Tray columns limiting flow rates

The rate-based models suggested up to now do not take liquid back-mixing into consideration. The only exception is the nonequilibrium-cell model for multicomponent reactive distillation in tray columns presented in Ref. 169. In this work a single distillation tray is treated by a series of cells along the vapor and liquid flow paths, whereas each cell is described by the two-film model (see Section 2.3). Using different numbers of cells in both flow paths allows one to describe various flow patterns. However, a consistent experimental determination of necessary model parameters (e.g., cell film thickness) appears difficult, whereas the complex iterative character of the calculation procedure in the dynamic case limits the applicability of the nonequilibrium cell model. 

Figure 6.6 is a typical tray stability diagram. The area of satisfactory operation (shaded) is bound by the tray stability limits. These limits are discussed in the following sections. The upper capacity limit is the onset of flooding. At moderate and high liquid flow rates, the entrainment (jet) flooding limit is normally reached when vapor flow is raised, while the downcomer flooding limit is normally reached when liquid flow is raised. When flows are raised while the column operates at constant LIV (i.e., constant reflux ratio), either limit can be reached. At very low liquid rates, as vapor rate is raised, the limit of excessive entrainment is often reached. 

The internal flow of liquid and vapor must be re-evaluated from the standpoint of column capacity, both in the design and performance studies of columns. The physical dimensions of a column can handle only limited ranges of vapor and liquid flow rates. The objective of this chapter is to evaluate the hydraulic aspects of fluid flow in trayed columns. The column performance is examined with regard to factors such as flooding, entrainment, pressure drop, mass transfer, and tray efficiency. 

The downflowing liquid is transported from a tray to the tray below by means of conduits called down. comers, and it is evident thet if the downcomer is not sufficiently large to handle (he required liquid load, the pressure drop associated with liquid flow will serve as a constriction and a point of flow rale limitation. In fact, downcomers usually serve to bottleneck operations or high-pressure fractionators and absorbers. They must be sized such that they do not fill completely under the highest flow rates expected for the column. As will be shown, the vapor flow rate contributes toward the liquid cupacity limitation. 

A sieve-tray column with 15 plates is used to prepare 99 percent methanol from a feed containing 40 percent methanol and 60 percent water (mole percent). The plates have 8 percent open area, in. holes, and 2-in. weirs with segmental downcomers, (a) If the column is operated at atmospheric pressure, estimate the flooding limit based on conditions at the top of the column. What is the F factor and the pressure drop per plate at this limit (Z>) For the flow rate calculated in part (a) determine the F factor and the pressure drop per plate near the bottom of the column. Which section of the column will flood first as the vapor rate is increased ... 

We need to discuss some of the limiting conditions in distillation systems. The minimum number of trays for a specified separation corresponds to total reflux operation. If the column is mn under total reflux conditions, the distillate flow rate is zero. Therefore, the reflux ratio is infinite, and the slope of the operating lines is unity. This is the 45° line. Thus, the minimum number of trays can be determined by simply stepping up between the 45° line and the VLB curve (see Fig. 2.8). 

Many distillation column use reboiler heat input as a primary manipulated variable, usually to control a temperature on an appropriate tray. This means that reboiler heat input is not constrained during normal operation with normal feed flow rates. However, as the feed flow rate to the column is decreased, less vapor boilup is required to achieve the same separation. If the feed drops to the point where the low vapor-boilup limit is encountered, the control structure must change. Three alternative control structures for achieving stable operation at minimum vapor flow rates are discussed in the following. 

In this chapter, all of the process flow rates were considered to be constrained by zero flow and the maximum flow allowable by the valve size and span of the flow measurement. The main column constraint because of flooding is associated with the vapor traffic and pressure drop across the trays or packing in the distillation column. The mass transfer rate limit for stripping light key impurity from the bottoms stream was presented. The mass transfer rate limit for absorbing heavy key impurity from the overhead vapor stream was also presented. 

With higher bottoms flow rates, lower reboiler duties are required at minimum reflux because less liquid must be vaporized. For any given bottoms rate, a minimum amount of reflux is required to maintain both liquid and vapor phases on all trays. Curve AB in Figure 6.5 is a plot of the minimum reflux ratio. This minimum is not to be confused with the minimum reflux ratio required to bring about a specified separation with a given number of stages. The curve characteristics depend mainly on the feed thermal conditions. Below this curve either the liquid or the vapor dries up on some of the trays. The upper limit of the reflux ratio is determined by practical considerations such as duty limitations and column diameter. 

A general, rigorous, multi-component rate-based model takes into account mass and energy transfer between the phases, coupled with the usual inter-stage flows included in the equilibrium stage-based models discussed in previous chapters. Rate-based models are not limited to packed columns but can be apphed to trayed columns as well. 

The deethanizer in a naphtha-fed olefins plant also is a column with a smaller diameter upper section than lower section. This column operates at a pressure between 340 and 410 psia with a top temperature of 0° to 20°F. A trayed column typically contains 45 to 70 actual trays. Again the liquid surface tension is very low, and the density difference between the liquid phase and the vapor phase is small. Therefore, the capacity of a trayed column usually is limited by the downcomers. In a trayed tower, the cross-sectional area is rather large compared to vapor flow rate. Replacing trays with an IMTP system increases the capacity by about 35% above the design rate for the trays. 

There are diree factors that limit the accuracy of the preceding analysis. The first of these relates to the phenomenon of inverse response discussed in Chapter 13. It is characteristic of valve tray columns and some sieve tray columns operating at low boilup rates. It exercises its most serious effect in those columns where base level is controlled via steam flow. If the level becomes too high, the level controller increases the steam flow. But this causes a momentary increase in base level due to the extra liquid coming down the column (also due to thermosyphon reboiler swell ). Without proper design the level controller can become very confused. This is discussed in detail in Chapter 16. 

As in two-product columns, total reflux in multiple product columns is a limiting condition where the colnmn internal liqnid and vapor flows are very large compared to each of the products and feed(s). Multiproduct columns are considered to consist of column sections defined by the product locations. Each section is bonnded by two products, one at its top and the other at its bottom. Thus, a column with. y sections has h- 1 products. As in two-product columns, multiproduct columns operating at total reflux achieve the maximum separation possible with a given number of stages in each section and for a given set of product rates. Conversely, if the separation between the different products is specified, the minimum trays required in each section are evaluated by total reflux calculations. 

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