Efficiency, tray processes

Mean drying time for the oven tray process is 19.2 hr. All 15 batches were dried within the specified time of 16 to 20 hr. No seasonal influence was apparent. The average moisture content of these batches is 1.2% the standard deviation is 0.3%. The 15 batches dried using the fluid bed dryer had a residual moisture of 0.8% (SD = 0.1%). Drying time is mechanically controlled and not recorded. The statistics favor the fluid bed process it is more efficient and uniform. There is nothing in these data to disqualify the oven tray dryer from further use, however. 

Urea Casale SA Urea Ammonia (NH ) and carbon dioxide (CO ) The Casale split flow loop process is an improved CO stripping process. It produces urea from NH and CO by applying Casale s highly efficient full condenser, Casale-Dente high-efficiency trays and Casale high-efficiency hydrolyzer used in the process condensate treatment unit. 4 NA... 

Calculation of distillation efficiency requires process simulation. From the number of theoretical stages (A eq). column seetion and overall efficiency can be determined, respectively based on actual number of trays (A act)- For a simple tower with two products, one from the top and one from the bottom together with one condenser and one reboiler, the separation efficiency can be calculated via... 

When you set up a computer model for any distillation process, you must select a tray efficiency. Tray efficiencies (for mechanically intact trays) vary from 30 to 90 percent. Experience is the only guide in making the least wrong selection. 

Nonisothermal Gas Absorption. The computation of nonisothermal gas absorption processes is difficult because of all the interactions involved as described for packed columns. A computer is normally required for the enormous number of plate calculations necessary to estabUsh the correct concentration and temperature profiles through the tower. Suitable algorithms have been developed (46,105) and nonisothermal gas absorption in plate columns has been studied experimentally and the measured profiles compared to the calculated results (47,106). Figure 27 shows a typical Hquid temperature profile observed in an adiabatic bubble plate absorber (107). The close agreement between the calculated and observed profiles was obtained without adjusting parameters. The plate efficiencies required for the calculations were measured independendy on a single exact copy of the bubble cap plates installed in the five-tray absorber. 

Fractional equihbrium stages have meaning. The 11.4 will be divided by a tray efficiency, and the rounding to an integral number of actual trays should be done after that division. For example, if the average tray efficiency for the process being modeled in Fig. 13-36 were 80 percent, then the number of actual trays required would be 11.4/0.8 = 14.3, which would be rounded to 15. 

The second classification is the physical model. Examples are the rigorous modiiles found in chemical-process simulators. In sequential modular simulators, distillation and kinetic reactors are two important examples. Compared to relational models, physical models purport to represent the ac tual material, energy, equilibrium, and rate processes present in the unit. They rarely, however, include any equipment constraints as part of the model. Despite their complexity, adjustable parameters oearing some relation to theoiy (e.g., tray efficiency) are required such that the output is properly related to the input and specifications. These modds provide more accurate predictions of output based on input and specifications. However, the interactions between the model parameters and database parameters compromise the relationships between input and output. The nonlinearities of equipment performance are not included and, consequently, significant extrapolations result in large errors. Despite their greater complexity, they should be considered to be approximate as well. 

The actual number of trays needed for a particular separation duty depends on the efficiency of the plate, and the packings if they are used. Thus, any factors that cause a decrease in tray efficiency will also change the performance of the colunm. Tray efficiencies are affected by such factors as fouling, wear and tear and corrosion, and the rates at which these occur depends on the properties of the liquids being processed. Thus the proper materials of construction must be selected for tray construction. 

As vapor rates decrease, the tray activity also decreases. There eventually comes a point at which some of the active devices (valves or bubble caps) become inactive. Liquid passing these inactive devices gets very little contact with vapor. At very low vapor rates, the vapor activity will concentrate only in certain sections of the tray (or, in the limit, one bubble cap or one valve). At this point, it is possible that liquid may flow across the entire active area without ever contacting a significant amount ot vapor. This will result in very low tray efficiencies for a distillation process. Nothing can be done with a bubble cap tray to compensate for this. 

An LTX unit is not a very efficient stabilizer because the absence of trays or packing keeps the two phases from approaching equilibrium at the various temperatures that exist in the vessel. In addition, it is difficult to control the process. Typically, for a 100-psi to 200-psi operating pressure, a 300°F to 400°F bottoms temperature is required to stabilize completely the condensate. The heating coil in an LTX separator is more Uke-... 

To provide this finished water quality, a system incorporating flocculation, settling, and mixed-media filtration is used. Fig. 6 schematically illustrates the flow of the system used. After flocculation, the coagulated water is processed through a shallow-depth sedimentation basin using tube settlers. Tube settlers provide efficient sedimentation in greatly reduced detention times. The tube settlers are essentially a honeycomb of 1-in. trays operating in parallel. The... 

H. Chan and J.R. Fair, Prediction of point efficiencies on sieve trays. Binary systems, lnd. Engng. Chem. Process Des. Dev., 23 (1984) 8L4-819. 

Distillation trays in a fractionator operate between 10 and 90 percent efficiency. It is the process person s job to make them operate as close to 90 percent efficiency as possible. Calculating tray efficiency is simple. Compare the vapor temperature leaving a tray to the liquid temperature leaving the trays. For example, the efficiency of the tray shown in Fig. 1.2 is 100 percent. The efficiency of the tray in Fig. 1.3 is 0 percent. 

It is a characteristic of process equipment, that the best operation is reached, at neither a very high nor a very low loading. The intermediate equipment load that results in the most efficient operation is called the the best efficiency point. For distillation trays, the incipient flood point corresponds to the best efficiency point. We have correlated this best efficiency point, for valve and sieve trays, as compared to the measured pressure drops in many chemical plant and refinery distillation towers. We have derived the following formula ... 

Next to the efficiency of the trays in a distillation tower, the efficiency of a fired heater is the most critical factor in saving and/or making money for the process plant. The primary objectives in operation of a fired heater are to... 

In order for a process to be controllable by machine, it must represented by a mathematical model. Ideally, each element of a dynamic process, for example, a reflux drum or an individual tray of a fractionator, is represented by differential equations based on material and energy balances, transfer rates, stage efficiencies, phase equilibrium relations, etc., as well as the parameters of sensing devices, control valves, and control instruments. The process as a whole then is equivalent to a system of ordinary and partial differential equations involving certain independent and dependent variables. When the values of the independent variables are specified or measured, corresponding values of the others are found by computation, and the information is transmitted to the control instruments. For example, if the temperature, composition, and flow rate of the feed to a fractionator are perturbed, the computer will determine the other flows and the heat balance required to maintain constant overhead purity. Economic factors also can be incorporated in process models then the computer can be made to optimize the operation continually. 

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