Material and Energy Balances
An EXCEL Spreadsheet for Steady-State Simulation
of a Methanol Production Process
Brice Carnahan – University of Michigan
Introduction
Methanol is a major industrial chemical with worldwide production in excess of 20 million tons annually. Its
principal uses are as a raw material for other chemicals, in particular, formaldehyde, acetic acid, and octaneenhancing gasoline additives, as a solvent, and potentially as a liquid fuel.
Methanol was once produced by destructive distillation of wood, hence its common name, "wood alcohol". It is
now synthesized from pressurized mixtures of hydrogen, carbon monoxide, and carbon dioxide on the surface of
metal (copper, zinc, chromium) oxide catalysts supported on inert carrier particles. The principal reactions are:
CO + 2 H2 = CH3OH
CO2 + 3 H2 = CH3OH + H2O
(1)
(2)
The synthesis gas (sometimes called syngas) reactant mixture of CO, CO2, and H2 is created by complex
catalyzed reactions of methane and steam collectively known as steam reforming. The CH4 is compressed to
about 300 psia, mixed with steam, and heated to 1650 oF in reformer reactor tubes packed with nickelimpregnated ceramic particles inside a gas-fired furnace. Although the reactions are mechanistically quite
complex, steam reforming can be approximated for engineering purposes with two simple reactions:
CH4 + H2O = CO + 3 H2
CH4 + 2 H2O = CO2 + 4 H2
(3)
(4)
About three moles of CO are formed for every mole of CO2, so it is convenient to describe the stoichiometry of
these two independent reactions in the reformer with the following "combined" chemical equation:
4 CH4 + 5 H2O = 3 CO + CO2 + 13 H2
(5)
Fresh methane and steam are mixed in stoichiometric molar ratio 4:5 and reacted essentially completely.
The reactant synthesis gas mixture from reformer reaction (5) is cooled to a temperature of about 500 oF, mixed
with a cool recycle stream containing unreacted H2, CO, and CO2, compressed to a pressure of 1500 psia, and
fed to the methanol synthesis reactor where methanol is produced by reactions (1) and (2). The mixed input to the
reactor has a high fraction of hydrogen that, with the high reactor pressure, tends to drive equations (1) and (2) to
the right (remember LeChatlier's principle?).
The product gases from the synthesis reactor, containing methanol, water, and substantial fractions of unreacted
carbon monoxide, carbon dioxide, and hydrogen, leave the reactor at a temperature of about 725oF. The gas
temperature rises in the synthesis reactor because both reactions (1) and (2) are exothermic (give off heat).
The synthesis reactor effluent is cooled to about 105 oF so that almost all of the water and most of the methanol
condense, while the unreacted hydrogen and carbon monoxide and most of the carbon dioxide does not. Much of
the H2, CO, and CO2, and small amounts of water and methanol are recycled and mixed with the fresh feed
syngas. Because the amount of hydrogen from the reforming process exceeds the amount required for the
methanol synthesis, it is essential to have a purge stream in the methanol synthesis part of the process (why?).
The product mixture of liquid methanol and water is then distilled and separated into nearly pure methanol and
water streams. Typically, the water bottoms stream containing small amounts of methanol is sent to a biological
waste pond where bacteria metabolize (eat) any remaining methanol, thus avoiding water pollution problems.
Note that the flowsheet below shows only the processing units involved in reaction and separation (compressors,
heat exchangers, utilities, etc. are not included).
1
Purge
Methanol
Methane
Distillation
Column
Steam
Reformer
Synthesis
Reactor
Condenser
Water
Synthesis of Methanol from Methane
Excel Flowsheet/Spreadsheet and Steady-State Simulator
Prepare an Excel spreadsheet for simulating steady-state operation of the methanol (CH3OH) synthesis process
described in the introduction.
Spreadsheet layout
Create your spreadsheet in the style of the chloroprene example handed out and discussed in class, and available
for downloading on the website. In particular, the sheet should include:
1. A drawing of the flowsheet for the process that includes:
a) a labeled (“reformer”, “condenser”, etc.) box for each process unit for which material balance equations
must be solved (including mixers and splitters).
b) a labeled (integer, #) line for each process stream, with an arrow indicating the direction of mass flow.
2) A bordered set of cells near each stream containing stream parameter values, similar in appearance to:
F#
CH4
H2O
CO
CO2
H2
CH3OH
Total
2000.00
1.0000
0.0000
0.0000
0.0000
0.0000
0.0000
1.0000
Here, # is a stream number, and F# represents the flow rate (in lbm moles/hr) for stream #. The entries following
the six components are the component mole fractions. The entry following “Total” should be the computed sum of
the mole fraction entries (will equal 1.0, for properly computed mole fractions). Note: Please use the component
names and order shown above for all streams in your process spreadsheet.
