US2461115A - Method of dewaxing oils - Google Patents

Description

Feb. 8, 1949. c. F. HOLM ET AL METHOD OF DEWAXING OILS 2 Sheets-Sheet 1 Filed March 8, 1945 Cuff 5 Patented Feb. 8, 1949 I UNITED STATES METHOD or DEWAXING OILS Curt Filip Holm, Angby, and Bruno Kuno Engel, Nockeby, Sweden, assignors to Aktiebolaget Separator-Nobel, Stockholm, Sweden, a corporation of Sweden Application March 8, 1945, Serial No. 581,658 In Sweden March 16, 1944 2 Claims. 1

This invention relates to improvements in a process for dewaxing mineral oils, tars, distillation products or other products obtained therefrom etc.. In the performance of such processes, the wax-bearing stock is generally dissolved in a diluent, the wax is precipitated by chilling the solution and is then removed by centrifuging, filter pressing or cold settling. According to prior art and practice the expense of chilling is reduced by using the cold dewaxed oil solution as a chilling medium in heat exchangers for partly chilling the diluted stock. The ultimate chilling to the dewaxing temperature then takes place in separate chillers, working e. g. with direct ammonia expansion.

So as to facilitate the precipitation of wax, the

diluent should possess a higher solubility for oil than for wax, at low temperature. As examples of commonly used diluents may be mentioned: naphtha, halogenated hydrocarbons such as ethylene dichloride, trichlorethylene etc., acetone, methylethylketone etc., or mixtures including such solvents. The purpose of diluting the stock is also to lower the viscosity of the chilled liquid, and to effect the necessary specific gravity differential between the oil solution and the precipitated wax, if the latter is to be removed by centrifuging or cold settling. In such a case the wax may be removed either as the heavier component, e. g. when using naphtha as a diluent, or as the lighter component, e. s. when using ethylene dichloride as a diluent.

Oil and diluent in suitable proportions, e. g. 1:3 by volume, are usually first mixed at a temperature sufficiently high to effect complete solution of the oil and wax in the diluent. The solution is then chilled and the precipitated wax is removed. Although the following description refers to the chilling of a mixture of oil and diluent, the present invention is not limitedto such dewaxing processes but may to advantage be applied also in the dewaxing of certain light, lowviscosity oils such as gas oil, spindle oil, transformer oil etc., without the aid of a diluent, as may sometimes be practiced.

The temperature differential between the diluted stock and the chilling medium should not exceed certain limits, e. g. 0., since the wax might otherwise precipitate (crystallize) in such a condition that it would be difiicult to remove. This danger especially exists within a certain temperature range duringthe chilling in the heat exchangers. The critical temperature range and the maximum allowable temperature differential vary for difierent stocks and diluents.

For practical reasons a dewaxing plant will have to be used for different kinds of stock, and the heat eXchangers should be dimensioned and arranged so that the chilling surface can under all conditions be fullyutilized by maintaining the maximum allowable temperature differential between the diluted stock and the chilling medium. In the customary use of a simple heat exchanger equipment this requirement is generally not fulfilled, since the cold dewaxed oil solution used as a chilling medium will be of less volume, because of the removal of wax therefrom, than the diluted wax-bearing stock to be chilled. When these two liquid streams are simply passed in counterfiow through the heat exchangers, the temperature differential will thus be smaller at the beginning than at the end of the heat exchanger chilling step. This increase of the tem perature difierential during the heat exchanger chilling step is further accentuated by the fact that each unit volume of the dewaxed oil solution is capable of absorbing less heat than must be removed from each similar unit volume of the diluted stock which contains to be preeipitated by withdrawal also of the latent heat of fusion of the wax. Consequently, if the smallest temperature differential is allowed at the end of said chilling step, as is often the case, the greater part of the chilling surface is badly utilized, 1. e. unnecessarily large heat exchangers have to be installed, which means unnecessary expense.

It is an object of the present invention to avoid these difficulties incident to prior art and practice of chilling the wax-bearing diluted stock in direct heat exchange relation to cold. dewaxed'oil solution. In accordance with the invention, the maximum temperature differential between the two liquids, consistent with the precipitation of the wax in a form favorable for its removal, is maintained by withdrawing part of the dewaxed oil solution from the heat exchanger system,

chilling same-preferably to the temperature prevailing at the point of the system Where it is returned-and returning it to the system. The invention may also be practiced by adding dilucut to the stock to be chilled at one or several points within the heat exchanger system.

If, for instance, a constant temperature differential is to be maintained throughout the heat exchanger system, a portion of the dewaxed oil solution, corresponding to the difference in heat capacity between the two counterflow liquid streams, is withdrawn, chilled and returned to the system. If, on the other hand, the temperature difierential should be greater at the beginning than at the end of the heat exchanger chilling step, a still larger proportion of dewaxed oil solution is circulated in the manner described.

The manner in which the invention is performed is illustratedin the accompanying diagrams, of which Fig. l is a flow-diagram of a dewaxing plant according to the invention,

Fig. 2 is a COIIESDO'QGLlng chilling diagram,

Figs. 3-5 are alternative chilling diagrams.

