Module 5 Process Integration of Heat and Mass

Module 5: Process Integration of Heat and Mass Chapter 10 David R. Shonnard Department of Chemical Engineering Michigan Technological University of Texas at Austin Michigan Technological University 1

Module 5: Outline The environmental performance of a process depends on both the performance of the individual unit operations, but also on the level to which the process steams have been networked and integrated l Educational goals and topics covered in the module l Potential uses of the module in chemical engineering courses l Review of heat integration concepts l Introduction to the tools of mass integration and synthesis of mass exchange networks - Chapter 10 l Cast study - heat integration of the MA flowsheet University of Texas at Austin Michigan Technological University 2

Module 5: Educational goals and topics covered in the module Students will: l learn about efficient utilization of waste streams as raw materials through application of source/sink mapping l are introduced to graphical tools of mass exchange network synthesis, composition interval diagrams and load line diagrams. l apply mass exchange network synthesis to simple flowsheets University of Texas at Austin Michigan Technological University 3

Module 5: Potential uses of the module in chemical engineering courses Mass/energy balance course: • dilute contaminant balance calculations around process units • source/sink matching of energy streams Continuous/stagewise separations course: • applications to in-process recovery and recycle of contaminants Design course: • graphical design tools for mass integration of waste streams University of Texas at Austin Michigan Technological University 4

Module 5: Analogies between process heat and mass integration Heat Integration the optimum use of heat exchangers and streams internal to the process to satisfy heating and cooling requirements. Tools: 1. Temperature interval diagram 2. Heat load diagram (pinch diagram) Mass Integration the optimum use of mass exchangers and streams internal to the process to satisfy raw material requirements, maximize production and minimize waste generation (water recycle/reuse applications). Tools: 1. Source/sink mapping and diagrams 2. Composition interval diagram 3. Mass load diagram (pinch diagram) University of Texas at Austin Michigan Technological University 5

Module 5: Heat exchange networks key features T - Heat Load Diagram Heat exchange network • internal • external • composite curves • pinch analysis • minimum external utilities [(m. Cp)1 + (m. Cp) 2]-1 89% reduction in external utilities Seider, Seader, and Lewin, 1999, “Process Design Principles”, John Wiley & Sons, Ch. 7 University of Texas at Austin Michigan Technological University 6

Module 5: Heat exchange networks Illustrative example - before heat integration per sec 1 kg/s, Cp = 1 k. J/(kg-˚C) 2 kg/s, Cp = 1 k. J/(kg-˚C) per sec University of Texas at Austin Michigan Technological University 7

per sec Module 5: Heat exchange networks Temperature - load (pinch) diagram Placement of each load line vertically is arbitrary 2 kg/s Cooling load for external network, 160 k. J/s 1 kg/s Heat transfer load by internal network, 140 k. J/s Heating load for external network, 30 k. J/s 10 ˚C minimum temperature difference defines the pinch University of Texas at Austin Michigan Technological University 8

Module 5: Heat exchange networks Illustrative example after heat integration 82. 4% reduction in cooling utility per sec 140 k. J/s transferred per sec University of Texas at Austin 46. 7% reduction in heating utility Michigan Technological University 9

Module 5: Mass integration: objectives and methods Methods 1. Segregation objective is to prepare source streams to be acceptable to sink units within the process or to waste treatment avoid mixing of sources 2. Recycle Pollutant-rich streams Pollutant-lean streams direct sources to sinks 3. Interception selectively remove pollutants from source 4. Sink/generator manipulation adjust unit operation design or operation El-Halwagi, M. M. 1997, “Pollution Prevention Through Process Integration: Systematic Design Tools”, Academic Press University of Texas at Austin Michigan Technological University 10

Module 5: Motivating example: Chloroethane process before mass integration Mass balance in terms of CE, the minor component Objective is to reduce the concentration of CE sent to biotreatment to < 7 ppm and a load of < 1. 05 x 10 -6 kg CE/s El-Halwagi, M. M. 1997, “Pollution Prevention Through Process Integration: Systematic Design Tools”, Academic Press University of Texas at Austin Michigan Technological University 11

Module 5: Motivating example: Chloroethane process after mass integration Interception CE load to biotreatment = 2. 5 x 10 -7 kg/s Recycle El-Halwagi, M. M. 1997, “Pollution Prevention Through Process Integration: Systematic Design Tools”, Academic Press University of Texas at Austin Michigan Technological University 12

Module 5: Mass Integration Tools: Source-sink mapping the purpose of source-sink mapping is to determine if waste streams can be used as feedstocks within the process - direct recycle A range of acceptable flowrates and composition for each sink , “S” Recycle source “a” directly or mix sources “b” and “c” to achieve the target flowrate composition using a Lever Rule - like calculation El-Halwagi, M. M. 1997, “Pollution Prevention Through Process Integration: Systematic Design Tools”, Academic Press University of Texas at Austin Michigan Technological University 13

