CHAPTER 9 FLOWSHEET ANALYSIS FOR POLLUTION PREVENTION.

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CHAPTER 9 FLOWSHEET ANALYSIS FOR POLLUTION PREVENTION

Transcript of CHAPTER 9 FLOWSHEET ANALYSIS FOR POLLUTION PREVENTION.

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CHAPTER 9

FLOWSHEET ANALYSIS FOR POLLUTION PREVENTION

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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.

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Content• Qualitative analysis

– Material Flow Analysis– Frameworks for Analyzing Flowsheets– Hierarchical Design Procedures

• Quantitative analysis– HAZ-OP Recap– Process Mass Integration– Process Energy Integration

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Qualitative Analyses

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Material Flow AnalysisEstablish :

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?

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Frameworks for Examining Flowsheets of Existing Processes for Pollution Prevention

OpportunitiesPrincipal steps for evaluating an existing process’

flowsheet1. Establish Process System Boundaries

2. Waste Stream Audit (mass balance or other)

3. Examine PP Options for Waste Streams

4. Examine PP Options for Unit Operations

5. Evaluate Impact of Each Potential PP Option on the Entire Process and on the Finished Product

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Hierarchical Design Procedures for Pollution Prevention

1. Input information : type of problem2. Input/output structure of the flowsheet3. Recycle structure of the flowsheet4. Specification of the separetion system

1. General structure : phase splits2. Vapor-recovery system3. Liquid-recovery system4. Solid-recovery system

5. Energy integration6. Evaluation of alternatives7. Flexibility and control8. Safety

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Quantitative Analyses

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http://www.uscg.mil/hq/gm/risk/e-guidelines/html/vol3/08/v3-08-cont.htm

HAZ-OP Recap

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http://www.uscg.mil/hq/gm/risk/e-guidelines/html/vol3/08/v3-08-cont.htm

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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)

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PEI : Pinch Diagramexample

• For a better understanding of the topic, please check María Elena González’s module

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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

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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

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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.

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PMI : Source Sink Mapping diagram Example

Let's consider the following table:

Sources Sinks

Flow rate, kg/s

Concentration of X, ppm

LabelFlow rate,

kg/sConcentration

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

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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 two-dimensional diagram because just one contaminant is being treated.

PMI : Source Sink Mapping diagram Example

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PMI : Source Sink Mapping diagram Example

1

2

C

A

B

02 4 6 8 10 12 14 16

1

2

3

4

5

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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.

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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.

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PMI : Source Sink Mapping diagram Example

The concentration of compound X in the combined streams is calculated as follows:

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.

rate flow rate flowionconcentrat * rate flow ionconcentrat * rate flow

2stream 1stream

2stream 2stream 1stream 1stream

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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

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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