Top-rated ScreenCasts

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05.2 - The Rankine cycle Click here. 100 1

Thermal Efficiency with a 1-Stage Rankine Cycle. (uakron.edu, 12min) Steam from a boiler enters a turbine at 350C and 1.2MPa and exits at 0.01MPa and saturated vapor; compute the thermal efficiency (ηθ) of the Rankine cycle based on this turbine. (Note that this is something quite different from the turbine's "expander" efficiency, ηE.) This kind of calculation is one of the elementary skills that should come out of any thermodynamics course. Try to pause the video often and work out the answer on your own whenever you think you can. You will learn much more about the kinds of mistakes you might make if you take your best shot, then use the video to check yourself. Then practice some more by picking out other boiler and condenser conditions and turbine efficiencies. FYI: the conditions of this problem should look familiar because they are the same as the turbine efficiency example in Chapter 4. That should make it easy for you to take your best shot.

Comprehension Questions:

1. The entropy balance is cited in this video, but never comes into play. Why not?

2. Steam from a boiler enters a turbine at 400C and 2.5 MPa and exits a 100% efficient turbine at 0.025MPa; compute the Rankine efficiency. Comment on the practicality of this process. (Hint: review Chapter 4 if you need help with turbine efficiency.)
17.06 Determining the Spontaneity of Reactions Click here. 100 1

Which way will a reaction go? (3:40) (msu.edu)

When both reactants and products are present in a reactng mixture, the direction the reaction will proceed is not necessarily indicated by the sign of ΔGo or Ka. Rather, it is determined by ΔG. This screencasts provides guidance for understanding this concept.

Comprehension Questions: (Hint: review Example 17.1 before answering.)

1. CO and H2 are fed in a 2:1 ratio to a reactor at 500K and 20 bars with a catalyst that favors only CH3OH as its product. When the conversion of H2 is 32%, will the reaction go forwards towards product or back to reactants?
2. CO and H2 are fed in a 2:1 ratio to a reactor at 500K and 20 bars with a catalyst that favors only CH3OH as its product. When the conversion of CO is 52%, will the reaction go forwards towards product or back to reactants?
3. CO and H2 are fed in a 1:1 ratio to a reactor at 500K and 20 bars with a catalyst that favors only CH3OH as its product. When the conversion of H2 is 42%, will the reaction go forwards towards product or back to reactants?
4. CO and H2 are fed in a 1:1 ratio to a reactor at 500K and 20 bars with a catalyst that favors only CH3OH as its product. When the conversion of H2 is 42%, will the reaction go forwards towards product or back to reactants?
17.05 - Effect of Pressure, Inerts, Feed Ratios Click here. 100 1

How to push, pull, persuade a reaction (3:32) (msu.edu)

Pressure can be used to influence conversion for reactions where gas phase species are present. Feed ratios, inerts, or simultaneous reactions can also be used.

Comprehension Questions:

