Valorization of waste heat in the food industry

Abstract

This study provides suggestions to decrease hot utility use and impact on the environment of a skim milk powder (SMP) processing plant. These suggestions are based on the integration of waste streams with processes in the production of SMP. To identify appropriate waste and process streams, process integration has proven to be a successful method. This concept addresses the issue of waste heat of a process by pointing out to the importance of the relation between unit-processes. In this study, the integration of waste streams is accomplished by heat exchangers, heat pumps and a zeolite wheel. The general assumption underlying this research, is that recovering heat from unused waste streams decreases hot utility use. Multiple methodologies were used in this study. To accomplish process integration, the pinch analysis forms an essential tool. This analysis was used to identify potential heat recovery schemes in the SMP plant. Furthermore, to fully understand these schemes, a thermal, environmental and economical analysis was done. These analyses were based on mass and energy balances, (in)direct CO2 emissions and investment and operational costs. Several conclusion can be drawn from the pinch analysis. First, the pinch temperature is 50◦C. This represents the dividing line between processes which require heating and cooling. Second, the heating and cooling demand is known. The SMP production process requires 6.8 MW of external heating and 1.7 MW of external cooling. Third, the amount of recoverable heat is equal to 5.3 MW. Fourth, appropriate heat sink and sources are identified. The heat sources, i.e. waste streams, are the spray dryer exhaust and the the condensate streams from two evaporators. The supply temperatures of the exhaust and the condensate streams are respectively 77, 65 and 55◦C. The appropriate heat sinks are the spray dryer air inlet and the evaporator product inlet streams. The target temperature of the spray dryer inlet and evaporator inlet streams are respectively 190, 75 and 70◦C. Seven heat recovery setups are proposed. In terms of saved energy, the spray dryer exhaust has the most potential for heat recovery. The zeolite wheel is the best performing heat recovery technology in the spray dryer process, with a recovery of 3.5 MW. Furthermore, a heat exchanger-heat pump combination and a stand-alone heat exchanger recover respectively 1.8 and 1.6 MW. In contrast, the evaporation process has less potential for heat recovery. The best performing configuration is a heat pump, recovering 1.4 MW from the evaporator condensate. The environmental performance is strongly related with the amount of energy saved. This is because the most decisive parameter in the environmental analysis is the carbon intensity of natural gas and electricity. The best performing configurations in terms of saved CO2 emissions are the zeolite wheel (3806 ton/yr), the heat exchanger-heat pump combination (1934 ton/yr) and the stand-alone heat exchanger (1593 ton/yr) in the spray dryer process. In the evaporation section, the heat pump saves 1485 ton CO2 per year. The total annual costs are derived from the equivalent annual costs of the asset, carbon savings and utility savings/costs. The best performing configuration is the heat pump in the evaporation process. This technology has an annual return of 145 ke per year. Other configurations with a high return are the stand-alone heat exchanger (143 ke/yr), the heat exchanger-heat pump combination (128 ke/yr) and the zeolite wheel (123 ke/yr).