Answer any FIVE questions

Transcription

1 School of Chemistry Pietermaritzburg November 2010 CTEC343 EXAMINATION 100 MARKS; 3 HOURS INDUSTRIAL CHEMISTRY External Examiner: Dr. Ian Love Dept. of Chemistry & Chemical Technology National University of Lesotho Internal Examiners: Dr. Desigan Reddy & Dr. Colin Southway School of Chemistry University of KwaZulu Natal INSTRUCTIONS: Answer any FIVE questions Non programmable calculators are permitted for calculations. To ensure accurate marking, students are required to write neatly and clearly. This paper consists of 19 pages, including: this cover sheet, 5 question pages, 2 Figure pages, 1 Periodic Table and 10 data sheets. Please ensure that you have ALL pages. Your answer sheets MUST be handed in with your answer books. Please ensure that you have written your student number on all submitted material. CTEC343 EXAMINATION, November 2010 Chemistry: UKZN Pietermaritzburg Page 1 of 19

2 QUESTION ONE A flow diagram for a sulfur burning contact process sulfuric acid plant is included as Figure 1. The reactions that occur in the plant are: S(l) + O 2 (g) SO 2 (g) H c = kj SO 2 (g) + ½O 2 (g) SO 3 (g) H c = kj SO 3 (g) + H 2 O(l) H 2 SO 4 (l) H abs = kj a) The formation of sulfur dioxide in the sulfur furnace goes to completion. If the flow rate of elemental sulfur to the furnace is 3.2 kg s 1, and the flow rate (at 25 C and 1.00 atm) of dry air is 22 m 3 s 1, what will be the composition of the gas mixture leaving the furnace? Show your working. (The composition of dry air is 21% by volume.) (7) b) Before the gas mixture enters the converter, will it be necessary to add any more air? Explain briefly. c) The converter shown in the flowsheet is a double absorption converter. The chart below shows how the conversion efficiency in the converter depends on the temperature. (2) Typical conversion efficency curve for production of SO 3 % conversion to SO Temperature ( C) (i) Explain clearly and briefly the shape of the graph. (1) (ii) Explain clearly why four beds of catalyst are used in the converter and what happens to the gas mixture as it passes through the converter. Use a simplified diagram, showing the flows into and out of the converter, to illustrate your answer. Describe what happens to the flows while they are outside the converter. (7) d) Typically, dual absorption plants for the production of sulfuric have an overall conversion efficiency of 99.7%. The unreacted sulfur dioxide is emitted to the atmosphere. For a plant with a sulfur feed rate of 3.2 kg s 1 : (i) What is the rate of production (in kg s 1 ) of 100% H 2 SO 4? (2) (ii) What is the rate of emission (in g s 1 ) of SO 2? (1) [20] CTEC343 EXAMINATION, November 2010 Chemistry: UKZN Pietermaritzburg Page 2 of 19

3 QUESTION TWO a) Saturated steam is to be used to heat a stream of water from 25 C to 90 C (at 1 bar). The saturated steam is fed into the heat exchanger at a gauge pressure of 300 kpa and the saturated condensate leaves through a steam trap at the same temperature and pressure. (i) What is the temperature of the saturated steam and the saturated condensate? (1) (ii) What mass of steam would be required to heat 1 kg of water? Show your working. (3) b) The gases leaving the furnace of a sulfur burning plant (see Figure 1 for the flow sheet) at a temperature of 900 C are cooled to 420 C in Boiler 1 where superheated steam at a pressure of 40 atm and a temperature of 400 C is produced from water at an inlet temperature of 90 C and a pressure of 1 atm. If the molar flow rate of the gas mixture entering the boiler is 900 mol s 1 and if the average molar heat capacity of the gas is 33.1 J mol 1 K 1, what is the rate of steam production in g s 1? Show your working clearly. (10) c) What is the value for H c for the oxidation of sulfur dioxide at a temperature of 500 C? Show your working. At 25 C: SO 2 (g) + ½O 2 (g) SO 3 (g) H c = kj (6) [20] QUESTION THREE a) The diagram below show a distillation column. You have been asked to do a mass balance analysis on this column to determine the composition of the bottoms and the rate of production of the distillate. (Vapour) Heat Exchanger Cooling 1000 kg/h Feed 10% EtOH Distillation Column (Reflux) Distillate (Product) P kg/h 60% EtOH 40% H 2 O Heat Bottoms (Waste) B kg/h EtOH H 2 O (i) (ii) Sketch the diagram and include, with a brief reason, a suitable system boundary. Is it possible to complete the mass balance with the information given? Explain briefly. (1) (3) CTEC343 EXAMINATION, November 2010 Chemistry: UKZN Pietermaritzburg Page 3 of 19

