Problem Description
Consider that you are part of a team responsible for debottlenecking the process shown in Fig.1 to make it more energy-efficient. In the PFD, 9 streams have been highlighted (RED for hot streams and BLUE for cold streams. Hot streams are labelled H1 to H4 and cold streams are labelled C1 to C5 ). The information for the highlighted streams – mass flowrate, specific heat capacity, start and target temperature (i.e.: Ts and Tt) are given in Table 1. The minimum approach temperature for this process is 10oC .
Table 1 Stream information (Ignore that the streams are not in mass balance)
| Streams | m (kg/s) | Cp (kJ/kgoC) | Ts (oC) | Tt (oC) |
| C1 | 15 | 4.0 | 60 | 170 |
| C2 | 35 | 2.0 | 35 | 164 |
| C3 | 70 | 2.5 | 80 | 125 |
| C4 | 80 | 2.5 | 140 | 300 |
| C5 | 50 | 2.0 | 100 | 300 |
| H1 | 50 | 4.0 | 160 | 45 |
| H2 | 30 | 2.0 | 220 | 60 |
| H3 | 25 | 4.0 | 327 | 30 |
| H4 | 80 | 2.0 | 220 | 160 |
Fig.1 Current Process Flow diagram
- Provide an evaluation of the external utility cost (cooling water and steam) for the heat exchangers (HX1 to HX9) as shown in Fig 1
- Illustrate a strategy for reducing external utility use in the process. This should be shown using the HEN grid.
- Provide an evaluation of the external utility cost (cooling water and steam) for the modified process.
N/B:
- Assume (and justify) appropriate inlet and outlet cooling temperature for each cooler
- Assume (and justify) appropriate steam conditions for each heater
- Assume cost of energy/type of fuel as appropriate for estimating utility costs
- Provide detailed references for all data used in your calculation
- Units must be shown explicitly in all your calculations, tables and figures and all assumptions must be fully justified.
Marking criteria
| Task | Marking criteria to achieve the maximum mark | Maximal achievable mark |
| i | Presentation – properly organised in sections, showing assumptions and justifications, clear descriptions and arguments to support each point, neatly labelled diagrams, units | 15% |
| ii | References – Harvard Style | 5% |
| iii | Clear evaluation of external utility cost based on original PFD | 20% |
| iv | Illustration of strategy for reducing external utility use using HEN diagram including cascade diagram for pinch | 40% |
| vi | Clear evaluation of the external utility cost based on your proposed integration | 20% |
The mark distribution of the individual task has already been indicated above. The marking criteria shown below describe the required criteria to achieve a maximum mark for an individual task.