The diluted stock to be dewaxed is fed through the piping i to the three heat exchanger chillers 2 provided with scraping machinery for maintaining the chilling surfaces relatively free of wax and thus ensuring a high heat transfer coefiicient. During its passage through the chillers 2 the stock is chilled from e. g. +50 C. to (2., according to the diagram Fig. 2. From the last heat exchanger chiller 2 the oil passes to the two direct ammonia expansion chillers 3, which are likewise provided with scraping machinery, and these chillers the temperature is further reduced to e. g. 32 0., as indicated in. Fig. 2. From the last chiller 3 the liquid is conducted to the centrifuges 4, working in parallel, where the wax is removed and discharged through the piping 5. The dewaxed cold oil solution flows through the piping 6 to the surge tank I, and by means of the pump 8 it is passed through the piping 9 into the heat exchanger chillers 2, in counter-current flow with the liquid to be chilled.

The number of the chillers 2 and 3 and the centrifuges t can of course be varied.

From the warmest end of the heat exchangers 2' the dewaxed oil solution is discharged through the piping it. At l! a portion of same is withdrawn by the pump i2 and passed through the direct ammonia expansion chiller l3 and the piping M, 92 and 58 back to the coldest end of the heat exchangers 2. The liquid stream through the piping it thus joins the liquid stream through the piping 9, the rate of flow of the chilling medium through the exchangers thus exceeding the rate of flow through the piping 9.

The dewaxed oil solution which is not withdrawn at H is discharged through the piping l5, and in continuous operation the rate of flow through the piping 55 will naturally be equal to the rate of flow through the piping 9. l2l 3l4 thus forms shunt-system through which a stream of chilling medium, i. e. dewaxed oil solution, is continuously circulated.

In the example shown in Fig. 2 the diluted stock introduced through the piping i gives ofi 286,000 kg.=cal./hr. to the dewaxed oil solution passing in counter-flow through the heat exchangers 2. However, for reasons already mentioned, the liquid stream entering the heat exchangers through the piping 9 can absorb only 190,000 kg.=cal./hr. at the maximum allowable temperature differential of 15 C. The remaining 98,000 kg.=cal./hr. are absorbed by the liquid introduced through the piping [4 at approximately the same temperature as the liquid entering through 9. By the shunt-system 12-4 3id a certain amount of heat, in this case 96,000 kg. cal./hr. is thus transferred to the chiller l3.

In Fig. 2, a is the temperature curve for the liqold to be chilled, b the temperature curve for the chilling medium at a constant temperature differential throughout the period of heat exchange,

4 c and d the temperature curves for the chilling medium in case the temperature difierential is decreasing or increasing, respectively, in the direction in which the chilling medium is flowing through the heat exchangers. In the latter cases neither the curves 0 and d nor the curve a will, in reality, be straight'lines.

The curve at represents a customary heat exchange chilling procedure. On account of the decreasing temperature differential (the vertical distance between the curves a and d) at increasing temperatures, the 286,000 kg.=cal./hr. cannot be absorbed by the chilling medium, and the rates of flow thus have to be considerably reduced in order to obtain a chilling as per the diagram. The chilling surface will thus be less utilized than if the chilling is carried out according to the invention (the curves 1) and c).

If the liquid stream through the shunt-systern l2-!3--i4 is increased, so that the curve 0 is valid, a still larger amount of heat than 286,000 kg.=cal./hr. can be removedin the heat exchanger chillers, the surface of which is thus still better utilized than according to curve I). This is conditional, however, on a gradual increase of the temperature differential above 15 C. being permitted.

The curves e and ,f in Fig, 2 correspond to the chillers 3 in Fig. 1, which are operating at ammonia evaporating temperatures of 33 C. and -l0 0., respectively.

As already mentioned, the heat exchanger chillers'Z have to be provided with scraping machinery. The cost of such chillers is about 5 times higher than the cost of ordinary tube and shell chillers. This latter comparatively inexpensive type of chiller can to advantage be used for chilling the circulating stream of oil solution, i. e. as chiller l3 in Fig. I, seeing that the oil solution is dewaxed and no accumulation of wax on the chilling surfaces need thus be feared. Furthermore, there is no danger of shock chilling and thus no limitation of the temperature differential between the oil solution and the chilling medium (ammonia), and the heat transfer coefficient for the direct ammonia expansion tube and shell chiller is about three times higher than for heat exchanger chillers with scraping machinery. Consequently the necessary surface of the chiller [3 will be relatively small.

By thus utilizing as far as possible, according to the present invention, the surface of the relatively expensive heat exchanger chillers provided with scraping machinery, and employing relatively inexpensive chillers for part of the chilling, the'plant investment costs are considerably reduced. 7

A further economy may be obtained by chilling the oil solution circulating through the shuntsystem 12-13-44 in two successive steps, at different ammonia evaporating temperatures (in this case 33 C. and -40 0.). Owing to the low cost per chillin surface and the high heat transfer coeiiicient, it is also possible to use only the higher ammonia evaporating temperature (in this case 33" C.) which means lower operatin costs. Brine may of course also be employed as a chilling medium, although experience has proved that direct ammonia expansion is more economical.

