EEC 503Spring 2009
Homework 5
1.Constructed wetlands have been proposed as wastewater treatment systems. Plants
produce enzymes that break down organic pollutants. Both extracellular and intracellular degradation are known to occur. Research is being conducted to study the feasibility of this method for removing explosives and ammunitions at numerous defense sites. The system involves a gentle downflow of water over sloping vegetation with a slow accompanying water evaporation rate. Consider a 1 km x 0.1 km x 0.2 m wetland. A nonvolatile pesticide runoff of concentration 10-2 mol/m3 is introduced into the wetland at a flow rate of 0.05 m3/min and undergoes a first order biochemical reaction with a rate constant of 10-5 min-1.
What is the steady state concentration of the pesticide at the outflow from the wetland? Consider water to be in plug flow. Compare the result if you assume CSTR behavior. Which is more likely?
2.A type of waste incinerator in use for combusting pollutants is called a rotary kiln.
It is a cylindrical reactor inclined to the horizontal, and undergoing slow rotation to achieve adequate mixing of the reactants in any cross section. Consider a pesticide (DDT) containing waste being incinerated in the kiln at a temperature of 300˚C. If its decomposition is first order with a rate constant, k of 15 sec-1 at 300˚C, determine the reactor volume required for 99.99% destruction of DDT from the waste-stream flowing into the kiln at a rate of 0.01 m3/s. Consider the kiln to be first a CSTR. Repeat your calculations for a PFR. If the stack gas emission guidelines for HCl from the incinerator is stipulated to be less than 1 kg/h, is the unit in compliance? The feed concentration of DDT is 1 x 10-4 mol.m-3.
3.An impurity A must be reduced in a product stream to make the product acceptable. For a liquid phase reaction, A products, you have measured reactant concentration as a function of time in an isothermal, batch reactor and the data are given below.
You want to design a plug flow reactor to process the feed of 100 (mol/lit) and containing CA0 = 10 (mmol/lit) and to achieve conversion of 80%. What should the volume of the plug flow reactor be? The PFR operates at the same temperature for which batch data are given.
4.Gas Phase reaction A4 Pwas studied in a batch autoclave at constant volume and at constant temperature of 200˚C. The information obtained for the rate of disappearance of reactant A is presented in a tabular form below:
You are to operate a continuous flow jet-stirred reactor for this reaction which you can assume behaves as a continuous flow stirred tank reactor CSTR. The feed consists of 50% (molar) A and 50% inerts and is at a total pressure of 10 atm and 200˚C. We want to achieve 90% conversion of A. Find the reactor volume necessary for a production rate of 4 kmol P/min. You can safely assume that the reactor operates isothermally at 200˚C and that ideal gas law can be used for the gas mixture.
5.Dreamerite is a fermentation product obtained from alcolite. The reaction is catalyzed by a culture of micro organisms (locally within the company known as bugs) which grows and multiplies by using negligible amount of alcolite and other reactants available in excess and in the process produces dreamerite. The reaction stoichiometry is:
The rate of dreamerite production at the desired temperature of 60˚C was found to be:
The rate of growth of micro organisms at that temperature is (this rate of growth is sustained by even small amounts of alcolite):
where NB is the number of bugs. Experience shows that the maximum concentration of bugs that can be handled is 103 bugs/L. The local market could easily absorb the production of 5250 metric tons of dreamerite a year. The molecular weight of dreamerite is 110 .
The feed is available at 1 molar concentration of alcolite. Alive bugs could not be separated from the reaction mixture but they could readily be handled with the reaction mixture. The separation of dreamerite from the unconverted alcolite is inexpensive; thus, the outlet conversion of reactant is not important, as the unused portion could be readily recycled. The limiting factor is the cost of micro organisms. It is costly to pay the Biochem Company which is constantly supplying new cultures for batch production.
. Within the above constraints evaluate whether the process can be performed continuously at steady state and which reactor type should be used. What reactor volume is going to be needed? Remember that a good reactor choice should achieve the desired production, should not exceed the permissible concentration of micro organisms in the outflow, should minimize (cut out?) the use of fresh micro organism culture and minimize the required reactor volume. Calculate the needed reactor space time and volume.
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