Embed Size (px)
Citation preview
MEEBAL Exam 3 December 2012 Show all work in your blue book. Points will be deducted if steps leading to answers are not shown. No work outside blue books (such as writing on the flow sheets) will be considered. No outgoing text messages are allowed during the exam. You must use given stream numbering in the problems (no points given if different numbering system is used). Report all answers with three significant digits. You are allowed to use three pages of notes and a calculator (no textbooks, computers or tablets such as iPads are allowed). You must pass in your test sheet (with your name on the sheet) with your blue book for your exam to be graded. You may request your final exam grade from the TAs (do not email Dr. Tessier) on or after January 2nd. There are no regrades for final exams, and final exams are not returned to students. Dr. Tessier or the TAs do not provide breakdowns of your final grade (so don’t bother requesting this). 1. (10 points)
A mixture of C4H10 and C5H12 (Stream 1) is burned completely with pure oxygen (Stream 2). Stream 3 contains 53.5% water. After removal of all the water, the residual Stream 3 contains 93.55% CO2 and the rest is O2. The molar flow rate of Stream 3 is 100 mol/s.
Calculate:
a. (5 pts) Composition (mole fractions) of Stream 1 b. (5 pts) Flow rate of Stream 2 (mol/s)
1
2
3
Furnace
O2
C4H10 C5H12 100 mol/s
53.5% H2O CO2 O2
MEEBAL Exam 3 December 2012 2. (20 points)
A mixture of 50 wt% acetone (A), 30% acetic acid (B) and 20% acetic anhydride (C) is distilled at 1 atm. The mass flow rate of Stream 1 is 100 kg/h, and the temperature of this stream is 348 K. Stream 2 is pure A. Stream 2 enters a condenser at 329 K and emerges as liquid at 303 K. The condenser heat duty is 55718 kJ/h. Half of the condensate is withdrawn as Stream 5 and the rest is refluxed back as Stream 4. Stream 3 enters a steam-heated reboiler where it is partially vaporized. Vapor from the reboiler returns to the column as Stream 6 and the liquid leaves as Stream 7. Both Streams 6 and 7 are at 398 K. The steam that heats the reboiler (which does not mix with Streams 3, 6 or 7) enters the reboiler as a saturated vapor and leaves as a saturated liquid. Calculate:
a. (5 pts) Flow rate of Stream 5 (kg/h) b. (5 pts) Composition (mass fractions) of Stream 7 c. (5 pts) Reboiler heating requirement (kJ/h) d. (5 pts) Flow rate (kg/h) of Steam at 15 bar gauge into the reboiler to meet the heating requirement
Thermodynamic Data:
For A: Cpl = 2.30 kJ/(kg-K) Cpv = 0.459+3.15x10-3 T- 0.79x10-6 T2 kJ/(kg-K) ΔHv (329 K) = 520.6 kJ/kg, Tb=329 K
For B: Cpl = 2.18 kJ/(kg-K)
For C: Cpl = 2.35 kJ/(kg-K)
ΔHv (saturated steam at 15 bar gauge) = 1945 kJ/kg
Column
Condenser
Reboiler
33
2
1
3
4 5
7
6
A (l), B (l), C (l) 398 K
A (l), 303 K
A (v), 329 K
100 kg/h 50 wt% A (l) 30 wt% B (l) 20 wt% C (l) 348K
Steam (15 bar gauge) for heating reboiler
MEEBAL Exam 3 December 2012 3. (20 points)
CS2(g) is formed by reacting CH4(g) with S(g) in the following reaction:
𝐶𝐻! 𝑔 + 4𝑆 𝑔 → 𝐶𝑆!(𝑔)+ 2𝐻!𝑆(𝑔) Stream 1 (100 mol/s) containing CH4(g) and S(l) in stoichiometric proportions at 125 °C is fed to a heat exchanger where the reactants are heated to 700 °C. At this temperature Stream 2 enters the reactor. Heat is removed from the reactor at a rate of 20 kJ per mol of feed. The reaction products (Stream 3) emerge from the reactor at 750 °C. Do not assume complete conversion unless specified below. Calculate:
a. (5 pts) Fractional conversion of CH4 achieved in the reactor b. (5 pts) Heat added to the reactants in the heat exchanger QH (kW) c. (5 pts) QR (kW) when 100% fractional conversion is achieved in the reactor d. (5 pts) Temperature of Stream 3 (oC) when no heat is transferred to/from the reactor (QR=0) and the
