module 5. process integration of heat and mass (ppt)

University of Texas at Austin Michigan Technological University1

Module 5: Process Integration of Heat and Mass

Chapter 10

David R. ShonnardDepartment of Chemical Engineering

Michigan Technological University


Module 5: OutlineThe environmental performance of a process depends on both theperformance 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


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


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


Module 5: Analogies between process heat

and mass integration

Heat Integrationthe optimum use of heat exchangers and streams internal to the process to satisfy heating and cooling requirements.

Tools: 1. Temperature interval diagram2. Heat load diagram (pinch diagram)

Mass Integrationthe 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 diagrams2. Composition interval diagram3. Mass load diagram (pinch diagram)


Module 5: Heat exchange networks -key features

Seider, Seader, and Lewin, 1999, “Process Design Principles”, John Wiley & Sons, Ch. 7

Heat exchange network • internal • external

T - Heat Load Diagram • composite curves • pinch analysis • minimum external utilities

[(mCp)1 + (mCp) 2]-1

89% reduction in external utilities


Module 5: Heat exchange networks -Illustrative example - before heat

integration

1 kg/s, Cp = 1 kJ/(kg-˚C)

2 kg/s, Cp = 1 kJ/(kg-˚C)

per sec

per sec


Module 5: Heat exchange networks -Temperature - load (pinch) diagram

per s

ec

Placement of each load line vertically is arbitrary

10 ˚C minimum temperature difference defines the pinch

2 kg/s

1 kg/s

Cooling load for external network, 160 kJ/s

Heat transfer load by internal network, 140 kJ/s

Heating load for external network, 30 kJ/s


Module 5: Heat exchange networks -Illustrative example after heat integration

46.7% reduction in heating utility

82.4% reduction in cooling utility

140 kJ/s transferred

per sec

per sec


1. Segregationavoid mixing of sources

2. Recycledirect sources to sinks

3. Interceptionselectively remove pollutants from source

4. Sink/generator manipulationadjust unit operation design or operation

Module 5: Mass integration: objectives and methods

Pollutant-rich streams

Pollutant-lean streams

objective is to prepare source streams to be acceptable to sink units within the process or to waste

treatment

Methods

El-Halwagi, M.M.1997, “Pollution Prevention Through Process Integration: Systematic Design Tools”, Academic Press


Module 5: Motivating example: Chloroethane process before mass

integration

C2H5OH +HCl→ C2H5Cl+H2O

Chloroethanol (CE) is byproductC2H5OCl

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 < 1.05x10-6 kg CE/s



Module 5: Motivating example: Chloroethane process after mass

integrationInterception

Recycle

CE load to biotreatment = 2.5x10-7 kg/s



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



Module 5: Source-sink mapping: acrilonitrile (AN) process before recycle

C3H6 +NH3 +1.5 O2catalyst⏐ → ⏐ ⏐ C3H3N+ 3H 2O

450 ˚C,2 atm

mass fraction of AN always equal to 0.068

2-phase stream always with 1 kg/s H2O but no H2O in the AN layer

NH3 equilibriumCW = 4.3 CAN

0 ppm NH3 0 ppm ANrequired

NH3 partitioningCSTEAM = 34 CPRODICT

≤ 10 ppm NH3

may contain AN


Module 5: Source-sink map acrilonitrile (AN) process

Sinks for water Sources

for water


Module 5: Flow rates of condenser and fresh water sent to Scrubber

Water Mass Balance

0.5 kgs+ x + y=6.2 kg

sNH3 Balance

0.8kgs× 0 ppm+ x ×14 ppm+y× 0 ppm

0.8 kgs

+ x +y=10 ppm

x = flow rate of condensate stream sent to Scrubber

= 4.4 kgs = 4.0

kg H2Os

+ 0.4 kg AN

s

y = flow rate of fresh water sent to Scrubber = 1.0 kg H2O

s


Module 5: Mass balances on AN units for remaining flow rates and compositions

Scrubber

to decanter? kg/s H2O? kg/s AN? ppm NH3

From fresh water supply1.0 kg/s H2O0 kg/s AN0 ppm NH3

Aqueous streams from condenser and distillation column4.7 kg/s H2O0.5 kg/s AN12 ppm NH3

Gas stream from condenser0.5 kg/s H2O4.6 kg/s AN39 ppm NH3


Module 5: Flow rates and compositions from Scrubber to Decanter

Water Mass Balance

0.5 kgs+1.0 kg

s+ 4.0 kg

s+ 0.7 kg

s=6.2 kgH2O

sAN Mass Balance

4.6kgs+ 0.4

kgs+ 0.1

kgs

=5.1kgAN

sNH3 Balance

5.1 kgs× 39 ppm+ 0.8 kg

s× 0 ppm+1.0 kg

s× 0 ppm+ 4.4 kg

s×14 ppm

5.1kgs+ 0.8

kgs+1.0

kgs

+ 4.4kgs

=23 ppm

And similarly for other units


acrilonitrile (AN) process after recycle

60% of original

freshwater feed 30% of original

rate of AN sent to biotreatment is 85% of original

AN production rate increased by 0.5 kg/s; $.6/kg AN and 350 d/yr = $9MM/yr


Module 5: Mass exchange network (MEN) synthesis

1. Similar to heat exchange network (HEN) synthesis2. 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. Constraintsi. Positive mass transfer driving force between rich and lean process streams established by equilibrium thermodynamicsii. Rate of mass transfer by rich streams must be equal to the rate of mass acceptance by lean streamsiii. Given defined flow rates and compositions of rich and lean streams


