Page 148 - Sustainability in the Process Industry Integration and Optimization

Fu r t h e r A p p l i c a t i o n s o f P r o c e s s I n t e g r a t i o n 125

thereby forming the Hydrogen Pinch. Separating the hydrogen
source and sink parts then determines the target value for the
hydrogen utility minimum flow rate.
The procedure for calculating the supply target requires varying
the flow rate of gas supplied to the system until a Hydrogen Pinch
is found. The sources from hydrogen-consuming processes or
from processes generating hydrogen as a secondary product
(dehydrogenation plants) have flow rates that are determined by
normal process operation; these rates are assumed to be fixed for the
purposes of designing a hydrogen network. However, process
hydrogen sources with variable flow rates can be regarded as
imports from external suppliers and from processes (i.e., steam
reformers or partial oxidation units) that generate hydrogen as a
main product. Those sources are hydrogen utilities.
One approach to minimizing hydrogen utility consumption is to
increase the purity of one or more sources. A hydrogen purification
system introduces an additional sink (feedstock for purification) and
two sources (purified stream and residue stream), resulting in new
targets. By employing Hydrogen Pinch Analysis, an engineer can
make the best use of hydrogen resources in order to meet new
demands and improve profitability.

6.2 Oxygen Pinch Analysis

Another extension of Process Integration is Oxygen Pinch Analysis
(Zhelev and Ntlhakana, 1999). The idea is to analyze the problem so
that targets are derived prior to designing a system for minimizing
oxygen consumption of the micro-organisms used for waste
degradation. The next step is to design a flowsheet that achieves the
targets. In most cases, oxygen is supplied through agitation.
Aeration requires energy, so an analysis based on the Oxygen Pinch
eventually leads back to the original application of energy
conservation. Using the chemical oxygen demand or COD (Monod,
1949) as the baseline range for organic contaminants allows one to
set quantitative targets (for oxygen solubility, residence time, and
oxidation energy load) as well as additional qualitative targets—
namely, the growth rate that is a direct indicator of the age and
health of micro-organisms (Zhelev and Bhaw, 2000). Analyzing the
information in Figure 6.2 and then matching the oxygen supply line
to the CC (so they touch at the Pinch point) yields targeting information
on the growth rate of micro-organisms, oxygen solubility, residence
time, and oxidation energy load.
In the Oxygen Pinch approach, the method recurs to energy but
also incorporates extra information concerning environmental
issues. An important contribution of this method is its ability to
target—in parallel with the concentration of oxygen and the total