Distillation columns vapor mixing

After leaving the reactor the reaction mixture is passed to a settling tank where the denser HF is deposited in the lower phase. The organic phase is mixed gently with HF the HF phase contains tar components and traces of benzene. From the HF phase a side stream is refined. This side stream is heated in a preheater, partially vaporized, and separated into two components in a distillation column HF and benzene are distilled over the top while tar components are taken away at bottom. The top product is condensed, cooled, and collected in a settle tank. The bottom product is neutralized using potassium... 

For a puncture, break, or pressure relief valve (PRV) opening from a reactor or distillation column, there may be no clear-cut level distinguishing the liquid and vapor phases. That is, the system is initially mixed. In this case, noncondensable gases, condensable vapors, and liquid plus solids are initially discharged. The value of (Xq is nonzero and less than unity, reflecting the contributions of the gases and vapors. 

Heat Requirement of the Process. Heat is required for vaporization in the extractive distillation column, and for the reconcentration of magnesium nitrate solution. Overall thermal effects caused by the magnesium nitrate cancel out, and the heat demand for the complete process depends on the amount of water being removed, the reflux ratio employed, and the terminal (condenser) conditions in distillation and evaporation. The composition and temperature of the mixed feed to the still influence the relative heat demands of the evaporation and distillation sections. For the concentration of 60 wt% HNO3 to 99.5 wt% HNO3 using a still reflux ratio of 3 1, a still pressure of 760 mm Hg, and an evaporator pressure of 100 mm Hg, the theoretical overall heat requirement is 1,034 kcal/kg HNO3. 

In the process using sulfuric acid (see Figure 1) this acid was, and in many instances still is, added to the weak nitric acid produced by an AOP before the mixed acid was fed to the top of a distillation column. The feed has been preheated in some processes to minimize the vapor load in the distillation column. Enough sulfuric acid was added to the feed so that the vapor leaving the top of the column was at least 98% nitric acid. Live steam was added to the base of the column to provide the heat for the column and the stripping vapor required to... 

In the Mohil-Badger vapor-phase process, fresh and recycled benzene are vaporized and preheated to the desired temperature and fed to a multistage fixed-bed reactor. Ethylene is distributed to the individual stages. Alkylation takes place in tile vapor phase. Separately, file polyethylbenzene stream from the distillation section is mixed with benzene, vaporized and heated, and fed to the transalkylator, where polyethylbenzenes react with benzene to form additional ethylbenzene. The combined reactor effluent is distilled in the benzene column. Benzene is condensed in the overhead for recycle to the reactors. The bottoms from the benzene column are distilled in the ethylbenzene column to recover the ethylbenzene product in the overhead. The bottoms stream from the ethylbenzene column is further distilled in the polyefitylbenzene column to remove a small quantity of residue. The overhead polyethylbenzene stream is recycled to the reactor section for transalkylation to ethylbenzene. 

Another feature of a residue map we would like to illustrate is the representation of systems that form two liquid phases. In Fig, 6.3 we show how mixtures of vinyl acetate and water form two liquid phases with drastically different compositions. We can take advantage of this nonideality to help produce pure acetic acid from a single distillation column. In Fig. 6.4 we show how the net feed to a column can be changed by mixing the original feed with the vinyl acetate rich reflux. The new feed composition contains less acetic acid acid and water and more vinyl acetate. When we look at the residue curves that pertain to the new feed composition, we find that they move over areas with little water. Most of the feed water is rejected with the overhead vapors... 

A thermal cracking unit for waxes consists of a furnace, a primary separation column, a stabilization column and a distillation section. The feedstock is vaporized, mixed with steam to 40 per cent weight, and enters a tubular furnace in which the residence time is a few seconds (2 to 10 s) at 500 to 600°C. Once-tbrougb cbnversion is relatively low (15 to 30 per cent) to avoid side reactions. Operation is at atmospheric pressure or ghtly above. Direct quench, or quench with a heat transfer fluid, generates steam. Primary fiactionation allows the recycling of the unconverted part of the feedstock. 

The flowchart shown here depicts a multi-unit separation process. Three liquid streams are mixed adiabatically the product stream is pumped through a heater to a distillation column, and the overhead product from the column is partially condensed to yield liquid and vapor products. Using the blocks MIX (mix two streams to form a third), PUMP, HEAT, DISTILL, and CNOS, construct a block diagram for the simulation of this process. 

By defining a mixed A -valuc model, programs developed for solving vapor-liquid distillation columns have been successfully modified and used for simulating three-phase distillation (Schuil and Bool, 1985). In this method a mixed /( -value is defined as the ratio of the mole fraction of a component in the vapor to its mole fraction in the mixed liquid phase (Section 2.3.3). The column is solved using the mixed /(-values instead of the usual vapor-liquid /(-values to determine the temperatures, compositions, and flow rates of the vapor and total liquid on all the trays. The liquid phase split is then calculated on the basis of /(-values for each liquid phase to determine the compositions and flow rates of the two liquid phases. 