Module 5: Mass integration motivating example - Phenol-containing wastewater

to wastewatertreatment

to wastewatertreatment

Mass separating agents

Outlet streams for recycle or sale

- Minimize transfer to waste treatment -



Module 5: Outline of MEN synthesis1. 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


Module 5: Hypothetical set of rich and lean streams - stream properties

Rich Stream Lean Stream

Stream FlowRate, kg/s

yin yout Stream FlowRate, kg/s

xin xout

R1

R2

R3

5

10

5

0.10

0.07

0.08

0.03

0.03

0.01

L 15 0.0 0.14

Equilibrium of pollutant between rich and lean streams

y = 0.67 x


Module 5: Composition interval diagram - a tool for MEN synthesis

x scale matched to y scale using y = 0.67 x


Module 5: Mass transfer loads in each interval

Rich Streams

Region 1 and 2 = (yout −yin)× RiStreamsi∑ = (0.08- 0.1)× 5 kg/ s = - 0.1 kg/ s

Region 3 = (0.07- 0.08)× (5 kg/ s+ 5 kg/ s) = - 0.1 kg/ s

Region 4 = (0.03- 0.07)× (5 kg/ s+10 kg/ s+ 5 kg/ s) = - 0.8 kg/ s

Region 5 = (0.01- 0.03) ×(5 kg/ s) = - 0.1 kg/ s

negative mass load denotes transfer out of the stream


Module 5: Composite load line for the rich stream

Region 1 & 2

Region 5

Region 3

Region 4


Module 5: Combined load line for rich and lean

streams

Rich Stream can be moved vertically

mass load to be added to lean stream externally

mass load to be transferred internally

mass load to be removed from rich stream by externalMSA


Module 5: Stream matching in MEN synthesis


Module 5: Heat integration of the MA flowsheet

Without Heat Integration

9.70x107 Btu/hr-9.23x107 Btu/hr

2.40x107 Btu/hr

-4.08x107 Btu/hr

Reactor streams generate steam


Module 5: Heat integration of reactor feed and product streams

0.E+00

2.E+07

4.E+07

6.E+07

8.E+07

1.E+08

1.E+08

0 100 200 300 400 500 600 700 800 900

Temperature (F)

Q (Btu/hr)

Hot Stream Cold Stream

Internal load9.251x107 Btu/hr

External load0.468x107 Btu/hr

(795 ˚F, 9.72x10 7 Btu/hr)

(805 ˚F, 9.72x10 7 Btu/hr)

(165.3 ˚F, 0 Btu/hr)

(215 ˚F, 0.468x10 7 Btu/hr)

ΔΤmin = 10 ˚F


Module 5: Heat integration of absorber outlet and recycle streams

0.E+00

1.E+07

2.E+07

3.E+07

4.E+07

5.E+07

100 200 300 400 500

Temperature (F)

Q (Btu/hr)

Hot Stream Cold Stream

Internal load2.321x107 Btu/hr

External load1.73x107 Btu/hr

(400 ˚F, 4.05x10 7 Btu/hr)

(445.6 ˚F, 4.05x10 7 Btu/hr)

(228.1 ˚F, 1.73x10 7 Btu/hr)

(100 ˚F, 0 Btu/hr)

ΔΤmin = 15 ˚F


Module 5: Maleic anhydride flowsheet with heat integration


Module 5: Heat integration

summary

Energy Duty Energy (Btu/hr)

No HI HI

Compressor 1.52x107 1.52x107

Reactor (Er1) -9.40x107 -9.40x107

Reactor (Er2) -9.40x107 -9.40x107

Reactor (Er3) -9.40x107 -9.40x107

Rxn. prod. cooler (E4) -9.23x107

Abs. out heater (E5) 2.40x107

Purge heater (E6) 2.36x106 2.36x106

Condenser (E7) -8.81x106 -8.80x106

Reboiler (E8) 1.28x107 1.28x107

Recycle pump (E9) 2.50x104 2.50x104

Recycle cooler (E10) -4.08x107 -1.68x107

Feed heater (E11) 9.70x107 4.69x106

Total Inputs 15.1x107 3.51x107

Total Outputs 42.4x107 30.8x107

76.8% reduction

27.4% reduction

Greater energy reductions are possible when steam generated from the reactors is used for the reboiler, purge and feed heaters


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

module 5. process integration of heat and mass (ppt)

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