4.1.3. Case 3: Candy Processing and Packaging

The process of candy processing and packaging was evaluated for HP integration. The stream data were taken from Miah et al. [37] and are listed in the Appendix A (Table A2). The ΔTmin of the process is 5 ◦C. As can be seen from the GCC in Figure 10, the Pinch temperature of this process is 19.5 ◦C. The hot utility required is 1.82 MW and the cold utility required is 0.33 MW. It is assumed that all the source energy is used to heat the sink when the process is integrated with a HP, fixing the source duty to 0.33 MW. The pressure differences of the heat exchangers on the source side and sink sides are both 50 kPa. The setting ranges of the independent optimisation variables and optimisation results of a process of candy processing and packaging integration HP are shown in Table 5.


**Table 5.** Variable settings and optimisation results of a process of candy processing and packaging integration 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, -.

**Figure 10.** GCC of case 3 and integration options using (**a**) JCHP-Ar, (**b**) JCHP-CO2, (**c**) VCHP and (**d**) TCHP.

For intuitive display of the results and analysis, the GCC of the process combined with the HPs is shown in Figure 10. As can be seen from Table 5, the four HPs (JCHP-Ar, JCHP-CO2, VCHP and TCHP) can save 29%, 25%, 24% and 37% of the hot utility by improving the waste heat quality of the process. The ranking of the HP COPs is VCHP > JCHP-CO2 > JCHP-Ar > TCHP when integrating with this process. The reason can be seen in Figure 10, stemming from the fact that the ΔTin between the source and the sink is approximately the same as the ΔTout. The slopes of source and sink are both small in the GCC. As the working fluid of the JCHP does not undergo a phase change remaining gas, the ΔT between inlet and outlet of the working fluid in JCHP changes significantly in the heat exchange with both the source and the sink. The slope of the working fluid is relatively large in the GCC (Figure 10a,b). Due to the phase changes of the working fluid of the VCHP, the ΔT between the inlet and outlet of the working fluid in the VCHP change very little. The slope of the working fluid is small in the GCC, see Figure 10c. For the TCHP, the slope of the working fluid is small in the heat exchange with source in the GCC, whereas the slope of the working fluid is large in the heat exchange with sink in the GCC, see Figure 10d.

In this case, the average temperature between working fluid and source/sink in VCHP is small, so the energy loss of the heat exchangers is lower, the heat exchange efficiency is higher and affects the COP positively. Although the average temperature between working fluid and source/sink in JCHP and TCHP is large, and therefore the energy loss is higher, the heat exchange efficiency is smaller and affects negatively the COP. The performance of JCHP and TCHP are both weak. This process is more suitable for heat integration with a VCHP, which is consistent with the conclusion of Section 3.

## 4.1.4. Case 4: Methanol Distillation Process

The methanol distillation process was evaluated for HP integration based on the stream data from a 4-column double-effect methanol distillation process of a chemical plant [38]. The data are given in the Appendix A (Table A3). The ΔTmin of the process is 15 ◦C. As can be seen from the GCC in Figure 11, the Pinch Temperature of this process is 74.26 ◦C. The hot utility required is 138.48 MW and the cold utility required is 139.90 MW. It is assumed that the heat duty of the heat exchanger at the sink side is fixed 20.86 MW. The pressure differences of the heat exchangers at source side are set 50 kPa, and at the sink, the side is set 0 kPa. The setting range of independent variables and optimisation results of a 4-column double-effect methanol distillation with an integrated HP are shown in Table 6. Finally, for more intuitive display the results, the GCC of a 4-column double-effect methanol distillation process integrated with different types of HPs is given, as shown in Figure 11. As can be seen from Table 6, the HPs can save 15% of the hot utility by improving the waste heat quality of the process. The ranking of the HP COPs is VCHP > JCHP-CO2 > JCHP-Ar > TCHP when integrating with this process. The reason can be seen in Figure 11 and is related to the observation that the ΔTin between source and sink is small, while the ΔTout is too significant. The slopes of source and sink are both steep in the GCC.


**Table 6.** Variable settings and optimisation results of a methanol distillation 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> T5: The outlet temperature of the expansion valve in the HP cycle, MPa. <sup>4</sup> R: The compression ratio of the compressor.

The ΔT between the inlet and outlet of the working fluid in JCHP changes significantly in the heat exchange with both the source and the sink. The slope of the working fluid is relatively large in the GCC, as shown in Figure 11a,b. For the VCHP, in the heat exchange with the source and the sink, the ΔT between the inlet and outlet of the working fluid in VCHP change very little. The slope of the working fluid is small in the GCC—see Figure 11c. The TCHP shows a different behaviour due to the transcritical nature of the heat release part. The slope of the working fluid is small in the heat exchange

with source in the GCC, while the slope of the working fluid is steep in the heat exchange with sink in the GCC, as shown in Figure 11d.

**Figure 11.** GCC of case 4 and integration options using (**a**) JCHP-Ar, (**b**) JCHP-CO2, (**c**) VCHP and (**d**) TCHP.

In this case, the average ΔT between working fluid and source/sink in VCHP is small, so the energy loss of the heat exchangers is lower, the heat exchange efficiency is higher, and COP is affected positively. Although the average ΔT between working fluid and source/sink in JCHP and TCHP is massive, and thus the energy loss is higher, the heat exchange efficiency is smaller, and this affects the COP negatively. The performance of the JCHP and the TCHP are both weak. In addition, the outlet pressure of the compressor in TCHP is too high (25.71 MPa). This means high-pressure requirements for equipment of TCHP, with high equipment investment cost. The TCHP economy is likely to be poor. This process is more suitable for heat integration with VCHP, which is consistent with the conclusion of Section 3.

For the optimal COP of JCHP, the reason for the large ΔT between the working fluid and the source/sink after heat exchange can be seen in Figure 12. The figure shows the relationship between power consumption and COP of JCHP with compression ratio. In the JCHP, the heat load of the sink-side heat exchanger Qh and the outlet pressure of the expander are fixed. By changing the outlet pressure of the compressor, a series of work required by the compressor, work produced by the expander and COP are obtained. It can be seen from Equation (1) that the COP is inversely proportional to the power consumption of the HP when Qh is constant.

**Figure 12.** The relationship between power consumption and COP of JCHP with compression ratio.

As can be seen from Figure 12, with the increase of the compression ratio of the compressor (that is, the increase of the outlet pressure of the compressor compared to inlet pressure), the power consumption of the JCHP first decreases and then increases. The COP of JCHP increases first and then decreases with the increase of compressor compression ratio. That is, there is an optimal pressure for the optimal COP of the JCHP. When the outlet pressure of the compressor is lower than the optimal pressure, although the outlet temperature of the compressor decreases (that is, the inlet temperature of the working fluid exchanging heat with the source decreases and the temperature difference decreases), the COP of JCHP is not optimal. The same is true for the sink side.
