METHOD FOR REDUCING THE CLOUD POINT OF MATERIALS USING AN ULTRAFILTRATION SEPARATION PROCESS

Abstract: A method is disclosed for reducing the cloud point of materials comprising the steps of chilling the waxy oil so as to achieve submicron crystallization followed by ultrafiltration thereby effecting separa­ tion of a permeate or reduced cloud point from a retentate. Chilling to produce submicron crystalliza­ tion is to a temperature about 3°C or less above the cloud point of the material, preferably from 0.5 to 2°C above the cloud point of the material, most preferably between 1 to 2°C above the cloud point of the material. In general, any mixture, solution or melt containing components which crystallize at different temperatures can be separated by this procedure. The method is especially useful for reducing the cloud point of waxy hydrocarbon oils. The ultrafiltration membrane used will be one which is inert in and non-reactive with the components of the solution, mixture or melt, will not melt at the temperatures employed, which possess an effective pore size smaller than the submicron crystals formed upon chilling and which does not have a strong chemical affinity for the crystallized component of the feed mixture, solution or melt.

Description: Background of the Invention The separation of a mixture into its components has been performed by any number of techniques in the past. Such separation techniques include distillation,extraction, chilling/filtration, and more recently, reverse osmosis through a membrane which in general depends upon molecular type differences between components in a stream, perstraction and pervaporation which rely upon differences in molecular type and solubility in the membrane, and ultrafiltration which, in general, relies upon size differences to effect separation. Distillation is typically employed when the boiling points of the components in the solution, mixture or melt are sufficiently different so that a relatively clean separation of the components can be achieved. Materials of different molecular type but similar boiling point are not readily separable by distillation. Extraction effects the separation of components in a mixture by utilizing the differences in solubility of the components in some added, easily separated/recovered solvent. For example, aromatic hydrocarbons are separated from generally similar boiling point aliphatic hydrocarbon by taking advantage of the higher solubility of aromatics in an extraction solvent such as phenol, furfural or NMP. The extraction solvent selective separates the aromatics from the mixture; the aromatics themselves are easily separated from the extraction solvent. Crystallization from a solution or melt followed by filtration is a well established separation technique. It has found application in oil dewaxing. In general, the oil to be dewaxed with or without added dilution solvent is chilled to a temperature at which the wax crystallizes producing a wax/oil slurry. This slurry is subsequently filtered to yield a dewaxed oil. Thus, large wax crystal size is desirable for efficient filtration. This chilling is usually to a temperature well below the desired pour point of the oil and is called the filter temperature. Thus, to achieve a pour point of about -15°C, one would have to chill the oil to a temperature lower than -15°C. Reverse osmosis separates components by molecular type and employs applied pressure to overcome the osmotic pressure and selectively permeate one component through a non-porous membrane. Perstraction and pervaporation separate components on the bases of molecular type and rely upon the selective dissolution of one component in the membrane and migration of the component across the membrane in response to a concentration gradient. Ultrafiltration separates molecules on the basis of difference in molecule particle size. In the present invention it has been discovered that in effecting an ultrafiltration separation of components of different melting/solidification point temperature it is not necessary, and in fact, is detrimental to ultrafilterability to cool the mixture to a temperature at or below the phase transition temperature of the higher melting component to be separated. The phase transition temperature, for the purposes of this application and the appended claims, is defined as the temperature at which macroscopic crystals are formed, and operating temperature is that at which the component to be removed is just in the crystallite or submicro crystal form. Thus, operation must be at just above the phase transition temperature. Description of the Figures Figure 1 presents the interrelationship which exists between permeation rate, permeate cloud point and feed cloud point as a function of feed temperature. Figure 2 presents the effect of time on stream on the ultrafiltration of Bright Stock. Figure 3 present the effect of time on stream on the ultrafiltration of a 600N oil. Figure 4 presents the effect of pressure on the ultrafiltration of a 600N oil. Description of the Present Invention Mixtures, solutions or melts containing components which crystallize at different temperatures can be separated by chilling said solution, mixture or melt to a temperature 3°C or less above the liquid/solid phase transition temperature of the higher melting component, preferably between 0.5 to 2°C above the phase transition temperature, most preferably between 1 to 2°C above the phase transition temperature, and ultrafiltering the resulting chilled mixture, solution or melt and recovering a permeate containing less of the higher melting component than the feed and a retentate enriched in the higher melting component in comparison to the feed. The exact temperature or temperature range within the above recited elements would depend on the particular feed mixture solution or melt and on the pore size of the membrane used relative to the crystallite size. The phase transition temperature of the higher melting point component corresponds to the cloud point of the feed mixture, solution or melt. Ultrafiltration is conducted at a pressure between 172.4 and 1034.3 kPa (25 and 150 