Distillation columns liquid holdup

Consider the binary batch distillation column, represented in Fig. 3.58, and based on that of Luyben (1973, 1990). The still contains Mb moles with liquid mole fraction composition xg. The liquid holdup on each plate n of the column is M with liquid composition x and a corresponding vapour phase composition y,. The liquid flow from plate to plate varies along the column with consequent variations in M . Overhead vapours are condensed in a total condenser and the condensate collected in a reflux drum with a liquid holdup volume Mg and liquid composition xq. From here part of the condensate is returned to the top plate of the column as reflux at the rate Lq and composition xq. Product is removed from the reflux drum at a composition xd and rate D which is controlled by a simple proportional controller acting on the reflux drum level and is proportional to Md-... 

Packed fractional distillation columns run in the batch mode are often used for low-pressure drop vacuum separation. With a trayed column, the liquid holdup on the trays contributes directly to the hydraulic head required to pass through the column, and with twenty theoretical stages that static pressure drop is very high, e.g., as much as 100-200 mm Hg. 

Volumetric liquid holdups in the reflux drum and column base are held perfectly constant by changing the flow rates of bottoms product B and liquid distillate product. ... 

A. LIQUID HOLDUPS. The most common and most important trade-off is that of specifying holdup volumes in tanks, column bases, reflux drums, etc. From a steadystate standpoint, these volumes should be kept as small as possible because this will minimize capital investment. The more holdup that is needed in the base of a distillation column, the taller the column must be. In addition, if the material in the base of the column is heat-sensitive, it is very desirable to keep the holdup in the base as small as possible in order to reduce the time that the material is at the high base temperature. Large holdups also increase the potential pollution and safety risks if hazardous or toxic material is being handled. 

The older tall oil distillation columns used bubble cap trays. In new columns, structured packing is preferred. Because of the low pressure drop of structured packing, steam injection is no longer necessary. The low liquid holdup of this packing minimizes the reactions of the fatty and resin acids. A specific distillation sequence for vacuum columns using structured packing of Sulzer has been described (26). Depitching is carried out at a vacuum of... 

To increase product recovery in batch distillations, as a result to the lower liquid holdup in a packed column. 

FIG. 14-73 Liquid holdup, air-water data by Billet ( Packed Column Design and Analysis, Ruhr University, Bochum, Germany), preloading regime. (From Kister H. Z., Distillation Design, copyright by McGraw-Hill reprinted with permission.)... 

In a steady state continuous distillation with the assumption of a well mixed liquid and vapour on the plates, the holdup has no effect on the analysis (modelling of such columns does not usually include column holdup) since any quantity of liquid holdup in the system has no effect on the mass flows in the system (Rose, 1985). Batch distillation however is inherently an unsteady state process and the liquid holdup in the system become sinks (accumulators) of material which affect the rate of change of flows and hence the whole dynamic response of the system. 

Batch distillation. Because of the smaller liquid holdup of packed columns, a higher percentage of the liquid can be recovered as top product. 

Number of chemical species Distillate flow rate Diffusion coefficient Efficiency Energy flux Energy transfer rate Eeed flow rate Column height Enthalpy Liquid holdup Height of a transfer unit Vapor-liquid equilibrium ratio (K value)... 

The basic theory of batch distillation is given in Hart (1997), Perry et al. (1997), Richardson et al. (2002) and Walas (1990). In the simple theoretical analysis of batch distillation columns, the liquid holdup in the column is usually ignored. This holdup can have a significant effect on the separating efficiency and should be taken into account when designing batch distillation columns. The practical design of batch distillation columns is covered by Hengstebeck (1976), Ellerbe (1997), and Hart (1997). 

Packed columns are used conventionally to obtain a low pressure drop or low liquid holdup when there is practically no heat to remove or supply or when the gas or the liquid is corrosive. They are not used when solids are present in the feed or are formed in the reaction. Although packed columns or reactors can be operated cocurrently, their operation is usually countercurrent. In particular, countercurrent use is preferred when a higher concentration driving force is needed, that is, for distillation or for most physical absorption. However, when irreversible reaction occurs between dissolved gases and the absorbent, the mean concentration driving force is the same for both modes of operation. In this case the capacity of cocurrent columns is not limited by flooding, and at any given flow rates... 

