Transport of catalyst

For a fixed bed reactor it is very important that the pressure drop over the bed is as low as possible. This condition is usually fulfilled by using pellets, extru-dates or spheres with a diameter greater than 3 mm. A fixed bed can have a height of ten meters or more. For this reason the catalyst particles in a fixed bed must have a high mechanical strength, otherwise the particles in the lower part of the bed will break under the weight of the upper half of the catalyst bed. In the riser reactor there is a continuous transport of catalyst pellets. Here it is necessary that... 

Catalyst particles are usually cylindrical in shape because it is convenient and economical to fonii tliem by extmsion—like spaghetti. Otlier shapes may be dictated by tlie need to minimize tlie resistance to transport of reactants and products in tlie pores tlius, tlie goal may be to have a high ratio of external (peripheral) surface area to particle volume and to minimize the average distance from tlie outside surface to tlie particle centre, witliout having particles tliat are so small tliat tlie pressure drop of reactants flowing tlirough tlie reactor will be excessive. 

Shale oil contains large quantities of olefinic hydrocarbons (see Table 8), which cause gumming and constitute an increased hydrogen requirement for upgrading. Properties for cmde shale oil are compared with petroleum cmde in Table 10. High pour points prevent pipeline transportation of the cmde shale oil (see Pipelines). Arsenic and iron can cause catalyst poisoning. 

When a relatively slow catalytic reaction takes place in a stirred solution, the reactants are suppHed to the catalyst from the immediately neighboring solution so readily that virtually no concentration gradients exist. The intrinsic chemical kinetics determines the rate of the reaction. However, when the intrinsic rate of the reaction is very high and/or the transport of the reactant slow, as in a viscous polymer solution, the concentration gradients become significant, and the transport of reactants to the catalyst cannot keep the catalyst suppHed sufficientiy for the rate of the reaction to be that corresponding to the intrinsic chemical kinetics. Assume that the transport of the reactant in solution is described by Fick s law of diffusion with a diffusion coefficient D, and the intrinsic chemical kinetics is of the foUowing form... 

When two reactants in a catalytic process have such different solubiUty properties that they can hardly both be present in a single Hquid phase, the reaction is confined to a Hquid—Hquid interface and is usually slow. However, the rate can be increased by orders of magnitude by appHcation of a phase-transfer catalyst (40,41), and these are used on a large scale in industrial processing (see Catalysts, phase-TRANSFEr). Phase-transfer catalysts function by faciHtating mass transport of reactants between the Hquid phases. Often most of the reaction takes place close to the interface. 

Figure 10 shows that Tj is a unique function of the Thiele modulus. When the modulus ( ) is small (- SdSl), the effectiveness factor is unity, which means that there is no effect of mass transport on the rate of the catalytic reaction. When ( ) is greater than about 1, the effectiveness factor is less than unity and the reaction rate is influenced by mass transport in the pores. When the modulus is large (- 10), the effectiveness factor is inversely proportional to the modulus, and the reaction rate (eq. 19) is proportional to k ( ), which, from the definition of ( ), implies that the rate and the observed reaction rate constant are proportional to (1 /R)(f9This result shows that both the rate constant, ie, a measure of the intrinsic activity of the catalyst, and the effective diffusion coefficient, ie, a measure of the resistance to transport of the reactant offered by the pore stmcture, influence the rate. It is not appropriate to say that the reaction is diffusion controlled it depends on both the diffusion and the chemical kinetics. In contrast, as shown by equation 3, a reaction in solution can be diffusion controlled, depending on D but not on k. 

Intraparticle mass transport resistance can lead to disguises in selectivity. If a series reaction A — B — C takes place in a porous catalyst particle with a small effectiveness factor, the observed conversion to the intermediate B is less than what would be observed in the absence of a significant mass transport influence. This happens because as the resistance to transport of B in the pores increases, B is more likely to be converted to C rather than to be transported from the catalyst interior to the external surface. This result has important consequences in processes such as selective oxidations, in which the desired product is an intermediate and not the total oxidation product CO2. 

Catalyst design is in a primitive stage. There are hardly any examples of tme design of catalysts (42). However, development of improved catalysts has been guided successfully in instances when the central issues were the interplay of mass transport and reaction. An example is catalysts used for hydroprocessing of heavy fossil fuels. 

L oss of Catalyst by Vapor Transport. The direct volatilisation of catalytic metals is generally not a factor in catalytic processes, but catalytic metal can be lost through formation of metal carbonyl oxides, sulfides, and hahdes in environments containing CO, NO, O2 and H2S, and halogens (24). 

FIG. 23-24 Reactors with moving catalysts, a) Transport fluidized type for the Sasol Fischer-Tropsch process, nonregenerating, (h) Esso type of stable fluidized bed reactor/regeuerator for cracldug petroleum oils, (c) UOP reformer with moving bed of platinum catalyst and continuous regeneration of a controlled quantity of catalyst, (d) Flow distribution in a fluidized bed the catalyst rains through the bubbles. 

The distance above the catalyst bed in which the flue gas velocity has stabilized is refened to as the transport disengaging height (TDH). At this distance, there is no further gravitation of catalyst. The center-line of the first-stage cyclone inlets should be at TDH or higher otherwise, excessive catalyst entrainment will cause extreme catalyst losses. 

Large yields of polymer seem to be obtained only when polymerization proceeds on the outer catalyst surface, because the transport of high molecular polyethylene from catalyst pores is impossible (112). The working part of the specific surface of the catalyst can be expected to increase with diminishing strength of links between catalyst particles (112). Therefore, to obtain a highly active catalyst a support with large pore volume should be used (e.g. silica with pore volume >1.5 cm8/g). 

Mtllf.r and Gidaspow 82 have studied transport of 75 pm catalyst particles in a 75 mm diameter vertical pipe. The flow was characterised by a dilute rising core and a dense annular region at the walls which tended to move downwards. The fractional volumetric concentration of the solids was from 0.007 to 0.04 in the core and up to 0.25 in the annular region. 

The Industrial Revolution was made possible by iron in the form of steel, an alloy used for construction and transportation. Other d-block metals, both as the elements and in compounds, are transforming our present. Copper, for instance, is an essential component of some superconductors. Vanadium and platinum are used in the development of catalysts to reduce pollution and in the continuing effort to make hydrogen the fuel of our future. 

Bulk transport of the reactants to the vicinity of a catalyst particle. 

Transport of the reactants into the catalyst particle by diffusion through the pores. 

The selection of reactor type in the traditionally continuous bulk chemicals industry has always been dominated by considering the number and type of phases present, the relative importance of transport processes (both heat and mass transfer) and reaction kinetics plus the reaction network relating to required and undesired reactions and any aspects of catalyst deactivation. The opportunity for economic... 

---