Second liquid phase

SURFACE-BASED SOLID-LIQUID SEPARATIONS INVOLVING A SECOND LIQUID PHASE... 

Distribution of the solids into the bulk second liquid phase... 

This article deals enly with reflux drums. Use only the larger vessel volume determined. Do not add two volumes such as reflux plus product. If a second liquid phase is to be settled, additional time is needed. For water in hydrocarbons, an additional 5 minutes is recommended. 

When a small quantity of a second liquid phase is present, a drawoff pot (commonly called a bootleg) is provided to make separation of the heavy liquid (frequently water) easier. The pot diameter is ordinarily determined for heavy phase velocities of 0.5ft/min. Minimum length is 3 ft for level controller connections. Minimum pot diameter for a 4 to 8 foot diameter reflux drum is 16 inches. For... 

Reflux drums usually are horizontal, with a liquid holdup of 5 min half full. A takeoff pot for a second liquid phase, such as water in hydrocarbon systems, is sized for a linear velocity of that phase of 0.5 ft/sec, minimum diameter of 16 in. 

Intelligent engineering can drastically improve process selectivity (see Sharma, 1988, 1990) as illustrated in Chapter 4 of this book. A combination of reaction with an appropriate separation operation is the first option if the reaction is limited by chemical equilibrium. In such combinations one product is removed from the reaction zone continuously, allowing for a higher conversion of raw materials. Extractive reactions involve the addition of a second liquid phase, in which the product is better soluble than the reactants, to the reaction zone. Thus, the product is withdrawn from the reactive phase shifting the reaction mixture to product(s). The same principle can be realized if an additive is introduced into the reaction zone that causes precipitation of the desired product. A combination of reaction with distillation in a single column allows the removal of volatile products from the reaction zone that is then realized in the (fractional) distillation zone. Finally, reaction can be combined with filtration. A typical example of the latter system is the application of catalytic membranes. In all these cases, withdrawal of the product shifts the equilibrium mixture to the product. 

The deliberate imposition of a second liquid phase should allow higher conversions from a thermodynamic standpoint compared to those which can be realized in either of the two... 

The benefit of a carefully selected second liquid phase is particularly great when the desired intermediate product is capable of undergoing further facile, undesirable reactions. In the conversion of HO(CH2)60H to HO(CH2)6Br with aqueous HBr, by using a hydrocarbon solvent the desired product is obtained in high yields and the dibromoproduct formation is reduced or even eliminated. 

The role of a second liquid phase in photochemical sulphoxidation of paraffins with SO2 and O2 mixtures may also be cited, where water is used to extract sulphonic acid to prevent formation of di- and poly-sulphonic acids (Fischer, 1978). 

The adoption of a second liquid phase has also proved useful in the hydroformylation reaction of propylene for which Ruhrchemie and Rhone-Poulenc have used Rh based water... 

An interesting example of the use of a second liquid phase is to facilitate catalyst recycle when the reactor is operated as a slurry reactor. 

A second liquid phase may be deliberately employed in an emulsified form to gain advantages similar to those cited earlier for organic processes. Such two-phase systems, and even two-phase enzymatic reactions, allow both the electrochemistry and organic chemistry to take place in their optimum medium. Further, the aqueous phase allows acidity to be controlled in the organic medium and the organic phase allows the desired intermediate product to be extracted to improve yields. 

The reaction mass consists of two liquid phases and one solid phase no solvent is required. The major liquid phase is the crude amine product itself. The solid phase is promoted sponge nickel catalyst. Surrounding the catalyst is a second liquid phase consisting of concentrated caustic and water. Water and caustic are added continuously to make up for losses leaving in the crude product. The ratios of water, caustic, and catalyst in the reaction mass are controlled to produce high yields of product amine and very low catalyst usages. High catalyst concentrations are employed in the reaction mass to keep the concentration of unreacted nitriles very low the upper limit on the catalyst concentration is the point where the circulation rate is inhibited. 

BDS process. The pore size of the filter (0.2-1.0 xm) is selected such that the liquid phase, which is miscible with the liquid that is used to wet the filter, passes through the filter, while the second liquid phase remains. Thus, an aqueous filter is wet with a liquid, which is miscible with water, but immiscible with oil. The flow rate is chosen so as to prevent solid deposition through the filter. Although, such a separation process can be applied to any oil/water emulsion, it was particularly envisioned as part of a BDS process. One may ask, whether it would be more efficient to break a macroemulsion by filtering than it is by any other means Second, in the case of microemulsions, how efficient would such a filtration process be ... 

