Chemical Reactor Systems: STR and PFR with Temp & Flow Control, Study notes of Chemistry

Download Chemical Reactor Systems: STR and PFR with Temp & Flow Control and more Study notes Chemistry in PDF only on Docsity! BROWN INDUSTRIES REACTION CELL Department of Chemical Engineering University of Michigan Ann Arbor, MI. 48109 August 16, 2008 5 — 1 EQUIPMENT DESCRIPTION The reactor cell equipment, located in room 3000 of the G. G. Brown building, contains two small pilot scale reactors. A stirred Tank Reactor (STR) that can be used as a batch, semi- batch and continuously stirred tank reactor (CSTR). A small plug-flow reactor (PFR) that can be used as a differential reactor for the study of reaction kinetics. Figure 1 is a piping and Instrumentation diagram of the CSTR system and Figure 2 is a diagram of the PFR system. COMPONENTS Stirred Tank Reaction Vessel (STR): Four vessels are available to be used with the tank reactor cell. Either one of the four vessels can be installed in the cell. The top of the cell is common to all 4 vessels and contains openings for the following items: 1. Top-entry variable-speed agitator 2. Thermocouple (TC) for reaction temperature monitoring. 3. Sampling loop connections that allows for continuous monitoring of pH and Conductivity as well as a syringe sample-withdrawal port. 4. Two feed inlet ports. 5. One product discharge port. 6. One manual feed port (to accommodate a plastic funnel with a narrow stem) 7. One vent port. The four vessels available for the tank reactor are:  Three cylindrical jacketed Pyrex glass tanks of approximately 0.5-liter, 1-liter, and 3-liter capacity. The jacket can be connected to the Heating/cooling loop. The reactor currently installed is a "Chemglass" brand model CG-1931-12 with inside dimensions approximately 97 mm diameter by 320 mm high. The impeller is an all-Teflon 6-blade turbine "Chemglass" model CG-2092-01 approximately 50 mm diameter.  A cylindrical jacketed Pyrex glass tank of approximately 1-liter capacity. The Jacket in this vessel was sealed under vacuum to minimize heat loses through the reactor walls. This vessel has no bottom valve so it must be drained through the siphon tube connected to the drain pump. The agitator can be turned on or off from the main control panel, and from a local switch located on top of the motor. The speed of the agitator can be varied from a potentiometer located on the motor. Jacket Recirculation Loop: A centrifugal pump and associated piping to produce a high flow of liquid in the jacket of the reactor. The recirculating loop allows for heating and cooling of the stirred tank reactor jacket or, alternatively, one of the feed streams of the plug-flow reactor (PFR). A steam-injection heater is provided to heat the recirculating water. A controlled cold water supply is provided to cool the recirculating water. The recirculating loop is equipped with 2 TCs to monitor the temperature of the water entering and leaving the jacket. A flow switch and a temperature switch are installed to shut off the steam flow when there is insufficient recirculation flow or when the temperature in the jacket exceeds 95oC. This is done for safety reasons, and to prevent boiling of the water in the recirculation loop. Feed System: Two variable-speed variable-stroke positive-displacement piston pumps with a maximum capacity of approximately 200 ml/min for pump #1 and approximately 120 ml/min for pump #2. These pumps can withdraw liquid from two distinct feed reservoirs and can be controlled manually and through the controller, by selecting the appropriate position of the switches on the control panel. These switches have 3 positions: 1. ON, the pump motor runs at full speed regardless of the signal from the controller; 5 — 2 Plug Flow Reactor (PFR): In addition to the four tank reactors, there are 2 plug-flow reactors (PFR) available for use with this equipment. One reactor consists of a 4-mm inside diameter glass tube 47-cm long filled with 1-mm diameter rasching rings. The other reactor available is a KENICS1 model no. 37-03-075 in-line static mixer 3.4-mm inside diameter 19-cm long with 27 helical mixing elements. Although the PFR does not have temperature control, the temperature of the feed can be controlled by the same heating/cooling control system used in the jacket of the CSTR (see Fig. 2 for Jacket Recirculating Loop component details). With the PFR set-up, the recirculating loop functions as a pre-heating system to warm the feed stream #1 before it is fed to the reactor. The feed system for the PFR set-up is the same as the one for the CSTR. The temperature of the PFR feed preheater can be controlled by the software installed in the computer for data acquisition. Choose the PFR data acquisition program when operating the plug flow reactor. This program uses the same control algorithm used for isothermal STR control, and it maintains the temperature of the feed stream #1 at a desired set point by either heating or cooling the water in the recirculating loop. As