(83e) Rotating Zigzag Bed Application in Extractive Distillation Process of THF-Methanol-Water System

Rotating bed, as a kind of process intensification equipment, was first developed in 1980s and has been commercialized in VOC removal from groundwater, deoxygenation of oilfield injected water, nanoparticles preparation, waste gas purification and reactive separation. However, studies on rotating bed in distillation processes were relatively few and industrial applications were even less. Since first successfully commercialized in 2004, rotating zigzag bed (RZB) has found about 300 application cases in distillation and stripping processes, with excellent economic and social benefits.

This paper was focused on a pharmaceutical plant, where THF-methanol-water system was required to separate in limited area and space. The boiling point temperature between THF and methanol was only 1.5°C and they also form a homogeneous azeotrope. It is very difficult to produce high-purity THF, e.g. methanol residue less than 100ppm, using conventional distillation. Extractive distillation was employed and a unit of RZB was designed and applied. Due to presence of water in this system itself, water was selected as extractant so as not to introduce additional substance.

Process design parameters were obtained using Aspen Plus simulation software, where NRTL was chosen as property package. According to gas and liquid loads on each theoretical plate, the designed RZB contains three rotors with a diameter of 850mm. In the RZB the extractant was introduced between the stationary baffles in the upper rotor. And the feed entered between the upper and middle rotor.

During commission, the actual composition of the feed was THF82.8%, methanol10.5% and water 6.7%. The operation parameters were: feed flow rate 500kg/h, distillate flow rate 432kg/h, reflux flow rate 432kg/h. Analysis results of the distillate were THF94.9%, water 5.1% and methanol 24ppm. When the system was running, measuring data of samples agreed well with designed values. Especially, at total reflux stage, the methanol concentration in the distillate was only about10ppm.

If the same extractive distillation task was performed in a packed tower containing Sulzer BX500 structured packing, the tower had a diameter of 0.5m and a height of 10m (exclusive of skirt). The height and the volume of RZB is only about 1/7 and1/2 compared with the packed column. The most obvious difference is that the RZB was far lower than the packed tower. The RZB is of significance for the plants where the area and space was relatively devoid.

Successful applications of the RZB in industrial distillation processes fully demonstrated its advantages such as high gas-liquid mass transfer rate and small size. Because multiple feeds can be easily installed on the RZB, it was applicable to extractive distillation, which requires two middle feeds. In the application case of THF-methanol-water system, the methanol concentration in THF product was only 24ppm, satisfying the requirement (<100ppm) set by the customer.