GB2128490A

GB2128490A - Vacuum distillation method and apparatus

Info

Publication number: GB2128490A
Application number: GB08325259A
Authority: GB
Inventors: Masaharu Takada
Original assignee: Sasakura Engineering Co Ltd
Current assignee: Sasakura Engineering Co Ltd
Priority date: 1982-09-25
Filing date: 1983-09-21
Publication date: 1984-05-02
Also published as: KR840005974A; FR2533455B1; JPS59130592A; KR870000756B1; GB2128490B; FR2533455A1; GB8325259D0

Abstract

A method of gas extraction from a vacuum distiller comprises the steps of collecting a gas in a vacuum distiller (1), extracting (2) the gas, cooling (3) the gas extracted from said distiller, pumping the cooled gas using an oil- sealed vacuum pump (5), and introducing atmospheric air into the pump in its compression stage. The disclosure further relates to apparatus for carrying out the foregoing method. <IMAGE>

Description

SPECIFICATION Vacuum distillation method and apparatus This invention relates to method and apparatus for vacuum distillation of brine.

In prior art apparatus for distilling brine in a vaccum, the non condensable gas, such as air, entering into the apparatus must be vented or purged together with steam, and this may be done relatively efficiently using an oil-sealed rotary-blade vacuum pump, for example.

However, if such a vacuum pump were used with a brine distiller which discharges a relatively large amount of steam mixed with a much smaller amount of the noncondensable gas, water droplets produced from the steam in the pump would be mixed with the sealing oil. As a result, the oil would become highly viscous, and the volume of the oil-water mixture would increase, with the result that the oil would be discharged from the gas outlet. Since such a pump has this advantage, it has not been used normally for this purpose.

By contrast, in the situation where a relatively small amount of steam and a larger amount of noncondensable gas are pumped, an efficient oilsealed type vacuum pump could be used. Further, if the water content is small, an oil-sealed vacuum pump with a gas-ballasting device could be used, which is supplied with ambient air. The temperature of the air could be raised in the pump so that the specific humidity at saturation becomes high. By utilizing this characteristic and phenomenon, the pump could discharge, in the form of gas, the water to be condensed in the pump or some water contained in the oil.

It is a general object of this invention to provide a method of venting gas from a vacuum distiller, which method conserves energy by enabling the use of an oil-sealed vacuum pump with the distiller.

This invention relates to a method of gas venting from a vacuum distiller, and comprises the steps of collecting a gas in a vacuum distiller, extracting the gas, cooling the gas drawn from said distiller, pumping the cooled gas using an oilsealed vacuum pump, and introducing fresh atmospheric air into the pump during its compression state.

Another feature of the invention resides in apparatus for carrying out the foregoing method.

Preferred embodiments of this invention are shown in the accompanying figures of the drawings, wherein: Fig. 1 is a flow chart illustrating a first embodiment of the invention; Fig. 2 is a flow chart illustrating a second embodiment; and Fig. 3 is a schematic view of a conventional gas-ballast oil-sealed rotary-blade vacuum pump for use with a distiller.

With reference first to Fig. 3, a conventional pump includes a first-stage pump B for compression of gas from a high vacuum equal to that in a distiller A, to an intermediate vacuum, and a second-stage pump C for compression of the gas from the intermediate vacuum up to atmospheric pressure. A predetermined amount of ambient fresh air is introduced into the secondstage pump C through an opening D. Lubricating and sealing oil F recirculates from the secondstage pump to the first stage through a pipe E.

