Fluidized Bed Unit (FBU) Operating Manual. Fall 2008

Transcription

1 Fluidized Bed Unit (FBU) Operating Manual Fall 2008

2 Background Fluidized bed technology is based on the fluid like behavior of a bed of solid particles when subjected to the buoyant forces exerted by a gas or liquid. Though composed of an inhomogeneous mixture of fluid and solids, the bed fluidized by the gas or liquid behaves like a fluid (i.e., exhibiting hydrostatic surface properties and an effective bulk density lower than the original solids). The outcrop of this behavior is a host of attractive benefits including enhancement of heat and mass transport, high contacting efficiency for reactants, and improved flow and transport options for the solids. Figure 1: Bed of glass beads fluidized by air Understanding the behavior of fluidized beds is important to realizing these and other benefits. This behavior can be seen in the short video captured in Figure 1, where a bed of small glass beads moves from the slumped condition through bubbling fluidization and back again as the air flow rate is first increased then decreased. Description of Facilities The FBU is formally a part of the Permeameter Unit skid (see Figure 2) but is separable from that unit from both operations and data collection points of view. The unit is controlled and operated entirely in manual fashion using valves, a regulator, and rotameters with the exception of observation and data collection which rely on the Honeywell Experion system. Figure 2: Schematic diagram of the FBU portion of the PERM unit skid

3 Using the Honeywell Experion Process Knowledge System (PKS) Honeywell s Experion PKS is a distributed control system (DCS) widely used in industry and on many lab experiments here at LSU. In Experion, each process variable is represented by an entity called a Control Module (CM). Each CM is a collection of Function Blocks (FB) and each FB consists of many values called parameters. Within a CM (and sometimes between CMs), the FBs are wired together in various ways to monitor and control the process. Desired values of many parameters may be entered via the computer keyboard. The purpose of the next few sections is to explain how to use Experion to observe and collect data from the Fluidized Bed Unit (FBU). Logon to a Honeywell Experion Equipped PC On a PC loaded with Honeywell Station software, logon using your normal ID, password and domain. Open the Honeywell Station software by going to Start/Programs/Honeywell Experion PKS/Client Software/Station. If your ID is authorized for use of the Honeywell PKS system, the Station program full menu will appear. From the Unit item on the menu, select PERM. The Permeameter/FBU P&ID schematic will appear. You will be using this schematic to follow the operation of the FBU. This schematic has NOT been updated to reflect recent modifications to the unit on the FBU portion of the skid. Rely on the included drawing in this manual (Figure 2) to provide you some of the missing details. Controlling from the PID Schematic (At present, there are no controllers on the PERM schematic for use in controlling the FBU. The FBU is controlled using manual devices at the unit skid itself.) Every controller is represented by a small colored circle containing the tagname of the CM, with the values of the setpoint (backlit in green) and the process value (backlit in cyan) near the circle. Any standalone transmitter (those without controllers) is represented by a small white circle containing the tagname of the transmitter, with the process value (backlit in cyan) near the circle. The first letter indicates the type of measurement F for flow rate, P for pressure, or T for temperature. To change any analog value from a schematic, you must click on it (if it can be changed, its backlighting will change), and then enter the new desired value. The changeable objects in this display are the values near each controller for the Permeameter, and there are additional objects on the faceplates, all of which are explained below. The main value associated with a controller or transmitter is the measured input, or process value (PV). In addition to the PV, controllers have several additional values, the most important of which is the setpoint, or SP. This is just like the speed setting on a cruise control the controller will manipulate its output (the throttle position in this case) to move the PV to the SP and hold it there. The SP (in green) and the PV (in cyan) are shown immediately to the side of the circle representing the controller. Clicking on any of the small colored circles the controllers will bring up the respective controller s faceplate, on which you can see the tagname, description, engineering units, and several of the most important parameters on the controller. You can also change many of these parameters. Near the bottom of the faceplate is a combo box labeled MD, which can be used to select the mode of the controller. Immediately above the mode are the OP (changeable), the PV (not changeable) and the SP

