Abstract
This paper describes the chemical reactor with a fluidized bed of catalyst, its advantages and disadvantages. The works also includes design elements and technologies of creation of the fluidized bed. The separate section describes the observed similarity of the behavior of the liquid in the fluidized bed. The conclusion describes the main spheres of application of the chemical reactor. During the study, it was concluded that this type of chemical reactor is widespread due to the large number of advantages that largely compensate the disadvantages. In general, it is difficult to name the industry, where the fluidized bed is not used or cannot be applied in the future. At the same time, it is one of the most difficult environments for the implementation of chemical processes, and the list of failed attempts of its use is quite large.
Introduction
Industrial chemical process is economically and environmentally efficient production of the desired product from the feedstock. Chemical engineering process includes several consecutive stages: physical operations, preparing the initial substances for a chemical reaction (e.g., grinding, heating, etc.); the actual chemical transformation; then the reaction products and unreacted reagents are processed, using various methods of separation, purification, etc. In most cases, the chemical stage is the most important part of the process. Therefore, the "heart" of the process is the chemical reactor.
The choice of the type and design of the chemical reactor, calculation, creation of a system of control of its operation are important targets of chemical technology. The design of the reactor does not lend itself to the template, and for the process, many different designs can be offered. In search of the optimal design it is not necessary to stay at the cheapest. The reactor may have a low cost, but additional processing of the resulting products will be quite expensive. Therefore, when designing, it is better to consider the efficiency of the overall process.
Industrial reactors can be very diverse: a simple tank or a container with a stirrer, a hollow, or a column with a nozzle, a blast furnace or a complex apparatus with catalyst, the atomic reactor, and many others. A variety of chemical reactors makes it difficult to completely classify them. Depending on the criterion that forms the basis of classification, the same reactor can be assigned to different classes.
The most used are the following features of classification of chemical reactors: hydrodynamic situation, the conditions of heat transfer, phase composition of the reaction mixture, a method of organization of process, the nature of the process variations in time, structural characteristics.
Reactor with fluidized catalyst bed
Fluidization is the conversion of the layer of granular material under the influence of the rising gas or liquid flow, or other physical and mechanical impacts to the system, solid particles of what are in a suspended state, and they are resembling the properties of a liquid-fluidized bed. Due to external similarity with boiling liquid, it is often called fluid bed (Yates, 4).
Some basic concepts. Types and ways of creating fluid systems.
The simplest fluid system is created by filling a vertical unit by granular material layer, through the bottom of which, uniformly across the section, inert fluidizing agent (gas or liquid) is injected. At low speed W the granular layer is stationary; with increasing the layer, the height begins to increase (the layer extends). When W reaches a critical value at which the force of hydraulic resistance layer upward flow of fluidizing agent equals the weight of solid particles, the layer acquires fluidity and shifts in a fluid state. The corresponding linear velocity of the fluidizing agent is called the speed of onset of fluidization or its first critical speed Wk [for small (size ≤0.1 mm) particles Wk ~d2, for large (≥1 mm) — Wk ~ √(d )where d is the particle diameter](). (Gupta et al.,3)
The latter decreases with increasing of density upstream.
With further increase of W, the hydraulic resistance of the layer remains constant until it collapses or the intensive removal of granular material flow from the apparatus will not start. Corresponding to this state of the layer flow velocity is called the speed of ablation or second critical speed of fluidization (Wуh), exceeding Wk in ten times. If the speed of the fluidizing agent is greater than the speed of ablation of particles of the largest of the lowering material, the layer is completely taken by the flow.
With increasing of W, layer porosity (fraction of volume, taken by fluidizing agent) increases, so the average concentrations of solid particles per unit volume of the layer are reduced. In the case of fluidization of gas, hollow movable heterogeneity - bubbles (heterogeneous layer) appear. During the fluidization with liquid, the layer expands and remains much more homogeneous by local concentrations of particles (homogeneous layer). In the case of fluidization with gas, elevated pressures create the fluidized bed of the intermediate type.
Spouted layer is a variation of fluidized bed. In this case, the gas (fluid) is introduced into the lower part of the granular layer in the jet. Solid particles are picked up and make it to the upper part of the layer. On the periphery of the jet (usually at the walls of the apparatus) dense layer of particles is moving downward, i.e. they are continuously circulating. In gushing layer, only a part of the solid particles is in suspension.
