SU1490111A1 - Method and apparatus for evaporating aqueous solutions of urea - Google Patents

Abstract

The method of evaporation of aqueous solutions of urea and the device for its implementation can be used in the chemical and food industries, in the production of mineral fertilizers, in the evaporation of thermally unstable substances and allows to increase the final concentration of the evaporated solution and reduce the content of the products of thermal decomposition of urea in the finished product. The method includes feeding the initial solution and heating steam into the evaporation chamber, separating the resulting vapor-liquid mixture into separate streams of one stripped off solution and superheated secondary steam in a separating device installed in the bulk separator, discharging the one stripped off urea solution, condensate of the heating steam and superheated secondary steam. After the vapor-liquid mixture is separated into separate phases in a bulk separator in its near-wall region, moistened water vapor is introduced into the superheated secondary steam flow with a degree of dryness of 0.7-0.9. The evaporator for carrying out the method comprises a tubular heating chamber 1, a volume separator 4, rectangular nozzles 6, guide plates 7, wedge-shaped inserts, annular manifolds 12 with nozzles 13 and truncated cones. Saturated water is supplied to the collectors 12, which flows through the nozzles 13 to the surface of the separator 4 and the steam line 11, moves along their inner surface, preventing the formation of a layer of crystalline urea deposits and decomposition products on them. 2 sec. f-ly, 3 ill.

Description

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solution and heating steam into the evaporation chamber; separation of the resulting vapor-liquid mixture into separate streams of one stripped off solution and superheated secondary steam in a separating device installed in a bulk separator; discharge of one stripped off urea solution, heating steam condensate and superheated secondary steam. After the vapor-liquid mixture is separated into separate phases in a bulk separator in its near-wall region, moistened water vapor with a degree of dryness of 0.7-0.9 is introduced into the stream of superheated secondary steam. The evaporator for carrying out the method comprises a tubular heating chamber 1, a volume separator 4, rectangular nozzles 6, guide plates 7, wedge-shaped inserts, annular collectors 12 with nozzles 13 and truncated cones. Into the collectors 12 the overhead water vapor is supplied, which through the nozzles 13 flows to the surface of the separator 4 and the steam line 11, moves along their inner surface, preventing the formation of a layer of crystalline urea deposits and decomposition products on them. 2 sec. f-ly, 3 ill.

This invention relates to a process for the production of crystalline substances and may be used for evaporation of urea aqueous solutions.

The purpose of the invention is to increase the final concentration of the evaporated solution and reduce the biuret content in the finished product by preventing the formation of a layer of deposits on the unheated surfaces.

To achieve this goal, in the method that includes feeding the initial solution into a tubular evaporator and heating steam into its annular space, separating the vapor-liquid mixture into separate streams of superheated secondary steam and one stripped off solution in a separator installed in the bulk separator above the tubular evaporator, discharging one stripped off urea solution, superheated secondary steam and heating steam condensate according to the invention after separating the vapor-liquid mixture into separate phases in a bulk separator in its The humidified water vapor is introduced into the area with a degree of dryness of 0.7-0.9.

The use of a protective layer of moistened steam in the interior of the bulk separator leads to the elimination of the formation of a layer of deposits on unheated surfaces, which improves the quality characteristics of the finished product.

FIG. 1 shows an apparatus for carrying out the proposed method.

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general form; in fig. 2 shows section A-A in FIG. one; in fig. 3 - node I in figure 1.

The attachment apparatus contains a tubular heating chamber 1 with fittings 2 and 3 for supplying and discharging heat-transfer agent (heating steam), a volume separator A located above it, a cover 5 with rectangular nozzles 6, guide plates 7, wedge-shaped inserts 8, fitting 9 for supplying the solution, nozzles for removal of one stripped off solution 10, the steam pipe of the secondary superheated steam 11, annular manifolds 12 with nozzles 13, truncated cones 14 and a fitting 15 for introducing unnumbered water vapor.

