Fixed-bed extractor

During the extraction an unsteady process prevails. The present paper presents an unsteady state mathematical model for a fixed bed extractor (model 1). The overall mass transfer coefficients were calculated by matching the calculated and experimental values of oil loading in CO2. The results are compared with those obtained by the model developed by Catchpole et al, 1994 (model II). Good agreement between both models results and our experimental measurements were obtained, although the model II allows the best fit over the entire extraction curve. 

If a deep bed u used in a reflux extractor the exlractur can be analyzed tike fixed-bed extractors operating with dtiwnftow and yj0 0. In such a case V = Ee. However, if the bed is Hooded and consequently part of the reflux bypasses the bed. V should be based on an Eh that barely results in bed saturation rather than the actual Efe. If the refluxed solvent b not passed throngh a bed but is edded 10 a well-mixed batch of extract and solid and the rate of extract discharge is E . the extractor can be treated like a diflcrtmriel extractor. Reflux extractors in which very short beds are used can also be treated like differential extractors as a mesonablc approximation. 

In continuous and fixed-bed extractors there is a critical through pot rate beyond which flooding will occur. The viscosity of extract usually iucreases as it moves toward the fresh solids end of extraction systems. Houce, in crossflow extractors nod slalioanry-basket extractors, progressive changes in recirculation rates may have to he used to compensate for such changes in viscosity. 

Improved selectivity in the liquid-phase oligomerization of i-butene by extraction of a primary product (i-octene C8) in a zeolite membrane reactor (acid resin catalyst bed located on the membrane tube side) with respect to a conventional fixed-bed reactor has been reported [35]. The MFI (silicalite) membrane selectively removes the C8 product from the reaction environment, thus reducing the formation of other unwanted byproducts. Another interesting example is the isobutane (iC4) dehydrogenation carried out in an extractor-type zeolite CMR (including a Pt-based fixed-bed catalyst) in which the removal of the hydrogen allows the equilibrium limitations to be overcome [36],... 

In the second step, the dioxanes are vaporized, superheated, and then cracked on a solid catalyst (supported phosphoric acid) in the presence of steam. The endothermic reaction takes place a about 200 to 2S0°C and 0.1 to OJ. 10 Pa absolute. The heat required is supplied by the introduction of superheated steam, or by heating the support of the catalyst, which operates in a moving, fluidized or fixed bed, and, in this case, implies cyclic operation to remove the coke deposits formed. Isoprene selectivity is about SO to 90 mole per cent with once-through conversion of 50 to 60 per cent The 4-4 DMD produces the isoprene. The other dioxanes present are decomposed into isomers of isoprene (piperylene etc.), while the r-butyl alcohol, also present in small amounts, yields isobutene. A separation train, consisting of scrubbers, extractors and distillation columns, serves to recycle the unconverted DMD, isobutene and fonnol, and to produce isoprene to commercial specifications. 

The concept of combining membranes and reactors is being explored in various configurations, which can be classified into three groups, related to the role of the membrane in the process. As shown in Figure 25.12, the membrane can act as (a) an extractor, where the removal of the product(s) increases the reaction conversion by shifting the reaction equilibrium (b) a distributor, where the controlled addition of reactant(s) limits side reactions and (c) an active contactor, where the controlled diffusion of reactants to the catalyst can lead to an engineered catalytic reaction zone. In the first two cases, the membrane is usually catalytically inert and is coupled with a conventional fixed bed of catalyst placed on one of the membrane sides. 

Extraction equipment can be classified by the method used to contact the solid with the solvent. Two general extraction methods are used for extracting plant raw materials dispersed-solids extraction and percolation extraction. During dispersed-solids extraction, enough solvent is used to suspend the solids within the liquid. A stirred tank is the simplest example of a dispersed-solids extraction. In a percolation extractor, solvent flows through a fixed bed of ground raw material. 

Fixed-bed extraction systems m which supercritical gas is used as the solvent employ various means to remove the extracted solute from the gas before it is repassed through the bed. In terms of solute concentration in the solids these extractors act in the same way as extractors in which solvent distillation is used. Depending on the way solutes are removed from the supercritical gas, concentrated extracts may or may not be obtained. 

Y for extract entering fixed-bed or continuous countercurrent extractor (kg/m3)... 

Y for extract leaving fixed bed of solids, Y for extract leaving continuous extractor (kg/m3) the solubility liotii of a solute (kg/m3)... 

SFE from solids is carried out by continuously contacting the solid substrate with the supercritical solvent. In most cases the solid substrate forms a fixed bed. During extraction, the supercritical solvent flows through a fixed bed of solid particles and dissolves the extractable components of the solid. The solvent is fed to the extractor and evenly distributed to the inlet of the fixed bed. The loaded solvent is removed from the extractor and fed to the precipitator. 

The direction of flow of the supercritical solvent through the fixed bed can be upwards or downwards. At high solvent ratios (ratio of flow of supercritical solvent to the amount of solid) the influence of gravity is negligible. The shape of the fixed bed can also be a matter of design consideration. Height to diameter for cylindrical fixed beds, cylindrical shaped layers of solid material, and combination of extractor and precipitator are some possible variations. 

The solid supported extractants can be employed in fixed bed contactors to extract metal ions from solutions. Other geometries include slurry extractors and moving bed adsorbers. We consider a fixed bed geometry. In this case the following mass transfer processes may be present (1) interpellet mass transfer, which refers to the diffusion and mixing of metal ion in fluid occupying the spaces between pellets (2) interphase mass transfer, which is the transfer of metal ion across the fluid peUet interface and (3) intraparticle mass transfer, which is the diffusion of metal ions in... 

---