Thermodynamics of Clean Production

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Oct 27, 2013 (3 years and 11 months ago)

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CHE 572100 Clean Chemical Process and Technology


Thermodynamics of Clean Production

D.S.H. Wong

Department of Chemical Engineering

National Tsing Hua University

2007/12/18

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Thermodynamics of Clean Production

CHE 572100 Clean Chemical Process and Technology


Contents


Resource Thermodynamics


Efficiency


Dispersion


Alternate Energy Source


Biofuel


Fuel Cell


Clean Production Technology


Pinch


Integration


Intensification

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Resource Thermodynamics

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Problems of the 21st Century
--

Lack of Resource

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Problems of the 21st Century
--

Pollution of Environment

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Twin Brothers of Trouble
--

Energy and Pollution

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Laws of Thermodynamics


Energy is conserved





Processes are irreversible


This is our source of problem

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Efficiency of a Process

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Irreversibility of Mixing

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Solution to the Energy Problem


Alternate energy source


Biofuel


Fuel cell


...


Energy saving technology


Pinch


Integration


Intensification



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Energy Efficiency

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V. Energy Issues

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Energy Consumption by Sector

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Energy Saving Technology
--

Pinch, Integration and Intensification


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Thermodynamics of Clean Production

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Basic Theory of Pinch

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A heat recovery problem


Consider a system with two hot streams to be cooled and
two cold streams to be heated. To obey the Second Law,
thermal energy transfer is limited by the temperature
difference. Assume that there must be a 5 K difference in
temperature for heat to be efficiently transferred. What are
the minimum heating and cooling that must be supplied
from external utilities (i.e.: high pressure steam and cooling
water?


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Simple heat balance


Heat supplied


due to H1 = 1000*(250
-
120) = 130,000


due to H2 = 4000*(200
-
100) = 400,000


total = 530,000


Heat demand


due to C1 = 3000*(150
-
90) = 180,000


due to C2 = 6000*(190
-
130)= 360,000


total = 540,000


Constraint due to second law has not been considered:
heat must be transferred from high to low temperature

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Heat exchanger

T
hi

T
ho

T
ci

T
co


The smaller

Tis,helessishehearecvere,anhe
less is heat exchange area required. This leads to
decrease in capital cost but increase in utility used


A rule of thumb is to let

T
min

= 5 K


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Heating composite curve

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Cooling composite curve

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Pinch and minimum utility

Minimum cooling

Minimum heating

Pinch point

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Heat cascade

Pinch

Heating

Utility

Cooling

Utility

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Design Heuristics Based on Pinch
Theory

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Do not exchange heat across pinch

-
X

-
X

-
X

-
X

+X

+X

+X

+X

-
X

+X

-
X

+X

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Power generation by waste heat recovery
below pinch is most desirable

-
X

-
X

+
X
-
W

-
X

+
X
-
W

-
W

-
X

-
W

-
X

-
X

-
W

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Cogeneration above pinch is also
acceptable

-
X

-
X

+
X
-
W

-
X

+
X
-
W

-
X

-
X

+
X

+
W
-
X

+
W
-
X

+
W
-
X

-
W

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Cogeneration across pinch is not
worthwhile

-
X

-
X

+
X
-
W

-
X

+
X
-
W

-
W

-
X

-
X

+
X

-
X

-
X

+
X
-
W

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Pinch in Separation

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Gas treating


The theory of pinch can be extended to design a separation
network, one such application is in the removal of hydrogen
sulfide from coke oven gas


El
-
Halwagi and Manousiouthakis, AICHE J. 1989m v.35, p.1233
-
1244


The basic process is shown in the following diagram


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Utility and process resource


Specifications


Ammonia is available at 2.3 kg/s as a byproduct of the process


Chilled methanol is available as a external mass separating agent


Henry’s law of hydrogen sulfide in ammonia and methanol are
1.45 and 0.26 respectively


Targets for cleaning and inlet conditions of solvent are as
follows:







What is the minimum amount of methanol required


Stream

Flow

Inlet H
2
S

Outlet

R1

0.9

0.070

0.0003

R2

0.1

0.051

0.0001

Ammonia

2.3

0.0006

Methanol

??

0.0000

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Saturation Targets


Due to mass transfer limit


since ammonia is responsible for cleaning the bulk gas, therefore its
saturation target cannot be greater than the equilibrium composition of the
richest inlet composition
0.070/1.45=0.0483


since inlet ammonia contains 0.0006
H
2
S, the cleanest target it can achieve is
greater than 0
.0006*1.45=0.00087








What is the minimum extra capacity of ammonia that is left


What is the maximum amount of methanol required

Stream

Flow

Inlet
H
2
S

Outlet

R1

0.9

0.070

0.0003

R2

0.1

0.051

0.0001

Ammonia

2.3

0.0006

0.0
483

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Composition levels

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Gas Phase

Source

Liquid Phase

Source

0.070

R1 Inlet

0.0483

R1 Inlet/H
A

0.051

R2 Inlet

0.0352

R2 Inlet/H
A

0.00087

H
A
*Process
Ammonia Inlet

0.0006

Process Ammonia
Inlet

0.00030

R1 Outlet

0.00010

R2 Outlet



0.

H
M
*Methanol Inlet

0.

Methanol Inlet

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Composition cascade


Extra ammonia capacity = 2.3
-
0.04242/(0.0483
-
0.0006)=1.41


Maximum methanol needed = 0.00059/(0.0001/0.26
-
0.0000) =1.53



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Water Pinch

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Water Demand and Source

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Water Cascade Table


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Summary


Pinch targeting is the estimation of minimum resource
required for accomplishing a task, thus identifying wasteful
processes



Pinch analysis also provide guidelines of good design
practice



Although pinch targeting and analysis do not provide actual
solutions of the design problem, they are still very helpful in
reducing energy and other resource usages.




