The paper "Industrial waste heat recovery technologies: An economic analysis of heat transformation technologies" defines waste heat all forms of heat (latent as well as sensible)
Figure 3.23: Cycles for two refrigerants at the same operating temperatures and pressures.
Chapter 3
that are not the purpose of the system. The heat sources can be furnaces, wastewater from washing, drying or cooling processes, motors. The paper also refers to the potential of waste heat, in fact it is necessary to distinguish between the theoretical and physical potential, technical and economic potential. In fact, the theoretical potential only considers physical constraints. What emerges from the paper is that the common method to transform heat from low to high grade is through the use of a mechanically driven heat pump that allows a condensation temperature that varies from 120 to 140 °C for reasons related to materials, subjects at high pressure and temperature. The temperature difference is also limited by the compression ratio of the compressor. The advantages of mechanically driven heat pump can be summarized as: highly established technology, relatively high efficiency that decreases with increasing the temperature lift, well known controls and compact machines that take up little space.
The paper "High temperature heat pumps: Market overview, state of the art, research status, refrigerants, and application potentials" focuses on the analysis of heat pumps operating at high temperatures; it is therefore interesting to report what emerges from the authors' work.
In fact, it goes to report more than 20 high temperature heat pumps that can operate at temperatures above 90 °C and it goes to report the working fluids that can be used. The capacity range goes from 20 kW to 20 MW. The COP reported ranges from 2.2 for lift temperatures of 70 °C up to 5.8 for temperature differences of 30 °C. The working fluids treated are R1336mzz, R718, R245fa, R1234ze, R600, and R601. The fluid R1336mzz can reach 160 °C. In the article it emerges that, although heat pumps are able to produce heat avoiding combustion and consequently the production of carbon dioxide, there are underlying problems:
• Low level of awareness of the technical possibilities and the economically feasible application potentials of high temperature heat pumps among users, consultants, investors, plant designers, producers, and installers.
• Lack of knowledge regarding their integration with industrial processes.
• Long payback period, it is often over 3 years and many companies do not want to invest in the long term.
• Competition with traditional boilers capable of producing more energy at lower costs.
The Table 3.25 shows the main heat pumps available on the market capable of operating at temperatures above 90 °C.
Table 3.25: Main heat pumps with a temperature above 90 °C.
Manufacturer Product Refrigerant Max. heat sink
Chapter 3
Another work worth mentioning is "Analysis of technologies and potentials for heat pump-based process heat supply above 150 °C" in which the authors analyse two possible solutions to provide heat above 100 °C. The two cycles that are a multi-stage cycle using R-718 as refrigerant, which can be constructed as an open or closed system, and a reversed Brayton cycle using R-744 as refrigerant are proposed.
The first proposal is to make a cycle work on three different pressure levels. There are two heat pumps in the bottom of the evaporator, however, if the waste heat temperature is high enough, they can be dispensed with. The high temperature cycle consists of a multi-stage vapor compressor in which after each compression, the working fluid is cooled. This system can produce steam at 210 °C using a temperature of 90 °C for evaporation. The authors show that this temperature can be increased if more compressors are added in series or by increasing the evaporation pressure. In the Figure 3.24 it is possible to see this type of system.
The second proposal is the use of a reversed Brayton cycle with the R744 fluid in supercritical conditions in a pumped heat electricity storage system. Molten salt in liquid condition is used as a means of storing heat. The COP is around 1.3 for a temperature of 465
°C and drops to 1.2 if it reaches 565 °C. The COP is low as the compressor uses a pressure jump of 140 bar and requires a lot of electrical power. The Figure 3.25 shows the flowsheet.
Figure 3.24: Flowsheet of a cascade heat pump with a multi-stage R-718 cycle for steam generation or closed loop heat supply at different temperature levels.
Figure 3.25: Flowsheet of reversed Brayton cycle.
Case Study: Reference plant for the validation model
In Europe there are different types of refineries, from the simplest with a capacity of 100000 bbl/d to the more complex ones that process up to 350000 bbl/d. A refinery with a capacity of 100000 bbl/d will be considered in this work. The refinery in question is a hydro-skimming refinery, i.e. it is composed of primary distillation units, a gasoline block to meet the specifications of gasoline, a kerosene sweetening unit to produce jet fuel and middle-distillates hydrodesulphurization units for the production of diesel for land propulsion. and marine and heating oil. In this type of plant, the residue that remains from the vacuum distillation unit is partly sold as bitumen and partly sent to the visbreaking unit for a partial conversion into more valuable distillates with a reduction in viscosity to comply with the specifications of the fuel oils. The hydrogen rich gas that is produced in the heavy naphta catalytic reformer is compressed and sent into a pressure swing absorber (PSA) which increases the hydrogen concentration and is then used in the desulfurization and hydrotreating units. There is no steam methane reformer in this refinery to produce hydrogen. Crude atmospheric distillation and vacuum distillation are not thermally integrated because they are built in different points of the area. Sea water is used as the condensation fluid, there are no cooiling towers. The Table 4.1 shows the characteristics of the crude oils that are used as raw material in the refinery, while the Table 4.2 shows the products and raw materials used in the refinery.
Table 4.1: Properties of crude oils processed in the refinery.
Raw materials Origin °API Sulphur content wt.
Ekofisk Norway 42.4 0.17
Bonny light Nigeria 35.0 0.13
Arabian light Saudi Arabia 33.9 1.77
Urals medium Russia 32.0 1.46
Arabian heavy Saudi Arabia 28.1 2.85
Maya blend Mexico 21.7 3.18
Table 4.2: Overall material balance.
Products Annual production kt/y
LPG 110.7
Petrochemical naphtha 24.2
Gasoline U95 Europe 614.6
Gasoline U92 USA export 263.4
Jet fuel 450.0
Road diesel 1372.9
Marine diesel 183.0
Heating oil 276.6
Low sulphur fuel oil 806.2
Medium sulphur fuel oil 0.0
High sulphur fuel oil 518.0
Bitumen 125.0
Sulphur 13.5
Subtotal 4756.1
Raw materials Consumptions, kt/y
Ekofisk 1272.8
Bonny light 1226.9
Arabian light 460.0
Urals medium 1390.0
Arabian heavy 139.0
Maya blend 244.0
MTBE 59.8
Natural gas 121.8
Biodiesel 86.7
Ethanol 31.9
Subtotal 5033.0
Fuels and Losses 276.9
Chapter 4