Page 142 - Process Equipment and Plant Design Principles and Practices by Subhabrata Ray Gargi Das

5.7 Threshold problems 139

Deliverables
(a) Targets for energy saving in a process
- Potential saving in process fuel demand
- Potential saving in utility levels (e.g. replacing HP steam with LP steam use)
- Potential power savings or scope for power generation (e.g. power saving in refrigeration
system)
- Additional energy saving due to process modifications (e.g. change in the reactor
temperature, change in distillation column pressures, etc.)
(b) Targets for energy savings at site-wide level
- Potential savings in site-wide fuel demand and changes in co-generation with existing central
utility system.
- Potential benefits in fuel or co-generation due to the central utility system modifications (such
as introduction of new steam mains, changes in steam mains pressure, etc.)
(c) Capital cost targets for a process
- Approximate investment in the heat exchanger capital to achieve the targeted energy savings,
applicable for certain processes.
(d) Targets for pay-back period (economic analysis).
For certain applications (e.g. crude preheat train) it is possible to estimate approximate pay-back
period as a function of targeted energy savings.
Steps of design
a) Develop a Minimum Energy Requirement (MER) network
• Extract data from simulation, plant operations or design specification sheets and prepare
stream specification Table (Table 5.1). This involves the data extraction from the heat and
material balances of the process. The data should be consistent.
Divide problem at the Pinch
• Start at the pinch and move away
• Start with biggest stream “IN”
• Observe CP OUT CP IN , splitting streams where necessary following Fig. 5.8
• Place all pinch matches first
• Maximise loads on all pinch matches to minimise number of units (“tick-off” heuristics)
• Fill in the rest
• Merge hot end and cold end design
b) Evolve the MER network for network simplicity and capital-energy trade-off
• Analyse and act on heat load loops and heat load paths
c) Estimate further reduction of operating cost by (i) replacing high cost utility by lower cost ones
(e.g. high pressure steam with low pressure steam, refrigerant with cooling water, etc.) e using
grand composite curve and (ii) process modification

5.7 Threshold problems
In any problem, a pinch does not occur if ðDT min Þ is below a threshold value ðDT threshold Þ. Such
problems are called “threshold problems”. These can be understood from the composite curves
presented in Fig. 5.10.In Fig. 5.10A both hot and cold utility are required but as the composite curves
are moved closer to each other, the requirements of both utilities decreases until at