4 b) An evaporator is used to produce a 55 mass % sugar solution from a 30 mass % sugar solution. The feed rate of the raw sugar solution is 500 kg/h and the plant operates for 24 h/day. How much product sugar solution and how much water does the plant produce per day? (4) c) The salt can be removed from seawater by a process known as reverse osmosis. Essentially pure water can be obtained, along with a concentrated salt solution (brine). The diagram below shows a flow diagram for such a system. Some of the brine is recycled and added to the seawater feed. Brine recycle R kg/h 5.3% salt Sea water 1000 kg/h 3.1% salt 4.0% salt reverse osmosis cell Brine waste B kg/h 5.3% salt Pure water P kg/h 0% salt (i) (ii) Using a suitable system boundary (and showing that boundary in your answer), calculate the rate of waste brine production (B) and the rate of pure water production (P). (4) Using a suitable system boundary (and showing that boundary in your answer) calculate the rate of brine recycle (R). (4) d) Mixtures of acetonitrile and water are commonly used as mobile phases in liquid chromatography. Two different mobile phase mixtures have been prepared; one contains 12.0 % acetonitrile and the other contains 2.0 % acetonitrile. You now need to use these two mixtures to prepare 1.0 L of a mobile phase containing 4.0 % acetonitrile. What volume of each mixture should you take? (Assume that volumes are additive.) Show your working. (4) [20] CTEC343 EXAMINATION, November 2010 Chemistry: UKZN Pietermaritzburg Page 4 of 19

5 QUESTION FOUR Consider the following information obtained for a magnesium copper system at constant pressure: The melting points of magnesium and copper are 648 C and 1085 C respectively. Two congruently melting compounds, MgCu 2, which melts at 800 C, and Mg 2 Cu, which melts at 580 C, are formed. The three eutectics formed are at 9.4 wt % Mg (680 C), 34 wt% Mg (560 C) and 65 wt% Mg (380 C). (i) Using the graph paper provided, construct a detailed phase diagram for the system described above and clearly specify the identity of all phases present. (13) (ii) Give a detailed description of what occurs when a melt containing 25 wt% Mg is cooled from 900 C to 500 C. (2) (iii) Using the graph paper provided, construct with reasonable accuracy the cooling curves that would result for melts containing 14 wt% Mg, 34 wt% Mg and 90 wt% Mg when cooled from 1000 C to 300 C. (5) [20] QUESTION FIVE The isobaric, isothermal diagram for the system H 2 O / Li 2 SO 4 / (NH 4 ) 2 SO 4 is given in Figure 2. (i) Label all phase regions. (4) (ii) Describe in detail the sequence of events on evaporation of the solution of composition A. (4) (iii) For 100 g of solution of composition A, what would be the amounts of each species? (HINT* Consider your answer to (ii) above.) (3) (iv) Calculate the maximum number of grams of Li 2 SO 4 that can be added to 100 g of solution A, if ammonium sulfate is still to be the first solid to separates out on evaporation. Clearly explain your reasoning. (6) (v) Determine the composition of this new solution (as prepared in (iv) above) and mark it on the phase diagram. (3) [20] CTEC343 EXAMINATION, November 2010 Chemistry: UKZN Pietermaritzburg Page 5 of 19

