Survey
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Homework Assignment #4 ENE 802 Physico-Chemical Processes in Environmental Engineering Fall 2003 Dr. Susan J. Masten Due: Nov. 26, 2003 1. The hydrolysis of methyl bromide (CH3Br) at 25 oC in a buffered aqueous solution (pH 7.0) containing 5 mM Br-occurs by the following reaction: CH3Br + H2O CH3OH + Br- + H+ To what extent is CH3Br ultimately converted to CH3OH when assuming that CH3Br is initially present at low concentrations (i.e., [CH3Br] < 2 M)? In the literature you have found the following data (all for 25 oC): Species CH3Br (g) CH3OH (aq) H2O (l) H+ (aq) Br- (aq) 2. Gof (kJ mol-1) Po (atm) Cwsat (1 atm) (mol L-1) - 28.2 - 175.4 - 237.2 0 - 104.0 2.1 1.6 x 10-1 Yesterday, a train derailment resulted in the spill of approximately 13,000 gal of the soil fumigant sodium metham (Vapam) (http://www.alanwood.net/pesticides/metam.html) to the Sacramento River, 70 km upstream from Shasta Lake, California’s largest reservoir. The spill killed almost all aquatic life in the river. In an aqueous environment, metham (which is highly soluble) hydrolyzes to form methyl isothiocyanate (H3C-N=C=S) Rate constants for this reaction are ka 300 M-1s-1, k’n 1 x 10-8 M-1s-1. Metham also undergoes indirect photolysis to form methyl isothiocyanate, with a pseudo-first-order rate constant of approximately 1 x 10-4 sec-1. The California Regional Water Quality Control Board urgently needs your advice, both as to the maximum concentrations of metham that will be encountered at different points along the Sacramento River, and as to the total load that will enter Shasta Lake. a) Calculate the maximum concentration (in M) of Na+ (which behaves as a conservative tracer) and metham that will be encountered, both at the town of Dunsmuir (10 km b) c) d) e) f) 3. downstream from the spill) and at the point where the Sacramento River reaches Shasta Lake (70 km downstream from the spill). How important are the processes of hydrolysis and indirect photolysis in attenuating the peak concentration of metham between Dunsmuir and Shasta Lake? Quantify your answer. What are the half-lives for hydrolysis and indirect photolysis? The pH of the water is 8.1 How important are these two processes collectively in attenuating the total load of metham between Dunsmuir and Shasta Lake? Data are currently not available as to Henry’s Constant for metham. Would you recommend measuring this parameter or do you believe that you can neglect air-water exchange of metham? Explain your conclusion – it would be great if you could estimate the Henry’s Constant. Estimate the total mass of methyl isothiocyanate (in moles) that will be transported into Shasta Lake, ignoring losses from any processes (such as air-water exchange) that might remove this compound during its transport in the Sacramento River. Do you think this is a reasonable estimate – or do you think that other processes would be important in the removal of methyl isothiocyanate? If so, which processes are important and why? (This incident actually occurred on July 14, 1991.) If ion exchange is used to reduce the Ag+ concentration of a waste stream from 5.7 mg/L to 0.43 mg/L, determine the effluent pH. The initial pH is 8.3. Use a polystyrene base DVB 8% cross linked resin, sodium form, sulfonic acid. Since the 2 exchangeable ions are monovalent, assume that the activity coefficients of RAg and RH are equal (i.e., RAg = RH) Ag in the expression for the selectivity coefficient. The selectivity coefficients are, K Na = H 8.51 and K Na = 1.27. 