The present invention relates to a process for the separation of water and acetic acid in the presence of an immiscible organic dimethylbenzene compound such as -xylene or m-xylene and methyl acetate and apparatus therefor.

Typically crude terephthalic acid is produced by the oxidation of ^-xylene. The oxidation is generally conducted using acetic acid as solvent in the presence of a catalyst. The solution is then cooled in a stepwise manner to crystallise the terephthalic acid. Since water is produced in the reaction and acetic acid is used as the solvent, the major portion of the reactor overheads is acetic acid and water. There is therefore a need for an acetic acid/water separation system. Since large quantities of vapour are produced, small amounts of -xylene become entrained in the overhead and are carried forward to the distillation system.

A similar process is used for the preparation of isophthalic acid by the oxidation of w-xylene. In this process, small amounts of m-xylene become entrained in the overhead in the separation and are carried forward to the distillation system.

Fractional distillation of water and acetic acid, referred to as binary distillation, is a well known and well established procedure. In the context of the process for preparing pure aromatic dicarboxylic acids such as terephthalic acid it is desirable to obtain a water product in the distillation column that has as low a concentration of acetic acid as possible. This is because any acetic acid leaving the column in the overheads will typically be lost to effluent and thus represents an economic loss of solvent. The difficulty of achieving a pure water stream is illustrated by the x-y diagram for the acetic acid/water system of Figure 1.

From Figure 1 it can be seen that the x-y curve begins to pinch at high water purities. This means that a large number of stages are required in the distillation column and a large reflux will be required to achieve the desired low acetic acid content in the column overheads. This can result in a large duty in the reboiler.

In view of the problems associated with the separation, it has become commonplace to use entrainers to improve the separation by means of heterogeneous azeotropic distillation. Numerous different entrainers have been proposed including i-butyl acetate, n-propyl acetate and n-butyl acetate. These all offer the ability to separate acetic acid from water with fewer stages, less reflux, and hence less duty, than the binary equivalent. Depending on the entrainer selected and the column design, in theory entrainer based systems can require up to half the duty, in the form of steam, than their binary equivalents. Thus binary systems are considered uneconomic. For this reason commercial systems for the production of /j-xylene typically include, as standard, an heterogeneous azeotropic based distillation system using an entrainer for the water/acetic acid separation.

The idea of using entrainers is well known and has been described since the 1930s. US 1917391 and US 2050234 are examples of early processes for the recovery of acetic acid from water which use entrainers. These examples are not directed to the production of terephthalic acid production as they pre-date the commercialisation of the technology. However, they do illustrate that the use of entrainers to address separation difficulties have been utilised for many years. As acknowledged in GB 1576787, there are problems associated with the use of entrainers. The solution proposed in GB 1576787 is to control the temperature in the column by varying the feed rate thereto.

It was not until the 1990s that entrainers were adopted for use in processes for the production of terephthalic acid. One example of the application of a heterogeneous azeotropic distillation in the terephthalic acid production process is described in WO 96/06065. Specific entrainers are identified and methods for controlling the column by means of organic and aqueous reflux are discussed. A similar arrangement is discussed in W098/45239. Here the temperature of the condenser is controlled to keep the methyl acetate ester in the vapour phase so that it can pass through the reflux drum/decanter as a vapour to a downstream stripping column.

The biggest issue for the pure terephthalic acid process is that the column does not just contain acetic acid and water. Methyl acetate and -xylene will also be present which can result in very unstable column operation. CA 2580951 recognises that the presence of the p- xylene can be a problem as, due to its volatility, it tends to build up in concentration within the column, and affects the acetic acid/water separation. The solution proposed in CA 2580951 is to provide a side draw from the column that will contain the p-xylene. This p- xylene is then removed by distillation with recovered -xylene-free stream being injected back into the column. An alternative arrangement is discussed in US2007/027340. In this the -xylene itself acts as the entraining agent in the acetic acid/methyl acetate/water separation in a distillation column. Since a large quantity of -xylene is present, the overhead vapour from the distillation column will contain approximately the same amount of -xylene as water and hence when condensed, these will separate into two layers. The first layer will be an organic, /^-xylene, rich layer and the second layer will be an aqueous, water, rich layer. The majority of the organic layer is recycled to the distillation as reflux. A minor amount of the aqueous stream is also recycled to assist in the control of the column. Thus the reflux drum acts as a decanter.

