Vapor-liquid contacting

The bottom section of the main column provides a heat transfer zone. Shed decks, disk/doughnut trays, and grid packing are among some of the contacting devices used to promote vapor/liquid contact. The overhead reactor vapor is desuperheated and cooled by a pumparound stream. The cooled pumparound also serves as a scrubbing medium to wash down catalyst fines entrained in the vapors. Pool quench can be used to maintain the fractionator bottoms temperature below coking temperature, usually at about 700°F (370°C). 

Fleight equivalent to a theoretical plate (FIETP) for vapor-liquid contacting is 1.3-1.8 ft for 1 in. pall rings, 2.5-3.0 ft for 2 in. pall rings. 

Hydrotreating units in the past were built with one reactor, to meet the 500 ppm S specification however, a second reactor is needed to cope with the actual requirements. Several attempts have been made also to change the operating conditions to improve performance of the units and to achieve the stipulated level of desulfurization. From a co-current fluid-dynamics, the first modification went into countercurrent feeding. In a countercurrent reactor, where hydrogen is fed at one end and the feed in the other, the most difficult to-desulfurize compounds, will react under the higher hydrogen concentration. The countercurrent operation introduces some other problems, such as hot spots and vapor-liquid contact. 

The improvement of vapor-liquid contact is one of the strong contributors to the desired S removal levels. New developments in vapor-liquid distributors have been introduced in the market and some of them are already in use commercially. An advantage of enhancing conversion through this option is that it also increases catalyst useful life, because the gain is achieved by improving efficiency and not by increasing severity. 

The introduction of changes in the gas flow pattern of the unit could provide a means for improvement in both, single and two-reactor systems. It is important to keep in mind that in counter-current designs measures have to be taken to prevent hot spots within the reactor bed and to improve vapor-liquid contact. 

Packed column. A distillation column filled with an inert material to enhance vapor/liquid contact. The packing can be beads, pellets, Raschig rings, metal chains, or specifically shaped devices such as saddles, helices, or rings. 

The weir height on many trays is adjustable. We usually adjust the weir height to between 2 and 3 inches. This produces a reasonable depth of liquid on the tray, to promote good vapor-liquid contact. 

The net result of this unpleasant scenario is loss of both vapor-liquid contacting and tray efficiency. Note how the mechanical problems (i.e., levelness) of tray 1 ruins the tray efficiency of both trays 1 and 2. 

Wetted surface area—the number of square feet of packing surface area available for vapor-liquid contacting, per cubic foot of tower volume. 

In a trayed tower, vapor-liquid contact occurs, only on the 5 or 6 in above the tray deck, and the majority of the tower s volume is not used to exchange heat or mass between vapor and liquid. In a packed tower, the entire packed volume is used for this vapor-liquid contacting. 

One way out of this problem, is to increase APL, the pressure drop of the liquid flowing through the orifice holes. This could be done, by increasing the orifice hole liquid velocity. We could drill fewer orifice holes. Unfortunately, this would decrease the number of drip points per square foot of tower area (6 to 10 is a good target). This would reduce vapor-liquid contacting efficiency. Or, we could have smaller orifice holes. But too small a hole would probably plug with corrosion products. 

The broad classes of packings for vapor-liquid contacting are either random or structured. The former are small, hollow structures with... 

Literature data is almost entirely for small equipment whose capacity and efficiency cannot be scaled up to commercial sizes, although it is of qualitative value. Extraction processes are sensitive because they operate with small density differences that are sensitive to temperature and the amount of solute transfer. They also are affected by interfacial tensions, the large changes in phase flow rates that commonly occur, and even by the direction of mass transfer. For comparison, none of these factors is of major significance in vapor-liquid contacting. 

The dry Murphree efficiency calculated thus far takes into account the vapor and liquid resistances and the vapor-liquid contact patterns, but is uncorrected for the effects of entrainment and weeping. This correction converts the dry efficiency to a "wet or actual Murphree tray efficiency. Colburn [Eq. (14-98), under "Entrainment ] incorporated the effect of entrainment on efficiency, assuming perfect mixing of liquid on the tray. 

Maximize the specific surface area, i.e., the surface area per unit volume. This maximizes vapor-liquid contact area, and, therefore, efficiency. A corollary is that efficiency generally increases as the random packing size is decreased or as the space between structured packing layers is decreased. 

Spread the surface area uniformly. This improves vapor-liquid contact, and, therefore, efficiency. For instance, a Raschig ring (Fig. 14-48 ) and a Pall ring (Fig. 14-48c) of an identical size have identical surface areas per unit volume, but the Pall ring has a superior spread of surface area and therefore gives much better efficiency. 

Having downcomers configured, the tray active area (ft2) is calculated next. The tray active area is defined as that area of tray cross section open to vapor-liquid contact. Downcomer areas and their inlets and... 

Packed-tower efficiency and turndown are strongly dependent on the quality of initial liquid distribution. Uneven distribution may cause local variations in the liquid/gas ratio, localized pinch conditions, and reduced vapor-liquid contact. Figure 14 shows two common liquid distributor types, the ladder type (shown as the top distributor) and the orifice type (shown as the redistributor). The ladder type is a horizontal header of pipes, which are perforated on the underside. The orifice type is a flat perforated plate equipped with round or rectangular risers for gas passage. Other common types of distributors are a header equipped with spray nozzles (spray distributor) and a header of horizontal channels, with V notches cut in the vertical walls of the channels (notched-trough distributor). 

As an alternative to trays, especially at low volumetric liquid-to-vapor ratios, packing can be used to promote vapor-liquid contact. One approach is to dump specially shaped pieces of metal, glass, or ceramic material into the column, wherein they are supported on a grid. An example of dumped or random packing is shown in Fig. 7. 

Mass transfer. According to the classical model, mass transfer takes place by vapor-liquid contact in the froth. In the spray regime, mass transfer takes place on the surface of the drops (116). In order to appreciate the differences in mass transfer between the regimes, distillation systems have been classified into three types ... 

Most theoretical models incorporate the effects of vapor and liquid nonuniformity into the relationship between the Murphree and the point efficiency. Developing models for vapor-liquid contact on trays has been a fertile research area in the last couple of decades, with literally hundreds, maybe thousands of papers published on the subject. A thorough review is given by Lockett (12). 

Fall ring tongues, promoting vapor-liquid contact, mixing and liquid... 

Heat transfer constraints Heat must be transferred into the liquid in the reboiler to boil off the vapor needed to provide the vapor-liquid contacting in the column. If the base temperature becomes too high and approaches the temperature of the heating source, the heat transfer rate will decrease and vapor boilup will drop. The same result occurs if the reboiler fouls and the heat transfer coefficient drops. In the condenser, heat must be transferred from the hot vapor into the coolant stream to remove the heat of condensation. If the column is operating at its maximum pressure, capacity maybe limited by condenser heat removal. 

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