METHOD AND APPARATUS FOR REMOVING VOLATILE COMPONENTS FROM AN AQUEOUS MEDIUM

Abstract: A system and a method are disclosed for removing dissolved organic impurities, such as gasoline or other hydrocarbons, from contaminated water. The contaminated water (11) is fed into a stripper column (10), operated under a vacuum, to enable the water to enter the column at its boiling point. Steam (13), as a vaporizing gas, strips the impurities out of the water and the overhead vapor phase is compressed (17) to boost it to a higher pressure, so it can be condensed to vaporize part of the bottoms stream from the column. Since the bottoms stream is mostly water, the vapor generated is the steam used to strip impurities from the water.

Description: This invention concerns a method and apparatus for removing volatile components from an aqueous medium. More particularly, the invention is directed to removing dissolved impurities from water by steam-stripping the impurities under vacuum conditions. A major problem today is contamination of groundwater by hydrocarbons, such as gasoline, benzene, toluene, or xylene, that leak out of underground storage tanks. Groundwater can also be contaminated by chlorinated solvents, such as trichloroethylene, trichloroethane, perchloroethylene, and methylene chloride, which are used in degreasing operations and then dumped into open pits in the ground. One way to remove organic impurities from groundwater is by a technique referred to as steam-stripping. In a typical steam-stripping process, the contaminated water is first passed through a heat exchanger to bring its temperature up to about 180-200°F (82-93°C) and then directing it into the top of a stripper column. Steam, as the vaporizing gas, enters the column near the bottom and flows upwardly at a temperature of about 212-240°F (100-116°C). As the downwardly moving water and the upwardly moving steam make contact inside the tower, the steam and water are at about the same temperature until the steam begins to contact the cooler incoming feed water near the top of the column. At this point the steam condenses on the cooler feed water to heat the water to its boiling point. Since much of the steam is consumed in heating the cooler feed water, only a very small amount of steam is available for vaporizing the hydrocarbon impurities and carrying the impurities out of the column in the vapor phase. As the vapor phase leaves the column, it is passed through a heat exchanger to condense the vapors to a mixture of water and an organic liquid phase containing the organic impurities. The organic liquid phase is separated from the water in a decanter vessel and the organic liquid phase is discharged to an incinerator, or carried to another point for further processing. The water phase, which is still saturated with dissolved impurities, is recycled back to the column and stripped again to remove the impurities. The steam-stripping procedure described above has a major drawback; only a small amount of steam is available, as a vaporizing gas, to carry the impurities out of the stripper column. Therefore, the amount of steam required to operate under such conditions is relatively high and the process is costly to operate. The present invention overcomes this problem by operating the stripper apparatus at a reduced pressure, that is, under a vacuum condition, so that the feed water can enter the column at its boiling point. US-A-3 898 058 discloses a method for vacuum stripping of hydrocarbon contaminated water streams and an apparatus for carrying out said method. The apparatus comprises a contacting vessel (12) in which a contaminated stream and a high vapor pressure stripping gas may be introduced ; the vessel (12) communicates with a vacuum pump (18) ; the vacuum pump (18) communicates with a condenser (22) ; the condenser (22) communicates with a separating vessel (26). The separating vessel (26) communicates with the contacting vessel (12). The method and the apparatus according to the invention however differs therefrom. The present invention provides a method and an apparatus for removing volatile components from an aqueous medium, such as water. Examples of the volatile components are certain hydrocarbons and chlorinated solvents, such as, for example, trichloroethylene, trichloroethane, perchloroethylene, methylene chloride, benzene, toluene, xylene, gasoline, and others, which represent impurities in the water. In practice, the contaminated water is fed into a stripper column, usually at the top of the column. Vaporizing gas, in the form of steam, enters the column near the bottom from a reboiler located adjacent to the column, or from a steam generator that may be located remote from the column. As the steam flows upwardly in the column, it contacts the downwardly flowing water phase and vaporizes the volatile impurities. During the stripping operation a vacuum pump creates a negative pressure condition in that part of the system which includes the column. The overhead vapor phase containing the volatile impurities is directed into a vapor compressor, which compresses the vapor to raise its pressure to a point below atmospheric pressure. The pressurized vapor is then passed into a condenser, which condenses part of the vapor phase and thus forms a liquid phase and a vapor phase. The liquid and vapor phases are then passed into a receiver vessel. This vessel separates the phases, so that the liquid phase contains water and the condensed volatile impurities, and the vapor phase contains noncondensable components. From the receiver vessel, the vapor phase is drawn into the vacuum pump, which raises the vapor pressure to atmospheric pressure. The liquid phase from the receiver vessel is carried into a decanter vessel. When the vapor phase reaches atmospheric