A further difficulty associated with the entrainers generally used in the process is that they have a tendency to hydrolyse in the column which impacts on the separation and results in entrainer losses. In WO 2009/01 3623 a process is described which aims to minimise the entrainer loss by taking a side draw, which should contain a portion of the hydrolysed entrainer, from high up on the column and injecting it into a lower section of the column to promote conversion of the hydrolysed entrainer back to the original entrainer.

Thus much of the development of the art to address the problems associated with separating the water/acetic acid mixture has focussed on the use of entrainers. However, as indicated above, the use of entrainers gives rise to various problems. Indeed a typical column using entrainers will form numerous azeotropes. In particular, an azeotrope will be formed between the water and the entrainer. This azeotrope is desired as it allows an easier separation of the acetic acid and water. However, the entrainer can also form an azeotrope with acetic acid. This means that the column must be carefully controlled to prevent entrainer slipping into the bottoms or acetic acid into the top product. The problem is particularly acute in the process relating to the preparation of terephthalic acid due to the presence of methyl acetate and /?-xylene.

In the current entrainer based systems it is desirable to prevent the /?-xylene from entering into the column overheads. This is because its presence will increase the acetic acid concentration in the overheads as well as forcing entrainer into the column bottoms which represents a process loss. It is also desirable not to allow the / -xylene into the column bottoms as the configuration of modern terephthalic acid plants would allow quantities of p- xylene to be lost if it exits in the column bottoms. As seen from CA 2580951 a side draw can be taken from the column to minimise the problem. However, this results in the need for another distillation column to separate the -xylene from the rest of the column material. It can also be difficult to ensure that the side draw is in the correct position. A further problem is that the amount removed in the side draw has to be carefully monitored so as not to upset the operation of the column.

The methyl acetate present is a product of the oxidation reaction and represents conversion of acetic acid. In modern terephthalic acid plants methyl acetate can be recovered and returned to the oxidation reactor to inhibit further conversion of acetic acid. If methyl acetate is allowed into effluent streams it represents a process loss of acetic acid thus driving up production costs. In the acetic acid/water distillation system, the methyl acetate is the most volatile component and is therefore present in the highest concentration in the column overheads. This lowers the overheads temperature and can interfere with the ability of the entrainer/water azeotrope to leave in the column overheads. This requires careful monitoring of the overheads temperature and the amount of reflux provided to the column. Examples of systems in which methyl acetate is recycled are those described in US2006/0094901 and EP 0973717.

Since the entrainer is present in the column overheads, another column is required to separate the entrainer from the water such that the entrainer can be recycled back to the main column. This therefore adds to the capital and operating costs of the process.

Acetate based entrainers are commonly selected. The use of acetates in the presence of water can result in hydrolysis to the alcohol and acid. This is discussed in WO2009/013623. Whilst the proposed solution may address the problems, the arrangement requires a complicated column operation as it has the potential to de-stabilise column operation relatively quickly.

It will therefore be seen that in the azeotropic based systems, the reflux drum acts as a decanter and the incoming condensate from the overheads condenser is generally separated into two layers. There are then two liquid exit streams from the decanter, one which contains mostly the organic entrainer, i.e. the organic phase, the majority o f which will be returned to the distillation co lumn as reflux, and an aqueous phase which may be partial ly refluxed to the column to maintain co lumn control but the majority of which will be passed to another column to separate this stream into three main streams, a water stream comprising more than 99% water, a methyl acetate stream, comprising more than 95% methyl acetate, and an entrainer rich stream, which is recycled back to the reflux drum or the main column.

The amount of the organics present in the overhead vapour will generally be dependent on the azeotropic entrainer used but can be from about 55 wt% where the entrainer is p-xylene up to about 70 to 80 wt% for an acetate based entrainer.

It can be argued that the decision to select an entrainer-based system over the binary system is primarily based on duty since entrainer-based systems are more complex in terms of operation and require significantly more equipment. Therefore for a binary system to compete with an entrainer-based design, it must have an overall duty similar to that of the entrainer-based systems and have a way of handling the dimethylbenzene and methyl acetate.

It is therefore desirable to provide an arrangement which will enable the satisfactory separation of water and acetic acid in the presence of a dimethylbenzene compound and methyl acetate without the use of an entrainer. The process of the present invention utilises the fact that the dimethylbenzene compound is immiscible with water and thus can be removed by phase separation.