6 QUESTION SIX (i) For a binary system comprising the volatile components, A and B, Dalton s Law can be expressed as: and where the terms all have their usual meanings. Using this relationship, derive (showing all details of your derivation) an expression that would give the ratio of the masses of the two volatile components, A and B, in the vapour, i.e.,vapour, in terms of their mole,vapour fractions in the liquid, i.e. and, the vapour pressures of the pure components, i.e. and, and their respective molar masses. (HINT* Start with the,vapour,vapour term!) (5) (ii) Relative volatility (α) is often used in the design of large industrial distillation processes and provides a measure of the differences in volatility between two components and hence their boiling points. Through the use of a suitable equation, explain what is meant by relative volatility and state how the numerical value of α can be used to decide whether distillation is a suitable means of separation. Assume the two liquids behave ideally. (3) (iii) Industrial towers often use reflux to achieve a more complete separation of products. Explain what is meant by the term reflux, the effect it has in the distillation column and how this impacts on the efficiency of separation. (4) (iv) Draw a suitable flow diagram for an industrial distillation operation. Your diagram should include the following labels: feed, bottom stage, top stage, partial reboiler, reflux, distillate, rectifying section, stripping section and total condenser. (5) (v) The McCabe Thiele method is a graphical approach that uses vapour liquid equilibrium data to determine the theoretical number of stages required to affect separation of a binary system. The method assumes constant molar overflow. List three requirements of this condition. (3) [20] CTEC343 EXAMINATION, November 2010 Chemistry: UKZN Pietermaritzburg Page 6 of 19

7 Figure 1 Sulfur burning contact process sulfuric acid plant (see Q 1) CTEC343 EXAMINATION, November 2010 Chemistry: UKZN Pietermaritzburg Page 7 of 19

8 School of Chemistry Pietermaritzburg November 2010 CTEC343 EXAMINATION 100 MARKS; 3 HOURS INDUSTRIAL CHEMISTRY Student Number: Figure 2 CTEC343 EXAMINATION, November 2010 Chemistry: UKZN Pietermaritzburg Page 8 of 19

9 CTEC343 EXAMINATION, November 2010 Chemistry: UKZN Pietermaritzburg Page 9 of 19 The Periodic Table H He Li Be B C N O F Ne Na Mg Al Si P S Cl Ar K Ca Sc Ti V Cr Mn Fe Co Ni Cu Zn Ga Ge As Se Br Kr Rb Sr Y Zr Nb Mo Tc Ru Rh Pd Ag Cd In Sn Sb Te I Xe Cs Ba * La Hf Ta W Re Os Ir Pt Au Hg Tl Pb Bi Po (209) 85 At (210) 86 Rn (222) 87 Fr (223) 88 Ra (226) 89** Ac (227) 104 Db (261) 105 Jl (262) 106 Rf (263) 107 Bh (262) 108 Hn (?) 109 Mt (?) * Lanthanide Series 58 Ce Pr Nd Pm (147) 62 Sm Eu Gd Tb Dy Ho Er Tm Yb Lu ** Actinide Series 90 Th (232) 91 Pa (231) 92 U (238) 93 Np (237) 94 Pu (239) 95 Am (243) 96 Cm (247) 97 Bk (247) 98 Cf (252) 99 Es (252) 100 Fm (257) 101 Md (256) 102 No (259) 103 Lr (260)

