Distillation towers pressure drop, tray

In the reflux scheme, a column temperature controller manipulates a control valve in the reflux line. The reflux drum level controller manipulates a valve in the distillate line. The column base level controller manipulates a valve in the bottoms line. The feed and reboiler steam are each on flow rate control. In some cases, there is a controller for the pressure drop across the trays that manipulates the valve in the reboiler steam line. However, it is preferred to use a steam flow rate controller and simply monitor the tower pressure drop. With this scheme the separation power base is derived from the ratio of steam/feed. The distillate/feed material balance split is maintained by the MRT point controller. 

A common type of distillation contacting device used in refinery applications is the sieve tray. In the early 50 s and for many years before, the bubble cap tray was the mainstay of the distillation field. A sieve tray consists of a flat plate with regularly spaced holes, normally 1/2 to 1 inch in diameter. Liquid flows horizontally across the tray and into a channel, called a downcomer, which leads to the tray below. The sieve tray exhibits good capacity, excellent efficiency, low pressure drop, and good flexibility i.e., it will operate quite efficiently at tower loadings which are 1/2 to 1/3 of design values. 

The reason for the disparity in performance of such devices in the two services has been clearly outlined by Hachmuth (HI). Bubble-tray towers for distillation, for example, use as the source of energy for dispersion of the gas and for developing the desirable turbulent flow conditions both the expansion of the vapor as it experiences a pressure drop in flowing through the tray, and the liquid head available between trays. In liquid extraction only the liquid head is available. When it is considered that the difference in densities of the contacted phases in distillation may be of the order of 50 to 60 lb./cu. ft., whereas in extraction it is more likely to be of the order of 5 or less, it is easy to understand that in the latter case there is simply insufficient energy available from this source to provide for adequate dispersion and interphase movement. Interfacial area between phases remains small, turbulences developed are of a low order, and mass transfer rates are disappointingly small. 

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 ... 

Finally, the pressure drop of the vapor flowing through a packed tower will be an order of magnitude less than through a trayed tower. For vacuum distillation service, this is often of critical importance. 

The flange leak was taped over, and the exhaust-steam pressure dropped back to 100 mm Hg. The steam required to drive the turbine fell by 18 percent. This incident is technically quite similar to losing the downcomer seal on a distillation tower tray. Again, it illustrates the sort of field observations one needs to combine with basic technical calculations. This is the optimum way to attack, and solve, process problems. 

With the availability of economical and efficient packings, packed towers are finding increasing use in new distillation processes and for retrofitting existing trayed towers. They are particularly useful in applications where pressure drop must be low, as in low-pressure distillation, and where liquid holdup must be small, such as when distilling heat-sensitive materials whose exposure to high temperatures must be minimized. 

Baffle columns and shower tray columns, shown in Fig. 2.27, are characterized by relatively low liquid dispersion and very low pressure drops. The major application of this type of flow regime is in cooling towers, where the water flows across wooden slats and very large volumes of gas are handled. Here economics dictate that fans rather than compressors be used. Some gas absorption and vacuum distillation columns employ baffle or shower trays. 

The trays, installed in an 8-ft-dia., 6-stage distillation tower, were spaced 18 in apart. Each tray contained 69 angle elements (0.787 x 0.787 x 0.118 in. apiece) at 0.158 in. spacings. Slot area was 4.38 ft /tray. Distilling a mixture of cyclohexane and n-hexane with total reflux at 24 psi showed the Angle Tray s capacity factor is approximately 20% larger than sieve trays, while its pressure drop is less than half. Also, as vapor load increases, pressure drop rises slowly—which the developer considers another good result. 

In both downcomer back up and choke cases, downcomer liquid inventory increases and downcomer liquid backs up until the downcomer froth level reaches the tray above H > Hs). This phenomenon is called downcomer flood. When downcomer flood occurs to any tray, the whole tower will be flooded very quickly. A tower under downcomer flood provides virtually no distillation. In contrast, under tray flood, liquid can still leave the tower and the tower could stiU operate if the control system allows it although distillation efficiency suffers. Downcomer flood can be prevented in design by providing adequate downcomer area and clearance underneath the downcomer and minimizing tray pressure drop. Reducing reflux rate in operation could be effective in avoiding downcomer flood in operation. 

A common example of foam formation in the bottom of a fractionator inducing flooding occurs in a crude preflash tower. In this case, stable foam accumulates in the bottom of the column as a consequence of the "flow improver" chemicals added to crude oil. These chemicals reduce pressure drop in the crude pipelines. Once the foam level rises to the feed inlet nozzle, the trays flood and black distillate is produced. Please see Chapter 18 (Preflash Towers). 

Modern tower packings provide a much lower pressure drop per theoretical stage than trays thus packed columns have been accepted readily for vacuum separations. As the operating pressure is increased, pressure drop becomes less important as a basis for selection of the fractionation device. Therefore, we will examine the other characteristics of both packings and trays to ascertain the potential applications for tower packings in high-pressure distillations. 

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