The fluidized bed system can also be created in the following ways:
a granular bed is subjected to the impact of the mechanical vibration;
a granular bed is mechanically stirred, e.g. rotation of the completed device;
solid particles that have ferromagnetic properties are subjected to effects of electromagnetic fields, etc. These and other techniques can be combined with the fluidization gas or liquid.
Here are some general properties of the bed and the liquid.
1) Hydrostatical pressure in the layer by height H is the same as that of the liquid column
2) Shear waves may be experienced under mechanical action on the surface layer that resembles the surface of boiling water.
3) The behavior of foreign bodies in the layer obeys the law of Archimedes. For example, it is possible to say about the occurrence of a fluidized condition, if the bodies having a density less than the average density of the layer, pop up, and those with bigger - drown.
4) The solids flow from the hole in the side wall of the apparatus with fluidized bed that was introduced into the tubing, forming a jet, the initial velocity of a swarm W=√2gH where g is the acceleration of free fall.
5) Adjacent fluid layers behave as communicating vessels. Supporting medium density of solid particles in these layers due to differences in operating speeds in fluidizing gas, it is possible to organize the circulation of material. Trays in the horizontal layer flow as fluid in the channels.
6) The speed of ascent of bubbles in the layer and inviscid fluid at low velocity fluidizing gas is almost the same and is proportional to √d , where d is the equivalent bubble diameter (diameter equivalent to the ball having the same volume as the bubble).
The similarity between the fluid and the layer manifests itself when mixing devices are placed in it. Patterns of macromixing in a fluidized bed of solid particles and liquids are comparable when the bubbling of the gas. However, the analogy with the liquid is observed only when passing sufficient to fluidization gas amount through the granular layer. For example, if the gas is introduced unevenly over the cross section of the layer, there are zones where particles are immobile. Such immobile (stagnant) zones can be formed in various structural elements of the device (to internal heat-exchange devices, etc.). Undesirable side processes that occur in the agglomerates of solid particles etc. can occur in the stagnant zones. If in the course of chemical-technological process, the particles agglomerate, this may lead to the termination of fluidization.
The advantages and disadvantages of fluidized bed.
Depending on the characteristics of chemical-technological process, the same properties of the fluidized bed can be interpreted both as advantages and disadvantages (Oka et al.,14). Thus, the entrainment of fine particles from the layer complicates the implementation of catalytic processes, and when drying it is used for unloading the finished product; under vigorous stirring the field is leveled and the possibility of considerable local overheating is eliminated, i.e. the isothermal layer is reached (which is important, for example, in the processing of thermolabile materials), however, decreases the driving force and increases the heterogeneity of the treatment solids. Abrasion of them in the layer can result, for example, in increase of the consumption of catalysts, substantial costs of the dust cleaning of the exhaust gases; however, when roasting, chlorination or drying, followed by resinification of the surface of solid particles and walls of the apparatus, abrasion plays an important role.
The main advantages of the apparatuses with fluidized bed before applying them in the same chemically-technological processes with the machines with a fixed or moving bed of granular material and apparatus of the type "rotating drum": ease of load and travel of expected material and unloading of finished product; the possibility of placing heat-exchangers, gas distribution or mixing devices inside; the intensity of heat transfer between fluidized bed and surface structural elements; ease of sealing, even at high working pressures, etc. For many chemically-technological processes, unit capacity, including the apparatus with fluidized bed is almost infinite.
Application of reactors with fluidized catalyst
Fluidization in the flow systems is often used for heating and cooling, adsorption, drying, water degassing of polymers, coking, the recovery of Fe2O3 with hydrogen (Nauman,420). Usually, solid particles move downward the gas flow. The approach of flow patterns to the ideal displacement is achieved by means of partitions of the collapse type, arrays of transfer devices, design of fluidized bed in the form of a vertical cascade of series-connected devices.
Many reactor processes (including catalytic) are carried out in a fluidized layer. The most famous are oxidative chlorination of ethylene to dichloroethane; oxidative ammonolysis of propylene to obtain acrylonitrile; synthesis of vinyl acetate interaction of acetic acid with acetylene; the oxidation of naphthalene into phthalic anhydride and SO2 to SO3; various CHLOROSILANES, the interaction of powdered Si and its alloys with HC1, СН3С1, С2Н5С1, and С6Н5С1; the direct chlorination of hydrocarbons and chlorohydrocarbons. Very promising is the chlorination of metal oxides with obtaining of chlorides of Al, Ti, Fe, Si, etc.
