Continuous-flow separation system

The combination of ionic liquids with supercritical carbon dioxide is an attractive approach, as these solvents present complementary properties (volatility, polarity scale.). Compressed CO2 dissolves quite well in ionic liquid, but ionic liquids do not dissolve in CO2. It decreases the viscosity of ionic liquids, thus facilitating mass transfer during catalysis. The separation of the products in solvent-free form can be effective and the CO2 can be recycled by recompressing it back into the reactor. Continuous flow catalytic systems based on the combination of these two solvents have been reported [19]. This concept is developed in more detail in Section 5.4. 

Ultrafiltration has been used for the separation of dendritic polymeric supports in multi-step syntheses as well as for the separation of dendritic polymer-sup-ported reagents [4, 21]. However, this technique has most frequently been employed for the separation of polymer-supported catalysts (see Section 7.5) [18]. In the latter case, continuous flow UF-systems, so-called membrane reactors, were used for homogeneous catalysis, with catalysts complexed to dendritic ligands [23-27]. A critical issue for dendritic catalysts is the retention of the catalyst by the membrane (Fig. 7.2b, see also Section 7.5). 

Figure 2.1 — Variants of integrated reaction, separation and detection in continuous-flow analytical systems. (1) Reaction/separation. (2) Reaction/detec-tion. (3) Separation/detection. (4) Reaction/separation/detection.

Active flow-through (bio)chemical sensors include a microzone where a (bio)chemical reaction, a separation or both takes place. The active microzone may be located in the flow-cell itself (Figs 2.6.B and 2.6.C) or built into a probe sensor for insertion into a continuous-flow analytical system (Fig. 2.6.A). The external appearance of a sensitive microzone can be as widely different as the type of detector and process concerned. This is discussed in greater detail in the following section. 

In continuous-flow pyrolysis systems, the sample is heated rapidly in a steady flow of carrier gas. The volatile pyrolysis products are diluted by the carrier gas and quickly removed from the reaction zone into the separation column. The main drawback of this method is the comparatively poor reproducibility of the heating pattern. 

These experiments have been performed in a classical fixed bed continues flow reactor system [5]. The catalyst sample (0.2 g) diluted with SiC was charged in a 12.6 mm i.d. tubular stainless-steel reactor, between two layers of SiC. The thermocouple well was placed in the axis of the reactor. The various CsHj - O2 - N2 mixtures were prepared by means of mass flow controllers and the concentrations of products were determined by gas chromatography, using PORAPAK Q columns (2 m 1/8") for preliminary separation and CARBOXEN 1000 column (4m 1/8") for O2, N2, CO, CO2 and an other PORAPAK Q column (2 m 1/8") for CsHg and C3H6. 

There are two separate mechanisms for treating water by electroflocculation batch and continuous flow. Both systems involve a reaction chamber into which is placed a set of electrodes. In a batch system, a single chamber holds all the electrodes and water, as illustrated in Fig. 1. The water is pumped into the reactor, the current for the electrochemical reactions is passed, and the pollutants float to the surface. In this situation, they are best removed by raising the water level, which forces the floe out the top chute. The water rests in the chamber for a predetermined time before it is pumped out. With batch processes requiring time for the water to be pumped in and out, there is a practical volume limit of about 10 kL per batch, beyond which pump in/out times may be too long to be practical. 

An automatic method for the separation and determination of RF vitamin in food samples (chicken liver, tablet, and powder milk) is proposed by Zougagh and Rios [2], The method is based on the online coupling of supercritical fluid extraction (SFE) with a continuous flow-CE system with guided optical fiber fluorometric detection (CE-CE-ED). The whole SFE-CF-CE-FD arrangement allowed the automatic treatment of food samples (cleanup of the sample followed by the extraction of the analytes), and the direct introduction of a small volume of the extracted material to the CE-ED system for the determination of RF vitamins. Fluorescence detection introduced an acceptable sensitivity and contributed to avoidance of interferences by nonfluorescent polar compounds coming from the matrix samples in the extracted material. Electrophoretic responses were linear within the 0.05-1 pg/g range, whereas the detection limits of RE vitamins were in the 0.036-0.042 pg/g range. 

