WO1990015651A1

WO1990015651A1 - Method and apparatus for liquid-liquid separation

Info

Publication number: WO1990015651A1
Application number: PCT/US1990/003336
Authority: WO
Inventors: Roland E. Weber; John J. Pavlovich
Original assignee: Aurian Corp
Current assignee: Aurian Corp
Priority date: 1989-06-13
Filing date: 1990-06-13
Publication date: 1990-12-27
Anticipated expiration: 1991-12-13

Abstract

Small quantities of one or more liquids having a high boiling point dispersed with a large quantity of one or more immiscible liquids having a lower boiling point are separated in a rapid and efficient manner. A non-reactive gas (36) is heated to a temperature above the boiling point of at least one liquid but below that of at least one other liquid. The gas is combined (34) with the liquid mixture (37) to be separated, and the liquid-gas blend is introduced into a vessel (17) containing a pool (21) of at least one liquid having a boiling point higher than that of at least one other liquid, which pool has been heated to a temperature above the boiling point of at least one liquid but below its own boiling point. The resulting fractions of each component are then collected.

Description

METHOD AND APPARATUS FOR LIQUID-LIQUID SEPARATION

FIELD OF THE INVENTION

The present invention relates to separation of liquid mixtures, and in particular to a method and apparatus for separating small quantities of a liquid having a high boiling point dispersed within a large quantity of immiscible liquid having a lower boiling point.

BACKGROUND OF THE INVENTION

Efficient separation of emulsified liquid mixtures is necessary in a wide range of industrial environments. For example, the lubricants employed in equipment such as gas or air compressors combine with large volumes of water during operation, necessitating disposal of the entire liquid effluent. Because the effluent is considered hazardous waste due to the relatively small amount of lubricant present therein, the ability to separate the bulk liquid into hydrocarbon and liquid fractions permits restriction of expensive disposal measures to the hazardous material itself. In addition, pre-treatment of contaminated water prior to purification frequently benefits from initial separation of hydrocarbon fractions.

Reliable techniques for separating small quantities of hydrocarbons dispersed within a large quantity of immiscible liquid are few, and the apparatus which presently perform this function provide limited efficiency. Typical oil-water separators, for example, commonly employ grease-traps, baffles, skimmers and/or polishing filters to promote separation. Because they operate without phase changes, these systems require large quantities of energy, and typically cannot separate emulsified mixtures. Separators capable of operating on emulsions ordinarily utilize one or more emulsion-breaking chemicals, resulting in production of waste sludge that poses a solid-waste disposal problem.

The limitations associated with these oil-in-water systems derive from the respective physical properties of oil and water. The specific heat constant and heat of vaporization of water are much larger than those of typical organic compounds, including oils. Most oils boil at a higher temperature than that of water, however. Systems that rely on any form of distillation therefore require considerable energy input in order to raise the temperature of the mixture to 100 'C or above, because the overall specific heat of the mixture will be close to that of water; in addition, a further quantity of energy must be applied to induce boil-off of the water.

By contrast, systems designed to remove small quantities of water from large amounts of oil require significantly less energy; this is due to the lower specific heat of the mixture (which will be closer to that of the oil) , as well as to the smaller absolute amount of water that must be vaporized. Furthermore, such systems typically operate at a lower temperature than oil-in-water systems in order to minimize boil-off of organic compounds along with the water; such boil- off is rarely observed with oil-in-water systems.

Several systems have been developed for removing small quantities of water entrained within a heavy hydrocarbon, such as oil. U.S. Patent No. 4,789,461 describes a system wherein heavy crude oil containing water is sprayed onto a pool of heated oil that has been dehydrated; the water evaporated upon contact with the pool is evacuated through a vent. U.S. Patent No. 4,197,190 discloses a process for dehydrating tars and hydrocarbon oils containing water, wherein such material is heated prior to being sprayed into a holding vessel. Separate outlets are provided for removing the separate components, and the water component is also scrubbed prior to release. U.S. Patent No. 3,608,279 describes an apparatus for separating fatty acid distillates entrained in steam. The gaseous mixture is introduced into a vessel under temperature and vacuum pressure conditions that promote separation.