With all the variety of reactor designs, they represent devices with freely boiling or partitioned with a failure of the grating layers, that are provided with heat exchange elements; the latter are gas-distributors in the form of perforated plates or nozzles and bubblers. Often, the gas enters the reactor through the side fitting. Gaseous and liquid reagents are simultaneously injected. Ways to improve the contacting phases, as well as the impact on mixing in the reactors is fundamentally the same as for the systems of gas-liquid in a column apparatus. Thanks to the fluidity of the fluidized bed, such catalytic processes of oil secondary refining, cracking and reforming, are carried out in units of the combined reactor - regenerator, which allows to move from preperiodic production to continuous. This combination quickly spread to other reaction and mass-transfer processes (e.g., system reactor / adsorber).
The fluidized bed is also used: to obtain granular products by typing in the layer of atomized solutions or jets of gas, condensing with formation of solid products, such as mineral fertilizers, ice; coating heated parts with protective polymer film; for crystallization from fluids, leaching (fluidizing agent in the leaching solution), dissolution; as a high-temperature coolant, etc.
Conclusion
Fluidized (boiling) layer of catalyst has some properties similar to the properties of boiling liquids: fluidity, “toughness”, the ability to take the shape of its containing vessel bursts on the surface, skipping of the bubbles.
Granular (particle size from 4 to 0.1 mm) and pulverized (particle size less than 0.1 up to 0, 01 mm) catalysts are used for fluidized bed.
Advantages and disadvantages
1) fluidized bed method allows, first of all, to increase the intensity of the catalysts resulting from the use of the inner surface of catalyst grains (d < 1.5 mm), makes it possible to almost completely remove noticifation braking characteristic of the fixed bed.
2) Intensity of the catalyst and its lifetime also increases due to the removal of local overheating and hypothermia contact of the masses, so characteristic of fixed bed especially during sintering of granules in the form of lumps and crusts on the surface layers and heat exchange surfaces.
3) The use of the fluidized bed also attracts with the compactness of the contact devices and ease of automation.
4) The heat transfer area in contact devices during the processing of the concentrated gas is reduced in comparison with stationary layer in about 20 times.
5) Fluidized bed is not clogged with dust and hydraulic resistance during operation remains constant, whereas the hydraulic resistance of the fixed bed even with the fine cleaning of the gas increases during the year in 1,5-2 times.
The use of fine-grained solid catalyst provides easy pneumatic unloading of the contact apparatus.
The development of the reactors is on the path of capacity building. Contact apparatus for the oxidation of sulfur dioxide with the capacity of 50 - 100 t/day in terms of H2SO4 was first established in 60-ies; in the 80 — 200 - 350 t/day, and now a system operates with a capacity of 1000 tons H2SO4 per day.
Thus fluidized bed is different from fixed bed by the speed of the process, the stability of the layer in time, the possibility of replacing and regenerating the catalyst, means for supplying and discharging the heat in the area of catalysis.
The main disadvantages of fluidized bed are: the large number of dust-collecting means, catalyst attrition due to impact and friction of the grains against each other on the walls of the reactor and the heat exchanger elements that are placed in a layer, the presence of erosion of the walls of the device, which strongly depends on the ability of the abrasive grains (Kunii et al., 11).
For example, grain oxide-iron catalysts wear out faster than grains of silica gel. This is obviously connected with the well-known abrasiveness of iron oxide.
Works Cited
Gupta, C. K, and D Sathiyamoorthy. Fluid Bed Technology In Materials Processing. Boca Raton, Fla.: CRC Press, 1999. Print.
Kunii, Daizō, and Octave Levenspiel. Fluidization Engineering. New York: Wiley, 1969. Print.
Nauman, E. B. Chemical Reactor Design, Optimization, And Scaleup. New York: McGraw-Hill, 2002. Print.
Oka, S, and E. J Anthony. Fluidized Bed Combustion. New York: M. Dekker, 2004. Print.
Yates, J. G. Fundamentals Of Fluidized-Bed Chemical Processes. London: Butterworths, 1983. Print.
