Process Integration of Industrial Heat Pumps in Grass-Root and Retrofit Situations

Abstract: In several works, methods for process integration of heat pumps (HPs) have been developed, which in turn have been used for case studies in different industrial types. HP integration has thus proven to be a promising technique for utility savings and attractive payback periods. However, there is still a demand for better understanding of the need and costs for rearrangements of a heat exchanger network (HEN) due to HP integration and a demand for further development of more generalized methods. This thesis deals with the following methods for grass-root situations (1), and mainly for retrofit situations (2 6).

1. An optimization methodology to minimize the annual cost of a HEN with an integrated HP in the variables global .DELTA.T_min, heat pump size Q_cevaporator temperature T_E and condenser temperature T_C.

2. A scanning method to identify opportunities for heat pump integration on three levels of complexity in the HEN changes. To be used at an early stage to identify suitable parts of the process for further work.

3. A method for identifying necessary costs of HEN changes as a function of condenser/ evaporator size and temperature.

4. A rigorous method for identifying optimal and economically suitable heat pump installations.

5. A method for identifying the cheapest HEN changes needed for a given condenser/ evaporator size and temperature combined with a given degree of increased heat exchange.

6. A method for identifying optimal and other economically attractive mixes between heat pumping and improved heat exchange.

With the methods 3 and 5, all relevant costs such as piping, material, pressure drop etc. are considered. With the methods 4 and 6, optimal solutions can be identified in diagrams for any combination of payback period, maximum allowed investment cost and annual profit. If the optimal solution is not possible in practice, the economic results of off-optimum solutions are also shown, for the selection of second-best solutions. Different examples are studied with these methods.

General results can be summarized as follows. A better understanding of the need and costs for HEN rearrangements as functions of the condenser/evaporator size and temperature, and a better understanding of the parameters determining this need, are acquired. The interaction between HP integration and increased heat exchange is studied and it is shown that these techniques are both competitors and collaborators. The methods developed are shown to be useful tools for selecting the best heat pump installations in terms of HP type and size, evaporator temperature, condenser temperature and optimal mix between heat pumping and improved heat exchanging. For the same amount of utility savings, HP integration can often be accomplished with less HEN rearrangement than needed for only increased heat exchange. HPs can often be integrated in large industrial processes with complex HENs at favourable payback periods.