FR2533455A1 - VACUUM DISTILLATION PROCESS - Google Patents

Abstract

THE INVENTION RELATES TO A PROCESS FOR EVACUATING GAS FROM A VACUUM DISTILLER. THIS PROCESS CONSISTS OF: A.A COLLECTING THE GAS FROM A VACUUM DISTILLER; B.A EXTRACT THE GAS; C.A COOL THE GAS EXTRACTED FROM THE DISTILLER; D.A PUMP THE COOLED GAS USING AN OIL SEALED VACUUM PUMP; E.A INTRODUCE FRESH AMBIENT AIR INTO A PUMP COMPRESSION STAGE; F.A EVACUATE THE GAS PUMP.

Description

Process and apparatus for vacuum distillation

  The present invention relates to a method and an apparatus

  for vacuum distillation of brine.

  In prior art apparatus for vacuum distillation of brine, the non-condensable gas, such as air, entering the apparatus must be vented or purged with the steam, and this can be done However, if such a vacuum pump was used with a brine distiller which discharges a relatively large amount of mixed steam to a much smaller quantity of the brine, it is relatively efficient using a rotary pump with oil seal. In the case of non-condensable gas, droplets of water produced in the pump from the steam would mix with the oil in the seal. This would result in the oil becoming extremely viscous and the volume of the oil-water mixture would increase, with the result that the oil would be rejected by the gas outlet Due to this disadvantage, such a pump was not used normally for this purpose, In contrast, in the case where a quantity relatively low vapor and greater amount of a non-condensable gases are pumped, a vacuum pump of the seal type of oil can be used effectively, In addition, if the amount of water is low; an oil seal vacuum pump with a gas ballast device could be used, with an ambient air supply The temperature could be high in the pump, so that the vapor pressure at saturation is high, using this characteristic and this phenomenon, however, the pump could be rejected "in the form of gas, water to be condensed in the pump or water contained in the oil." A general object of the invention is to propose an evacuation method. of gas from a vacuum distiller which conserves energy by allowing an oil seal vacuum pump to be used with the distiller 1 - -2-

  For this purpose, the subject of the invention is a method for evaluating

  gas extraction from a vacuum distiller, which consists of recovering a gas in a vacuum distiller, extracting the gas, cooling the gas removed from said distiller, pumping the cooled gas using a vacuum pump with oil seal and introducing fresh atmospheric air into the pump during its compression step The invention also relates to an apparatus for the

  implementation of the method above.

  Preferred embodiments of the invention are illustrated by the accompanying drawings, in which: Figure 1 is a diagram of a first embodiment of the invention; FIG. 2 is a diagram illustrating a second form of implementation; FIG. 3 is a schematic form of a rotary vane, oil seal and ballast vacuum pump.

  gas intended to be used with a distiller.

  Referring first to Figure 3, which shows a conventional pump comprising a first pump stage B for compressing a gas from a high vacuum, equal to that of a distiller A, to an intermediate vacuum , and a second stage C for compressing the gas from the intermediate vacuum to the atmospheric pressure A predetermined amount of fresh ambient air is introduced into the second stage C of the pump through an opening D A sealant and lubricating oil F is recycled from the second stage of the pump to the first stage by a conduit E. If such a vacuum pump, having a capacity of 1.0 m / h, pumps only steam (at saturation temperature s 340 C) in distiller A, under a pressure of 39.9 Torr (mm Hg), that is to say 53.2 mbar, the specific volume of saturated vapor being 0.03762 kg / m 2, the quantity of water is flowing

  in the pump is 0.03762 kg / m × 1.0 m 3 / h = 0.03762 kg / h.

  Moreover, and by way of example, air at a temperature of 20 ° C. and with a specific humidity of 0.01468 kg / kg at saturation is introduced through the opening D and the rejection. is made at the outlet of the second stage C at a temperature of 700 C

  and with a specific humidity of 0.2763 kg / kg at saturation.

  In order to saturate the air introduced with the pumped quantity of 0.03762 kg / h and to evacuate it in gaseous form, the quantity of air required from the opening D can be calculated by the following equation: , 03762 kg / h = 0 J, 14378 lcg / hooo oo (1) (0.2763 0.01468) kg / kg On the other hand, if the pumped gas of the distiller A is a mixture of approximately equal amounts of water vapor and air, the amount of steam in 1.0 m 3 / h of the mixture is

  half of the above example, i.e. 0.03762 kg / h.