According to the present invention there is provided a process for the separation of acetic acid and water in the presence of a dimethylbenzene compound and methyl acetate comprising:

supplying a feed comprising acetic acid, water, dimethylbenzene , and methyl acetate to a distillation column;

operating the distillation column to separate the acetic acid and water;

removing a stream comprising acetic acid from the column as bottoms;

removing a stream comprising water, dimethylbenzene , and methyl acetate from the column as gaseous overhead stream;

passing the gaseous overhead stream through a condenser such that at least a portion is condensed;

passing the condensed overhead stream to a reflux drum; withdrawing a portion of the condensed aqueous overhead stream from a lower section of the reflux drum and returning said aqueous stream to the distillation column as reflux;

removing a portion of the condensed stream from the upper section of the reflux drum;

allowing said removed portion of the condensed overhead stream to separate into an upper layer comprising dimethylbenzene and a lower layer comprising water and methyl acetate;

decanting the upper layer comprising dimethylbenzene; and

removing the lower layer comprising water and methyl acetate;

and wherein no condensed organic overhead stream is returned to the distillation column as reflux.

The gaseous overhead stream removed from the distillation column will generally be substantially aqueous; that is to say it will comprise a low amount of organic component. In one arrangement it will comprise less than 5 wt% organic component. Lower amounts of organic component such as 2 wt% or 1 wt% or even less may be present.

The removed lower layer comprising water and methyl acetate will generally be passed to a methyl acetate column for separation by conventional means. At least some of any gaseous overhead stream not condensed will generally also be passed to the reflux drum. Any vapour in the reflux drum may also be removed from the reflux drum and at least a portion of it may be passed to the methyl acetate column where present. Where the vapour and the liquid streams from the reflux drum are passed to the same methyl acetate column, the distillation process is further streamlined and offers enhanced economics.

The methyl acetate and/or the dimethylbenzene may be recovered and recycled. Thus where the feed stream of the present invention is one formed in the production of terephthalic acid, the methyl acetate and the dimethylbenzene are generally recycled to the reactor in which the terephthalic acid is produced. Thus any loss to the system is minimised and generally completely removed.

Any suitable portion o f the condensed aqueous overhead stream may be returned to the distillation column as reflux. Generally the amount will be that required to optimise the operation of the distillation column. Whilst the process of the present invention occurs without an organic reflux stream to the distillation column it will be understood that minor amounts of organic material may be returned in the aqueous reflux stream however this is not preferred.

As the column overheads pass through the condenser, steam will be generated which can be used to generate power and therefore improve the economics of the process. The steam generated by the overheads condenser will generally be less than 5 barg. This ability to generate power further improves the economics of the system.

The aqueous overhead stream withdrawn from the reflux drum may be removed from any suitable position provided that is not from the uppermost level thereof since any dimethylbenzene will move to the top of the condensed liquid. In one arrangement the lower portion will be any area except the uppermost 10% of the height of the liquid. In an alternative arrangement, the lower portion will be the lowermost 50% of the height of the liquid. In a still further arrangement it will be the lowermost 20% or 10% of the liquid. In a preferred arrangement, the aqueous stream is removed from the bottom of the reflux drum.

A portion of the condensed stream removed from the upper section of the reflux drum may be removed in any suitable means and from any suitable position but will generally be removed from the top level of the condensed stream. In one arrangement, the removal will be by flowing over a baffle. The portion removed can be regarded as a purge and will include any dimethylbenzene which had been present in the condensed stream and which would have moved to the surface. The reflux drum cannot be considered as a decanter as the amount of organics in the drum is so small.

The apparatus in which the removed portion is allowed to separate may be included in the reflux drum. That is to say an improved reflux drum and decanter may be used. Thus in one arrangement, the reflux drum will be split into two sections which are separated by an internal baffle. This arrangement has the added advantage that any turbulence in the region of the removal of the reflux stream does not affect the settlement in the settlement zone. In one specific arrangement, there is a section of the drum into which the condensate flows and the portion required for reflux to the main column is drawn from the bottom of the drum. The remainder which is to be purged from overflows the internal baffle to pass to a separating section o f the drum. This arrangement ensures that any dimethylbenzene that did float to the liquid surface in the section is automatically swept over the internal baffle into a separating section where it is separated from the aqueous purge stream and collected for recycle to the reactor.

In this arrangement, it is ensured that dimethylbenzene cannot accumulate in either the reflux drum or the stripping column. As all of the dimethylbenzene that is fed to the main column is subsequently recovered in the separating section of the reflux drum, the financial and environmental losses associated with losing from the process.

The dimethylbenzene may be decanted from the remaining portion of the condensed overhead stream by any suitable means. In one arrangement, it may overflow a baffle provided in the reflux drum.

