Continuous Cyclic Distillation for Binary Solvent Exchange: The Batch Stack

Abstract

As the pharmaceutical industry adopts continuous processes, batch distillation for solvent exchange will need to be replaced with a continuous distillation process. Cyclic distillation is adapted to solvent exchange, and a simplified calculation method that does not assume constant molar overflow is developed for binary systems. A three-stage cyclic distillation for exchange of methanol with water designed for a bottoms product that is 90% water produces a 99.3% methanol distillate, uses 25.4% less solvent than constant volume batch distillation, and is roughly equivalent to a five-stage continuous distillation. When water is the feed solvent and is exchanged with methanol, a three-stage cyclic distillation uses 54.4% and 40.0% less solvent than constant volume batch distillation and a three-stage continuous distillation, respectively.

A mass and energy balance stage model for cyclic distillation

“…For pilot scale or industrial applications there are currently two draining methods available: A simultaneous draining, where all trays are drained to the tray below at the same time, and a sequential draining 27,28 , where the stages are drained and filled one-by-one starting from the bottom and to the top of the column. The simultaneous draining method is the most studied method and can be achieved with the Maleta trays 20 as previously mentioned.…”

Cyclic Operation of Flash and Column Flash Distillation

“…Because commercial simulators do not support cyclic distillation, , calculations were done with a spreadsheet using the constant latent heat method. , Instead of assuming that total molar vapor and liquid flow rates are constant, the constant latent heat method defines a latent heat unit that makes the latent heat of vaporization of the two components equal. The following assumptions are made: latent heats are constant, sensible heat changes are negligible, and the column is adiabatic.…”

“…Because this composition was guessed, backward batch integration can be done for the first stage using the final liquid product concentration as the starting point. The integrations were done with a spreadsheet using Simpson’s rule with the value of (vapor flow)/(inlet flow) forced to the (specified value)/(number of steps in integration) with Goal Seek. , For the bottom stage in the column (stage 1), the inlet vapor is the feed and the outlet vapor concentrations are calculated at different cumulative vapor flow rates. The equilibrium data from Aspen Plus using the NRTL VLE correlation at 1.0 atm was used to find vapor and liquid mol fractions, and these results were converted to VLE values in latent heat units x * and y * with the equations from Table .…”

“…Following the development of the Higee process, process intensification in distillation has drawn considerable attention . This interest in process intensification has revived interest in cyclic distillation. − Although first developed in 1961 by Cannon and his co-workers, , the process hibernated for a number of years because operating methods with more than a few stages were not reliable. However, reliable operating methods are now available. − The cyclic distillation mode of operation leads to increased column efficiency that is evident from either more separation with the same number of stages or fewer stages for the same separation. − In addition, energy consumption can be reduced.…”

“…The column will eventually reach a cyclic steady state. Recognition of the batch character of cyclic operation is used for the development of theoretical models to simulate the operation. , Currently, all of the theoretical models are equilibrium stage models. A mass balance model was developed that calculated a vapor flow rate for each stage based on the latent heat of vaporization .…”