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

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

2 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 Educational goals and topics covered in the module Potential uses of the module in chemical engineering courses Review of heat integration concepts Introduction to the tools of mass integration and synthesis of mass exchange networks - Chapter 10 Cast study - heat integration of the MA flowsheet 2

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

4 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 4

5 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) 5

6 Module 5: Heat exchange networks - key features Heat exchange network internal external T - Heat Load Diagram composite curves pinch analysis minimum external utilities [(mc p ) 1 + (mc p ) 2 ] -1 89% reduction in external utilities Seider, Seader, and Lewin, 1999, Process Design Principles, John Wiley & Sons, Ch. 7 6

7 Module 5: Heat exchange networks - Illustrative example - before heat integration per sec 1 kg/s, C p = 1 kj/(kg- C) 2 kg/s, C p = 1 kj/(kg- C) per sec 7

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

9 Module 5: Heat exchange networks - Illustrative example after heat integration 82.4% reduction in cooling utility per sec 140 kj/s transferred per sec 46.7% reduction in heating utility 9

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

11 Module 5: Motivating example: Chloroethane process before mass integration Mass balance in terms of CE, the minor component C 2 H 5 OH +HCl C 2 H 5 Cl +H 2 O Chloroethanol (CE) is byproduct C 2 H 5 OCl Objective is to reduce the concentration of CE sent to biotreatment to < 7 ppm and a load of < 1.05x10-6 kg CE/s El-Halwagi, M.M.1997, Pollution Prevention Through Process Integration: Systematic Design Tools, Academic Press 11

12 Module 5: Motivating example: Chloroethane process after mass integration Interception CE load to biotreatment = 2.5x10-7 kg/s Recycle El-Halwagi, M.M.1997, Pollution Prevention Through Process Integration: Systematic Design Tools, Academic Press 12

13 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 13

14 Module 5: Source-sink mapping: acrilonitrile (AN) process before recycle catalyst C 3 H 6 +NH O 2 C 3 H 3 N +3H 2 O 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 NH 3 equilibrium C W = 4.3 C AN NH 3 partitioning C STEAM = 34 C PRODICT 14

15 Module 5: Source-sink map acrilonitrile (AN) process Sinks for water Sources for water 15

16 Module 5: Flow rates of condenser and fresh water sent to Scrubber Water Mass Balance 0.5 kg s +x +y =6.2kg s NH 3 Balance 0.8 kg s 0 ppm +x 14 ppm +y 0 ppm 0.8 kg s +x +y =10 ppm x = flow rate of condensate stream sent to Scrubber = 4.4 kg s = 4.0 kg H 2 O s kg AN y = flow rate of fresh water sent to Scrubber = 1.0 kg H 2 O s s 16

17 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 Gas stream from condenser 0.5 kg/s H 2 O 4.6 kg/s AN 39 ppm NH 3 Scrubber to decanter? kg/s H 2 O? kg/s AN? ppm NH 3 17

18 Module 5: Flow rates and compositions from Scrubber to Decanter Water Mass Balance 0.5 kg s +1.0kg s +4.0kg s +0.7kg s =6.2kgH 2O s AN Mass Balance 4.6 kg s +0.4kg s +0.1kg s =5.1kgAN s NH 3 Balance 5.1 kg s 39 ppm +0.8kg s 0 ppm +1.0kg s 0 ppm +4.4kg s 5.1 kg s +0.8kg s +1.0kg s +4.4kg s 14 ppm =23 ppm And similarly for other units 18

19 acrilonitrile (AN) process after recycle freshwater feed 30% of original AN production rate increased by 0.5 kg/s; $.6/kg AN and 350 d/yr = $9MM/yr rate of AN sent to biotreatment is 85% of original 60% of original 19

20 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 20

21 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 - Minimize transfer to waste treatment - to wastewater treatment to waste water treatment 21

22 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 22

23 Module 5: Hypothetical set of rich and lean streams - stream properties Rich Stream Lean Stream Stream Flow Rate, kg/s y in y out Stream Flow Rate, kg/s x in x out R L R R Equilibrium of pollutant between rich and lean streams y = 0.67 x 23

24 Module 5: Composition interval diagram - a tool for MEN synthesis x scale matched to y scale using y = 0.67 x 24

25 Module 5: Mass transfer loads in each interval Rich Streams Region 1 and 2 = (y out y in ) Streams i R i = ( ) 5 kg/s = -0.1 kg/s Region 3 = ( ) (5 kg/s+5 kg/s) = -0.1 kg/s Region 4 = ( ) (5 kg/s+10 kg/s+5 kg/s) = -0.8 kg/s Region 5 = ( ) (5 kg/s) = -0.1 kg/s negative mass load denotes transfer out of the stream 25

26 Module 5: Composite load line for the rich stream Region 3 Region 1 & 2 Region 4 Region 5 26

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

28 Module 5: Stream matching in MEN synthesis 28

29 Module 5: Heat integration of the MA flowsheet 9.70x10 7 Btu/hr -9.23x10 7 Btu/hr 2.40x10 7 Btu/hr Reactor streams generate steam -4.08x10 7 Btu/hr Without Heat Integration 29

30 Module 5: Heat integration of reactor feed and product streams 1.E+08 Τ min = 10 ыf 1.E+08 (795 ыf, 9.72x10 7 Btu/hr) 8.E+07 Internal load 9.251x10 7 Btu/hr 6.E+07 (805 ыf, 9.72x10 7 Btu/hr) 4.E+07 2.E+07 (165.3 ыf, 0 Btu/hr) 0.E External load 0.468x10 7 Btu/hr (215 ыf, 0.468x10 7 Btu/hr) Temperature (F) Hot Stream Cold Stream 30

31 Module 5: Heat integration of absorber outlet and recycle streams 5.E+07 4.E+07 (400 ыf, 4.05x10 7 Btu/hr) 3.E+07 Internal load 2.321x10 7 Btu/hr (445.6 ыf, 4.05x10 7 Btu/hr) 2.E+07 (228.1 ыf, 1.73x10 7 Btu/hr) 1.E+07 Τ min = 15 ыf External load 1.73x10 7 Btu/hr 0.E+00 (100 ыf, 0 Btu/hr) Temperature (F) Hot Stream Cold Stream 31

32 Module 5: Maleic anhydride flowsheet with heat integration 32

33 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 Energy Duty Energy (Btu/hr) No HI HI Compressor 1.52x x10 7 Reactor (Er1) -9.40x x10 7 Reactor (Er2) -9.40x x10 7 Reactor (Er3) -9.40x x10 7 Rxn. prod. cooler (E4) -9.23x10 7 Abs. out heater (E5) 2.40x10 7 Purge heater (E6) 2.36x x10 6 Condenser (E7) -8.81x x10 6 Reboiler (E8) 1.28x x10 7 Recycle pump (E9) 2.50x x10 4 Recycle cooler (E10) -4.08x x10 7 Feed heater (E11) 9.70x x10 6 Total Inputs 15.1x x10 7 Total Outputs 42.4x x

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