Novel Solvent Exchange Distillation System for Multi-Component Solvents- a Comparison Study with Batch Separations

Authors: 
Fazlollahi, F., Brigham Young University

A novel continuous exchange distillation system is introduced, simulated for a number of exchange scenarios, and compared with batch distillation. The exchange system was designed for steady state operation during a campaign. Start-up will give higher concentrations of both products if liquid feed and replacement solvent vapor are added simultaneously to the top and bottom of the column, respectively. Thus, if operation is for a short campaign the exchange results under-predict the actual results. If the reaction system used to produce the solute dissolved in the feed solvent is converted to a continuous system, the steady-state exchange system will obviously be easier to interface with the other system components.

When the feed contains the more volatile solvent, both the continuous and batch exchange systems work well although the continuous exchange system clearly works best with 4 or more stages. Comparison of results of steady-state exchange with batch distillation with the same number of stages and reflux ratio of 1.0 for replacement of a pure more volatile feed solvent show that the exchange system uses less solvent and has a higher distillate mole fraction if there are 5 or more stages. Compared to batch distillation with a higher reflux ratio (R = 4) the steady-state exchange system produced a purer distillate product with less solvent with 5 or more stages for a pure methanol feed. For an 85% methanol feed the batch system could produce a purer distillate product. The lack of a rectifying section and operation without reflux limited the distillate concentration the steady state exchange system could achieve. In all comparisons the exchange system, which has no reflux, uses much less energy compared to batch distillation.

Since the purpose of exchange is to change solvents while the solvent remains dissolved, the distillation must have a path that liquid can go from the feed to the bottoms product without being all vaporized. The new exchange system meets this requirement. Since latent heats are not equal, the molar bottoms flow rate will be less than the molar feed flow rate if the feed solvent has a lower latent heat than the replacement solvent. This case was illustrated with the exchange of replacement solvent water for feed solvent methanol. If the solute solubility requires additional water this water can be added to the feed, to an intermediate stage or to the bottoms product. In the reverse case the latent heat of the replacement solvent is less than the latent heat of the feed solvent and the bottoms flow rate will be greater than the feed rate. This case was analyzed in Figure 5. With xB,W = 0.1 the resulting flow rates in kmol/h are F = 12.5,S = 27.42, B = 14.26, and D = 25.66.

Batch distillation would seldom be employed when the feed solvent is more volatile than the replacement solvent. On the other hand, the steady state exchange system appears to be quite useful when the feed solvent is less volatile than the replacement solvent. The operation with the more volatile component exiting in the bottoms is closer to a stripping operation than to most distillation systems. In order to force the more volatile replacement solvent to remain in the bottoms, the amount of solvent used is significantly greater than the amount required for replacement of a more volatile solvent. However, in many cases the amount of solvent will be less than that required by constant volume diafiltration with a membrane that is not selective for one of the solvents. The reason for the advantage of the steady state exchange system is that staging of distillation systems to reuse the energy (and in this case the added solvent) is much easier than staging membrane systems. As a result, we predict that the steady state exchange system will be competitive and often be less expensive than alternatives such as constant volume diafiltration when a less volatile feed solvent is replaced with a more volatile solvent.

Although the continuous exchange system can obviously be used with multicomponent systems, these systems have not been developed in literature. Since the flow rate and composition of the new solvent can be changed, the system can be optimized for any desired value function. For example, if the solute is desired in a new mixture of solvents the concentration of the new solvent vapor can be adjusted during the distillation to achieve the desired concentration. Alternatively, if preventing one of the new solvents from entering the distillate vapor is important (e.g., to avoid azeotropes when the distillate is purified) the operation can be designed to keep the unwanted component in the bottoms product. Multicomponent batch systems (pot only and with a column above the still pot) can also be optimized by adjusting the order of adding the components desired in the new solvent mixture. These applications will be compared to the exchange process.