Slurry reactor heat effects

Hydrogenations can be carried out in batch reactors, in continuous slurry reactors, or in fixed-bed reactors. The material of constmetion is usually 316 L stainless steel because of its better corrosion resistance to fatty acids. The hydrogenation reaction is exothermic and provisions must be made for the effective removal or control of the heat a reduction of one IV per g of C g fatty acid releases 7.1 J (1.7 cal), which raises the temperature 1.58°C. This heat of hydrogenation is used to raise the temperature of the fatty acid to the desired reaction temperature and is maintained with cooling water to control the reaction. 

Due to the consumption of reactants and the production or consumption of heat, concentration and temperature profiles can develop in the stagnant zone around and in the particle itself (Fig. 11). In the following paragraphs, criteria are derived to ensure that the effect of these gradients on the observed reaction rate is negligible [4, 27, 28]. In gas/liquid/solid slurry reactors, the mass transfer between the gas and liquid phase has to be considered, too (see Refs 9 and 29). 

If the catalyst deactivates rapidly or whenever good heat transfer properties are essential and where local hot spots cannot be accepted, the slurry reactor should also be considered. Finally, the slurry reactor can be an attractive alternative to multitube gas-solid packed-bed processes, particularly where large heat effects ask for thousands of thin tubes to control the reactor temperature in the packed beds. Also here, the much better heat transfer characteristics of slurries relative to gas-solid packed beds are deciding. 

Applying high energy input reactors such as venturi loop reactors may have an extra bcnificial effect. Hence, slurry reactors which fit onto a table and which still have a capacity of kilotons per year are probably within reach, provided the heat effects involved in the reaction can be handled. The heat transfer in slurry reactors using built-in tubes, has been studied [40-43, 124-127],... 

Shinn (1982) developed a mechanically agitated slurry reactor with induction heaters for coal liquefaction. While the induction heaters required large power input, they allowed the slurry to heat up to 400-450°C in few minutes, thus cutting down the heat-up period. In the use of such reactors, the effects of induction heating on the metal degradation and failure need to be carefully considered. Except for the induction heating system, the rest of the reactor was a conventional slurry reactor. The concept of induction heating is more practical for smaller-size reactors. 

When the catalyst is available in a small amount, a microreactor assembly is often used (Miller, 1987). This is a simple T-type reactor heated by a fluidized sand bath. The mixing is provided by mechanical agitation that shakes the reactor up and down within the fluidized bed. Because of the small amount of slurry, and an effective heat transfer in the fluidized sand bath, the heat-up period in such a reactor is small. The nature of mechanical agitation is, however, energy-efficient. The reactor provides only a small sample for the product analysis, which makes the usefulness of the reactor for detailed kinetic measurements somewhat limited. The reactor has been extensively used for laboratory catalyst screening tests in coal liquefaction. 

Slurry reactors are often used for intrinsic kinetic measurements. In order to alleviate the effects and complications of the initial heat-up period, as well as the induction period, on the kinetic measurements, novel designs have been introduced. Cup-and-cap reactors, falling-basket reactors, rapid-injection reactors, reactors with induction heaters, and microreactors are five such novel designs. Each of these reactors has been found to be successful the first three, however, consider both induction and heat-up periods. The last two reactors alleviate the complications due to the heat-up period only. All of these... 

Typical values for the heat transfer coefficient lie around 104 W mj"2 K-1. Heat transfer effects rarely affect rates measured in slurry reactors. 

THE REACTIONS OF INDUSTRIAL IMPORTANCE WHICH ARE CARRIED OUT IN BUBBLE COLUMNS AND SLURRY REACTORS AND ARE ACCOMPANIED BY LARGE HEAT EFFECTS... 

