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.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

Hierarchical Design Procedures for Pollution Prevention 1.Input information : type of problem 2.Input/output structure of the flowsheet 3.Recycle structure of the flowsheet 4.Specification of the separetion system 1.General structure : phase splits 2.Vapor-recovery system 3.Liquid-recovery system 4.Solid-recovery system 5.Energy integration 6.Evaluation of alternatives 7.Flexibility and control 8.Safety

Quantitative Analyses

HAZ-OP Recap

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: SourcesSinks Flow rate, kg/s Concentration of X, ppm Label Flow rate, kg/s Concentration of X, ppm Labelmaxminmaxmin A B C1.04

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

1 2 C A B

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: 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