(282e) Solvent Recovery by a New Batch or Semi-Batch Rectifying Still Unit

A batch or semi-batch distillation process for rectification of solvent waste mixtures has effectively been intensified by installing a new rotating stirrer into the jacketed tank evaporator. The stirrer named “Wall Wetter” consists of two inclined half-cut pipes (gutters) fixed to the central rotating shaft. The Wall Wetter stirrer can pump up a large volume of liquid through the gutters due to its centrifugal effect from the liquid pool and sprayed over the upper jacket surface above the liquid pool level. The upper heat transfer area is covered with a falling liquid film, so that the entire jacket surface can be utilized effectively over the whole term of batch distillation. This jacketed tank evaporator equipped with the Wall Wetter stirrer (Wall Wetter pot) can effectively reduce the distillation process time providing a very high rate of vaporization utilizing the entire area for heat transfer during the entire distillation term.

Original dual flow trays named “Lift Tray” are installed without a downcomer in the rectifier. This tray is a set of two perforated plates: the lower plate, i.e. a fixed perforated plate is covered from upside with a floating perforated plate which can move up and down depending on the vapor flow rate. Therefore this tray has a great advantage of keeping the pressure drop almost constant over a wide range of F-factor by automatically changing the perforation per cent (max.=36.4%, min.=9.1%) with the height of the floating plate above the fixed plate.

The solvent recovery test was conducted combining a trayed rectifier with the Wall Wetter pot as the new patented rectification still unit. The entire column operates as an enriching section. The experimental rectifier (inside diameter 200 mm) has seven Lift Trays with no downcomers and the stage-to-stage spacing is 400 mm. The solvent mixture feed contains 97.8 wt% of ethyl acetate, 2.1 wt% of n-butyl acetate, and 0.1 wt% of toluene with some non-volatile resin residues.

The primary purpose of distillation is to find an effective operation obtaining pure ethyl acetate as much as possible.

At the beginning, a batch (31.5 kg) of the feed mixture is charged into the Wall Wetter pot, and then the system is brought to and kept at quasi-steady state under total reflux until attaining the desired purity of the overhead product. Then a constant portion of the overhead product equal to the constant feed rate is continuously withdrawn in accordance with the reflux policy (reflux ratio = 1.0) of continuous operation with no discharge of the bottom product. Therefore less-volatile components are accumulated in the liquid pool of the Wall Wetter pot. The second purpose is to make the volume of residuum in the Wall Wetter pot as small as possible.

After the total reflux operation (85 min), the continuous operation was stably lasted satisfying the specification of the overhead product purity for 115 min.

It has successfully been confirmed that this batch rectification still unit installed with Wall Wetter stirrer has the following advantages:

(1) Stable highly-efficient distillation

(2) Maintenance of very high evaporation rate

(3) Prevention against fouling

(4) Easy washing and saving of solvent for cleaning

(5) Simple inside structure

(6) Easy operation and maintenance