Temperature profiles column distillation

Figures 6.52 and 6.53 illustrate selected modeling results on temperature profiles of distillation columns. These figures are similar to Figures 6.30 to 6.33 for the MP HCR.

Why not put new lyrics to an old tune This is an excellent idea, and many have done this very thing. Rice" started w ith the Smith-Brinkley raethod" used to calculate distillation, absorption, extraction, etc., overhead and bottoms compositions, and developed distillation equations for determining the liquid composition on any tray. This together with bubble point calculations yield a column temperature profile useful for column analysis. 

The method starts with an assumption of the column temperature and flow profiles. The stage equations are then solved to determine the stage component compositions and the results used to revise the temperature profiles for subsequent trial calculations. Efficient convergence procedures have been developed for the Thiele-Geddes method. The so-called theta method , described by Lyster et al. (1959) and Holland (1963), is recommended. The Thiele-Geddes method can be used for the solution of complex distillation problems,... 

Thermodynamics and mass transfer. Operating pressure, number of plates and reflux ratio in the distillation column temperature profile in the column equilibrium conditions in the reactor... 

The composition and temperature profiles in the RDC are shown in Figure 7. The ester product with traces of methanol is the bottom product, whereas a mixture of water and fatty acid is the top product. This mixture is then separated in the additional distillation column and the acid is refluxed back to the RDC. The ester is further purified in a small evaporator and methanol is recycled back to the RDC. 

The mathematical model comprises a set of partial differential equations of convective diffusion and heat conduction as well as the Navier-Stokes equations written for each phase separately. For the description of reactive separation processes (e.g. reactive absorption, reactive distillation), the reaction terms are introduced either as source terms in the convective diffusion and heat conduction equations or in the boundary condition at the channel wall, depending on whether the reaction is homogeneous or heterogeneous. The solution yields local concentration and temperature fields, which are used for calculation of the concentration and temperature profiles along the column. 

Figure 1. Extractive distillation column and its temperature profile...

In the case of temperature-sensitive separations the column temperature profile is constrained. Appropriate methods are stripping, liquid-liquid extraction, adsorption and crystallization, as well as vacuum distillation. 

The initial total flow rate and temperature profiles can make the difference between success and failure of a rigorous method. Usually for distillation columns, the condenser and reboiler temperatures are estimated and a calculation that assumes constant molal overflow Sec. 2.2.2) is used to initialize tbe internal vapor and liquid flow... 

Another phenomenon of highly nonlinear systems is parametric sensitivity. We illustrated this behavior for the temperature profile in the plug-flow reactor. Nonideal distillation systems can also show this sensitivity. For example, in Fig. 6.5 a small change in the feed composition or organic reflux flow can dramatically change the composition ( and t emperature) profile in the column. Instead of a vinyl acetate-rich profile in the top section, a water-rich profile can be present. 

Figure 6.12 illustrates this effect. As more LLK component comes in with the feed stream, more depression occurs in the rectifying section temperature profile. If we control a tray temperature near the top and the amount of LLK in the feed increases, the temperature on this tray will start to go down. We will increase heat input to drive it back to its setpoint value, and this will push more HK component out the top. Therefore holding a constant temperature on a tray near the top of the column w ould result in significant variations in the amount of heavy key component in the distillate product. All of the LLK components must go out the top of the column, and there is nothing we can do about it once these components enter the column. Action must be taken in the upstream column to keep LLK components out of this column. Similar effects occur in the stripping section and near the base when variations occur in the amount of HHK components in the feed. Now temperatures rise as more heavy components enter the column, and we drop more LK component into the bottoms product if we hold a constant temperature on a tray near the base of the column. 

Step 5. The final isobutane product is the distillate from the DIB column, and we want to keep the composition of the nC4 impurity at 2 mol %. Nothing can be done about the propane impurity. Whatever propane is in the fresh feed must leave in the product stream. Because the separation involves two isomers, the temperature profile is flat in the DIB column. Use of an overhead composition analyzer is necessary. 

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.

Figure 11.2 Temperature profiles, (at Azeotropic distillation column (b) reactor.

In the Smuda process the pyrolysis reactor temperature is 350°C and the operating pressure is 4-5 psi. The pyrolysis gases from the pyrolysis vessel are sent directly to a distillation column. The distillation column has a typical temperature profile as follows top 140°C, Sulzer 250Y middle 322°C, Sulzer 350Y and bottom 331°C. 

Figure 14.9. Liquid temperature profile in extractive distillation column.

