US2909475A - Solvent dewaxing plant with unitary compressor - Google Patents

Description

Oct. 20, 1959 J. D. BUSHNELL SOLVENT DEWAXING PLANT WITH UNITARY COMPRESSOR Filed Oct. 4, 1957 2 Sheets-Sheet 1 James D. Bushnell Inventor By 6. Attorney :0 Ill 5 86:8 oE 3 T T a ow mm N? hm bw 3:20 mm H N0 m om Al Eo2m mw H 9 E 12.. q Eo w 9 mm oh r v r v 1 I F m 1 z. hm J mm SwmmEEo x03 5 m 0 E 8 w 0 z 97 s A S Iv. mm |v 1 E35 A I mm mm mm howmoEEou hommwBEoo as .2 mm 520 United States Patent Ofice 2,909,475 Patented Oct. 20, 1959 SOLVENT DEWAXING PLANT WITH UNITARY COMPRESSOR James D. Bushnell, Berkeley Heights, N.J., assignor to Esso Research and Engineering Company, a corporation of Delaware Application October 4, 1957, Serial No. 688,310

5 Claims. (Cl. 208-35) This invention relates to improvements in the art of solvent dewaxing of petroleum oil fractions. The invention is particularly concerned with an improved process for dewaxing lubricating oil fractions with light hydrocarbon solvents such as propane. In accordance with this invention the conventional propane dewaxing plant is replaced by one in which the compressor facilities are unitized, thereby improving operating elficiency and markedly reducing investment costs.

In the refining of petroleum fractions from paralfinic type crudes, particularly lubricating oil fractions, it is necessary to remove wax therefrom in order that the finished product will have the desired properties. Thus for example wax must be removed from lubricating oils so that the lubricants will remain fluid at low operating temperatures. Wax recovery is also desirable because the purified parafiin wax is itself a valuable product.

Two solvent dewaxing processes are now widely used in industry. One is known as propane dewaxing and the other as ketone dewaxing. In both processes the solvent serves to reduce the viscosity of the chilled slurry of wax and oil and to provide favorable conditions for crystal growth so that satisfactory separation of the wax on a continuous rotary filter can be obtained. The present invention is concerned primarily with a propane dewaxing process. In this disclosure propane dewaxing refers to a dewaXing process in which any liquefied normally gaseous hydrocarbon is used, propane being the preferred hydrocarbon.

In a conventional propane dewaxing plant three separate compressor services are necessary. One multistage compressor, known as the chilling compressor, is used for autorefrigeration of the batch of propane and waxy oil feed. The chilling compressor normally operates under suction conditions that vary greatly from the be ginning to the end of the chilling cycle so that a complicated control system is required for that compressor. A second multistage compressor, known as the holding compressor, operates at constant suction pressure and picks up vapors that. are generated in the various low temperatures vessels of the dewaxing plant. The chilling and holding machines must compress these vapors to a pressure at which they can be condensedwith cooling water. A third compressor, which is known as the blowgas compressor, handles a large volume of vapor but has a relatively'low compression ratio. The blow-gas from this compressor is not condensed, but is employed for creating the necessary pressure differential in the rotary filter, and, for blowing the Wax cake loose from the filter drum on the discharge side. f

In the present invention a single multistage compressor takes care of all of the three compressor services referred to above. To accomplish this the vapors from the chiller are throttled down to the pressure at which the holding compressor normally takes suction. While this increases somewhat the horsepower requirement over part of the chilling cycle, there are several benefits mentioned later in the specification which overcome this slight drawback. The filter feed drum and filter shell are held at a slightly higher pressure (2 to 10 psi.) than the filter rundown tank, which operates at the same pressure as the compressor suction. Blow-gas for the filter is supplied from an intermediate stage of the compressor.

The nature and objects of the invention will be better understood when reference is made to the accompanying drawings in which:

Figure 1 is a flow plan of a conventional plant for conducting a dewaxing process; and

Figure 2 is a flow plan of that portion of a propane dewaxing plant which incorporates the improvements of the present invention.