  The specific volume being 0.8540 m 3 Acg x 760 = 16.267 m 3 / kg, the remaining quantity of air, obtained by mgotpliant by 1/2 the suction capacity of 100 m 3 / h of the pump, is

3 3

  0.5 m / h x (I il) kg / m 3 0.03074 kg / h & which gets into

16.267

  the pump, with: 0,8540 m 3 / kg: specific volume of air at a temperature of 34 C under atmospheric pressure

  760 (Torr): atmospheric pressure (10013 bar).

  39.9 (Torr): suction gas pressure (53.2 mbar)} Similarly, from equation (1), the amount of air that must be introduced through the opening-D is = ( 0.03762 / 2) 0.03074 = 0.04116 kg / ho (2)

(0.2763 0.01468)

  The results of the calculations above show that, if the vapor content of the gas mixture sucked from the still A decreases, the amount of air introduced through the opening D can be very small, such as 1 / 3.5, as it emerges

  the comparison of equations 1 and 2.

  Figure 1 shows the invention in its application -

  a distiller for low temperature evaporation, which

uses the concepts above.

  In Fig. 1, a conventional distiller comprises an enclosure forming an evaporation chamber. Chamber 1 is under a relatively high vacuum and steam at a temperature of about 34 ° C. is produced. vapor of the chamber 1 is condensed by a condenser 6, in which circulates ordinary water at a temperature of 25 C, and the condensate is collected in a bowl 16 and discharged by a pump 17 The rest of the non-condensable gas

  is evacuated via a discharge pipe 2.

  The enclosure 1 contains a brine 18 which is heated by a heating medium circulating in a conduit 19 In a conventional apparatus of this type, the supercooling of the evacuation gas is ordinarily about 2 ° C. and the temperature of the exhaust gas in duct 2 is about

32 C.

  Under these conditions, the vapor-to-air ratio in the exhaust gas is calculated as follows: x = 0.622 Ps = 0.622 x 35 7 = 5.29 (3), p Ps 39.9 35.7 or Ps denotes the saturated vapor pressure at 32 C, under 7 Torr (47.6 mbar), and where P is the vapor pressure

  saturating at a temperature of 34 C, 39.9 Torr (53.2 mbar).

  Therefore, the vapor rate for 1 kg of air is about 5.3 kg. If a gas ballast vacuum pump is used, a large quantity of air must therefore be introduced through the opening of the device. of Figure 3, to absorb the high rate of water vapor This would require the pump a high energy consumption to reject this large amount of air, resulting in a lowering

the efficiency of the system.

  According to the invention, the mixture of gases to be discharged, which comprises a large quantity of vapor and a smaller quantity of non-condensable gas (which will be referred to hereinafter as air), is fed through the duct. 2 in a direct contact heat exchanger or cooler 3, wherein the 32 C gas mixture is cooled to 26 C by direct contact with a fraction of ordinary water, which is at 250 C and a wide fraction of the vapor of the mixture

  condensed The resulting gas mixture, which has a percentage

  The cooler 3 receives the ordinary water through a conduit 21 and sprays the water into the gaseous mixture, the water and the condensate are recovered and, in this example , are conveyed by a conduit 22 to the chamber 1 l A pump 23 withdraws water from

the room 1.

  Under these conditions, the ratio of water vapor to air in the gaseous mixture is 2 × = 0.622 Ps = 0.622 × 25.2 = 1.063,000, oo (4) P Ps 39.9 25.2 Ps = saturation vapor pressure at a temperature of 260 ° C. under 25.2 Torr (33.6 mbar) P denotes the saturated vapor pressure at a temperature of 34 ° C. under 39 p 9 Torr

(53.2 mbar).

  In equation (4), compared with the situation illustrated by equation (3), the ratio of water content is about 1/5 and the absolute amount of gas to be discharged is reduced by (5929 + 1 ) a (1.0663 + 1) and is about 1/3 The capacity of the vacuum pump 5 can, therefore, be low and the amount of air to be introduced from the outside into the pump 5 can This also allows a significant reduction in the amount of energy consumed by the pump. FIG. 2 illustrates the invention in its application to a distiller for evaporation at a temperature.

much higher.

