This report analyses the improvement potential of zeotropic refrigerant mixtures on the performance of vapour compression heat pumps integrated into industrial dryers. The report reviews the scope of total global energy consumption and the percentage of this attributable to industry to demarcate application area.

It proposes that by targeting industrial process heating, significant reductions to energy consumption and carbon emission by industry can be achieved. Heat pumps are introduced as the preferred system to accomplish this for temperatures up to 200 ◦C. A performance limitation is identified in the form of heat transfer between process streams that reject and receive heat at non-constant temperatures.

Zeotropic refrigerant mixtures are introduced in heat pumps to improve the interaction with non-isothermal heat sources and-sinks. The considered mixture compounds are limited to future-proof refrigerants and the mixtures are limited to binary ones. Humid air encountered in industrial drying is targeted as one such non-isothermal process stream with a large potential for waste heat recovery. The most prominent industries that use dryers are identified and evaluated as to provide representative operating conditions at which a heat pump integrated dryer would have to work. Heat pump cycles are described both on a cycle scale as well as per component and the most important design aspects and considerations are presented. The methods used to allow heat pumps to interact favourably with temperature glides are presented namely zeotropic mixtures and trans-critical heat pump cycles.

A modeling strategy is proposed to simulate heat pump integrated dryers in large numbers in order to investigate the effects of dryer inlet and outlet conditions as well as refrigerant mixture composition both in terms of constituents and mixing ratio. The total process flow diagram is given and divided into definable thermodynamic states for both the (humid) air as well as the refrigerant. Governing equations for each component are presented and the calculation method for the thermodynamic states is discussed in terms of known state variables and used thermodynamic libraries. Finally a novel heat pump cycle is proposed, motivated by a desire to decouple the glide matching in the evaporator from that in the condenser, that attempts to use the fractionation risk present in zeotropic mixtures as an advantage instead. The criteria placed on refrigerant are presented both within the context of the future-proof limitation, legislative limitation as well as those introduced by the process requirements.
The most promising refrigerant candidates from literature are presented for applications in the defined process conditions. Finally a selection is made of 13 refrigerants namely water, ammonia, (iso)butane,
(iso)pentane, methane, ethane, propane, CO2, propylene, ethylene and hexane. The thirteen refrigerants are combined into 78 refrigerant pairs and modelled using the described modeling strategy. The modelling reveals that using zeotropic mixtures does improve the COP for many refrigerants when compared to their pure cycle performance. For drying at 180 ◦C two heat pumps are proposed at different outlet temperatures. The cycles use 87.5%mol Isobutane mixed with Ethane and 87.5%mol NH3 mixed with Propane for a high temperature and lower temperature outlet respectively. COPs of 3.38 and 3.44 were calculated with PRs of 16.53 and 6.84 for the Isobutane-based and NH3-based cycles respectively. A lower drying temperature of 120 ◦C was explored and here CO2-based mixtures were identified as highly desirable refrigerants due to their non-toxic, non-flammable nature as well as low global warming and ozone depletion potential. The mixtures 87.5%mol CO2-Isopentane and 90%mol CO2-Isobutane were proposed, both feasible with single stage compression and possessing a COP of 3.96 and 4.02. Zeotropic mixtures improved the COP in at least 48 out of 78 possible refrigerant combinations when compared to the pure cycle COPs and allowed the dampening of flammability and toxicity where the pure refrigerant possesses those properties.

It is clearly demonstrated that future-proof binary zeotropic mixtures increase the performance of VCHP-integrated dryers across all relevant temperature ranges, by as much as 21.47%. The highest COPs are found when zeotropic refrigerant mixtures are used together with trans-critical operation. It is concluded that zeotropic mixtures improve the COP of vapour compression heat pump integrated dryers and should be utilised for non-isothermal processes like drying.