In order that the improvements provided by the present invention may be understood, it is first necessary to describe the essential portions of a conventional propane dewaxing plant. The flow plan of such a plant is shown in Figure 1. It will be understood that in an actual plant, for economy of operation, a number of the streams are run through heat exchangers to give up heat to, or to take heat from, other streams. To simplify the drawing, however, some of these heat exchangers have been omitted in this presentation.

The warm waxy oil feed for the plant enters the system through line 11. Preferably a small proportion of a crystal modifier or dewaxing aid (0.02 to 0.2 percent of the feed) is added to the feed. A suitable dewaxing aid comprises a Friedel-Crafts condensation product of chlorinated paraffin wax and naphthalene, for example. The presence of this material during chilling ensures a wax crystal structure that promotes filter rate and wax cake porosity so as to enable more elficient washing of the wax cake. The blend of oil feed and dewaxing aid is pumped by means of pump 13 into a mixer 17 where it is mixed with warm propane that is pumped from storage vessel 15 through line 14 by means of pump 16.

The mixture of propaneand feed passes through line 18 and cooler 19 into a surge tank 20 for temporary storage and is charged from there through line 21 into one of a pair of batch chillers 22. To simplify the drawing, only one such chiller is actually shown. In these chillers the. mixture .is cooled at a controlled rate by gradual reduction of pressure and vaporization of propane from the solution. The propane leaving the batch chiller flows through line 27 ,to the intake side of chilling compressor 28. The desired propane-to-oil ratio is maintained in the chiller by introducing liquid makeup propane through line 24. The makeup propane is pumped into the line 24 from the warm propane storage vessel 15 by means of pump 23 and/or from cold propane storage drum 67 by pump and line 71. The upper section of the batch chiller has a contacting section 25 wherein the makeup propane countercurrently contacts the outgoing propane vapors soas to ensure that makeup propane will enter the main portion of the chiller at the proper temperature and thereby avoid shock chilling of the slurry. Shock chilling is undesirable since it causes the formation of excessively fine crystals and consequently poor filtration.

. While a batch of slurry is being chilled in one of the batch chillers 22, the batch in the other chiller is emptied into the filter feed tank 32 through line 31. The chiller is then warmed by pressuring it with warm propane vapors so as to prevent the initial shock chilling of the next batch of slurry. The time required to chill each batch is normally about 30 to 35 minutes, during whichthe emptying, warming and filling steps can be undertaken for the other chiller.

From the filter feed tank 32 the cold slurry is pumped by means of pump 35 through line 36 to a continuous rotary pressure type filter 37. This filter comprises a cloth-covered filter drum that is enclosed in a pressure housing which also contains the slurry being filtered.

percent of its filter area submerged in the chilled slurry. The oil and solvent pass through the filter cloth into the filter drum and are conducted through line 40 into the filter rundown tank 41.

As the filter drum rotates, the Wax cake that is built up on the filter cloth emerges from the slurry and is washed with cold solvent, which is applied through distributor pipes located above the filter. The cold propane for this operation is supplied by line 42 and pump 43. The washed cake is loosened from the filter cloth by means of a blow back with gas supplied through line 45, this gas coming from the blow gas compressor 46. The loosened filter cake is removed with a knife blade that diverts the wax into a trough having a screw conveyor to carry the wax to the suction boot of a pump 47. The details of the rotary filter, including the provisions for washing, blow-back and screw conveying, are not shown in the drawing as these are well known in the art.

The slack wax from the filter is sent with the aid of pump 47 through line 48 into a slack wax evaporator 50 where propane is removed overhead into line 51. The slack wax itself is conducted through line 52 into a stripping vessel 53 where additional solvent is stripped off with steam. The stripped wax is removed through line 54 for further processing, while the steam and propane vapors are conducted through lines 55 and 61 to steam condenser 62.