  In general, an oil seal type vacuum pump is capable of producing a high vacuum, but if the still is hot, the vapor pressure will be high and the pressure difference between the high vacuum produced by the pump and the hot vapor pressure will be great If the distiller is connected by a low-resistance duct to a gauze-type vacuum pump that has a constant pumping volumetric capacity (volume / time), the pump will tend to -6 - To suck up too much water vapor to be dried by the air introduced from the outside as a gas ballast, because the specific volume of the steam decreases when the temperature of the distiller increases. water (weight / time) sucked by the pump will exceed its maximum dissolution capacity by the air introduced from the outside through the opening D (Figure 3) This problem is also solved

by the present invention.

  According to FIG. 2, a conventional high-temperature and vapor-compression type still 12 comprises a steam compressor 7 The hot steam compressed by

  the compressor 7 passes through a bundle of test tubes

  The ordinary water, at a temperature of about C, coming from a pipe 9 and a heat exchanger 10, is sprayed through a pipe 11 onto the tubes 8 and the

  resulting steam is admitted at the inlet of the compressor 7.

  In this embodiment, means are provided for reducing the amount of vapor drawn by a vacuum pump below the allowable amount for the pump. As seen in FIG. 2, the exhaust gas from the tubes 8

  is cooled by a cooler 3 and goes through a restric-

  or a throttling means 13, before supplying the pump 5 Even if the pressure of the gas is high in the distiller 12, the pressure at the inlet 14 of the pump is low because of the throttling means 13, so that the gas relaxes and increases in specific volume, while the volume flow rate sucked by the pump is approximately constant. As a result, the weight amount of vapor aspirated can be kept below the allowable value.

  for the rate of water vapor from the pump.

  According to another method, when the vapor pressure 1 rises, fresh air is admitted upstream of the inlet 14 of the pump 5 via a pipe 15 to raise the air content of the gas sucked, thus lowering the vapor content, to reduce the water content aspirated O 7 i A control system 24 is connected to the outlet of the gas cooler 3 and is sensitive to the temperature of the gas, in order to actuate an air valve 25, who controls the

  amount of air introduced through line 15.

  In this embodiment, the exhaust gas is cooled in the direct contact cooler 3, which is supplied with ordinary water. However, as the distiller

  operates at high temperature, any other type of cooling

  can be used instead and the cooling means

  It does not have to be ordinary water.

  The gas evacuation method according to the invention comprises the steps of cooling the gas discharged from a vacuum distiller and discharging gas into the atmosphere using a vacuum pump with a gasket. oil, which includes a fresh air suction device to the compressions stage In this way, the evacuation of gas can reach an efficiency of 80% and a large amount of energy can

  be saved compared to conventional methods.

  In addition, this process can completely prevent the rust caused by the condensation of water vapor which appears when an oil seal vacuum pump is used in a known manner, and avoid the increase in viscosity of

  d oil to a mixture of water with oil.

  In addition, the cooling of the exhaust gas by direct contact with a fraction of the ordinary water reduces the amount of gas. Consequently, a pump of reduced capacity can be used, which thus limits the consumption.

of electrical energy.

  Furthermore, a restriction means and / or an air introduction means is provided on a suction pipe or inlet pipe of the pump. This makes it possible to use a vacuum pump with a seal. Therefore, such a pump can be used in a wide range of applications.

temperatures.

REVEMM 1 ICATIONS

  1. Process for evacuating the gas from a vacuum distiller, characterized in that it consists of: a) collecting the gas from a vacuum distiller b) extracting the gas; c) cooling the gas extracted from said distiller: d) pumping the cooled gas using a vacuum pump & oil seal; e) introducing fresh ambient air into a compression stage of the pump;

f) evacuate the pumped gas.

  2. Method according to claim 1, wherein the distiller receives ordinary water, characterized in that the extracted gas is cooled in step c) by contacting

  with at least a fraction of the water supplying the distillate

tor.

  3. Method according to one of claims 1 and 2,

  characterized in that the extracted and cooled gas is passed through a restriction means before it

feeds the pump.

  4. Method according to one of claims 1 to 3,

  characterized in that air is introduced into the gas

  extracted and cooled before it feeds the pump.