US2006011A - Continuous propane dewaxing - Google Patents

Description

June 25, 1935. H, v ATwELL 2,006,011

CONTINUOUS PROPANE DEWAXING Filed April 24, 1933 2 SheetsSheet l INVENTOR Jfarold Vfltwell ATTORNEY June 25, 1935. v v ATWELL 2,006,011

CONTINUOUS PROPANE DEWAXING Filed April 24, 1933 2 Sheets-Sheet 2 W/kw INVENTOR JiQTOld VfitWG/U/ 201% if? ATTORNEY Patented June 25, 1935 UNITED STATES CONTINUOUS PROPANE DEWAXING Harold v. Atwell, White Plains, N. Y.,' assignor to Standard Oil Company, Chicago, 111., a corporation of Indiana Application April 24, 1933, Serial No. 667,564

7 Claims.

This invention relates to the separation of wax from oil by the use of a liquefied normally gaseous diluent, such as propane, and it pertains more particularly to a method and means whereby shock chilling may be avoided in a continuous refrigeration system.

The object of my invention is to avoid the use of batch chillers and at the same time to avoid shock chilling which has been inevitably the result of all continuous chillers heretofore proposed. In practicing the dewaxing process a wax-bearing oil must be dissolved in the liquefled gas at relatively high temperature under superatmospherlc pressure and the pressure is then reduced so that the chilling will be effected by utilizing the heat of vaporization of the liquefied diluent. 'If the oil-wax solution in liquefied diluent is passed from a high pressure-zone to a low pressure zone through a valve or restricted orifice the instantaneous drop in temperature causes the precipitation of wax which is so finely divided or is of such physical character that it cannot be readily filtered, settled or centrifuged; it causes the wax to separate in such fine particles that the waxy oil becomes a mushy or unctuous mass of substantially uniform consistency instead of well crystallized wax particles suspended in diluted oil. The object of my invention is to provide a continuous system of chilling the diluted oil by auto refrigeration (vaporization of diluent) without shock chilling. In practicing my invention I employ an insulated pipe coil for the adiabatic expansion of the refrigerating medium. By proper choice of length and cross section of such a pipe coil, the rate of cooling a mixture pumped through it can be fixed at any desired value for any given rate of flow. Thus, the mixture may be passed continuously through an elongated pipe coil of gradually increasing diameter so that the temperature of the liquid flowing through the coil will be gradually lowered at the rate of about one to eight degrees per minute, preferably about three or four degrees per minute, without at any time subject ing the liquid to shock chilling. Instead of using an elongated pipe of gradually increasing cross section I may employ a cooler in which the pipe is successively manifolded to increase the effective cross sectional area.

I may obtain the pressure drop necessary to effect the desired cooling by merely using an elongated pipe of relatively small diameter and extremely long length so that the decrease in pressure is due to the pressure drop which is, in; turn, due to a friction of flow through the pipe. The temperature of liquid propane or any other normally gaseous diluent will necessarily fall with a fall in pressure; in other words, if the pressure at the discharge end of the elongated pipe is reduced to atmospheric, then the temperature of the propane mixture must be about 40 F.- Suflicient propane must be employed in this case toabsorb by vaporization not only the frictional heat developed, but to lower the temperature of the mixture the desired 10 amount. In such a system I prefer to trap out propane vapor at intervals to prevent unduly high velocities in the cooling pipe, but it should be emphasized that the cooling is effected in the pipes instead of in the traps, there being no restricted orifices or valves for causing the pressure in the traps to be materially different from that in the pipes immediately before or after said traps.

The invention will be more clearly understood 0 from the following detailed description of a preferred embodiment taken in connection with the accompanying drawings which form a part of the specification, and in which- Figure 1 is a diagrammatic plan of my improved system; I

Figure 2 is a detail illustrating a manifolded pipe cooler;

Figure 3 is a modification of the pipe cooler of gradually increased cross sectional areas along the length thereof.

