Continuous vapor-liquid contact

Distillation in a packed tower continuous vapor-liquid contact in countercurrent flow... 

Most disdllations conducted commercially t rate continuously, with a more volatile fiaction recovered as distillate and a less volatile fiaction remaining as residue or bottoms. If a portion of the distillate is condensed and returned to the process to enrich the vapors, the liquid is called reflux. The apparatus in which the entichment occurs is usually a vertical, cylindrical vessel called a still or distillation column. This apparatus normally contains internal devices for effecting vapor-liquid contact the devices may be trays or packings. 

Continuous distillations usually take place in towers containing packing or plates to promote vapor-liquid contact. With vapor and liquid in countercurrent flow through the tower, analysis usually is in terms of the number of theoretical plates required to effect the desired separation. Figure 10.5.12 illustrates the concept of a theoretical plate. The vapor and liquid leaving the plate are in physical equilibrium. Distillation calculations then depend on accurate knowledge of the equilibrium relationship. When activity... 

When countercurrent vapor-liquid contacting for the purpose of inducing mass transfer between the phases is carried out in a continuous differential manner as in packed columns, phase equilibrium is not, in general, achieved at any point in the column. In a trayed column, the approach to equihbrium on each tray is determined by the tray efficiency. In a packed column, the separation... 

An interesting class of exact self-similar solutions (H2) can be deduced for the case where the newly formed phase density is a function of temperature only. The method involves a transformation to Lagrangian coordinates, based upon the principle of conservation of mass within the new phase. A similarity variable akin to that employed by Zener (Z2) is then introduced which immobilizes the moving boundary in the transformed space. A particular case which has been studied in detail is that of a column of liquid, initially at the saturation temperature T , in contact with a flat, horizontal plate whose temperature is suddenly increased to a large value, Tw T . Suppose that the density of nucleation sites is so great that individual bubbles coalesce immediately upon formation into a continuous vapor film of uniform thickness, which increases with time. Eventually the liquid-vapor interface becomes severely distorted, in part due to Taylor instability but the vapor film growth, before such effects become important, can be treated as a one-dimensional problem. This problem is closely related to reactor safety problems associated with fast power transients. The assumptions made are ... 

A vapor-recirculating equilibrium still similar to that described by Hipkin and Myers (1) was used to determine vapor-liquid equilibrium data for the system, water-MEK-THF. In this still shown schematically in Figure 1, a recirculating vapor is continuously contacted with a static liquid sample. The vapor-liquid system is enclosed by a jacket where... 

Vapor-liquid mass-transfer operations, such as absorption, stripping and distillation, are carried out in packed and plate columns. The key difference is that counterflowing vapor and liquid are contacted continuously with packings, and discretely with plates. The equilibrium and operating lines of packed and plate columns are identical under the same operating conditions—feed and product flowrates and compositions, temperature and pressure. Models for the design and analysis of packed columns are based on their close analogy to plate devices. 

The gas-liquid contact in a packed bed column is continuous, not stage-wise, as in a plate column. The liquid flows down the column over the packing surface, and the gas or vapor flows counter-currently up the column. In some gas-absorption columns, co-current flow is used. The performance of a packed column is very dependent on the maintenance of good liquid and gas distribution throughout the packed bed, and this is an important consideration in packed-column design. 

The distillation process utilizes the difference in vapor pressures of isotopic species. Because these differences are small, the process has to be repeated many times in a fractionating column, resulting in a cascade of several stages. Liquid is boiled at the bottom and vapor is condensed at the top, so that a continuous counter-current contact is established. 

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