The essence of the method and the principle of operation of the apparatus for its implementation are as follows.

Through shuder 3 (figure 1) in the annular space of the chamber 1 serves heating steam. The initial solution to be evaporated enters the evaporator tubes 1 through fitting 9 and, lifting upwards, boils. The heating steam, having given up its heat to the solution, is condensed and removed from the apparatus through shuder 2.

The vapor-liquid mixture coming from chamber 1 passes through rectangular nozzles 6, where, during movement, it separates under the action of centrifugal force into two streams — a stream of liquid (one stripped off solution) and a stream of secondary superheated steam. The fluid flow is pressed by the centrifugal force to the top of the rectangular nozzles 6, and the steam takes

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its lower part. The nozzles 6 have a curved configuration, the outlet openings are facing the bottom of the volume separator 4. The fluid from the nozzle 6

through its cross section between the upper and middle plates 7 and is directed downwards. The secondary superheated steam leaves the nozzle 6 through a section between the middle and lower plates 7, and is divided into two streams with wedge-shaped inserts 8, which are separated from the liquid flow. Further, the secondary steam flows rise to the upper part of the bulk separator and are removed from the apparatus through the steam lines 11, and the liquid solution through the nozzle 10. The absence of the sprinkling fluid when the vapor-liquid mixture is divided into separate phases eliminates the need for droppings.

The annular manifolds 12 are fed through the nozzle 15 by the fixed steam. When it flows out through the short nozzles 13 into the bulk separator 4, in which a deep vacuum is maintained, the steam parameters change along the constant entropy line and the steam goes from the saturated to the humidified state. For the spreading of jets of humidified steam across the entire sealing surface of the nozzle surface 13, they are inclined toward the surface and form an acute angle with it. Then, the stream of moistened water vapor, evenly distributed around the perimeter of the protected vertex, enters the next channel formed by the inner surface of the volume separator 4 with a truncated cone 14. In a narrowing channel, the flow of moistened steam stabilizes.

layer on a short stretch. The nozzles 13 are directed towards the velocity vector of the main flow of superheated steam, and the speed at the outer boundary of the protective layer should be equal to the speed of the main flow of superheated steam. The layer of moistened steam moves along the internal unheated surfaces of the volume separator 4 and the steam line 11, preventing the formation of a layer of crystalline carbamm and products on them

its decay.

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In the proposed method, the internal surfaces are protected.

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a volumetric separator, a secondary steam line and an ejector (not shown) by creating a protective layer of moistened water vapor around these unheated surfaces. Moisturized steam is formed from dry naschennogo steam, when the last of the short nozzles into the space with less pressure. To form a continuous protective layer, short nozzles from which the flow of moistened steam comes should be directed to the surface to be protected at an acute angle, the nozzles should be directed in the direction of movement of the main stream of secondary superheated steam. In order to form a stable stabilized flow of humidified steam, a truncated cone is installed in the protective layer above the nozzles, the base directed towards the main flow movement. The height of the truncated cone and the distance between the base of the cone and the surface to be protected is selected based on the condition that a developed boundary layer is formed of moistened steam, and the speed at its outer boundary should be equal to the velocity of the main stream of superheated steam. From this, the flow rate of the moistened steam is determined. The degree of cyxocTiY of the moistened steam is due to the pressure of the inferior steam until it expires through the nozzle.

The invention is illustrated by the following examples.

