US2757126A - Continuous countercurrent crystallization process - Google Patents

Description

July 31, 1956 R. P. CAHN 2,757,126

CONTINUOUS COUNTERCURRENT CRYSTALLIZATION PROCESS Filed June 30, 1953 LIQUID "NH RECYCLE IOVlll6 BED of III PARTICLES ROBERT F. CAHN INVENTOR BY'U). Q 7 ATTORNEY 2,757,126 Patented July 31, 1956 'CONTINUOUS COUNTERCURRENT CRYSTAL- LIZATION PROCESS Elizabeth, N. J., assignor to Esso Regmeermg Company, a corporation of Robert P. Cahn, search and E Delaware Application June 30, 1953, Serial No. 365,187 4 Claims. (Cl. 196-18) The present invention is concerned with an improved process for the crystallization of constituents and for the removal of the formed crystals from the mixture. The invention is particularly directed toward an improved continuous dewaxing process wherein wax crystals are continuously formed and removed from a waxy distillate containing the same. In accordance with the present process the dewaxing operation is conducted with the aid of heavy solids, preferably having a low melting point, which solids countercurrently contact the upflowing distillate.

In the refining of hydrocarbon oils such as petroleum oils, it is known to segregate paraflin waxes from so-called parafiin distillates, waxy lubes and the like. The segregation of these waxes is secured by a number of processes. For example, it is known to chill the selected wax containing fraction in order to secure crystallization of the wax and to remove thewax crystals from the oil by filtering, centrifuging and the like. use various dewaxing solvents such as liquid normally gaseous hydrocarbons, such as propane, as well as other solvents, such as methylethyl ketone and the like. It is also known to utilize in dewaxing operations solvent mixtures wherein one solvent comprises a wax precipitating solvent while the other comprises a solvent having a high solubility for oil. A solvent mixture of this character, for example, comprises 40% by volume of toluene and 60% by volume of methylethyl ketone.

The wax segregated from the hydrocarbon oil, usually termed, slack wax, contains from about 10% to 40% of oil. The slack wax is refined usually by conventional sweating to produce crude scale wax in a manner to reduce the oil content to less than about 5% by weight. The slack wax may be distilled to obtain the desired boiling range wax prior to sweating, if desired. This crude scale wax generally has an oil content of about 2% to 3% by weight. In order to remove this oil from the crude scale wax to produce a refined wax having an oil content below about .5 usually below about 3%, various procedures have been proposed and employed.

While these conventional batch processes are entirely satisfactory the equipment required is relatively expensive. Furthermore, the preferred operating conditions of these processes are diflicult to control. The processes also require expensive refrigeration, filtering and heat exchange equipment. In accordance with the present process, these disadvantages are overcome by utilizing a countercurrent crystallization tower wherein a liquid is introduced at the top of the tower. This liquid crystallizes as it countercurrently contacts up-fiowing liquid. The process of the present invention may be more fully understood by reference to the drawing illustrating one embodiment of the same.

Referring specifically to the drawing, a heated feed oil, which for the purpose of description is assumed to be a waxy distillate, is introduced into tower by means of feed line 1. If desired some solvent may be introduced by means of line 25. This oil is cooled in cooling zone 2 to a temperature in the range from about 75 F. to 150 It is also known to F., preferably to a temperature in the range from 85 F. to 100 F. Aliquid substance, as for example, magnesium nitrate hexahydrate is introduced into the top of countercurrent treating zone 10. The amount of magnesium nitrate hexahydrate added is in the range from about 40 to 200 volumes of magnesium nitrate hexahydrate per 100 volumes of waxy feed distillate. Preferred concentrations are in the range from about 50 to 100 volumes per 100 volumes of waxy feed. The magnesium nitrate hexahydrate is injected through appropriate spray nozzles 4 into the oil phase which is kept below the melting point of magnesium nitrate hexahydrate by a water coolant or equivalent pumparound stream. This pumparound stream is cooled in cooling zone 5 and returned to the top of the tower by means of line 6. The temperature at the top of the tower is in the range of about F. to 200 F.,

preferably in the range from about F. to F. The magnesium nitrate hexahydrate will thus solidify at the top of the tower into small particles which settle easily out of the liquid phase .due to the wide density difference and will begin descending downwardly through the oil phase. As these particles settle, they will countercurrently contact progressively cooler liquid rising through the column. In accordance with the present process the coldest point is reached at the mid-pumparound level. At this point upflowing liquid is withdrawn from tower 10 by means of line 7, passed through a refrigeration zone 8 and returned to the system by means of line 9. The temperature at which the oil is removed is about 0 F. to 40 F., and the temperature at which the oil is returned is about 10 F. to 50 F.

