Liquid Split

Figure 52 also shows that the actual recovery curve does not decrease below a certain level. This indicates that a certain amount of material is always recovered to the underflow and bypasses classification. If a comparison is made between the minimum recovery level of solids to the liquid that is recovered, they are found to be equal. Therefore it is assumed that a percent of all size fractions reports directly to the underflow as bypassed solids in equal proportion to the liquid split. Then each size fraction of the actual recovery curve is adjusted by an amount equal to the liquid recovery to produce the "corrected recovery" curve shown in Figure 52. As the Djoc point changes from one application to another, the recovery curves shift, along the horizontal axis. In order to determine a single graph which represents the corrected recovery curve, the particle size of each size fraction is divided by the Dj value and a "reduced recovery" curve can be plotted, as shown in Figure 53. Studies reported by Arterburn have shown that this curve remains constant over a wide range of cyclone diameters and operating conditions when applied to a slurry...

The first step is the split of the initial mixture in essentially monophase submixtures, as gas, liquid and solid. This operation, called the first separation step, can employ simple flash or a sequence of flashes, adsorption/desorption and reboiled stripping, or the combination of the above techniques. Next, the process synthesis activity can be further handled by specialized managers, namely gas split manager (GSM), liquid split manager (LSM) and solid split manager (SSM). 

The liquid split manager deals with two selectors zeotropic and azeotropic mixtures. A second decomposition may take place as a function of mixture composition and sensitivity to temperature. The first criterion generates dilute and bulk separations, while the second one leads to temperature-sensitive and temperature-... 

The phase diagram for the partially miscible binaries water/2-ethylhexanol and water/lauric acid can be described satisfactorily by UNIQUAC with binary interaction parameters from LLE data plus the azeotropic point This procedure allows accurate prediction of the liquid-liquid split, while preserving sufficient accuracy for VLE. The interaction parameters are given in Tables 8.5 and 8.6. 

The reactor/separator/recycle structure is decided by considering the physical properties of the species found in the reactor effluent (Table 9.1). The catalyst and the organic phase are immiscible. Therefore, they can be separated by liquid-liquid splitting. The separation of the organic components by distillation seems easy. In a direct sequence, the inert and any light byproduct will be removed in the first column. The second column will separate the reactants, which have adjacent volatilities. Therefore, there will be only one recycle for both reactants. The third column will separate the product from the heavies. The reactor/separation/ recycle structure of the flowsheet is presented in Figure 9.2. 

The reactor-outlet stream contains a dispersion of hydrocarbons in sulfuric acid. The first separation step is therefore a liquid-liquid split The sulfuric-acid phase contains some amounts of sec-butyl acid sulfate, which decomposes at higher temperature (15 °C) to produce conjuct polymers dissolved in the acid and a mixture of C4-C1( isoparaffins with low octane number (pseudoalkylate) that separates as a second liquid phase. The hydrocarbon phase contains a small amount of di-isoalkyl sulfates. These need to be removed before entering the distillation units otherwise they will decompose and release sulfuric acid. The sulfates are removed by washing with either dilute caustic or sulfuric acid. In the first case, sulfates are converted to salts that are discarded. With sulfuric acid, sulfates are converted to isoalkyl acid sulfates that can be recycled to the alkylation reactor [15, 10]. 

The considerations developed so far allows setting up the final conceptual flowsheet, as displayed in Figure 11.9. After reaction and quench the off-gas is submitted to a first separation of acrylonitrile by low-temperature cooling, at 10 °C. In the decanter the liquid splits into two phases. If the acetonitrile concentration is negligible, the organic phase containing acrylonitrile can be sent directly to the first purification column (Heads). The aqueous phase is sent to the acrylonitrile recovery. The off-gas from flash is compressed at 4.5 bar and submitted to absorption in cold water of 5 °C. In this way higher acrylonitrile recovery may be achieved (over 99.8%) with reduced water consumption. 

Azeotropes can also be homogeneous (single liquid phase) or heteroge-neous (multiple liquid phases). In a heterogeneous azeotrope the repulsive forces between different molecules in the liquid phase are strong enough to overcome the entropy increase due to mixing such that the liquid splits into two or more separate liquid phases. At equilibrium the chemical potential for each component is still the same in all phases ... 

In a l-l. three-necked flask equipped with a dropping funnel, a thermometer, and an efiicient fractionation column fitted with either a vapor- or liquid-splitting head (Note 1) is placed 400 ml. of mineral oil. The oil is heated to 240-270 and dicyclopen-tadiene (Note 2) is added at the rate of 5-10 ml. per minute. The reflux ratio and the rate of addition of dicyclopentadiene are adjusted to maintain the distillation head temperature at 40°. The cyclopentadiene is collected in a Dry Ice-acetone receiver (Note 3). 

The results for the more asymmetric propane and methanol (Galivel-Solastiuk, Laugier, and Richon 1986) mixture at 313 K is shown in Figure 3.4.4. In this case the correlation with the IPVDW model is poor, giving a false liquid-liquid split and underpredicting the pressure over the whole concentration range. Similar results are obtained at other temperatures for this system with this mixing rule. 

Difficulties are also encountered when water and alcohol mixtures are considered. The correlation of the 2-propanol and water binary system at 353 K is shown in Figure 3,4.5. Here we see that at a temperature of 35 3 K, and also at lower temperatures, the IPVDW mixing rule gives a false liquid split and poorly represents the VLE data. At higher temperatures, the results for this system improve somewhat, as shown in Figure 3.4.6 for 523 K, but the correlation is still not acceptable for industrial design. 

Flash2 - rigorous vapor-liquid split or vapor liquid liquid split FlashS - rigorous vapor-liquid-liquid split Decanter - separate two liquid phases Sep - use split fractions... 

This phenomenon, where the liquid splits into two phases, could occur on a number of trays or in the entire column. From a practical standpoint and for better control of the column operation, one of the liquid phases is usually withdrawn when it forms. 

Although the general model focuses on single-pass trays, multi-pass trays are also used. In multi-pass trays the liquid splits and flows in opposite directions so that any liquid element travels only a certain fraction of the tray width. Liquid flows down from one tray to the tray below through more than one downcomer, except on alternating trays in two-pass trays where liquid flows down one central downcomer every other tray. Figure 14.3 illustrates tray arrangements and liquid paths in two-, three-, four-, and five-pass trays. 

Activity coefficients are generally predicted by one of the Wilson, UNIQUAC, NRTL, or van Laar methods. The Wilson and UNIQUAC methods are presented briefly here. Most chemical engineering thermodynamics textbooks have a section on phase equilibria that can provide more detailed descriptions. The Wilson equation [1] is only used with miscible fluids. For highly non-ideal fluids and for systems in which liquid-liquid splitting occurs, the NRTL method is applicable [2], When no experimental data are available, the UNIQUAC method can be used [3,4]. 

---