Drying tray columns

In general, plants using SO2 gas derived from metallic sulfides, spent acids, or gypsum anhydrite purify the gas stream before drying it by cold, ie, wet, gas purification. Various equipment combinations including humidification towers, reverse jet scmbbers, packed gas cooling towers, impingement tray columns and electrostatic precipitators are used to clean the gas. 

Entrainment Corrections. Above about 80% of flood, the recirculation of liquid as entrainment between trays undermines the countercurrent action of the tray column, and efficiency therefore suffers. This is a particular problem in vacuum distillation where it may be optimum to allow a certain amount of liquid entrainment in initial design. Figure 13.41 shows an approximate method for entrainment correction to column efficiency or Murphree efficiency. The abscissa scale is the same parameter used for flooding prediction (Figure 13.32(b)). The ordinate value is used to correct from a dry to a wet efficiency (with entrainment) ... 

This type of unit features the main steps encountered in partial condensation. The available gas mixture, previously dried on molecular sieves, is cooled to around — 120 C by heat exchange in a series of piaicexchangers operating in countercurrent flow on the cold purified products, and is then introduced at the bottom of a tray column with a downflow of liquid methane. The operation takes place at a pressure of abont 1.6.10 Pa absolute. It yields hydrogen with a purity better than 98.5 per cent volume, and which contains less than 10 ppm of CO. After expansion and partial vaporization, the extract feeds a distillation colnmn operating at 0.2. 10 Pa absolute, which separates the carbon monoxide at the top with a yield of at least 80 per cent at a purity over 99 per cent volume, and liquid methane at the bottom. This methane provides reflux to the scrubbing column and partly to the feed cooling uuiL... 

Similar guidelines apply when column capacity restriction is primarily related to column vapor load [e.g., a downcomer backup restriction when backup is primarily due to dry tray pressure drop (199) or in packed columns in which flooding is induced by excessive vapor loads (195)]. The guidelines do not apply when column capacity restriction is primarily caused by excessive liquid load (e.g., downcomer choke or downcomer backup restriction when the backup is primarily... 

Ap across the column < design instrument fault/[low boilup rate], see Section 3.3/dry trays/low feedrate/feed temperature too high. 

During some operations, the vapor appears to pulse upward one to three trays at a time. It is also possible to see one or two dry trays in the column while the other sections have a good liquid level above the tray. If the column is being pushed too hard, the upper trays may flood and cause problems In the lower section of the column. It is also possible to see the transfer of heat energy in the condenser as fluid passes in opposite directions. In a glass distillation bench-top operation, it is easy to observe occurrences and record data that are not visible on a large-scale operation. 

The idea is best explained with an example. Suppose the base level in a distillation column is normally held by bottoms product withdrawal as shown in Fig. 8.4a. A temperature in the stripping section is held by steam to the rcboiler. Situations can arise where the base level continues to drop even with the bottoms flow at zero (vapor boilup is greater than the liquid rate from tray 1). if no corrective action is taken, the reboiler may boil dry (which could foul the tubes) and the bottoms pump could lose suction. 

The tray temperatures in our preflash tower, shown in Fig. 4.4, drop as the gas flows up the tower. Most of the reduced sensible-heat content of the flowing gas is converted to latent heat of evaporation of the downflowing reflux. This means that the liquid flow, or internal reflux rate, decreases as the liquid flows down the column. The greater the temperature drop per tray, the greater the evaporation of internal reflux. It is not unusual for 80 to 90 percent of the reflux to evaporate between the top and bottom trays in the absorption section of many towers. We say that the lower trays, in the absorption section of such a tower, are drying out. The separation efficiency of trays operating with extremely low liquid flows over their weirs will be very low. This problem is commonly encountered for towers with low reflux ratios, and a multicomponent overhead product composition. 

Water dried, carbon dioxide and hydrogen sulphide free gas i.e. wet-gas, is chilled (typically to about -35°C) and enters the bottom of the absorber tower there are usually two absorber towers. Condensate separated in the chiller unit leaves the bottom of the tower, the gas rises against a chilled falling solvent that has entered the top of the tower (the lean-oil). The solvent absorbs the heavier constituents while the lighter sales gas rises to the top of the absorber and exits the top of the tower. The now rich-oil is collected on an absorber tray above the gas entry point and passes via heat exchangers to a column (ROD). 

A feed that is superheated at Tf and Pf has a negative XHp and, therefore, a negative q. A superheated feed will increase the vapor rate above the feed tray (Vr > Vj) and decrease the liquid rate below the feed tray (Lj < L ). If the feed is sufficiently superheated, it may dry up the liquid in the lower column section, making Lj approach zero. 

The column operable range is determined in part by the requirement that no tray be allowed to dry up, that is, liquid and vapor must exist on each tray to maintain phase equilibrium. This range may be defined by the limits over which the condenser and reboiler duties may vary. As such, the condenser and reboiler duties are considered the two independent variables required to define the column performance (Sections 3.2.3 and 5.2.1). Alternatively, other pairs of variables may be chosen as the independent variables defining the column performance and each set can vary... 

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