The dewaxed oil in the filter rundown tank 41 is pumped by means of pump 56 through line 57 into a dewaxed oil evaporator 58 where propane is removed overhead and sent through line 51 and cooler 30 to the propane storage vessel 15. A portion of this propane is cooled by cooler 59 and by heat exchanger 49 and is then sent through line 44 to drum 67, which is a combined cold propane storage drum and blow gas knockout drum. Additional propane is removed from the dewaxed oil by steaming in the dewaxed oil stripper 60, the mixture of steam and the propane thereby removed being sent through line 61 to steam condenser 62. Stripped oil leaves stripper 60 through line 63 for further processing. The separated propane from condenser 62 is conducted through line 39 to knockout drum 34.

In order to produce dewaxed oil having a pour point of 15 to 20 F., it is necessary to dewax with propane at temperatures of the order of -10 to --35 .F. The desired temperatures are maintained in the filter feed tank and the filter rundown tank by taking ofi propane vapors by means of the holding compressor 65. Propane from the filter feed tank passes to the holding compressor through line 33 and compressor knockout drum 34. Propane from the filter rundown tank flows into the blow-gas knockout drum and cold propane storage tank 67 through line 68 and from there to the holding compressor through line 69. The compressed propane from the chilling compressor and the holding compressor is condensed at 29 before being sent through line 51 to the propane storage vessel 15.

Holding compressor 65 runs at essentially constant conditions and may be either of the reciprocating type or they centrifugal type. Under typical conditions the holding compressor may take suction at to 15 p.s.i.g. and discharge at 175 to 225 p.s.i.g. The chilling compressor 28 operates under a wide range of suction pressures, e.g. from 5 to 150 p.s.i.g. and is of the reciprocating type. The discharge pressure of the chilling compressor is also in the order of 175 to 225 p.s.i.g. Usually the chilling compressor is equipped with devices for varying its volumetric capacity as the suction pressure varies, so as to use the full horsepower except at the start of the chilling cycle. The blow-gas compressor 46 in a typical run will have suction pressures of from 5 to 15 p.s.i.g. and discharge pressures of from 25 to 40 p.s.i.g.

Having in mind the above description of a typical propane dewaxing plant of the prior art, the improvements provided by the present invention will be appreciated from the flow plan of Figure 2 and the accompanying description. Where the same elements or their equivalents appear in Figure 2 as in Figure 1, they are given the same reference numerals. Figure 2 does not show the complete plant but only that portion which is necessary for a full understanding of the modifications supplied by the present invention.

Referring now to Figure 2 it will be noted that a multistage compressor takes the place of the three compressors 28, 46 and 65 of Figure l. The high pressure discharge from the compressor normally flows through line 126 and condenser 129 to the propane storage drum, combining with the propane returning from the slack wax and dewaxed oil flash drums or towers. The hot propane vapor from the high pressure discharge is also used for warming the chillers, as will be explained later. As will also be shown, use is made of lower pressure propane taken from the interstage 101 of the compressor.

The functions performed by the single compressor will be best understood by describing the operational procedure for handling the feed of oil and propane in the unit. The mixture of feed oil and'propane enters the warm solution tank through line 18. The chilling of the mixture is done batchwise in chiller 122, which operates on a regular cycle as for example a one hour cycle. Although only one chiller is shown, two chillers 122 can be employed alternately, as in a conventional plant. Assuming that the chiller has been warmed to 60 F. after the previous chilling, the cycle operates as follows.

Warm solution is permitted to flow through line 21 into chiller 122 by opening valve 106. Flow is promoted by the fact that the pressure in the tank 120 is greater than in the chiller. When the desired level has been reached in the chiller, valve 106 is closed. Then valve 109 is opened and propane is bled off from the chiller through line 127, knockout drum 134 and line 169, the latter line leading to the intake of the multistage compressor 100. When the desired chilling temperature is reached, for example, -10 F., valve 109 is closed and valves 107 and 110 are opened. Propane vapors from the interstage 101 of the compressor flow through line 124 into the chiller and force the chilled batch through line 31 into the filter feed tank 32. When the chiller has been emptied, valves 107 and 110 are closed and hot propane vapor from the high pressure discharge of the compressor under control of valve 111 flows through line 126a into the chiller and the vessel is warmed back to 60 F. The warm chiller is then ready to repeat the cycle.

