Distillation columns with heat pumps

Figure 3.17. Distillation column with heat pump (a) Separate refrigerant circuit (b) Using column fluid as the...

Figure 16-2 Distillation column with heat pumping by overhead vapor compression.

FIG. 13-2 Complex distillation operations with single columns, a) Use of intermediate heat exchangers, (h) Coupling of intermediate heat exchangers with heat pump, (c) Heat pump with external refrigerant, (d) Heat pump with vapor compression, (e) Heat pump with hottoms flashing. 

Consider the hypothetical perfect separation of a mixture of ethylene and ethane into pure products by distillation as shown below. Two schemes are to be considered conventional distillation and distillation using a heat pump with reboiler liquid flashing. In both cases the column will operate at a pressure of 200 psia, at which the average relative volatility is 1.55. A reflux ratio of 1.10 times minimum, as computed from the Underwood equation, is to be used. Other conditions for the scheme using reboiler liquid flashing are shown below. Calculate for each scheme ... 

Figure 16-1 Distillation column with conventionai heat pump.

Distillation column with top vapour recompression heat pump The flow diagram of the top vapour recompression scheme is shown in Fig. 2. The top column outlet stream is compressed with compressor to raise its temperature and promoting its energy content to be more usable. When the top column pressure is 21.6 bar, the temperature is increased from 81 to 139.9 C and also the pressure is increased from 21.6 to 52.6 bar. The compressor polytropic efficiency was assumed to be 70%. After the compressor, the heat exchanger allows transfer of the energy of this stream to boil up the bottom column outlet stream. With the same top column pressure and reflux ratio as before, the compressor outlet stream is condensed and cooled to 109.2 C, while the bottom column outlet stream is partially boiled. 

Distillation column with bottom flashing heat pump... 

One approach to overcoming the temperature mismatch is to consider a flowsheet that features a distillation column with external side reactors. The column is operated at a pressure that is favorable while stiU permitting separation, which is typically as low as possible for the use of cooling water in the condenser. There is no catalyst on the trays in the column, so there is no chemical reaction occurring in the column. Several total liquid trap-out trays are located in the middle section of the column. Liquid is collected and pumped up to a sufficiently high pressure so that it remains liquid when heated to the higher temperatures that are favorable for reaction. This configuration permits the temperature for separation and the temperature for reaction to be set independently. 

By virtue of its chemical and thermal resistances, borosilicate glass has superior resistance to thermal stresses and shocks, and is used in the manufacture of a variety of items for process plants. Examples are pipe up to 60 cm in diameter and 300 cm long with wall tliicknesses of 2-10 mm, pipe fittings, valves, distillation column sections, spherical and cylindrical vessels up 400-liter capacity, centrifugal pumps with capacities up to 20,000 liters/hr, tubular heat exchangers with heat transfer areas up to 8 m, maximum working pressure up to 275 kN/m, and heat transfer coefficients of 270 kcal/hz/m C [48,49]. 

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. 

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. 

With this assumption, and using a modified Neumann model for HI/I2/H20 mixtures description, CEA (Leybros, 2009) devised a flow sheet for the iodine section which decomposes almost all incoming HI and therefore returns relatively pure products (the iodine return flow contains only 4 molar% water and less than. 3 molar% HI) to the Bunsen section, an important feature for the counter-current reactor. Secondary helium heat is provided to the boiler of the column (235 kj/mol), whereas all other heat needs are fulfilled through internal heat recovery, with the help of a heat pump which transfers heat from the products of the distillation column to its feed. Mainly because of the presence of this heat pump, the iodine section uses 60 kj/mol of electric power on top of the helium heat. 

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