Now, if such a vacuum pump having a capacity of 1.0 m3/h pumps only steam (at saturation temperature: 340 C) from the distiller A, having a pressure of 39.9 Torr (mmHg), the specific volume of saturated steam being 0.03762 Kg/m3, the water content flowing into the pump is 0.03762 Kg/m3x 1.0m3/h=0.03762 Kg/h. On the other hand, and by way of example, air at a temperature of 200C and a specific humidity of 0.01468 Kg/Kg at saturation is introduced at the opening D, and the discharge is made at the outlet of second pump C at a temperature of 700C and a specific humidity of 0.2763 Kg/Kg at saturation.In order to saturate the introduced air with the pumped water content of 0.03762 Kg/h and discharge it in gaseous form, the amount of air required from the opening D is calculated by the following equation:

By contrast, if the gas pumped from the distiller A is a mixture of about equal amounts of water vapor and air, the amount of vapor in 1.0 m3/h of the mixture is half the above example, i.e.,

The specific volume being

the remaining amount of air is, by multiplication by 1/2 of the pump suction capacity of 1.0 m3/h, 1 0.5 m3/hx( )Kg/m3=0.03074 Kg/h 16.267 which flows into the pump, wherein: 0.8540 m3/Kg: Specific volume of air at a temperature of 340C under the atmospheric pressure 760 (Torr): Atmospheric pressure 39.9 (Torr): Pressure of the sucked gas.

Similarly from equation (1), the amount of air which must be introduced through the opening D is

The results of the above calculations show that, if the ratio of the amount of vapour in the gas mixture sucked from the distiller A lowers, the amount of air which should be introduced through the opening D may be very small, such as 1/3.5, as will be apparent from a comparison of equations (1) and (2).

Fig. 1 shows the invention as applied to a distiller for evaporation at a low temperature, which utilizes the foregoing concepts.

In Fig. 1, a conventional distiller includes a housing forming an evaporation chamber 1. The chamber 1 is under a relatively high vacuum, and vapor at a temperature of about 340C is produced therein. A major portion of the vapor in the chamber 1 is condensed by a condenser 6, through which raw water at a temperature of 250C flows, and the condensate is collected by a pan 1 6 and removed by a pump 17. The remaining non-condensible gas is vented through a vent pipe 2.

The chamber 1 has brine 18 therein which is heated by a heating medium flowing through a pipe 1 9. In conventional apparatus of this nature,the amount of supercooling of the vent gas is ordinarily about 20C, and the temperature of the vent gas in pipe 2 is therefore about 320C.

Under these conditions,the vapor-to-air ratio in the vent gas is calculated as follows:

where Ps: Saturated vapor pressure at a temperature of 320C=35.7 Torr P : Saturated vapor pressure at a temperature of 340C=39.9 Torr Thus, the ratio of the amount of vapor of 1 Kg of air is about 5.3 Kg. If a gas-ballast vacuum pump is used, then a large amount of air must be introduced through the opening D in Fig. 3 in order to absorb the large rate of water vapor. This would necessitate a large pump power consumption to discharge this large amount of air, thereby lowering the efficiency of the system.

In accordance with this invention, the vent gas mixture, which is a mixture of a large amount of steam and a smaller amount of noncondensible gas (hereinafter referred to as air), is passed through the pipe 2 into a direct contact heat exchanger or cooler 3, where the gas mixture at 320C is cooled down to 260C by direct contact with a portion of the raw water which is at 250C, and a large portion of the steam in the mixture is condensed. As a result, the gas mixture having an increased percentage of air is fed to a vacuum pump 5. The cooler 3 receives raw water through a pipe 21 and sprays the water into the gas mixture. The water and condensate is collected and, in this example, is led through a pipe 22 to the chamber 1. A pump 23 removes water from the chamber 1.

Under this condition, of the ratio of the amount of water vapor to air in the gas mixture is

where Ps: Saturated vapor pressure at a temperature of 260C=25.2 Torr P : Saturated vapor pressure at a temperature of 340C=39.9 Torr.

In the equation (4), as compared with the condition shown in equation (3), the ratio of the water content is about 1/5, and the absolute amount of vent gas is reduced to (1.0663+1) from (5.29+1) and is about 1/3. The capacity of the vacuum pump 5 used can therefore be small, and the amount of air to be introduced into the pump 5 from the outside can also be small. This enables a great reduction in the amount of power consumed by the pump.

Fig. 2 shows the invention as applied to a distiller for evaporation at a much higher temperature.

