By Kirsten Sinclair Rosselot and David
T. Allen
The environmental performance
of a process flowsheet depends on both the performance of the individual
unit operations that make up the flowsheet and on the level to which
the process streams have been networked and integrated. While Chapter
9 describes methods for improving the performance of individual operations,
this chapter examines methods for assessing and improving the degree
to which the unit operations are integrated. Specifically, Section
10.2 examines process energy integration and Section 10.3 examines
process mass integration. The methods presented in these sections,
and the case study presented in Section 10.4, will demonstrate that
improved process integration can lead to improvements in overall mass
and energy efficiency.
Before examining process integration in detail,
however, it is useful to review the methods that exist for sytematically
assessing and improving the environmental performance of process
designs. A number of such methods are available. Some are analogous
to Hazard and Operability (HAZ-OP) Analyses (e.g., see Crowl and
Louvar, 1990).
Section 9.7 briefly describes how a HAZ-OP
analysis is performed; to summarize, the potential hazard associated
with each process stream is evaluated qualitatively (and sometimes
quantitatively) by systematically considering possible deviations
in the stream. Table 10.1-1 gives the guide words and examples of
deviations used in HAZ-OP analysis. Each guide word is applied to
each relevant stream characteristic, the possible causes of the
deviation are listed, and the consequences of the deviation are
determined. Finally, the action(s) required to prevent the occurrence
of the deviation are determined.
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Table 10.1-1
Guide words and deviations in HAZ-OP analysis. |
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Guide Word
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Example Deviations
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NO or NOT
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No flow for an input stream.
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MORE
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Higher flow rate, higher temperature,
higher pressure, higher concentrations.
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LESS
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Lower flow rate, lower temperature,
lower pressure, lower concentrations.
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AS WELL AS
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Extra phase present, impurity
present.
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PART OF
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Change in ratio of components,
component missing.
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MORE THAN
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Extra phase present, impurity
present.
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REVERSE
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Pressure change causes a vent
to become and INLET.
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OTHER THAN
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Conditions that can occur during
startup, shutdown, catalyst changes, maintenance.
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For a single pipeline taking
fluid from one storage tank to another, there may be several possible
deviations, such as:
- no flow
- more flow
- more pressure
- more temperature
- less flow
- less temperature
- high concentration of a particular component
- presence of undesirable compounds
Note that each deviation may have more than
one possible cause so that this set of deviations would be associated
with dozens of possible causes. It would be difficult to consider
all the deviations and their consequences without a structured system
for analyzing the flowsheet. A similar analysis framework has been
employed, in a series of case studies, to identify environmental
improvements in process flowsheets (DuPont, 1993). In these case
studies, a series of systematic questions are raised concerning
each process stream or group of unit processes. Typical questions
include:
- What changes in operating procedures might reduce wastes?
- Would changes in raw materials or process chemistry be effective?
- Would improvements in process control be effective?
Process alternatives, such as those defined
in Chapter 9, can be identified, and in this way the environmental
improvement opportunities for the entire flowsheet can be systematically
examined. (See, for example the cases from the DuPont report described
by Allen and Rosselot,1997).
Other methods for systematically examining
environmental improvement opportunities for flowsheets have been
developed based on the heirarchical design methodologies developed
by Douglas (1992). The hierarchical levels are shown in Table 10.1-2.
Note that Level 1 in this Table applies only to processes that are
being designed, not to existing processes. The hierarchy is organized
so that decisions that affect waste and emission generation at each
level limit the decisions in the levels below it.
As an example of the use of hierarchical
analysis procedures, consider a case study drawn from the AMOCO/US
EPA Pollution Prevention Project at AMOCO's refinery in Yorktown,
Virginia (Rossiter and Klee, 1995). In this example, the flowsheet
of a fluidized-bed catalytic cracking unit (FCCU) is evaluated for
pollutioin prevention options. A flowsheet of the unit is shown
in Figure 10.1-1.
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Figure 10.1-1
Process flow diagram of a fluidized-bed catalytic cracking unit. |
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Beginning with Level 2 of
the heirarchy listed in Table 10.1-2, (input-output structure), the
following pollution prevention strategies were generated:
1) Improve quality of the feed to eliminate
or reduce the need for the vapor line washing system shown in the
upper right-hand corner of Figure 10.1-1.
2) Reduce steam consumption in the reactor
so that there is less condensate to remove from the distillation
system.
3) Within the catalyst regeneration system,
the loss of fines (upper left hand corner of Figure 10.1-1) is partly
a function of the air input rate. A reduction in air flow (e.g.
by using oxygen enrichment) is a possible means of reducing the
discharge of fines.
Two ideas were generated during review of
the recycle structure (level 3):
1) The reactor uses 26,000 lb/hr of steam.
This is provided from the utility steam system. If this could be
replaced with steam generated from process water, the liquid effluent
from the unit would be reduced. Volatile hydrocarbons contained
in the recycled steam would be returned directly to the process.
Catalyst regeneration consumes more than 11,000 lb/hr of steam.
It may be possible to satisfy this duty with "dirty steam"
as well, since the hydrocarbon content would be incinerated with
the coke in the regenerator.
2) Used wash water is collected at several
points and then purged from the process. If it could be recovered
and recycled instead, or if recycled water from other sources could
be used for washing in place of fresh water, fresh water usage and
wastewater generation could both be reduced by about 10,500 lb/hr.
