Pressure services reboiler

At atmospheric pressure, and in vacuum services, the static head produced by the gas is small and can be neglected. However, in high pressure fractionators requiring large numbers of theoretical stages, the static head of vapor can be appreciable from condenser to reboiler. This correction should be added to the calculated pressure drop to determine the bottom column pressure and reboiler temperature. 

The fractionator shell itself should often have some extra trays. Conventional instrumentation alone cannot always be expected to handle all the things that can happen to a fractionation system, such as changes in feed composition, reboiler steam pressure, or coolant temperature (especially for an air condenser during a sudden cold front). Experience for a given service is the best guide for extra trays. 

Figure 10-97A compares horizontal and vertical units in the same hydrocarbon boiling service at low pressures and shows that the horizontal units are more favorable in the same service than vertical units and even more so when the mean temperature difference is low. Figure 10-97B compares horizontal and vertical thermosiphon units with kettle reboilers when boiling the same hydrocarbon mixture also see Fair, Jacobs, and Rubin. ... 

The tube lengths used for vertical thermosyphon reboilers vary from 1.83 m (6 ft) for vacuum service to 3.66 m (12 ft) for pressure operation. A good size for general applications is 2.44 m (8 ft) by 25 mm internal diameter. Larger tube diameters, up to 50 mm, are used for fouling systems. 

Water in reflux tends to get trapped in the tower, if the tower-bottom temperature is above the boiling point of water, at the tower s operating pressure. The water trickles down the tower and revaporizes off of the hot reboiler tubes. As the water may be saturated with corrosive salts and gases, reboiler tube corrosion can be rapid. In almost every petroleum refinery service, refluxing water is a quick route to reboiler tube leaks. 

In vacuum distillation, excessive pressure drop causes excessive bottom temperatures which, in turn, increase degradation, polymerization, coking, and fouling, and also loads up the column, vacuum system, and reboiler. In the suction of a compressor, excessive pressure drop increases the compressor size and energy usage. Such services attempt to minimize tray pressure drop. Methods for estimating pressure drops are similar for most conventional trays. The total pressure drop across a tray is given by... 

A column flooded prematurely after being switched to a new service. Flooding was caused by an excessive pressure drop in a kettle reboiler circuit backing up liquid to above the rdboiler return nozzla Elevating vigKn inlet above liquid level solved the problem. Ladr of level indication made diagnosing the problem difficult. 

If the velocity in the riser is too low, the two phases will separate. A head of liquid will build up and create back pressure against the distillation tower. Periodically, a slug of liquid will be pushed through the riser and relieve the back pressure. The riser then gradually refills with liquid. This type of slug flow causes fluctuating tower pressure. Other instances of slug flow in distillation service are thermosiphon reboiler outlets and partially vaporized feed lines. 

On one unit a depropanizer was reboiled with 30-lb steam. One morning a small feed preheater, also using 30-lb steam, was put in service. Soon after, the ability to control the depropanizer pressure was lost. Figure 18-7 illustrates the process flow. 

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