pressure, it passes into a second condenser, where part of it is condensed to form a liquid phase and a vapor phase. The vapor phase in this condenser is discharged from the condenser and the liquid phase passes into a decanter vessel. The decanter vessel separates part of the condensed volatile impurities from the water phase, and these components are discharged from the decanter vessel. The water phase, which still contains some of the condensed volatile components, is also discharged from the decanter vessel. More particularly, the present invention resides in an apparatus for removing volatile components from an aqueous medium, which comprises a column (10) for receiving an aqueous medium containing volatile components, and for receiving steam as a vaporizing gas; a vapor compressor (17) in communication with the column (10) and a reboiler (12); the reboiler (12) communicates with the column (10) and a receiver vessel (18); the receiver vessel (18) communicates with a vacuum pump (21); the vacuum pump (21) communicates with a condenser (22); the condenser (22) communicates with a decanter vessel (26); the decanter vessel (26) communicates with the receiver vessel (18) and the column (10); wherein, in operation, the vacuum pump (21) causes a negative pressure condition in the column (10), and in that part of the system between the column (10) and the vacuum pump (21); the aqueous medium containing volatile components and the steam enter the column and flow countercurrent to each other; the steam vaporizes the volatile components, and the vapor is compressed by the vapor compressor (17) to raise its pressure to a point below atmospheric pressure; the reboiler (12) condenses part of the steam, which provides a liquid phase and a vapor phase; the receiver vessel (18) separates the liquid phase and the vapor phase, wherein the liquid phase contains water and the condensed volatile components, and the vapor phase contains noncondensable components; the vacuum pump (21) raises the pressure of the vapor phase from the receiver vessel (18) to atmospheric pressure; the condenser (22) condenses part of the vapor phase from the vacuum pump (21), which provides a liquid phase and a vapor phase, and the vapor phase is discharged by vent line (31) from the condenser (22); the liquid phase from the receiver vessel (18) and the liquid phase from the condenser (22) are carried into the decanter vessel (26), the decanter vessel (26) separates part of the condensed volatile components from the water phase and these components are discharged from the decanter vessel (26), and the water phase containing the remaining condensed volatile components is discharged from the decanter vessel (26). The invention also resides in a method for removing volatile components from an aqueous medium, which comprises: passing into a column an aqueous medium containing volatile components, and steam as a vaporizing gas; flowing the aqueous medium and the steam vapor through the column countercurrent to each other; contacting the aqueous medium with the steam vapor to vaporize the volatile components, and causing a negative pressure condition in the column; passing the steam vapor containing the volatile components into a vapor compressor, and compressing the vapor to raise its pressure to a point below atmospheric pressure; passing the pressurized steam vapor into a reboiler, to condense part of the vapor phase, and thereby form a liquid phase and a vapor phase; passing the liquid phase and vapor phase into a receiver vessel, to separate these phases, such that the liquid phase contains water and the condensed volatile components, and the vapor phase contains noncondensable components; passing the vapor phase from the receiver vessel into a vacuum pump to raise the pressure of this phase to atmospheric pressure; passing the liquid phase from the receiver vessel into a decanter vessel; passing the vapor phase from the vacuum pump into a condenser, to condense part of the vapor phase, and thereby form a liquid phase and a vapor phase; discharging the vapor phase from the condenser; passing the liquid phase from the condenser into the decanter vessel, to separate part of the condensed volatile components from the water phase; discharging the condensed volatile components from the decanter vessel; and discharging the water phase containing the remaining condensed volatile components from the decanter vessel. The drawing is a schematic illustration of a steam-stripping apparatus according to this invention. In the description that follows, the term "aqueous medium" means water containing a volatile component, or a mixture of volatile components. The term "volatile components," as used herein, means compounds having a relative volatility to water of greater than 1.0 when they are dissolved in water. Examples of these components include hydrocarbons, such as gasoline, benzene, toluene, or xylene; chlorinated solvents, such as trichloroethylene, trichloroethane, perchloroethylene, or methylene chloride, or mixtures thereof. Referring to the drawing, the steam-stripping apparatus illustrated therein includes a stripper column 10. In the practice of this invention, it is preferred to use a conventional packed tower, or tray tower of the type used for distillation or rectification. Water contaminated with the organic impurities described above is carried into column 10 through the inlet line 11 at the top of the column. A reboiler 12 is connected to the column by a steam line 13 near the bottom of the column. A water inlet line 14 is connected to a discharge line 15 from column 10 and to the bottom of reboiler 12. A vapor compressor 17 is installed in line 16 between the column and the reboiler. The reboiler 12 is a conventional shell and tube heat exchanger, or