Distillation columns vapor dynamics

Vertical in-tube condensers are often designed for reflux or knock-back application in reactors or distillation columns. In this case, vapor flow is upward, countercurrent to the hquid flow on the tube wall the vapor shear ac4s to tliicken and retard the drainage of the condensate film, reducing the coefficient. Neither the fluid dynamics nor the heat transfer is well understood in this case, but Sohman, Schuster, and Berenson [J. Heat Transfer, 90, 267-276... 

Any of the global Newton methods can be converted to a relaxation form in Ketchum s method by making both the temperatures and the liquid compositions time dependent and by having the time step increase as the solution is approached. The relaxation technique should be applied to difflcult-to-solve systems and the method of Naphtali and Sandholm (42) is best-suited for nonideal mixtures since both the liquid and vapor compositions are included in the independent variables. Drew and Franks (65) presented a Naphtali-Sandholm method for the dynamic simulation of a reactive distillation column but also stated that this method could be used for finding a steady-state solution. 

Tables 11.5 to 11.7 contain process stream data. These data come from the TMODS dynamic simulation and not from a commercial steady-state simulation package. The corresponding stream numbers are shown on the flowsheet in Fig. 11.1. Tables 11.8 to 11.10 list the process equipment and vessel data. In the simulation, all gas is removed in a component separator prior to the distillation column. This involves the liquid from the separator and the absorber. The gas is sent back and combines with the vapor product from the separator to form the vapor feed to the absorber. Figure 11.2a shows the temperature profile in the azeotropic distillation column.

We have designed and implemented a reactive divided wall distillation column for the production of ethyl acetate from acetic acid and ethanol. Important aspects derived from steady state simulation were considered for instance, a side tank was implemented in order to split the liquid to both sides of the wall and a moving wall inside the column that allows to fix the split of the vapor stream. The dynamic simulations indicate that it is possible to control the composition of the top and bottoms products or two temperatures by manipulating the reflux rate and the heat duty supplied to the reboiler, respectively. The implementation of the reactive divided wall distillation columns takes into account important aspects like process intensification, minimum energy consumption and reduction in Carbon Dioxide emission to the atmosphere. 

The distillation column used in this study is designed to separate a binary mixture of methanol and water, which enters as a feed stream with flow rate F oi and composition Xp between the rectifying and the stripping section, obtaining both a distillate product stream D oi with composition Ad and a bottom product stream 5vo/ with composition Ab. The column consists of 40 bubble cap trays. The overhead vapor is totally condensed in a water cooled condenser (tray 41) which is open at atmospheric pressure. The process inputs that are available for control purposes are the heat input to the boiler Q and the reflux flow rate L oi. Liquid heights in the column bottom and the receiver drum (tray 1) dynamics are not considered for control since flow dynamics are significantly faster than composition dynamics and pressure control is not necessary since the condenser is opened to atmospheric pressure. 

This example is intended to demonstrate the process dynamics methodology as implemented on a single equilibrium stage. A stream of light hydrocarbons is sent to a distillation column where the C3 s and lighter components are separated from the C4 s. Since the feed composition fluctuates substantially, it is sent to a flash drum located upstream of the column in order to attenuate the composition fluctuations and thereby improve the column controllability. The vapor and liquid products from the flash drum are then sent to different trays in the column. 

Many industrial separation processes are based on phase equilibria. By this we mean that the various components of the mixtures present in the (vapor, liquid, solid) phases are in equilibriinn. This is a dynamic equilibriinn and equal mnnbers of components are being transferred continuously from one phase to the other thus the concentrations at equilibriinn do not change. To design the separation processes in industry, e.g., finding the height and number of trays of a distillation column, we need to know the concentrations at equilibrium at any temperature and pressure. 

Figure 21.2 shows the LV configuration for the two-point composition control of a binary distillation column discussed in Example 20.9. After assigning manipulated variables to regulate the vapor and liquid inventories, the boilup rate, V, and the reflux flow rate, L, remain available to control the distillate and bottoms product compositions, and Xg, respectively. To assess the controllability and resiliency of this configuration, the disturbances are taken to be the feed composition, Xp, and the flow rate, F. The column dynamics are approximated by a linear model in transfer function form (Sandelin et al., 1990) ... 

Step 3 Decomposition into component parts. It has been demonstrated that a first-order lag is a reasonable approximation for the dynamics of a distillation column (Skogestad, 1987). Thus, the LSF configuration is decomposed into two component parts, one for each column. Four intermediate variables are identified to model the information transfer between the component parts Xbh B, Tg, and Qch (= Qrl)- Note that both Tg and Qch are needed for the energy balance in the reboiler because partial vaporization occurs. 

In chapter 16 the liquid dynamics for a distillation column were derived. In section 16.8 it was shown that the response of the bottom hold-up to vapor flow changes can be written as ... 

The phenomenon of overshoot or inverse response results from the zero in the above example and will not occur for an overdamped second-order transfer function containing two poles but no zero. These features arise from competing dynamic effects that operate on two different time scales (ti and T2 in Example 6.2). For example, an inverse response can occur in a distillation column when the steam pressure to the reboiler is suddenly changed. An increase in steam pressure ultimately will decrease the reboiler level (in the absence of level control) by boihng off more of the liquid. However, the initial effect usually is to increase the amount of frothing on the trays immediately above the reboiler, causing a rapid spillover of liquid from these trays into the reboiler below. This initial increase in reboiler liquid level, is later overwhelmed by a decrease due to the increased vapor boil-up. See Buckley et al. (1985) for a detailed analysis of this phenomenon. 

Table 5,14 gives a digital computer FORTRAN program for this three-component batch distillation dynamic simulation. The specific example is a column with 20 trays and relative volatilities of 9, 3, and 1. The vapor flow rate is constant at 100 mol/h. 

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. 

Scheme 3 (Figure 3.17) indirectly adjusts the material balance through the two level loops. If the steam flow is increased, then the sump level controller decreases the bottom flow. As the additional vapors go overhead and condense, the reflux accumulator level control increases the distillate flow a like amount. The separation is held constant by manually setting the reflux flow to maintain a relatively constant energy per unit feed. This scheme is recommended for columns with a small energy per unit feed (VfF < 2). This scheme also offers the fastest dynamics. 

For example, in distillation we generally do not attempt to control a temperature or composition in the base of the column by manipulating reflux. There is typically a liquid hydraulic lag of 6 seconds per tray, so a change in reflux to the top of a 50-tray column does not change the liquid flow at the bottom of the column for about 5 minutes. The dynamic performance of this loop is poor, so we do not pair bottoms composition with reflux no matter what the RGA tells us to do. On the other hand, the vapor boilup affects all sections of the column quite quickly, so it can be paired with a controlled variable at the top of the column with no dynamic problem. 

A very full bag of distillation dynamic simulation techniques has been developed and demonstrated in this chapter. The example considered is a simple binary ideal vapor-liquid equilibrium (VLB) column. As the remaining chapters in this book demonstrate, these techniques can be readily extended to much more complex flowsheets and phase equilibrium. 

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