Transcript Chapter 9

CHAPTER 9
FLOWSHEET ANALYSIS FOR
POLLUTION PREVENTION
Objective:
To discuss methods for assessing and
improving the degree to which unit
operations are integrated. Improving
process integration could lead to
improvements in overall mass and energy
efficiencies.
Content
• Qualitative analysis
– Material Flow Analysis
– Frameworks for Analyzing Flowsheets
– Hierarchical Design Procedures
• Quantitative analysis
– HAZ-OP Recap
– Process Mass Integration
– Process Energy Integration
Qualitative Analyses
Material Flow Analysis
Establish :
1. System Boundaries :
-
Must be clearly defined
Their specification directly impacts the project
There is no correct or incorrect choice
2. Level of Detail Required (relevant or
irrelevant)
-
Does a detailed mass balance analysis yield
significant information about waste flows?
Frameworks for Examining Flowsheets of
Existing Processes for Pollution Prevention
Opportunities
Principal steps for evaluating an existing process’
flowsheet
1.
2.
3.
4.
5.
Establish Process System Boundaries
Waste Stream Audit (mass balance or other)
Examine PP Options for Waste Streams
Examine PP Options for Unit Operations
Evaluate Impact of Each Potential PP Option on the
Entire Process and on the Finished Product
Hierarchical Design Procedures for
Pollution Prevention
1.
2.
3.
4.
Input information : type of problem
Input/output structure of the flowsheet
Recycle structure of the flowsheet
Specification of the separetion system
1.
2.
3.
4.
5.
6.
7.
8.
General structure : phase splits
Vapor-recovery system
Liquid-recovery system
Solid-recovery system
Energy integration
Evaluation of alternatives
Flexibility and control
Safety
Quantitative Analyses
HAZ-OP Recap
http://www.uscg.mil/hq/gm/risk/e-guidelines/html/vol3/08/v3-08-cont.htm
http://www.uscg.mil/hq/gm/risk/e-guidelines/html/vol3/08/v3-08-cont.htm
Process Energy Integration (PEI)
• Goal : integrate energy that would otherwise be
wasted in order to improve environmental and
economic costs.
• Thermodynamic constraints to heat transfer :
– Quantity of heat absorbed by cold stream = quantity
heat lost by the hot stream
– Heat flows from higher temperature streams to lower
temperature streams
• Pinch diagram (see following slide)
PEI : Pinch Diagram
example
• For a better understanding of the topic,
please check María Elena González’s
module
Process Mass Integration (PMI)
• Goal : integration of materials that would
otherwise be wasted.
• 3 tools
– Source-Sink Mapping
– Strategy for Determining Optimum Mixing,
Segregation and Recycle Strategies
– Mass Exchange Network (MEN) synthesis
PMI : Source-Sink Mapping
• Identify sources (wastewater streams that have the
material) and sinks (processes that require the material)
of material for which integration is desired
• Identify the flowrates of both sources and sinks
• Keep in mind that ranges of flowrates can be used for
sinks
• Account for the feasibility of using “contaminated”
streams in sinks
• Determine the concentration of contaminants that are
identified as being potentially significant problems for the
sinks
PMI : Source Sink Mapping
diagram
It is necessary for creating a source-sink mapping diagram:
 Identifying the sources and sinks of the material for which
integration is desired.
 To know the flow rates of the sources and sinks (range of
flow rates).
 To identify contaminants present in the source streams that
pose a potential problem for the sinks.
 To know the tolerance of each sink for the contaminants.
 To know the concentration of contaminants identified as
being significant problems for the sinks.
PMI : Source Sink Mapping
diagram Example
Let's consider the following table:
Sources
Sinks
Flow rate,
kg/s
Concentration of
X, ppm
Label
Flow rate,
kg/s
Concentration
of X, ppm
Label
max
min
max
min
A
3.0
7
1
4.8
4.0
5
0
B
5.0
15
2
2.5
2.1
1
0
C
1.0
4
PMI : Source Sink Mapping
diagram Example
The table specifies the flow rates of water for sources and
sinks (ranges of flow rates), the concentration of
contaminant X in the sources and the limits of this
contaminant on the sinks.
In order to construct the source-sink diagram, it is necesary
to plot flowrates of sources and sinks on the y-axis and
contaminant concentration en the x-axis.
The next slide shows the resulting diagram. It is a twodimensional diagram because just one contaminant is being
treated.
PMI : Source Sink Mapping
diagram Example
5
B
1
4
3
A
2
2
1
C
0
2
4
6
8
10
12
14
16
PMI : Source Sink Mapping
diagram Example
In this case, sources A, B and C appear as points because the
flow rate and contaminant concentrations are point values, and
sinks 1 and 2 are represented by shaded areas because the flow
rate and the acceptable concentration of contaminant X are
ranges of values.
As observed, the only direct reuse opportunity would be to use
stream C to partially satisfy the water demand for stream 1
because its concentration falls within the range allowed for
stream 1.
PMI : Source Sink Mapping
diagram Example
Because source streams whose concentration of
contaminants is too high for feeding to any sinks can be
combined with low concentration sources to lower their
concentration, we can blend different streams and try to
satisfy the contaminant constraint for stream 1.
The flow rate of the combined streams is the sum of the
flow rates of trhe individual streams.
PMI : Source Sink Mapping
diagram Example
The concentration of compound X in the combined streams is
calculated as follows:
flow r a tes t r eam1 * c onc entr aion
t s t r eam1  flow r a tes t r eam2 * c onc entr aion
t s t r eam2
flow r a tes t r eam1  flow r a tes t r eam2
If it is not possible to obtain the desired composition for the
sink by mixing the different source streams, then a clean
stream (without pollutants) should be consider for blending.
It should be kept in mind that all streams consider for mass
process integration should be economically feasible.
Optimizing Strategies for Segregation
Mixing and Recycle of Streams
This technique is used when processes become
more complicated, and uses a combination of
mathematical optimization techniques and
process simulation packages. These tools can
identify opportunities for recycle, segregation
and mixing of streams
Mass Exchange Network Synthesis
• Goal : mass efficiency and does not achieve mass
integration through re-routing of process streams but
through direct stream exchanges.
• Tools :
– Composition interval diagrams
– Load lines
• Mass balance and equilibrium constraints.
– The total mass transferred by rich stream (the stream from
which a material is to be removed) is to be equal to that
received by the lean stream (the stream receiving the
material)
– Mass transfer is possible only if a positive drinving force
exists for all rich stream/lean stream matches