Excessive entrainment

In the froth regime, column throughput is usually restricted by flooding due to excessive entrainment, excessive downcomer backup, or downcomer choke. Excessive entrainment or excessive pressure drops may occasionally (but infrequently) restrict throughput in the froth regime. 

Entrainment occurs when spray or froth formed on one tray enters the gas passages in the tray above. In moderate amounts, entrainment will impair the countercurrent action and hence drastically decrease the efficiency. If it happens in excessive amounts, the condition is called priming and will eventually flood the downcomers. 

Substituted Amides. Monosubstituted and disubstituted amides can be synthesized with or without solvents from fatty acids and aLkylamines. Fatty acids, their esters, and acid halides can be converted to substituted amides by reaction with primary or secondary aLkylamines, arylamines, polyamines, or hydroxyaLkylamines (30). Di- -butylamine reacts with oleic acid (2 1 mole ratio) at 200—230°C and 1380 kPa (200 psi) to produce di-A/-butyloleamide. Entrained water with excess -butylamine is separated for recycling later (31). 

An excess of crotonaldehyde or aUphatic, ahcyhc, and aromatic hydrocarbons and their derivatives is used as a solvent to produce compounds of molecular weights of 1000—5000 (25—28). After removal of unreacted components and solvent, the adduct referred to as polyester is decomposed in acidic media or by pyrolysis (29—36). Proper operation of acidic decomposition can give high yields of pure /n j ,/n7 j -2,4-hexadienoic acid, whereas the pyrolysis gives a mixture of isomers that must be converted to the pure trans,trans form. The thermal decomposition is carried out in the presence of alkaU or amine catalysts. A simultaneous codistillation of the sorbic acid as it forms and the component used as the solvent can simplify the process scheme. The catalyst remains in the reaction batch. Suitable solvents and entraining agents include most inert Hquids that bod at 200—300°C, eg, aUphatic hydrocarbons. When the polyester is spHt thermally at 170—180°C and the sorbic acid is distilled direcdy with the solvent, production and purification can be combined in a single step. The solvent can be reused after removal of the sorbic acid (34). The isomeric mixture can be converted to the thermodynamically more stable trans,trans form in the presence of iodine, alkaU, or sulfuric or hydrochloric acid (37,38). 

Entrainment losses by flashing are frequently encountered in an evaporator. If the feed is above the boiling point and is introduced above or only a short distance below the liquid level, entrainment losses may be excessive. This can occur in a snort-tube-type evaporator if the feed is introduced at only one point below the lower tube... 

As a general nile, the direct-heat units are the simplest and most economical in construction and are emploved when direct contact between the solids and flue gases or air can be tolerated. Because the total heat load must be introduced or removed in the gas stream, large gas volumes and high gas velocities are usually required. The latter will be rarely less than 0.5 m/s in an economical design. Therefore, employment of direct rotating equipment with solids containing extremely fine particles is likely to result in excessive entrainment losses in the exit-gas stream. 

In general, this equipment offers an economical heat-transfer area for first cost as well as operating cost. Capacity is hmited primarily by the air velocity which can be used without excessive dust entrainment. Table 12-32 shows hmiting air velocities suitable for various sohds particles. Usually, the equipment is satisfactory for particles larger than 100 mesh in size. [The use of indirect-heated conveyors eliminates the problem of dust entrainment, but capacity is limited by the heat-transfer coefficients obtainable on the deck (see Sec. 11)]. 

Plate-Column Capacity The maximum allowable capacity of a plate for handling gas and liquid flow is of primaiy importance because it fixes the minimum possible diameter of the column. For a constant hquid rate, increasing the gas rate results eventually in excessive entrainment and flooding. At the flood point it is difficult to obtain net downward flow of hquid, and any liquid fed to the column is carried out with the overheaa gas. Furthermore, the column inven-toiy of hquid increases, pressure drop across the column becomes quite large, and control becomes difficult. Rational design caUs for operation at a safe margin below this maximum aUowable condition. 

The distance between trays Zt should be larger than h, sufficient so that (1) the streamers of dispersed hquid from the holes break up into drops before coalescing into the layer of liqmd on the next plate, (2) the linear velocity of continuous liquid is not greater than that in the down out to avoid excessive entrainment, and (3) the tower may be enterea through handholes or manholes in the sides for cleaning. 

The distance above the catalyst bed in which the flue gas velocity has stabilized is refened to as the transport disengaging height (TDH). At this distance, there is no further gravitation of catalyst. The center-line of the first-stage cyclone inlets should be at TDH or higher otherwise, excessive catalyst entrainment will cause extreme catalyst losses. 

Hats on chimney tray excessively restricted vapor resulting in liquid entrainment. Design error. 

Economic considerations dictate that a commercial fluid bed operates at as high a gas velocity as practicable. A frequent limiting factor is entrainment from the bed which is a very strong ftmction of gas velocity. A well defined fluid bed can be maintained even at gas velocities well in excess of the free fall velocity of the biggest particles... 

The diameter of a tower is established by the volume of vapors which must be handled and by the maximum allowable vapor velocity which can be tolerated without encountering excessive entrainment of liquid from one plate to the plate above. Entrainment can occur by splashing and/or suspension of small droplets in the vapor as a mist. It tends to defeat the purpose of fractionation even a small amount may be serious when rigid specifications on color or impurities must be met. 

Another important consideration in tower design is tray downcomers size. At high ratios of liquid flow to vapor flow a proportionally greater area on the tray must be allotted to the downcomer channel opening. Downcomers are designed from basic hydraulic calculations. If the downcomer is inadequately sized and becomes filled with liquid, liquid level will build on the tray above. This unstable situation will propagate its way up to the tower and result in a flooded tower condition. Excessive entrainment can also lead to this same condition and, in fact, is usually the cause of flooding. 

The usefulness of the Grignard reagent of 3-bromothiophene is somewhat limited as it can only be prepared by the entrainment method. The simultaneous formation of Grignard reagents from 3-bromothiophene and a fivefold molar excess of ethyl bromide gave, however, a 55% yield of 3-thiophenecarboxylic acid upon carbona-tion. With acetaldehyde, a 55% yield of methyl 3-thienyl carbinol... 

Because it is known that the entrainment from perforated trays is considerably less than for bubble caps, the 2-ft, 8-in. diameter would be very conservative and perhaps excessively large. 

Because reciprocating compressors should be supplied with clean gas, inlet filters are recommended in all applications. They cannot satisfactorily handle liquids entrained in the gas, although vapors are no problem if condensation within the cylinders does not take place. Liquids will destroy the lubrication and cause excessive wear. 

The inside of the reservoir generally will have baffles to prevent excessive sloshing of the fluid and to put a partition between the fluid return line and the pump suction or inlet line. The partition forces the returning fluid to travel farther around the tank before being drawn back into the active system through the pump inlet line. This aids in settling the contamination and separating air entrained in the fluid. 

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