Packings monoliths

In this section of the studies, the flow patterns of two-phase flow in the investigated packings (monoliths, Sulzer DX , and Sulzer katapak S ) are discussed. Further, the resulting residence time distribution (RTD), flooding limits, and mass transfer behavior are compared. 

A column is essentially a device that holds a stationary phase in place, allowing the mobile phase to carry an injected sample through and allowing analytes to interact with available surface. As we discussed in Section 3.2, the efficiency is mainly dependent on the column type (packed, monolithic, or capillary) and particle size for packed columns, or through-pore diameter for monolithic columns. 

Carbon aerogels were first synthesized by Pekala in 1989 through pyrolysis of organic aerogels [28,29]. They display three-dimensional random close-packed monolithic structure [30,31] of slightly overlapping carbon spheres (3 to 30 nm)... 

Sphere-packed monolith reactors for catalytic gas/liquid reactions were studied by Bauer et al. [31]. The authors used this concept for a structured trickle bed reactor and observed a significant increase in the reactor performance, which can be explained by an increased specific surface area and a uniform wetting of the particles. This in turn diminishes the risk of local catalyst overheating, which may harm the catalyst stability and the product selechvity. 

Bauer, T, Haase, S., Al-Dahhan, M.H., and Lange, R. (2007), Paper 559d - Monolithic reactor and particle-packed monolithic reactor for three-phase catalytic reactions , in The 2007 Annual Meeting, Salt Lake City, UT, AIChE. 

Use of the peUetted converter, developed and used by General Motors starting in 1975, has declined since 1980. The advantage of the peUetted converter, which consists of a packed bed of small spherical beads about 3 mm in diameter, is that the pellets were less cosdy to manufacture than the monolithic honeycomb. Disadvantages were the peUetted converter had 2 to 3 times more weight and volume, took longer to heat up, and was more susceptible to attrition and loss of catalyst in use. The monolithic honeycomb can be mounted in any orientation, whereas the peUetted converter had to be downflow. AdditionaUy, the pressure drop of the monolithic honeycomb is one-half to one-quarter that of a similar function peUetted converter. 

From isotherm measurements, usually earried out on small quantities of adsorbent, the methane uptake per unit mass of adsorbent is obtained. Sinee storage in a fixed volnme is dependent on the uptake per unit volume of adsorbent and not on the uptake per unit mass of adsorbent, it is neeessary to eonvert the mass uptake to a volume uptake. In this way an estimate of the possible storage capacity of an adsorbent can be made. To do this, the mass uptake has to be multiplied by the density of the adsorbent. Ihis density, for a powdered or granular material, should be the packing (bulk) density of the adsorbent, or the piece density if the adsorbent is in the form of a monolith. Thus a carbon adsorbent which adsorbs 150 mg methane per gram at 3.5 MPa and has a packed density of 0.50 g/ml, would store 75 g methane per liter plus any methane which is in the gas phase in the void or macropore volume. This can be multiplied by 1.5 to convert to the more popular unit, V/V. 

The need for higher bed conductivity has lead to research aimed at producing carbons that combine high packing density and improved conductivity. If a monolithic block of carbon adsorbent can be produced which eliminates void spaces there are several advantages ... 

The three principal catalyst bed configurations are the pellet bed, the monolith, and the metallic wire meshes. An open structure with large openings is needed to fulfill the requirement of a low pressure drop even at the very high space velocities of 200,000 hr-1. On the other hand, packings with small diameters would provide more external surface area to fulfill the requirement for rapid mass transfer from the g .s stream to the solid surface. The compromise between these two ideals results in a rather narrow range of dimensions pellets are from to 1 in. in diameter, monoliths have 6 to 20 channels/in., and metallic meshes have diameters of about 0.004 to 0.03 in. 

The analysis of the transient behavior of the packed bed reactor is fairly recent in the literature 142-145)- There is no published reactor dynamic model for the monolith or the screen bed, which compares well with experimental data. 

There is a general trend toward structured packings and monoliths, particularly in demanding applications such as automotive catalytic converters. In principle, the steady-state performance of such reactors can be modeled using Equations (9.1) and (9.3). However, the parameter estimates in Figures 9.1 and 9.2 and Equations (9.6)-(9.7) were developed for random packings, and even the boundary condition of Equation (9.4) may be inappropriate for monoliths or structured packings. Also, at least for automotive catalytic converters. 

Control of emissions of CO, VOC, and NOj, is high on the agenda. Heterogeneous catalysis plays a key role and in most cases structured reactors, in particular monoliths, outperform packed beds because of (i) low pressure drop, (ii) flexibility in design for fast reactions, that is, thin catalytic layers with large geometric surface area are optimal, and (iii) attrition resistance [17]. For power plants the large flow... 

Static mixing catalysts Operation Monolithic reactors Microreactors Heat exchange reactors Supersonic gas/liquid reactor Jet-impingement reactor Rotating packed-bed reactor... 

The kinetic parameters estimated by the experimental data obtained frmn the honeycomb reactor along with the packed bed flow reactor as listed in Table 1 reveal that all the kinetic parameters estimated from both reactors are similar to each other. This indicates that the honeycomb reactor model developed in the present study can directly employ intrinsic kinetic parameters estimated from the kinetic study over the packed-bed flow reactor. It will significantly reduce the efibrt for predicting the performance of monolith and estimating the parameters for the design of the commercial SCR reactor along with the reaction kinetics. 

Reactors with a packed bed of catalyst are identical to those for gas-liquid reactions filled with inert packing. Trickle-bed reactors are probably the most commonly used reactors with a fixed bed of catalyst. A draft-tube reactor (loop reactor) can contain a catalytic packing (see Fig. 5.4-9) inside the central tube. Stmctured catalysts similar to structural packings in distillation and absorption columns or in static mixers, which are characterized by a low pressure drop, can also be inserted into the draft tube. Recently, a monolithic reactor (Fig. 5.4-11) has been developed, which is an alternative to the trickle-bed reactor. The monolith catalyst has the shape of a block with straight narrow channels on the walls of which catalytic species are deposited. The already extremely low pressure drop by friction is compensated by gravity forces. Consequently, the pressure in the gas phase is constant over the whole height of the reactor. If needed, the gas can be recirculated internally without the necessity of using an external pump. 

Bubble column Loop reactor Packed column Plate column Monolithic reactor... 

If, from tran.sport-reaction considerations, a particle size smaller than 1 mm is chosen, slurry or monolith reactors will be considered. For larger particles, packed-bed reactors are more suitable. 

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