1. The principle by which a change in temperature, pressure, or concentration leads to a counteracting change in equilibrium is known as:_____.
2. For the reaction: CO + 2H2 = CH3OH, an increase in pressure will cause the products to: ___ (decrease, increase, or be unaffected). Explain.
2. For the reaction: CH4 + H2O = CO + 3H2, an increase in pressure will cause the products to: ___ (decrease, increase, or be unaffected). Explain. (FYI: this reaction, known as "steam reforming" is an important step in making chemicals from natural gas.)
3. For the reaction: CO + 2H2 = CH3OH, adding an inert component will cause the products to: ___ (decrease, increase, or be unaffected). Explain.
4. We discuss temperature effects in detail later, but for now you should be able to make predictions based on ____ principle (cf. #1 above). An exothermic reaction gives off heat. Therefore, adding heat to an exothermic reaction (ie. raising the temperature) will cause the products to: ___ (decrease, increase, or be unaffected). Explain.
5. For the reaction: H2O + CO = H2 + CO2, an increase in pressure will cause the products to: ___ (decrease, increase, or be unaffected). Explain. (As a first approximation, you may neglect deviations from ideal gas behavior, but then discuss the effect these deviations would have if you did take them into account. Which component's fugacity would be most affected by these deviations and how do these deviations change with pressure?)
6. For the reaction: coal + H2O = CO + H2, an increase in pressure will cause the products to: ___ (decrease, increase, or be unaffected). Explain. (Hint: carbon in the form of coal is solid and does not exist in the vapor phase. cf. section 17.14. It might be helpful to think of the reverse reaction, known as coking, where the solid carbon precipitates from the gas. This is a very simple example of simultaneous reaction and phase equilibrium.)
7. For the reaction: CO + 2H2 = CH3OH, adding an inert liquid to the reactor through which all products are removed will cause the products to: ___ (decrease, increase, or be unaffected). Explain. (Hint: this is a bit more sophisticated example of simultaneous reaction and phase equilibrium. How will the inert liquid alter the concentrations in the vapor? Remember that the fugacities are proportional to the gaseous partial pressures.)
08.08 - Reference States Click here. 100 1

This sample calculation shows how to compute the liquefaction in the Linde process for methane as the operating fluid. (uakron, 8min) The Linde process is a slight variation on the OVC cycle wherein the liquefied fraction exiting the throttle is captured as product and removed from the process. There is also heat integration in the sense that the cold vapor is used to precool the feed to the throttle.

FYI: Since natural gas is mostly methane, this process could be easily adapted to the production of liquefied natural gas (LNG) or liquified petroleum gas (LPG, mostly propane). Liquefied gases may seem impractical when you first encounter them, but they are more efficient for transport because they are so much more dense than the gases. Keeping them as liquids is basically a reflection of the effectiveness of the insulation. If any gas leaks from the relief valve (~1.1 bar), then liquid must evaporate to fill the space. The requisite heat of vaporization in that case cools the remaining below the boiling temperature. No heat = no vaporization.
17.05 - Effect of Pressure, Inerts, Feed Ratios Click here. 100 1

Partial pressures and reactor sizing are among the keys to chemical engineering calculations (uakron.edu, 7 min, review from Section 1.6). Partial pressures (uakron.edu, 7 min) also play an essential role in reaction equilibrium calculations. Partial pressure calculations basically involve straightforward mass balances, but specific vocabulary and a need for systematic precision can cause difficulty. The calculations involve six elements that must be carefully computed:

(1) Stoichiometry - the reaction equation must be stoichiometrically balanced such that the number of atoms of each element are the same on both sides of the equation. This balance is achieved by adjusting the stoichiometric coefficients. The change in the number of moles of each component must be in correct stoichiometric proportions relative to the "key component." Inert compounds (see below) are NOT included in the stoichiometric equation. For the example in this presentation, the objective of the reactor is to oxidize carbon monoxide (CO) in a catalytic converter by reacting it with oxygen (O2). So, CO + 0.5 O2 = CO2.
(2) Limiting reactant (aka. "key component") - It is common to feed an excess of one of the components in order to promote complete conversion of the other components. The limiting reactant is the component that is NOT in excess. For this example, O2 is fed in excess so that CO conversion can be promoted. CO becomes the limiting reactant in that case and conversion must be computed relative to CO, NOT O2. If you think about it, expressing the conversion with respect to the excess component would mean that 100% conversion could result in a negative mole number for the limiting reactant. Such an implication is obviously physically impossible (and potentially embarrassing if you appear not to know that).
(3) %Excess - The number of moles of an excess component in the feed is (1+Xs) times the stoichiometric amount relative to the key component, where the stoichiometric amount is the number of moles necessary to perfectly balance the key component, and Xs is the fractional form of the %excess. For this example,  the stoichiometric ratio of CO:O2 would be 1:0.5 and for 50% excess, Xs = 0.50, and the actual ratio would be 1:0.75.
(4) %Conversion - the %conversion is the fraction of the entering amount of the limiting reactant that is transformed into product(s). Note that this might be different from the "extent of reaction," ξ. For example, if 50 moles/h of CO enter the reactor and the conversion is 90%, then 5 moles of CO exit the reactor. If you express the number of moles of CO as 50-ξ, you might conclude that the moles of CO exiting the reactor is 49.1. Take a minute to think about what the words mean before you start to calculate, then make a mental estimate of what the results should be, then get out your calculator. Another common mistake is to apply the % conversion to all the components, wrongly including the excess component. For example, if 45 moles of CO react, then 22.5 moles of O2 react. With 50% excess O2 in the feed, the O2 exiting should be 37.5-22.5=15, NOT 3.75. This is what it means to be careful and systematic. You must compute the conversion of limiting reactant first, then compute the conversion of other components relative to the limiting reactant.
(5) Inerts - These are components that may enter the reactor by coincidence or convenience but do not participate in the reaction. Therefore, their number of moles exiting the reactor is simply equal to their number of moles entering the reactor. A common mistake is to apply the %conversion to all components entering the reactor, including the inerts. In this example, the source of O2 is air, with roughly 4:1 ratio of nitrogen (N2) to O2. The N2 is inert.
(6) Total Pressure - Once the mole numbers and mole fractions have been computed, don't forget to multiply the mole fractions by the total pressure to get the partial pressure. The partial pressure is equal to the mole fraction only in the case that the reactor operates at 1.00 bar.