4 (changeable). To change the OP or SP, single click the value, type in the new value and press ENTER. The SPs and tuning constants on the schematic can be changed the same way. There are also special buttons on the schematic to accomplish various functions for the Permeameter unit. These are not needed to run the FBU. Controller Modes (At present, there are no controllers on the PERM schematic for use in controlling the FBU. The FBU is controlled using manual devices at the unit skid itself.) As mentioned above, a controller has an SP and an OP. The OP is always given in percent (0 100%) and the SP has the same engineering units as the PV (in this experiment, the flow rates are in ml/min or SCFM, the temperatures are in C, and pressures are in inches of water column). When the controller mode is MANual, the OP is held until the operator changes it. When you want to change it, simply click the OP in the faceplate, type in the new value, and press ENTER. The new OP will be held until you change it again. Note that you may enter an OP only while a controller is in MAN. An OP value of 6.9% is known as tight shutoff in the Experion system. Notice the small bar under the control valve on the schematic its length is proportional to the output. When the mode is not MAN, the controller uses the PV, SP and tuning constants to calculate the OP. When the mode is AUTOmatic, you may enter a new SP to be used for control. Note that changing an SP affects the OP only while a controller is in AUTO. The circle representing a controller is filled with a color which indicates the current mode of the controller. Yellow means the mode is MAN, and white means the mode is AUTO. Sampled Data Control Unlike dedicated analog instrumentation for process control, the Honeywell Experion uses microprocessor based digital computers to perform measurement and regulatory control tasks. It uses workstation and server PCs to configure the system, build control strategies and schematics, gather and display data, etc. The calculations in such systems are typically performed at regular intervals of time. The time between two consecutive data readings is referred to as the sampling time and such systems are referred to as sampled data systems. The sampling time used in Experion is one (1) second. All data are collected each second, control algorithms processed, and all outputs resent each second. Display Navigation When you first opened the Station software, you used an item from the menu bar to call up the main schematic. There are several additional ways to go from one display to another. For example, you can enter the tagname of a controller in the Command field at the top of the screen and press F12 to call up the detail display. For a controller, the detail display has 7 tabs. The one labeled LOOP TUNE is useful for tuning controllers.

5 Most of the toolbar buttons are used for navigation some require a name or number to be entered, and some go directly to the display. Most of the same functions are on the function keys. For example, to return to the previous display, click or press F8. To return to the display before that, do it again. From most displays (both system displays and custom schematics such as PERM), double clicking any value associated with a CM will take you to its detail display. From a detail display, click or press F2 to return to the main PERM schematic. On most custom schematics there may also be buttons to quickly get you from one display to another. Using Trends There is one button labeled Trend 50 on the main PERM schematic that is used to call up a trend. This trend button displays the PVs of all the controllers and transmitters on the Permeameter/FBU. The only variable on this display of real interest to FBU users is P501, the Fluidized Bed Pressure Drop. At the bottom of the trend is the legend with all the tag.block.parameters associated with the traces. The checkboxes in the Pen column indicate which traces are currently on the trend. Click on the chart area of the trend and a white hairline cursor appears on the chart and the values at the hairline cursor appear in the Reference Value column of the legend. Along the bottom of the chart area is a horizontal scroll bar which allows you to scroll the chart area back and forth. Along the left axis are listed the low and high range of the selected trace. These allow you to change the range of the trace for the selected parameter. Practice by changing the range of the P501 to 0 to 20. Immediately above the left side of the chart area is a combo box which allows you to select one of the traces (you may also click anywhere on the line for this trace in the legend area). When you select an active trace, it is highlighted (thicker) in the chart area. Above the right side of the chart area is the Period combo box which allows you to select how much data, on a time wise basis, is displayed in the chart area. To the right of that is the Interval combo box which allows you to select the interval between points in the chart area. Practice changing to a different period and interval. Leave the period set to 1 day and the interval at 1 minute for now. For practice, scroll back until some variation in some of the traces appears. Notice that the timestamps below the chart area change as you scroll. Find some local max or min in one of the traces and click or drag the hairline to it. Now change the period back to one hour and notice that the cursor is centered on (or at least near) the local maximum or minimum. If necessary, move the hairline so it is exactly on the peak or valley and notice that the values, as well as the date and time, are shown in the Reference Value column in the legend. Now return the trend to the current time by clicking. All changes you make to the trend can be saved by clicking the familiar Windows Save icon just above the right end of the chart area next to the word (Modified). Saving Process Variable Values into Excel Normal Speed Data Collection Successful completion of most experiments on the FBU will require the analysis of a good deal of data. To collect these data, an Excel workbook containing a Visual Basic Add in is provided.