In certain cases it may be advantageous to withdraw or return the oil solution circulatin through the shunt-system at other points of the heat. exchanger chillersystem than bereinbeiore described. Thus the oil in piping 14 may be returned to the system between two of the chillers 2, as, for example, through pipe 9% to the piping between the middle and right hand coolers 2, Valve 93 being opened while valve 94 may be closed. The temperature curves will then differ from those shown in Fig. 2, the minimum or maximum vertical distance between the curves being located somewhere between their extreme ends.

In Fig. 3 the curves g and h represent the cases in which the circulating oil is returned and. withdrawn, respectively, between the two last chillers 2, the temperature differential between the two counteriiow oil streams (a-g and a--h, respectively) thus having a, minimum and maxi-, mum, respectively, at that point. The best mode of operation will of course depend on the temperature diiierentials which can be allowed at difierent stages of the chilling, consistently with the precipitation of the wax in the desired form.

The curves a, g and h in Fig. 3 will in reality not be straight lines.

As will be seen from Fig. 2, the dewaxed oil solution conducted to the heat exchanger chillers has a somewhat (2 C.) higher temperature than the dewaxing temperature (in this case 32 C.). This slight temperature increase is due to the heat radiation losses in the centrifuges, piping etc., and to the heat generated in the pump 8. The circulating dewaxed oil solution should therefore preferably be chilled somewhat below the temperature of the liquid in the pipe 9, viz. down to the dewaxing temperature, which can be done without any danger of wax accumulating on the chilling surfaces of the chiller 13.

It has heretofore been assumed that the liquid to be chilled is passed through the heat exchangers at a constant rate of flow (the curve a in Figs, 2 and 3). In many cases, however, it is preferable to adjust the temperature differential between the two counterflow liquid streams also by successively increasing the rate of flow of the liquid to be'chilled. As already mentioned, this liquid consists to a considerable part of a diluent. However, it is often not necessary to add this diluent, or in any case not the total quantity thereof, to the wax-bearing stock already prior to the chilling, but the diluent may :be added in several successive steps in the course of the chilling.

Fig. 4 shows an example of such a mode of options of diluent are added, the second portion' after having first been separately chilled to or below the temperature prevailing in the heat exchangers at B. The curve is represents the dewaxed cold oil solution which is withdrawn at A, chilled and returned at C, in the manner already described. According to Fig. 4, the rate of circulation is adjusted so that the temperature differential i-k is maintained constant between C and B, whereafter it is successively increased with rising temperature.

In many cases it has proved suitable to add the last portion or diluent after the heat exchanger chillers 2, immediately before the chillers 3 (say at C in Fig. 1 and Fig. 4).

In the practice of this method of successively adding diluent, it is generally of economical advantage to chill the diluent e. g. or C. below the temperature of the wax-bearing stock at the point of the chilling system where the diluent is introduced. The curve I in Fig. 5 represents such a mode of operation. The diluent must not be chilled to such low temperatures, however, that the crystallisation of the wax is impaired by shock chilling.

By these modifications of the invention the chilling surface of the relatively expensive heat exchanger chillers with scraping machinery can always be fully utilized, and a substantial part of the chilling can be carried out in relatively inexpensive chillers, whereby the economy of the plant is improved.

When carrying out the dewaxing process in two or several subsequent stages, i. e. when the oily wax obtained in the first stage according to Fig. 1 is deoiled by being washed in a second stage with a further quantity of solvent, the dilute oil solution thus obtained from the second stageis generally used as a diluent in the first stage after having first passed through a heat exchanger for chilling the fresh solvent used in the washing operation in the second stage.

While in the claims we have used the generic term wax-bearing mineral oil, it will be under stood that this term includes oil to which a solvent may or may not have been added and comprehends also tars, distillation products and similar products, containing wax, to all of which the art commonly applies the term oil.

We claim:

1. The process of dewaxing mineral oils which comprises chilling the wax-bearing oil in a plurality of stages in countercurrent heat-exchange relation to a flowing heat-absorbing liquid, separating from the chilled wax-bearing oil wax and substantially wax-free oil and utilizing the latter as a heat-abs0rbing liquid in all said stages, Withdrawing wax-free oil from the heat-exchange system and separately cooling the wax-free oil so withdrawn and, at a point between two of said stages, mixing the so withdrawn and cooled waxfree oil with the wax free oil flowing from the locus of separation. V 1

2. The process of dewaxing mineral oils which comprises chilling the wax-bearing'oil in a plurality of stages in countercurrent heat-exchange relation to a flowing heat-absorbing liquid, separating from the chilled wax-bearing oil wax and substantially wax-free oil and utilizing the latter as a heat-absorbing liquid in all said stages, withdrawing wax-f-ree oil from the heat-exchange system and separately cooling the wax-free oil so withdrawn and mixing the so withdrawn and cooled wax-free oil with the wax-free oil flowing from the locus of separation after the latter oil has passed part way through the heat-exchange system.

CURT FILIP HOLM. BRUNO KUNO ENGEL.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS 2,366,792 Kirkbride Jan. 9, 1945

Images