Steam strippers

The use of steam to remove lower-boiling or lighter components from a liquid is one of the oldest methods of distillation. Sometimes called steam distillation, this technique relies on a combination of two simple effects. 

The first effect is illustrated when we blow across a bowl of hot soup to cool the soup. Our breath displaces the steam vapors that are on top of the soup. This encourages more molecules of steam vapors to escape from the soup that is, the vapor pressure of the steam above the liquid soup is diminished, because steam is pushed out of the soup bowl with air. The correct technical way to express this idea is to say, The partial pressure of the steam, in equilibrium with the soup, is diminished.  

But our breath itself does not remove heat from the soup. The evaporation of steam from the soup, promoted by our breath, takes heat. Converting one pound of soup to one pound of steam requires 1000 Btu. This heat of evaporation comes not from our breath, but from the soup itself. The correct technical way to express this second effect is, The sensible-heat content of the soup is converted to latent heat of evaporation.  

For example, if we have 101 lb of soup in a rather large bowl, and cause one pound to evaporate by blowing across the bowl, the soup will lose 1000 Btu. This heat of evaporation will come at the expense of the temperature of the remaining soup in the bowl that is, each pound of soup will lose 10 Btu. If the specific heat of our soup is 1.0 Btu/[(lb)( F)], the soup will cool off by 10 F. 

A steam stripper, as shown in Fig. 15.1, works in the same way. The diesel-oil product drawn from the fractionator column is contaminated 

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In a steam stripper, steam is introduced into a packed tower, which causes volatiles to be removed in the vapor phase. An a2eotropic mixture is formed, resulting in a separation of the volatiles from the water. An effluent recycle is usually employed to reduce volatiles in the Hquid effluent. 

The process has two main sources of waste water. These are the condensate streams from the steam strippers. The principal pollutant in both wastewater streams is phenol. Phenol is of concern primarily because of its toxicity, oxygen depletion, and turbidity. In addition, phenol can cause objectionable taste and odor in fish flesh and potable water. 

The effluent streams are currently combined and sent to a steam stripper to remove NH3 from wastewater. The environmental discharge limit for NH3 is 30 ppm. 

Alkylation generates relatively low volumes of wastewater, primarily from water washing of the liquid reactor products. Wastewater is also generated from steam strippers, depropanizers, and debutanizers, and can be contaminated with oil and other impurities. Liquid process waters (hydrocarbons and acid) originate from minor undesirable side reactions and from feed contaminants, and usually exit as a bottoms stream from the acid regeneration column. The bottoms stream is an acid-water mixture that is sent to the neutralizing drum. The acid in this liquid eventually ends up as insoluble calcium fluoride. 

The req cle screamSj especially the cyclohexane that has come through the steam stripper must be thoroughly dried. It doesn t take much more than trace impurities to poison the fresh catalysts. 

Catalytic cracking units are one of the largest sources of sour and phenolic wastewaters in a refinery. Pollutants from catalytic cracking generally come from the steam strippers and overhead accumulators on fractionators, used to recover and separate the various hydrocarbon fractions produced in the catalytic reactors. 

Regeneration of spent catalyst in the steam stripper may produce enough carbon monoxide and fine catalyst particles to constitute an air pollution problem. 

A vacuum stripper removes any unwanted butadiene, and the steam stripper following it removes the excess styrene. Neither the styrene nor butadiene is recycled. Solids are removed from the latex by filters, and the latex may be concentrated to a higher solids level. 

Figure 2. General reaction scheme for SOx reduction applied to Additive R. Amount of sulfur released in reactor and steam stripper.

A steam stripper, as shown in Fig. 10.1, works in the same way. The diesel-oil product drawn from the fractionator column is contaminated with gasoline. The stripping steam mixes with the diesel-oil product on the trays inside the stripper tower. The steam reduces the hydrocarbon partial pressure and thus allows more gasoline to vaporize and to escape from the liquid phase into the vapor phase. The heat of vaporization of the gasoline cannot come from the steam, because the steam (at 300°F) is colder than the diesel oil (at 500°F). The heat of vaporization must come from the diesel-oil product itself. 

Many side-stream steam strippers, of the type shown in Fig. 10.1, do not work very well. Operating personnel report that the stripping steam is not effective in removing undesirable lighter components from the stripper feed. Why could this be so ... 

As discussed in Chap. 2, tray deck dumping also greatly reduces tray efficiency. Unfortunately, steam strippers can have widely varying vapor rates, between the top and bottom trays of a column. 

Figure 10.2 Vapor loads highest on top tray of steam stripper.

Removing benzene and other aromatic compounds from a plant s effluent water is an increasingly common environmental requirement. This is typically achieved with a steam stripper. There is a rather neat trick, which can increase the stripper s efficiency adding saltwater to the stripper feed. Aromatics, especially benzene, are far less soluble in brine than they are in freshwater. But, of course, the brine will be more corrosive than salt-free freshwater. 

A side stream is withdrawn at the tenth tray from the top of T-2 and proceeds to steam stripper T-3 equipped with five trays. Steam is fed below the bottom tray. The combined steam and oil vapors return to T-2 at the eighth tray. Stripper bottoms are pumped with J-6 through E-2A (on crude) and E-2B (on cooling water) and to storage as heavy gasoline. ... 

It is fed to an absorber where 75% of the propane is recovered. The total amount absorbed is 50 mol/hr. The absorber has four theoretical plates and operates at 135psig and 100°F. All of the absorbed material is recovered in a steam stripper that has a large number of plates and operates at 25 psig and 230°F. 

The most extensive application of fluidization has been to catalytic cracking of petroleum fractions. Because the catalyst degrades in a few minutes, it is circulated continuously between reaction and regeneration zones. Figure 17.31(a) is a version of such equipment. The steam stripper is for the removal of occluded oil... 

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