Mel’niсk V. N., Karachun V.V.

National Technical University of Ukraine «KPI»

THE PROBLEMS OF MASS EXCHANGE AND AERATION

The feature of heterogeneous-catalytic reactors is the presence of solid catalyst. It differs the reactors with fixed, movable and with boiling ball of catalytic. Both reactors can be two-phase or three-phase. Two-phase reactors are divided into gas-phased and liquid-phased. Under such conditions, the process occurs in system gas-solid phase or liquid-solid phase. In three-phase reactors process usually occurs in system gas-liquid-solid phase.

There are two types of heterogeneous-catalytic reactors with fixed ball of catalyst: reactors with watering catalyst and reactors with flooded catalyst.

The work of chemical reactors and the description of methods of their calculation is the sense of numerous textbooks and monographs. However, if to take as a point of departure the classification of chemical reactors, previously given, it is ease to convince that in every without exception textbooks and monographs contemplated only a couple types of chemical reactors. Maim attention is spared to homogeneous and two-phased heterogeneous-catalytic reactors. It is also necessary to mention that, although some authors emphasize the difference between gas-phased and liquid reactors, features of homogeneous and heterogeneous-catalytic liquid-phase reactors are not specially conditioned at present time.

Heterogeneous and three-phase heterogeneous-catalytic reactors are being examined rarely than other. Apparatus of this type in general nomenclature of chemical reactors occur enough frequently. For example, appeal to processes of hydroformylation, hydrodesulfurization, liquid-phase oxidation, liquid-phase hydrogenation, synthesis of polyatomic alcohols, synthesis of isoprene etc. This list can be considerably expanded. But in textbooks and monographs it is inadmissible attention to methods of computation of reactors for implementation of reactions in two-phase systems – liquid-liquid or liquid-gas and in three-phase systems gas-liquid-solid phase.

Let’s analyze specific features of reactors of examined types and correlate them with features of other types of reactors. First of all, it becomes obvious the presence of principal disarrangement between heterogeneous and heterogeneous-catalytic reactors. At same time, three-phase heterogeneous-catalytic reactors in certain degree unit features of heterogeneous and heterogeneous-catalytic reactors with certain advantage of features of the last. So what are the features of heterogeneous process.

At first, in heterogeneous liquid-phase reactor process occurs in liquid-phase, that frequently serves to the necessity of accounting the features of kinetics of reactions of this class.

At second, in heterogeneous reactor one of transport phases is simultaneous the reactionary phase, that limits residence time of this phase in apparatus.

At third, in reactors for implementation of heterogeneous and heterogeneous-catalytic processes it occurs the distinctive aspect of interphase boundary, type of mass-exchange, and thus, its mechanism, although in all cases the heat transfere through the interphase boundary precedes the chemical reaction.

The work of heterogeneous chemical reactor may be characterized in such way. Two phases come into the apparatus: continuous and dispersed, each of them contains one or several reagents. To come into chemical interaction, reagents, which come into apparatus by means of transference phase, are due to move in reactionary phase at first. Products that form in the course of reaction are distributing among phases.

If to analyze different factors that influence on the work of heterogeneous reactor, they can be united in five groups:

- thermodynamic factors. These are constants of chemical and physical balance. Thermodynamic factors describe the direction of reaction, technological parameters of its carrying out, influence on the velocity and selectivity of process;

- kinetic factors. This group includes constants of velocity and energy of activation all general and collateral reactions, that occur in system, and also real and imaginary order of reaction;

- mass-exchange factors. These are mass-transference coefficient of initial and intermediate substances, and final product of the reaction.

- heat-exchange factors. Current group consists of heat-transference coefficient, between phases and heat-transference coefficient between environment and heat-exchange devices. Furthermore, surface area of external heat-exchange can be rated as thus group;

- hydrodynamic factors. Formally characteristics of interfacial area and mixing in solid and dispersed phases are a part of this group. However role of hydrodynamic factors is considerably wider, because hydrodynamic situation in reactor determines finally not only integral velocity of heat- and mass-exchange, but also the integral velocity of chemical reaction.

It can be separated constructional factors into special group, but, as it was marked, they do not play independent role. Constructional features of apparatus are only the influence implement on physical processes in reactor. In the first place, on hydrodynamic factors.

The main stage in producing any product of microbiological synthesis, on which the effectiveness of technology depends a lot, is the stage of cultivation of microorganisms in aerobic or anaerobic conditions. In industry underlying methods of cultivation are used and surface methods are used not so often. Accordingly to conditions and methods of cultivation, all industrial reactors and plants may be divided into three groups: reactors for underlying aerobic cultivation, reactors for underlying anaerobic cultivation, and plants for surface cultivation.

Principle of underlying cultivation of populations of microorganisms in aerobic conditions consists in permanent arrival of air into reactor environment receptacle – air at simultaneous intensive mixing of nutrient environment. Meanwhile processes on micro- and macro-level are being elapsed in reactor. To micro-level are conditionally attributed biochemical and physics-chemical phenomenon, which occur on the level of cells and do depend on morphological and physiological features of microorganisms. To macro-level belong hydrodynamical, heat and other phenomenon, which much determines constructive features of reactors. But such a dividing is relative, because processes that occur in reactor have germaneness. Phenomenons, which have a position on micro- and macro-levels, have a great role at modeling, scaling and creating high-intensive reactors.

Ferment environment is complex many-phase system, which condition depend on nature of microorganisms, their morphological and physiological features, substratum, rate and intensity of aeration, mixing etc. The complicacy of mathematical formulation of occurring in fermenter processes prevents from creating a strict mathematical model. The identification of similar to real models, will make it possible to calculate with using ECM optimal industrial specimens with the smallest expenditure.

At once, scale conversion from laboratorial to industrial fermanter without using intermediate stages became possible only for its certain assemblies today. It is created a great number of simplified and complex structural mathematical models of fermenters.

It exists a great number of monographs and tutorials, devoted to physic-chemical bases of computation of chemical reactors and their mathematical modeling. However the problems of computation of reactors for liquid-phase processes are elucidated in them either fragmentary or totally untouched.

Firstly it concerns heterogeneous reactors for implementation of reactions in two-phase systems liquid-liquid or liquid-gas, and also in three-phase systems gas-liquid-solid catalyst. Thereby, the computation of similar reactors is quite specific and in majority of its cases it essentially differs from computation of apparatus for homogeneous processes.

In spite of increasing role of multy-phase reactors in chemical and petrochemical industries, the extent of treatment remains deficient for a while. It can be explained not only by the general complicacy of task, but by definite disadvantages it methods of studying and description of certain sides of process, such as laws of forming and hydrodynamics of two-phase systems, conditions of mass-exchange between two phases etc. Thus the rate of substantiation and reliability of computation reveals to be different for different variants of processes and constructions of reactors.