RU2396129C1 - Swirl heat exchange separator for cleaning gas from vapours of admixtures - Google Patents

Abstract

FIELD: process engineering. ^ SUBSTANCE: invention is intended for separation and can be used for cleaning gas phase of uranium hexafluoride of admixtures in the form of fluorocarbons. Proposed separator comprises casing with bundle of heat exchange tubes fixed in tube plate, overflow tubes, feed and discharge branch unions and condensate drain union. Overflow tubes are located coaxially inside heat exchange tubes and have free end on the side of lower edges of heat exchange tubes. Gap between free end of overflow tube and lower edge of heat exchange tube makes at least 1.2 and does not exceed 1.5-1.7 of heat exchange tube ID. ^ EFFECT: stable process characteristics, ease of production and efficient separation. ^ 3 cl, 2 dwg

Description

The invention relates to the production of low enriched uranium hexafluoride and can be used to clean the gas phase of uranium hexafluoride from impurities in the form of fluorocarbon vapors.

The chemical purity of uranium hexafluoride (HFC), used as a raw material in the production of enriched uranium, should ensure the normal operation of gas centrifuges that are very sensitive to the presence of impurities and the nuclear purity of uranium dioxide obtained from low enriched uranium hexafluoride and used to prepare pelletized nuclear fuel.

The presence of fluorocarbons (FU) in the HFC process streams of isotope separation plants is due both to their delivery from sublimation plants in HFC feedstock and to the use of lubricants in equipment components (for example, booster compressor bearings). Fluorocarbon grease is a product of sequential fluorination and vacuum distillation of carbon oil [Maksimov B.N. and other Industrial organofluorine products. Reference / L .: Chemistry, 1990. - 464 p.]. The mass spectrum of fluorocarbon greases begins in the region of 500 ÷ 550 amu (compounds of type C _n F _2n-14 , ..., C _n F _2n-20 ) and lasts up to ~ 1300 amu (compounds of the type C _n F _2n-8 , ..., C _n F _{2n + 2} ), and the homologs have approximately the same volatility. The boiling point of the homologous mixture of fluorocarbon grease is ~ 130 ° C (at a pressure of 1.33 kPa). For reference: the temperature of the desublimation of HFCs at a pressure of 101.3 kPa is 56.4 ° C.

The low-temperature purification of the gas phase of HFCs from FU vapors is complicated by the fact that supercooled uranium hexafluoride is easily desublimated in the form of crystals and there is a danger of blocking the machine pipelines of technological equipment. This limits the choice of apparatus design, the operation of which is associated with low-temperature separation and low-temperature gas separation. In addition, the geometric dimensions of the apparatus are limited by nuclear safety requirements.

For gas purification from vapors of impurities, a device is known [DE No. 2316570, IPC B01D 5/00, 1973] (analogue), consisting of a working chamber operating under pressure, with a gas inlet and gas outlet. The gas to be cleaned is fed tangentially into the chamber. The gas outlet is equipped with a heater. Nozzles were introduced tangentially into the chamber to supply an inert gas (nitrogen) in the liquid-phase state.

Also known installation [RU No. 2298424 C1, IPC B01D 5/00. Publ. 10/05/2007] (analogue), designed to purify gases from solvent vapors, containing a sealed chamber with a gas outlet in its upper part and a drain pipe in the lower bottom, a gas inlet and an inlet pipe equipped with a swirl with a radial outlet. An annular collector for cryogenic liquid with an inlet pipe is attached coaxially with the inlet pipe. The installation also includes a contact heat exchanger and an annular filter located on the upper part of the chamber. A heating heat exchanger and a bottom filter are located at the bottom of the chamber. The installation allows, with an increase in the concentration of solvent vapor in the gas to be cleaned, to increase both the flow rate of the cryogenic liquid and the flow area for discharging the resulting aerosol, and to reduce the flow rate of the cryogenic liquid when the concentration is reduced.

The disadvantages of analogues include the use for the transfer of vapor of impurities to the condensate of an additional cryogenic liquid spent on gas cooling, condensation of solvent vapors and cooling of the heated chamber walls.