Example 1. At a plant for producing 1000 u / d of urea with a capacity of 1000 tons / day, an aqueous solution of urea with an initial concentration of 90-93% enters the second-stage evaporator and rises through the tubes of the heating chamber 1. At the same time, heating steam with a temperature of 150-160 ° C is supplied to the annular space. The heating steam, which gives up heat to the solution, condenses and is removed from the apparatus. The solution boils and the resulting vapor-liquid mixture enters the chamber with rectangular nozzles 6, where it is divided into streams of a liquid one stripped off solution and secondary superheated steam. The liquid phase is discharged from the bulk separator through the nozzle 10, and the secondary superheated steam through the steam line 11. Through the fitting 15 into the collector 12, saturated water vapor is fed with a pressure of P 1.4 eta. The flow through the short nozzles 13 into the space of the volume separator 4, where the pressure P 0.026 atm is maintained, the steam changes its parameters at a constant entropy and goes into a humidified state with a degree of dryness X 0.83, i.e. 17% of moisture is contained in a wet couple. The moisture content of the finished product according to Fisher is 0.25%, the content of biuret in the finished product is 0.77%. For condensation of 1320 kg / h of injected steam, 21.4 m / h of circulating water must be supplied.

Example 2, The method is carried out analogously to example T, only the manifold t2 is fed with the primary steam with a pressure of 25 atm. The pressure in the bulk separator decreases to 0.015 atm. When passing through the nozzles 13, the steam is transferred to a humidified state with a degree of dryness of X 0.715. The amount of wet steam is equal to SID.R 880 kg / h, the amount of condensed moisture introduced into the protective layer G, 250 kg / h. The maximum concentration on the walls 320 / (320-1-250) is 56%. The moisture content of the finished product according to Fisher is 0.2%, the content of biuret in the finished product is 0.7%. The flow rate of cooling water is 14.25 m- / h.

EXAMPLE 3. The method is carried out in a manner similar to Example 1, only the collector 12 is supplied with secondary water vapor from the first stage of the pump, which has a pressure of 0.4 at. The pressure in the bulk separator is maintained at 0.03 atm. The steam passing through the nozzles 13 goes into a wetted state with a degree of dryness of 0.885. The amount of wet steam in this case is G. 2240 kg / h, the amount of dropping moisture applied is G, 257.6 kg / h, the maximum concentration on the walls is 320 / (320; f257.6) 56%. The moisture content of the finished product according to Fisher is 0.3%, the content of biuret is 0.8%. Additional cooling water supply is not required.

A comparison of the proposed method and apparatus with the known technical solutions shows that the use of the saii iTHoro layer of moistened steam near the internal surfaces of the bulk separator leads to the elimination of the formation of a layer of deposits on the plate.

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There are a lot of surfaces that adversely affect the quality characteristics of the finished product (concentration of one stripped off solution and biuret content).

Claims (2)

1. A method of evaporating urea solutions, which includes feeding the initial solution and heating steam into a tubular evaporator with a volumetric separator, separating the vapor-liquid mixture in the separator after concentrating under centrifugal forces into separate streams of superheated secondary steam and evaporated solution, withdrawing the evaporated urea solution, condensing the heated steam and superheated secondary steam, o tl and h aa yu, and so that, in order to increase the final concentration of one stripped off solution and decrease the content of biuret a finished product by preventing the formation of layer depositions on unheated surfaces, a stream of superheated vapor after separation of vapor-liquid mixture into the separator volume in its near-wall Oblas, be introduced into the humidified water vapor with a degree of dryness of 0.7 to 0.9, 2. An external apparatus for evaporation of aqueous solutions of urea, containing a tubular heating chamber, a volumetric separator, a vapor-liquid flow separator installed in it in the form of a lid with two branch pipes directed towards the bottom of the separator, equipped with three guide plates and wedge-shaped inserts, shuts of introducing the initial solution, heating steam, evacuation of the evaporated solution, condensate of heating steam and the steam line of the first secondary steam, characterized in that increasing the final concentration of the solution and reducing the biuret content in the finished product, it is equipped with devices for creating a protective layer of steam on the surfaces of the separator and the steam line placed above the cm and a steam line, each of which is made in the form of an annular manifold with nozzles directed toward the surface at an acute angle, and mounted on it a conical member, a large base facing upwards and forming with the walls of the separator and steam pipe annular passage for vapor flow. fu2. PU2.3