In general, an oil-sealed type vacuum pump is able to produce a high vacuum, but if the distiller 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 pipe of low resistance with a gas-ballast type vacuum pump, which has a constant pumping volume capability (volume/time,) the pump would likely suck too much water vapor to be dried up with the air introduced from outside as gas-ballast, because the specific volume of the steam decreases as the temperature of the distiller rises.

In other words, the amount of water content (weight/time) sucked by the pump would exceed its maximum ability of dissolution with the air introduced from the outside through the opening D (Fig.3) This problem is also overcome by the present invention.

In Fig. 2, a conventional high temperature vapor-compression type distiller 12 includes a vapor compressor 7. The hot vapor compressed by the compressor 7 is passed through a bundle of evaporative tubes 8. Raw water at a temperature of about 250C passed through a pipe 9 and a heat exchanger 10 is sprayed from a pipe 11 over the tubes 8, and the resulting generated vapor enters the inlet of the compressor 7.

In this embodiment, means is provided for reducing the amount of vapor sucked by a vacuum pump 5 below the allowable value of the pump. As shown in Fig.2, the vent gas from the tubes 8 is cooled by a cooler 3, and is passed through a restriction or throttling means 13 before being fed to the pump 5. Even if the gas pressure in the distiller 12 is high, the pressure at the pump inlet 14 is low because of the throttling means 13, so that the gas expands and increases its specific volume, while the volume flow rate sucked by the pump is approximately constant. As a result, the amount (by weight) of the sucked vapor can be below the allowable value of water vapor rate for the pump.

In still another method, when the vapor pressure rises, fresh air is fed at the upstream of the inlet 14 of pump 5 through a pipe 15 to raise the percentage of the amount of air in the sucked gas, thereby lowering the percentage of steam to reduce the sucked water content. A sensorcontrol 24 is connected to the outlet of the vent cooler 3 and senses the gas temperature, and it operates an air valve 25 which controls the amount of air fed through the pipe 15.

In this embodiment, the vent gas is cooled by the direct-contact cooler 3 which is fed with raw water. However, since the distiller operates at a high temperature, any other type of cooler can be used instead, and the cooling medium does not need to be raw water.

The method of gas venting according to this invention includes the steps of cooling the gas vented from a vacuum distiller, and the releasing the gas into the atmosphere using an oil-sealed vacuum pump which has a device to suck fresh air in a compression stage.

Consequently, the gas venting can reach an efficiency of 80%, and a large amount of energy can be saved as compared with the conventional methods.

Also, this method can completely prevent rust caused by the condensation of water vapor when an oil-sealed vacuum pump was conventionally used, and prevents the viscosity of the oil from increasing due to water mixing with the oil.

Further, the cooling of the vent gas by direct contact with a portion of the raw water reduces the amount of the gas. Consequently, a reduced capacity pump can be used, thereby saving electric power consumption.

Additionally, throttle means and/or means for introducing air is provided in a pipe on the suction or intake side of the pump. This enables an oilsealed vacuum pump to be available for a distiller which is operated at a high temperature. Thus, such a pump can be utilized over a wide range of temperatures.

Claims

1. A method of gas venting from a vacuum distiller, said method comprising the steps of a) collecting a gas in a vacuum distiller; b) extracting the gas; c) cooling the gas extracted from said distiller; d) pumping the cooled gas using an oil-sealed vacuum pump; e) introducing ambient fresh air into a compression stage of the pump; and f) releasing the pumped gas.

2. A method according to Claim 1 , wherein the distiller receives raw water,and the extracted gas is cooled in step c) by being brought into contact with at least a portion of raw water being fed to said distiller.

3. A method according to Claim 1, and further comprising the step of passing the extracted and cooled gas through one or both of throttling means and means for supplying air before feeding the gas into the pump.

4. A method according to Claim 2, and further comprising the step of passing the extracted and cooled gas through one or both of throttling means and means for supplying air before feeding the gas into the pump.

5. Apparatus for carrying out the method of Claim 1.

6. Apparatus for performing the method of Claim 1, substantially as described with reference to Figure 1 or Figure 2 of the accompanying drawings.