plate and frame heat exchanger which can also be considered as the first condenser; and the vapor compressor 17 is a rotary lobe compressor. Adjacent to reboiler 12 is a receiver vessel 18, which is connected to the reboiler through inlet line 19. An outlet line 20 connects the receiver vessel 18 to the suction side of a vacuum pump 21. The discharge side of vacuum pump 21 is connected to a condenser 22 by a discharge line 23. For the vacuum pump 21 it is preferred to use a single pass oil lubricated pump. The condenser 22 is a conventional shell and tube heat exchanger, or a plate and frame heat exchanger. In the shell and tube heat exchanger, cooling water enters the shell side of the heat exchanger through an inlet line 24, and is discharged through a discharge line 25. Condenser 22 is connected to a decanter vessel 26 by line 27. Receiver vessel 18 is connected to the inlet side of a pump 28 by line 29, and the outlet side of the pump is connected by line 30 to the decanter vessel 26. Condenser 22 also includes a vent line 31, for venting waste products to the atmosphere, or to a location of use. A discharge line 32 and a vent line 33, from decanter vessel 26, are connected to the vent line 31, to equalize the pressure in decanter 26 with that in condenser 22. Discharge line 32 is also connected to the inlet line 11 (for column 10) by a recycle line 34. The decanter vessel 26 also includes an outlet fitting 35 near the top of the vessel. In a typical operation of the steam-stripping apparatus of this invention, toluene is removed from a supply of groundwater to make the water suitable for human consumption. In the operation of this apparatus, the water contaminated with toluene is fed into the stripper column 10 through inlet line 11 at a rate of about 150,000 lb/hr (67,500 kg/h). The temperature of the incoming water is about 50°F (10°C) and the concentration of toluene dissolved in the water is about 10 ppm. The water flows downwardly in the column, as indicated by the downwardly directed arrows. At the same time, steam from the reboiler 12 flows upwardly and countercurrent to the flow of water in the column. When the upwardly moving steam makes contact with the downwardly moving water, the steam vaporizes the more volatile toluene and the vapor phase is carried overhead through outlet line 16 into the vapor compressor 17. During the stripping operation, the vacuum pump 21 creates a negative pressure condition, of 10 mm Hg absolute (1,300 Pa) in the stripper column 10 and that part of the system between the column and the compressor 17. As the vapor phase enters the compressor 17, it is compressed to raise the pressure to 25 to 50 mm Hg absolute (3325-6650 Pa). Increasing the pressure of the vapor phase allows the steam to condense at 78 to 100°F (25 to 38°C) when it enters the reboiler 12. When the vapor phase is condensed in reboiler 12, the resulting mixture contains inert materials, which are noncondensable, and it also contains toluene and water vapor. The noncondensable components consist mostly of air that is dissolved in the feed water or that the steam picks up as it moves through the stripper apparatus. The condensate (liquid phase) formed in the reboiler is also saturated with the toluene impurities. From reboiler 12 the condensate and the vapor phase are carried into receiver vessel 18. Inside the receiver vessel the two phases separate and the vapor phase is drawn into the suction side of vacuum pump 21. The pressure condition in the apparatus between the column and the vacuum pump is controlled by a pressure indicator controller 36 (PIC), which includes an automatically-operated valve 37 (pneumatic or electrical operation). In the vacuum pump 21, the pressure of the vapor phase is boosted up to 760 mm Hg absolute (101 x 10³ Pa), i.e., atmospheric pressure. From pump 21 the vapor phase is discharged into condenser 22 on the tube side, and cooling water enters the condenser on the shell side through inlet line 24 and leaves the condenser through discharge line 25. When the vapor phase contacts the cooler tubes, part of the vapor phase condenses to a liquid mixture of water and toluene, which is passed into decanter vessel 26 through line 27. The condensate also contains some of the oil lubricant which passes through the vacuum pump 21. That part of the vapor phase containing the noncondensable components is discharged from condenser 22 into the atmosphere, or to a location for use through vent line 31. Pump 28 draws the condensate phase out of the receiver vessel 18 through line 29 and pumps it into decanter vessel 26 through line 30. The liquid level in receiver 18 is controlled by a liquid indicator control 38 (LIC), which includes an automatically-operated valve 39. Inside the decanter 26 the toluene and the oil lubricant are separated from the water in the condensate phase and the toluene and oil are disposed of through the outlet fitting 35. The water, which is saturated with toluene, is discharged into recycle line 34 by line 32 and returned to the stripper column 10 through the inlet line 11. After the contaminated water passes down through the stripper column 10, the concentration of dissolved toluene is reduced to about 4 ppb, so that the water is essentially clean. Pump 40 withdraws part of the clean water from stripper column 10 through discharge line 15 and pumps it through line 41 to a location for use or storage. The liquid level in column 10 is regulated by a liquid indicator control 42 (LIC), which includes an automatically-operated valve 43. Some of the clean water in line 15 passes into reboiler 12 through line 14, where it is vaporized to provide the steam used in the stripping operation. The water that does not vaporize drops into line 15 and is pumped out of the column by pump 40.