Comprehension Questions:

1. What is the value of the total pressure (bar) applied in the presentation of this example?
2. What equation is used to compute the mole number of O2? What is the final overall equation used to compute PO2?
3. Suppose 100 moles/h of ammonia (NH3) at 100bars is to be produced from N2 and hydrogen (H2) with 10% excess N2. Methane (CH4) is included with the N2+H2 as a result of the synthesis process with a ratio of 1:10 CH4:H2. (a) Write a stoichiometrically balanced equation (b) Identify the limiting reactant (c) Calculate the number of moles and partial pressures of each component entering the reactor. (d) Calculate the number of moles and partial pressures of each component exiting the reactor assuming 25% conversion.
10.07 - Nonideal Systems Click here. 100 1

This screencast shows how to quickly visualize Pxy phase diagrams for nonideal systems using Excel (5min, uakron.edu). These sample calculations for methanol+benzene apply the simplest nonideal solution model: ΔHmix = A12*x1*x2. Rigors of this model are discussed in Chapter 11. Nevertheless, its basic elements are simple enough that they can be understood in Chapter 10. When x1=0 or x2=0, a pure fluid is indicated, corresponding to no mixing and zero heat of mixing. When A12=0, the ideal solution approximation is recovered. When A12>0, the model indicates an endothermic interaction (like 2-propanol+water, Fig. 10.8c), giving rise to "positive deviations from Raoult's Law." When A12<0, the model indicates an exothermic interaction (like acetone+chloroform, Fig. 10.9c), giving rise to "negative deviations from Raoult's Law." With this spreadsheet, you can quickly change your components and A12 values to see how the phase diagram changes and gain "hands-on" familiarity with the principles discussed in Section 10.7. 

Note: This is a companion file in a series. You may wish to choose your own order for viewing them. For example, you should implement the first three videos before implementing this one. Also, you might like to see how to quickly visualize the Txy analog of the Pxy phase diagram. If you see a phase diagram like the ones in section 11.8, you might want to learn about LLE phase diagrams. The links on the software tutorial present a summary of the techniques to be implemented throughout Unit3 in a quick access format that is more compact than what is presented elsewhere. Some students may find it helpful to refer to this compact list when they find themselves "not being able to find the forest because of all the trees."

