DRYING OF GRANULATED AMMONIUM SULFATE IN A FLUIDIZED BED

A. D. Alekseev, V. M. Vitogin, Yu. A. Yanovich

(Presented by the Scientific-Methodological Seminar of the Faculty of Chemical Technology)

In connection with the planned significant increase in the volume of production of mineral fertilizers, questions of improving the methods of their production acquire particular importance. Existing technological schemes provide for obtaining ammonium sulfate in crystalline form. The question of further processing the crystalline material and obtaining it in granulated form has not yet found a satisfactory solution.

The aggregation of ammonium sulfate crystals and the formation of their more or less stable bonds occur when a certain amount of water or an aqueous solution of ammonium sulfate is added to the crystalline material. In this case it becomes necessary to remove excess moisture from the granules and to fix the form that they acquire as a result of granulation. Consequently, solving the problem of obtaining strong ammonium sulfate granules must also provide simultaneously for solving the problem of the drying method.

Positive experiments in the use of a fluidized bed for drying various materials [1—5] make it possible to believe that, among the simple drying methods known and tested in practice, drying in a fluidized bed is the most promising.

In order to verify the possibility of practical implementation of the process of drying ammonium sulfate granules in a fluidized bed, a model of a batch apparatus was made, and preliminary studies were carried out to investigate the main regularities of the behavior of the bed when it was acted upon by cold and heated air streams.

The scheme of the experimental installation is shown in Fig. 1. The experimental installation consists of compressor 1, receiver 2, supply pipelines 3, 4 with built-in diaphragms 5, 6, delivery pipeline 7, an electric air heater with temperature-control system 8, and apparatus 9. The supply pipelines are equipped with shutoff devices that ensure their independent connection to the line. The pressure in the receiver was monitored by manometer 10 and maintained at the required level by regulating valve 11.

Later it became necessary to install flow interrupter 15 with drive 16 and calming vessel 17.

The apparatus for forming a fluidized bed and for visual observation of its behavior consists of two parts—a glass column 12 with a diameter of 35 mm and a height of 500 mm, and a distribution box 13. The column was connected to the distribution box by seating it on a lapped conical surface.

Fig. 1. Diagram of the experimental setup.

The system of piezometers and differential manometers made it possible to measure averaged pressure values at all points of interest. The temperature of the heat carrier upstream of the distributor grid, in the bed, and at its outlet was measured with thermometers.

The experiments were carried out with granulated and crystalline ammonium sulfate, as well as with artificially prepared compositions containing different proportions of both components. The starting components used were commercial-grade crystalline ammonium sulfate with an average particle size of 0.5 mm and a bulk density \(\rho_n = 0.84\ \text{g/cm}^3\), and dry granules of the same material with an equivalent diameter of 3.6 mm and a bulk density \(\rho_n = 0.76\ \text{g/cm}^3\).

The first stage of the study was aimed at determining the general pattern of bed behavior under the action of a stationary stream of cold and heated heat carrier and at experimentally determining the range of velocities for the stable existence of the fluidized bed.

In Fig. 2, dashed lines show the fluidization curves for a bed consisting of crystalline ammonium sulfate (\(f = 0\), left part of the graph) and a bed composed of dry granules of the same material (\(f = 0\), right part of the graph).

It was established that, when a loose bed of dry material is blown through by a stationary air stream, there is a range of gas-flow velocities in which a stable fluidized state is observed. For a bed consisting of crystalline material, the velocity of transition to the fluidized state is minimal and is numerically equal to 0.3 m/s, which corresponds to a degree of expansion \(K_o = 1.2\). The fluidization velocity of a bed composed solely of granules is maximal and equal to 1.8 m/s. The fluidization velocity of compositions composed of

of both components, depends on the percentage content of the corresponding component and lies within the indicated limits.

![Figure 2 graph]

Fig. 2. Dependence of the bed resistance on the air velocity and the frequency of dynamic actions. \(f=0\). Stationary flow. \(f=3.35;\ 10;\ 16.7\)—frequencies of dynamic actions, Hz.

In Fig. 3 the dashed lines show the change in the degree of expansion of the bed for the limiting cases of its existence.

![Figure 3 graph]

Fig. 3. Dependence of the bed expansion coefficient on the air velocity and the frequency of dynamic actions. \(h\)—bed height in the operating state, \(h_{\mathrm{n}}\)—bed height corresponding to the bulk weight, \(f=0\). Stationary flow. \(f=3.35;\ 10;\ 16.7\)—frequency of dynamic actions, Hz.

An increase in the temperature of the heat carrier led to a certain shift of the fluidization curves toward higher velocities, but did not introduce a qualitative change in the behavior of the bed. In the case of adding to the mass consisting of dry product a certain amount of wet material, a fluidized bed is also formed, but for

considerably greater expenditures of the fluidizing medium are required for its existence. An increase in velocity is accompanied by the rapid destruction of the moist components of the bed, whose strength is much lower than the strength of the main mass of the material, and by their entrainment from the apparatus.

All attempts to bring a fixed bed of moist granulated material into a fluidized state under the action of a steady stream of hot heat-transfer medium in the temperature range 350–420°K did not lead to the desired results, because when the moist material was introduced into the heated gas stream, an intensive process of moisture evaporation from the surface of the granules began, leading to their rapid hardening and sticking to one another.

