Distillation column vapor recompression

Various heat pumping schemes have been proposed as methods for saving energy in distillation. Of these schemes, use of the column overhead vapor as the heat pumping fluid is usually the most economically attractive. This is the vapor recompression scheme shown in outline in Fig. 14.6. 

For heat pumping to be economic on a stand-alone basis, it must operate across a small temperature difference, which for distillation means close boiling mixtures. In addition, the use of the scheme is only going to make sense if the column is constrained to operate either on a stand-alone basis or at a pressure that would mean it would be across the pinch. Otherwise, heat integration with the process might be a much better option. Vapor recompression schemes for distillation therefore only make sense for the distillation of close boiling mixtures in constrained situations3. 

A distillation column operates with vapor recompression. 

Vapor recompression is another means of improving energy efficiency. As shown on the left in Figure 2.88, the overhead vapor from the distillation column is compressed to a pressure at which the condensation temperature is greater than the boiling point of the process liquid at the tower bottoms. This way, the heat of condensation of the column overhead is reused as heat for reboiling the bottoms. This scheme is known as vapor recompression. 

In a vapor recompression process, heat for the reboiler of a distillation column is supplied by the vapor distillate. The temperature of the vapor distillate is raised by compressing it isentropically, and the heated vapor is used as the heat source for the reboiler. If this is the only heat source for the column, and the column feed is of fixed flow rate, composition, and thermal conditions, determine the number of degrees of freedom for an existing column. What control loops can be used to control the column performance ... 

In some respects this patent is like the deFillippi and Vivian patent, U.S. 4,349,415 although the deFillippi patent claims all near-critici liquids and supercritical fluids for extracting all organics while this one claims propylene as the extractant for ethanol. Both patents describe that the solvent and the extracted organic are separated by distillation. The major difference is in the supply of energy to the reboiler in the solvent distillation column. U.S. 4,349,415 utilized vapor recompression while this one uses a heat pump. 

Fig. 2-21. Methods of connecting columns in a ternary mixture distillation unit, without vapor recompression. (Representation according to ScHLtiTER and Schmidt [2.24], and IfeDDER [2.25]).

Dansinger, R. S., "Distillation Columns With Vapor Recompression", Chemicai Engi-... 

An NGL plant was selected to analyze several distillation assisted heat pump processes when compared to conventional distillation. The depropanizer column which is the third column of the NGL plant was suitable for retrofitting by heat pump systems. This conventional process, along with top vapour recompression, bottom flashing and absorption heat pumps, were simulated using the Aspen Plus software, in order to determine economically the best alternative. Distillation with both top vapor recompression and bottom flashing heat pumps allows reduction of operation (energy) costs by 83.3% and 84%, respectively. This improves the economic potential (incorporating capital costs) by 53% and 54%, respectively. 

Energy recovery in a distillation column means, practically speaking, recovaing or reusing heat contained in the column produa streams, whether they are liquid or vapor. A number of schemes have appeared in the literature. The two chief ones involve (1) multiple effect distillation, analogous to multiple effect evaporation, and (2) vapor recompression. But, regardless of the scheme, there are five design factors that must be considered ... 

In the past vapor recompression ( heat pumps ) has often been considered for distillation of materials boiling at low temperatures. The incentive in many instances was to be able to use water-cooled condensers, thus avoiding the expense of refrigeration. Another factor favoring vapor recompression is a small temperature diference between the top and bottom of the column. 

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