Conventional tray column

Whereas homogeneously catalyzed reactive distillation can be carried out in conventional tray columns (sometimes modified to ensure sufficient residence time of the reactants), a heterogeneous catalyst has to be fixed in the reactive section with the help of special internals. These internals have to combine good wetting characteristics to achieve a good contact between the Hquid and vapor phases with a large amount of catalyst that is readily accessible by the liquid in order to avoid macro-kinetic influences. 

In vacuum distillation, excessive pressure drop causes excessive bottom temperatures which, in turn, increase degradation, polymerization, coking, and fouling, and also loads up the column, vacuum system, and reboiler. In the suction of a compressor, excessive pressure drop increases the compressor size and energy usage. Such services attempt to minimize tray pressure drop. Methods for estimating pressure drops are similar for most conventional trays. The total pressure drop across a tray is given by... 

The prices shown for distillation columns are misleadingly low. Most of the cost of a conventional distillation column is associated with the reboiler, condenser, pumps, reflux drum, and internal trays or packing. Equipment costs can be ratioed by the 0.6 power with size to obtain rough prices for larger or smaller sizes. 

Recently there has been an increasing trend to replace the conventional trays depicted in Fig. 5 by trays having receiving pans that terminate some 15 cm above the tray deck. This provides more column cross-sectional area for vapor flow and allows increased vapor capacity. Even greater vapor capacity can be obtained from trays that utilize localized, upward co-current flow of vapor and liquid. But, as each tray then requires a vapor-liquid separation device, they are more expensive and are used only in specialized applications. 

FIG. 19-31 Some examples of bubble column reactor types, (a) Conventional bubble column with no internals. (6) Tray bubble column, (c) Packed bubble column with the packing being either an inert or a catalyst. [From Mills, Ramachandran, and Chaudhari, Multiphase Reaction Engineeringfor Fine Chemicals and Pharmaceuticals, Reviews in Chemical Engineering, 8(1-2), 1992, Figs. 2, 3, and 4.]... 

In nonequilibrium models, as in the other models, the subscript j is for the stage. In a trayed column, it is the actual tray. In a packed column, j is a section of packing. By convention, transfer is to be from the liquid to the vapor with the mass transfer rate to the vapor, Nf, taken as positive. 

Distillation columns have a large inventory in the reboiler, and typically, an inventory several times greater in the column itself. Column holdup may be reduced by using low holdup internals. Conventional trays and packings differ by a factor of about 10 in inventory per theoretical plate (Kletz, 1985a). An estimate of holdup per theoretical plate is shown (Lees, 1980). 

The first and third packed sections are similar to those of a conventional distillation column, and only distributor and support trays are required for the operation of the DWDC. A reboiler (Figure 5, element 9) that can be charged with the reacting mixture in the batch operation fashion was placed at the end of the third packed section. Finally, several thermocouples were implemented in the DWDC to register the temperature... 

Change tray configuration, or consider high-efficiency packing rather than conventional tray-type columns (this reduces pressure drop and decreases reboiler temperatures). 

A refinery aqueous stream flows at 600 gal/min and contains 15 Ib/hr benzene. The benzene is to be removed at a stripping efficiency of 99.95%. A tray column is to be used. Consider two operating pressures, 2.0 psia and 14.7 psia. With no attempt at optimization, evaluate the parameters that should be considered for design. In keeping with refinery practice, use conventional English units. 

The first type of internals is the so-called tea bag (Smith, 1984), which consists of wire-mesh bags that can be filled with the catalyst. These bags are stored on the trays of conventional distillation columns. The main drawback of this packing type is that transport Hmitations cannot be avoided, thus leading to smaller reaction rates. Furthermore the bags on the trays result in high additional pressure drops. 

Column hardware choice can have a significant influence on the conversion and selectivity such aspects can be properly described only by the NEQ cell model, or by a still more sophisticated model based on computational fluid mechanics (such models have yet to be developed). It is insufficiently realized in the literature that, say, for tray RD columns, the tray design can be deliberately chosen to improve conversion and selectivity. Even less appreciated is the fact that the design methodology for RD tray columns is fundamentally different from that of conventional trays. Liquid residence time and residence time distributions are more important in RD. The froth regime is to be preferred to the spray regime for RD applications this is opposite to the design wisdom normally adopted for conventional distillation. For relatively fast reactions, it is essential to properly model intra-particle dif... 

Figure 7.1 Conventional equipment used for fluid-fluid reactions (columns (a) multistage agitated column, (b) packed column, (c) sieve tray column, (d) buss loop reactor, (e) tubular reactor, and (f) static mixer)...

Going one level up in aggregation we consider the structures of complete columns. Conventionally, RD columns are classified in hybrid and non-hybrid units, depending on the distribution of reactive trays inside the column. Thus, hybrid columns are distillation columns with a reactive core and nonreactive sections (rectifying and/or stripping sections). Non-hybrid RD columns denote columns where all trays including condenser and reboiler are reactive (Giittinger, 1998). Hybrid columns are commonly... 

In this chapter, a suitable entrainer for this acetic acid dehydration system will be selected from several candidate acetates. Steady-state tray-by-tray column simulation will be used to determine the best entrainer with minimum total annual cost (TAC). Optimum process design and operating conditions will be determined to keep a high-purity bottom acetic acid composition and also to keep a small acetic acid loss in top aqueous draw. The overall control strategy of this column system will also be proposed to hold both bottom and top product specifications in spite of feed rate and feed composition disturbances. In the control study, conventional control strategy using only tray tempaature measurements will be considered so that the result of this study can easily be used directly in industry. 

The operation of a conventional tray fractionation column, and the associated variables, is sketched in Figure 7.15. Although the process has most of the standard properties of a staged countercurrent operation we have seen before, it does display a number of new features that require special mention ... 

The choice between trays and packing is somewhat arbitrary because eitho- can usually be designed to do an adequate job, and the overall economics ate seldom decisively in favor of one or the other. At tUs time, sieve tray columns ate probably the most popular for both absorbs and strippers in conventional, large commercial amine plants while packed columns are often used for revamps to increase edacity or efficiency and for special applications. 

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