IF~ROVING EVAPORATORS FOR CRYSTALLIZING SOLUTIONS Vo N. Korobanov, V. N. Kibitkin, E. M. Mitkevich, N. I. Gaidash, and V. N. Nikolenko

UDC 66.048.54

In the production of calcium chloride from still waste in soda plants containing (wt. %): 9-12 CaCI= and the crystallizing admixtures: 5-6Nat1, 0.1-0.13 CaSO~, 0.1-0.15 Ca(OH)2~ new evaporators with forced circulation and a heat-exchange surface of 630 m 2 are at present being introduced [i]. The evaporator (Fig. i) is made with outlying boiling zone and outlying heating chamber. The separator with 3200-mm diameter has a tangential inlet of the boiling pipe; the height of the steam space up to the built-in centrifugal spray separator is about 3 m. In the solution part of the separator there is a baffle installed intended for separating skins of incrustations and large fractions of the crystalline product falling off the walls from the circulating flow. In the lower cone of the separator a perforated pipe is mounted to prevent skins of incrustations falling into the unloading unit. The heating chamber of the evaporator is made of titanium alloy, the separator and the circulation circuit are made of carbon steel, The circulation pump has a discharge of 4300 m3/h, a head of 2.7 m, the electric motor is for ii0 kW. Parts of the pump coming into contact with the solution are made of corrosionresistant steel. Evaporators with a heat-exchange surface of 630 m = are built into two-body evaporator installations; in three-body installations these apparatuses are used as the third body. Operating Regimes of Apparatuses under Conditions of Industrial Operation Two-body

Installation Body Pressure, kPa: of the heating steam in the separator Boiling point of solution in separator, ~ Concentration of calcium chloride in solution, wt. %

First

Second

Third

270-390 100-140

100-140 18-35

110-140 27-42

111-122

84-95

93-99

22-25

35-43

35-43

1:7-1:13

1:7-1:13

1330-1410

1330-1410

Ratio of solid to liquid phase in unloaded suspension Density of solution in the apparatus, kg/m 3

Three-body

1220-1250

The first bodies of two-body installations operate under conditions leading to the deposition of scale of calcium sulfate (anhydrite) on the inner surfaces of the apparatuses: separators, pipes of heating chambers, circulation pipes. The second bodies of two-body installations and the third bodies of three-body installations operate under conditions of crystallization of sodium chloride from the solution of calcium chloride. In these a p p a ratuses scale does not form but on the inner surface of the separator in the boiling zone incrustations of sodium chloride form. Evaporation of crystallizing and scale-forming solutions is accompanied, in addition to scale formation in the pipes of the heating chamber, by clogging of the heating pipes with shells of scale and incrustations which form upon destruction of the salt incrustations and scale on the inner surfaces of the separator and of the circulation pipes and are transported by the circulating flow to the heating pipes. In pipes partly clogged with scale~ the normal Translated from Khimicheskoe i Neftyanoe Mashinostroenie, No. 12 D pp. 10-12, December, 1984.

0009-2355/84/1112-0581508.50

9 1985 Plenum Publishing Corporation

581

I

Fig. I

b, g/kg

~', WI(m2-~ 20DIJ

I000~ ' ~

7

2 i ~.mm

Fig. 2

40

/ / 45

50 ~ h,% Fig. 3

Fig. i. Evaporator: i) spray separator; 2) separator; 3) baffle; 4) perforated pipe; 5) heating chamber; 6) circulation pump. Fig. 2. Dependence of the heat-transfer coefficient K of the apparatus of the first body on the thickness 6 of the scale on the walls of the heating pipes. Fig. 3. Dependence of the entrainment b of calcium chloride by secondary steam on the level h of the solution in the vacuum apparatus (according to the scale of the level gauge). circulation of the evaporated solution is prevented, the suspended crystalsare deposited, and after some time this leads to complete blockage of the pipes. It follows from Table i, which presents the results of the investigation of the first bodies of two-body installations, that the formation of patches of scale is connected with the general dynamics of scale formation, and the clogging of the heating pipes with scale begins after four months of operation when the layer of scale in the evaporator is more than I mm thick. In apparatuses operating in the regime of the last bodies of two-body or three-body installations, we find different intensity of the process of clogging of the heating pipes by salt scale, depending on the output and the length of operation between cleaning of the apparatus. An apparatus of the third body, whose output in evaporated water is 12-20 tons/h and length of operation between cleanings is 15-20 days, has about 5% clogged heating pipes after operating for more than one year. The heat-transfer coefficient is 1560 • ii0 W/(m 2. ~ In the second body of two-body installations, which have an output in evaporated water

582

TABLE i Length o f operation o f evaporator, months

Thickness of scale o n walls o f pipes of heating c h a m b e r , m m 0,3 1,2

5,5 8

1,7 ~3,0

N u m b e r o f blocked pipes o f heating chamber, %

None None, S o m e pipes contain patches o f I scale next to w h i c h sludge begins to be deposited ~ 20 ~4fl