Composition and temperature profiles are shown in Figures 14.13 and 14.14. In this example we see that the vapor and liquid temperatures are rather different. This result is quite typical of absorption columns indeed, it is possible for temperature differences to be over the order of 20 K. The McCabe-Thiele diagram is shown in Figure 14.15 and the component efficiencies in Figure 14.16. The efficiencies tend to be lower than in the distillation operations considered above. 

Figure 14.22. Calculated temperature profiles in experimental extractive distillation column.

In conclusion, the temperature profile in conventional distillation columns is the result of both phase equilibrium relations and enthalpy balances. In narrow-boiling mixtures, the phase equilibrium effect is generally more pronounced, while in wide-boiling mixtures, the enthalpy balances are more significant. The importance of the distinction between the two effects is twofold. First, different mathematical solution algorithms are better suited for each situation, as will be discussed in Chapter 13. Second, the understanding and prediction of column performance is enhanced when the two effects are recognized. Examples 7.1 and 7.2 illustrate the two cases. 

EXAMPLES To demonstrate the effect of the holdup specifications on the steady state solution of a batch distillation column at total reflux (a column operating at total reflux of type 2 D — 0, B = 0, F = 0), Examples 10-2 and 10-3 are presented in Table 10-6. The temperature profiles given in Table 10-7 were found by solving Examples 10-2 and 10-3 by use of the calcula-tional procedure described above.. 

To initiate the calculational procedure for the determination of the product distribution for specified reflux and distillate rates, a number of plates between the two pinches is selected. (As discussed in a subsequent section, too few plates but not too many plates may be selected.) Next L/V and temperature profiles for the plates between and including the two pinches as well as the distillate and bottoms temperatures are selected. Next the components of the feed are classified according to the above criteria. Since it is supposed that the complete definition of the feed, the reflux and distillate rates, as well as the column pressure and type of condenser are specified, the component-material balances can be solved for the component-flow rates throughout the column. The component-material balances may be simplified by taking advantage of the unique characteristics of the three classes of components, the distributed components, the separated lights, and the separated heavies. 

For all examples column pressure = 400 lb/in2 abs, partial condenser, thermal condition of the feed is boiling-point liquid. Initial temperature profile linear between the pinches (252-282°F) for all examples. For Examples 11-7 and 11-8 the initial temperature profile was taken to be linear between 227 and 242°F for plates 1 through 5 and the temperature of the distillate was taken to be 217°F. Initial vapor rates V — V2 for all j. In Example 11-7 the liquid sidestream is withdrawn from plate 6 (the accumulator is assigned the number 1), and in Example 11-8, it is withdrawn five plates above the feed plate. Use the equilibrium and enthalpy data given in Tables B-2 and B-24 of the Appendix. 

However, in many applications the temperature profile is quite flat (very little temperature change per tray) near the top of the column if the distillate product is of... 

Tolliver and McCune (402, 403) proposed an improved procedure, based on the sensitivity of the column temperature profile to material balance variations. Accordingly, column temperature profiles are evaluated at different distillate-to-feed (J IF) ratios, while either reboiler duty or reflux flow is kept constant. The best temperature control point is where temperature variations are largest and most symmetrical (403). The analysis can be readily performed using steady-state computer simulation. 

Luyben, W. L., "Profile Position Control of Distillation Columns with Sharp Temperature Profiles, AIChE J. 18, 1972, p. 238. 

It can then be stated that the liquid profile along a continuously operated packed distillation column operating under total reflux conditions will follow a residue curve. The exact residue curve will depend on the composition of the liquid at any point in the column. Furthermore, in accordance with the temperature profile along a residue curve, it can be deduced that the hottest temperature in a column occurs in the reboiler, while the coldest temperature is in the condenser. 

At first, the distillation still is charged with methanol - the low boiling reactant - and heated under total reflux until steady-state conditions are achieved. At this moment, acetic acid - the high boiling reactant - is fed above the reaction zone to the second distributor. After 30 min the reflux ratio is turned from infinity to two and the product withdraw at the top of the column begins. During the column operations, the liquid-phase concentration profiles along the column and the temperature profiles are measured. For the determination of the liquid-phase composition, two methods are applied simultaneously. On the one hand, samples... 

Guideline 11. Select measurement points that are sufficiently sensitive. Consider, for example, the indirect control of the product compositions from a distillation column by the regulation of a temperature near the end of the column. In high-purity distillation columns, where the terminal temperature profiles are almost flat, it is preferable to move the temperature measurement point closer to the feed tray. 

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