Claims: A method for removing volatile components from an aqueous medium, which comprises: passing into a column an aqueous medium containing volatile components, and steam as a vaporizing gas; flowing the aqueous medium and the steam vapor through the column countercurrent to each other; contacting the aqueous medium with the steam vapor to vaporize the volatile components, and causing a negative pressure condition in the column; passing the steam vapor containing the volatile components into a vapor compressor, and compressing the vapor to raise its pressure to a point below atmospheric pressure; passing the pressurized steam vapor into a reboiler, to condense part of the vapor phase, and thereby form a liquid phase and a vapor phase; passing the liquid phase and vapor phase into a receiver vessel, to separate these phases, such that the liquid phase contains water and the condensed volatile components, and the vapor phase contains noncondensable components; passing the vapor phase from the receiver vessel into a vacuum pump to raise the pressure of this phase to atmospheric pressure; passing the liquid phase from the receiver vessel into a decanter vessel; passing the vapor phase from the vacuum pump into a condenser, to condense part of the vapor phase, and thereby form a liquid phase and a vapor phase; discharging the vapor phase from the condenser; passing the liquid phase from the condenser into the decanter vessel, to separate part of the condensed volatile components from the water phase; discharging the condensed volatile components from the decanter vessel; and discharging the water phase containing the remaining condensed volatile components from the decanter vessel. The method of Claim 1 in which the pressure of the steam in the column is about 10 mm Hg (1300 Pa). The method of Claim 2 in which the steam is compressed to raise its pressure to a range of from 25 to 50 mm Hg (3325 to 6650 Pa), and most of the water in the vapor is condensed at from 78 to 100°F (25 to 38°C). An apparatus for carrying out the method for removing volatile components from an aqueous medium according to Claim 1, which comprises: a countercurrent column (10) for receiving by an inlet line (11) an aqueous medium containing volatile components, and for receiving steam by a steam line (13) as a vaporizing gas ; a vapor compressor (17) in communication with the column (10) and connected by a line (16) with a reboiler (12) and able to raise a pressure below atmospheric pressure ; the reboiler (12) communicating by the steam line (13) with the column (10) and by an inlet line (19) with a receiver vessel (18) and able to condense part of the vapor phase; the liquid/vapor separating receiver vessel (18) communicating by lines (29,30) with a decanter vessel (26) and communicating by an outlet line (20) with a vacuum pump (21) able to raise the vapor pressure to atmospheric pressure and to cause a negative pressure condition in the column (10), and in the part of the system between the column (10) and the vacuum pump (21) ; the vacuum pump (21) communicating by a discharge line (23) with a condenser (22) ; the condenser (22) communicating by a line (27) with the decanter vessel (26) and having vapor phase discharging means (31) ; the decanter vessel (26) separating part of the condensed volatile components from the water phase and having discharging means (35) and communicating by lines (29,30) with the receiver vessel (18) and by a discharge line, a recycle line and the inlet line (32, 34,11) with the column (10).

Inventors: ROBBINS, LANNY A.
Applicants: THE DOW CHEMICAL COMPANY
Application year: 1987
Main classification: B01D3
IPC codes: B01D3, B01D1, C02F1