Since very little, if any, wax is present in the upflowing liquid at this point, exchanger fouling due to the wax will not occur. On the other hand, autorefrigeration may be used in the pumparound section if a diluent such as propane is used. As the particles flow downwardly in the area below the pumparound section, the magnesium nitrate hexahydrate particles act as a coolant and crystallization nuclei for the feed material entering the column as illustrated. In the section between the pumparound section and the feed point, the magnesium nitrate hexahydrate particles absorb heat from the liquid and the solidified wax. The liquid feed is cooled from the feed temperature to the pumparound temperature. The feed temperature is in the range from about 75 F. to 150 F., while the pumparound temperature is in the range from about 0 to 50 F. Furthermore, the wax crystallizes out on the magnesium nitrate hexahydrate particles. In this'zone the wax undergoes a certain amount of sweating between the pumparound level and the feed level where the concentration of wax in the liquid corresponds to the feed composition.

In the area of the column below the point of feed introduction, it is preferred to introduce a solvent by means In accordance with the present invention the magnesium,

nitrate hexahydrate crystals and the wax pass into a heating zone 12 heated by heating coils 13 or equivalent means. This area is maintained at a temperature preferably in the range of about 210 F. to 250 F. Under these conditions the magnesium nitrate hexahydrate and wax liquefy and form two phases. The Wax phase plus solvent is removed by means of line 14, while the liquid magnesium nitrate hexahydrate phase is removed by means of line 3 and preferably recycled to the top of the tower. The wax phase is passed to a distillation unit 15 wherein temperature and pressure conditions are adjusted to segregate the solvent which is removed overhead by means of line 11 and preferably recycled to the system. The wax phaseis removedby-means of-line;16 and further handled and refined as desired. The dewaxed oil is removed by means of line 20 and passed to distillation zone 21. Temperature and pressure conditions 1 in zone '21 are "adapted to remove-over'headby means ofline '22 the solvent and to segregate by-means of line 23 the dewaxed oil.

The present inventionemploys a solid carrier to act as a nucleus for the wax. in order to avoid filtration and eliminate solid "handling, this solid is preferably a low melting inorganic salt which is immiscible with the organic substance processed in the unit.

Satisfactory salts are for example:

Spec. '1\

A particularly desirable salt comprises -magnesium nitrate hexahydrate, the important properties ,of which are as follows:

Formula Mg. ,(NO3)2.6H20 Melting point 100 C.

Heat of fusion 38.2-'cal./g. Specific heat 0.8'87-cal./g./ "C. Density 1.464 g./cc.

Its reasonably high density and very high specific heat, low -heat.of fusionand .a melting point in the neighborhood of wax, make it well suited for the particular application.

While the invention has been described with respect to a dewaxing operation, it is to be understood that it may be applied to other crystallization processes andseparations by ureacornplex formation andthe like.

What is claimed is:

-l.'-Improved crystallization process which comprises introducing ,afeed liquid containing a constituent-recoverable by crystallization at an intermediate point of ,a vertical countercurrent treating tower, introducing .into the top .ofsaid .tower, in a plurality of small streams, .a liquid law melting inorganic salt immiscible withthe feed which has a greater specific gravity in its liquid andsolid phases than that of said feed liquid and which has a freezingpoint lower than the temperature at which said constituent of said feed liquid crystallizes, maintaining the top of said tower at a temperature below the freezing point of said salt, whereby said salt is frozen to "small solid particles which settle downwardly through said tower countercurrent to and in contact with the upflowing liquid feed in an intermediate portion of said tower, whereby said feed liquid is cooled and said constituent crystallizes on said solid particles, withdrawing the solid particles on which said constituent has been crystallized from the bottom of said tower, heating said particles to a temperature at which two immiscible liquid phases form, separating the respective liquid phases and recovering and recycling said liquid salt to the top of said tower.

