Apr-2008

Process for ultra-purification of paraxylene

A new crystallisation process for paraxylene, which is a parent material for PET, is based on Sulzer Chemtech’s heat pump crystalliser technology, and features two crystallisers and a refrigeration plant. Energy consumption and investment costs are claimed to be lower than with conventional systems

Oskar Fischer and Bernhard J Jans
Sulzer Chemtech Ltd

Article Summary

Paraxylene is a very important basic material in aromatics chemistry for the manufacture of terephthalic acid, dimethyl terephthalate and polyethylene terephthalate (PET) – intermediate products for fibre production in the manufacture of synthetic textiles such as Terylene, Trevira, Diolen and Dacron. Further products include films and plastic bottles, the latter undergoing rapid growth (Figure 1). Annual production is currently about 10 million tonnes and is growing at 5–7 per cent a year.

The main growth areas are the Far East, the USA and South America. Plant capacity is also being increased and production facilities are being improved in Europe too.

Increased pressure on the chemical industry to reduce waste during the various process stages necessitates an increased purity of the starting materials for more and more basic chemicals. The p-xylene manufactured at present has a purity of 99.5 to 99.7 per cent. However, the market is beginning to demand purities of 99.9 per cent and above.

The Sulzer Chemtech fractional crystallisation process makes use of falling film technology, and an entirely new process has been developed.

Traditional aromatics plants produce an isomer mixture with a para content of approximately 19 to 22 per cent. Two methods are used to increase the purity of this product to an acceptable value:
— About 40 per cent of current global production is purified using suspension crystallisation of various technologies. This crystallisation process can, in principle, be used to attain any desired degree of purity required today. In addition to crystallisers the plant also needs costly separating devices for the liquid and crystals – for instance, centrifuges or filters.

— The greater part of current production is purified using molecular sieves (eg, UOP). These plants were built to achieve purities of 99.2 to 99.7 per cent. Higher purities are possible, but with reduced capacities.

In principle, p-xylene is well suited to purification by fractional crystallisation. But since it is normally produced at a very low concentration, and Sulzer’s falling film technology has proven itself, especially in the highly purified area, this market was somewhat neglected by the company until recently.

Prospects
The degree of purity currently demanded by the market provided the impetus to try again and the company believes there are opportunities in different market segments:
— Expansion (debottlenecking) of existing plant by taking over a relatively highly-purified product and purifying it to the required degree. This allows a high throughput with the existing plant and at the same time the purity required by the market.

— Taking over feedstock from mobile selective toluene disproportionation (MSTDP) from which purities of 80 to 90 per cent are possible. This is ideal for falling film crystallisation and can be ultra-purified to 99.9-plus per cent with maximum yield (Figure 2).

Process and plant description
Falling film crystallisation principle
The process is characterised by three phases. At the start of the first phase the collecting tank is filled with feed and the pumps for product and refrigerant are started. A layer of crystals builds up in the crystalliser S1, as a result of which the level in the collecting tank falls with increasing crystalline layer thickness.

The crystallisation process is interrupted once a predefined level and corresponding end temperature has been reached.

The liquid remaining in the collecting tank, now containing considerably more impurities, is discharged via the residue line or pumped into the corresponding stage tank.

In the second phase the crystal layer is tempered to drive out impurities adhering to or contained within the layer. This liquid (also called partial melt) is collected in the collecting tank and once a predefined quantity is attained it is discharged via the residue line or pumped into a stage tank.

In the third phase the remainder of the crystal layer is fully melted. This melt is either the desired end product or an intermediate product which is then crystallised again in a following stage. The product purity depends on the quantity of residual melt which remains on or within the crystal layer at the end of the second phase.

Any desired degree of product purity can be reached by repeated crystallising, sweating and melting. In the same way a higher yield is achieved by recrystallising the residue of the first phase. The entire process takes place within the plant, so that no additional equipment is required.

Since energy consumption plays an important role in the purification of p-xylene, as in all processes today, and which in the case of p-xylene has to be at a very low level, new ways have been investigated to conserve energy as much as possible.

These considerations first resulted in a move away from the indirect cooling and heating used at present to a new principle of direct evaporation and condensation of a refrigerant (Figure 3).

The key components of the plant are the crystallisers. Two or more crystallisers of the same size are used, depending on the size of the plant.