Liquid plate columns

Plate column Cross-flow, countercurrent Integral Liquid and/or gas Absorption, rectification, stripping... 

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. 

Entrainment Entrainment in a plate column is that liquid which is carried with the vapor from a plate to the plate above. It is detrimental in that the effective plate efficiency is lowered because hquid from a plate of lower volatility is carried to a plate of higher volatility, thereby diluting distillation or absorption effects. Entrainment is also detrimental when nonvolatile impurities are carried upward to contaminate the overhead product from the column. 

Most plate columns operate under conditions such that gas is completely mixed as it flows between the plates, but few operate with pure plug flow of liquid. Departure from plug flow of liquid has been studied by Gautreaux and O Connell [Chem. Eng. Pi oq., 51, 232 (1955)] by assuming that hquid mixing can be represented as occurring in a series of stages of completely mixed liquid. For this model,... 

Ah.soi ption. Sec. 4, Thermodynamics and Sec. 18, Liquid Gas Systems. For plate columns, see Sec. 18, Gas-Liquid Contacting Plate Columns. For packed columns, see Sec. 18, Gas-Liquid Contacting Packed Columns. ... 

Calculation of column diameter (for packed columns, this is usually based on flooding conditions, and, for plate columns, on the optimum gas velocity or the liquid-handling capacity of the plate)... 

Fractionally distd through a 12 plate column and fraction b 77"/4mm was collected. Also wet the acetylthiophene in order to remove and free thiophene which forms an azeotrope with H2O, b 68°, Store in a brown bottle and the clear colourless liquid remains thus for extended periods. [Org Synth 28 1 1948 J Am Chem Soc 69 3093 1947.] The red 4-nitrophenylhydrazone crysts from EtOH, m 181-182°. 

Column diameter for a particular service is a function of the physical properties of the vapor and liquid at the tray conditions, efficiency and capacity characteristics of the contacting mechanism (bubble trays, sieve trays, etc.) as represented by velocity effects including entrainment, and the pressure of the operation. Unfortunately the interrelationship of these is not clearly understood. Therefore, diameters are determined by relations correlated by empirical factors. The factors influencing bubble cap and similar devices, sieve tray and perforated plate columns are somewhat different. 

The program is rather slow in execution and therefore the model is limited to an eight-plate column, which is rather unrealistic for this multicomponent separation. The program is therefore given only for example purposes and a real simulation should involve rather more plates. As in BSTILL, the speed of calculation is also very sensitive to the magnitude of the liquid holdup on the plates, which again are large compared to normal practice. 

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 vapour, 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. 

Plate columns can be designed to handle a wider range of liquid and gas flow-rates than packed columns. 

Plate columns can be designed with more assurance than packed columns. There is always some doubt that good liquid distribution can be maintained throughout a packed column under all operating conditions, particularly in large columns. 

If the liquid causes fouling, or contains solids, it is easier to make provision for cleaning in a plate column manways can be installed on the plates. With small-diameter columns it may be cheaper to use packing and replace the packing when it becomes fouled. 

For corrosive liquids a packed column will usually be cheaper than the equivalent plate column. 

The liquid hold-up is appreciably lower in a packed column than a plate column. This can be important when the inventory of toxic or flammable liquids needs to be kept as small as possible for safety reasons. 

Having estimated the number of trays, the column diameter can then be estimated. This is usually estimated with reference to the flood point for the column. The flood point occurs when the relative flowrates of the vapor and liquid are such that the liquid can no longer flow down the column in such a way as to allow efficient operation of the column14. For plate columns, there are a number of different mechanisms that can create flooding, but in one way or another, either14 ... 

A mixture of ammonia and air is scrubbed in a plate column with fresh water. If the ammonia concentration is reduced from 5 per cent to 0.01 per cent, and the water and air rates are 0.65 and 0.40 kg/m2s, respectively, how many theoretical plates are required The equilibrium relationship may be written as Y = X, where X is the mole ratio in the liquid phase. 

Absorption is a commonly applied operation in chemical processing. It is used as a raw material or a product recovery technique in separation and purification of gaseous streams containing high concentrations of organics (e.g., in natural gas purification and coke by-product recovery operations). In absorption, the organics in the gas stream are dissolved in a liquid solvent. The contact between the absorbing liquid and the vent gas is accomplished in countercurrent spray towers, scrubbers, or packed or plate columns. 

Liquid-liquid extraction is carried out either (1) in a series of well-mixed vessels or stages (well-mixed tanks or in plate column), or (2) in a continuous process, such as a spray column, packed column, or rotating disk column. If the process model is to be represented with integer variables, as in a staged process, MILNP (Glanz and Stichlmair, 1997) or one of the methods described in Chapters 9 and 10 can be employed. This example focuses on optimization in which the model is composed of two first-order, steady-state differential equations (a plug flow model). A similar treatment can be applied to an axial dispersion model. 

Distillation may be carried out in plate columns in which each plate constitutes a single stage, or in packed columns where mass transfer is between a vapour and liquid in continuous countercurrent flow. Plate columns are now considered, and packed columns are discussed in Section 11.11. 

These three types of trays have a common feature in that they all have separate downcomers for the passage of liquid from each tray to the one below. There is another class of tray which has no separate downcomers and yet it still employs a tray type of construction giving a hydrodynamic performance between that of a packed and a plate column. Two examples of this type of device are the Kittel plate and a Turbogrid tray(53). Design data for these trays are sparse in the literature and the manufacturer s recommendations should be sought. 

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