Chemistry Question
CEEGR/ENSC 3420 Homework #4 Name __________________________ Due: Sunday, May 19 at 5 PM 1. a. Calculate the mass of KCl required per day (kg/d) to precipitate silver (Ag) from 20 mg/L to 0.1 mg/L, given a wastewater flowrate of 1500 m3/d. There is a very small amount of Cl- initially in the water = 1 mg/L (2.8×10-5 M Cl- ). b. Would the KCl begin to precipitate before you reached this concentration? (Ksp of KCl = 21.7) 2. At what Ca2+ concentration in mg/L will precipitation of Ca3(PO4)2 occur in a solution containing 10-6 M PO43-. Use Ksp value from the CRC Ksp Table. 3. Nickel (Ni2+) is present in wastewater at 13 mg/L, and is to be precipitated out of solution until the Ni2+ reaches the MCL of 0.1 mg/L. The wastewater is initially at pH= 7 and has a flowrate of 3 MGD a. Write the solubility product equilibrium expression b. Solve for the pH required to achieve this treatment goal c. Calculate the mol OH- /d required to increase [OH-] from initial to final levels d. Calculate the mol OH- /d required to neutralize [H+] from initial to final levels e. Calculate the mol OH- /d required to form the solid (precipitate) f. Calculate the kg NaOH/d to supply the sum of parts c-e. g. If we use 25% wt/wt NaOH solution (density = 1.3 kg/L), and its unit cost is $0.90/gallon, estimate the cost per year to supply the caustic soda solution. 4. Write the units for k (using mol, L, and hr) for each order of reaction, and indicate what you would plot (i.e. what is y and what is x) for the linearized form of each of these equations in a batch reactor. a. 0th order b. 1st order c. 2nd order 5. Using the data below, determine the order of reaction, and then calculate the volume of CSTR required to reduce the contaminant from 100 to 5 mM for Q = 1000 m3/d. Show your tables, and r2 values, as well as CSTR calculation. C (mM) Time (hr) 245 0 188 1 152 3 124 5 96 8 60 20 6. Using the data below, determine the order of reaction and predict the maximum influent concentration allowed if this reaction is employed in a 2500 m3 CSTR with treatment goal of 20 µg/L and flowrate of 20,000 m3/d. Show your tables, and r2 values, as well as CSTR calculation. Time C (µg/L) (min) 1430 0 1275 15 985 25 820 60 542 170 431 220 208 340 7. The Verylittleturbulence River is best modeled as a PFR. A WWTP effluent pipe discharges into the river, resulting in a BOD concentration of 20 mg/L. Given the 1st order rate coefficient of 0.2/h, and a river velocity of 3 km/h, calculate the distance downriver where the BOD concentration = 1 mg/L. 8. A CSTR with one inflow and one outflow is used to catalytically degrade TCE (C2HCl3). However, there are two separate reactions that occur in this process, one with k1 = 0.05/hr, and the other with k2 = 0.002 mmol/L*hr. Groundwater contaminated with TCE at 15 mg/L is pumped at 500 m3/d into a 1000 m3 reactor. Beginning with the general material balance equation, derive the CSTR design equation and estimate the TCE concentration in the effluent. 9. Biodegradation of BOD in a lake after a rain event is observed to follow 1st order kinetics. During one episode, the BOD increased to 15 mg/L after a rain event and was reduced to 2mg/L in 1.5 days. After another rain event, BOD increased to 19 mg/L and decreased to 1.5 mg/L in 2.1 days. Use this data to estimate the reaction rate coefficient, k, and predict how long on average it would take to achieve 95% reduction of BOD. 10. An oil-storage area was abandoned 19 years ago. Oil spilled on the ground had saturated the soil at a concentration of 400 mg/kg of soil. A fast-food chain now wants to build a restaurant there, and samples the soil for contaminants, only to discover that the soil still contains oil residues at a concentration of 20 mg/kg. The local engineer concludes that because the oil must have been destroyed by the soil microorganisms at a rate of 20 mg/kg each year (i.e. 19 years at 20 mg/kg removed per year = 380 mg/kg removed, and 20 mg/kg remaining), that in one more year the site will be free of all contamination. a. Is this a good assumption? Why or why not? b. How many more years would you estimate that it will take for the soil to reach the acceptable contamination level of 1 mg/kg? 11. A tracer is used to model the fluid dynamics of a reservoir. The reservoir has two inputs, Q1 and Q2, and one effluent, Q. a. Derive the design equation for C as a function of time, in terms of the flowrates Q1 and Q2 b. Prepare a plot of C/C0 vs. t (t ranging from 0-60 d, in 3d increments), for Q1 = 5000 gal/d and Q2 = 20,000 gal/d, and reservoir volume = 1 MG c. Repeat but for Q1 = 50,000 gal/d, and Q2 = 20,00 gal/d. What happens to the concentration profile? Does this make sense, and why? 12.Given the following system, which can be modeled as a CSTR, derive the material balance equation for species ‘A’. You should start from the general material balance equation and clearly substitute for all terms, stating why you set any equal to 0. The reaction rate is best modeled as 1st order, with k = 0.2/hr. ‘A’ is degraded in the reactor and leaves in both effluents (C variables for reactor and effluents not indicated on figure, use your own labeling). a. Substitute for the following using variables only. accumulation = inflow = outflow = generation (use appropriate sign, + or -) = Final equation = b. Using your answers from part a, and given a goal for the concentration in stream four of 2 mM, what reactor volume is required?
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