Known apparatuses for gas-dynamic separation of the purified gas from aqueous and / or hydrocarbon components [RU No. 2291736 C2, IPC B01D 45/12. Publ. 01/20/2007] (analogue), in which a swirling flow of a high-pressure multicomponent hydrocarbon gas into the nozzle is used, condensed gas expands with cooling during outflow at a supersonic or supersonic speed into a cylindrical refrigerating chamber, condensation of components in an expanded and cooled rotating stream, precipitation of condensate from a rotating cooled gas stream on a solid surface of the refrigerator with the formation of a liquid film on it and removal of the last a coaxial slotted hole in the zone of reduced pressure which is created by the recirculation ejecting therefrom the gas phase source gas, purified gas stream increase braking pressure in the diffuser and its further feed consumer.

When the gas expands in the nozzle with a transonic or supersonic flow velocity (300 ÷ 360 m / s and above), the potential energy - gas pressure - passes into kinetic energy, while the gas cools and acquires a static temperature of about minus 50 ÷ 100 ° С (at initial gas temperature + 10 ° С).

The analogue provides for gas-dynamic separation with iso-enthalpy expansion of high-pressure gas flows flowing from the nozzle at a transonic or supersonic speed. This is not possible for uranium hexafluoride process streams. The use of devices of a similar design is also problematic because of the likely desublimation of HFCs during intensive cooling of the gas stream.

For purification and drying of natural gas by the method of low-temperature separation, a device is known [SU No. 516409, B01D 45/06. Publ. 06/05/1976. Bull. No. 21] (analogue), comprising a housing with inlet and outlet fittings for high and low pressure gas, and high and low pressure separators with liquid collectors installed in it, a shell-and-tube heat exchanger with U-shaped pipes communicating with the distribution chamber. The separation device in order to reduce the gas resistance is made in the form of a battery of direct-flow cyclone elements, and the low-pressure separator is placed in the casing of the heat exchanger.

Heat transfer intensification is achieved by installing swirl elements in the pipes of the heat exchanger from the inlet side of the high-pressure gas. U-shaped heat exchanger tubes are needed to cool the high pressure gas with a counter flow of low pressure cold gas. The gas discharged from the high-pressure chamber is sent for throttling for additional cooling and then enters the low-pressure chamber for final cleaning.

In cleaned, drained and heated form, the gas is sent to the main gas pipeline.

Thus, the combination of heat transfer and separation processes allows more efficient separation of the flow into phase components, provided hydrate formation is prevented in apparatuses with direct-flow centrifugal separation elements and swirlers. To exclude the precipitation of crystalline hydrates of impurity components during gas cooling in the apparatus, a hydrate inhibitor is additionally sprayed in the high-pressure chamber.

The disadvantages of the analogue: the need to use antihydrate inhibitors at the stages of cooling and gas separation.

As the closest analogue to the claimed device by technical essence, a vortex shell-and-tube heat exchanger [SU No. 281490, F28D 7/00. Publ. 09/14/1970. Bull. No. 29] (prototype) for cooling and purifying exhaust gases from impurity vapors with a tubular bundle fixed in the gratings and vortex energy separators of the cooled gas flow installed at the inlet to the pipes and made in the form of bushings placed in the pipes with a conical surface at the site of their contact with the pipes equipped with a screw thread, forming channels for introducing gas with pipes.

Gas (vapor-air mixture or gas-condensate mixture) under excess pressure is fed through the nozzle into the gas separation cavity of the heat exchanger and then through the screw nozzles of the bushings enters the tubes of the heat exchanger. In nozzles, the gas flow is rotationally driven. As a result of temperature separation of the flow inside the pipes, intense vapor condensation occurs. The resulting condensate flows down the walls of the pipes to the bottom and is discharged from the heat exchanger. Refrigerant circulates in the annulus. From the bottom of the heat exchanger, the gas flow moves to the upper receiver through transfer pipes, in which it is additionally cooled and moisture condensed. Cooled and condensate-free gas is discharged from the apparatus through a fitting in the upper part of the heat exchanger.

However, when operating a vortex shell-and-tube heat exchanger as a separator, hydrodynamic conditions are violated due to freezing of the holes of the bushes of the energy separator. For this reason, the use of a known apparatus for cleaning easily desublimated uranium hexafluoride from impurity vapors is practically impossible. In addition, the proposed design of a vortex shell-and-tube heat exchanger in geometric dimensions cannot be adapted to nuclear safety requirements.

