Holdup catalyst

Another approach to evaluate the performance of a trickle-bed reactor (particularly a pilot-scale reactor) is to incorporate the RTD with intrinsic kinetics. Since the liquid holdup, catalyst wetting, or the degree of axial dispersion can all be obtained from the RTD, this approach is not exclusive of the ones described above. For a first-order reaction, if the residence-time distribution E(t) and the degree of conversion are known, they can both be related by an expression... 

In this version, high-purity ethylene (99.8 per cent volume) and oxygen (99.5 per cent volume), mixed with dilution steam, are introduced at different levels at the base.of a titanium reactor more than 20 m high, containing 10 to-45 perforated trays and holdup catalyst solution. Conversion takes place at 0J to 5. 106 Pa, absolute, at a temperature kept at around 120 to 130°C by the vaporization of a fraction of the reaction medium (especially water), which removes the heat liberated by the oxidation of ethylene. 

High-hqiiid holdup trays designed with catalyst bed extending None specified Yeoman et al., Int. Pat. Appl., WO 9408679... 

Two complementai y reviews of this subject are by Shah et al. AIChE Journal, 28, 353-379 [1982]) and Deckwer (in de Lasa, ed.. Chemical Reactor Design andTechnology, Martinus Nijhoff, 1985, pp. 411-461). Useful comments are made by Doraiswamy and Sharma (Heterogeneous Reactions, Wiley, 1984). Charpentier (in Gianetto and Silveston, eds.. Multiphase Chemical Reactors, Hemisphere, 1986, pp. 104—151) emphasizes parameters of trickle bed and stirred tank reactors. Recommendations based on the literature are made for several design parameters namely, bubble diameter and velocity of rise, gas holdup, interfacial area, mass-transfer coefficients k a and /cl but not /cg, axial liquid-phase dispersion coefficient, and heat-transfer coefficient to the wall. The effect of vessel diameter on these parameters is insignificant when D > 0.15 m (0.49 ft), except for the dispersion coefficient. Application of these correlations is to (1) chlorination of toluene in the presence of FeCl,3 catalyst, (2) absorption of SO9 in aqueous potassium carbonate with arsenite catalyst, and (3) reaction of butene with sulfuric acid to butanol. 

Liquid holdup is made up of a dynamic fraction, 0.03 to 0.25, and a stagnant fraction, 0.01 to 0.05. The high end of the stagnant fraction includes the hquid that partially fills the pores of the catalyst. The effective gas/liquid interface is 20 to 50 percent of the geometric surface of the particles, but it can approach 100 percent at high hquid loads with a consequent increase of reaction rate as the amount of wetted surface changes. 

The retention time of the non-adsorbing methane (ti) is the measure of the column void volume or holdup. Ethylene is adsorbed by the catalyst, hence it does not reach the detector until the available surface is saturated, at which point ethylene breaks through and is detected by the sensor (t2). The adsorbed volume of ethylene is given simply by ... 

Riser termination. Good riser termination devices, such as closed cyclones, minimize the vapor and catalyst holdup time in the reactor vessel. This reduces unnecessary thermal cracking and nonselective catalytic re-cracking of the reactor product. The benefits are a reduction in dry gas and a subsequent improvement in conversion, gasoline octane, and flexibility for processing marginal feeds. 

Ross (R2) measured liquid-phase holdup and residence-time distribution by a tracer-pulse technique. Experiments were carried out for cocurrent flow in model columns of 2- and 4-in. diameter with air and water as fluid media, as well as in pilot-scale and industrial-scale reactors of 2-in. and 6.5-ft diameters used for the catalytic hydrogenation of petroleum fractions. The columns were packed with commercial cylindrical catalyst pellets of -in. diameter and length. The liquid holdup was from 40 to 50% of total bed volume for nominal liquid velocities from 8 to 200 ft/hr in the model reactors, from 26 to 32% of volume for nominal liquid velocities from 6 to 10.5 ft/hr in the pilot unit, and from 20 to 27 % for nominal liquid velocities from 27.9 to 68.6 ft/hr in the industrial unit. In that work, a few sets of results of residence-time distribution experiments are reported in graphical form, as tracer-response curves. 