The foregoing systems, designed primarily for separating small quantities of water from large samples of oil, exhibit a number of disadvantages. These would be magnified if applied to an oil-in-water mixture. Because none provides any mechanism for intrasystem temperature maintenance, a large amount of energy must be introduced by external equipment to ensure that the temperature of the mixture at the point of separation remains at or above the distillation temperature; an oil-in-water system would require significantly greater energy input. All three of the foregoing systems also utilize spray nozzles to introduce the mixture into a separation vessel. Although spraying tends to break surface tension and thereby promote separation of the components, the utility of this technique is limited by the amount of mechanical shear that may be introduced. Furthermore, it has been found that use of spray nozzles at the temperatures necessary for efficient oil- in-water separation results in "searing," a phenomenon whereby the interior of the vessel becomes coated with solid material that acts as an unwanted insulation layer and interferes with heat transfer.

Possibly to increase mechanical shear, the '190 reference provides for high-velocity flow rates at the heat-transfer stage. However, the high specific heat of water prevents economical use of high rates of flow for oil-in-water mixtures.

SUMMARY OF THE INVENTION

In accordance with the present invention, a process and apparatus are provided for chemical-free and cost-effective separation of small quantities of a liquid having a relatively high boiling point (e.g. a hydrocarbon) dispersed within a large quantity of immiscible liquid having a lower boiling point (e.g. water) . The process may be sequentially repeated to separate liquid mixtures containing more than two components, and would be expected to operate successfully on virtually any two liquids (or mixtures) having a boiling point differential of at least 1 "C.

The first step of the process consists of heating a gas (such as air) that will not react with any of the components of the mixture to a temperature above the boiling point of at least one of the liquid components but below that of at least one other liquid component. The heated gas is then combined with the mixture to be separated in such a fashion as to produce a mist. This liquid-gas blend is introduced into a vessel containing a heated pool of at least one liquid component having a boiling point higher than that of at least another component. The temperature of the pool should be maintained at a temperature above the boiling point of at least one component but below its own boiling point. Upon introduction into this environment, the high-boiling-point component or components merge with the pool while the low- boiling-point component or components quickly vaporize. The resulting gaseous fraction of the high-boiling-point component or components may then be collected, and the pool periodically drained. (Hereinafter, the liquid mixture will be referred to as consisting of high-boiling-point and low-boiling-point components, it being understood that additional subcomponents may be present therein, and that repetition of the procedure may be necessary.) In the preferred embodiments, the liquid- gas blend is introduced into the vessel by means of a nebulizer configured to produce a mist, and the non-reactive gas is heated by drawing it through the heated pool prior to combination with the mixture.

Nebulizing the liquid mixture with a pre-heated gas has been found to greatly enhance mechanical shear. By breaking up the liquid into the smallest possible droplets consistent with acceptable throughput rates, maximum surface area is developed. This high surface area is exploited by use of the heated pool, a key feature not found in the prior art. This pool acts as a constant-temperature heat sink, effectively spreading the applied heat energy uniformly and thereby keeping the atmosphere of the vessel at a high temperature. When liquid droplets are expelled from the nebulizer, this high ambient temperature encourages immediate separation into liquid and gaseous components. The nebulizer directs the liquid-gas blend toward the pool to prevent stray droplets of the high-boiling- point liquid from being collected along with the gaseous component. Because the temperature of the pool is maintained above the boiling point of the low-boiling-point liquid, the pool traps droplets of the high-boiling-point liquid but resists the immiscible vapor. Another function of the pool is to prevent searing when the high-boiling-point liquid is a hydrocarbon. If a source of heat is applied directly to an outer wall of the vessel, as in the prior art techniques, the hydrocarbon fraction will adhere to the inner walls and undergo searing.

