Unit Operations Laboratory - Kinetics Experiment
Dr. Valerie Young

Background (Return to top)

Your company has recently begun to manufacture Crystal Violet Dye (CVD).  In polar solvents, it forms a bright purple solution.  The wastewater stream from the process (2000 gallons per day at 70 °F and pH 7.0) contains 5 x 10^-4 M CVD.  Your plant has an environmentally-friendly biological treatment facility for wastewater, which consists of a series of outdoor ponds.  These ponds are populated with microscopic organisms that metabolize CVD and similar molecules, releasing CO₂ and N₂O.  Although this system has been approved by the appropriate regulatory agencies, the company is concerned about public perception if the discharge to the ponds is bright purple.  You have been asked to propose a process to decolor the wastewater before it is discharged to the treatment ponds.  Discharge to the ponds must have a temperature between 40 and 90 °F and a pH between 6 and 8 to preserve the microorganism population.

Internal company literature provides the following information.
Crystal Violet Dye (CVD) is a dark powder of formula weight 408.0 g/mole.  CVD in aqueous solution strongly absorbs light at 592 nm, allowing the use of UV/Vis spectroscopy to measure concentrations below about 2.5 x 10^-5  M.  In aqueous solution, OH- reacts with CVD, turning the purple solution clear. The reaction is believed to be elementary, i.e., the order of the reaction is an integer.

A recently-deceased research engineer was testing the reaction of CVD with NaOH in water.  Her laboratory notebook was also consumed in the spontaneous combustion event which tragically ended her life. However, she left two Excel files: "Calibration December 2001.xls" and "Experiment December 2001.xls". Dr. Young will e-mail you these files upon request. You should use the information in these Excel files.

Produce a preliminary design for a reactive process to use NaOH to decolor the wastewater stream for discharge into the treatment ponds.  Include all operations/equipment necessary to make the specified wastewater stream meet the criteria for discharge to the bioponds. Select from the three ideal reactor types (plug flow, continuous stirred tank, stirred batch) for your design and justify your selection. Specify the approximate size of the reactor and fundamental operating variables such as temperature, pressure, feed flow rate, and feed composition.  (You do not need to size any other equipment.) Assume the reactors represent the major contribution to capital costs for this project. In order to produce a preliminary design, you will need some quantitative information about the CVD + NaOH reaction. You should decide what this information is (your experimental objective) and determine it experimentally.

Available equipment includes a jacketed, stirred reactor (3 liter total volume) that may be run in either batch or continuous mode, a diode-array UV/Vis spectrometer with a flow-through cell, and a constant-temperature water bath.  Reaction mixture may be continuously withdrawn from the reactor, passed through the spectrometer cell, and returned to the reactor using small-diameter Tygon tubing and a "sipper" pump supplied for this purpose.  Available supplies include tap water, distilled water, solid CVD, 1 N NaOH in aqueous solution, methanol, typical laboratory glassware and a balance accurate to 0.0001 g.  You may also use any other equipment or supplies available in the unit operations laboratory.

• A thorough understanding of the relationship between intrinsic properties of a reaction (rate coefficient, reaction order, Arrhenius parameters) and the equations describing the operation of idealized reactors (stirred batch, continuous stirred tank, plug flow).
• Careful planning of experiments to efficiently obtain both results and a quantitative measure of their accuracy.

These are in addition to the standard guidelines in the syllabus.
 

Introduction

Include a rate equation for the reaction, written as a differential equation in [CVD] (i.e., the left side of the equation should be d[CVD]/dt, where [CVD] is the concentration of crystal violet dye, and t is time). Discuss how the parameters in this equation are used to design a reactor.

Experimental Methods

Consider how best to measure each component of the reactor feed to minimize experimental error.  CVD is a very impressive dye.  Do not use plastic labware.  Glassware can be cleaned by rinsing with water, then with methanol and then with water again.  Allow time at the end of lab to clean up.  Do not include the step-by-step procedure for UV/Vis operation.  Assume anyone trying to repeat your work can read these instructions.  Include the concentrations of the reactants for each run and the amounts of each reactant added for each run in your test matrix. Typically, students are able to complete 5-6 batch runs per lab period.

Expected Data and Results

Include a plot of [CVD] vs. reaction time for a batch experiment based on the data in "Experiment December 2001.xls" and "Calibration 2001.xls". Tell whether the reaction appears to be zero, first, or second order. Justify. Tell how the plot would differ if the temperature or the initial concentrations of the reactants were changed. Justify. Present the quantitative information about the CVD + NaOH reaction that can be obtained from "Experiment December 2001.xls" and "Calibration 2001.xls".

Prelab Meeting

Test your data analysis procedure on "Experiment December 2001.xls". Bring your calculations to the prelab meeting.

1. The reactor can be run in batch or CSTR mode. In theory, it should be possible to collect more data more quickly in CSTR mode. In practice, students have been unsuccessful in achieving and maintaining steady state in CSTR mode.
• If you want to run in batch mode, take precautions to prevent carryover of reaction mixture from one run to the next. Also, think carefully about how to mix the reactants so that the effect of dilution does not confound your observations of concentration changes due to reaction.  Remember that the reaction begins as soon as the reactants contact each other.
2. Remember that the rate expression includes the concentrations in the reactor, not the concentrations of the feed solutions.
3. Assume that the reaction between CVD and NaOH is an elementary reaction, i.e., the order of the reaction is an integer.
4. Remember that reaction is happening in the flow cell and its associated tubing, as well as in the reactor. When the residence time in the flow cell is significant compared to the reactant lifetime, the data become unreliable, because a significant portion of the reaction happens in the tubing instead of in the stirred tank reactor. In this case, you must change the reaction conditions to slow down the reaction.  (You cannot change the flow through the tubing.)
• As a rule of thumb for batch mode, it should take at least 5 minutes for the absorbance to fall from ca. 2 to ca. 0.1. Experience has shown that NaOH concentrations greater than about 0.05 M lead to trouble.
5. An air bubble in trapped in the flow cell can produce a sudden jump in the absorbance. To avoid air bubbles, don't run the sipper pump unless the inlet tube is submerged, and avoid excessive stirrer speed.  (Set the stirrer at about 10). Gently tapping the flow cell and/or changing its orientation will free a trapped air bubble. Gently is the key. Rough handling will break the seal in the cell and air will leak in constantly. Leave the UV/Vis running while you dislodge the air bubble. You will still be able to use the data from before and after the air bubble appeared.
6. It is nearly impossible to dissolve more than 0.5 g of CVD in a liter of water.

(Last modified on 12/13/01)