Our objective here is to study quantitatively how these external physical processes affect the rate. Such processes are designated as external to signify that they are completely separated from, and in series with, the chemical reaction on the catalyst surface. For porous catalysts both reaction and heat and mass transfer occur at the same internal location within the catalyst pellet. The quantitative analysis in this case requires simultaneous treatment of the physical and chemical steps. The effect of these internal physical processes will be considered in Chap, 11. It should be noted that such internal effects significantly affect the global rate only for comparatively large catalyst pellets. Hence they may be important only for fixed-bed catalytic reactors or gas-solid noncatalytic reactors (see Chap. 14), where large solid particles are employed. In contrast, external physical processes may be important for all types of fluid-solid heterogeneous reactions. In this chapter we shall consider first the gas-solid fixed-bed reactor, then the fluidized-bed case, and finally the slurry reactor. 

To determine the real effect of periodic operation on a FTS catalyst, it is important to do the assessment at conditions that can be compared to a normal steady-state assessment of the catalyst. Idealy, these conditions could include operation in a slurry reactor where heat removal is optimiun (isothermal operation) for a minimum of 100 hours before a forced periodic operation is... 

Slurry reactor Effective utilization of catal t Good liquid—solid mass transfer Good heat transfer Moderate gpas—liquid mass transfer Catalyst separation is difficult and a filtration step is required Low conversion and selectivity in continuous mode following backmixing... 

Fluidized beds give relatively higher performance, but within a narrow operating window. Another type of reactors, the slurry reactor, effectively utilizes the catalyst because of their small particle size in the micrometer range. However, catalyst separation is difficult and a filtration step is required to separate fine particles from the product. Moreover, when applied in the continuous mode, backmixing lowers the conversion and usually the selectivity [2]. Conventional continuous tubular reactors are used as falling film or wall reactor with catalyst coated on the wall however, supply/removal of heat and often broad residence time distribution because of large reactor diameters are two main drawbacks commonly encountered with such reactors. 

Though the term "slurry refers to a suspension of fine solid particles in a liquid, the term slurry reactor is often used for a three-phase system, where both gas bubbles and solid particles are suspended in a liquid phase. For a solid/liquid/gas process, slurry reactors have two obvious advantages the possibilities for very large solid/liquid surface areas and for good heat transfer to the reactor wall. Therefore the volumetric capacity of slurry reactors can be relatively large. However, effective separation of the fine catalyst from the liquid phase may offer considerable technical problems. One possibility is an external separation, e.g. with centrifuges or hydrocyclones, and a transport of a concentrated catalyst slurry back into the reactor. More often internal filters are used, usually consisting of porous tubes (sintered stainless steel, or ceramics), that are cleaned every few minutes by a periodic reversal of the flow. 

The slurry reactor has two significant advantages it has the highest volumetric capacity, and the best possibilities for heat transfer. It can often be operated under isothermal conditions (see section 8.3). A consequence is that scaling up is not too difficult. The combined effects of mass transfer and chemical reaction were presented in section S.S.2, eqs. (S.61) and (S.62). These can be used to estimate the quantitative effects for first order surface reactions. For other reaction orders, the same principle can be applied, but the calculations become more complicated. In practice, gas/liquid mass transfer is often the limiting factor, that is when a sufficient amount of finely divided catalyst is used. Therefore effective gas dispersion is essential (see section 4.6.4). 

Fixed- and Ebulliating-Bed Processes Intraparticle Diffusion Limitations in FT Catalysts. In a fixed-bed mode of operation, pressure drop considerations will dictate a minimum particle size, which in general is of the order of one or a few millimetres. Heat removal and minimization of temperature gradients in the bed rely on the effective heat conductivity in the bed, which is favoured by high fluid velocities and large particles. In an ebulliating bed, too, catalyst particles should not be too small lest they be entrained by the fluid as in a slurry reactor. 

As pointed out earlier, the major external resistance is that of mass transfer, and therefore, the effect of external heat transfer can be neglected. Furthermore, internal (intraparticle) transport effects can be neglected in slurry reactors except under some unusual reaction conditions since the size of the catalyst particles is of the order of 100 microns. In trickle-beds, however, both the internal heat and mass transport effects can be important. 

Fig. 2-8. Effect of blanket concentration on breeding ratio and wall power density of two-region slurry reactors, i = 2.25, total reactor power = 100 Mw (heat), pressure vessel =10 ft ID, core diameter =7 ft, poison fraction =0, temperature = 280°C.

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