The invention will be described as applied to the dewaxing of S. A. E. 50 midcontinent distillate untreated lubricating oil stock. It should be understood, however, that the invention is equally applicable to the dewaxing of any waxbearing oil whether it is an overhead stock or a residuum, whether it is acid treated or untreated, and whether or not it has been treated with selective solvents, such as dichlorethyl ether, nitrobenzene, chlor-aniline, sulfur dioxide and benzol, etc. As a diluent, I will describe the use of propane which contains slight amounts of isobutane, butane, ethane and unsaturated hydrocarbons of about the same boiling range. The invention is equally applicable to the use of any normally gaseous diluent, and it is specially applicable to the use of normally gaseous ethers, halogenated compounds such as methyl chloride, and to slightly miscible or immiscible refrigerants, such as ammonia and carbon dioxide.

About six to ten volumes of propane from storage tank IU are passed through line H to mixer l2, wherein it is intimately mixed with one volume of wax-bearing oil from pipe IS. The mixture should be at such a temperature that both the oil and the wax will be completely dissolved in propane and, if necessary, an auxiliary closed steam coil or other heating means may be inserted in the mixer or in the propane and/or oil lines. Assuming that the mixture is in complete solution at F., the gage pressure may be about 155 pounds, and the solution at this temperature and pressure is introduced by pipe |4 into continuous pipe cooler I5. This pipe may be extremely long and of rather small cross section so that as the mixture flows through the pipe the pressure of the liquid will be gradually decreased. The use of such a system, however, will necessarily require additional cooling to compensate for the friction of fiow through the pipe, and the liquids at the discharge end of the pipe will have a high velocity which may be detrimental to the physical structure of the wax crystals. I may balance the frictional heat by circulating a cooling fluid around the pipes at a temperature sufiicient to counteract the frictional heat but insufficient to effect any appreciable transfer of heat from the liquid in the pipes. If heat is abstracted from the waxy oil solution through the pipes there will be a tendency for the wax to adhere to the pipes and there will likewise be a tendency for the formation of an undesirable crystal structure, and the outstanding feature of my invention is the continuous cooling of the waxy mixture by virtue of the pressure drop within the pipe and not by virtue of the temperature differential between the inside and outside of the pipe.

As above stated, the use of an elongated pipe of small cross sectional area to obtain the pressure drop is open to some objections and I there fore prefer to employ continuous pipe chillers of the type illustrated in Figures 2 and 3. In these modifications the effective cross sectional area of the pipe is gradually increased so that adiabatic expansion of the propane is permitted without appreciably changing the velocity of flow through the pipe. By regulating the flow through the pipe the amount of propane vaporized is regulated which, in turn, regulates the rate of cool ing and in this way the temperature of the mixture may be gradually reduced at about three or four degrees per minute as the solution fiows through the pipe. In Figure 2 pipe I5 is connected by manifold Hi to pipes l1 and I8 which is, in turn, connected by manifold l9 to pipes 20, 2|, 22, etc. If the pressure of mixture entering pipe I5 is 155 pounds gage it may be 135 gage at manifold I6, 115 pounds gage at manifold I9, pounds gage at manifold 23, 80 pounds gage at manifold 24 and 70 pounds gage at manifold 25. Alternatively, as disclosed in Figure 3, a single elongated pipe |5' of gradually increased cross sectional area, may be employed so that as the liquid flows along the pipe more and more propane will be vaporized, the temperature will be gradually lowered, and the vaporized propane will fiow along through the enlarged part of the pipe. In any case, the pressure of the'propane-oil-wax mixture is reduced in pipe chiller l5 to about 10 pounds gage, at which pressure the mixture will be at a temperature of about 40 F. By this time a. relatively large volume of propane gas will have accumulated and I therefore introduce the mixture into a gas separation trap 26, in which the gases are withdrawn through pipe 21 by means of compressor 28, the compressor being driven by motor 29 which is regulated by the pressure in trap 26 so that a pressure of about 16 pounds gage is maintained in this gas separation trap.

