Page 143 - Design of Simple and Robust Process Plants

128 Chapter 4 Process Synthesis and Design Optimization
. Install a gas turbine for power generation in front of the furnace.
. Install an air pre-heater at the furnace.
. Replace the furnace by a steam reboiler through evaluation of the following
alternatives:
± Lower pressure operation in the bottom by ensuring less pressure drop over
the column and/or a lower top pressure.
± Allow more lights in the bottom.
± Install a side reboiler and utilize the superheat of steam for bottom heating.
± Install a side reboiler and utilize an electric heater for bottom heating.

Temperature and pressure and concentration profile On occasion, designers prefer
to make a profile of different conditions flowing through the process. (Grievink).
Normally, the conditions monitored are those of pressure, temperature, and concen-
tration, plotting of which renders visible how often there are ªjumpsº in processing
of the material. In particular, ªdoº, ªundoº and ªredoº situations are made visible.
These plots are made only rarely, but clearly help to identify improvements.
In conclusion, pinch and exergy analysis are complementing tools with which the
engineer can analyze and improve the energy situation. The challenge is to develop
ideas to minimize energy consumption by:
. identification of opportunities for minimization of energy targets by exergy
analysis;
. energy integration supported by pinch analysis.
Although the tools show targets and losses, the creativity for improvements must
originate from the engineer.

4.2.3.4 The design of an energy structure
The development of an energy structure around a process is subject to a certain
strategy. A basic approach consists of the following sequential steps:
1. Divide the process into independent operational sections and take care of
back-up provisions for input and output streams of an independent opera-
tional section
2. Develop the constraints for integration (as discussed in Section 4.2.3.1).
3. Analyze the process with exergy and pinch techniques and explore options
for process improvements. This might be time-consuming, but it is worth-
while. Process integration carried out after the process streams have been
fixed has limited impact compared with process modifications. The results of
the improvements are reflected in lower targets.
4. Develop a preliminary process energy structure in relation to the site energy
structure selection of levels of exchange on steam temperature and pressure,
drivers (electrical versus steam) and generation of steam and power, (Town-
send and Linnhoff, 1983). Basically the site utility levels are the first choice for
reasons of back-up and exchange between processes, Chapter 7. In case pro-