Catalyst packings, examples

Vanadium phosphoms oxide-based catalysts ate unstable in that they tend to lose phosphoms over time at reaction temperatures. Hot spots in fixed-bed reactors tend to accelerate this loss of phosphoms. This loss of phosphoms also produces a decrease in selectivity (70,136). Many steps have been taken, however, to aHeviate these problems and create an environment where the catalyst can operate at lower temperatures. For example, volatile organophosphoms compounds are fed to the reactor to mitigate the problem of phosphoms loss by the catalyst (137). The phosphoms feed also has the effect of controlling catalyst activity and thus improving catalyst selectivity in the reactor. The catalyst pack in the reactor may be stratified with an inert material (138,139). Stratification has the effect of reducing the extent of reaction pet unit volume and thus reducing the observed catalyst temperature (hot... 

Membrane reactors are defined here based on their membrane function and catalytic activity in a structured way, predominantly following Sanchez and Tsotsis [2]. The acronym used to define the type of membrane reactor applied at the reactor level can be set up as shown in Figure 10.4. The membrane reactor is abbreviated as MR and is placed at the end of the acronym. Because the word membrane suggests that it is permselective, an N is included in the acronym in case it is nonpermselective. When the membrane is inherently catalytically active, or a thin catalytic film is deposited on top of the membrane, a C (catalytic) is included. When catalytic activity is present besides the membrane, additional letters can be included to indicate the appearance of the catalyst, for example, packed bed (PB) or fluidized bed (FB). In the case of an inert and nonpermselective... 

There are two basic types of packed-bed reactor those in which the solid is a reactant, and those in which the solid is a catalyst. Many examples of the first type can be found in the extractive metallurgical industries. 

The pressure loss coefficient C (cq. 2) can be determined for typical packing forms, for example, according to [13], A comparison of different catalyst packings with respect to pressure drop is contained in Fig. 7. 

Unconsolidated porous media are loose packings. Examples include packed colunms, sand, and loose fibrous materials. Conversely, consolidated media have a fixed, interconnected solid phase. Examples include sintered materials, reservoir rocks, and certain catalyst supports. The terms fibrous vs. granular media denote... 

In conventional fixed-bed reactors the fluid phases move on a packed bed of catalyst particles. Since the two phases are not miscible and both need to be in contact with the catalyst, different operation modes are applied to improve the contact between the fluid phases and the catalysts. For example, in a trickle-bed reactor both fluid phases are introduced from the top in a vertically positioned reactor. Unfortunately poor distribution of the fluid phases gives rise to poor performance, local hot spots and sintering of the catalyst. 

Most steam reformers are process furnaces, transferring the required endothermic heat from a firebox to the process inside catalyst-packed tubes. Some steam reformers are auto-thermal , getting their heat from oxidation reactions occurring within the eatalyst bed. Secondary reformers in ammonia plants are an example of auto-thermal steam reformers. 

ILLUSTRATIVE EXAMPLE 18.17 Determine the pressure drop of 60°F air flowing through a 3-inch diameter 10-ft packed bed with 0.24-inch protruded catalyst packing made of 316 stainless steel. The superficial velocity is 4.65 ft/s. The protruded packing has a fraction void volume, effective particle diameter and to surface area per unit packing of 0.89, 0.0078.5 ft, and 3305 ft , respectively. 

An example of the use of extractor-type PlMRs in reactions other than esterification is the gas-phase decomposition of MTBE catalysed by tung-stophosphoric acid. Lee et reported the use of closed-loop recycle membrane reactors by using polycarbonate, polyarylate or cellulose acetate membranes to selectively permeate the formed methanol in a flat membrane reactor configuration, with the catalyst packed in the retentate side and by using helium as sweep gas in the permeate side. The authors also used a tube-and-shell reactor configuration with the catalyst packed in the shell side being the sweep gas fed to the tube side. 

The term porosity refers to the fraction of the medium that contains the voids. When a fluid is passed over the medium, the fraction of the medium (i.e., the pores) that contributes to the flow is referred to as the effective porosity of the media. In a general sense, porous media are classified as either unconsolidated and consolidated and/or as ordered and random. Examples of unconsolidated media are sand, glass beads, catalyst pellets, column packing materials, soil, gravel and packing such as charcoal. 

The photochemical behaviour of 7 OEt is the first example in which the reaction of achiral molecules in an achiral crystal packing does not occur at random but stereospecifically, resulting in a syndiotactic structure. As no external chiral catalyst exists in the reaction, the above result is a unique type of topochemical induction , which is initiated by chance in the formation of the first cyclobutane ring, but followed by syndiotactic cyclobutane formation due to steric repulsions in the crystal cavity. That is, the syndiotactic structure is evolved under moderate control of the reacting crystal lattice. 

Speed-up of mixing is known not only for mixing of miscible liquids, but also for multi-phase systems the mass-transfer efficiency can be improved. As an example, for a gas/liquid micro reactor, a mini packed-bed, values of the mass-transfer coefficient K a were determined to be 5-15 s [2]. This is two orders of magnitude larger than for typical conventional reactors having K a of 0.01-0.08 s . Using the same reactor filled with 50 pm catalyst particles for gas/Hquid/solid reactions, a 100-fold increase in the surface-to-volume ratio compared with the dimensions of laboratory trickle-bed catalyst particles (4-8 mm) is foimd. 

Jensen gives several examples for his present highly integrated chip systems [101], including a gas-phase reactor, a liquid-phase reactor, a catalyst-testing reactor, and a packed-bed multi-phase reactor. In addition, he provides the vision of a multiple micro-reactor test station (see Section 1.5.5.2). 

---