CHE 415: Chemical Engineering Laboratory Winter 2018
PLUG FLOW REACTOR
A continuous-flow plug flow reactor (PFR) for liquid-phase reactions is located in the NE corner of JOHN 214. The reactor consists of a clear glass tube filled with glass beads. The reactor is inclined slightly so that liquid will completely fill the tube. Two liquid feed streams are pumped at independent flow rates into the reactor. Each feed stream has its own pump and feed reservoir. The reactor effluent flows into the sewer. Underneath each feed reservoir is a shut-off valve. When the shut-off valve is opened, the liquid contents of the feed reservoir empty into the drain. Lab equipment for characterizing reaction kinetics in batch mode is also available. Conductivity probes and recording equipment are available.
Figure 1. The CHE 415 PFR system in Johnson Hall 214 consists of two 17.5 L reservoirs for dilute ethyl acetate and sodium hydroxide solutions, pumps and instrumentation to record conductivities and temperatures. The feed solutions are mixed in a header and reactor effluent is plumbed to the storm sewer.
Overview of Experimental Procedures
Note: Personal protective equipment is of paramount importance in this lab. In addition to the lab minimum of protective eyewear and a lab coat, you will wear an apron and face shield during concentrated solution preparation (see below) and nitrile gloves if operating or working around the batch reactor or PFR. Be sure to identify roles so that the dry working lab space and computer are not exposed to gloves or chemicals.
Conductivity Probe Calibration
Note: It is critical that the conductivity probe at the PFR outlet does not block the flow of your reactor. Consult an available instructor or TA to ensure the probe is positioned correctly before conducting your PFR experiments. Consult an available instructor or TA for assistance to assemble and connect the probe correctly and to locate the appropriate standards.
On your computer desktop, launch LoggerPro. At the top menu bar, select Experiment > Calibrate > Channel 1: Conductivity Probe. This will launch a dialogue box to set your conductivity standards. Ensure that your sensitivity setting is set to 0 – 20,000 µS/cm2 both on your conductivity probe and on your calibration dialogue box. Once verified, sufficiently submerge the probe in a vessel of water and establish low level point at 0 µS/cm2 and select Keep,
Decant approximately 30 mL of (2764 µS/cm2) standard solution into a small glass beaker from the stock bottle. Dry the probe before inserting into the standard. Establish high level point as previously described. Leave the probe in the standard solution to verify calibration by the in-time conductivity read out. Discard your standard solutions to the drain after use.
Equipment is provided for conducting batch experiments to characterize reaction kinetics. Select useful stock concentrations for aqueous solutions of ethyl acetate (CH3COOC2H5) and sodium hydroxide (NaOH) and prepare them at your lab bench using process water and the beakers provided. The equipment includes hot plates, external temperature controller, 250 mL and 400 mL beakers, graduated cylinders, Vernier conductivity probes and Vernier data recording equipment. Hot plates and external temperature controller are used to heat the solution at a constant temperature. Literature data should be used to inform your laboratory plans (runs, temperatures, durations, etc.). Consider having one of your team members at a time working to familiarize yourselves with the PFR process equipment, valving, etc. That will help ensure productivity in your second lab session.
Figure 2. Hot plates with external temperature controller and magnetic stirrers are used for batch experimentation. The external temperature probe (the orange wire) is used to the control the temperature of your solution. Always submerge the temperature probe to the middle of the liquid height in order to accurately measure the temperature of your solution. Note that operating temperature is approximately 3°C higher than setpoint.
!!!! CAUTION !!!! CAUTION !!!! CAUTION !!!! CAUTION !!!!
The heating surface can be extremely hot and cause severe burns!
The external temperature controller, temperature probe and conductivity probe wire should not come into contact with heating surface!
The external temperature controller should always be placed in vessel full of liquid, whenever the heater control is on. If not, this will cause the hot plate to heat up to 550°C!
· Remove minor exterior liquid spills promptly.
· Disconnect the power cord before moving or cleaning the unit.
· DO NOT touch the top surface even after disconnecting the cord because the top surface may still be hot enough to cause severe burns.
· Use tongs to handle hot glassware
Hot plate symbols
Caution – Hot Surface: Cautions that the top plate is too hot to touch.
Indicates that the accessory external temperature controller is properly plugged into the unit.
Heating instructions for temperature-controlled hot plates
· Connect the External Temperature Controller to the connector on the back of the unit. – Temperature Probe in Use Indicator: This will illuminate when External Temperature Controller is properly connected.
· Fill vessel with solution to be heated.
· place stir bar into vessel.
· Place vessel in the center of the top surface.
· Insert the tip of the External Temperature Probe into the solution.
· Secure the position of the External Temperature Controller by using a ring stand/support rod and clamp. – Assure that the cable of the External Temperature Controller does not come into contact with the heating surface.