psig), preferably 172.4 to 689.6 kPa (25 to 100 psig), most preferably 344.8 to 517.2 kPa (50 to 75 psig). The ultrafiltration membrane has a pore size in the range 0.001µ to 0.5µm, preferably 0.001 to 0.1µm, most preferably 0.005 to 0.1 µm. In particular, waxy hydrocarbon oils can be dehazed by chilling the waxy oil to a temperature of about 3°C or less above the haze point, preferably between 0.5 to 2°C above the haze point, most preferably 1 to 2°C above the haze point to achieve submicron crystallization yielding particles on the order of 0.001 to 0.1 µm in size and ultrafiltering the chilled oil to produce an oil permeate of reduced cloud point resulting from the removal of higher melting point wax. The oil permeate produced by the process of the present invention has been discovered to possess a haze point 10 to 15°C lower than the haze point of the original waxy oil feed. Chilling to a temperature more than about 3°C above the haze point produces a permeate having a haze point only about 1°C lower than that of the original waxy oil feed while chilling to a temperature at or below the haze point produces a wax/oil slurry which fouls the membrane and reduces the permeation rate. Ultrafiltration is performed using a membrane of adequate pore size to permit free passage of the uncrystallized component while retarding passage of the crystallized component. Ultrafiltration membranes are typically described as those membranes possessing pore size in the 0.001 to 0.5 µ range, preferably in the 0.001 to 0.1 µm range. The membrane should also be inert/non-reactive with the components of the solution, mixture or melt, should not itself melt or disintegrate at the temperature to which it is exposed and at which it is employed. To avoid fouling, the membrane should not have a strong chemical affinity for the crystallized components of the mixture solution or melt. In the process of the present invention, it is preferred that the chilling of the feed be conducted under conditions which produce crystals which are spherical in shape. Crystals of such spherical morphology are less susceptible to fouling and are more easily rejected. Better overall membrane performance will be the result. Thus, when the process of the present invention is practiced on waxy hydrocarbons, the chilling to a temperature of 3°C or less above the cloud point is preferably performed under conditions of the DILCHILL® process. The DILCHILL® process and its various variations are described in detail in U.S. Patent 3,775,288, U.S. Patent 3,773,650, U.S. Patent 3,850,740. In general terms, in the DILCHILL dewaxing process, the waxy hydrocarbon, with or without the addition of a quantity of predilution solvent, is introduced into a multi-stage chilling zone. Chilled dewaxing solvent is introduced into a plurality or all of the stages in the multi-stage chilling zone. In each stage in which cold solvent is introduced a condition of intense agitation is maintained so that substantially instantaneous mixing occurs at each point of solvent introduction. The wax crystals produced under these conditions are spherical in nature. The chilling of the waxy hydrocarbon under the above described conditions of intense agitation can be to the temperature necessary to practice the present invention. Alternatively, dewaxing can be practiced first using any of the dewaxing procedures which exist, including the aforementioned DILCHILL® process, after which the dewaxed product can be dehazed by being subjected to high agitation chilling to a temperature 3°C or less above the dewaxed feed cloud point followed by ultrafiltration, to remove higher melting point wax and thereby further reduce the cloud point. In any of these embodiments, if the temperature to which the feed is chilled to achieve a feed temperature 3°C or less above the feed cloud point is high enough so that the feed viscosity is low enough, it may be possible to eliminate addition of dilution solvent in the dehazing process. When dealing with naphthenic oil which inherently possess low pour points, the major cause for product quality concern is haze. Because of the very fine nature of the wax crystals constituting haze in such oils, dehazing typically involves the deliberate addition to the oil of a significant quantity of wax so as to facilitate removal of the wax contributing to haze. By practice of the present invention, dehazing can be achieved much more simply and economically. The present invention is illustrated by the following non-limiting examples. Experimental Example 1 A waxy 600N raffinate having a cloud point of 49°C was used as the feed. An ultrafiltration membrane of polysulfone having a pore size of 0.07 µm was used. The oil was chilled, without solvent dilution and without use of DILCHILL® type chilling in the following manner. The pressure employed was 1034.3 kPa (150 psig). Cloud points of the feed samples and recovered permeates were determined. The results are present in Table 1. The feed was first heated to 60°C and circulated through the ultrafiltration cell across the surface of the ultrafiltration membrane. During this circulation the feed temperature was controlled by regulating the heat from heating tapes on the cell unit. The permeation rate and cloud point data were plotted as a function of the feed temperature in Figure 1. Results from run no. 3 (Table 1) at 51°C are not shown because this run was preceded by two others at lower temperature which fouled the membrane. Figure 1 indicates that, at feed temperatures 54°C and above, the permeate cloud point was constant and only 1°C lower than the feed; at 50°C and below, the cloud point difference was greater than 13°C. Thus, at 54°C the wax crystals were either not yet formed or still too small to be rejected. Further, there exists a transition temperature between 50 and 54°C where the wax crystals reach the size required to be rejected by the membrane. There was a decrease in permeation rate with decrease in feed temperature, as expected from the increase in viscosity. However, the permeation rate decrease in the vicinity of 50°C was accelerated which reflected the formation/growth of