of 19-26 tons/h and length of operation between cleanings is more than 20 days, there are about 30-40% clogged pipes after six months of operation. The heat-transfer coefficient after four months of operation was 980 • 160 W/(m2,~ Improvements in the design of evaporators, e.g., the installation of various grids in the circulation pipes to prevent transport of scale to the heating pipes [2], the establishment of settlement zones with baffles in the apparatus for trapping large pieces of scale [I~ 3], make it possible to reduce the clogging of the pipes of the heating, chambers with pieces of scale and of salt incrustations, but they do not solve the problem of completely eliminating the formation of scale and its clogging the pipes. In our opinion, this problem can be solved on the basis of investigations of the processes of crystallization of scale-forming and encrusting admixtures under conditions of evaporation, the devising of technological methods ensuring reduced formation of scale and incrustations and their complete removal by flushing from all surfaces of the evaporator, determination of the length of operation of the evaporators during which scale and incrustations are not yet broken up into pieces, and working out a graph of cleaning of these apparatuses with a view to these data. The data of Table i indicate that the formation of scale and clogging of the pipes begin after four months of operation when scale more than i mm thick forms. It follows from Fig. 2 that the heat-transfer coefficient of an apparatus of the first body of a two-body installation is reduced to 50% when the scale is i mm thick. It is consequently advisable to clean the apparatas thoroughly and remove scale every four months of operation of the apparatus. Vacuum evaporatorsare distinguished by considerable entrainment of the evaporated solution by the secondary steam. The data (Fig. 3) indicate that the entrainment depends to a considerable extent on the level of the solution in the vacuum apparatus (the level was measured with a differential manometer with a pressure gradient of up to 40 kPa with an output of the apparatus of 18-24 tons/h and a pressure of 18-22 kPa; entrainment was determined according to the increase in concentration of calcium chloride in water behind the mixing condenser). When the level of the solution drops from 60 to 43% on the scale of the level gauge (i.e., by about 0.5 m), the entrainment of calcium chloride decreases from 6 to I g/kg (the level of 43% is close to the minimum level of the solution in the apparatus determined from the increased load on the motor of the circulation pump). The entrainment of solution from the first evaporators of two-body installations also depends to a considerable extent on the level of the evaporated solution: When the level drops from 70 to 60% according to the scale of the level gauge, the entrainment of calcium chloride decreases from 110-120 to 13-18 mg/kg condensate of secondary steam. The output in evaporated water then amounted to 15-20.5 tons/h, the pressure of the secondary steam was 120-130 kPao The difference in entrainment of calcium chloride in the first and second bodies is due to the different speeds of the secondary steam (i and 4.8-6.9 m/sec, respectively). In perfecting the apparatuses with the object of reducing entrainment, it is necessary, first of all, to increase the height of the separator, and also its diameter. It is advisable to provide the evaporators (especially those operating under vacuum) with outlying scale separators because when crystallizing solutions are evaporated, it is possible that the operation of transport pipes and unloading units is impaired, and this leads to a change in level of the solution in the evaporator and to increased entrainment of scale. On the basis of generalized experience with the operationj an evaporator has now been designed with forced circulation, with a heat-exchange surface of 630 m 2, with a separator of improved design, and an outlying scale separator. It is envisaged to make the heating chamber and some units of the circulation circuit of corrosion-resistant materials.

583

LITERATURE CITED I.

.

3. 4.

.

~. M. Mitkevich, V. N. Korobanov, N. I. Gaidash, et al., "Improving evaporators and crystallizers in the production of CaCI2," Khim. Neft. Mashinostr., No. 2, 7-8, TsINTIkhimneftemash, Moscow (1976). V. I. Leverash, S. I. Golub, and V. Ts. Gonionskii, Inventors' Certificate 719648 (USSR); "Evaporator for crystallizing and scale-forming solutions," Byull. Izobret., No. 9 (1980). A. V. B~mfert, Industrial Crystallization [in Russian], Khimiya, Moscow (1969). E. M. Mitkevich, V. N. Korobanov, N. I. Gaidash, et al., "Improving the process of evaporating calcium chloride solutions with crystallizing admixtures," Khim. Prom., No. i0, 47-48 (1980). V. A. Postnikov, G. P. Baranov, V. A. Misharin, et al., "Nonequilibrium superheating of solution in circulation vacuum crystallizers," Khim. Prom., No. ii, 859-861 (1974).

INVESTIGATION OF THE SCALE SEPARATORS OF GENERAL-PURPOSE EVAPORATORS V. F. Sushchenko and E. M. Kovalev

UDC 66.048.54 66,074.1

When the separator and the heating chamber are arranged coaxially in general-purpose evaporators, then scale separators are used operating on the principle of centrifugal and linear separation [I]. Such scale separators do not block up the steam space of the separator; they are simple to produce, and they are therefore particularly promising for apparatus with large unit power. On a prototype model of an evaporator, staff members of the Ukrainian Research Institute of Chemical Machinery investigated the efficiency of operation and the hydraulic resistance of scale separators (Fig. i). The scale separator is an annular cavity in the upper part of the separator, bounded by the outer rim of the separator and by the inner rim mounted coaxially with it. The scale separator operates in the following manner. Secondary steam enters the annular cavity through a rectangular opening, is given rotary motion, and after having carried out a complete rotation, it leaves the evaporator through the outlet pipe. Purification of the secondary steam is attained through the change of direction of the steam after it has entered the scale separator, through the appearance of centrifugal forces in the motion

Steam~~

~A A-A 5

ate

Fig. i. Diagram of the experimental evaporator: i) separator; 2) differential manometer; 3) replaceable scale separator; 4) compound pressure and vacuum gauge; 5) thermometer; 6) heating chamber.

Translated from Khimicheskoe i Neftyanoe Mashinostroenie, No. 12, pp. 12-13, December, 1984. 584

0009-2355/84/1112-0584508.50

9 1985 Plenum Publishing Corporation