The best known and most used empirical method is that of O Connell (Figure 12.60), for distillation columns and absorbers. The curves are based on plant data for several bubble-cap columns plus a few pilot-scale units. Efficiency is related to two properties of the feed mixture liquid viscosity and relative volatility a. Higher values of the p a product indicate larger liquid-side mass transfer resistance and hence a lower efficiency. For a vapor feed or a mixed vapor-liquid feed, the correlating viscosity should be that of the feed tray liquid. 

The vaporization plate efficiency is defined in a manner analogous to the vaporization point efficiency. Let /be the fugacity of component i in the perfectly mixed vapor phase which leaves any plate j of a distillation column and /j- be the fugacity of component i in the liquid phase leaving plate j. If/JJ is unequal to fji (where both / and / j- are nonzero, finite and positive), then there exists a positive number such that... 

Since few liquid mixtures are ideal, vapor-liquid equilibrium calculations are somewhat more complicated than for the cases in the previous section, and the phase diagrams for nonideal systems can be more structured than Figs. 10.1-1 to 10.1-6. These complications arise from the (nonlinear) composition dependence of the species activity coefficients. For example, as a result of the composition dependence of yt, the vapor-liquid equilibrium pressure in a fixed-temperature experiment will no longer be a linear function of mole fraction, so that no.nideal solutions exhibit deviations from Raoult s law. However, all the calculational methods discussed in the previous section for ideal mixtures, including distillation column design, can be used for nonideal mix-, tures, as long as the composition dependence of the activity coefficients is taken into account. 

When chromatographic separations (7) are operated in a batch mode, a portion of the mixture to be separated is introduced at the column inlet. A solute-free carrier fluid is then fed continually through the column, the solutes separating into bands or zones. Some industrial operations such as mixed-vapor solvent recovery and sorption of the less volatile hydrocarbons in natural gas or natural gasoline plants are being carried out on pilot plant and semiworks scales. Continuous countercurrent systems designed along the basic principles of distillation columns have been constructed. 

The ammonolysis of phenol (61—65) is a commercial process in Japan. Aristech Chemical Corporation (fomiedy USS Chemical Division of USX Corporation) currendy operates a plant at Haverhill, Ohio to convert phenol to aniline. The plant s design is based on Halcon s process (66). In this process, phenol is vaporized, mixed with fresh and recycled ammonia, and fed to a reactor that contains a proprietary Lewis acid catalyst. The gas leaving the reactor is fed to a distillation column to recover ammonia overhead for recycle. Aniline, water, phenol, and a small quantity of by-product diphenyl amines are recovered from the bottom of the column and sent to the drying column, where water is removed. 

Even the most complex process consists of simple units interconnected through streams. The simple units we encountered in this chapter encountered heat exchangers, pumps, compressors, turbines, throttling valves, vapor/liquid separators, and mixing points. Other common units in chemical plants are distillation columns, absorption towers, and chemical reactors. These are also analyzed by the general tools of the hemodynamics however, they involve mixtures of multiple components, and we will postpone their discussion until we develop methods for the calculation of mixture properties. 

It is standard practice in the modeling of distillation columns to assume that the liquid is incompressible and perfectly mixed on the trays, and that the vapor and liquid have the same temperature on the tray (thermal equilibrium), whereas the phases could be considered nonequilibrium and some vapor-phase efficiency is sometimes adopted in such cases (e.g., Murphree efficiency). Beyond these possible variants, the basic distillation column model can be reduced to total... 

Dl. A distillation column with two feeds is separating ethanol from water. The first feed is 60 wt % ethanol, has a total flow rate of 1000 kg h, and is a mix of liquid and vapor at 81°C. The second... 

D30. A distillation column is separating methanol from water. The column has a total condenser that subcools the reflux so that 1 mole of vapor is condensed in the column for each 3 moles of reflux. Lq/D = 3. a liquid side stream is withdrawn from the second stage below the condenser. This side stream is vaporized to a saturated vapor and then mixed with the feed and input on stage 4. The side withdrawal rate is S = 500 kmol/h. The feed is a saturated vapor that is 48 mol% methanol. Feed rate is F = 1000 kmol/h. A total reboiler is used, which produces a saturated vapor boilup. We desire a distillate 92 mol% methanol and a bottoms 4 mol% methanol. Assume CMO. Equilibrium data are given in Table 2-7. Find ... 

D6. We are separating water from n-butanol in a stripping column. The feed [F = 100 kmol/h, z = 0.65(mole fraction water), a saturated vapor] is mixed with the vapor leaving the top of the column before the combined stream is sent to the total condenser and then to a liquid-liquid setder. The column has a partial reboiler and CMO is valid. The top layer from the liquid-liquid settler (x, = x = 0.573 mole fraction water) is sent as a saturated liquid reflux to the distillation column. The bottom layer (xp = = 0.975 mole fraction water) is the distillate product. 

A small distillation column separating benzene and toluene gives a Murphree vapor efficiency of 0.65 in the rectifying section where 17V = 0.8 and x z = The tray is perfectly mixed and has a liquid... 

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