Page 285 - Sustainability in the Process Industry Integration and Optimization

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to be $17,519 per year, which results in a reasonable payback period
of about 2.4 years (or less if energy prices increase).
This case study of a combined problem in heat recovery and
process flexibility within a distillation unit’s preheating system
combined advanced Pinch-based retrofit methodologies with
additional heuristic rules. The result was that the flexibility goals
were better met by the introduction of loops in the network topology.
In short, the new and modified paths enabled a significant improvement
in heat recovery. Another benefit was that the more expensive steam
heating was partially replaced by slightly increased furnace duty in
the case of higher heat demand. This interesting trade-off between
two hot utilities yields an economic benefit owing to the lower cost
of furnace fuel.

11.3 Minimizing Water and Wastewater in
a Citrus Juice Plant
This case study describes a water and wastewater minimization
project designed for a citrus plant located in Argentina (Thevendiraraj
et al., 2003); the study proceeded by applying the Water Pinch
technology (Wang and Smith, 1994). Citrus juice–processing plants
consume large quantities of freshwater. The principal objective of
this study was to reduce both the freshwater consumed and the
wastewater produced by the plant. The citrus processing plant housed
the following processes: selection and cleaning, juice extraction, juice
treatment, emulsion treatment, and peel treatment.
Water minimization was achieved by maximizing water reuse
and identifying regeneration opportunities. Water-using operations
were represented by the maximum inlet and outlet contaminant
concentrations, which are functions of equipment corrosion,
fouling limitations, minimum mass transfer driving forces, and
limiting water flow rate through an operation. Targets determined
the minimum freshwater requirement using the Limiting
Composite Curve for the water design network. The graphical
Pinch methods that are based on single contaminants can be
extended to cover multiple contaminants. When dealing with a
number of operations, multiple contaminants, and multiple water
sources, the problem becomes more complex and so algorithms
using the basic Pinch principles have been developed for solving
by MPR-based methods (see Smith, 2005).
The study began with data extraction: each stream had to be
characterized by its contaminant concentration, inlet and outlet
concentration levels, and limiting flow rate through each operation.
The data were provided in a schematic flow diagram of the citrus
plant that incorporated a simplified water distribution network and
the mass balance of the plant’s water streams. Eleven freshwater-
using operations were identified. Chemical oxygen demand (COD)