Distillation thermodynamic efficiency

Other Separation Techniques. Under some circumstances, distillation is not the best method of separation. Among these instances are the following when relative volatiHty is <1.05 when <1% of a stream is removed, as in gas drying (adsorption or absorption) or C2H2 removal (reaction or absorption) when thermodynamic efficiency of distillation is <5% and when a high boiling point pushes thermal stabiHty limits. A variety of other... 

Thermodynamic efficiency is hurt by the large ATbetween the temperatures of melting and freezing. In an analogy to distillation, the high a comes at the expense of a big spread in reboiler and condenser temperature. Erom a theoretical standpoint, this penalty is smallest when freezing a high concentration (ca 90%) material. 

In this chapter, we examine the thermodynamic efficiency of the propane-propylene separation process by distillation. The tools necessary for this analysis are developed using the first and second laws of thermodynamics. Sources of thermodynamic inefficiency are pinpointed and, finally, some options are discussed to improve the efficiency of the separation. 

Fonyo-Z, R. E., "The Thermodynamic Efficiency and Energy Conservation of Industrial Distillation Systems," In reference 18, p. 298 (1981). 

Distillation is a unit operation that has been around for a long time and continues to be the primary method of separation in processing plants, in spite of its inherently low thermodynamic efficiency, The preeminence of distillation for the separation of fluid mixtures is not accidental, but fundamental, and therefore unlikely to be displaced. The reasons are both kinetic and thermodynamic. 

From a thermodynamic viewpoint, a typical thermodynamic efficiency of a distillation system is about 10 percent (4). This can be enhanced if intercondensers and interreboilers are used, In fact, it has been shown that conceptually, a distillation system can be devised which requires only the minimum work of separation. Although a thermodynamic efficiency of 10 percent appears low, not many other processes are more efficient (4). 

Use the Aspen Plus simulator with the following input summary to estimate the thermodynamic efficiency of the distillation column ... 

Intermediate Reboilers and Condensers A distillation column of the type shown in Fig. 13-2a, operating with an interreboiler and an intercondenser in addition to a reboiler and a condenser, is diagramed with the solid lines in Fig. 13-38. The dashed lines correspond to simple distillation with only a bottoms reboiler and an overhead condenser. Total boiling and condensing heat loads are the same for both columns. As shown by K ihan [Am. Inst. Chem. Eng. J. Symp. Ser. 76, 192, 1 (1980)], the adclition of interreboilers and intercondensers increases thermodynamic efficiency but requires additional stages, as is clear from the positions of the operating lines in Fig. 13-38. 

As a Rule, Favor Splits that Yield Equal-Sized Parts. Splitting the feed stream into equai-si2ed parts leads to a better thermodynamic efficiency in a simple distillation column. This is true because the column traffic in the sections above and below the feed will have better balance. 

Continuous distillation is a thermodynamically efficient method of producing large amounts of material of constant composition. However, when small amounts of material of varying product composition are required, batch distillation has several advantages. In batch distillation a charge of feed is heated in a reboiler, and after a short startup period, product can be withdrawn from the top of the equipment. When the distillation is finished, the heat is shut off and the material left in the reboiler is removed. Then a new batch can be started. Usually, the distillate is the desired product. 

An important point to keep in mind is that the losses in availability resulting from each individual irreversibility are multiplied together and hence it is not surprising that the over-all thermodynamic efficiency of even the most modem distillation process does not exceed 10% and many operate at much lower efficiency. 

Another point which further confuses the issue is that most distillation processes operate with heat energy and not with mechanical or electrical work, and the figure of 3 kw.-hr. per 1000 gallons is not a proper basis for a thermodynamic efficiency in such cases. One must deal with the availibility of heat, which depends on two temperature levels, that of the heat source and that of the heat sink. The 3 kw.-hr. is directly converted to 10,245 B.t.u. using only the first law of thermodynamics. One can relate minimum work to minimum heat through the well-known relation based on the first and second laws of thermodynamics,... 

Distillation remains the most used separation method, but is penalised by low thermodynamic efficiency in stand-alone operation. Therefore, energy saving in distillation is a priority topic in integrated process design. In this subchapter we present a number of techniques that can be applied to improve the energetic efficiency of distillation systems by integration with the whole process. 

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