ENGR101: Chemical and Biomolecular Engineering (CBE)
Lab 1: Catalytic Reactions
Helium Carrier Gas
Many important chemicals and materials are produced by chemical reactions. These reactions can be made faster by using catalysts. We will use a common industrial reaction, hexane
cracking, as a model system to explore how catalytic reactors work. The “cracking” reaction is
important in petroleum processing as this is one of the steps in the conversion of the large hydrocarbons that exist in crude oil to gasoline, other fuels, and more valuable feedstocks.
The reactor in this experiment is a simple quartz tube, packed with 0.5 grams of H-ZSM-5
catalyst. A diagram of the experiment is shown in Figure 1. The tube is surrounded by a clamshelltype tube furnace that controls
Catalyst
the reaction temperature. A
steady stream of helium is fed
into a bubbler containing the nHeater
Gas
hexane. This mixture of n-hexane
Chromatograph
and helium is then carried into a
stainless steel network of tubes
allowing the flow to be either
passed through the reactor or byHexane
Bubbler
pass it via the valves. Once the
feed is passed through the reactor, the subsequent product leav- Figure 1: Schematic of the n-hexane cracking experiment. Helium
ing the bed is passed through a gas is the carrier in the GC and for the hexane feed to the reactor.
gas-sampling valve on the gas
chromatograph (GC) for analysis.
Many reactions can take place on zeolite catalysts like H-ZSM-5, as listed in Table 1. The reactions are called “cracking reactions” because the relTable 1: Some of the possible reactions
atively large hexane “cracks” into two smaller mole- that may occur in the cracking reactions.
cules.
Methane seldom survives but rather comYou can change the concentration of the hexane bines with an olefin in a secondary reaction.
fed to the reactor by changing the temperature of the
𝐶6 𝐻14 → 𝐶5 𝐻10 + 𝐶𝐻4
bubbler. The warmer the water bath that the bubbler
𝐶6 𝐻14 → 𝐶4 𝐻10 + 𝐶2 𝐻4
is immersed in, the higher the concentration of hexane
𝐶6 𝐻14 → 𝐶3 𝐻8 + 𝐶3 𝐻6
in the reactor feed. Changes in this concentration can
Secondary reaction
affect the rate of reaction.
𝐶𝐻4 + 𝐶𝑛 𝐻2𝑛 → 𝐶𝑛+1 𝐻2𝑛+4
You can change the reaction temperature by adjusting the temperature on the tube furnace. At higher
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reaction temperatures, the hexane conversion will increase, but the selectivity
He
Detector
to the various products will also change.
The goal in reactor design is to produce
Columns
as much of the desired products as effiSample
ciently as possible. In some cases, you
may want to maximize the production of
C4 hydrocarbons and in others you may
prefer to produce C3 hydrocarbons.
The compositions of the reactor
feed and product are measured by gas
chromatography (GC). The GC separates
Figure 2: Schematic of a GC and typical output. The area
the components of a mixed stream under the curve for each component is proportional to the
based on how those compounds inter- mol fraction of that component. The proportionality constant is dependent on the thermal conductivity of the comact with a packing material on a long ponent. In general, you will only calibrate the hexane comcoiled column in the GC. When you turn position.
the sampling valve on the GC, a defined
volume of vapor is injected into the helium carrier gas, which then passes through a packed column. A schematic of a GC and results from a typical experiment are shown in Figure 2. Each of
the components of the vapor sample interact differently with the packing in the column and,
therefore, leave the column at different times. The area under the peak in the detector signal
vs. time is proportional to the amount of that species in the sample.
The first step in using a gas chromatograph is to calibrate the retention times for your samples. This involves injecting pure samples into the GC and noting the retention times (i.e. how
long it takes for each species to exit the column). The retention times for the components in your
system have been determined for you in that the GC output will label peaks as C1, C2, C3, etc for
methane, ethane/ethene, propane/propene, etc. You may see multiple C2, C4 and C5 peaks.
These peaks correspond to the olefin, paraffin, and other possible isomers. However, the current
GC program does not achieve separation of the propane and propene molecules, so these two
compounds elute together as a single C3 peak.
You will be given some experimental data so that you can calculate the conversion of hexane
under different conditions. The conversion of hexane (C6) can be calculated by comparing the
mol fraction of hexane present in the feed to the mol fraction of hexane present after reaction:
𝑥
𝑋𝐶6 = 𝐶6,𝑓𝑒𝑒𝑑
𝑥
−𝑥𝐶6,𝑝𝑟𝑜𝑑𝑢𝑐𝑡
𝐶6,𝑓𝑒𝑒𝑑
(2)
where 𝑥𝐶6,𝑓𝑒𝑒𝑑 is the mol fraction of hexane in the feed, 𝑥𝐶6,𝑝𝑟𝑜𝑑𝑢𝑐𝑡 is the mol fraction of hexane
that remains in the product stream after reaction, and 𝑋𝐶6 is the fractional conversion of hexane.
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Conversion is often expressed as a percent. Be careful with the chemical engineering norm that
lower case x’s are mol fractions and uppercase X’s are conversions.
You will be given an excel spreadsheet of GC data for several conditions.
CBE Lab Assignment Part 1
1. Catalytic Reaction (10 points). Define catalytic reaction and describe how it is used in
chemical operations. (150-word limit)
2. Hexane Conversion (10 points). For each reaction condition calculate the hexane conversion. Present your results in a table.
3. Bubbler Temperature (8 points). At a fixed a reaction temperature of 400℃, plot the
conversion of hexane as a function of bubbler temperature. Present your plot at Figure 1
with a figure caption.
