Post on 03-Jun-2018
transcript
8/12/2019 Lecture 3f
1/21
.
Lecture 3
1
8/12/2019 Lecture 3f
2/21
A Hierarchical Decomposition for Process
Synthesis
To guide the selection of process alternatives, Douglas formalized aDecision Hierarchy as a set of levels, where more detail in theprocess flowsheet is successively added to the problem.
These levels are classified according to the following process
decisions: Level l: Batch versus continuous
Level 2: Input-output structure of the flow sheet
Level 3: Recycle structure of flow sheet
Level 4: Separation system synthesis 4a: Vapor recovery
4b: Liquid recovery
Level 5: Heat recovery network
2
8/12/2019 Lecture 3f
3/21
First level:
we consider batch processes only if at least one of the
following holds.
We must get the process operational in a few months.
The product is one where the first company to market
wins an enormous competitive advantage.
We need only a few days production for a year's supply.
We have little design information and the process is
sensitive to upsets and variations.
The product will likely have a total lifetime of one to twoyears before some other product will come out that
replaces it.
The value of the product overwhelms the cost to
manufacture it. 3
8/12/2019 Lecture 3f
4/21
level 2: we consider the number of raw materialand product streams and their overall relation tothe process.
We also consider the presence of by-products andinert components in the process and how they
participate in the reaction chemistry.An important question is the recovery of these
compounds. At this level, a process recycle maybe needed for the reactor, and the designer
needs to consider the addition of purge streamsto avoid the buildup of inert components or by-products.
4
8/12/2019 Lecture 3f
5/21
Level 3 Explores the recycle structure of theflow sheet and focuses more closely on the
reactor itself.
We consider the number of separate reactor
networks in the flowsheet and their
interactions through recycle streams.
We also consider the effects of reactor
conditions on the rest of the flow sheet. These
could include the effect of inerts as a diluent
in the reactor feed and the effects of
equilibrium in choosing pressure, excess
components, and adiabatic operation for the
reactor. 5
8/12/2019 Lecture 3f
6/21
Level 4 is divided into two decision stages: vaporand liquid recovery.Raw materials from this step will
be recycled to the reactor while products and by-products are generally processed further andremoved.
In vapor recovery, the more expensive stage, we also
need to consider the effect of purge streams and theremoval of components based on their value andtheir effect on the reactor if they are recycled.
In the liquid recovery stage, we prefer to use
distillation, as this is often the least expensiveseparation. Design decisions at this stage includesequencing of the separators and determining theiroperating conditions.
6
8/12/2019 Lecture 3f
7/21
Level 5 deals with the heat recoverynetwork (HEN) once all of the otherflowsheeting decisions have been made.
7
8/12/2019 Lecture 3f
8/21
Case study: Ethylene and Water to Ethanol
Water
Ethylene
Methane
Propylene
R
E
A
C
T
O
R
Methane(Waste)
Ethylene(Recycle)
Propylene (Waste)
Diethyl ether (Bi product)
Ethyl alcohol (Product)
Iso propyl alcohol (waste)
Water(Recycle)
535-575 K, 68 atm
5-7% conversion
W/E-4/1 but due to lowconversion per pass, we
choose small water ratio
0.6-1
10% M excess prevent coking
Excess water push
equilibrium towards ethanol
production and back to
reactant (ethanol)
Propylene to IPA 0.5-
0.7% conversion
8/12/2019 Lecture 3f
9/21
Maximum Potential Profit
Depending on the purity and composition of rawmaterial, product and market rate with inflations ,we calculate the maximum profit potential for thesetup.
Estimate the gross profit : Depending on themarket price of product and requirement andpurity
Calculate the cost of raw material
Profit =gross profit-cost of raw material
Equipment cost/3 + annual operating costprofit(assume payout time and depreciable life)
8/12/2019 Lecture 3f
10/21
BP (oC) Tcritical (oC) Pcritical (atm)
Water 100 374.4 217.6
Ethyl alcohol 78.4 243.1 63.1
Ethylene -103.7 9.6 50.7
Di ethyl ether 34.6 193.8 35.5
Methane -161.5 -82.1 45.8
Propylene -47.7 91.4 45.4
Isopropyl alcohol 82.4 235.16 47.0
Physical property data for speciesChemical formula, MW, Sp gravity, Melt Pt., enthalpy, VP (using antoine eq parameters)
M, E-M E, PL- E
8/12/2019 Lecture 3f
11/21
Ethylene + waste gases
Water
Diethyl
ether
Ethanol
Water+
waste
Water
Ethylene
Absorber
DC DC DCFlash
Reactor
Compressor
Compressor
4. Condensible and
non condensible
3. Recycle structure of
flow sheet
Purge
stream
8/12/2019 Lecture 3f
12/21
Sepn SepnREACTOR
SepnSepn
Sepn
REACTOR
REACTOR
W
ELPL
M
W
W
EL
PL
M
EL
PL
M
EL
PLM
EL
M
PL
EL
DEE
EAW
ELPL
M
DEE
EA
W
EL
MDEE
EA
W
EL
W
EL
W
EL
W
M
M PL IPA
EA
DEE
EA
DEE
EA
DEE
Alternative separation Schemes
8/12/2019 Lecture 3f
13/21
GENERAL PROCEDURE FOR
MATERIAL-BALANCE PROBLEMS
Procedure
Step 1. Draw a block diagram of the process.
Show each significant step as a block, linked by lines and
arrows to show the stream connections and flowdirection.
Step 2. List the available data.
Show on the block diagram the known flows (orquantities) and stream compositions.
Step 3. List all the information required from the balance.
Step 4. Decide the system boundaries.
8/12/2019 Lecture 3f
14/21
Step 5. Write out the chemical reactions involved for
the main products and byproducts.
Step 6. Note any other constraints, such as specifiedstream compositions, azeotropes, phase or reaction
equilibrium, tie substances
Step 7. Note any stream compositions and flows that
can be approximated.
Step 8. Check the number of conservation (and other)
equations that can be written, and compare with
the number of unknowns. Decide which variablesare to be design variables;
Step 9. Decide the basis of the calculation.
8/12/2019 Lecture 3f
15/21
calculate the stream flows for aproduction rate of 10,000 kg/h.
8/12/2019 Lecture 3f
16/21
8/12/2019 Lecture 3f
17/21
8/12/2019 Lecture 3f
18/21
8/12/2019 Lecture 3f
19/21
8/12/2019 Lecture 3f
20/21
8/12/2019 Lecture 3f
21/21
Chemical Process Design
After economical and technical feasibility; mass and
energy balance are taken care.
Synthesis of process involves two steps
I. Individual steps are selected
II. These steps are interconnected
Leads to flowsheet structure of the process
Now the simulation of process is carried out usingmathematical models to predict the flow rates,
compositions, temperature and pressure of product
21