6 Open Excel 2003 and navigate to: C:\Program Files\Honeywell\Experion PKS\Client\PermRecorder.xls This file is used for recording normal speed (i.e., no faster than 1Hz) process data from the Perm and the included FBU subsystem while the experiment is running. Double click it and enable macros. The workbook will open with a Start button, the experiment name, a collection frequency Combo Box, and a Stop button on the top line. Click on the Start button, and the workbook will start collecting the relevant data at the specified collection frequency. While the workbook is collecting data, it may be scrolled, but you should not attempt to do anything else in this instance of Excel until after you click on the Stop button. If you do, the collector may stop and you may lose valuable data. When you finish a run, click on the Stop button and cut or copy whatever data you need into your daily workbook in a separate instance of Excel. Fast Data Collection Some experiments using the FBU will require the collection of data at faster than normal speeds. To collect these data, an additional Excel workbook containing a Visual Basic Add in is provided. Open Excel 2003 and navigate to: C:\Program Files\Honeywell\Experion PKS\Client\FluidBedRecorder.xls This file is used for recording fast data i.e., 20 Hz or 50 ms data from the FBU differential pressure transmitter while the experiment is running. Double click it and enable macros. The workbook will open with a Start button, the experiment name, a non functional collection frequency Combo Box, and a Stop button on the top line. Click on the Start button, and the workbook will start collecting the relevant data at a fixed frequency of 20 Hz. While the workbook is collecting data, it may be scrolled, but you should not attempt to do anything else in this instance of Excel until after you click on the Stop button. If you do, the collector may stop and you may lose valuable data. When you finish a run, click on the Stop button and cut or copy whatever data you need into your daily workbook in a separate instance of Excel. Startup Procedure To establish a desired flow of air through the FBU, execute the following procedure: 1. Ensure that the air regulator is closed (off) by turning the round black handle knob counterclockwise until the regulator gage reads 0 psig. 2. Ensure that both rotameter variable flow valves are closed. Doing so ensures that there is no air flow UNTIL desired by operations personnel. Doing so guards against having both rotameters open simultaneously and thereby getting a misleading total air flow rate through the FBU.

7 3. Ensure that the block valve just before the dry test meter is closed. The dry test meter in not in service at this time. Additionally, the maximum air flow deliverable by the large rotameter exceeds the capacity of the dry test meter. Exceeding the capacity of this meter will result in erroneous readings and can seriously damage the meter. Opening these block valves (BV12, BV13, BV14, and BV15) lines up potential air flow to the FBU. Doing so establishes an initial pressure for air flow. 4. Open block valves BV12, BV13, BV14, and BV Adjust the air regulator by turning the large black handle knob clockwise until the gage reads 5 psig. 6. Adjust the desired rotameter variable flow Doing so establishes an initial air flow value. valve until the desired reading is obtained. 7. Readjust the air regulator to 5 psig. You will notice that the regulator pressure had changed after performing Step Repeat Steps 6 and 7 until both regulator pressure and air flow reading are at the desired values. Changing the setting on either valve changes the reading at the other device, requiring that these two steps are repeated until both objectives are achieved. Shutdown Procedure To place the FBU is a safe shutdown mode, execute the following procedure: 1. Ensure that the air regulator is closed (off) by turning the round black handle knob counterclockwise until the regulator gage reads 0 psig. 2. Ensure that both rotameter variable flow valves are closed. 3. Ensure that the block valve just before the dry test meter is closed. 4. Close block valves BV12, BV13, BV14, and BV15. Doing so ensures that there is no air flow to the FBU. Doing so guards against having both rotameters open simultaneously and thereby getting a misleading total air flow rate through the FBU. The dry test meter in not in service at this time. Additionally, the maximum air flow deliverable by the large rotameter exceeds the capacity of the dry test meter. Exceeding the capacity of this meter will result in erroneous readings and can seriously damage the meter. Closing these block valves (BV12, BV13, BV14, and BV15) isolates FBU from potential air flow. Special Procedures Calibrating Rotameters There are two rotameters available to meter air into the FBU a larger one and a smaller one. If there is any uncertainty regarding the rated performance of either of these two instruments, calibration (or at least a check of the calibration) might be warranted. The accuracy of an off the shelf rotameter is typically ± 5% of the full scale reading. Improving on that accuracy would require calibration using