The liquid propane-oil-wax mixture is then passed through another pipe cooler 30, which is identical in structure and in operation to cooler I5. In this case the pressure drop from one end of the pipe cooler to the other may be about 42 pounds gage so that the temperature of propane-oil-wax mixture leaving the pipe cooler will be about 0 F. Here again it is desirable to remove accumulated propane vaporized and I therefore pass the mixture into a second gas liquid separator trap 3|, the propane being withdrawn through pipe 32 by means of compressor 33 driven by motor 34, which is regulated by the pressure in trap 3|. The cold slurry in the bottom of trap 3| is then passed through a third continuous pipe chiller 35 which is designed along the same lines and operates on the same principles as the chillers l5 and 30. In pipe chiller 35 the pressure is dropped from 28 pounds gage to about four pounds gage so that at the discharge of this pipe the propane oil-wax mixture will be at a temperature of about 40 F. Here again the mixture is passed into a trap 36, which serves the double function of a gas separatorand a storage tank for feeding the filter press. Propane is removed from the top of tank 36 through pipe 31 by means of compressor 38 driven by motor 39, the motor being regulated by the pressure in trap 36. Liquid slurry from the base of tank 36is forced by pump 40 into a continuous rotary filter press 4|, the filtrate being continuously accumulated in tank 42 and the wax being continuously forced by screw conveyor or pump 43 into drum 44. It should be understood that instead ofusing a continuous filter I may use a series of batch filters and direct the slurry consecutively from one filter to another while the remaining filters are being washed, discharged or repaired.

The filtrate from tank 42 is forced by pump 45 to stripper tower 46 which is heated by steam coil 41, the propane being driven out through pipe 48 and the finished oil being withdrawn through pipe 49 to atmospheric or vacuum stripper 50 which may likewise be provided with a suitable heating element 5|. The finished oil is withdrawn by pipe 52 and pump 53 to a suitable storage.

In a similar manner propane is removed from Wax in drum 44 by means of steam heater 54, the propane being removed through pipe 55 and the hot wax being withdrawn through pipe 56 and flashed through reducing valve 51 into low pressure stripper 58. The finished wax from this stripper is withdrawn through pipe 59 and pump 66 to suitable storage. Propane from strippers 50 and 58 are removed by pipes 6| and 62 respectively, picked up by compressor 63 and introduced, together with propane, from pipes 48 and 55 into pipes 64 and 65, thence into condenser 66 in which propane is liquefied and from which it is returned to storage tank l0. Compressors 28, 33 and 38 likewise discharge propane into pipe 65.

It will be noted that each of these compressors operates continuously with a uniform load and a constant pressure differential. This is an important improvement of my continuous process over the batch cooling process heretofore employed because in the batch processes the compressors must operate over a wide pressure range and must be adjusted with the decreasing temperatures to make up for the increase in the volume of vapors which must be removed. Such variable compressors must necessarily be inefiicient over the greater part of the cooling range,

they require unduly high initial costs for equipment and unduly high operating expenses due to greater power consumed. By operating continuously with uniform load and uniform pressure drop compressors 28, 33 and 35 may be relatively small and they will require a minimum amount of power and attention.

In practicing my invention as above described, I found that while allowing about 15 minutes for passage through each cooling pipe, (a total time of transfer of about 45 minutes) I obtained a final wax slurry in which the proportion of oil to propane was about one to three. I obtained a filter rate of about .4 gallons of undiluted heavy oil per square foot per hour and a cold test on the finished oil of about zero to five degrees Fahrenheit. These results, however, were obtained by the use of pipe coolers of uniform effective cross sectional area, and it is apparent that the modifications of the chiller shown in Figures 2 and 3 would make it possible to employ more rapid chilling rates and at the same time obtain higher filter rates and lower cold tests.

In the accompanying drawings I have shown three separate traps for trapping off propane vapors. It should be understood that I may dis pense entirely with these intermediate propane traps or I may greatly increase the number thereof. The important thing is to obtain the desired wax crystal formation by the gradual reduction of temperature in a pipe cooler due to adiabatic expansion of a portion of the diluent due to the pressure drop in the pipe. It should be particularly noted that at no time is the propane oil-wax mixture passed through a valve or restricted orifice to a zone of materially lower pressure, nor is it subjected to unduly turbulent agitation from throttling, etc. I prefer to insulate the pipe coolers and prevent any heat exchange between them and external substances. I may, however, as hereinabove pointed out, enclose the pipes in housings or heat exchangers so that I may cool the outside of the pipes sufficiently to compensate for the heat of friction generated by the flow of fluids in the pipe. For this purpose I prefer to employ the cold propane vapors from the succeeding vapor trap, passing them around the coil countercurrently to the flow of solution within.