Module 5: Source-sink mapping: acrilonitrile (AN) process before recycle ≤ 10 ppm NH 3 may contain AN 0 ppm NH 3 0 ppm AN required 450 ˚C, 2 atm 2 -phase stream always with 1 kg/s H 2 O but no H 2 O in the AN layer mass fraction of AN always equal to 0. 068 University of Texas at Austin NH 3 equilibrium CW = 4. 3 CAN Michigan Technological University NH 3 partitioning CSTEAM = 34 CPRODICT 14

Module 5: Source-sink map acrilonitrile (AN) process Sinks for water University of Texas at Austin Sources for water Michigan Technological University 15

Module 5: Flow rates of condenser and fresh water sent to Scrubber University of Texas at Austin Michigan Technological University 16

Module 5: Mass balances on AN units for remaining flow rates and compositions Aqueous streams from condenser and distillation column 4. 7 kg/s H 2 O 0. 5 kg/s AN 12 ppm NH 3 From fresh water supply 1. 0 kg/s H 2 O 0 kg/s AN 0 ppm NH 3 Scrubber Gas stream from condenser 0. 5 kg/s H 2 O 4. 6 kg/s AN 39 ppm NH 3 University of Texas at Austin to decanter ? kg/s H 2 O ? kg/s AN ? ppm NH 3 Michigan Technological University 17

Module 5: Flow rates and compositions from Scrubber to Decanter And similarly for other units University of Texas at Austin Michigan Technological University 18

acrilonitrile (AN) process after recycle freshwater feed 30% of original rate of AN sent to biotreatment is 85% of original University of Texas at Austin AN production rate increased by 0. 5 kg/s; $. 6/kg AN and 350 d/yr = $9 MM/yr 60% of original Michigan Technological University 19

Module 5: Mass exchange network (MEN) synthesis 1. Similar to heat exchange network (HEN) synthesis 2. Purpose is to transfer pollutants that are valuable from waste streams to process streams using mass transfer operations (extraction, membrane modules, adsorption, . . 3. Use of internal mass separating agents (MSAs) and external MSAs. 4. Constraints i. Positive mass transfer driving force between rich and lean process streams established by equilibrium thermodynamics ii. Rate of mass transfer by rich streams must be equal to the rate of mass acceptance by lean streams iii. Given defined flow rates and compositions of rich and lean streams University of Texas at Austin Michigan Technological University 20

Module 5: Mass integration motivating example Phenol-containing wastewater El-Halwagi, M. M. 1997, “Pollution Prevention Through Process Integration: Systematic Design Tools”, Academic Press Outlet streams for recycle or sale Mass separating agents to waste water treatment - Minimize transfer to waste treatment University of Texas at Austin to wastewater treatment Michigan Technological University 21

Module 5: Outline of MEN synthesis 1. Construct a composition interval diagram (CID) 2. Calculate mass transfer loads in each composition interval 3. Create a composite load line for rich and lean streams 4. Combine load lines on a combined load line graph 5. Stream matching of rich and lean streams in a MEN using the CID University of Texas at Austin Michigan Technological University 22

Module 5: Hypothetical set of rich and lean streams stream properties Equilibrium of pollutant between rich and lean streams y = 0. 67 x University of Texas at Austin Michigan Technological University 23

Module 5: Composition interval diagram a tool for MEN synthesis x scale matched to y scale using y = 0. 67 x University of Texas at Austin Michigan Technological University 24

Module 5: Mass transfer loads in each interval Rich Streams negative mass load denotes transfer out of the stream University of Texas at Austin Michigan Technological University 25

Module 5: Composite load line for the rich stream Region 1 & 2 Region 3 Region 4 Region 5 University of Texas at Austin Michigan Technological University 26

Module 5: Combined load line for rich and lean streams mass load to be added to lean stream externally mass load to be transferred internally Rich Stream can be moved vertically University of Texas at Austin Michigan Technological University mass load to be removed from rich stream by external MSA 27

Module 5: Stream matching in MEN synthesis University of Texas at Austin Michigan Technological University 28

Module 5: Heat integration of the MA flowsheet -9. 23 x 107 Btu/hr 9. 70 x 107 Btu/hr Reactor streams generate steam 2. 40 x 107 Btu/hr -4. 08 x 107 Btu/hr Without Heat Integration University of Texas at Austin Michigan Technological University 29

Module 5: Heat integration of reactor feed and product streams University of Texas at Austin Michigan Technological University 30

Module 5: Heat integration of absorber outlet and recycle streams University of Texas at Austin Michigan Technological University 31

Module 5: Maleic anhydride flowsheet with heat integration University of Texas at Austin Michigan Technological University 32

Module 5: Heat integration summary Greater energy reductions are possible when steam generated from the reactors is used for the reboiler, purge and feed heaters 76. 8% reduction 27. 4% reduction University of Texas at Austin Michigan Technological University 33

Module 5: Recap l Educational goals and topics covered in the module l Potential uses of the module in chemical engineering courses l Review of heat integration concepts l Introduction to the tools of mass integration and synthesis of mass exchange networks - Chapter 10 l Cast study - heat integration of the MA flowsheet University of Texas at Austin Michigan Technological University 34