Towers drop zone

Tower Internals and Equipment Modification. Tower capacity expansion can be achieved through the use of random or stmctured packing, or through the use of higher capacity trays such as the UOP multiple downcomer tray. Packing has been used in the gasoline fractionator, water quench tower, caustic and amine towers, demethanizer, the upper zone of the deethanizer, debutanizer, and condensate strippers. Packing reduces the pressure drop and increases the capacity. 

Other reactor types are also used for gas-liquid reactions, but they are not very common in fine chemicals manufacture. Spray towers and jet reactors are used when the liquid phase is to be dispersed. In spray towers the liquid is sprayed at the top of the reactor while the gas is flowing upward. The spray reactor is useful when a solid product, possibly suspended in the liquid, is formed, or if the gas-phase pressure drop must be minimized. In a jet reactor, the liquid is introduced to the reaction zone through a nozzle. The gas flows in, being sucked by the liquid. 

A wide variety of random packing and structured packing are available. For new construction, except for especially corrosive services, pressure drop critical applications, and special cases, economics drives fractionation equipment selection toward trays. Special cases include applications of very low liquid rates, and where equipment size is critical. Examples of critical equipment size include units on offshore platforms, towers in severe earthquake zones, and towers to be housed inside buildings. 

Effective steam stripping is indicated by a 30°F-50°F temperature drop between the flash zone and the tower bottoms. A lower AT means that the steam is not effectively contacting the resid. This can be due to upset or corroded trays or tray flooding. Often, poor level control in the bottom of the tower will permit resid to back up and flood the stripping trays. 

The key tool in troubleshooting flash-zone pressure problems is a vacuum-tower pressure survey. The time to initiate this survey is just after start-up when the trays, demister, and ejector system are clean and in good condition. Pressures are best measured with a portable mercury-filled vacuum manometer. Using a vacuum pressure gauge will reduce the accuracy of observed pressure drops. Relying on permanently installed gauges for pressure drop data will not give reliable results. 

Reduce the number of wash trays in the vacuum tower. In one installation, two bubble-cap trays replaced four tunnel-cap trays. The flash-zone pressure was therefore reduced. The net effect was beneficial in that the lower pressure drop was more useful than the loss in fractionation represented by the two trays removed. 

If overall tower pressure drop is too low from the flash zone to the top then there may be damage to the tower internals or hydraulic problems. If the pressure drop is too high then flooding, plugging or internal damage may be indicated. 

Many times the design of the vacuum crude tower involves only determining the packed depth in each section because the column already exists. In the case of a new installation, the column diameter must be fixed based on the pressure drop. Generally, the total pressure drop from column top to feed flash zone will be 10 to 15 mm Hg. The pressure drop will decrease significandy from the bottom to the top of each heat trans-... 

Most atmospheric towers have 25 to 35 trays between the flash zone and the tower top. Recommended ranges for the number of trays in various sections of the tower are given in Table 2.1. Using these numbers of trays and reasonable levels of heat recovery will provide adequate fractionation between products. The allowable pressure drop for... 

In order to determine the flash zone pressure, it is first necessary to allocate trays and/or other internals to various sections of the tower and then to assume pressure drops across these sections. The number of trays between lube tower draw trays is normally 3 to 5. Modern fuels towers use grid sections. Pressure drop values recommended for design purposes are given in Table 3.1. Having assumed the internal configuration of the tower, the flash zone pressure is then calculated arithmetically. 

This analysis assumes essentially no stripping of the feed flash liquid in the base section of the tower, although, in a later step, a temperature drop across the bottoms stripping zone will be set. Nelsons correlations show very little stripout at the steam rates usually employed, but experience shows that temperature drops as higli as 30 degrees F between flash zone and bottoms do occur. This is of little real importance, however, since the heat input to the system can be calculated independently of the absolute thermal condition of the feed. 

In practice, the value of these discrepancies is quite small in comparison to the total values of these heat quantities in question. The most conservative design approach would be to assume a zero temperature drop for the liquid from the flash zone to the base of the tower. This would maximize both feed heat input and heat removal duties. Conversely, assuming a high temperature drop, say 30 degrees F, would be a tighter, more competitive approach. 

In addition, the preseure drop caused by the change in kinetic energy content of the stream of fluid [Eq. (13-5)] must be computed. The change in momentum from the entrance to the zone, where the velocity is 218 ft per s, to the tower vaporizer where the velocily is substantially zero is... 

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