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Process Integration

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Example of Process Integration
--

Gas Composite Cycle

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Basic Rankine Cycle


Most traditional power plants employed a standard Rankine
cycle, the process flow diagram of which is shown below.






Typical operating conditions are as follows


The working fluid is water


The compressor and pump operate adiabatically and reversibly


There is no heat loss


Boiler produces saturated steam at 10 MPa


Condenser produces recycled water at 310 K and 0.1 MPa.

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Boiler

Condenser

Turbine

Pump

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Rankine cycle efficiency


Water at 310 K and 0.1 MPa
~ saturated water at 310 K


H=155 kJ/kG, S=0.5319
kJ/(kG K), V
L
=0.001007
M
3
/kG


Pumped to water at 10
MPa


W~V

P~0


Boiled to saturated steam
at 10 MPa


T~585.15 K;


H=2725 kJ/kG, S=5.611
kJ/(kG K),


Q=(2725
-
155)=2570 kJ/kG



Expanding to wet steam at
0.1 Mpa


T~373.15 K; S=5.611 kj/(kG K


SL=1.306 kJ/(kG K);SV=7.355
kJ/(kG K) :
x
=0.712


HL=419 kJ/kG;HV=2676
kJ/kG: H=2025 kJ/kG


W=2725
-
2025 kJ/kG = 700
kJ/kG


Cooling to 310 K


Q=2025
-
155=1870 kJ/kG


Efficiency


700/2570x100%=27%


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Rankine cycle TS plot


Rankine cycle


1
to 2: pump


2 to 3: boiler


3 to 4: turbine


4 to 1: condenser


Total work is the back box


1

2

3

4

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Rankine Cycle vs Carnot Cycle


To provide heat for the boiler, a fuel is burned and produces
a flue gas at 1000 K. To cool the low pressure steam in the
condenser, the cooling water is available at 300 K.







Carnot efficiency between 1000 and 300 K is 70%


Area outside the black
-
lined region but inside the red box is the
lost work,
thermo
-
efficiency is 39%

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Improving the Rankine Cycle


It is well known that the efficiency of Rnakine cycle can be
improved by superheating and waste heat preheating

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Basic Gas Cycle


In automobile, air planes, and
sometimes small power generator,
power is obtained using a
gas cycle



Combustion

Chamber at 1000 K

Air in

Fuel

Exhaust gas

Compressor

Turbine

300
K

Mixing with
ambient air

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Combine cycle
--

TS


A combined cycle proposed to integrate the two process to
achieve higher efficiency

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Combine cycle
--

Process

Combustion

Chamber at 1000 K

Air in

Fuel

Exhaust
gas

Compressor

Turbine

Boiler

Condenser

Pump

Turbine

Heat

exchanger

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Other Examples of Process
Integration
--

Divided Wall Column,
Reactive Distillation

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Concept of Divided Wall Column

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Reactive Distillation


Improve Selectivity


Reduce Raw Materials Usage


Reduce Byproducts Prevent
Pollution


Reduce Energy Use


Utilize Heat of Reaction for
Separation


Handle Difficult Separations


Avoid Separating Reactants


Eliminate/Reduce Solvents


Enhance Overall Rates


“Beat” Low Equilibrium Constants

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Traditional and Reactive Distillation
Process for Methyl Acetate Production

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Process Intensifications

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What is Process Intensification


The term “Process Intensification” was coined in the 1980s
to describe some step out thinking about process unit
operations, in particular gas/liquid mass transfer. This new
approach ... led to the prospect of
much smaller

(i.e.
intensified) chemical plants that would be significantly
cheaper and safer
that existing ones.

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Keys to Better Mass Transfer


Well mixed liquid and gas phases


Lots of interfacial surface area


Thin liquid film


Counter
-
current operation


In general gases mix well anyway, as do low viscosity liquids in thin
films. Simple geometry teaches us that smaller, finer packing give us
more and more surface area so that would be the obvious way to go
-

a column with very fine packing with counter
-
current gas flow.
However a liquid film running through a bed of fine material is
problematic when the liquid film thickness is around the same as the
clearance between the bits of packing. Liquid flow essentially stops
and column floods.


The key therefore is the thickness of the liquid film and what controls
that.


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Rotating Disc Packed Bed (High
-
G)

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Energy Expenditure of an Absorption
Process

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Effect of Mass Transfer on Solvent Rate


Use of High
-
Gee absorber
make possible a lot of
equilibrium stages in limited
space


Hence the energy
expenditure can be much
reduced

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Keys to Heat Transfer


One of the keys to performance is heat transfer area so it is
surprising that many heat exchangers are based on pipes
that have a minimum surface area!


Clearly the plate heat exchanger is a much more effective
way of providing area, albeit with some mechanical
downsides.

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Polymer Thin Film Heat Exchanger


Although the thermal conductivity of the polymer is not as
high as metals, the PFCHE made of 100 micron thick PEEK
films offers negligible thermal resistance when the heat
transfer coefficient is less than 4000 W/m
2
K.


Flexibility of polymer film make it possible to packed large
amount of heat transfer area into a small volume
--

reduce
the minimium driving force for heat transfer

T
min



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Conclusions


The actual thermodynamic limit of process efficiency is
achieved when there is a pinch point (no driving force for
heat, mass or material transfer).


A real pinch point required an infinite equipment size


In practice, the finite achievable driving force is determined
by mechanical design of the equipment


Process intensification is a set of technologies that try to
reduce size of the equipment as the driving force approaches
zero

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