the wax crystals in this temperature range. The plot of Figure 1 can be used to determine the best operating point. Due to the low permeation rate and high fouling potential at 49°C, the cloud point, the best overall membrane performance would be at a temperature above the cloud point, preferably in the vicinity of 50°C. At this temperature compared to 49°C the permeate cloud point is only 1°C higher while the permeation rate is almost three times greater. Also, the smaller wax crystals at 50°C are less susceptible to membrane fouling. Example 2 Additional runs were conducted using the 600N oil raffinate of Example 1 but employing different membranes. Teflon, polysulfone, sulfonated polysulfone, polyimide, cellulose acetate and polycarbonate membranes were evaluated for use in the present process. A pressure of 1034.3 kPa (150 psig) was employed as was a circulation rate of 3.8 L/min (1 gallon per minute). The feed was cooled to approximately 1°C above the feed cloud point using the technique described in Example 1. The results are presented in Table 2. Example 3 The effect of temperature control on the cloud point reduction of Bright Stock Oil using ultrafiltration was investigated for three membrane samples, two Teflon samples and a polysulfone sample. The feed was cooled to approximately 1°C above the feed cloud point again using the technique described in Example 1. The results of this experiment are presented in Table 3. Example 4 This Example is presented to illustrate the differences in performance which were obtained between different samples of the same materials obtained from the same suppliers. In this case a 600N raffinate with a cloud point of 51°C was used at feed. The performance of 3 samples of 2 different membranes was tabulated and averaged. The results are presented in Table 4. Example 5 The effect of diluting the oil with solvent to produce a feed to the ultrafiltration process is as investigated. A volume of Bright Stock oil diluted with an equal volume of MIBK solvent was used as feed. The actual oil/solvent ratio of the feed in the ultrafiltration experiment could have been slightly different, however, due to evaporation during the preliminary steps. The cloud point of the feed mixture was measured as 59.8°C, compared to 63°C for the Bright Stock oil. A Teflon DSI/K-100 membrane was used for the ultrafiltration experiment at the feed conditions: approximately 1°C above the feed mixture cloud point, 344.8 kPa (50 psig) and 3.8 L/min (1 GPM). The retentate and permeate samples were stripped from the solvent to measure the oil cloud point. A 4.4°C cloud point reduction was observed with the permeate. On an oil basis, the permeation rate was 25 L/m²/day. Comparing with solvent dilution to without solvent dilution (Table 2) the cloud point reduction is less and the permeation rate of oil slightly higher. However, considering the application and membrane performance variation illustrated in Example 4 coupled with some differences in test conditions (i.e. , a lower operating pressure), these differences in membrane performance are not significant. Example 6 - Effect of Time and Pressure on Ultrafiltration of Oil Runs were conducted to determine the effect of time and pressure on the ultrafiltration of oil. The membrane used were samples of Teflon, 0.02µm from Gore/Goretex®. In Run A a Bright Stock oil having a cloud point of 63°C was permeated through the membrane at 100 psig, 1 GPM and 1°C above the cloud point for a period of about 5 hours. Permeate cloud point, retentate cloud point and permeation rates were determined at intervals during the run. The results are presented in Figure 2. It is seen that over time the permeation rate steeply decreases. Permeate cloud point exhibited continuous improvement (i.e. increased decrease) over time but appeared to be reaching a steady, bottomed out level. Retentate cloud point also appeared to decrease but this was attributed to the loss of wax to the unit walls. It is apparent that in unit operation periodic cleaning treatments like back washing or back pulsing may be needed to recover permeation rate. Similar results were obtained when a run was conducted using 600N oil as the feed. The results are presented in Figure 3. Permeation rate falls off dramatically. Permeate cloud point experiences a slight improvement over time but quickly reaches a bottomed out level. The effect of pressure on the ultrafiltration dewaxing of an oil was also investigated using a 600N oil as feed. Increasing the process pressure resulted in a decrease in permeation rate with no significant improvement in permeate cloud point. The results are plotted in Figure 4. From this it is clear that the process of the present invention is preferably conducted at low pressure.

Claims: A method for separating the components of mixtures, solutions or melts containing said components which crystallize at different temperatures by chilling said mixture, solution or melt to a temperature of 3°C or less above the liquid/solid phase transition temperature of the higher melting component and ultra-filtering the resulting chilled mixture, solution or melt to recover a permeate containing less of the higher melting component than the feed and a retentate enriched in the higher melting component in comparison to the feed. The method of claim 1 wherein the mixture, solution or melt is chilled to a temperature which is from 0.5 to 2°C above the phase transition temperature of the higher melting component in the mixture, solution or melt. The method of claim 2 wherein the mixture, solution or melt is chilled to a temperature which is from 1 to 2°C above the phase transition temperature of the higher melting component in the mixture, solution or melt. The method of any one of claims 1 to 3 wherein the mixture, solution or melt is of a waxy hydrocarbon oil. The method of any one of claims 1 to 4 wherein the ultra-filtering is conducted at a pressure between 172.4 to 1034.3 kPa (25 and 150 psig).

Inventors: LAFRENIERE, LUCIE YOLLANDE
Applicants: EXXON RESEARCH AND ENGINEERING COMPANY
Application year: 1989
Main classification: B01D61
IPC codes: B01D61, C10G73