4.1.1. Case 1: Milk Spray Drying Process

The spray drying process of the milk powder was integrated with the HP. The stream data were only adopted the spray drying process from Atkins et al. [35], as shown in Table 2. The ΔTmin of the process is 20 ◦C.


**Table 2.** The stream data from a spray drying process.

As can be seen from the GCC in Figure 8, the Pinch Temperature of this process is 65 ◦C. The hot utility required is 17.66 MW and the cold utility required is 5.36 MW. It is assumed that all the source energy is used to heat the sink when the process is integrated with a HP. The heat duty of the heat exchanger at the source side for the HP is fixed 5.36 MW. The allowed range of the independent variables and the optimisation results of a spray dryer with an integrated HP (maximising the COP) are shown in Table 3.


**Table 3.** Variation settings and optimisation results of a spray drying with an integrated heat pump.

<sup>1</sup> P2: The outlet pressure of the compressor in the HP cycle, MPa. <sup>2</sup> P5: The outlet pressure of the expander or expansion valve in the HP cycle, MPa. <sup>3</sup> R: The compression ratio of the compressor, -.

For evaluation and interpretation, the GCC of the process integrated with different types of HPs is given, as shown in Figure 8. As can be seen from Table 3, the four HPs (JCHP-Ar, JCHP-CO2, VCHP and TCHP) can save 47%, 46%, 66% and 69% of the hot utility by improving the waste heat quality of the process. The ranking of best COP of the HPs is JCHP-CO2 > JCHP-Ar > VCHP > TCHP when integrating with this process. The reason can be seen in Figure 8, showing that the inlet temperature difference ΔTin between source and sink is too small, while the outlet temperature difference ΔTout is too large. That is, the slopes of source and sink are both steep in the GCC.

In the HP cycle, the working fluid of the JCHP does not undergo a phase change and remains in the gas phase. As a result, the ΔT between the inlet and the outlet of the working fluid in the JCHP changes significantly in the heat exchange with source or sink. The slope of the working fluid is relatively large in GCC, as shown in Figure 8a,b.

The working fluid of VCHP is evaporated during heat exchange with the source and is condensed during heat exchange with the sink, so the phase transition occurs. Therefore, in the heat exchange with the source or sink, the ΔT between the inlet and the outlet of the working fluid in the VCHP changes a little. The slope of the working fluid is small in GCC, as shown in Figure 8c. The reason for the temperature difference in the red dashed line in Figure 8c is that the working fluid becomes a superheated gas after increasing the pressure by the compressor. In the heat exchanger hot-side-HX, the working fluid is cooled to a saturated gas and then condensed to a liquid. Therefore, the red dashed line is tilted first and then becomes horizontal. However, when the working fluid is a gas that cools down from the superheated state to the saturated state, the CP is small, and the heat exchange efficiency is low. At the same time, the sink is a gas that the CP is small and the sink slope is large during the heat exchange, so the oblique part of the red dashed line is longer.

**Figure 8.** GCC of Case 1 and Process Integration using (**a**) JCHP-Ar, (**b**) JCHP-CO2, (**c**) VCHP and (**d**) TCHP.

The working fluid of TCHP is evaporated during exchanging heat with the source, while is supercritical fluid during heat transfer with the sink. As the slope of the working fluid is small during the heat transfer with the source in the GCC, whereas the slope of the working fluid is significant in the heat exchange with sink in the GCC, as shown in Figure 8d. In this case, the average temperature between working fluid and source/sink in JCHP is small, so the energy loss of the heat exchangers is lower, the heat exchange efficiency is higher, and affects the COP positively. The average temperature between the working fluid and the source/sink in VCHP and TCHP is large, so the energy loss is higher, the heat exchange efficiency is smaller and affects the COP negatively. The performance of VCHP and TCHP are both weak. In addition, the compression ratio of the compressor in VCHP is 17.40 too high for a single stage. This means that multiple stages of compression would be required, resulting in a substantial increase in the cost of the compressor and a higher cost for VCHP. The outlet pressure of the compressor in TCHP is very high (18.65 MPa). This means high-pressure requirements for equipment of TCHP, with very high equipment investment costs. The economy of the VCHP and TCHP are both weak, and this process is more suitable for Heat Integration with JCHP, which is consistent with the conclusion of Section 3. It can be seen that the method proposed in this study is feasible and effective.
