Page 286 - Sustainability in the Process Industry Integration and Optimization

I n d u s t r i a l A p p l i c a t i o n s a n d Ca s e S t u d i e s 263

was chosen as a component to proxy for all contaminants for two
reasons: first, COD measures the most significant contaminant
load in the majority of water streams; and second, COD exhibits
significantly high values.
The overall mass balance is closed by using the assumption
that evaporation losses from the steam system amount to 1 t/h
(otherwise, there is an “inconsistency” of 1 t/h of water that must
be accounted for). The extracted data on contaminant concentrations
and water flow rate made it possible to establish the amount of
water gained or lost by each operation in the process. The total
mass load picked up by the freshwater through each operation was
then calculated. The eleven water-using operations—together with
the water flow rates entering and leaving each operation—can be
represented in the form of a simplified water network, as shown in
Figure 11.6.
The freshwater COD concentration level for the plant is 30 ppm.
There was an existing water reuse between processes currently in
the plant, and these reuse streams were left unchanged. The
simplified water network presented in Figure 11.6 shows the
freshwater-using operations with existing water reuse streams “built
in” to each identified operation. The current total freshwater consumed
and wastewater generated by this citrus plant were, respectively,
240.3 and 246.1 t/h.
The existing water network provided a base starting point for the
Water Pinch Analysis. The freshwater target was evaluated by using
the Composite Curves. The maximum concentration levels were
based on the constraints and limitations dictated by process
conditions and requirements. This data were represented in the
WATER software tool (2005) with identified constraints. The process
restrictions on water type permissible for each operation indicated
that operations 2, 4, 5, and 10 can use only freshwater as input. Hence,
the minimum freshwater required by plant operations was
164.4 t/h.
Operation 1 is a batch process for which the analysis assumed
freshwater must always be available. This increases the plant’s total
minimum freshwater requirement [t/h] to 164.9 = 164.4 + 0.5 for
operation 1. The current total freshwater feed to the plant is 240.3 t/h.
These figures can be used to calculate the maximum theoretical
freshwater reduction (MTFWR) that is achievable:

MTFWR 240.3 164.9 ˜ 100 31.4 percent (11.2)
240.3

The Water Pinch Analysis was then carried out for the existing
water network with maximum concentration levels. The overall
freshwater target was calculated using the maximum reuse analysis.
Figure 11.7 shows the modified water network represented as a
conventional diagram, and the Limiting Composite Curve is plotted