(428a) A Multi-Effect Membrane Distillation Process for Preconcentration of Aqueous Solutions of Sugar in the Production of Bioethanol

Bioethanol production is generally regarded as a promising alternative way to substitute the traditional petroleum-based liquid fuels. Lignocelluloses, which are abundant in nature and independent of the market for food and cattle feed, are preferable to conventional starch or sugar containing feedstocks as raw materials for large scale production. The reported concentration of fermentable sugars in the cellulosic hydrolysate prior to fermentation is relatively low because of some process restraints during hydrolysis to obtain high sugar yield. However, in order to decrease the size of footprint of the entire processing plant and also energy consumption in downstream separation/purification, the feed to fermentation requires high concentration of sugars. To resolve this contradiction, an economically efficient and well performed preconcentration unit is necessary to added between the hydrolysis and fermentation step.

In the present study, multi-effect membrane distillation (MEMD), a new membrane based distillation process, has been developed, which combines the advantages of both membrane distillation (MD) and multistage flash (MSF) by equipping air gap membrane distillation (AGMD) with internal heat recovery. A novel separation device in the form of hollow fibers is fabricated to test its separation performance, which is identified as MEMD module. The water vapor flux (J) and energy efficiency in term of performance ratio (PR) and thermal efficiency (η) are the most important indicators for evaluation of module performance. J indicates the productivity of the membrane module; PR tells how much energy is recovered by internal configuration and η shows how much energy is lost due to conduction according to the second law of thermodynamics. Experiments were conducted using the dilute salt aqueous solution as a tracer to investigate the influences of operating variables including inlet temperatures of two sets of different fibers (Th:70-90°C and Tc: 25-45°C) and flow rate (F:16-48L/h) on these above three performance parameters. During the experiment, no leakage is detected and the distillate is of good quality which means the separation efficiency is almost 100%. It is found that the most important determinant parameter is flow rate and there exists trade-off phenomenon between flux (J) and energy efficiency (PR and η) under experimental ranges, that is, the maximum flux will be obtained with high temperature Th, low temperature Tc and high flow rate F while the maximum PR or η is obtained with high temperatures Th and Tc, as well as low flow rate F. Two kind of modules with different hollow fiber diameter and length (M1 and M2 fabricated in Chembrane, China) are also compared. Response surface method (RSM) is carried out to build an empirical quadratic model for prediction of the separation performance and optimization, which agrees well with the experimental results. In general, the typical measured water permeation flux in this study is around 2.0 - 9.0L/m2 h and the value of PR is 4 - 12 and the value of η is 0.80 - 0.95. Flux decline together with reduction of PR and the decrease of η were observed with the increased sugar concentration (up to about 500g/L) mainly because of its high viscosity and there existed slightly differences among three sugars (glucose, xylose and sucrose). But it should be noted that the performance of this new device is still appreciated even at higher concentration. This MEMD process was successfully applied in preconcentrating sucrose aqueous solution from 150g/L to 600g/L and also obtains 14-fold of initial concentration of model cellulosic hydrolysate (mixture of glucose and xylose aqueous solution) in a very efficient way.