· Turn Heat Control Knob until the Heating Temperature Display shows the desired heating temperature.
· Flashing Display: The number shown on the Heating Temperature Display will FLASH when the actual heating temperature is not at the set temperature.
· Constant Display: The number shown on the Heating Temperature Display will remain constantly ON when the measured solution temperature is at the set temperature. – Hot Top Indicator: The Hot Top Indicator will be ON at all times when the temperature of the top surface is too hot to touch (greater than ~60°C). – The Hot Top Indicator will FLASH when the Heat Control Knob is turned OFF but the top surface is still too hot to touch. – The Hot Top Indicator will be OFF when the temperature of the top is less than ~60°C.
Plug Flow Reactor Experiments
Prepare concentrated solutions of ethyl acetate (CH3COOC2H5) and sodium hydroxide (NaOH) feed solutions in the fume hood adjacent to the PFR using process water and the 1 L labeled Nalgene bottles provided. The feed reservoir volumes are both 17.5 liters. Determine in advance the required mass of sodium hydroxide pellets and ethyl acetate. Prepare concentrated stock solutions to be diluted in each reservoir to the desired feed concentrations. Safety note: ethyl acetate vapor is flammable and presents a serious fire risk if spilled or if solvent is allowed to openly vent to the lab. Ethyl acetate also has a strong odor when concentrated. Take measures to minimize fumes as breathing ethyl acetate vapor may cause respiratory irritation and headache.
Figure 3. After calibration of the conductivity probe. Place the conductivity probe as shown in the picture
Figure 3. The fume hood in JOHN 214 NE is shared by the CSTR and PFR teams for preparation of concentrated solutions. Be extra careful and communicative while working in close quarters with others. Leave the workspace as you found it, with stir plate, stir bar, graduated cylinders, etc.
CAUTION! Significant heat evolves when NaOH dissolves in water! Start with cold water and continuously stir while slowly adding pellets to the water to dissipate the heat of solution. The resulting concentrated NaOH solution is also extremely caustic – wear appropriate PPE (gloves, goggles, face shield, and lab coat) at all times when mixing and handling the solution. Always use a rubber bucket or other secondary containment when transporting containers of strong acids or bases.
Make sure the NaOH pellets have completely dissolved. Carefully pour each concentrated solution into the respective feed reservoir while continuing to fill the reservoir with water from the available hose (wear your PPE and pour along the side of the reservoir to prevent splashing). NOTE: Do not walk away from a reservoir as you fill it with a hose. Fill the reservoir with 17.5 L. Determine when you think it’s sufficiently well-mixed.
Calibrate each pump separately by measuring the liquid volume accumulated over a given time at a fixed “frequency control” setting. Repeat and get an average flow rate (liquid volume/time) at a given frequency control setting. Vary the frequency control setting to obtain a calibration curve of volumetric flow rate vs. frequency control setting. The calibration curve is valid only at a fixed stroke setting. Therefore, record the stroke setting. If you change the stroke setting even slightly, then you must re-calibrate the pump. If desired, you may calibrate the pump at a different stroke setting to change the range of flow.
Remember, you are conducting a reaction kinetics experiment. Periodically measure room, effluent stream, and feed reservoir temperatures.
At a fixed set of operating conditions, it is a good idea to take several samples of reactor effluent at various times to insure that steady-state operation has been achieved. Remember, the residence time in the reactor is set by the inlet liquid flow rates. Also, since the reaction rate is temperature dependent, be sure that you measure the temperature over the course of your experiments.
Drain each feed reservoir to the sewer. Rinse the reservoirs with tap water then pump at least 1 L of water through each pump and the reactor to thoroughly flush the system. Drain the remaining water in the reservoirs to the sewer. Pour the remainder of the concentrated solutions into the nearby lab sink simultaneously in order to neutralize each. It is important that you leave the lab station as you found it.
Design: 274 cm length
2.54 cm inner diameter
Packing: 0.6 cm beads, void volume 550 mL
Feed: 0.05-0.2M ethyl acetate (aq)
0.05-0.2M NaOH (aq)
Flow Rates: ethyl acetate feed solution:
134 to 255 mL/min at a stroke setting from 30 to 100
NaOH feed solution pump:
115 to 236 mL/min at a stroke setting from 30 to 100
Temperature: ambient (record feed, effluent, and air temperatures)
Appendix: Questions to Consider
The following is a list of questions that might be useful to discuss with your group:
How was steady-state operation verified?
Was the reactor system really isothermal? How was the isothermal condition verified?
How did the rate constant calculated from experimental data compare with literature values at the same temperature?
Was the reactor truly in plug flow?
What was the residence time for 90% conversion based on your measurement?