The method of the present invention may also be carried out by means of an apparatus, the operation and structure of which will be understood more readily from the following detailed description taken in conjunction with the accompanying drawings, in which:

BRIEF DESCRIPTION OF THE DRAWINGS

Fig. 1 is a schematic depiction of the apparatus;

Fig. 2 shows the construction of a preferred form of nebulizer; and

Fig. 3 presents a complete separation system based on the preferred embodiment of the present invention.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT

Referring first to Fig. 1, reference numeral 15 denotes generally a separator apparatus constructed in accordance with and embodying the invention. The separator apparatus 15 consists of a main chamber 17, which is surrounded by insulation layer 19. The chamber contains a pool 21 of the liquid from the mixture having the higher boiling point. Pool 21 is heated by heater 23, which lies below and preferably in contact with bottom plate 25. With this configuration, it is desirable to leave a small air gap 27 between heater 23 and the bottom of insulation layer 19; this both enhances the effect of the insulation as well as protecting it from the effects of direct contact with heater 23. The heater can be designed differently (one alternative form being a pad surrounding chamber 17 within insulation layer 19) so long as the heat is delivered evenly over pool 21. Localized heating has been found to adversely affect efficiency.

Heater 23 is adjustable, and must be set (manually or by means of a feedback temperature measurement circuit) to maintain the temperature of pool 21 so as to fall between the boiling points of the two liquids in the mixture. The temperature of pool 21 is monitored by thermometer probe 30, which couples to a visible readout or to a feedback circuit.

Mounted on top member 37 of chamber 17 is an injection point 36 for an inert gas, such as air, which is introduced under low pressure into heat-conductive (e.g. copper) tubing 38 and thereafter into nebulizer 34. Tubing 38 is oriented so as to pass through pool 21 and thereby allow the gas to be heated substantially to the temperature of pool 21. Tubing 38 may be coiled to facilitate sufficient contact time between the gas and pool 21, and the flow rate at which the gas is introduced must also be low enough to allow adequate heating. However, this flow rate cannot be so low as to retard nebulization. In one embodiment, constructed to accomplish separation of approximately .75 gallon/hour of liquid-liquid mixture, the flow rate was about .003 ft³/minute. An additional length of tubing 39 may be branched from tubing 38 in advance of nebulizer 34, with the open end thereof descending below the surface of pool 21. Although the addition of tubing 39 is not essential for proper operation, gas flowing therethrough promotes a slight turbulence in pool 21; this action assists heat transfer from heater 23 to pool 21, and from pool 21 to the atmosphere within chamber 17.

Also mounted on top member 37 of chamber 17 is an injection point 32 for the liquid-liquid mixture to be separated. The liquid-liquid mixture is immediately directed to nebulizer 34, which combines the mixture with the heated inert gas. Nebulizer 34 directs the resulting vapor downwardly through output bore 36, thereby facilitating contact between the vapor and pool 21.

The process of separation begins at nebulizer 34 and is completed when the vapor reaches pool 21. Droplets of the high-boiling-point liquid merge with pool 21, while the component with the lower boiling point evaporates. Upon evaporation, this latter component exits through duct discharge 40 and may, if desired, be condensed at outlet 42 for collection and possible reuse.

The level of pool 21, which increases in volume as liquid is drawn from the influent, is maintained at a relatively constant height by periodic or continuous removal through conduit 44 for collection at outlet 48. Conduit 44 may be equipped with anti-siphon device 45, which is essentially an opening to the atmosphere, to prevent the level of pool 21 from being drained to the level of outlet 50. Instead, the level of pool 21 is best controlled by the height of conduit 44, anti- siphon device 45 acting to maintain equivalence therebetween. Alternatively, removal may be accomplished using automatic withdrawal means responsive to the level of pool 21.

If composed of more than two components, the liquid collected at outlet 48 can be subject to further separation by another separator unit or by the same unit after the first phase of separation has been completed.