We will first provide a very brief illustration of the governing equations for mass transport and the operating line for a two-phase continuous cocurrent separation system in a conventional chemical engineering context. This will be followed by a brief treatment of the multi-component separation capability of such a system. Cocurrent chromatographic separation in a two-phase system, where both phases are mobile and in cocurrent flow, will be introduced next. The systems of interest are micellar electrokinetic chromatography (MEKC) chromatography with two mobile phases, a gas phase and a liquid phase capillary electrochromatography, with mobile nanoparticles in the mobile liquid phase. Continuous separation of particles from a gas phase to a cocurrent liquid phase in a scrubber will then be illustrated. Finally, cocurrent membrane separators will be introduced. 

Other above-ground continuous flow systems have been designed and operated for SCWO processes. A system developed by ModeU Development Corp. (Modec) uses a tubular reactor and can be operated at temperatures above 500°C. It employs a pressure letdown system in which soHd, Hquids, and gases are separated prior to pressure release. This simplifies valve design and material selection on the Hquid leg. 

Electrodialysis. In electro dialysis (ED), the saline solution is placed between two membranes, one permeable to cations only and the other to anions only. A direct electrical current is passed across this system by means of two electrodes, causiag the cations ia the saline solution to move toward the cathode, and the anions to the anode. As shown ia Figure 15, the anions can only leave one compartment ia their travel to the anode, because a membrane separating them from the anode is permeable to them. Cations are both excluded from one compartment and concentrated ia the compartment toward the cathode. This reduces the salt concentration ia some compartments, and iacreases it ia others. Tens to hundreds of such compartments are stacked together ia practical ED plants, lea ding to the creation of alternating compartments of fresh and salt-concentrated water. ED is a continuous-flow process, where saline feed is continuously fed iato all compartments and the product water and concentrated brine flow out of alternate compartments. 

In some systems, known as continuous-flow analy2ers, the reaction develops as the sample —reagent mixture flows through a conduit held at constant temperature. In such systems, the reaction cuvettes are replaced by optical reading stations called flow cells. In most analy2ers, whether of discrete- or continuous-flow type, deterrnination of electrolyte tests, eg, sodium and potassium levels, is done by a separate unit using the technique of ion-selective electrodes (ISE) rather than optical detection. 

Level 1 Batch versus continuous Level 2 Input—output stmcture of the flow sheet Level 3 Recycle stmcture of the flow sheet Level 4 Separation system specification... 

A similar catalytic dimerization system has been investigated [40] in a continuous flow loop reactor in order to study the stability of the ionic liquid solution. The catalyst used is the organometallic nickel(II) complex (Hcod)Ni(hfacac) (Hcod = cyclooct-4-ene-l-yl and hfacac = l,l,l,5,5,5-hexafluoro-2,4-pentanedionato-0,0 ), and the ionic liquid is an acidic chloroaluminate based on the acidic mixture of 1-butyl-4-methylpyridinium chloride and aluminium chloride. No alkylaluminium is added, but an organic Lewis base is added to buffer the acidity of the medium. The ionic catalyst solution is introduced into the reactor loop at the beginning of the reaction and the loop is filled with the reactants (total volume 160 mL). The feed enters continuously into the loop and the products are continuously separated in a settler. The overall activity is 18,000 (TON). The selectivity to dimers is in the 98 % range and the selectivity to linear octenes is 52 %. 

Table 11.4 lists reactors used for systems with two fluid phases. The gas-liquid case is typical, but most of these reactors can be used for liquid-liquid systems as well. Stirred tanks and packed columns are also used for three-phase systems where the third phase is a catal5hic solid. The equipment listed in Table 11.4 is also used for separation processes, but our interest is on reactions and on steady-state, continuous flow. 

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