2. Improved process for the removal'of waxy constituents from a waxy feed oil which comprises introducing said waxy oil at an intermediate point of a vertical countercurrent treating tower, introducing into the top of said tower, in a plurality of fine streams, a liquid low melting inorganic salt immiscible with the feed which has a greater specific gravity in its liquid and solid forms than said waxy oil and which has a freezing point lower than the temperature at which said waxy constituents are crystallized from said waxy oil, maintaining the top of said tower at a temperature below the melting point of said salt, whereby said salt is frozen to small solid particles which settle downwardly through said tower countercurrent to and in contact with the upflowing waxy oil in an intermediate portion of said tower, crystallizing substantially all of said waxy constituents of said oil onto said solid particles in said intermediate zone, withdrawing dewaxed oil from said tower, cooling said dewaxed oil 'to a temperature below the freezing point of said salt outside of said tower, re-introducing the cool dewaxed oil into the top of said tower, whereby said liquid salt is solidified, Withdrawing solid particles on which wax has been crystallized from the bottom of said tower, heating said particles above the melting point of said wax to form separate phases of wax and liquid salt, separating the respective phases, and recycling said liquid salt to the top of said tower.

3. Process as defined by claim 2 wherein the salt comprises magnesium nitrate hexahydrate, wherein the temperature of the waxy oil is'in the range from about to 150 F. and wherein the temperature at the top of the tower is in the range from about to 200 F.

4. Process vas defined by claim 3 wherein propane is introduced into said tower at a point below the point of injection of the feed stream.

ReferencesCitedin the file :of this patent UNITED STATES PATENTS 1,779,287 Pfafi et val. Oct. 21, 1930 2,062,355 Ellis Dec. 1, 1936 2,141,361 Pilat et al Dec. 27, 1938 2,322,438 Henryet .al June 22, 1943 2,622,114 Carney Dec. 16, 1952

Claims (1)

1. IMPROVED CRYSTALLIZATION PROCESS WHICH COMPRISES INTRODUCING A FEED LIQUID CONTAINING A CONSTITUENT RECOVERABLE BY CRYSTALLIZATION AT AN INTERMEDIATE POINT OF A VERTICAL COUNTERCURRENT TREATING TOWER, INTRODUCING INTO THE TOP OF SAID TOWER, IN A PLURALITY OF SMALL STREAMS, A LIQUID LAW MELTING INORGANIC SALT IMMISCIBLE WITH THE FEED WHICH HAS A GREATER SPECIFIC GRAVITY IN ITS LIQUID AND SOLID PHASES THAN THAT OF SAID FEED LIQUID AND WHICH HAS A FREEZING POINT LOWER THAN THE TEMPERATURE AT WHICH HAS SAID CONSTITUENT OF SAID FEED LIQUID CRYSTALLIZES, MAINTAINING THE TOP OF SAID TOWER AT A TEMPERATURE BELOW THE FREEZING POINT OF SAID SALT, WHEREBY SAID SALT IS FROZEN TO SMALL SOLID PARTICLES WHICH SETTLE DOWNWARDLY THROUGH SAID TOWER COUNTERCURRENT TO AND IN CONTACT WITH THE UPFLOWING LIQUID FEED IN AN INTERMEDIATE PORTION OF SAID TOWER, WHEREBY SAID FEED LIQUID IS COOLED AND SAID CONSTITUENT CRYSTALLIZES ON SAID SOLID PARTICLES, WITHDRAWING THE SOLID PARTICLES ON WHICH SAID CONSTITUENT HAS BEEN CRYSTALLIZED FROM THE BOTTOM OF SAID TOWER, HEATING SAID PARTICLES TO A TEMPERATURE AT WHICH TWO IMMISCIBLE LIQUID PHASES FORM, SEPARATING THE RESPECTIVE LIQUID PHASES AND RECOVERING AND RECYCLING SAID LIQUID SALT TO THE TOP OF SAID TOWER.

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