Another disadvantage of the prototype is its purpose for the purification of high-pressure gases, while the pressure of the gas phase of HFCs in process streams does not exceed 5.3 kPa at temperatures up to 40 ° C. With such pressures of uranium hexafluoride at the inlet to the heat exchange pipes, studies have shown that the twist loses its intensity at distances of about 7 ÷ 9 of the inner diameter of the pipe and then becomes ineffective for intensifying heat transfer of the gas stream with the channel walls.

The technical problem solved by the present invention is to increase the efficiency of gas-dynamic separation and stabilize the hydrodynamic regime of the heat exchange separator when cleaning gases from impurity vapors having close boiling / sublimation temperatures, for example, when cleaning the gas phase of uranium hexafluoride from fluorocarbon vapors.

An additional technical task is to simplify the manufacturing technology of the vortex heat exchange separator.

These technical problems are solved due to the fact that in the vortex heat exchange separator for purifying gas from impurity vapors, which contains a casing with a bundle of heat exchange tubes fixed in tube sheets, transfer pipes, inlet and outlet nozzles, as well as a condensate drain fitting, transfer pipes are installed coaxially inside the heat exchange pipes and have a free end from the side of the lower end of the latter, while the gap between the free end of the transfer pipe and the lower end of the heat transfer pipe is not less than 1.2 and not more than 1.5 ÷ 1, 7 inner diameter of the heat exchanger pipe.

Technical problems are additionally solved due to the fact that the vortex heat exchange separator contains four heat exchange and transfer pipes, and that the supply nozzle is additionally equipped with a distribution manifold, which provides tangential gas inlet into the heat exchange pipes.

Figure 1 shows a General view of the described vortex heat exchange separator in the composition of four heat transfer and transfer pipes; figure 2 is a cross section of a separator in the area of the inlet fitting.

The vortex heat-exchange separator contains a casing 1, four heat-exchange pipes 2, mounted in the tube sheets 3 and 4. Inside each heat-exchange pipe 2, a transfer pipe 5 is coaxially installed, with a distance from the free end to the lower end of the heat-transfer pipe not less than 1.2 and not more than 1 , 5 ÷ 1.7 of the inner diameter of the latter. Pipelines 6 forming a distributing collector connect the heat exchange pipes 2 to the supply nozzle 7 and provide a tangential gas input into the annular gaps formed between the walls of the heat exchange 2 and the transfer 5 pipes. In the upper part of the separator, transfer pipes 5 through bushings 8 are connected to the discharge nozzle 9. The lower ends of the heat exchange tubes 2 are equipped with fittings 10 for draining the condensate of fluorocarbon grease. The annulus of the separator is cooled by water, which is supplied through the nozzles 11 and discharged through the nozzle 12, which ensures uniform distribution of refrigerant over the annulus with no stagnant zones.

In addition, swirls can be additionally installed in the annular gap at the gas inlet to the heat exchange pipes to impart a stable rotational motion to the gas.

Vortex heat exchange separator operates as follows.

A stream of uranium hexafluoride containing pairs of fluorocarbons from a machine pipeline (not shown in FIG.) Enters a supply nozzle 7, and then through pipelines 6 into an annular gap formed by heat exchange 2 and transfer 5 pipes. Due to the tangential inlet, the gas in the annular annular gap acquires a rotational motion. In the annular gap, the HFC is cooled by heat exchange on the one hand with the water-cooled wall of the outer pipe 2 and on the other hand with the colder wall of the transfer pipe 5, along which the HFC stream, having a lower temperature, is counter-moving. The cooling of HFCs in the annular gap is accompanied by condensation of the fluorocarbon vapors on the walls of the heat exchange tube 2, which flow into the oil receiver (not shown) in the form of a liquid film through the nozzle 10, which is located under the separator. Aerodynamic separation of the fluorocarbon film from the gas stream occurs when the direction of gas movement changes by 180 ° in the lower part of the heat exchange tubes 2. In this section of the heat exchange tube, the gas is additionally cooled due to the reversal, since the flow reversal is accompanied by an abrupt change (decrease) in the gas static pressure.

In addition, in the channel of the transfer pipe 5, as a result of the flow reversal, strong pulsations of static pressure and the longitudinal component of the gas velocity occur. At the entrance to the transfer pipe, a pulsating radial velocity component appears in the stream. As a result, the fluorocarbon aerosol, which can be introduced into the transfer pipe by gas from the flow reversal section, coagulates and is dropped onto the wall in the form of a drop liquid, and then flows to the free end of the transfer pipe in the form of a film.