3) Labeled cells (with borders) for user entry of the following equipment parameters:
a) fractional conversion (reaction 5) of CH4 feed in the reformer (normally will be 1.0).
b) per-pass fractional conversion (by reaction 1) of CO in the methanol synthesis reactor.
c) per-pass fractional conversion (by reaction 2) of CO2 in the methanol synthesis reactor.
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d) fraction of the condenser overhead stream that is to be purged.
4) Labeled cells (with borders) for user entry of:
a) the fraction of each component entering the condenser that leaves in the overhead stream.
b) the fraction of each component entering the distillation column that leaves in the overhead stream.
c) a trigger cell (content is either a 0 or 1) for controlling iterative solution of the balance equations.
5) A labeled cell for automatically counting the number of iterations required for solution of the material balance
equations. The cell should be initialized at 0 (when the trigger cell is set to 0), and be incremented by 1.E-8 for
each iteration (when the trigger cell is set to 1).
Spreadsheet calculations
For each unit, your spreadsheet should include cell formulas for computing all output stream parameters, given
values for all input stream parameters and all equipment parameters for the unit. This “simulation model”
approach is equivalent to the sequential-modular algorithm used in commercial chemical process simulators.
Since the flowsheet contains a recycle stream, initial values for the recycle stream parameters must be guessed,
and iteration will be required for solution of the process material balance equations. When the trigger cell is set to
0, all stream parameters for the recycle stream should be set to zero. When the trigger cell is set to 1, all stream
parameters in the process should be computed (including the recycle stream parameters, calculated from the
purge unit balance equations).
Simulation Runs
In the calculation control tab (Preferences entry in the Tools menu), set the convergence tolerance to 0.00001
(for convergence, relative changes of all stream parameter values from one iteration loop to the next should be no
larger than 0.00001). Set the maximum number of iterations for recycle calculations to 600.
Base case
Use feed stream and equipment parameter values as follows:
Feed streams:
fresh feed to reformer process
Reactors:
fractional conversion of CH4 in reformer reactor for reaction (5)
fractional conversion of CO in synthesis reactor for reaction (1)
fractional conversion of CO2 in synthesis reactor for reaction (2)
Condenser:
fraction of each component in feed to condenser that leaves in the overhead stream
for CH4
for H2O
for CO
for CO2
for H2
for CH3OH
=
=
=
=
=
=
= 2000 lb mol/hr of methane (only)
= 2500 lb mol/hr of steam (only)
= 1.00
= 0.20
= 0.15
1.000
0.010
1.000
1.000
1.000
0.020
Distillation column: fraction of each component that leaves in the overhead stream (note that values must
be supplied even for components that do not appear in feed to the column):
for CH4
for H2O
for CO
for CO2
=
=
=
=
1.000
0.005
1.000
1.000
3
for H2
= 1.000
for CH3OH
= 0.998
Fraction of condenser overhead purged = 0.5
Set the trigger cell to 0 and do the following:
a) print the spreadsheet showing all cell content (on a single page, if everything is readable).
b) save your spreadsheet under another name, set the sheet for display of all cell formulas and column and
row headings (may require changing column widths) and print it (may require more than one page).
In the original sheet, set the trigger cell to 1 and find the iterative solution. Once convergence is achieved and you
are satisfied that your base-case results are correct, print the spreadsheet showing all cell content.
Purge fraction study
Keeping the other base-case parameters, change the fraction of the condenser overhead that is purged from 0.5
to the following new values, in turn: 0.25, 0.1, 0.05, 0.025. In each case, iterate to find the solution and print the
spreadsheet.
Conversion fraction study
With the purge fraction set to 0.025 and other parameters as in the base case, double the fractional conversion for
the two synthesis reactions (this might be accomplished by redesigning the synthesis reactor, for example). Find
the iterative solution and print the spreadsheet.
Report
Prepare a short report that contains the following:
1.
All printed spreadsheets from the base case, purge fraction study, and conversion fraction study.
2.
Three Excel plots for the purge fraction study showing:
a) Recycle flow rate vs fraction of condenser overhead purged.
b) The total production rate of methanol for the process vs fraction of condenser overhead purged.
c) Total moles of CO and CO2 purged vs fraction of condenser overhead purged
3.
Answers to the following questions:
Does the fraction of the overhead condenser stream that is purged significantly affect the amount of
methanol produced?
How do you think the capital (construction) cost of the plant would vary (qualitatively) with decreasing purge
rate?
What affect does a doubling of the conversion rate for the synthesis reactions have on the overall
production rate of methanol?
BC
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