Comprehension Questions:
1. Make a Pxy diagram for cyclohexane+toluene at 80C and A12=200. What kind of system is this?
2. Make a Pxy diagram for cyclohexane+benzene at 80C and A12=200. What kind of system is this?
3. Why does the system's qualitative behavior change so much when the components and model parameters are changed so little?
11.02 - Calculations with Activity Coefficients Click here. 100 3

Bubble Temperature (2:43) (msu.edu)

The culmination of the activity coefficient method is application of the fitted activity coefficients to extrapolate from limited experiments in a Stage III calculation. The bubble temperature is the easiest after bubble pressure. The recommended order of study is 1) Bubble Pressure; 2) Bubble Temperature; 3) Dew Pressure; 4) Dew Temperature. Note that an entire Txy diagram can be generated with bubble temperature calculations; no dew calculations are required.
07.11 - The molecular basis of equations of state: analytical theories Click here. 100 1

Nature of Molecular Parking Lots - RDFs(20min, uakron.edu) Molecules occupy space and they move around until they find their equilibrium pressure at a given density and temperature. Cars in a parking lot behave in a similar fashion except the parking lot is in 2D vs. 3D. Despite this exception, we can understand a lot about molecular distributions by thinking about how repulsive and attractive forces affect car parking. For example, one important consideration is that you should not expect to see two cars parked in the same space at the same time! That's entirely analogous for molecular parking. Simple ideas like this lead to an intuitive understanding of the number of molecules distributed at each distance around a central molecule. From there, it is straightforward to multiply the energy at a given distance (ie. u(r) ) by the number of molecules at that distance (aka. g(r) ), and integrate to obtain the total energy. A similar integral over intermolecular forces leads to the pressure. And, voila! we have a new conceptual route to developing engineering equations of state.
Comprehension questions:
1. Sketch u(r)/epsilon and g(r) vs. r/sigma for square well spheres at a very low density. Use a solid line for g(r) and a dashed line for u(r)/epsilon.
2. Sketch u(r)/epsilon and g(r) vs. r/sigma for hard spheres at a high density. Use a solid line for g(r) and a dashed line for u(r)/epsilon.
3. Sketch u(r)/epsilon and g(r) vs. r/sigma for square well spheres at a high density. Use a solid line for g(r) and a dashed line for u(r)/epsilon.
03.3 - Introduction to Mixture Properties Click here. 100 1

Props.xlsx has a lot of data, but usually we are only interested in a few components at a time. Adding a few lines at the top and applying the VLookup function makes it easy to tabulate the properties you need. (8min, uakron.edu)

Comprehension questions

1. Download the latest version of Props.xlsx from sourceforge. Add lines to support 8 components of interest and cells to compute Psat given T as input and Tsat given P as input by appropriately arranging Eqn. 2.47. Add a column for computing Hvap at Tsat for each component by Eqn. 2.45.

2. Insert a sheet(tab) called Hrxn in Props.xlsx. Types the names for components in the reaction CO+0.5O2=CO2. Use VLookup to tabulate the Hf values for each component. To the left of the name column, insert cells to represent the stoichiometric coefficients. Then calculate the heat of reaction by using the sumproduct() function applied to the stoichiometric coefficients and Hf values. Check your result with a hand calculation.

3. Download the latest versions of PREOS.xls and Props.xlsx from sourceforge. Update the Props tab appropriately. Then implement the VLookup function on the ThermoProps tab of PREOS so all you need to do is type the name of the compound of interest in order to update the ThermoProps sheet to all properties of interest. We discuss how to use PREOS.xls to solve problems in Unit II.
07.02 Corresponding States Click here. 100 2

Principles of Corresponding States (10:02) (msu.edu)
An overview of use of Tc and Pc and acentric factor to create corresponding states correlation. The relation between acentric factor and deviations from spherical fluids is highlighted.

Comprehension Questions:

1. What is the value of the reduced vapor pressure for Krypton at a reduced temperature of 0.7? How does this help us to characterize the vapor pressure curve?

2. Sketch the graph of vapor pressure vs. temperature as presented in this screencast for the compounds: Krypton and Ethanol. Be sure to label your axes completely and accurately. Draw a vertical line to indicate the condition that defines the acentric factor.

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