was the case with the isothermal control, the maximum and minimum achievable loop temperatures are 65 C and the temperature of the city water supply (8 to 25 C depending on the season) respectively. This controller consists of a master Proportional- Integral-Derivative (PID) control loop which monitors the temperature of the flow leaving the preheater, and a slave PID control loop which uses the output of the master controller as a set point and in turn controls the temperature of the recirculating loop. The PFR is equipped with its own thermocouples (in addition to the 2 on the recirculating loop) for reaction temperature measurements. Thermocouple #6 measures the temperature of the feed #1 stream just prior to its entry into the reactor. Thermocouple #14 is located at the outlet of the PFR. Additional Instrumentation:  Two thermocouples for temperature measurements independent of the reactors (#4, and #5). The following on-line instrumentation is available to measure product properties in the CSTR:  One pH transmitter (Cole-Parmer model H-05656-10) that displays pH values on the panel- mounted meter and sends a current signal to the computer. A Cole-Parmer model E-05662-53 combination pH electrode is supplied with the transmitter.  One conductivity transmitter (Cole-Parmer model E-19350-10) that displays conductivity values on the panel-mounted meter and sends a current signal to the computer. A Cole-Parmer model E-19500-58 conductivity cell with a cell constant of approximately 1 cm-1 is provided. A computer is connected to the reactor hardware and instrumentation for purposes of Process Control and Data Logging. The computer monitors the output of all the thermocouples, flow meters, feed weights, pH meter and conductivity meter using “LabVIEW”2 software. The conductivity cell and the pH electrode are mounted in the recirculating sample loop operated with a peristaltic pump. When this peristaltic pump is operating, a continuous flow of liquid from the reactor circulates through the loop allowing for continuous monitoring of the pH, and conductivity and provides a sample port to withdraw samples using a syringe. 1 Manufactured by Chemineer-Kenics, North Andover, Mass. 2 Produced by National Instruments, Austin, Texas. 5 — 5 ABBREVIATIONS: FM: Conductivity Probe Flow Meter FS: Conductivity Probe Flow Switch SSP: Conductivity Probe Syringe Sample Port TC: Conductivity Probe Thermocouple TS: Conductivity Probe Temperature switch UV: Conductivity Probe UV/VIS Spectrophotometer Controller Input signals Output signals Feed pump #1 Feed pump #2 Feed #1 Feed #2 TC2 Feed #1 Preheater TC14 TC6 Drain Loop OverflowTS TC3 Cold water supply Drain FS TC1 Recirculation pump Jacket recirculation loop To CSTR Steam supply To CSTRTo CSTR SSP PFR Figure 2 PFR Piping and Instrumentation Diagram 5 — 6 DATA ACQUISITION AND DATA LOGGING The analog signals from the thermocouples are sent to a MCC3 PCI-DAS TC data acquisition card, which converts the signals into calibrated and linearized outputs that are sent to the computer as temperatures in degrees Celsius. The signal from the pH meter and Conductivity meters are converted to equivalent analog signals ranging from 0 to 5 volts, and sent to a MCC PCI-DAS 1002 analog-to-digital (A/D) conversion card in the computer. The “LabVIEW” program converts the digital signal corresponding to the volts to the value of the measured variables in the desired engineering units (i.e. pH units, mS, etc). The Computer is also equipped with an 8-channel digital-to-analog (D/A) conversion card. This card converts the digital output signals generated by the LabVIEW program to analog voltage signals from 0 to 5 volts, and sent to the field instruments. Four of these channels are currently used to actuate the cooling water flow control valve, the heater, and the two feed pumps. Monitoring and Controlling Reactor Conditions. “LabVIEW” operates under Microsoft WINDOWS XP® operating system. To start the program, turn the computer on (if it is not already) and select the “Chelab” user account. Double- click on the "REACTOR CELL" folder if it is not open already. This folder contains the following icons for the control setup: “Adiabatic.vi” and “Isothermal.vi” for control of the CSTR and “PFR.vi” for control of the PFR feed temperature, and a file folder named “DATA” for storage of the data files generated during the experimental runs. To start the data acquisition and control functions, double-click on the appropriate icon; a product identification screen will appear, and the LabVIEW setup will start the data acquisition program. The setup produces a display screen with the values for all the sensor outputs on the right- hand side of the screen, and analog traces for some of the sensor outputs on the left-hand side of the screen. All values displayed on the screen are updated every second. A file named “Adiabatic_nnn.xls”, “Isothermal_nnn.xls”, or “PFR_nnn.xls”, is generated every time the data acquisition program is invoked. The “nnn” indicate the