Second Examination: March 9, 2005; 12:55 PM – 1:45 PM
Total – 100 points
For the multiple choice questions, justify your answer with as quantitative a reasoning as possible
- An endothermic reaction is taking place in a CSTR. The steady state temperature in the reactor is 270C. The reactor is surrounded by a steam jacket, where the saturation pressure of steam is 11.5 psia. The heat transfer coefficient between the jacket and the reactor is with a heat transfer area of 15 ft2. At steady state, what is rate of heat transfer from/to the jacket to/from the reactor in kJ/hour?
(Additional information: The saturation temperature of steam at the given pressure is 2000F).
10 points
Heat Transfer = U.A.(Ts –T)=
2. For the reaction conditions shown below, if the heat of reaction increases from -50 kJ/mole to -80 kJ/mole,
a. The slope of the XEB versus T line increases
b. The slope of the XEB versus T line decreases
c. The slope of the XEB versus T line is unchanged
10 points
Answer is b. As the heat of reaction increases, you will have higher temperature for every conversion.
- The following reaction is taking place in a PFR.
An engineer, Mr. Reck Less contends that the reactor must be run at as high a temperature as possible to obtain the highest possible conversion. His colleague, Mr. Lim It says that a conversion limit will be reached and multiple reactors with interstage cooling will be required to achieve high conversion. Who is right and why?
10 points
Mr. Reck Less is right. For endothermic reactions, there is no limit on conversion since Xe increases with T.
- For the given reaction conditions, as UA increases, the XEB versus T line
a. Does not change
b. Increases in slope and shifts to the right
c. Shifts to the left
d. Decreases in slope
10 points
Answer is b. The slope increases and the line shifts to the right.
- The following reaction is taking place in a CSTR. In order to ensure a high-conversion steady state, the engineer in charge should:
a. Increase the feed temperature.
b. Increase UA.
c. Decrease the feed temperature.
Justify your answer with a qualitative plot.
10 points
The answer is a. At higher feed temperature, the line intersects only the higher part of the G(T) curve.
- The gas-phase reaction is taking place in a PFR. Pure A enters the reactor at a pressure of 4 atmospheres and 350 K. The heat of reaction which can be assumed to be independent of temperature is -27 kJ/mol of A. The rate constant at 350 K is 0.2 s-1 and the activation energy is 18 kJ/mol. The volumetric flow rate is 10 liters/s. When the conversion of A is plotted versus the reactor volume, the following plot is obtained. Determine the temperature and concentration of A in the reactor at the mid-point (V=50) and at the exit of the reactor (V=100).
20points
- The following second-order liquid-phase reaction is taking place in a CSTR.
A and B are fed to the reactor at rates of 4 mol/min and 2 mol/min respectively at a temperature of 300 K. The total volumetric flow rate is 10 liters/min. Specific heats (in J/mol-K) of A, B, C and D are 125, 100, 130 and 135 respectively. The reactor is jacketed by water at a temperature of 400C. The overall heat transfer coefficient has been estimated at 200 J/(m2.s.K), while the heat transfer area is 0.5 m2. Mixing is ensured through an agitator, which contributes a work of 20 KW to the reactor. The heats of formation of A, B, C and D (at 298 K) are -45 kJ/mol, -30 kJ/mol, -50 kJ/mol and -60 kJ/mol respectively. The rate constant at 300 K is 0.1 and the activation energy is 30,000 J/mol. Find the steady-state temperature in the reactor for 90% consumption of the limiting reactant. Find the volume of the reactor to achieve this conversion.
30 points
1