Minimizing Mass-Transfer Resistances

Experiments were carried out with different sizes of catalysts, and it was found that for sizes of 0.25 mm and smaller the HDM and HDS conversions are almost constant, so that this size of particle was used to minimize intraparticle concentration gradients. 

Minimization of mass-transfer resistances between liquid and solid catalyst particles was studied by varying the liquid/amonnt of catalyst ratio (Perego and Paratello, 1999). Experiments were conducted at two space velocities (0.33 and 1.5 h ) and two temperatures (360°C and 420°C). It was found that conversions did not change appreciably at LHSV = 1.5 h when lOOmL of catalyst was used. However, for LHSV = 0.33 h and for the same amount of catalyst, the conversions were not constant. This means that at high flow rates, interphase mass resistance is minimal but at low flow rates it is still present. To increase liquid flow rate, keeping constant the LHSV, it is 

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One approach to minimize mass-transfer resistance in a stagnant mobile phase employs specially designed particles with a bimodal network of pores. The larger pores (>1000 A) facilitate convective transport of the mobile phase inside the particles, whereas the small pores (<500 A) are explored by the sample components by diffusion only and provide the necessary surface area for adequate sorption capacity. 

To study the kinetics of immobilized enzymes a recirculation reactor may be used. This reactor allows one to perform kinetic measurements with defined external mass transfer effects, reached by establishing a high flow rate near the catalyst, minimizing mass transfer resistance. The reactor behaves as a differential gradientless reactor allowing initial-rate kinetic measurements to be made. 

Metal interconnect-supported. Lawrence Berkeley National Laboratory (66), Argonne National Laboratory, and Ceres (67) have pioneered metal-supported cells to minimize mass transfer resistance and the use of (expensive) ceramic materials. In such cells, the electrodes are typically 50 im thick and the electrolyte around 5 tol5 im. While the benefits are obvious, the challenges are to find a materials combination and manufacturing process that avoids corrosion and deformation of the metal and interfacial reactions during manufacturing as well as operation. 

Polyelectrolyte layers typically have a thickness of approximately a nanometer, allowing ultrathin interfaces to be fabricated that have minimal mass-transfer resistance. LBL self-assembly using polyelectrolytes is thus well-suited for biosensor development studies, because, by minimizing mass-transfer resistances, the intrinsic enzyme kinetics can be measured. LBL assemblies of polyelectrolytes have been used to develop nanostructured biosensor interfaces that encapsulate enzymes (Sokolovskaya et al., 2005 Kohli et al., 2006, 2007a Hassler et al., 2007) and functional nanoparticles (Kohli et al., 2005,2007b). 

When the mass transfer resistances are eliminated, the various gas-phase concentrations become equal a/(/, r, z) = j(r, z) = a(r, z). The very small particle size means that heat transfer resistances are minimized so that the catalyst particles are isothermal. The recycle reactor of Figure 4.2 is an excellent means for measuring the intrinsic kinetics of a finely ground catalyst. At high recycle rates, the system behaves as a CSTR. It is sometimes called a gradientless reactor since there are no composition and temperature gradients in the catalyst bed or in a catalyst particle. 

As can be concluded from this short description of the factors influencing the overall reaction rate in liquid-solid or gas-solid reactions, the structure of the stationary phase is of significant importance. In order to minimize the transport limitations, different types of supports were developed, which will be discussed in the next section. In addition, the amount of enzyme (operative ligand on the surface of solid phase) as well as its activity determine the reaction rate of an enzyme-catalyzed process. Thus, in the following sections we shall briefly describe different types of chromatographic supports, suited to provide both the high surface area required for high enzyme capacity and the lowest possible internal and external mass transfer resistances. 

Solid particles are in the range of 0.01 to 1.0 mm (0.0020 to 0.039 in), the minimum size limited by filterability. Small diameters are used to provide as large an interface as possible to minimize the liquid-solid mass-transfer resistance and intraparticle diffusion limitations. Solids concentrations up to 30 percent by volume may be handled however, lower concentrations may be used as well. For example, in hydrogenation of oils with Ni catalyst, the solids content is about 0.5 percent. In the manufacture of hydroxylamine phosphate with Pd-C, the solids content is 0.05 percent. 

Bubble column reactors are quite commonly employed in the petrochemical industries for many oxidation and hydrogenation reactions (1 ). This type of reactor is ideal for reactions occurring in the slow reaction regime in which relatively low energy input is required to minimize the effect of mass transfer resistance. Nevertheless, attention has been drawn to the... 

Chiyoda and UOP jointly developed an improved methanol carbonyl-ation process on the basis of this supported rhodium complex catalyst the process is called the Acetica process. This process for the production of acetic acid has found several industrial applications in Asia. The process description emphasizes the use of a three-phase reactor, a bubble column, or gas-lift reactor. The reactor column contains a liquid, a solid catalyst, and a bubbling gas stream containing CO efficient dissolution of the gas in the liquid is ensured by the design, which minimizes gas-liquid mass transfer resistance. 

A historical perspective on aqueous-organic extraction using membrane contactor technology is available in Refs. [1,6,83]. The mechanism of phase interface immobilization was first explored in Ref. [84], while application of membrane solvent extraction for a commercial process was first explored in Ref. [85]. Two aspects of liquid-liquid contact in membrane contactors that are different from typical gas-liquid contact are (1) the membrane used could be hydrophobic, hydrophdic, or a composite of both and (2) the membrane mass transfer resistance is not always negligible. Ensuring that the right fluid occupies the membrane pores vis-a-vis the affinity of the solute in the two phases can minimize membrane resistance. These aspects have been discussed in detail in Refs. [6,86,87]. 

The reaction takes place on the catalyst housed in three stationary beds in the reactor. The catalyst used for the l-hexene isomerization reaction is a commercial E-302 reforming catalyst, supplied by Engelhard corporation. The bifunctional catalyst is composed of 0.6 wt% Platinum supported on 1/16" right cylindrical gamma-alumina extrudates. To minimize external mass-transfer resistances and to achieve CSTR behavior, the fluid phase containing the reactants is kept mixed by an impeller powered by a 0.75 hp MagneDrive assembly that can provide stirring speeds up to 3,000 rpm. Unconverted reactant, product and the SCF medium exit via a port located at the top of the reactor. 

Figure 9.2 shows the scanning electron microscopic (SEM) image of a cross-section of the membrane on the GE E500Amicroporous polysulfone support. It can be seen that the membrane consisted of two portions. The top portion was a dense active layer, which provided separation, and the bottom portion was a microporous polysulfone support, which provided mechanical strength. This composite structure minimizes the mass transfer resistance while maintaining sufficient mechanical strength. 

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