4. MSU has decided to treat the water used by the university for cooling and heating purposes with ion exchange (i.e., the cold water lines will not be treated). The water supply for the university is groundwater obtained from wells that are approximately 400 ft. deep. The raw water has the following characteristics: total iron: total manganese total hardness: temperature: 1.3 mg/L 0.5 mg/L 440 mg/L as CaCO3 50 oC The desired effluent hardness is 85 mg/L as CaCO3. The flow is to be calculated based upon the following criteria: 25,000 resident students at 50 gal/capita/day 34,000 non-resident students and faculty/staff at 25 gal/capita/day. Design an ion exchange treatment system which could be used to remove the hardness from this water. Describe (you do not need to design) any treatment you might need before ion exchange. A strong acid cation exchange column is to be used in the sodium mode for the removal the hardness ions. Sodium chloride (10% aq. solution) is to be used as the regenerant. The manufacturer’s specifications give the following information: a) Softening capacity: Salt (lb/ft3) Capacity (in kgrains (as CaCO3) per ft3 4 17.8-19.5 6 21.5-23.1 10 27.5-28.1 18 31.5-32.2 b) Leakage: A maximum of 0.75% of the untreated water hardness c) Bed depth: 30-48 in. d) Back-wash bed expansion: at backwash rate of 6.0 gpm/ft2, 75% e) Effective size of the resin beads: 0.45 mm f) Operating conditions: Operation Service Backwash Regeneration Rinse Rate 2-5 gpm/ft3 6-8 gpm/ft2 at 50-75 oC 0.25-1.0 gpm/ft3 1-5 gpm/ft3 Solution Water Water Time (min) ---5-15 10-20% NaCl Water 30-60 10-40 The time for rinse should be sufficiently long as to pass 20-35 gal water/ft3 resin through the column. In designing the system you are to determine the hardness leakage, the bypass fraction (if used), the quantity of water to be treated each day, the number of columns required, the cross-sectional area and volume of each column, the exchange capacity of each column, the service time of operation, the weight of salt and volume of salt solution, the regeneration flow rate and cycle time required. 5. A water at pH 6.7 contains 8.9 x 10-4 M total carbonate (CT). For coagulation, 30 mg/L alum (MW = 594 g/mole) is added. a) b) What will be the final pH if the system is closed (i.e., no gas transfer is allowed)? How much lime (CaO) is required to maintain the pH at 6.7? Rework the above problem if the system were in equilibrium with the air. ( PCO2 Error! Switch argument not specified.= 0.00033 atm) (KH = 10-1.45 at 25°C) 6. You are to chemically treat a wastewater to remove suspended solids using coagulation followed by sedimentation. The wastewater has the following characteristics: Q = 5 x 105 m3/day Total suspended solids (TSS) = 260 mg/L Alk = 220 mg/L as CaCO3 Temperature = 15 oC pH = 8.2 You performed a jar test to determine the mean velocity gradient required to induce coagulation resulting in a decrease in 95% of the turbidity. A dosage of 100 mg/L ferric chloride was required. The flocculation apparatus you used is described below: 4 flat rotary paddles Cd = 1.20 Paddle dimensions: 12.5 cm long and 2.5 cm wide Velocity of paddles: 35 cm/sec Relative velocity between water and the paddles: 75% of the paddle velocity Volume of water in flocculation basin = 45 L Based upon the design criteria obtained from the jar tests, along with “conventional wisdom”, you decided to design a flocculation system that has a retention time of 45 min. (a) What torque must be applied if the paddles are to rotate at 2.5 rpm? (b) If the collision efficiency factor is 0.5, what is the value for the volume fraction of colloidal particles per unit volume of suspension, ? 7. You are to design a dissolved air flotation system to treat a waste stream, of 300 gpm, having a temperature of 103 oF. This waste stream contains significant quantities of nonemulsified oil and non-settleable suspended solids. The concentration of oil and grease, as measured by the hexanes extractable test, was found to be 125 mg/L and the suspended solids concentration was 140 mg/L. Since the oil material is of mineral origin and not easily biodegraded, it was deemed necessary by the Michigan DNR that you must reduce the effluent levels of suspended solids to 20 mg/L and the oil and grease to 25 mg/L. Note: Cs = 18.6 mg/L at 103 oF. Laboratory studies provided the following additional information: optimum alum dosage: 50 mg/L pressure: 50 psig f = 0.5 sludge: 3% by weight Calculate: (a) the recycle rate, (b) the surface area of the flotation unit, and (c) sludge quantities generated in lb dry weight/day and in gal/day.