Adiabatic trickle bed reactors

Trickle-bed reactors, in which gas and liquid flow co-currently downwards through a bed of catalyst particles, are commonly used for hydrogenation of various hydrocarbons. These reactors can develop local exotherms in which undesirably high temperatures occur. In this paper a rule for prevention of these hot spots is developed. Its form has been derived from theory and its two parameters have been estimated from real process data. Its use assures safe operation of adiabatic trickle-bed reactors. ... 

J Hanika, K Sporka, V Ruzicka, J Hrstka. Measurement of axial temperature profiles in an adiabatic trickle bed reactor. Chem Eng J 12 193-197,1976. 

Hanika J., K. Sporka, V. Ruzicka and J.H. Rstka Measurement of Axial Temperature Profiles in an Adiabatic Trickle Bed Reactor . Chem Engn J. 12, 193 (1976). 

Hanika, J., Lange, R., Dynamic aspects of adiabatic trickle-bed reactor control near the boiling point of reaction mixture. Chemical Engineering Science, 1996, 51(11), 3145-3150... 

In general, the recommendation for adiabatic trickle bed reactors is to avoid internals that obstruct flow and to try to prevent localized partial plugging of the bed through good flow distribution. For reactions that require heat transfer radially through a wall such as in tubular reactors, it is recommended that the pilot plant reactor be run at conditions similar to the commercial reactor. Tests at the low mass velocities typical for pilot plants may be too conservative to get an economic design. 

Stanek, B. and J. Hanika. The Effect of Liquid Flow Distribution on Catalytic Hydrogenation of C clohexene in an Adiabatic Trickle-Bed Reactor. Chem. Eng. Sci. 37. (1982)... 

Compare Equation (11.42) with Equation (9.1). The standard model for a two-phase, packed-bed reactor is a PDE that allows for radial dispersion. Most trickle-bed reactors have large diameters and operate adiabatically so that radial gradients do not arise. They are thus governed by ODEs. If a mixing term is required, the axial dispersion model can be used for one or both of the phases. See Equations (11.33) and (11.34). 

Temperature Fluctations The above physical description of a trickle-bed reactor does not include the assumption that the temperature in the "normal" region is uniform in a cross section. In an adiabatic reactor, all fluctations grow in amplitude and size, even the infinitesimal ones. A mottled temperature structure cannot be avoided, and a set of temperature sensors at a given height will not necessarily be consistent. A band of temperature readings will always occur. 

Activation energy, stability in trickle-bed reactors, 76 Activation overpotential, cross-flow monolith fuel cell reactor, 182 Activity balance, deactivation of non-adiabatic packed-bed reactors, 394 Adiabatic reactors stability, 337-58 trickle-bed, safe operation, 61-81 Adsorption equilibrium, countercurrent moving-bed catalytic reactor, 273 Adsorption isotherms, countercurrent moving-bed catalytic reactor, 278,279f... 

Figure 1.1 Examples of reactors with fixed catalyst beds (a) adiabatic packed bed (b) cooled tubular reactor (c) cocurrent trickle bed reactor (d) packed bubble column.

The commercial trickle-bed reactors, such as hydrodesulfurization and hydrocracking reactors, are often operated adiabatically. The temperature rise in such reactors is often controlled by the additions of a quench fluid (normally hydrogen) at one or more positions along the length of the reactor. A schematic of an adiabatic trickle-bed HDS reactor with a single quench is shown in Fig. 4-7. 

It will be worthwhile to compare the efficiency of the pulsed trickle bed reactor with that of a trickle bed reactor in the gas continuous flow mode and with a monolith reactor. Based on its good heat and mass transfer characteristics the suggestion is that for reaction with a relatively small adiabatic temperature rise and a low reaction rate the trickle bed reactor in the gas continuous mode can be used. For high reaction rates and low thermal effects the monolith reactor is promising. In between the two a trickle bed reactor with shell-catalyst establishes itself... 

Most commercial trickle-bed reactors operate adiabatically at high temperatures and high pressures and generally involve hydrogenation, oxidation, desulfurization, hydrocracking, etc. The most important hydrodynamic properties for trickle-bed reactors are i) the liquid holdup that controls the liquid-to-gas reactant... 

Large-scale hydroprocessing trickle-bed reactors normally operate under adiabatic conditions therefore, heat effects caused by the reaction must also be included. Shah [61] showed that in this case the critical Bodenstein number for elimination of axial dispersion effects is a function of a heat parameter as well as a modified Damkohler number. For low Damkohler numbers smaller critical Bodenstein numbers than in isothermal reactors are sufficient to eliminate axial dispersion in adiabatic reactors, whereas the inverse is true for large Damkohler numbers. 

Shah YT, Paraskos JA. Criteria for axial dispersion effects in adiabatic trickle bed hydroprocessing reactors. Chem. Eng. Sci. 1975 30 1169-1176. 

The oil feed is mixed with hydrogen-rich gas and then preheated to the proper reactor inlet temperature. The combined fe (oil+hydrogen) enters the top of the fixed bed reactor, called a trickle-bed reactor. Generally the catalyst bed is divided into several beds, which are separated by quenching zones where cold hydrogen-rich streams are injected in order to control the temperature inside the reactor. ITie catalytic beds are adiabatic and therefore temperature increases along each bed since the reactions are exothermic. 

In most applications of trickle-flow reactors, the conversions generate heat that causes a temperature rise of the reactants, since the industrial reactors are generally operated adiabatically. In the cocurrent mode of operation, both the gas and the liquid rise in temperature as they accumulate heat, so there is a significant temperature profile in the axial direction, with the highest temperature at the exit end. When the total adiabatic temperature rise exceeds the allowable temperature span for the reaction, the total catalyst volume is generally split up between several adiabatic beds, with interbed cooling of the reactants. In the countercurrent mode of operation, heat is transported by gas and liquid in both directions, rather than in one direction only, and this may increase the possibility of obtaining a more desirable temperature profile over the reactor. 

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