Heat Pumping in Distillation

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 economical on a stand-alone basis, the 

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. 

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Figure 14.6 Heat pumping in distillation. A vapor re-compreasion scheme. (From Smith and Linnhoff, Trans. IChemE, ChERD, 66 195, 1988 reproduced by permission of the Institution of Chemical Engineers.)...

Diez, E., Langston, P., Ovejero, G., and Romero, M.D. (2009). Economic feasibility of heat pumps in distillation to reduce energy use. Appl. Therm. Eng., 29, 1216-1223. 

Pumping over a large temperature interval is expensive in term of capital, as well as in term of energy, when this is exclusively from Imported electricity. However, when cheap mechanical energy can be found, the use of heat pumping in distillation becomes a viable solution for energy saving. 

Figure 17.10. Use of heat pumps in distillation, (a) Heat pump with external refrigerant, (h) Heat pump with compression of overhead vapor, (c) Heat pump with reboiler liquid flashing. [H. R. Null, Chem. Eng, Progr., 72 (7), 58-64 (1976).]...

In this subsection we describe heat pumps, multieffect distillation of binary mixtures, synthesis of multicomponent distillation systems with heat integration, and multieffect distillation for thermally coupled configurations. 

Desuperheating. Vapor entering distillation condensers is usually saturated, unless the column is heat pumped. In the latter case, the condenser first desuperheats the vapor, then condenses it. In the desuperheating zone, heat is removed from the vapor by sensible heat exchange. 

Unfortunately, the overall design problem is even more complex in practice. Spare driving forces in the process could be exploited equally well to allow the use of moderate utilities or the integration of heat engines, heat pumps, etc. in preference to distillation integration. 

Ethyl phenylethylmalonate. In a dry 500 ml. round-bottomed flask, fitted with a reflux condenser and guard tube, prepare a solution of sodium ethoxide from 7 0 g. of clean sodium and 150 ml. of super dry ethyl alcohol in the usual manner add 1 5 ml. of pure ethyl acetate (dried over anhydrous calcium sulphate) to the solution at 60° and maintain this temperature for 30 minutes. Meanwhile equip a 1 litre threenecked flask with a dropping funnel, a mercury-sealed mechanical stirrer and a double surface reflux condenser the apparatus must be perfectly dry and guard tubes should be inserted in the funnel and condenser respectively. Place a mixture of 74 g. of ethyl phenylmalonate and 60 g. of ethyl iodide in the flask. Heat the apparatus in a bath at 80° and add the sodium ethoxide solution, with stirring, at such a rate that a drop of the reaction mixture when mixed with a drop of phenolphthalein indieator is never more than faintly pink. The addition occupies 2-2 -5 hoius continue the stirring for a fiuther 1 hour at 80°. Allow the flask to cool, equip it for distillation under reduced pressure (water pump) and distil off the alcohol. Add 100 ml. of water to the residue in the flask and extract the ester with three 100 ml. portions of benzene. Dry the combined extracts with anhydrous magnesium sulphate, distil off the benzene at atmospheric pressure and the residue under diminished pressure. C ollect the ethyl phenylethylmalonate at 159-160°/8 mm. The yield is 72 g. 

Equipment Constraints These are the physical constraints for individual pieces of eqiiipment within a unit. Examples of these are flooding and weeping limits in distillation towers, specific pump curves, neat exchanger areas and configurations, and reactor volume limits. Equipment constraints may be imposed when the operation of two pieces of equipment within the unit work together to maintain safety, efficiency, or quahty. An example of this is the temperature constraint imposed on reactors beyond which heat removal is less than heat generation, leading to the potential of a runaway. While this temperature could be interpreted as a process constraint, it is due to the equipment limitations that the temperature is set. 

Tube and shell heat exchangers, small distillation columns, reactors, valves, pumps and other items are available in impregnated grapliite. Graphite can be joined only by cementing, which embrittles on aging. It is prone to mechanical damage, particularly when subjected to tensile stresses. 

Heat pumps are increasingly finding applications in the process industries. A typical application is the use of the low grade heat from the condenser of a distillation column to provide heat for the reboiler see Barnwell and Morris (1982) and Meili (1990). Heat pumps are also used with dryers, heat being abstracted from the exhaust air and used to preheat the incoming air. The use of a heat pump with an evaporator is described in Volume 2, Chapter 14. 

The following cheap procedure serves for the preparation of large amounts of methylamine hydrochloride (Brochet and Gambier, Bull. Soc. chim., 1895 [iii.], 13, 533). Heat together in a distilling flask attached to a downward condenser 250 g. of ammonium chloride and 570 c.c. of 35 per cent formaldehyde solution. With the thermometer in the liquid, slowly raise the temperature to 104° and maintain at this point until distillation ceases (about 4-5 hours from the start). By then 100-120 g. of water and methyl alcohol will have collected in the receiver. Cool the flask, remove the ammonium chloride which separates by filtration at the pump, and evaporate the filtrate to half its volume on the water bath. Again remove ammonium chloride by filtration and concentrate the filtrate until a film of crystals forms on the surface. Cool and separate the methylamine hydrochloride by Ultra-... 

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