Question 1: A water tower has been cleaned with a chlorine solution, and the chlorine vapors in the tower exceed the allowable concentrations for the work crew to enter and finish repairs. If the initial chlorine concentration is 15 mg/m3, the volume of the tower is 1900 m3, and the allowable chlorine concentration is 1.5 mg/m3, how long must the tank be vented with clean air flowing at 2.0 m3/s before the workers can enter, if the tank operates as a CMFR?
Question 2: A railroad tank car derails and ruptures, releasing 380 g of pesticide into a creek flowing at a velocity of 0.10 m/s with an average cross-section of 3 m2. Twenty km downstream of the spill, the creek discharges into a pond containing 40,000 m3 of water. Assume that the pesticide is nondegradable in the timeframe available, that the spill can be treated as a spike input into the creek, and that the creek and pond behave as a PFR and CMFR, respectively.
(a) What will the maximum concentration of pesticide be in the pond, and how long after the spill will that concentration occur?
(b) How long will be required from the time of the spill until 99% of the pesticide has been flushed out of the pond?
Question 3. Ethylene dibromide (EDB) is a small organic compound that, if ingested over long periods, can cause diseases of the stomach, liver, kidneys, and reproductive systems. It used to be used in automobile gasoline and is still used in aviation fuels, and it has been found in trace concentrations in some water supplies. The USEPA has set the maximum contaminant le vel for EDB in drinking water at 50 parts per trillion (!).
An industry has a small wastewater flow (2.5 L/min) that contains 3mg/L each of EDB and sodium (which is nonreactive). The stream is treated in a 3000-L CMFR under conditions where the rate constant for the first-order decay of EDB is 7 d-1. The reactor has been operating at steady state, but a process upset has occurred, causing the influent concentrations of both EDB and sodium to jump to 30mg/L, where they remain for 24 hours.
(a) What is the effluent EDB concentration prior to the upset?
(b) Write the mass balance for sodium starting from an instant after the process upset, and determine the effluent concentration 6 hours after the upset.
(c) Repeat part (b) for the EDB.
Question 4: A 40-gallon electric water heater is being used to heat tap water that enters at a temperature of 10oC. When the heating level is set at its maximum, the heating element uses 5 kW of electricity. The heater is almost 100% efficient at converting electrical energy to heat, and heat losses from the unit are negligible.
(a) If several people take consecutive showers using hot water at a rate of 2 gal/min and the unit reaches steady-state operation, what will be the temperature of the water as it exits the heater?
(b) After the last person showers, how long will it take before the water in the heater warms up to 55oC?
Question 5: Calculate the minimum rate at which make-up water from a river at 15°C must be pumped to evaporative cooling towers for a 1000-MW nuclear power plant, if the plant is 32% efficient and all of the waste heat is dissipated via evaporative cooling.
Question 6: The data from last week’s demonstration is provided below. The volume of water in the ‘reactor’ for the Spike Input experiment was 500 mL in both sections. Test the results for your section to determine whether the data for binding of the dye to activated carbon fit a first-order kinetics expression and whether the reactor in the Spike Input experiment could be modeled as a CFMR. Find the best-fit values for the first-order rate constant for removal of the dye by binding to activated carbon, and for the average hydraulic residence time and flow rate in the mixing experiment.
