Page 139 - Design of Simple and Robust Process Plants

124 Chapter 4 Process Synthesis and Design Optimization
± A split into stages above and below the pinch which are not thermally linked,
or into two parallel staged systems, might be favorable.
± The ultimate simple evaporator is a one-stage evaporator below the pinch,
and driven by excess heat, or above the pinch where the overhead heat is
used for other applications (this is not this a simple design).
. Generic points
± The minimum number of heat exchangers to be installed is:N min units =
(Streams above pinch ± 1) + (Streams below pinch ± 1)
± The pinch technology is also effectively applied for the development of site
energy system (Dhole and Linnhoff, 1992) (see also Chapter 7).

Pinch analysis is also applied to other service streams such as water and hydrogen,
while the technology is further extended to mass exchanger networks.
The water pinch analysis is based on the quality term expressed in impurity level,
and the quantity aspect expressed in mass flow (Wang and Smith, 1994). These ana-
lyses received attention as water consumption goes hand in hand with waste water
production ± which is not appreciated environmentally. The technique determines
water flow targets, and shows the effect of regeneration for reuse and recycle to
minimize water consumption and waste water flows. In refineries the technique is
also applied to hydrogen, as this is a critical component due to its limited availabil-
ity.

Exergy analysis This is another technique used to optimize design (Kotas, 1995).
While pinch analysis was developed to support the development of heat integration
in process plants, exergy analysis addresses the overall energy efficiency to mini-
mize energy targets through identification and minimization of exergy losses. The
quality of energy is expressed in exergy, which is the maximum amount of work that
can be obtained from a given form of energy using the environmental parameters as
the reference state. Different forms of energy to be recognized are:
. Ordered energy
. Disordered energy
Ordered energy includes potential, kinetic, and electrical energy, and is fully converti-
ble to other forms of ordered energy, such as shaft work. Examples are hydraulic
power converted into electric power, which in turn is converted into mechanical
power for lifting a weight, or any other mechanical activity. These conversions of
energy can, in principle, be performed at 100% efficiency, although in practice some
losses will occur
Disordered energy includes thermal energy and chemical energy. Thermal energy
can be converted to work to the maximum with the Carnot cycle, the maximum
achievable efficiency
being h T h T 0
carnot ˆ
T h
where T h is high inlet temperature in K, and T o is the outlet temperature of the
Carnot cycle.