaseous stream comprising HCI, acetic acid, acid chlorides and acid anhydrides and optionally monochloroacetic acid is led via an upper outlet (4) to a condenser (1 1 ). In the condenser (1 1 ) acetic acid, acid chlorides, acid anhydrides and any monochloroacetic acid are condensed and recycled via outlet 14 to the chlorination reactor (1 ). The purified HCI is led to the lower inlet (12) of the stripper (6).

The resulting liquid stream comprising monochloroacetic acid, dichloroacetic acid, acid chlorides, acid anhydrides and optionally un reacted acetic acid is led via a lower outlet (5) to the upper inlet (7) of a stripping tower (6) which is fed with HCI gas via lower inlet (12). In the stripping tower (6) a liquid stream is formed which comprises monochloroacetic acid, dichloroacetic acid. Said liquid stream is led via outlet 9 for further processing in the dechlorination unit. The resulting liquid stream comprises less than 0.1 wt% acid anhydrides and less than 0.1 wt% acid chlorides, based on total liquid stream. By carefully controlling the pressure in the stripper the presence of any acid anhydride in the resulting liquid stream can be avoided altogether. For instance, at a temperature of 155°C and a pressure of 3.2 bara, no acid anhydride was present in the resulting liquid stream.

The gaseous stream formed in the stripper comprises hydrogen chloride, acid chlorides and traces of acid anhydrides. Said gaseous stream is led to a cooler (9) via the upper outlet (8) of the stripper (6). In the cooler (9) the gaseous stream is cooled to a temperature between 10 and 60°C (preferably around 35°C). Said cooled stream is fed to an absorber (10) with an acetic acid inlet and a HCI outlet (13), wherein the acid chlorides and acetic anhydrides are absorbed in acetic acid and combinedly fed to the chlorination reactor. The recovery rate of the catalyst is thus more than 99 wt% in the process according to the invention.

As is clear from the schematic view, the condenser and absorption tower are disposed above the chlorination reactor and the stripping tower is located below the chlorination reactor, so as to make optimal use of the effect of gravity on the flow of materials, reducing the complexity of the plant.