The preferred design for nebulizer 34 is shown in Figure 2. The nebulizer must produce droplets within an acceptable size range, and should also resist clogging. Optimal performance occurs when the droplets emerge in the form known in the art as a mist. Output as a spray, which consists of droplets larger than those in a mist, does not result in good separation. Conversely, output in the form of a fog, wherein droplets are smaller than those of a mist, permits escape of more than one component through the outlet intended for discharge of vapor.

Inlet conduit 60 admits the liquid-liquid mixture to be separated into hollow chamber 68, which is surrounded by corrosion-resistant body 62. One end of body 62 is coupled to the conduit containing the preheated gas. The other end of body 62 is capped by solid plug 66. The liquid-liquid mixture and preheated gas react within hollow chamber 68 and are ejected through output bore 36. The diameter of output bore 36. should be large enough to discourage blocking due to buildup of liquid residue, but small enough to restrict the droplet size of the ejected output to the proper range.

The dimensions of the nebulizer are largely dictated by the desired flow rate. Optimal performance has been found to occur when the liquid-liquid mixture is introduced at low pressure, approaching a gravity feed. The cross-sectional area of inlet conduit 60 must therefore be large enough to accommodate the desired flow rate at this low pressure. The distance 72 between inner edge 70 of solid plug 66 and the point of contact with inlet conduit 60 must be sufficiently long to permit the liquid-liquid mixture to react with the preheated gas before ejection; however, if the distance is too great, the reaction will be insufficient for nebulization to occur.

The foregoing criteria have been found to be satisfied where the interior diameter of inlet conduit 60 is approximately one-third the diameter of interior chamber 68 and approximately one-half the diameter of output bore 36. Representative dimensions for 3/4 gallon/hour system include a distance 72 of 13/16", an inlet conduit 60 diameter of 1/8", and an interior chamber 68 diameter of 1/4".

The present invention may be used in conjunction with other components to provide a complete and convenient system for separating emulsified liquid mixtures. Such a system is illustrated in Figure 3. Bulk quantities of the liquid mixture may be delivered to collection barrel 80 through inlet port 82. Barrel 80 is equipped with two electrodes 84 and 86, which are of different lengths. These serve as contacts in double-pole single-throw relay 88. Relay 88 controls the operation of pump 90 and air solenoid valve and air pump assembly 92. The pump of assembly 92 can be eliminated if the liquid mixture source produces a suitable gas output; compressors typically emit sufficient quantities of air at adequate pressures, permitting the air solenoid valve to be connected directly to the compressor's output.

When the liquid level in barrel 80 rises to the level of shorter electrode 84, the relay circuit is completed (assuming the liquid is conductive) , causing relay 88 to close and actuate pump 90 (which transmits the liquid mixture from barrel 80 to inlet 32 of separator unit 15) and assembly 92, which draws air and transmits it at low pressure to the nebulizer (not shown) , as hereinabove described. The differing lengths of electrodes 84 and 86 act as a delay timer, and the lengths must therefore be adjusted with respect to one another, the flow rate through port 82 and the depth reached by pump 90 within barrel 80 so as to allow the desired level of liquid to build up in barrel 80 prior to separation. Alternatively, pump 90 and assembly 92 can be actuated by means of a float switch disposed within barrel 80, which triggers at a pre-set liquid level.

Separator unit 15 is equipped with manual heat controller 92, which controls the output of heater 23 (depicted in Fig. 1) . Controller 92 is set by the user so as to maintain the temperature of the pool within separator unit 15 at a level falling between the boiling points of the two liquids to be separated. The user can monitor this temperature on the display of thermometer 94. Thermometer 94 may be any standard analog or digital model capable of providing accurate readings at the temperatures necessary for proper operation. Thermometer 94 communicates with the pool by means of probe 30 • (shown in Fig. 1) .

Separator unit 15 is placed on collection barrel 96, which may be removed and replaced as necessary.

The terms and expressions which have been employed are used as terms of description and not of limitation, and there is no intention, in the use of such terms and expressions, of excluding any equivalents of the features shown and described or portions thereof, but it is recognized that various modifications are possible within the scope of the invention claimed.

