Page 218 - Membranes for Industrial Wastewater Recovery and Re-Use

Sgstem desigri aids 18 7

4.2.2 Water pinch: the history
The pinch concept was developed at the Department of Process Integration at the
IJniversity of Manchester Institute of Science and Technology (UMIST), UK, in
1970 as a method to reduce energy demand by recovering or transferring heat
by empIoying heat exchangers at critical junctures of a process. This pinch
concept was then taken and applied to processes using water. The fundamental
theoretical formulations for the application of the pinch concept to wastewater
problems were amongst others pioneered by El-Halwagi and co-workers (1 992,
1995), Smith and co-workers (1991a,b, 1994, 1996) including Kuo and Smith
(1997, 1998), Alva-Argaez et al. (1998a,b) and Wang and Smith (1994a,b,
1995). A large number ofstudies applying the concept ofwater pinch have since
been performed. Mathematical programming approaches have been formulated
(e.g. Rossiter and Nath, 1995; Doyle and Smith, 1997), and several software
tools are now available (Section 4.2.3). Two such software tools are
WaterTargetB , a commercial software provided by Linnhoff March - a division of
KBC Process Technology Limited - and Water, a package provided by UMIST to
the members of its own research consortium. In the following sections, these
software tools will also be referred to as the LM and UMIST software.
WaterTarget@ is a software suite comprising WaterTrackerm and WaterPinchm’.
Whereas the latter is the heart of the program for defining the optimal water
network, the first is used to set up the water balance. In the following sections,
the program will be referred to as WaterTarget ”, although Waterpinch@ is the
part used and elaborated on. The graphical plot used in WaterTarget tL does not
represent concentration versus mass load, but concentration versus flow rate.
This methodology and the interpretation of the curves generated can be found in
Buehner and Rossiter (1 996).

42.3 Methodology
Every problem definition begins with accurately identifying every unit
operation using and producing water, including processes as well as
utility operations such as steam production. The existing water network is
thus obtained and, for given measured flow rates, the water balance can be
checked. The accuracy of this balance determines to a great extent the result
and usefulness of a pinch analysis. Indeed, a substantial imbalance of water
would strongly indicate either unaccounted for water-consuming unit
operations, leaks, and/or an ignorance of flow rates through some or all of the
selected units. A water pinch analysis can only uscfully procccd if the
imbalance is less than 10%.

Relaxing the inlet and outlet concentration
It is most often assumed, in the first instance, that all processes are fed with pure
water, such that the minimum water usage is obtained through summing
the flow rates through all the units. It is necessary to stipulate, for all units, the
maximum inlet and outlet concentrations for the different curltarninants of