Studies on Development of Hybrid Systems for Ethanol Distillation
Even though distillation and dehydration are well tested and mature technologies, there is plenty of room for further improvements. One major drawback in current technology is the amount of energy required to distill ethanol out of beer, which normally contains only 10-12% ethanol and yet can be as high as 40% of the energy content of the ethanol [6]. Therefore, energy saving improvements in distillation is still a high demand research area. Several interesting developments are reported in recent literature providing insight into new directions in ethanol distillation research. Much of the new research on ethanol distillation is in application of pervapora — tion techniques, or incorporation of pervaporation into distillation.
Pervaporation is a method for separation of liquids by partial vaporization through a membrane. This technique allows preferential vaporization of the more volatile component, which is ethanol, and therefore pervaporation is less energy consuming than straight distillation. Many research groups have attempted to develop hybrid systems with pervaporation and distillation. Generally, two types of hybrid processes have been investigated. Several studies have attempted to generate optimal designs for using pervapora- tion as the dehydration stage following distillation. Other studies have integrated the pervaporation process directly with other separation processes using complex recycle streams and energy integration. Progress in this area till 1999 has been reviewed by Lipnizki et al. [7]; different approaches to integrate pervaporation with ethanol distillation process is included in their review paper. Then in 2010, Frolkova and Raeva provided a comprehensive review of methods available for ethanol dehydration [8]. Their discussion also included a hybrid pervaporation-distillation process for breaking the ethanol-water azeotrope. The applications of optimization methods for the distillation process are rare in the literature reports. However, in one study Szitkai et al. attempted to optimize the performance of a hybrid distillation-pervaporation process for ethanol separation [9]. Their study employed a mixed-integer nonlinear programming (MINLP) approach to minimize the total annual cost.
Various approaches for integration of pervaporation for the recovery of biomass fermentation products are reviewed in a 2005 review article [10]. The review also presented several original processes employing both hydrophobic and hydrophilic membranes in conjunction with distillation to improve process efficiency. More recently, this group proposed an innovative process which combines distillation and vapor permeation to improve process energy efficiency [11, 12]. Several variations were proposed but the general idea was to exploit the selective nature of the vapor permeation membrane together with vapor compression to improve separation performance and reduce energy load. Then, Del Pozo Gomez et al. proposed a novel pervaporation process, in which both vapor and liquid streams were fed to a modified pervaporation module [13, 14]. The vapor and liquid phases were separated by a conductive wall and only the liquid was exposed to the membrane surface. As the liquid permeates through the membrane, heat is lost. Conventionally, this would cause a temperature drop, decreasing the permeation flux. However, in the proposed process, the heat lost due to permeation is supplied by the partial condensation of the vapor stream.
There are few examples of similar approaches; in one case Fontalvo et al. suggested that a two-phase vapor-liquid mixture be contacted directly with a membrane surface [15, 16]. Again, the goal was that condensation of the vapor should provide energy to augment the pervaporation process. Further, the presence of both phases would increase the turbulence at the membrane surface, thereby decreasing concentration polarization effects.
In another example, Haelssig and coworkers proposed a hybrid membrane separation system to replace the rectifying column and dehydration system in the ethanol recovery process [3]. A schematic representation of this separation system is shown in Figure 14.4.
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Figure 14.4 Overview of the proposed hybrid separation process. (Adapted with permission from reference [3]; copyright 2012 Elsevier). |
In the new process, the vapor stream leaving the beer column enters the bottom of a vertically oriented membrane unit and flows upwards through the module. As in a rectifying column, the vapor partially condenses and refluxes in the system at the top of the membrane unit. The liquid reflux flows down the surface of the membrane through the action of gravity. This leads to countercurrent contacting of the vapor and liquid phases, allowing enrichment of the volatile components in the vapor phase. A vacuum is maintained on the permeate side of the membrane to keep a driving force for the selective pervaporation of water. The pervaporation process is associated with a heat loss, since the permeating species must be vaporized. Thus, an energy flux also drives the partial condensation of the vapor phase. Clearly, the process includes aspects of distillation, dephlegmation and pervaporation. For this reason, the process will be referred to as membrane dephlegmation.
Haelssig and coworkers anticipated that the membrane dephleg — mation process is capable of producing a concentrated ethanol stream above the azeotropic composition. Of course, the combination of the rectifying column and the dehydration system in the conventional separation process also produces dehydrated ethanol. However, two separate units are required and it is expected that the use of a single unit will reduce the capital investment and simplify the whole process [3].
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