Claims

1. A method of separating a mixture of at least two liquid components, a first component having a higher boiling point than a second component, comprising the steps of:

a. providing a vessel accommodating a pool containing at least a first component of said mixture having a boiling point higher than at least a second component of said mixture; b. heating the pool to a temperature above the boiling point of the second component of the mixture; c. in a nebulizer, combining the mixture with a non- reactive gas that has been heated to a temperature above the boiling point of the second component to produce a nebulized liquid-gas blend; d. introducing the nebulized liquid-gas blend into the vessel as a mist, whereby the first component merges into the pool and the second component vaporizes; and e. withdrawing the separated components from the vessel.

2. The method of claim 1 wherein said nebulizer comprises:

a. gas input means; b. liquid input means; c. an interior chamber located adjacent to said gas and liquid input means; and d. an output bore,

3. The method of claim 2, wherein said gas is air.

4. The method of claim 2, wherein one of the liquids is water.

5. The method of claim 2, wherein one of the liquids is a hydrocarbon.

6. The method of claim 4, wherein the other liquid is an oil.

7. The method of claim 2 wherein the mixture and the gas are introduced into the nebulizer at substantially right angles to one another.

8. The method of claim 2 wherein the cross-sectional area of the gas input means is approximately one-third the cross- sectional area of the interior chamber and approximately one- half the cross-sectional area of the output bore.

9. The method of claim 2 wherein the hollow chamber extends a sufficient axial distance from the gas and liquid input means to facilitate nebulization.

10. The method of claim 2 wherein the cross-sectional area of said output bore is sufficient to retard clogging.

11. The method of claim 1 further comprising repeating steps (a) through (e) for the liquid remaining in the pool if said remaining liquid comprises more than one component.

12. An apparatus capable of separating a mixture of at least two liquid components having different boiling points, comprising:

a. a chamber for accommodating a pool comprising at least a first component of said mixture having a boiling point higher than at least a second component of said mixture; b. means for heating said chamber to a temperature above the boiling point of at least one of the components to be separated; c. means for heating a non-reactive gas to a temperature above the boiling point of at least one component; d. a nebulizer for combining the heated gas with said mixture to produce a nebulized liquid-gas blend; e. means for introducing the nebulized liquid-gas blend into the chamber as a mist, whereby the first component merges into the pool and the second component vaporizes; and f. means for withdrawing the separated components from the chamber.

13. The apparatus of claim 12 wherein said nebulizer comprises

14. The apparatus of claim 13, wherein said gas is air.

15. The apparatus of claim 13, wherein one of the liquids is water.

16. The apparatus of claim 13, wherein one of the liquids is a hydrocarbon.

17. The apparatus of claim 13, wherein the other liquid is an oil.

18. The apparatus of claim 13, further comprising means for causing turbulence within said pool.

19. The apparatus of claim 13 wherein the mixture and the gas are introduced into the nebulizer at substantially right angles to one another.

20. The apparatus of claim 13 wherein the cross-sectional area of the gas input means is approximately one-third the cross- sectional area of the interior chamber and approximately one- half the cross-sectional area of the output bore.

21. The apparatus of claim 13 wherein the hollow chamber extends a sufficient axial distance from the gas and liquid input means to facilitate nebulization.

22. The apparatus of claim 13 wherein the cross-sectional area of said output bore is sufficient to retard clogging.

23. A nebulizer comprising:

wherein the relationship between the cross-sectional areas of said gas input means, said interior chamber and said output bore are such that the liquid will be ejected through said output bore as a mist.

24. The nebulizer of claim 23 wherein the cross-sectional area of the gas input means is approximately one-third the cross- sectional area of the interior chamber and approximately one- half the cross-sectional area of the output bore.

25. The nebulizer of claim 23 wherein the hollow chamber extends a sufficient axial distance from the gas and liquid input means to facilitate nebulization.

26. The nebulizer of claim 23 wherein the cross-sectional area of said output bore is sufficient to retard clogging.