The chilled mixture in the filter feed tank 32 is held at the desired temperature, e.g. -10F. by bleeding off propane through line 133 under control of valve 108. The mixture is fed by gravity to the filter 37 through line 36. A level control valve (not shown) maintains the filter drum submerged at about the 50 percent level in the filter vat or housing. Filtering, washing and blow-back are conducted in the same manner as described in conjunction with Figure 1. The filtered oil flows through line 40 into filter rundown tank 41, while the wax is conducted to a slack wax flash drum through line 148.

To obtain the blow-gas for the filter, a stream of gas from the interstage 101 of the compressor is conducted by means of line 103 through flow control valve 104 into line leading to the rotary filter trunnion valve. Liquid propane in the propane gas storage tank 15 is at a high pressure and high temperature, e.g. 210 p.s.i.g. and 110 F., and the gas flowing through line 103 is at a lower pressure and temperature, e.g. 90 to F. and 35 p.s.i.g. A smallstream of liquid propane is admitted from the storage drum through line 145 and temperature control valve 105 where it mixes with the propane vapor from line 103. The pressure reduction through the valve 105 causes the liquid propane to vaporize and eflects a cooling action on the vapor stream. Thus the combined streams may be cooled to 15 F. for example.

The following specific operating conditions are presented by way of example for the operation of a process using the flow plan of Figure 2. Figures are given for both a light feed stock and a heavy feed stock, each of which has been phenol extracted and hydrofined before entering the dewaxing unit feed tankage. These feedstocks have the inspections shown in Table I.

Table 1 Light Heavy Feedstock Feedstock Typical operating conditions for dewaxing both of these feedstocks to a pour point of 30 F. and a cloud point of 35 F. are given in Table II. Feeding 725 barrels per stream day (b./s.d.) of light feedstock into the plant should yield 73 b./s.d. of slack wax and 658 b./s.d. of dewaxed .oil of 120 SSU viscosity at 100 F. and 41 SSU viscosity at 210 F. With the heavy feedstock 570 b./s.d. should yield 60 b./s.d. of slack wax and 515 b./s.d. of dewaxed oil of 945 SSU viscosity at 100 F. and 83 ,SSU viscosity at 210 F.

Table 11 Light Heavy Feedstock Feedstock Representative pressures and temperatures in the various units of Figure 2 are presented in Table 111.

Table III Press, p.s.1.g.

Propane Storage Tank 15 Warm Solution Tank 20...

Chiller 122 Filter Feed Tank 32 Filter 37 Filter Rundown Tank 41 117 max. 16 min. 16

Intake to the compressor 100 is at 9 p.s.i.g. while the output through line 126 is at 220 p.s.i.g. and ISO-260 F. temperature. At the interstage 101 of the compressor the pressure is 60 psig. and the temperature 90-160 F. Pressure control valve 102 drops the pressure to 35 p.s.i.g. in lines 103 and 124.

The manner in which the compressor 100 functions as a chilling compressor and as a blowback compressor should be well understood from the foregoing description. The manner in which it also functions as a holding compressor may not be readily apparent. A better understanding of this will be reached if consideration is given to the functions of a holding compressor. First of all, the holding compressor operates to build up to a usable pressure all of the propane that is recovered from the system at low pressures. For example, vapors from the dewaxed oil stripper 60 and the slack wax stripping vessel 53 of Figure 1, find their way to the compressor intake in Figure 2 through line 139 leading to knockout drum 134. A second function of the holding compressor is to main tain proper temperatures in the various low temperature vessels by autorefrigeration, removing from those vessels propane vapors generated by heat pickup from the atmosphere. Thus a certain proportion of the propane flowing from filter 37 through line 40 and combined with propane flowing from rundown tank 41 through line 168 to the knockout drum represents autorefrigeration vapors. The refrigeration of the chilled mixture in the filter feed tank 32' by bleeding off propane through line 133 has already been described.

A third function is to remove from the filter feed tank 32 the vapors that are blown into that tank from the chiller following the transfer of a chilled batch from the chiller to the filter feed tank. The completion of the batch transfer will be indicated by a sudden pressure buildup in the filter feed drum, and the excess vapors contributed by this buildup must be removed.

