King Fahd University of Petroleum & MineralsCHEMICAL ENGINEERING DEPARTMENT

CHE 203Final Exam

ThermodynamicsTerm 071

Instructor : Dr. MUATAZ ALI ATIEHDate : 22 January 2008Duration : 3 hoursLocation : 4-105

Name of student: ID#: Section: 2

Section Maximum Marks Marks ObtainQ-1 25Q-2 25Q-3 25Q-4 25

TOTAL 100

Q1 [25 marks]Superheated steam at 40 bar absolute and 500 oC flows at a rate of 250 kg/min to an adiabatic turbine, where it expands to 5 bar. The turbine develops 1500 kW. From the turbine the steam flows to a heater, where it is reheated isobraically to its initial temperature. Neglect Kinetics energy changes.

(a)Write an energy balance on the turbine and use it to determine the outlets stream temperature.

(b)Write an energy balance on the heater and use it to determine the required input (kW) to the steam.

(c) Verify that an overall energy balance on the two-unit process is satisfied.

(d)Suppose the turbine inlet and outlet pipes both have diameter of 0.5 meter. Show that it is reasonable to neglect the change in kinetics energy for this unit.

Q2 [25 marks]

A mixture that contains 46 wt% acetone (CH3COCH3), 27% acetic acid (CH3COOH), and 27% acetic anhydride [(CH3CO)2O] is distilled at P = 1 atm. The feed enters the distillation column at T = 348 K at a rate of 5,000 kg/h. The distillate (overhead product) is essentially pure acetone, and the bottoms product contains 1% of the acetone in the feed. The vapor effluent from the top of the column enters a condenser at 329 K and emerges as a liquid at 303 K. Half of the condensate is withdrawn as the overhead product, and the remainder is refluxed back to the column. The liquid leaving the bottom of the column goes into a steam-heated reboiler, in which it is partially vaporized. The vapor leaving the reboiler is returned to the column at a temperature of 398 K, and the residual liquid, also at 398 K, constitutes the bottoms product.

(a) Calculate the molar flow rates and compositions of the product streams. (b) Calculate the condenser cooling requirement Q(kJ/h). (c) Use an overall energy balance to determine the reboiler heating requirement Qr(kJ/h). (d) If the reboiler heat is provided by the condensation of saturated steam at 10 bar gauge, at what rate must steam be fed?

5000 kg/h

Q3 [25 marks]

In the production of many microelectronic devices, continuous chemical vapor deposition (CVD) processes are used to deposit thin and exceptionally uniform silicon dioxide films on silicon wafers. One CVD process involves the reaction between silane and oxygen at a very low pressure.

SiH4(g) + 02(g) Si02(s) + 2 H2(g)

The feed gas, which contains oxygen and silane in a ratio 8.00 mol 02/mol SiH4, enters the reactor at 298 K and 3.00 torr absolute. The reaction products emerge at 1375 K and 3.00 torr absolute. Essentially all of the silane in the feed is consumed.

(a) Taking a basis of 1 m3 of feed gas, calculate the moles of each component of the feed and product mixtures and the extent of reaction, (mol).

(b) Calculate the standard heat of the silane oxidation reaction (kJ/mol). Then, taking the feed and product species at 298 K (25°C) as references, prepare an inlet—outlet enthalpy table and calculate and fill in the component amounts (mol) and specific enthalpies (kJlmol). (See Example 9.5-1.)

Data (Hf

o)siH4(g) = —61.9 kJ/mol, (Hfo)sio2(S) = —851 kJ/mol

(Cp)siH4(g)[kJ/(mol K)] = 0.01118 + 12.2 X 10-5 T — 5.548 x 10-8T2 + 6.84 X 10-12T3

(CP)so2(S)[kJ/(mol K)] = 0.04548 + 3.646 X 10-5T — 1.009 x 103/T2

The temperatures in the formulas for Cp, are in kelvin.

(c) Calculate the heat (kJ) that must be transferred to or from the reactor (state which it is). Then determine the required heat transfer rate (kW) required for a reactor feed of 27.5 m3/h.

Q4 [25 Marks]

(a)Nitrogen gas tank contains nitrogen gas at high purity ( 99 %). The specific volume of the gas in the tank is 0.01 m3/kg at a temperature 175K. Determined the pressure of nitrogen in a tank be using the ideal gas, Compressibility Chart, van der Waals, and Pittezer corolation equations. The error involved in each case is to be determined.

(b)Carbon dioxide initially at 50 kPa, 600 K undergoes a process in a closed system until its pressure and temperature is 2 MPa and 1000 K, respectively. Assuming ideal gas behavior, find the entropy change of the carbon dioxide by assuming constant specific heats.