Biphasic systems that contain the catalyst in the supercritical phase and the substrates/products in a second liquid phase can also be implemented. With water as the polar phase, these inverted systems are particularly attractive for the conversion of highly polar and/or low-volatile hydrophilic substrates with limited solubility in typical SCFs such as scC02. 

Because of the physical equilibrium, the association in the liquid phase is determinded by that in the vapour phase. Therefore no additional association constants are required for the liquid phase. In the case of liquid-liquid equilibrium calculations, an analogous procedure was adopted using convergence test (5), with y. referring to the second liquid phase. 

This section describes catalytic systems made by a heterogeneous catalyst (e.g., a supported metal, dispersed metals, immobilized organometaUic complexes, supported acid-base catalysts, modified zeolites) that is immobilized in a hydrophilic or ionic liquid catalyst-philic phase, and in the presence of a second liquid phase—immiscible in the first phase—made, for example, by an organic solvent. The rationale for this multiphasic system is usually ease in product separation, since it can be removed with the organic phase, and ease in catalyst recovery and reuse because the latter remains immobilized in the catalyst-philic phase, it can be filtered away, and it does not contaminate the product. These systems often show improved rates as well as selectivities, along with catalyst stabilization. 

The examples described in Sects. 2-4 all use carbon dioxide as the phase to dissolve the substrates and/or products. Under continuous operation, the CO2 phase thus serves as the mobile phase. However, one may also envisage a so-called inverted scenario, where SCCO2 becomes the stationary catalyst phase and a second liquid phase contains substrates and products. This allows the processing of components that are not or only very poorly soluble in SCCO2. Furthermore, as it uses a liquid as continuous phase, energy-demanding compression cycles of the CO2 phase are avoided. A necessary prerequisite is the use of a sufficiently C02-philic catalyst as outlined below. 

The presence of fine solid particles or a finely dispersed second liquid phase in the continuous absorbent phase can have a very strong effect on the mass transfer rate between the gas and the continuous phases. The mass transport into the solid particles or liquid drops can essentially alter the concentration gradient and, consequently, the absorption rate [27-36]. The qualitative explanation of this phenomenon is that the particle absorbs oxygen in the oxygen-rich hydro-dynamic mass transfer film, after which, desorption of oxygen takes place in the oxygen-poor bulk of the liquid. 

There is a definite need, therefore, for systems that combine the advantages of high activity and selectivity of homogeneous catalysts with the facile recovery and recycling characteristic of their heterogeneous counterparts. This can be achieved by employing a different type of heterogeneous system, namely liquid-liquid biphasic catalysis, whereby the catalyst is dissolved in one liquid phase and the reactants and product(s) are in a second liquid phase. The catalyst is recovered and recycled by simple phase separation. Preferably, the catalyst solution remains in the reactor and is reused with a fresh batch of reactants without further treatment or, ideally, it is adapted to continuous operation. 

CoacervatlOil, A phenomenon associated with colloids wherein dispersed particles separate from solution to form a second liquid phase is termed coacervation. Gelatin solutions form coacervates with the addition of salt such as sodium sulfate [7757-82-6], especially at pH below the isoionic point. In addition, gelatin solutions coacervate with solutions of oppositely charged polymers or macromolecules such as acacia. This property is useful for microencapsulation and photographic applications (56—61). 

In the previous section (2.1.2) we were concerned with phase transitions between liquid and vapor and discussed the various techniques for effecting such changes. In this section we will look at transferring solute components from one liquid phase to a second liquid phase. This technique is referred to as liquid-liquid extraction (LLE). The main restriction on this separation technique is that the two phases must be immiscible. By immiscible liquids we mean two liquids which are completely insoluble in each other. A little reflection will reveal it is very difficult to have two liquids that are mutually insoluble. If such a system were achievable, then the total pressure, P, of the system would be defined by. 

Commonly their orientation is horizontal. When a small amount of a second liquid phase (for example, water in an immiscible organic) is present, it is collected in and drawn off a pot at the bottom of the drum. The diameter of the pot is sized on a linear velocity of 0.5ft/sec, is a minimum of 16 in dia in drums of 4-8ftdia, and 24 in. in larger sizes. The minimum vapor space above the high level is 20% of the drum diameter or 10 in (Sigales, 1975). 

---