Waste water minimization using pinch analysis

Abstract

This thesis lays out the basic principles for analyzing a water using operation and then compares the freshwater and wastewater flowrates for the system with and without
reuse. First, the system is defined as a mass transfer problem in which the contaminant is transferred from a contaminant rich process stream to a water stream. Next, the system is analyzed treating each water-using operation separately. Finally, the minimum freshwater requirement for the integrated system is determined by maximum water reuse subject to constraints such as minimum driving force for mass transfer. For this analysis, the concentration composite curve, the concentration interval diagram and the freshwater pinch are introduced. The methods of regeneration reuse and recycle are also discussed.
The approach for single contaminant problem is extended to multiple contaminants problem with multiple constraints. The preliminary mass exchange network is designed
on the basis of concentration interval diagram and further simplification is achieved by loop breaking. The basic concepts of each method are formulated into a mathematical
code to obtain computer-aided solution to a problem.
Two industrial case studies are discussed to illustrate the significance of wastewater minimization and the results obtained are compared with that predicted using analytical method. The first one is a SO2 extraction problem from four process streams and the second is a petroleum refinery complex problem. An average reduction of about 20% in the freshwater requirement is achieved with water reuse while a reduction of about 60 % is achieved by regeneration reuse. There is also a reduction in the number of units in the mass exchange network by four units with water reuse.