The gap between the free end of the transfer pipe 5 and the lower end of the heat transfer pipe 2 is selected at least 1.2 internal diameter of the heat transfer pipe for:

- prevention of the formation of a vortex gas flow with a smaller gap near the lower end of the heat exchange tube 2. When a vortex flow is formed, the liquid film will break off the wall and fluorocarbon aerosol will be thrown into the transfer pipe channel 5;

- the implementation of continuous HFU subsonic flow from the lower part of the heat exchange pipe to the transfer pipe channel, which excludes gas overcooling with HFC desublimation during a turn;

- increase the time of adsorption condensation of fluorocarbons on the walls of the channel with a minimum temperature of the gas stream in the lower part of the heat exchange pipe;

- ensuring minimum hydraulic resistance of the headland.

At the same time, the gap between the free end of the transfer pipe and the lower end of the heat exchanger pipe is no more than 1.5–1.7 of the internal diameter of the heat exchanger pipe, since a larger gap will cause a stagnant zone in the vicinity of the lower end of the heat transfer pipe, which will reduce the rate of condensate removal of fluorocarbon grease to oil receiver.

When moving in the transfer pipe 5, the gas is heated by a counter-flow of HFCs, which eliminates the formation of desublimate on the walls of the discharge nozzle 9. The cleaned and heated uranium hexafluoride passing through the separator through the discharge nozzle 9 is again sent to the intermachine pipelines of the uranium plant.

Based on the studies, it was found that the conditions of the highest heat removal, the least hydraulic resistance, subject to restrictions on the diameter of the separator (~ 0.4 m) for nuclear safety requirements, meets the design of the separator with four heat exchange and transfer pipes.

Pilot tests of the design of the vortex heat-exchange separator showed that at an initial pressure of the gas phase of the HFC at the inlet of the separator ~ 4680 Pa, the outlet pressure is ~ 2290 Pa, while the pressure drop of the gas phase of the HFC at the turn of the flow reaches ~ 2150 Pa, the pressure drop at rectilinear sections of the annular gap and transfer pipe ~ 150 Pa and ~ 90 Pa, respectively. The HFC flow velocity at the entrance to the annular gap was ~ 15 m / s. After a turn, the flow velocity increased to ~ 30 m / s. After passing through the annular gap, the temperature of the gas phase of HFCs decreased by 5–6 ° C; during the passage of the turn due to gas expansion, the temperature further decreased by ~ 1–2 ° C.

During the tests, industrial water with a temperature of 2 ÷ 27 ° С was used as a refrigerant in the separator for cooling the walls of heat-exchange pipes.

The length of the heat exchange and transfer pipes (respectively, the heat exchange surface area) is determined by the mass flow rate of HFCs to be cleaned and the gas pressure at the inlet / outlet of the separator. In the practical implementation of the vortex heat exchange separator, the ratio of the length of the heat exchange pipe to its inner diameter was more than 10.

It has been experimentally established that the presence of a distributing collector with piping 6 in the design of the vortex heat exchange separator with pipelines 6 for tangential gas entry into the annular gap of the heat exchange 2 and transfer 5 pipes increases the separator efficiency by about 25%.

The results of pilot tests showed that the design of the vortex heat exchange separator has stable operating characteristics, is simple in manufacturing technology and provides effective cleaning of the gas phase of uranium hexafluoride from fluorocarbon vapors. The design provides the temperature regime of the separator, eliminating the hypothermia of the gas, which can cause the desublimation of uranium hexafluoride.

Claims (3)

1. A vortex heat exchange separator for purifying gas from impurity vapors, comprising a casing with a bundle of heat exchange tubes fixed in the tube sheets, transfer pipes, supply and discharge nozzles, and a condensate drain fitting, characterized in that the transfer pipes are installed coaxially inside the heat transfer pipes and have a free end from the side of the lower end of the latter, while the gap between the free end of the transfer pipe and the lower end of the heat transfer pipe is not less than 1.2 and not more than 1.5 ÷ 1.7 of the inner diameter of the heat BMENA pipe. 2. The vortex heat exchange separator according to claim 1, characterized in that it contains four heat exchange and transfer pipes. 3. The vortex heat exchange separator according to claim 1, characterized in that the inlet fitting is additionally equipped with a distributing manifold, providing tangential gas inlet into the heat exchange pipes.

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