Installing Temperature and Composition Controllers

Installing temperature and composition controllers is somewhat more involved than installing level and flow controllers because of three issues. First, we need to include additional dynamic elements in the loop. Temperature and composition measurements 

CONVERTING FROM STEADY-STATE TO DYNAMIC SIMULATION 

the appropriate location for the temperature or composition sensor may not be obvious. Some method for making a good selection must be used. As discussed in Chapter 6, there are several ways to approach this problem. These include looking at the shape of the temperature profile in the column, calculating steady-state gains and using SVD analysis. 

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Once the file has been exported into Aspen Dynamics, controllers are installed to achieve the desired control structure and dynamic simulations are mn to check the stability and performance of the control system. Various types of disturbances should be imposed on the system, such as throughput changes, feed composition changes, and changes in the set-points of the product-quality controllers (temperature and composition controllers). In this section we demonstrate, in a detailed step-by-step fashion, how these operations are performed in Aspen Dynamics. 

An alternative simulation was developed using the Flash3 model, as shown in Figure 8.40b after exporting and installing a control structure. In Aspen Plus, a vapor line with a valve is added. The decanter is specified to be adiabatic and at a fixed pressure (0.6 atm). The temperature specified in the upstream condenser HX2 is adjusted to 320 K to give a very small vapor flow rate (3% of the feed). After the file is exported, a pressure controller is inserted on the decanter, but it is put on manual and the vapor valve is closed. Now decanter pressure varies with temperature and composition. Its steady-state value is 0.366 atm with the decanter temperature controller set point set at 313 K so that a direct comparison with the previous case can be studied. 

In the selection of control equipment, the most important waste-gas characteristics are volumetric flow rate, concentration and composition of organic compounds in the waste-gas, waste-gas temperature and humidity, and rbe content of particulate matter, chlorinated hydrocarbons, and toxic pollutants. Other factors influencing the equipment selection are the required removal efficiency, recovery requirements, investment and operating costs, ease of installation, and considerations of operation and maintenance. The selection of a suitable control method is based on the fundamental selection criteria presented as well as the special characteristics of the project. 

The bypassed vapor heats up the liquid there, thereby causing the pressure to rise. WTien the bypass is closed, the pressure falls. Sufficient heat transfer surface is provided to subcool the condensate, (f) Vapor bypass between the condenser and the accumulator, with the condenser near ground level for the ease of maintenance When the pressure in the tower falls, the bypass valve opens, and the subcooled liquid in the drum heats up and is forced by its vapor pressure back into the condenser. Because of the smaller surface now exposed to the vapor, the rate of condensation is decreased and consequently the tower pressure increases to the preset value. With normal subcooling, obtained with some excess surface, a difference of 10-15 ft in levels of drum and condenser is sufficient for good control, (g) Cascade control The same system as case (a), but with addition of a TC (or composition controller) that resets the reflux flow rate, (h) Reflux rate on a differential temperature controller. Ensures constant internal reflux rate even when the performance of the condenser fluctuates, (i) Reflux is provided by a separate partial condenser on TC. It may be mounted on top of the column as shown or inside the column or installed with its own accumulator and reflux pump in the usual way. The overhead product is handled by an alter condenser which can be operated with refrigerant if required to handle low boiling components. 

Test for any leaks from shell and heating coils/tubes. Check that all the bubble cap trays are installed properly. AU the instruments for Uow measurement and control, temperature, pressure, and level indication on trays including the composition control unit (reflux ratio controUer) are to be calibrated, and their probes shaU be installed correctly. 

Feedforward on feed composition can be a valuable enhancement but may not be practical. Firstly it requires an on-stream analyser on feed. Few plant owners would install this as standard and there may not be sufficient economic justification to add it later. Secondly it may not be possible to acquire an analyser that responds quickly enough. If the change in feed composition affects tray temperatures and/or inferentials before being reported by the analyser then the feedback controller(s) will take corrective action. A delayed measurement of feed composition would then be less valuable than having no measurement. 

Upstream control systems require continuous composition data for stream F. It is impractical to install an analyzer on F, but an existing analyzer is already installed on stream V,. In addition, the following data are measured on a continuous basis flow rate, temperature, and pressure of stream F, flow rates of L L3, and V3, and the stage pressures Pi, Pi, and P3. Outline a computational procedure that could be used to determine, at any given moment, the composition of stream F, as well as that of products L Lj, and V3, assuming steady-state conditions exist. 

Reference electrodes must be used in anodic protection systems because the potential of the vessel to be protected has to be carefully controlled. The reference electrode must have an electrochemical potential that is constant with respect to time and that is minimally affected by changes in temperature and solution composition. Several reference electrodes have been used for anodic protection, including those listed in Table 12.9. The reference electrode has been a source of many problems in anodic protection installations because of its fragile nature. 

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