By adding small amounts of moist material to the fluidized mass of dry product, it was possible in individual cases to maintain the bed in a normal state. But this was achieved at the cost of a considerable increase in the velocity of the stream and was accompanied by appreciable entrainment of fine components from the bed. Such a bed is highly sensitive to the uniformity of feeding, and small overloads lead to disruption of the regime and cessation of its “boiling.” A further increase in the intensity of the stream’s action on the particles, associated with an increase in its velocity, is undesirable because of excessive entrainment.

In order to increase the effectiveness of the action of the gas stream on the bed while simultaneously reducing the consumption of the fluidizing medium, the character of the motion of the stream itself was changed. In the section between the calorifier and the apparatus, a mechanical interrupter of the rotary type was installed, driven in rotation by a controllable electric motor.

The second stage of the investigations was carried out with the aim of elucidating the character of the influence of unsteady actions on the behavior of a bed of crystalline and granulated ammonium sulfate and of compositions made up of these materials.

It was established that there is a qualitative and quantitative difference in the behavior of the bed when it is acted upon by an unsteady air stream as compared with steady actions. The qualitative difference consists in the fact that, at a certain intensity of dynamic actions sufficient to destroy the least strong bonds between particles, their rearrangement begins. It is accompanied by a decrease in the distance between particles and leads to an overall decrease in the volume of the bed and to its compaction. The degree of compaction of the bed depends on a number of factors, the principal ones being: the initial packing density of the particles in the bed, the intensity of the dynamic action, and the ability of the material to compact. The process of bed compaction is accompanied by a certain increase in the resistance it offers to the moving stream. For this period it is characteristic that, for a bed consisting of particles of constant size, with sufficient duration of the dynamic actions and with a definite intensity of them, there exists a quite definite degree of their compaction. This was established by visual observations and verified by immersing in the bed a lead ball suspended on a flexible thread. It was noted that the relative mobility of particles in the compacted bed is small, and washing out of fine fractions of the material into the upper regions of the bed is not noticeable. For each pulsation frequency there exists such an intensity of dynamic action at which the maximum compaction of the bed is observed. A subsequent gradual increase in the velocity of the fluidizing medium increases the distance between particles; their mobility increases, and the bed acquires the properties of fluidity, analogous to what occurs in the case of the exis—

by exposure to a stationary flow of the fluidizing medium. A feature of nonstationary actions of the gas flow is the possibility of forming a fluidized bed at lower average flow rates of the fluidizing medium. This is illustrated by the plots in Figs. 2 and 3. In the left-hand part of the plots are shown the fluidization curves for a bed consisting of crystalline ammonium sulfate, and in the right-hand part—for a bed of dry granules.

Experiments were carried out on the formation of a fluidized bed from wet granulated ammonium sulfate under nonstationary action by a heat carrier with a temperature of 350–420°K. The material was introduced directly into the heated apparatus, countercurrent to the heat carrier. The fluidized state of the bed was established and maintained stably over a wide range of flow-pulsation frequencies at low flow velocities and in the absence of entrainment. The residence time in the apparatus of material with an average granule size of 3.5 mm, an initial moisture content of 6.0%, and a final moisture content of 0.5%, at the temperatures indicated above, is 3–4 minutes, which indicates the desirability of using a fluidized bed formed by nonstationary actions of the flow of the fluidizing medium for carrying out a whole series of mass-transfer processes.

Preliminary granulation of ammonium sulfate into aggregates 1–3 mm in size, in combination with nonstationary actions of the heat carrier, has a positive effect on drying in a fluidized bed, since it ensures stability of the hydrodynamic regime of the process and practically eliminates entrainment of material with the gases. However, these advantages can be achieved only if sufficiently strong wet granules are fed to the dryer. Weak wet granules of ammonium sulfate are destroyed before they enter the bed, already in the dryer feeder.

In this connection, it is advisable to combine the operations of granulation and drying of ammonium sulfate in one apparatus. For this purpose, we propose carrying out granulation of premoistened ammonium sulfate on a vibrating surface located between the screw feeder and the working chamber of the fluidized-bed dryer, so that the stream of wet granules is delivered by the vibrating surface directly into the ascending stream of hot gases.

Conclusions

1. It has been shown that, in order to bring a bed of wet material (ammonium sulfate), both ungranulated and granulated, into a stable fluidized state, it is necessary to use a nonstationary flow of drying gases.

2. Combining the operations of granulation and drying of ammonium sulfate in one unit ensures the effective course of both processes.

Literature

1. N. I. Syromyatnikov, V. F. Volkov. Processes in a Boiling Bed, Sverdlovsk, Metallurgizdat, p. 109, 1959.

2. Max Leva. Fluidization, Moscow, Gostoptekhizdat, p. 285, 1961.

3. P. G. Romankov, N. B. Rashkovskaya, V. E. Babenko, A. D. Goltsiker. Dryers with a fluidized bed, Chemical Industry, No. 11, 42(822)–47(827), Goskhimizdat, 1962.

4. B. S. Sazhin. New developments in the technology of drying wet materials in a suspended bed, Chemical Industry, No. 11, 50(830)–55(835), Goskhimizdat, 1962.

5. P. G. Romankov, N. B. Rashkovskaya, L. L. Sinelnikova. Drying of certain polymer materials in a fluidized bed and by the aerofountain method, Chemical Industry, No. 11, 41(841)–43(843), Goskhimizdat, 1963.