4. First Order Reaction (5 points). First-order reactions have reaction rates that are proportional to the amount of hexane available to react. At higher bubbler temperatures, there
is more hexane available to react. Consider how the data in your Figure 1 is or is not indicative of a first-order reaction. (200-word limit)
5. Reaction Temperature (12 points). At a fixed bubbler temperature, plot the conversion
of hexane as a function of reactor temperature. Present your plot as Figure 2 with a figure
caption. Discuss how reaction temperature impacts hexane conversion. (200-word limit)
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Lab 2: Dialysis
Dialysis involves the diffusion of low-molecular weight compounds from solution across a thin
porous membrane to a region of lower concentration. A familiar application of dialysis is the removal of waste products from the blood stream by artificial kidneys. Here, blood is passed
through hollow-fiber cellulosic membranes on the shell side while saline solution is circulated
counter-currently on the other (tube) side. Urea and other small molecules diffuse through the
membrane, and proteins and cells are retained in the blood. The dialysis apparatus in the Unit
Operations Lab is depicted in Figure 3.
Figure 3 Schematic of the Dialysis Apparatus. Flow into the dialyzer on both
the shell and tube sides is controlled by
the pumps. Flow out of the dialyzer on
both the shell and tube sides is controlled by needle valves on the flow
meters.
The dialyzer is a cartridge of thousands of hollow tubes and functions as a tube in shell mass
transfer unit. For this lab exercise, the dialyzer is just a unit operation (black box) in which potassium chloride (KCl) moves from one stream to another. You will be given the flow rates and concentrations of all the streams as shown in Table 2 and evaluate the material balance.
Table 2: Dialysis raw data. Notice that the columns in the table are arranged to match the arrangement of the instruments in the experimental apparatus.
Conductivity
Flow Rate
Conductivity
(mS/cm)
(gallons per hour)
(mS/cm)
Trial
Tube in
Tube
Tube
Tube
Shell
Shell
Shell in
Shell
out
out
in
out
in
out
1
0
2…
0
The conductivity data can be converted to concentration of potassium chloride by the formula given in Equation (1). This equation is an experimentally acquired calibration and has an
error of +/- 5%.
𝑚𝑜𝑙
𝑀 (𝑙𝑖𝑡𝑒𝑟
)=
𝐶(𝑚𝑆⁄𝑐𝑚)
4
100
(1)
Use volume conversions to convert your flow rates from gallons per hour to liters per hour using
Equation (2). The flowmeters of an error of ± 0.1 GPH.
𝐹 (𝑙𝑖𝑡𝑒𝑟
) = 3.79 × 𝐹(𝐺𝑃𝐻)
ℎ𝑟
(2)
Record results or your calculations on Table 3.
Table 3: Dialysis final data. Notice that the columns in this table are arranged in a manner to facilitate the material balance calculations.
Tube Side
Shell Side
In
Out
In
Out
Trial
Flow
(l/hr)
Concentration
(mol/l)
Flow
(l/hr)
Concentration
(mol/l)
Flow
(l/hr)
Concentration
(mol/l)
Flow
(l/hr)
Concentration
(mol/l)
0
0
Under isothermal conditions and at low potassium chloride concentrations (<0.1M), you may
assume that the liquid density is constant at 1.00 g/ml and independent of KCl concentration.
The molar mass of KCl is 74.55 g/mol.
CBE Lab Assignment Part 2
6. Dialysis (10 points). Define dialysis and describe how it is used in chemical operations.
(150-word limit)
7. Dialysis Final Data (10 points). Using Table 3 as an example, organize and convert the
raw data provided into the final data in preparation for mass balance analysis. Present
you results in a table.
8. Mass Flow Rates of KCl (10 points). Calculate the total mass flow rate of KCl (g/min) entering the dialyzer and the total mass flow rate of KCl exiting. Show the equations you
developed and display you answers a new table.
9. Mass Flow Rates of Liquid (10 points). Calculate the total mass flow rate of the liquid
(g/min) entering and exiting the dialyzer. Show the equations you developed and display
you answers by a new table (simply add columns to your table from #8).
10. Percent Differences (5 points). In an ideal steady state process, the mass flow rates entering and exiting are equal. In most cases, the entering and exiting mass flow rates of
KCl and the liquid in the dialyzer will be similar. You can quantify this by calculating the
difference in the material balances as a percentage using equation (3).
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% 𝑑𝑖𝑓𝑓𝑒𝑟𝑒𝑛𝑐𝑒 =
𝑡𝑜𝑡𝑎𝑙 𝑚𝑎𝑠𝑠 𝑜𝑢𝑡 𝑜𝑓 𝑠𝑝𝑒𝑐𝑖𝑒𝑠 𝑖−𝑡𝑜𝑡𝑎𝑙 𝑚𝑎𝑠𝑠 𝑖𝑛 𝑜𝑓 𝑠𝑝𝑒𝑐𝑖𝑒𝑠 𝑖
𝑡𝑜𝑡𝑎𝑙 𝑚𝑎𝑠𝑠 𝑖𝑛 𝑜𝑓 𝑠𝑝𝑒𝑐𝑖𝑒𝑠 𝑖
(3)
A positive valued means that more of a given species exits the unit than enters while a
negative error means that more enters than exits. Calculate the percent differences in the
mass flow rates of KCl and the liquid. Show your results in a new table (simply add columns to your table from #9)
11. Discussion of Results (10 points). Experimental error can be random or systematic. Random error is a result of the inherent accuracy of the experimental measurements and the
average error of many data points should be near zero. Systematic error results if one or
more of the measurements is systematically incorrect due to a problem with the calibration of the equipment, a leak, or some other repeated error. In most cases, random and
systematic errors coexist. Based on the four trials in this experiment, which do you think
is more important in this experiment? (150-word limit)
12. Comment on this Lab (not graded). The teaching team would appreciate any feedback
you have about this lab exercise.
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