Page 149 - Sustainability in the Process Industry Integration and Optimization

126 Cha p te r S i x

Process Dissolved Pinch
stream O stream
2
COD COD I/COD Substrate
supply line
Dissolved Process
Slope = μ m /S
O stream stream Intercept = I/S
2
D I/D I/D
μ -Specific growth Slope ~ growth rate, O solubility,
m 2
S-Saturation residence time, oxidation energy load
D-Diluation
FIGURE 6.2 Oxygen Pinch method (after Zhelev, 2007).

energy required—a quality characteristic: the micro-organisms’
health as assessed by their rate of reproduction.

6.3 Combined Analyses, I: Energy-Water, Oxygen-Water,
and Pinch-Emergy

6.3.1 Simultaneous Minimization of Energy and Water Use
Water savings can be achieved through the strategic implementation
of water reuse between water-using operations. Further minimization
of freshwater usage is possible by regenerating water, which is then
recycled. The design methodology developed by Smith and colleagues
(Wang and Smith, 1994; Kuo and Smith, 1997; Gunaratnam et al.,
2005) has proven to be effective in process industries because it
provides a systematic means of establishing realistic minimum water
requirements for a site as well as conceptual design guidelines for
de-bottlenecking water systems.
In some process industry sectors (e.g., the food industry), water
use is closely linked to energy systems. The diagram shown in
Figure 6.3 explains the basic concept and the importance of
considering water and energy systems concurrently. In Figure 6.3(a),
freshwater is supplied to two water-using operations and then is
discharged in a parallel arrangement. The necessary heating or
cooling (usually through a heat exchanger) is provided according to
process requirements. This conventional practice can be significantly
improved by implementing a design for simultaneous water reuse
and heat recovery, as shown in Figure 6.3(b). Water reuse between
operations reduces water consumption, and the proposed heat
recovery between streams will reduce the need for utilities (e.g.,
steam, cooling water).
When the problem is considered jointly, finding the best energy
recovery options and water reuse schemes is an extremely complex
task because there are strong design interactions between systems