Packed towers, separations

After the SO converter has stabilized, the 6—7% SO gas stream can be further diluted with dry air, I, to provide the SO reaction gas at a prescribed concentration, ca 4 vol % for LAB sulfonation and ca 2.5% for alcohol ethoxylate sulfation. The molten sulfur is accurately measured and controlled by mass flow meters. The organic feedstock is also accurately controlled by mass flow meters and a variable speed-driven gear pump. The high velocity SO reaction gas and organic feedstock are introduced into the top of the sulfonation reactor,, in cocurrent downward flow where the reaction product and gas are separated in a cyclone separator, K, then pumped to a cooler, L, and circulated back into a quench cooling reservoir at the base of the reactor, unique to Chemithon concentric reactor systems. The gas stream from the cyclone separator, M, is sent to an electrostatic precipitator (ESP), N, which removes entrained acidic organics, and then sent to the packed tower, H, where SO2 and any SO traces are adsorbed in a dilute NaOH solution and finally vented, O. Even a 99% conversion of SO2 to SO contributes ca 500 ppm SO2 to the effluent gas. 

If a waste contains a mixture of volatile components that have similar vapor pressures, it is more difficult to separate these components and continuous fractional distillation is required. In this type of distillation unit (Fig. 4), a packed tower or tray column is used. Steam is introduced at the bottom of the column while the waste stream is introduced above and flows downward, countercurrent to the steam. As the steam vaporizes the volatile components and rises, it passes through a rectification section above the waste feed. In this section, vapors that have been condensed from the process are refluxed to the column, contacting the rising vapors and enriching them with the more volatile components. The vapors are then collected and condensed. Organics in the condensate may be separated from the aqueous stream after which the aqueous stream can be recycled to the stripper. 

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. 

A typical amine system is shown in Figure 7-4. The sour gas enters the system through an inlet separator to remove any entrained water or hydrocarbon liquids. Then the gas enters the bottom of the amine absorber and flows counter-current to the amine solution. The absorber can be either a trayed or packed tower. Conventional packing is usually used for 20-in. or smaller diameter towers, and trays or structured packing for larger towers. An optional outlet separator may be included to recover entrained amines from the sweet gas. 

In a trayed absorber the amine falls from one tray to the one below in the same manner as the liquid in a condensate stabilizer (Chapter 6, Figure 6-4). It flows across the tray and over a weir before flowing into the next downcomer. The gas bubbles up through the liquid and creates a froth that must be separated from the gas before it reaches the underside of the next tray. For preliminary design, a tray spacing of 24 in. and a minimum diameter capable of separating 150 to 200 micron droplets (using the equations developed in Volume 1 for gas capacity of a vertical separator) can be assumed. The size of packed towers must be obtained from manufacturer s published literature. 

There are several types of wet collectors including spray towers, packed towers, and wet centrifugal collectors. The spray tower is a cylindrical or rectangular tower into which the incoming air is passed. Highspeed water sprays in the tower impact and remove the dust that is subsequently separated from the droplets by various types of eliminators. Spray towers are effective for all kinds of dust and even moisture-laden gases. 

Case Study 1. Pump-and-Treat System with a Packed-Tower Air Stripper, McClellan Air Force Base Superfund Site, California, Operable Units B/C, 1987. The costs associated with pump-and-treat system used at the site were estimated in 1994. Costs were approximately 80 per pound of removed volatile organic compounds (VOCs) based on operating costs alone and approximately 150 per pound when capital costs were included (D141286, p. 135). It should be noted that the operation and maintenance costs for the an air stripper could not be separated from the total cost of the project. Capital cost and operating cost information for this project are summarized in Case Study 1. 

As the potentialities of liquid extraction as a separation method were developed, the need for efficient, continuously operated, multistage equipment became apparent. It was natural therefore to turn to devices which had been so successful in other similar fluid-contacting operations, such as the bubble-tray tower and the packed tower of distillation. These devices have proved to be disappointing in liquid-extraction service, however for example, bubble-tray towers provide tray efficiencies in liquid-extraction operations of less than 5% (S7), and conventional packed towers show heights of transfer units of 10 to 20 ft. or more (T3). 

The very first continuous distillation column was the patent still used to produce Scotch whiskey in the 1830s. It had 12 bubble-cap trays with weirs, downcomers, tray decks, and bubble caps with internal risers. Current trayed towers are quite similar. As most distillation towers have always been trayed rather than packed, one would have to conclude that trayed towers must have some sort of inherent advantage over packed towers. And this is indeed true, in a practical sense even though, in theory, a packed tower has greater capacity and superior separation efficiency than a trayed column. 