- Other holding compressor functions include removing from the low pressure system, and recovering, both the blowback gas and the propane vapors generated by the liquid propane quench admitted into the blowback line and used for cooling the blowback gas before it enters the filter. Also recovered are the excess vapors generated when the filter is cooled following a periodic hot Wash to remove wax particles that have plugged the filter cloth. This hot Wash is conventionally done with hot kerosene, for example, at periods of from 2 to 24 hours, depending on the need. 7

The following differences between the conventional plant as typified by the flow plan of Figure 1, and the improved plant, as typified by Figure 2, should be noted. In the conventional plant thefilter feed tank and the filtrate drum (filter rundown tank) are maintained at the same pressure, while the shell of the rotary filter is maintained at a pressure 2 to 10 pounds per square inch higher. This requires that the feed be pumped into the filter. Pumping at this point is undesirable, because once the clystals have formed the less handling of the crystals before filtration the better the separation obtained. Thus it has been found that agitation of the crystals by the pump tends to cause mechanical fracture of the crystals, increasing their tendency to clog the filter cloth. Additionally, in conventional practice the slurry in the vat of the filter usually tends to heat up several degrees because the action of the holding compressor in the conventional plant cannot be utilized to keep the temperatore down to normal. This temperature rise is undesirable because it enables part of the precipitated wax to become redissolved in the slurry.

Contrasted with the above, in the present invention the filter feed tank and the shell of the rotary filter are maintained at the same pressure. As a result the filter feed can flow into the filter by gravity, and the slurry in the filter vat does not heat up. The difierential pressure needed to eflfect filtering is obtained by maintaining the filter rundown tank at about 2 to 10 pounds lower pressure than the filter shell. This causes the filtrate to flash to a temperature lower than that existing in the filter vat. The cold filtrate can be used expeditiously to cool the wash propane stream.

Additional advantages of this invention over previous practice are as follows:

(1) In large plants it allows a single centrifugal compressor to replace a number of reciprocating compressors, thus reducing investment and maintenance costs.

(2) In small plants it reduces the total number of reciprocating compressors, since fewer services are involved. This also simplifies the problem of providing spare compressors for each service.

(3) The number of suction knockout drums and the amount of piping, headers, and instrumentation are substantially reduced.

place due to the heat capacity of the suction lines and headers of the chilling compressor.

As a result, liquid propane condenses in the header at the beginning of a chilling cycle when the suction line is still cold, but at a relatively high pressure. This liquid propane can cause damage to the compressor. This is avoided'in the system disclosed by this invention, since the compressor suction line is always at low pressure.

In conventional plants the throughput may be limited by either chilling, holding, or blow-gas compressor capacity, and the relative horsepower requirements for the different services will change with different feed stocks due to different solvent dilution ratios required for high and low viscosity feed stocks. Thus it is impossible to use all of the compressor capacity effectively. In this invention, one compressor handles all three services, so that the loads may be effectively balanced to use all the available compressor horsepower.

The scope of this invention is not to be confined to the specific examples and embodiments herein presented and described, but is limited only by the claims appended hereto.

What is claimed is:

1. In the dewaxing of a waxy petroleum hydrocarbon fraction wherein said fraction is mixed while warm with a liquefied normally gaseous hydrocarbon under pressure, successive batches of said mixture are conducted to a prewarmed chilling zone and chilled to dcwaxing temperature by reducing the pressure in said chilling zone and thereby vaporizing a portion of said normally gaseous hydrocarbon, separated waxy constituents are removed from the chilled mixture by filtration through a continuous rotary filter, the filtrate being conducted to a filtrate tank, and the wax cake built up' on the filter is loosened by blowback with a portion of said normally gaseous hydrocarbon, the improvement which comprises throttling down the vapors from the chiller to essentially the same pressure as the vapors from the filtrate tank, compressing the aforesaid vapors in a multistage compressor, operating at an essentially constant low intake pressure, to provide a high pressure source of liquefied hydrocarbon for the chilling step and obtaining compressed vapors from an interstage of the compressor for use as blowback gas on the filter.

