Fixed bed design

Skeletal catalysts are usually employed in slurry-phase reactors or fixed-bed reactors. Hydrogenation of cottonseed oil, oxidative dehydrogenation of alcohols, and several other reactions are performed in sluny phase, where the catalysts are charged into the liquid and optionally stirred (often by action of the gases involved) to achieve intimate mixing. Fixed-bed designs suit methanol synthesis from syngas and catalysis of the water gas shift reaction, and are usually preferred because they obviate the need to separate product from catalyst and are simple in terms of a continuous process. 

Although both the BDST and two-parameter models fit the experimental data points so well, care should be taken when they are being used in fixed-bed design. Consequently, Chu [106] in contradicting an early work of Ko et al. [38], suggests that these models should not be used to predict breakthrough a priori. The modeling approach presented here is thus for the purpose of illustration only. 

Band-Aging - Especially with fresh catalysts, the reaction occurs over a relatively small zone in a fixed bed. This reaction front marches down the catalyst bed as the coke deposits first deactivate the front part of the bed (Figure 4). Use of a sufficient catalyst volume permits a fixed-bed design in which on-stream periods are long enough to avoid overly frequent regeneration cycles. 

Different types of reactors are utilized for a wide variety of pyrolysis applications, including processing of waste plastics. The worldwide waste plastic pyrolysis systems utilize the fixed-bed designs of vertical shaft reactors and dual fluidized-bed, rotary kiln and multiple hearth reactor systems. The type of reactor used is chiefly based on material to be pyrolyzed and expected products from the pyrolysis. Stainless steel shaking type batch autoclave and stainless steel micro tubular reactors have also been used extensively [14]. Fluidized-bed reactors have been extensively used in producing raw petrochemicals from the pyrolysis of waste plastics [22, 24]. 

Combined ion exchange and liquid-liquid extraction processes produce an oxide which is pure enough to be a direct source of nuclear grade metal. A later development (Porter-Arden) involved transferring resins between loading and elution stations which simulated a more efficient countercurrent operation but was not truly continuous. Fixed bed designs must employ a clarified feed if not to... 

Unlike fixed bed designs where scale up data may be obtained semi-empirically, continuous countercurrent ion exchange plant requires model hydrodynamic data for both the liquid and resin phases as well as predetermined equilibrium and kinetic data for a chosen system. A continuous cycle becomes particularly attractive when required to treat more highly concentrated liquors or operate at high treatment flowrates. 

The structure of a computer program for fixed bed design with the onedimensional model is outlined in ... 

The multi-tube reactor is more common than the other two fixed bed designs because many of the important heterogeneous catalytic processes require effective heat transfer between the mobile fluid, catalyst bed and heat-ing/cooling media. 

In fixed bed designs, the sorbent and catalyst particles are mixed together and formed into a stationary packed bed. The reaction gas mixture is passed through the vessel allowing the sorbent to remove CO2 as the catalyst further reacts with CO and H2O to form H2. At some point, however, the sorbent will become saturated with CO2 and start to contaminate the H2-product gas as shown in Figure 6.11. 

With fixed bed designs, there are some constraints on the system such as making sure the particles do not fluidise, but these are not sufficient to explicitly define the process design. It is only through the integration of the SERF process into the full fuel production and conversion process that the optimal cycle arrangement and operating conditions can be obtained. 

The most important fixed-bed designs are the nonisothermal, nonadiabatic, fixed- (or packed)-bed reactor (NINA-PBR) (also called the multitubular or heat-exchanger-type reactor), and the single or multistage adiabatic fixed-bed reactor (A-PBR), and it is important at the outset to note the difference between the approaches and the design of these two operational categories. 

With very exothermic reactions the number of beds would have to be uneconom-ically large to limit the temperature increase per bed. This problem has been solved by introducing the multi-tube reactor. A schematic illustration of a multi-tube reactor is shown in Fig. 11.3. A representative multi-tube reactor can contain hundreds or thousands of tubes with an inside diameter of a few centimeters [3]. The diameter is limited to this small size to avoid excessive temperature and hot spots. The multi-tube reactor is more common than the other two fixed bed designs because many of the important heterogeneous catalytic processes require effective heat transfer between the mobile fluid, catalyst bed and heating/cooling media. 

For the sake of simplicity many fixed bed designs incorporate the isothermal assumption which is normally valid when the adsorbable component concentration is low, or the heat of adsorption is low, or the thermal wave and the mass transfer zone are well separated at the end of the bed. A relatively simple method is available to test whether the last criterion is valid (Ruthven 1984). 

The fixed bed design was soon replaced by more convenient proeesses with eontinuous circulation of the eatalyst from the reactor to the regenerator and then back to the reactor. The new crackers had the advantage of using smaller vessels with less heat loss. They were also more flexible to operate because the catalyst itself acted as the heat transfer medium. 

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