A packed tower can successfully fractionate with a very small pressure drop, as compared to a tray. For a modern trayed tower, to produce one single theoretical tray worth of separation (that s like a single, 100 percent efficient tray), a pressure drop of about 6 in of liquid is needed. A bed of structured packing can do the same job, with one inch of liquid pressure drop, even when allowing for the vapor distributor. In low-pressure fractionators, especially vacuum towers used to make lubricating oils and waxes, this can be of critical importance. 

Continuous changes in compositions of phases flowing in contact with each other are characteristic of packed towers, spray or wetted wall columns, and some novel equipment such as the FHGEE contactor (Fig. 13.14). The theory of mass transfer between phases and separation of mixtures under such conditions is based on a two-film theory. The concept is illustrated in Figure 13.15(a). 

The most useful measure of the separating power of packed towers is the HETP, the height equivalent to a theoretical plate or stage. It is evaluated simply as the ratio of packed height used for a certain degree of separation to the theoretical number of stages. Its relation to the fundamental quantity, HTU, or the height of a transfer unit, is... 

When a sucrose- or other simple carbohydrate-based solution is mixed with yeast and oxygen in a fermenter, carbon dioxide vapour and alcohol are produced. The carbon dioxide can then be passed through a separator to remove any trace cany-over of foam. Once the foam has been removed the carbon dioxide is compressed. It is then scrubbed with water in a packed tower, removing water-soluble impurities such as alcohol, ketones and other aroma chemicals produced during fermentation. 

An extractor column is generally a tall, vertical packed tower that has two or more bed sections. Each packed bed section is typically limited to no more than 8 ft tall, making the overall tower height about 40 to 80 ft. Tower diameter depends fully upon liquid rates, but is usually in the range of 2 to 6 ft. Liquid-liquid extractors may also have tray-type column internals, usually composed of sieve-type trays without downcomers. These tray-type columns are similar to duoflow-type vapor-liquid separation, but here serve as contact surface area for two separate liquid phases. The packed-type internals are more common by far and are the type of extractor medium considered the standard. Any deviation from packed-type columns is compared to packing. 

A demethanizer fractionation tower is frequently positioned first. This tower is often operated at 34 x 105 Pa, with temperatures low enough to obtain liquid methane. This tower is usually a tray-type column, although more recently packed towers have been introduced. The noncondensible gases (hydrogen, nitrogen, and carbon monoxide) and relatively pure methane can thus be separated from the C2 and higher hydrocarbons. 

Rectification can be conducted in packed towers filled with Raschig rings. The rectification separates three fractions. Fraction I, which consists of trichlorosilane with a small amount of dichlorosilane, is separated at 35 °C at the top of the tower. Fraction II (a mixture of trichlorosilane and silicon tetrachloride) is separated at 35-36 °C at the top of the tower (this fraction can later be sent into the tower tank for repeated rectification. Fraction III (tank residue) mostly consists of silicon tetrachloride. Subsequent rectification of fraction II yields HSiCl3 (95-100%) and SiCl4 (to 5%). For obtaining phenyltrichlorosilane by high-temperature condensation, this condensate can be used without rectification. 

Like tetraethoxysilane, triethoxysilane can be synthesised in a bubble etherificator (see app.6 in Fig. 23) below 60 °C, and rectified in a packed tower. This allows to separate two fractions below 131 °C (unreacted alcohol with an impurity of triethoxysilane) and 131-135 °C (triethoxysilane). 

Enameled reactor 4 with inverse cooler 5, an agitator and a jacket is filled with a solution of quinoline in chlorobenzene from agitator 3 and loaded with ammonia chloride and phosphorus pentachloride through a hatch. The synthesis is conducted at 128-130 °C until the quantity of released hydrogen chloride is noticeably reduced. Hydrogen chloride is absorbed with water in packed tower 7. After the process is finished, the reactive mixture is cooled and filtered in nutsch filter 8 to separate muriatic quinoline and the excess of ammonia chloride. Phosphonitrilechloride can also be conducted in tetrachloroethane medium in this case the process is carried out at 135-140 °C. 

Distillation stage calculations are usually performed with ideal stages, The number of ideal stages required for the separation is divided by the overall column efficiency (Sec, 7,1,1) to obtain the required number of trays. In packed towers, the number of stages in the column is multiplied by the HETP (Height Equivalent of a Theoretical Plate, see Sec. 9.1,2) to obtain the packed height. 

---