10 Data Sheet Physical Constants Boltzmann constant k = x J K 1 Planck constant h = x J s Elementary charge e = x C Speed of light in vacuum c = x 10 8 m s 1 = x cm s 1 Avogadro constant L or N A = x mol 1 Gas constant R = kl = J K 1 mol 1 = L kpa K 1 mol 1 = L atm K 1 mol 1 Molar volume of an ideal gas = L mol 1 (at atm and K) V m = L mol 1 (at kpa and K) Faraday constant F = el = x 10 4 C mol 1 Atomic mass unit (amu) u = x kg Rest mass of electron m e = x kg Rest mass of proton m p = x kg Rest mass of neutron m n = x kg Vacuum permittivity ε υ = x J 1 C 2 m 1 Standard acceleration of free fall g = m s 2 Rydberg constant for the H atom R H = cm 1 Conversion Factors 1micron (µ) = 10 6 m = 1 µm 1 Ångström (Å) = 1 x m = 0.1 nm = 100 pm 1 L = 10 3 m 3 = 1 dm 3 1 atm = x 10 5 N m 2 = x 10 5 Pa = 760 mmhg = 760 Torr 1 bar = x 10 5 Pa 1 psi = kpa = bar = atm 1 J = cal = 1 Pa m 3 = 1 m 2 kg s 2 1 cal = J 1 ev = x J 1 L atm = J 1 Btu = 1055 J 1 W = 1 J s 1 1 ppm = 1 µg g 1 = mg kg 1 = 1 mg L 1 (dilute aqueous solutions only) 1 tonne = 1000 kg 1 lb = 454 g T C = (T F 32)/1.8 Unit Prefixes P T G M k d c m µ n p f peta tera giga mega kilo deci centi milli micro nano pico femto CTEC343 EXAMINATION, November 2010 Chemistry: UKZN Pietermaritzburg Page 10 of 19

11 Superheated Steam Tables - Imperial Units ( The table shows the total heat (enthalpy) of superheated steam, in Btu per pound Pressure Saturated psi Temp Total Temperature--Degrees Fahrenheit ( t ) Abs. Gauge t P ' P CTEC343 EXAMINATION, November 2010 Chemistry: UKZN Pietermaritzburg Page 11 of 19

12 Superheated Steam Tables - Imperial Units ( The table shows the total heat (enthalpy) of superheated steam, in Btu per pound Pressure Saturated psi Temp Total Temperature--Degrees Fahrenheit ( t ) Abs. Gauge t P ' P CTEC343 EXAMINATION, November 2010 Chemistry: UKZN Pietermaritzburg Page 12 of 19

13 Superheated Steam Tables - Imperial Units ( The table shows the total heat (enthalpy) of superheated steam, in Btu per pound Pressure Saturated psi Temp Total Temperature--Degrees Fahrenheit ( t ) Abs. Gauge t P ' P CTEC343 EXAMINATION, November 2010 Chemistry: UKZN Pietermaritzburg Page 13 of 19

14 Enthalpy (kj/kg) of compressed water at different temperatures and pressures Pressure (bar) T ( C) CTEC343 EXAMINATION, November 2010 Chemistry: UKZN Pietermaritzburg Page 14 of 19

15 Pressure Saturated Steam Tables - Metric Units Temp Saturated Water Specific enthalpy Evaporation Saturated Steam bar kpa C kj/kg kj/kg kj/kg absolute gauge CTEC343 EXAMINATION, November 2010 Chemistry: UKZN Pietermaritzburg Page 15 of 19

16 Pressure Temp Saturated Water Specific enthalpy Evaporation Saturated Steam bar kpa C kj/kg kj/kg kj/kg gauge CTEC343 EXAMINATION, November 2010 Chemistry: UKZN Pietermaritzburg Page 16 of 19

17 Specific enthalpy Pressure Pressure Saturated Water Evaporation Saturated Steam bar bar C kj/kg kj/kg kj/kg gauge CTEC343 EXAMINATION, November 2010 Chemistry: UKZN Pietermaritzburg Page 17 of 19

18 Specific enthalpy Pressure Pressure Saturated Water Evaporation Saturated Steam bar bar C kj/kg kj/kg kj/kg gauge CTEC343 EXAMINATION, November 2010 Chemistry: UKZN Pietermaritzburg Page 18 of 19

19 Specific Heat Capacity (kj kg 1 K 1 ) T ( C) oxygen sulfur dioxide sulfur trioxide CTEC343 EXAMINATION, November 2010 Chemistry: UKZN Pietermaritzburg Page 19 of 19