In reactive distillation, chemical reactions are assumed to occur mainly in the liquid phase. Hence the liquid holdup on the trays, or the residence time, is an important design factor for these processes. Other column design considerations, such as number of trays, feed and product tray locations, can be of particular importance in reactive distillation columns. Moreover, since chemical reactions can be exothermic or endothermic, intercoolers or heaters may be required to maintain optimum stage temperatures. Column models of reactive distillation must include chemical reaction... 

The reaction occurs in the liquid phase and the conversion is kinetical ly determined. The liquid holdup on the trays should, therefore, be carefully considered. By separating the ethyl acetate as it is formed, the reaction can be driven toward completion. The feed to the column is a mixture of acetic acid, ethanol, and water. The distillate is predominantly ethyl acetate and most of the unreacted ethanol, plus small amounts of water and unreacted acetic acid. The bottoms product contains most of the water and unreacted acetic acid, plus small amounts of ethyl acetate and unreacted ethanol. 

Depending on the tray geometry (diameter, weir height, downcomer design, etc.) in trayed columns or on the type of packing in packed columns, a certain amount of liquid is retained within the column as liquid holdup. An additional amount of liquid holdup is associated with the condenser assembly. The remaining space within the distillation system contains the vapor holdup. In continuous columns, holdup is usually not a factor since it does not affect the separation as long as the column operates at steady state. 

A three-stage batch distillation column is charged with a 100 kmol mixture containing 60% mole component 1 and 40% mole component 2. The column pressure is maintained at 100 kPa, and the distillate rate is 20 kmol/h. It is desired to produce two distillation cuts. The first cut will be produced by continuously adjusting the reflux ratio to maintain the distillate composition at 90% mole component 1. Production of the second cut starts when the L/V ratio is 0.80. The L/V ratio will be fixed at this value until the second cut cumulative composition is 75% mole component 1. Determine the amount of each cut. Assume negligible tray holdups and use vapor-liquid equilibrium data from Problem 6.1. 

For distillation columns Add sufficient trays to account for disturbances and anticipated expansion of production. The expected turndown ratios affect the choice of internals in a distillation column with a large turndown ratio, suggesting the use of bubble caps a low turndown ratio may point to structured packings. At the same time, safety and hazard analysis indicates that we want a minimum of liquid holdup in the system. 

Distillation control schemes may be analyzed either on a steady-stale (sensitivity analysis) or on a dynamic basis. The latter requires a dynamic model that takes into account the dynamic response of the column and the control loope. An example of a dynamic model is described by McCune and Gailier,6 but il should be apparent from the material presented enrlier that the holdup characteristics of distillation columa devices can vary widely, and snch variation should be accommodated by the model. The development of the naw high-efficiency packings has caused a new look at the system dynamics when the liquid holdup in the column is quite low, and thus the existing models for trays may not be adjustable to application to packings. The use of a tray-type dynamic model is described in the article by Gailier and McCune 7 so work to date has been reported for packed column dynamic models. 

Consider a binary mixture of components A and B, to be separated into two product streams using conventional distillation. The mixture is fed in the column as a saturated liquid (i.e., at its bubble point), onto the feed tray / (Figure 4.10), with a molar flow rate (mol/min) F/ and a molar fraction of component A, overhead vapor stream is cooled and completely condensed, and then it flows into the reflux drum. The cooling of the overhead vapor is accomplished with cooling water. The liquid from the reflux drum is partly pumped back in the column (top tray, N) with a molar flow rate FR (reflux stream) and is partly removed as the distillate product with a molar flow rate FD. Let us call Mrd the liquid holdup in the reflux drum and xD the molar fraction of component A in the liquid of the reflux drum. It is clear that xD is the composition for both the reflux and distillate streams. 

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