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Table 10.1-2
Levels for hierarchical analysis for pollution prevention (Adapted
from Douglas, 1992). |
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Design Levels
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Identify the material to be manufactured
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Specify the input/output structure of
the flowsheet
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Design the recycle structure of the flowsheet
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Specify the separation system
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General structure: phase splits
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Vapor recovery system
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Liquid recovery system
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Solid recovery system
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Process integration
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Integrate process heating and cooling
demands
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Identify process waste recycling and
water reuse opportunities
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Three options were identified
for separation systems (level 4):
1) Replace heating done by direct contacting
with steam by heating with reboilers.
2) Place additional oil-water separators
downstream of existing condensate collection points and recover
hydrocarbons.
3) Improve gas-solid separation downstream
of the regenerator to eliminate loss of catalyst fines. This might
simply require better cyclone and/or ductwork design, or electrostatic
precipitation.
These first four levels of the design
heirarchy lead us to the types of process improvements described
in Chapter 9 - improvements in the reactor, and improvements in
the separation system. As Table 10.2-1 notes, the next step in the
design process is to identify opportunities for process integration.
This is the main topic of this chapter and the next several sections
describe methods for process energy integration and methods for
identifying process waste recycling and reuse opportunities.
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Chapter 10. Example Problem
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Example 10.4
Constructing a Composition Interval Diagram |
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Construct a CID for the rich streams of Table
10.8. With the aid of this CID, calculate the mass transferred out
of the rich streams in units of kg/s within each region of the CID.
The mass transferred from the rich streams within each region is
equal to (yout-yin)×G
Ri, where yout and yin
are the exiting and entering rich stream mass fractions, respectively,
and G Ri is the sum of the rich stream flow rates
in the region. Note that mass transfer is negative for the rich
streams because they are losing mass.
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Table 10.8
Stream data for three rich streams and one lean stream. |
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Rich Stream
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Lean Stream
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Stream
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Flow Rate, kg/s
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y in
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y out
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Stream
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Flow Rate, kg/s
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x in
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x out
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R1
R2
R3
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5
10
5
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0.10
0.07
0.08
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0.03
0.03
0.01
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L
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15
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0.00
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0.14
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Solution
The rich streams are mapped from Table 10.8
to generate the CID shown in Figure 10.20. The mass transferred
in each region is:
region 1 = ( yout -yin)×E
Ri = (0.08-0.10)5 kg/s = -0.10 kg/s
region 2 = (0.07-0.08)(5+5) kg/s = -0.10
kg/s
region 3 = (0.03-0.07)(5+10+5) kg/s = -0.80
kg/s
region 4 = (0.01-0.03)5 kg/s = -0.10
kg/s
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Figure
10.20 Composition interval diagram for the rich streams of
Table 10.8. |
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In Figure 10.19, the
compositions of the rich and lean streams are on separate axes. These
axes can be combined by applying the equilibrium relationship. If
the equilibrium relationship in the region of interest for the species
considered in this problem is given by
y = 0.67 x*,
then a mass fraction of y = 0.1 in the rich stream is in
equilibrium with a mass fraction of x* = 0.15 in the lean
stream. By converting the lean stream compositions of Figure 10.19
to the rich stream compositions with which they are in equilibrium,
and vice versa, a combined CID with shared axes as shown in Figure
10.21 can be constructed.
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Figure
10-21 Combined composition interval
diagram for the streams of Table 10.7. |
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Chapter 10. Sample Homework Problem
1. Determine the amount of energy savings
that could be achieved by putting in place a Heat exchange network
in the hot water system for a typical house.
a.) Begin by identifying the hot water streams
from which heat can be extracted (e.g., dishwasher effluent, shower
effluent). For each of these streams, estimate a water exit temperature
and a daily flow rate.
b.) Assume that these hot streams will be
contacted with the cold supply water entering the hot water heater
and estimate the amount of energy that could be extracted from the
hot streams. Determine the annual energy savings if the home uses
an electric hot water heater and electricity costs $0.06/kwh. Make
reasonable assumptions about the efficiency of the hot water heater
(fraction of the electricity that goes into heated water used by
the homeowner).
c.) The cost of an installed, non-contact,
single tube, shell and tube exchanger for this application is approximately
$500. Assume that the hot water exit lines already pass near the
water heater so that little additional plumbing is required. Determine
the time required to repay the installation cost using money saved
in energy costs.
Note:
The following homework problem was inadvertently left out of the
textbook:
5. An industrial process uses four water
streams with various levels of acceptable contamination. The data
are listed below.
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Stream |
Flow rate
(10^3 kg/hr) |
Maximum allowable
contaminant concentration (ppm) |
1 |
50 |
20 |
2 |
100 |
50 |
3 |
80 |
100 |
4 |
70 |
200 |
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The same process generates
four wastewater streams with various levels of contamination. The
data are listed below. |
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Stream |
Flow rate
(10^3 kg/hr) |
Maximum allowable
contaminant concentration (ppm) |
1 |
50 |
50 |
2 |
100 |
100 |
3 |
70 |
150 |
4 |
60 |
250 |
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Using the source-sink
diagrams described in this chapter, identify possible water reuse
strategies for this facility. Compare your answers to those described
by Polley and Polley (Chemical Engineering Progress, February, 2000,
pp 47-52). |
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Chapter 10. Lecture Notes
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