Heat pump process

Schematic of heat pump process. (Adapted from Ullmann s Encyclopedia of Industrial Chemistry, Vol. A22, VCH Publishers, Inc., New York, 1993.) 

In the example discussed in this chapter (Sections 10.3-10.6), we restrict ourselves to the single-column process for the simple reason that the analysis is straightforward and illustrates the concepts best. In addition, data for this analysis are readily available [6]. 

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Cryogenic distillation has been used extensively ia the processiag of natural gas for nitrogen removal and for helium recovery (22—23). Two basic processes are now used for nitrogen rejection from natural gas— the single-column heat-pumped process and the double-column process. Eadier processes utilized multistage flash columns for helium recovery from natural gas (24). 

Fig. 8. Single-column heat pumped process for nitrogen rejection.

In these systems the converter is producing waste heat, which has to be released to the ambient connected to an entropy flow caused by the irreversibilities within the converter. The discharging process will be a heat pump process, where the entropy has to be taken from the ambient. Therefore it is obvious that these systems have to be coupled to the ambient conditions. Such a storage is not self-sufficient. These systems are called indirect thermal energy storages. 

The continually repeated statement that freezing processes in general have a much lower energy requirement than vaporization processes because the heat of melting is only one seventh of the heat of vaporization, is based on a false premise. All vaporization processes as well as freezing processes are essentially heat-pumping processes and the work requirement for heat pumping depends not only on the quantity of heat to be pumped but also on the temperature difference over which the heat is to be pumped. The net result of these two factors is that the work requirements for these two processes are about comparable. This can be demonstrated by some calculations. 

In the aforementioned process, the heat for the reboiler is usually available as waste heat from the steam cracker, for example, and is essentially cost-free. If this heat is not available, a heat pump can be used. The heat pump can upgrade the heat, at an exergetic cost, to the desired temperature level. If the separation is viewed in isolation, this means that the heat rejected by the condenser at relatively low temperature, can be upgraded to be the higher temperature heat input for the reboiler. A schematic of the heat pump process is given in Figure 10.2. The overhead vapors are heated slightly in the reflux subcooler, which enables these vapors to be compressed and cooled in the condenser-reboiler. 

Note in Table II that the heat pump alternative represents about a 30% saving on available energy input compared to the refrigerated process, and that both represent a very large reduction in the available energy requirement of the original process. The heat pump process does indeed save the losses at both condensers, but has some compensating losses elsewhere. 

If the heat pump process uses the product stream as a working medium this is known as direct vapor compression. Vaporization and condensation take place in the same apparatus. 

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. 

Thus the appropriate placement of heat pumps is that they should be placed across the pinch. Note that the principle needs careful interpretation if there are utility pinches. In such circumstances, heat pump replacement above the process pinch or below it can be economic, providing that the heat pump is placed across a utility pinch. Such considerations are outside the scope of the present text. 

Figure 6.38 Integration of heat pumps with the process.

Most refrigeration systems are essentially the same as the heat pump cycle shown in Fig. 6.37. Heat is absorbed at low temperature, servicing the process, and rejected at higher temperature either directly to ambient (cooling water or air cooling) or to heat recovery in the process. Heat transfer takes place essentially over latent heat profiles. Such cycles can be much more complex if more than one refrigeration level is involved. 

As with heat pumping, the grand composite curve is used to assess how much heat from the process needs to be extracted into the refrigeration system and where, if appropriate, the process can... 

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. 

Heat pumps are particularly suitable for recycling heat energy in the chemical-process industries. For the outlay of an additional fixed-capital expenditure Cec on a heat-pump system, a considerable reduction in the annual heating cost can be effected. 

A common process task involves heating a slurry by pumping it through a well-stirred tank. It is useful to know the temperature profile of the slurry in the agitated vessel. This information can be used to optimize the heat transfer process by performing simple sensitivity studies with the formulas presented below. Defining the inlet temperature of the slurry as T, and the temperature of the outer surface of the steam coil as U then by a macroscopic mass and energy balance for the system, a simplified calculation method is developed. 

Technology exists to recover heat from processes operating at all temperatures, from regenerators on high-temperature plant to heat pumps using low-temperature effluent as a heat source. The problem in many cases is to find a use for the heat recovered. The best solution is to recycle the heat within the same plant, as the supply will always be matched to the demand. An alternative is to use the heat recovered in associated plant (for example, the heat recovered from a melting furnace can be used to dry feedstock for the furnace). 

If the temperature of the water is not controlled it will come to the wet bulb temperature of the air passing through. Ignoring pump heat, the process is adiabatic. The psychrometric plot follows a wet bulb line. 

Pumps are a common feature in most heating and process applications. Where one pump only is provided this is sized for the maximum load (which may occur rarely). Two smaller pumps could be installed such that one pump carries the load for most of the time while the second is used at times of peak demand. 

A large variety of oils is available, and recommendations for any set of conditions, compressor type and refrigerant can be obtained from the refiners. They are naphthene or paraffin-based oils. Synthetic lubricants have been developed for ultra-low-and high-temperature systems, especially for process heat pumps. 

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