Each section consists of a reaction plate where the reaction mixture flows, surrounded by two cooling plates containing the UE. The reactants and catalyst are stored separately and put into contact at the opening of the first reaction plate. The pilot holdup is typically 1.5 1. The successive plates of the reactor can be represented as shown in Figure 12.1. Inside the reactive plate (RP), the environment of the reaction mixture is composed of PEEK. The UE flows between two stainless steel plates, the sandwich plate (SP) and the transition plate (TP). 

While having some advantages over riser reactors, downer reactors also suffer fixrm some serious shortcomings, such as a low solids holdup in the bed, difficulty in even distribution of injected residual on the catalysts, and a high sensitivity to the structure of the inlet [11,12]. Therefore, the development of a new coupled CFB reactor that can fully utilize the advantages of the riser and the downer is of interest. 

Implementation of MRI to quantify holdup and wetting in packings of porous packing elements (e.g., catalyst support pellets) must be performed with care. Difficulties in data acquisition and analysis arise because the signal we wish to... 

The success of periodic flow interruption is due to the liquid static holdup within the porous catalyst pellets and the interstices of the catalyst bed. 

Trickle Bed Reactors (2). A trickle bed reactor utilizes a fixed bed over which liquid flows without filling the void spaces between particles. The liquid usually flows downward under the influence of gravity, while the gas flows upward or downward through the void spaces amid the catalyst pellets and the liquid holdup. Generally cocurrent downward flow of liquid and gas is preferred because it facilitates... 

A fluidized-bed reactor consists of three main sections (Figure 23.1) (1) the fluidizing gas entry or distributor section at the bottom, essentially a perforated metal plate that allows entry of the gas through a number of holes (2) the fluidized-bed itself, which, unless the operation is adiabatic, includes heat transfer surface to control T (3) the freeboard section above the bed, essentially empty space to allow disengagement of entrained solid particles from the rising exit gas stream this section may be provided internally (at the top) or externally with cyclones to aid in the gas-solid separation. A reactor model, as discussed here, is concerned primarily with the bed itself, in order to determine, for example, the required holdup of solid particles for a specified rate of production. The solid may be a catalyst or a reactant, but we assume the former for the purpose of the development. 

In Figure 23.7, the bubble, cloud, and emulsion regions are represented by b,c + iv, and e, respectively. The control volume is a thin horizontal strip of height dx through tiie vessel. The overall depth of the bed is Lfl, which is related to the holdup of catalyst, Wcat. The performance equation may be used to determine Wcat for a given conversion /A (and production rate), or the converse. 

Equation 23.4-6 is one form of the performance equation for the bubbling-bed reactor model. It can be transformed to determine the amount of solid (e.g., catalyst) holdup to achieve a specified /A or cA ... 

Here, the dynamic liquid holdup (in m3/m3) refers to the portion of the void (available) bed volume that has been occupied by the liquid. There are also correlations for the static holdup, that is, when the flow rate is zero after wetting. Dynamic liquid holdup is normally between 0.03 and 0.25, whereas the static liquid holdup is between 0.01 and 0.05, and for nonporous catalysts, usually he s < 0.05 (see Section 3.6.3 Perry and Green, 1999). 

In a reactor completely filled with liquid, the wetting efficiency is 100% or, in other words, the external wetting of the catalyst is complete (Burghardt et al., 1995). While it is true that when a fixed bed is completely filled with liquid wetting is complete (wetting efficiency is unity), the opposite is not true in a trickle bed, a portion of the bed voids will be always occupied by the gas phase. Thus, while in a well-operated trickle bed the wetting efficiency could be unity, its total liquid holdup based on the void volume is always lower than the bed voidage, i.e. the bed is never completely filled with liquid. 

The use of the Akita-Yoshida correlation is justified since the catalyst loading is very low (2%), and the effect of the solids on gas holdup is expected to be minimal. The liquid volume is simply... 

---