fractional conversion is the same as in part (a) Heat capacities: Cp (S(l)) = 29.4 J/(mol⋅K) Cp (S(v)) = 36.4 J/(mol⋅K) Cp (CH4(g)) = 71.4 J/(mol⋅K) Cp (CS2(g)) = 31.8 J/(mol⋅K) Cp (H2S(g)) = 44.8 J/(mol⋅K) Tb,S = 125 °C Heat of vaporization of S(l) at 125 °C = 83.7 kJ/mol Heat of reaction (700 °C) = -274 kJ/mol
MEEBAL Exam 3 December 2012 4. (10 points)
Flue gas (Stream 1) is fed to a heat exchanger at a rate of 100 mol/s and is cooled from 1000 °C to 150 °C. This heat is used to generate saturated steam (Stream 4) in the heat exchanger at 100 °C. The flue gas has a heat capacity given by:
𝐶!𝑅 = 3.83+ 0.000551𝑇/𝐾
The heat of vaporization of water at 100 °C is 2256.9 kJ/kg. The surroundings are at a temperature of 27 °C. Neglect pressure changes for the flue gas and water/steam. Assume that the heat exchanger does not exchange heat with the surroundings. Calculate:
a. (5 pts) The mass flow rate of Stream 4 (kg/s) b. (5 pts) Lost work in the process (kW)
MEEBAL Exam 3 December 2012 5. (10 points)
100 mol/s of flue gas (Stream 1) enters a turbine at 9 bar and 900 °C and discharges at 1 bar (Stream 2). The turbine operates adiabatically with an efficiency of 77%. The heat capacity of the flue gas is 32 J/(mol K). Assume that the flue gas is an ideal gas. Calculate:
a. (5 pts) Work output of the turbine (kW) b. (5 pts) Temperature (oC) of the gas discharging from the turbine (T2)
MEEBAL Exam 3 December 2012 6. (15 points)
The operating conditions for the steam power plant are: P1 = P2 = 8500 kPa T2 = 600 °C P3 = P4 = 10 kPa η (turbine) = 0.80 η (pump) = 0.80 Net work obtained from the process = 70,000 kW For superheated vapor at 2, H2 = 3635.4 kJ/kg S2 = 6.9875 kJ/kg-K For saturated liquid and saturated vapor at P3=P4, Hliq = 191.832 kJ/kg Hvap = 2584.8 kJ/kg (specific heat of saturated vapor, not the heat of vaporization) Sliq = 0.6493 kJ/kg-K Svap = 8.1511 kJ/kg-K Vliq = 1.010 cm3/gm Calculate:
a. (5 pts) Flow rate of water (kg/s) b. (5 pts) Rate of heat transfer in the boiler (kW) c. (5 pts) Thermal efficiency of the power plant
Boiler
Turbine
Condenser
Pump
1 2
3 4
MEEBAL Exam 3 December 2012 7. (10 points)
C3H8 is recovered from 100 kmol/s of a gas mixture (Stream 1) containing 8 mol% C3H8 and 92% N2 by contacting the mixture with liquid C10H22 (Stream 2) in an absorber. In the process, 90% of entering C3H8 is absorbed in Stream 3. Assume that C3H8 in Stream 3 (liquid) is in equilibrium with Stream 1. The absorber functions at T=27 °C and P = 1 atm. Antoine’s equation for C3H8 is as follows: log10 p* (bar) = 4.53678 – 1149.36 / (T (K) + 24.906) Calculate
a. (5 pts) Molar flow rate of Stream 3 (kmol/s) b. (5 pts) Temperature (oC) needed to transfer 99% of the C3H8 from Stream 1 to Stream 3
MEEBAL Exam 3 December 2012 8. (15 points)
SiH4 (g) + O2 (g) à SiO2 (s) + 2H2 (g)
1 m3/s of feed gas containing O2 and SiH4 in the ratio of 6 mol O2 per mol SiH4 enters a reactor at 25 °C and 3.2 mm Hg absolute. Reaction products emerge as Stream 2 at 1102 °C and 3.2 mm Hg. All the SiH4 in the feed is consumed. Assume ideal gas behavior.
Data:
ΔHformation, SiH4(g) = -61.9 kJ/mol
ΔHformation, SiO2(s) = -851 kJ/mol cp, SiH4(g) = 0.01118 + 12.2x10-5 T – 5.548x10-8 T2 (kJ/mol-K)
cp, SiO2(s) = 0.04548 + 3.646x10-5 T – 1.009x10-8 T2 (kJ/mol-K) HO2, 1102 °C (specific enthalpy of O2 at 1102 oC) = 36.10 kJ/mol
HH2, 1102 °C (specific enthalpy of H2 at 1102 oC) = 32.35 kJ/mol
Calculate: a. (5 pts) Mole fractions of components in Stream 2 b. (5 pts) Volumetric flow rate of Stream 2 (m3/s) c. (5 pts) Rate of heat transfer to or from the reactor (kW)
1 2
Reactor 1 m3/s O2 (g) SiH4 (g)
SiO2 (s) H2 (g) O2 (g)