numerical order of the file and it is incremented by one each time a new file is saved. All the temperature, pH, conductivity, flows, controller output, and time values are stored in this file every 30 seconds. Note: The data that are written to the file are not the actual readings at every 30-second mark. Instead, they are the calculated average of the values read each second, over the previous 30 seconds. To Start the data acquisition click on the Start button (a small right-pointing arrow located on the leftmost position of the menu bar. See figure 3 for details). To stop the data acquisition click on the red “STOP” located on the upper left corner of the user interface. A new file is created each time the Start button is pressed, and saved when the Stop button is pressed. To retrieve these files at the end of your work, double-click on the “DATA” icon in the "REACTOR CELL" folder. The files list can be sorted by date with the most recently created file at the top of the list. Your files can be recognized by their creation date and time, and by the “File Name” shown in the user interface window. When you are done with the computer, please “Log Off” the Chelab account, but do not shut down the computer. There is an anti-virus program that is set to run every night at midnight so it is important that the computers be left on for this event. 3 Manufactured by Measurement Computing Corporation. 5 — 7 Providing cooling water. The air-to-close pneumatic-controlled valve that controls the flow of cooling water is normally open (it needs air pressure to the actuator in order to close). There is a cold water supply valve (located immediately below the cold-water flow-control valve) that needs to be open for the reactor to have cooling capabilities (see figure 5 for detail). Cooling water will flow trough the jacket unless the temperature controller is turned on and requires heating. Setting up the reactor temperature control system. 1) Open the cold water supply valve. Note that cold water must flow out of the loop overflow. 2) Make sure the heating/cooling recirculating loop drain valve is closed 3) Position the 3-way heating loop direction valve to the desired reactor (see figure 6 for details) This valve is located to the right of the CSTR and to the left of the PFR. The arrow in the valve handle should point to the desired reactor. 4) Turn on the jacket recirculation pump. Use the air valve located in the Air Motor switch valves selection panel to the right of the main indicator panel. 5) Set the “Cooling Water Control Valve” switch in the control panel to the “CONTROL” position. 6) Make sure that the air pressures on the actuators of the steam and water flow control valves is 3psig (or less), and that cold water is overflowing the recirculation loop, then turn the steam supply main valve (see figure 5 for detailed position of these pressure gauges). 7) When you are ready to apply heating, turn the heater switch on the main control panel to the “CONTROL” position. 8) DO NOT CHANGE THE STEAM SUPPLY PRESURE. THIS PRESSURE SHOULD BE SET AT 20 PSIG OR LESS. NOTIFY YOUR SUPPERVISOR IF YOU NOTICE THE STEAM SUPPLY PRESSURE ABOVE 20 PSIG. Now the reactor loop is ready for temperature control. Use the temperature setpoint slide in the computer user interface to select the desired operating temperature. Note: when controlling the temperature of the STR the liquid level must be at least 1 inch above the tip of the temperature sensor and the agitator should be turned on. There are two switches to operate the agitator, one in the main control panel, and one on the agitator motor itself. The agitator speed can be controlled from the knob located on the motor. It is recommended that the agitator speed be adjusted so it does not generate a large amount ob bubbles that may interfere with the analytical instruments (specially the spectrophotometer). 5 — 10 Figure 5: Heating and cooling control valves and indicating air-pressure gauges. Figure 6: Reactor Cell main components detail. 5 — 11 Cold Water Flow Control Valve Cold Water Main ValveSteam Flow Control Valve Steam Main Valve Control Valve Pressure Gauges Feed Pumps Air-Motors switch valves Feed Direction valves PFR STR Heating loop direction valve Control Panel Setting the Maximum Flow of the Feed Pumps. The two computer-controlled feed-pumps have a manually set stroke length. The stroke length determined the amount of fluid delivered per stroke by the pumps. The stroke length is shortest (nominally no length) when the pump head axis is parallel (or coaxial) with the pump motor. The stroke-length increases as the angle of the pump-head axis with the motor axis increases. Turning the collar that attaches the pump head to the pump motor can change this angle (see figure 7 for details). The user can set the desired maximum flow rates as follows: Figure 7: Detail of feed pump Maximum flow adjustment. Adjust the flow to the desired maximum flow by adjusting the pump head collar. Clockwise decreases flow rate; counterclockwise