Improved separations for polyester feedstocks

Dating back to 1941, the production of polyester is based on the polymerisation of either dimethyl-terephthalate (DMT) or purified terephthalic acid (PTA) with diols.

With an annual worldwide production of 17million tonnes, polyester is the world's major man-made fibre for textiles and industrial applications, so improving any stage of its production could be of significant benefit to producers.

In collaboration with the German engineering company H&G Hegmanns, the Swiss chemical engineering company Sulzer Chemtech has brought such benefits to the DMT process, making it more energy-efficient and increasing the purity of this polyester feedstock.

Conventionally, most DMT is manufactured from para-xylene by successive catalytic oxidation and esterification. The resulting crude ester is distilled to separate the DMT from heavy boiling components and light esters. The DMT is then further purified in a two-stage suspension crystallisation process, using methanol as solvent. Large amounts of solvent are needed for this stage, with consequently high energy costs for solvent recovery.

The purified DMT product can then, if required, be converted into PTA in a hydrolysis reactor, with the PTA again being further purified by crystallisation.

The improved DMT/PTA process developed by Hegmanns and Sulzer combines an enhanced oxidation reaction with efficient by-product and energy recovery, a revised raw ester and by-product distillation train, and a switch to melt crystallisation for the final DMT purification.

According to Sulzer Chemtech's Manfred Stepanski and Armin Rütti, these improvements allow new opportunities to optimise the process.

Sulzer's mass transfer technology and know-how played a key role in the redesign of the distillation and crystallisation units. After by-product recovery, the liquid stream from the oxidation reactor is sent to two vacuum distillation columns. The first recovers methyl p-toluate for recycle back to the oxidation reactor, while the second separates the DMT product from heavy components and any catalyst carry-over.

It is at this stage that the use of Sulzer's structured column packings has led to 'high separation efficiency and massive capacity increase in the distillation unit', say Stepanski and Rütti. Although esters of terephthalic acid (DMT and its isomers) can be separated by distillation, some of the isomers are close-boiling compounds and require special know-how. To optimise the process, Sulzer carried out comprehensive trials in a series of pilot columns. Today, its DMT columns are designed based on those trials, often using the high-capacity packing MellapakPlus.

Smaller equipment can be used, reducing investment costs, while the recovery of methyl p-toluate in the first column is optimised. Three of the largest DMT columns with Mellapak or MellapakPlus packings are in operation at Formosa Chemicals & Fibre in Taiwan. The biggest column here is 5m in diameter.

Sulzer's and Hegmanns's other major improvement to the DMT/PTA process is at the DMT crystallisation stage. Conventionally, the distilled crude DMT is purified to polymer-grade product in a two-stage counter-current suspension crystallisation in methanol. This crystallisation train and associated methanol recovery unit can typically account for nearly half of the total investment costs.

The improved process eliminates the suspension crystallisation and the use of methanol as a solvent altogether. Instead, Sulzer's melt crystallisation technology reduces both investment and operating costs.

The technology is based on layer crystallisation, which can be either static - or falling-film. In a falling-film unit, crystal layers are grown from a falling film of melt on the inside of cooled vertical tubes. Impurities are rejected from the crystals and concentrated in the remaining melt. Falling-film crystallisation is generally used for relatively pure feeds and high capacities, making it ideal for the purification of DMT to polyester grade.