1 CE 548 Introduction to Process Analysis and Selection.
Transcript of 1 CE 548 Introduction to Process Analysis and Selection.
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CE 548
Introduction to Process Analysis and Selection
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Reactors used for the treatmentReactors used for the treatmentWastewater treatment involving physical, chemical or biological
activities are carried out in vessels or tanks commonly known as “reactors”.
Types of reactors:
• Batch reactor: The flow enters, is treated and then is discharged and the cycle repeats. Once the processing commences flow does not enter or leave the vessel. Used for small operations.
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Reactors used for the treatmentReactors used for the treatmentTypes of reactors:
• Complete mix reactor: Complete mixing occurs instantaneously and uniformly throughout the reactor as fluid enters the reactor. Usually squar tanks, L~W. Used with most newer systems.
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Reactors used for the treatmentReactors used for the treatmentTypes of reactors:
• Plug-flow reactor: Fluid passes through the reactor with little or no longitudinal mixing and exit the reactor in the same sequence as they entered. Long rectangular tanks L>>W. Used in most older systems.
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Reactors used for the treatmentReactors used for the treatmentTypes of reactors:
• Complete-mix reactors in series (e)
• Packed-Bed reactors (f), (g)
• Fluidized-Bed reactors (h)
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Reactors used for the treatmentReactors used for the treatmentApplications of the reactors:
• The principle applications of reactors types used for the treatment of wastewater are reported in Table 4-1
• Operational factors that are considered in the selection of the type of reactor to be used:
The nature of the wastewater to be treated
The nature of the reaction
The reaction kinetics
The process performance requirements
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Table 4-1
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Reactors used for the treatmentReactors used for the treatmentHydraulic characteristics of reactors:
Ideal flow in complete-mix and plug-flow reactors for pulse (slug-dose)
= V/Q
Where;
= hydraulic detention time, T
V = volume of reactor, L3
Q = flowrate, L3
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Reactors used for the treatmentReactors used for the treatmentHydraulic characteristics of reactors:
Ideal flow in complete-mix and plug-flow reactors for step inputs (continuous injection)
= V/Q
Where;
= hydraulic detention time, T
V = volume of reactor, L3
Q = flowrate, L3
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Mass-Balance AnalysisMass-Balance Analysis Principle: conservation of
mass; mass neither created nor destroyed.
Rate of accumulation of
reactant within the system boundary
=
Rate of flow of reactant into the system boundary
-
Rate of flow of reactant out of
the system boundary
+
Rate of generation of reactant within
the system boundary
(1) (2) (3) (4)
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Mass-Balance AnalysisMass-Balance Analysis Preparation of Mass Balances
• Schematic
• Control volume
• List all data and assumptions
• List all rate expressions
• Select a basis for calculation
Application of mass-balance analysis
• Assumptions: Constant flowrate into and out of control volume No evaporation (constant volume) Complete mixing Reaction occurs within reactor
Rate of reaction is first-order (rc = -kC)
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Mass-Balance AnalysisMass-Balance Analysis Formulation of mass balance:
Accumulation = Inflow – outflow + generation
if the reaction is steady state, there is no accumulation (dC/dt = 0), Thus equation (4-6) can be written as:
VrQCQCVdt
dCco
kCrc
6)-(4 eq VkCQCQCVdt
dCo )(
VrQCQC co 0
)( oc CCV
Qr
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Modeling Ideal Flow in ReactorsModeling Ideal Flow in ReactorsModeling of the hydraulic characteristics of reactors is important because the results can be used to determine the actual amount of time a given volume of water will remain in the reactor.
Complete Mix Reactor:
Accumulation = Inflow – outflow + generation
Using tracer, nothing is being generated; generation = 0
For pulse (slug) input:
Integrating with limits C=Co to C=C, and t=0 to t= t yields:
QCQCVdt
dCo
CV
Q
dt
dc
0oC
eCeCeCC ot
oVQt
o/)/(
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Modeling Ideal Flow in ReactorsModeling Ideal Flow in ReactorsComplete Mix Reactor:
Accumulation = Inflow – outflow + generation
Using tracer, nothing is being generated; generation = 0
For step input:
Integrating with limits C=C to C=Co, and t=0 to t= t yields:
dtV
Q
CC
dC
CCV
Q
dt
dC
QCQCVdt
dC
o
o
o
)(
)(
)1()1()1( /)/( eCeCeCC ot
oVQt
o
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Modeling Ideal Flow in ReactorsModeling Ideal Flow in ReactorsPlug-flow Reactor:
Accumulation = Inflow – outflow + generation (generation = 0)
xxx QCQCVt
C
xx
CCQQCV
t
C
xx
CQxA
t
C
x
C
A
Q
t
C
x
C
x
C
A
Q
t
C
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Nonideal Flow in ReactorsNonideal Flow in Reactors Factors leading to nonideal flow in reactors: (Fig 4-6)
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Nonideal Flow in ReactorsNonideal Flow in Reactors Need for tracer analysis:
• Tracer analysis is used to assess the hydraulic performance of the reactor by measuring the residence time.
• Application of tracer studies include:
Assessment of short circuiting in sedimentation tanks and biological reactors
Assessment of contact time in chlorine contact basins
Assessment of the hydraulic approach conditions in UV reactors
Assessment of flow patterns in constructed wetlands and other natural treatment systems
Example 4-1 Page 236
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Reactions, Rates, and Coefficients Reactions, Rates, and Coefficients Types of reactions:
• Homogenous: the reactants are distributed uniformly throughout the fluid. (batch, complete-mix, plug-flow)
Rate of reaction;
• Heterogeneous: reaction occur at a specific site. (packed and fluidized bed reactors)
Rate of reaction;
dt
dCr
timearea
moles
dt
Nd
Sr
1
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Reactions, Rates, and Coefficients Reactions, Rates, and Coefficients Types of rate expressions:
• Typical rate expressions for selected processes: Table 4-6
• Integration and differential methods used to determine reaction rate coefficients: Table 4-7
• Example 4-5.
order)-mixed or n(saturatio
order)-(second
order)-(second
order)-(first
order)-(first
order)-(zero
CK
kCr
CkCr
kCr
CCkr
kCr
kr
BA
s
2
)(
2020
2121
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Modeling treatment process kineticsModeling treatment process kineticsBatch reactor:
kt
o
C
C
Co
eC
C
kCr
rdt
dC
Q
VrQCQCVdt
dC
0
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Modeling treatment process kineticsModeling treatment process kineticsComplete mix-reactor:
• Graphical solution: Example 4-6
no
no
oo
C
Co
nk
C
nQkV
CC
n
k
C
QVk
CC
kCr
VrQCQCVdt
dC
)/(1)/(1
1)/(1
:is expression the searies, in reactors for
:by given is 0) (dC/dt
conditions statesteady under 93)-(4 Eq to solution The
93)-(4
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Modeling treatment process kineticsModeling treatment process kineticsPlug-flow:
kQ
Vk
o
C
C
xx
Cxxx
eeC
C
kCr
Vrxx
CCQQCV
dt
dC
QC
VrQCQCVdt
dC
:by given is solution the 0), (dC/dt conditions statesteady under
:gives for form aldifferenti the ngSubstituti
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Modeling treatment process kineticsModeling treatment process kineticsPlug-flow with axial dispersion:
120-4 eq of use the facilitate to used be can 26-4 Figure
velocity fluid
dispersion axial of tcoefficien
factor dispersion
where;
120)-(4
:equation following theby given is solution The
u
D
uLDd
dka
daadaa
da
C
C
o
/
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)2/exp()1()2/exp()1(
)2/1exp(422
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