increases flow. Do not exceed the 4.0 setting on the pump dial (this correspond to approximately 3 ml/sec for pump #1 or 1 ml/sec for pump #2). NOTE: DO NOT ALLOW THE PUMP COLLAR TO BECOME COMPLETELY UNSCREWED FROM THE PUMP MOTOR (past number 4.0 in the stroke length indicator). THIS MAY CAUSE DAMAGE TO THE PUMP. IF THE COLLAR BECOMES TOTALLY UNSCREWED FROM THE MOTOR IMMEDIATELY TURN THE PUMP SWITCH ON THE CONTROL PANEL TO THE OFF POSITION. THEN SCREW THE COLLAR AT LEAST 1 FULL TURN BEFORE RESTARTING THE PUMP MOTOR. The flow rates on that pump can now be adjusted between zero and the maximum just chosen, by changing the voltage on the computer screen. Positioning Feed Materials on the Balances. It is important that the following precautions be taken before the start of the run: 1) Make sure that the oil feed dip-tube does not touch the bottom of the feed container. This will cause an additional weight and produce erroneous weight measures. 2) Make sure there are two clamps holding the dip-tube into the oil container. This will minimize the transmission of vibrations caused by the pump strokes to the balance. 3) To feed the Methanol/Na Methoxide mixture into the reactor, do not use an open container. Instead, place a cap with a quick-connect fitting and a dip-tube on a 1-liter reagent bottle and place the bottle on the balance. Allow the coiled portion of the tube to float freely and clamp the far end of the coil to minimize tube forces on the balance. This set-up will minimize the amount of methanol fumes evaporating into the lab area. 5 — 12 Stroke Length Indicator Scale Stroke Length Adjustment Collar 0 = lowest flow 4 = highest flow 6. Lift the inlet tubing from the beaker and allow air to displace the buffer from the electrode holder loop. Insert the pump inlet tubing into a beaker with 200 ml of DI water and pump the water through the electrode holder loop to rinse the electrodes. 7. Fill a 50 ml beaker with Standard buffer pH = 4. Insert the pump inlet tubing in the beaker and pump the buffer solution through the electrode holder loop until it completely fills with the buffer and approximately 10 ml have flowed to the drain. Then insert the drain tube into the beaker containing the buffer to allow the buffer to circulate through the electrode holder loop. 8. Once the reading is stable adjust the "SLOPE" screw in the lower meter until the pH value of 4 registers in the transmitter display. 9. Lift the inlet tubing from the beaker and allow air to displace the buffer from the electrode holder loop. Insert the pump inlet tubing into a beaker with 200 ml of DI water and pump the water through the electrode holder loop to rinse the electrodes. 10. Before connecting the sample loop to the reactor, flush most of the water in the loop with air, then with 50 ml of methanol, and then with air again. Dispose of the methanol-water wash into the 1-gallon waste container. 11. Reconnect the sample loop to the reactor. Note: pH 7 is the neutral point for the slope of the pH transmitter. That means that the pH reading at 7 should not change when adjusting the “SLOPE” screw. If calibration at points that do not include pH 7 is desired (i.e. between pH 8 and 10 or between pH 4 and1); use the buffer closer to pH 7 in step 1, the buffer farthest from pH 7 in step 3, and repeat steps 1 to 4 until both pH readings match the pH of the buffer solutions. Storage of the pH Electrode When all your daily experiments have been completed the pH electrodes must be stored in pH Electrode Storage Solution. In order to accomplish this use the following procedure: 1. Disconnect the two quick-connect fittings of the tubes connecting the pH/Conductivity sensor- loop to the reactor and connect them to short tubes to reach a beaker placed on top of the stirring plate (see figures 2 and 3 for clarity). 2. Flush most of the remaining biodiesel mixture from the sample loop with air, then with 50 ml of Methanol, then with air again. 3. Fill a 250 ml beaker with 200 ml of DI water and pump the water through the electrode holder loop in order to rinse both electrodes. 4. Lift the inlet tubing from the beaker and allow air to displace the water from the electrode holder loop. 5. Fill a 50 ml beaker with pH Electrode Storage Solution (or pH 4 buffer). Insert the pump inlet tubing in the beaker and pump the storage solution through the electrode holder loop until it completely fills with the solution and approximately 10 ml have flowed to the drain. Then insert the drain tube into the beaker containing the storage solution to allow the liquid to circulate through the electrode holder loop for a few seconds. 6. Turn the pump off. 5 — 15 Stirring Hot Plate Quick connectors Sample draw-tube Sample return Mixer motor Peristaltic pump pH CP Stirring Hot Plate Quick connectors Calibration Sample Mixer motor Peristaltic pump pH CP Fig 3: Conductivity, pH and UV. monitoring of reactor contents Fig 4: Hook-up for instrument probes calibration with external sample 5 — 16 Reaction Cell Notes: 5 — 17