Tube Foaming Evaporator

The vertical tube foam evaporator is used to evaporate cooling tower blowdown before disposal. It is essentially a conventional vertical tube, recirculating 

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Sephton, H, H., "Vertical Tube Foam Evaporator", Chemical Engineering Progress,... 

Mechanical cleaning is fairly easy with such units, and the capital investment is relatively low. Circulation stops, however, if the heat input is interrupted, creating the danger of the settling of any solids suspended in the liquor. This type of unit is not well suited to viscous liquids because of the low heat transfer coefficients associated with the low velocities of natural convec-tion.f Short-tube vertical evaporators have largely been surpassed by other types, particularly for applications involving liquors that foam, deposit excessive scale, are excessively viscous, or are heat sensitive. 

Black Liquor from Soda-pulp Mills.—One ton of pulp will produce about 3,300 gal. of 5 deg. liquor, and this is concentrated to 35 deg. in multiple-effect evaporators of the horizontal and film type, with a capacity of from 2 to 2K gal. per square foot, according to the steam pressure, which will be from 10 to 25 lb., with a vacuum of from 27 to 28 in. Special construction and separators are necessary on account of the excessive foaming. Evaporators must have cast-iron or steel shells with wrought-iron or steel tubes, as the liquors are strongly alkaline. 

Long-tube vertical evaporators are especially effective in concentrating liquids that tend to foam. Foam is broken when the two-phase mixture hits the baffle plate. 

The LT evaporator shown in Fig. 11-122/ is typical of those commonly used, especially for black hquor. Feed enters at the bottom of the tube and starts to boil partway up the tube, and the mixture of liquid and vapor leaving at the top at nigh velocity impinges against a deflector placed above the tube sheet. This deflector is effective both as a primary separator and as a foam breaker. 

The fusion is best carried out in vacuum, suitably in the vacuum baking apparatus used in the sulfonation of amines by the baking process. When the water is evaporated from the melt, the mass foams and, in order to avoid clogging the suction tube, this tube should be sufficiently wide and the tray should not be more than half filled. In order to obtain good yields, it is necessary to use about twice the sodium hydroxide required theoretically, i.e., about 8 moles per mole of sodium benzenedisulfonate. 

Dry NHj gas (passed through a drying tube packed with CaH ) was bubbled slowly through a solution of 2,6-dichloro-9-(2,3,5-tri-0-acetyl-/ -D-ribofuranosyl)purine (1.0 g, 2.2 mmol, dried at 60 C in vacuo) in anhyd DME (20 mL, distilled directly from NaH into the dried reaction flask) at rt for 8 h. The solution was evaporated and the colorless residue was treated with CIICI3 (10 mL). This suspension was applied to a column of silica gel (50 g, 2 x 45 cm). CHClj/acetone with an increasing gradient of acetone was used for elution. The appropriately pooled fractions were evaporated and the resulting colorless solid foam was dissolved in CHCI, /i-PrOH. The product crystallized upon slow evaporation of the solution. It was filtered, washed with EtjO, and dried yield 857 mg (91%) mp 151-152°C. 

The collets from the expander are hot and moist and are often cooled and dried prior to entering the extraction process. The dryers, which may be equipped with steam heating, typically remove about 2% moisture (most of that added by live steam in the expander) and reduce temperamre by about 40°C. The dryers also allow time for the collets to approach a more uniform moisture and temperature. Some plants report an increased tendency for the misceUa in the second effect evaporator to foam or to coat or plug the evaporator tubes. It is thought that this may be related to a lack of drying and a nonuniformity of the collet such that the center is hot and moist, causing water-related problems in extraction. 

Distilled Water.—Water will boil with a small temperature difference, and the apparatus is simple and cheap. It is therefore economical to combine a num ber of effects to one unit, and quadruple- and decituple-effect evaporators of the vertical-tube and film type are common practice. They are operated under pressure and vacuum, and the capacity will vary from 3 to 6 gal. per square foot according to the total temperature difference. Evaporator shells are made of cast iron or steel, and tubes of steel, brass or copper. There is no foaming, but generally all raw water contains considerable amounts of scale forming material and tubes have to be cleaned frequently. Sea water has to be used to a considerable extent for the manufacture of distilled water, and usually the Reilly coil type is used aboard ships, as it is claimed by the inventor that scale will not deposit on the tubes. Sometimes the 20°B6. liquor coming from the sea water stills is concentrated further for the recovery of sea salt in standard vertical-tube evaporators with salt filters. 

A properly purified and well-filtered solution will not give much trouble on account of foaming, but all sugar juices contain considerable amounts of lime and silica, which will form a hard scale on the tubes. This scale is usually removed by boiling with a weak soda ash and acid solution. The Standard type has the advantage over all other constructions as it can be cleaned mechanically and is therefore used to a large extent in the cane-sugar industry. Evaporator shells are usually made of cast iron and sometimes of steel. Tubes are of brass or copper. 

Glucose.—One bushel of corn will give about 14 gal. of liquor, and the first concentration is done in a triple-effect evaporator from 15 to 30°B6. with from 5 to 10 lb. of exhaust steam pressure and a vacuum of from 27 to 28 in. The capacity is from 2 i to 3 gal. per square foot. Usually the standard and the horizontal-type evaporators are used also the semi-film. The latter has the advantage that it will prevent foaming and facilitate cleaning. The second concentration from 30 to from 44 to 45°B. is done in a single-effect of the standard or horizontal type. The capacity is from % to. 1 gal. per square foot, and the horizontal type must be of special construction so as to leave sufficient space between the tubes for the discharge of the heavy syrup. On account of the calcium sulphate in the weak liquor, the tubes in the multiple effect will foul very rapidly, and have to be cleaned either chemically or mechanically. The liquors are apt to foam, and special precautions in the form of extra vapor space and separators are to be taken. Evaporators are usually built with cast-iron shells and copper tubes. 

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