2. Improved process as defined by claim 1 wherein said liquefied normally gaseous hydrocarbon comprises propane.

3. Improved process as defined by claim 1 wherein cooling of said blowback gas is effected by admitting into the gas a small stream of liquefied hydrocarbon from the said high pressure source.

4. In the dewaxingof a waxy petroleum hydrocarbon fraction wherein said fraction is "mixed while warm with a liquefied normally gaseous hydrocarbon under pressure, successive batches of said mixture are conducted to a prewarmed chilling zone and chilled to dewaxing temperature'by reducing'the pressure in said chilling zone and thereby vaporizing a portion of said normally gaseous hydrocarbon, separated waxy constituents are removed from the chilled mixture by filtration through a continuous rotary filter, the filtrate being conducted to a filtrate tank, and the wax cake built up on the filter is loosened by blowback with a portion of said normally gaseous hydrocarbon, the improvement which comprises continuously compressing said normally gaseous hydrocarbon in a multistage compressor operated at an essentially constant low intake pressure, throttling the vapors in said chilling zone down to said intake pressure during said chilling step, feeding said chilled mixture to said rotary filter at a pressure slightly above said intake pressure, operating said'filtrate tank at essentially said intake pressure, conducting vapors from said filtrate tank and low pressurevapors recovered elsewhere in the dewaxing :plant to the intake of said compressor and obtaining vapors for said blowback from the interstage of said compressor, whereby chilling service, holding service and blowback service loads are effectively balanced and all three of said services are handled by a single compressor.

5. Improved process as defined by claim 4 wherein said chilled mixture is held in a filter feed tank, prior to filtration, maintained at essentially the same pressure as the shell of said rotary filter and fed to said rotary filter by gravity, whereby to avoid mechanical fracture of the crystals.

Referenccs'Cited in the file of this patent UNITED STATES PATENTS 1,988,706 Swift Jan. 22, 1935 2,077,712 Roberts et al. Apr. 20, 1937 2,079,182 Petty et a1. May 4, 1937 2,115,211 Overbaugh Apr. 26, 1938 2,134,331 Gee Oct. 25, 1938

Claims (1)

1. IN THE DEWAXING OF A WAXY PETROLEUM HYDROCARBON FRACTION WHEREIN SAID FRACTION IS MIXED WHILE WARM WITH A LIQUEFIED NORMALLY GASEOUS HYDROCARBON UNDER PRESSURE, SUCCESSIVE BATCHES OF SAID MIXTURE ARE CONDUCTED TO A PREWARMED CHILLING ZONE AND CHILLED TO DEWAXING TEMPERATURE BY REDUCING THE PRESSURE IN SAID CHILLING ZONE AND THEREBY VAPORIZING A PORTION OF SAID NORMALLY GASEOUS HYDROCARBON, SEPARATED WAXY CONSTITUENTS ARE REMOVED FROM THE CHILLED MIXTURE BY FILTRATION THROUGH A CONTINUOUS ROTARY FILTER, THE FILTRATE BEING CONDUCTED TO A FILTRATE TANK, AND THE WAX CAKE BUILT UP ON THE FILTER IS LOOSENED BY BLOWBACK WITH A PORTION OF SAID NORMALLY GASEOUS HYDROCARBON, THE IMPROVEMENT WHICH COMPRISES THROTTLING DOWN THE VAPORS FROM THE CHILLER TO ESSENTIALLY THE SAME PRESSURE AS THE VAPORS FROM THE FILTRATE TANK, COMPRESSING THE AFORESAID VAPORS IN A MULTISTAGE COMPRESSOR, OPERATING AT AN ESSENTIALLY CONSTANT LOW INTAKE PRESSURE, TO PROVIDE A HIGH PRESSURE SOURCE OF LIQUEFIED HYDROCARBON FOR THE CHILLING STEP AND OBTAINING COMPRESSED VAPORS FROM AN INTERSTAGE OF THE COMPRESSOR FOR USE AS BLOWBACK GAS ON THE FILTER.

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