8 system components with even better accuracy. An independently calibrated dry test meter, for example, with ± 1% accuracy would be a good candidate for rotameter calibration. However, without a recent certified calibration certificate for the dry test meter itself or at least some knowledge that the dry test meter is measuring properly it s unclear that this method offers value. Another possibility for performing at least a single point flow check for a rotameter would be to pass a known volume of gas through the rotameter at a constant rate. A standard cylinder (i.e., known total volume) of compressed air can be used to provide such a flow and the change in pressure over a fixed time period can be used to estimate the volumetric flow rate that passes through the rotameter. Comparing this to that indicated by the rotameter would provide a single calibration check point. The FBU can be equipped to perform this test if necessary. Failing that, the rotameter faceplate calibrations are given on the meters themselves and can be found in this manual in the Instrumentation Specs section. Determining Distributor Pressure Drop The fritted metal plate used to distribute air uniformly to the FBU bed adds pressure drop to the observed total differential pressure across the unit. Increasing the air flow through the fritted metal plate increases this added pressure drop, which then becomes an increasingly larger percentage of the total observed differential pressure. If one needs to determine the fluidized bed differential pressure ex the added pressure drop due to the distributor, a pressure drop versus air flow measurement study should be conducted prior to introducing solids to determine needed correction(s). Determining Bed Void Fraction at Minimum Fluidization εmf Determining bed void fraction at minimum fluidization velocity requires some involved testing, measurement and calculations. A thorough discussion of this is best left to a literature reference (Subramani, Balaiyya and Miranda 2007). Laboratory Procedures Determining Average Particle Diameter using Sieves Fluidization behavior and virtually every correlation attempting to explain that behavior is greatly dependent on a representative average particle size of the fluidized solids. This average is determined in some manner from the distribution of particle sizes present in the admixture of solids. There are a variety of methods for determining particle size distribution but the most common method is a sieve analysis in which a known quantity of solids is shaken through a series of sieves of ever decreasing aperture. The distribution of solids retained on each screen is used to estimate an average particle size. A representative diameter can be determined as follows (Baker and Herrman 2002): d gw = log n 1 i= 1 ( W log d ) n i i= 1 W i i (1.1)

9 Where: dgw is the geometric mean, mass weighted diameter, W i d i is the mass of particles retained on the is the diameter of the th i th i sieve, and sieve in the stack, calculated from d = ( d d ) i u o 0.5 (1.2) d u th Where: = diameter opening through which particles will pass (sieve preceding i ), and do = diameter opening through which particles will not pass (i sieve). Determining Average Particle Sphericity Depending on the particular solids used in FBU, the particles may be shaped more like spheres or they may be quite angular. If the particles are large enough, a micrometer can be used to make multiple length measurements at different particle orientations. If the particles are too small for measurement by micrometer, a microscope and quantitative reticule can be used instead. Specifically, if one defines the smallest dimension of a particle to be r and the remaining two dimensions to be s and t respectively, then the sphericity of the particle φ s can be calculated as follows (Siwiec 2007): th φ = s r st (1.3) The average of a sufficient number of replicate measurements can produce a reasonable value which should compare favorably to that asserted by the manufacturer of the material. Determining Bed Void Fraction ε The bed void fractionε absent fluidization also described as the bed porosity is basically a property of the bulk solids and can be estimated ex situ by volume and mass measurements and a simple implicit calculation (Chase n.d.): ρ = ε ρ+ (1 ε) ρ (1.4) o Where: ρo is the bed bulk density as determined by volume and mass measurements, ρ is the fluid density (normally air, unless bulk density is determined in another other), ρ p is the solids particle density. p

10 Instrumentation Specs Rotameters Device Flow Specific Gravity Temperature Pressure Large rotameter SCFM = Scale x F 0 psig Small rotameter Scale = L/min C 760 mm Hg abs. Safety Considerations See fluidized bed solids Materials Safety Data Sheet for hazard information. Works Cited Baker, Scott, and Tim Herrman. "Evaluating Particle Size." Kansas State University (accessed October 22, 2008). Chase, George G. "Solids Notes 4." University of Akron. (accessed October 22, 2008). Siwiec, Tadeusz. "The Sphericity of Grains of Filtration Beds Applied for Water Treatment on Examples of Selected Minerals." Electronic Journal of Polish Agricultural Universities 10, no. 1 (2007). Subramani, Hariprasad J., M.B. Mothivel Balaiyya, and Lima Rose Miranda. "Minimum fluidization velocity at elevated temperatures for Geldart's group B powders." Experimental Thermal and Fluid Science 32 (2007):