Since the dimethylbenzene is removed by being decanted from the reflux drum, no additional equipment is required to separate out the dimethylbenzene and thus the economics of the process are not compromised. It will therefore be understood that the process of the present invention provides an improvement over the heterogeneous azeotropic distillation systems of the prior art. In particular, the binary process of the present invention offers a simplified approach when compared with the prior art arrangements. Further, the process of the present invention does not suffer from the various control problems noted with the entrainer systems of the prior art as the system of the present invention is inherently more stable.

Any suitable conditions for the distillation column may be used. In one arrangement, the column may have an operating pressure of from 0 to about 5 barg. The overheads temperature may be in the region of from about 99°C to about 160°C. The column may include from about 30 to about 50 theoretical plates. The composition of the bottoms stream which comprisies the acetic acid may be from about 90 to about 100% acetic acid.

According to a second aspect of the present invention there is provided a reflux drum for use in the separation of acetic acid and water in the presence of dimethylbenzene and methyl acetate comprising: an inlet; a reflux stream outlet; a settlement region in which liquid may separate into an upper layer comprising dimethylbenzene and a lower layer comprising water and methyl acetate; means for removing the upper layer from the settlement region; and an outlet for the lower layer.

The reflux drum may additionally include an outlet for any vapour which may enter the reflux drum with the liquid supplied via the inlet.

Any suitable means for removing the upper layer comprising the dimethylbenzene may be provided. In one arrangement the reflux drum may include a decanting overflow which enables the dimethylbenzene to be recovered. In one arrangement the decanting overflow will be provided by a tube which may be of any suitable cross-section, although a tube of circular cross-section is particularly preferred. The height of the overflow will be selected depending on the capacity of the system and will be selected such that only the dimethylbenzene containing layer will be decanted.

According to a third aspect of the present invention there is provided apparatus for the separation of acetic acid and water in the presence of dimethylbenzene and methyl acetate comprising: a distillation column, an inlet, a bottoms outlet and an overhead outlet; a condenser through which a stream removed from the distillation column via the overhead outlet can be passed; and a reflux drum according to the above second aspect of the present invention.

In each of the aspects detailed above the dimethylbenzene may be any suitable compound such as p-xylene or m-xylene.

The distillation column may be of any suitable configuration and may, as appropriate, contain packing.

The present invention will now be described by way of example with reference to the following drawings in which:

Figure 1 is an x-y diagram for acetic acid/water system;

Figure 2 is a schematic diagram for the apparatus in which the process of the present invention may be operated; Figure 3 is a schematic representation of the apparatus for the process of the present invention illustrating exemplary liquid levels; and

Figure 4 is an example of one arrangement of the decanting overflow

It will be understood by those skilled in the art that the drawings are diagrammatic and that further items of equipment may be required in a commercial plant. The provision of such ancillary items of equipment forms no part of the present invention and is in accordance with conventional chemical engineering practice.

The present invention will be discussed in detail in connection with -xylene it will be understood that it can be equally carried with other dimethylbenzene compounds such as m- xylene.

As illustrated in Figure 2, in the present invention a stream comprising water, acetic acid, p- xylene and methyl acetate is provided in line I to a distillation column 2. It will be understood that the stream may be multiple streams. A bottoms stream comprising acetic acid is removed from the column in line 3. Column overheads comprising water, -xylene and methyl acetate are removed in line 4 and passed through a condenser 5 where it is at least partially condensed. The liquid and any uncondensed vapour are passed in line 6 to the reflux drum 7.

A baffle 8 is included in the reflux drum 7 to separate the area of the reflux drum into a first area 15 from which a reflux stream is removed in line 9 and a settlement zone 10. The reflux stream removed in line 9 is returned to the distillation column 2. Liquid not required for reflux will fill the first area 1 5 until it has reached a level that enables it to overflow the baffle 8 and enter the settlement zone 10.

Liquid entering the settlement zone 10 will separate into two layers, an upper layer comprising /7-xylene which is immiscible with water, and a lower layer comprising water and methyl acetate. When the level of liquid in the settlement zone 10 reaches a sufficient level, the upper layer comprising the p-xylene will be removed over the decanting overflow 1 1 and via line 12. The liquid levels are illustrated schematically in Figure 3 and the configuration of one arrangement of the decanting overflow is illustrated in Figure 4. The lower liquid layer is removed from the reflux drum 7 in line 13 and passed to a water/methyl acetate separation column. Any vapour entering the reflux drum 10 will be removed in line 14 and then passed to the water/methyl acetate separation column, not shown.