As examples of other diluents equivalent to propane I may employ butane, butylene propylene, dimethyl ether, dichlor difluor methane, methyl chloride, methyl formate and the like. Although I do not use heat exchangers for chilling the waxy 'oil mixture and solidifying the wax, it should be clearly understood that heat exchangers are employed for utilizing the available refrigeration of the system; for instance, a heat exchanger may be employed in the line between pump 45 and stripping tower l6, and the refrigeration value of the cold filtrate may be used to cool wash propane (fresh propane used for washing filter cake). If desired, this heat exchanger may be employed to condense and/or cool propane vapors from any of the compressors hereinabove mentioned, thereby effecting a considerable saving in the required amount of condenser water.

One of the outstanding advantages of a continuous process of this type is the possibility of employing countercurrent heat exchangers in various parts of the system, since the temperatures at any one point are substantially constant at all times.

While I have described a preferred example of my invention it should be understood that I do not limit myself to the details hereinabove set forth except as defined by the following claims:

I claim:

1. In the method of dewaxing oils by means of liquefied normally gaseous diluents, the step of continuously chilling a diluted oil-wax mixture without passing said mixture through a restricted opening between a zone of high pressure and a zone of low pressure, which step comprises continuously passing said mixture through an elongated conduit whereby the rate of vaporization of propane is controlled by the gradual pressure drop incident to the flow of said mixture through said conduit.

2. The method of obtaining readily filterable wax crystals from a waxy lubricating oil stock which comprises dissolving said stock in an amount of liquefied normally gaseous diluent sufficient to supply both the diluent and refrigerant, passing said mixture through an elongated insulated conduit while gradually decreasing its pressure whereby refrigeration is gradually effected by direct vaporization of diluent from the mixture, removing the vaporized diluent from the resulting cooled diluted wax slurry, filtering the wax from the diluted oil, and separating the diluent from the wax-free diluted oil.

3. The method of continuously cooling a mixture of waxy oil with a normally gaseous diluent whereby separable wax crystals are formed, which method comprises passing said mixture through an elongated closed passageway to permit a gradual reduction of pressure with the consequent vaporization of diluent, trapping out liberated diluent vapors, continuing the flow of said mixture through an elongated closed passage for the further gradual vaporization of diluent and again trapping out liberated diluent vapors, there being no substantial difference in the pressure between trapout zones and the elongated closed passageways leading to and from said zones so that the major cooling is effected in the coils rather than in the trapout zones.

4. The process of continuously cooling a propane-oil-wax mixture to effect the solidification of wax in readily separable form, which comprises forcing said mixture through an elongated closed passageway of narrow cross section to effect a gradual and substantial pressure drop, allowing the gradual pressure drop to cause vaporization of propane for cooling the mixture to solidify the wax and maintaining the walls of the passageway at a sufficiently high temperature to prevent wax crystals from adhering thereto.

5. The method of continuously dewaxing oil by means of a liquefied normally gaseous diluent,

which comprises admixing a relatively large.

amount of said diluent with said waxy oil at a temperature sufficiently high to effect complete solution of both oil and wax, passing said solution through an elongated closed passageway whereby the pressure is gradually reduced and thereby permitting a gradual vaporization of diluent and the gradual cooling of the solution, separating the vaporized diluent from the cooled, diluted oilwax mixture, mechanically separating the wax from the diluted oil, and separating the wax-free oil from the diluent.

6. The method of claim 5 wherein diluent vapors are removed at a plurality of spaced points between the introduction of the mixture into the elongated closed conduit and the mechanical separation of wax from diluted oil.

'7. The method of continuously cooling an oilwax mixture diluted with a normally gaseous d11- uent for obtaining readily separable wax crystals, which comprises passing said mixture through an elongated, closed passageway, and removing vaporized diluent from the liquid at spaced points in said passageway by means of compressors operating at substantially constant pressure differentials, the pressure drop being gradually efaooaoi 1 fected throughout the entire elongated, closed passageway and the compressors serving to force vapors from. the trap against a higher pressure in a gas line whereby the major part of the cooling is efiected in the elongated conduit instead of in the traps.

HAROLD V. ATWELL.

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