Slack wax de-oiling process

An improved method for separating a slack wax into a wax fraction and a lube oil fraction is disclosed. The method comprises passing the slack wax into a first mixing zone and subsequently into a second mixing zone. Solvent selectively miscible with the lube oil fraction is added to the zones. The temperature of the solvent added to the second mixing zone is substantially higher than the temperature of the solvent added to the first mixing zone. This process produces a wax fraction having a relatively low lube oil content.

BACKGROUND OF THE INVENTION 
This invention is directed at the separation of a crystallized component 
from a slurry. More specifically the subject invention is directed at the 
separation of high melting point refined wax from a slack wax feed stream. 
In the production of lube oils and waxes it is important to effect a good 
separation of the lube oil from the wax. The presence of wax in lube oil 
adversely affects the pour point of the oil, while the presence of lube 
oil in wax is not desirable for several reasons. Since the unit price of 
lube oil products typically is higher than that of wax products, eonomic 
considerations dictate that the oil content of the wax be as low as 
possible. In addition, many refined wax products, such as those used in 
contact with food, require that the residual oil content be maintained 
below a predetermined value. The removal of oil from wax typically has 
involved the use of a chilling zone to precipitate the wax, solvent 
addition to remove some of the residual oil from the wax, and a separation 
zone to remove the wax from the oil and solvent. Where the oil content of 
the wax is not reduced sufficiently in one separation stage, it may be 
necessary either to further process the wax or to utilize the wax in lower 
quality applications. Reprocessing the wax, such as by passing the wax 
through one or more separation zones, may not be desired because of the 
additional operating and capital costs. Previous work has been directed at 
the separation of lube oil from wax. U.S. Pat. No. 4,146,461 is directed 
at the dewaxing of waxy lubricating oil stocks by the injection of cold 
dewaxing solvents at a plurality of points. The patent discloses adjusting 
the cold solvent addition rate to each stage to ensure that the 
temperature drop in the initial stages is greater than the temperature 
drop in the final stages. U.S. Pat. No. 3,644,195 is directed at the 
separation of a waxy oil stream by adding cold solvent to a multi-stage 
mixing zone to crystallize the wax. The wax, separated from the lube oil 
by rotary filters, is again mixed with solvent at a temperature sufficient 
to dissolve low-melting wax only, after which the high-melting wax is 
separated by another rotary filter. 
U.S. Pat. No. 2,284,607 is directed at a method of dewaxing oil. This 
patent discloses the chilling of the primary solvent and feed stream 
mixture and the subsequent addition of a secondary solvent at a higher 
temperature than the primary solvent-feed mixture. After the secondary 
solvent is added, the mixture again is chilled, after which the wax is 
separated. 
U.S. Pat. No. 4,169,039 also is directed at dewaxing an oil. This patent 
discloses the use of a multi-stage mixing and crystallization zone in 
which relatively small amounts of the components from the hot washing drum 
are recirculated to the mixing zone, but at a lower temperature than the 
material being processed in the mixing zone. 
In all of the patents noted above the separation of the oil from the wax 
requires the use of additional quantities of solvent and/or additional 
processing steps. Accordingly, it is desirable to provide a process which 
will reduce the residual oil content in wax to relatively low values 
without the use of additional processing equipment or additional 
processing steps. 
It also is desirable to provide a process which will reduce the residual 
oil content in wax to relatively low levels without the use of excessive 
amounts of solvent. 
It also is desirable to provide a process which will permit a decrease in 
the wash solvent addition rate without increasing the residual oil content 
of the wax above a predetermined limit. 
The subject invention is directed at a method for separating a first, 
crystallized component from a second, non-crystallized component by 
passing the feed stream comprising the first and second components through 
first and second mixing zones. Solvent is added to both mixing zones, with 
the temperature of the solvent added to the first zone lower than that 
added to the second mixing zone. More specifically, the subject invention 
is directed at reducing the residual oil content of a wax fraction by 
passing the wax-containing feed stream through a first mixing zone where 
the feed stream is contacted with a solvent at a lower temperature than 
the feed stream to precipitate the wax and form a wax slurry. The slurry 
is then contacted in a second mixing zone with solvent at a higher 
temperature than the solvent added to the first mixing zone to remove 
residual oil from the wax fraction. The slurry exiting from the second 
mixing zone is passed to a separation zone for separation of the wax 
fraction from the slurry. 
SUMMARY OF THE INVENTION 
A method for separating a crystallizable component from a 
non-crystallizable component in a multicomponent feed stream, said method 
comprising: 
A. adding solvent selectively miscible with the non-crystallizable 
component to a first mixing zone at a temperature below the temperature of 
the feed entering the first mixing zone to thereby crystallize at last a 
portion of the crystallizable component and form a slurry; 
B. passing slurry from the first mixing zone to a second mixing zone 
wherein the slurry is contacted with additional quantities of solvent, the 
temperature of the solvent added to the second mixing zone being 
substantially higher than the temperature of solvent added to the first 
mixing zone to thereby remove quantities of the non-crystallized component 
from the crystallized component; and, 
C. passing slurry from the second mixing zone to a separation zone wherein 
the crystallized component is separated from the non-crystallized 
component and solvent. 
The present invention is of particular utility where the feed stream is a 
slack wax which is to be separated into a wax fraction and a lube oil 
fraction. In such an application the present invention comprises: 
A. passing slack wax into a first mixing zone and contacting the slack max 
therein with a solvent selectively miscible with the lube oil, the solvent 
added to the first mixing zone at a lower temperature than the temperature 
of the entering slack wax is thereby crystallize at least a portion of the 
wax and form a slurry; 
B. passing slurry from the first mixing zone into a second mixing zone 
wherein the slurry is contacted with additional solvent, the temperature 
of the solvent added to the second mixing zone being higher than the 
temperature of the solvent added to the first mixing zone; and 
C. passing slurry from the second mixing zone into a separation zone 
wherein the slurry is separated into a crystalline wax fraction and a lube 
oil fraction. 
In a preferred embodiment of the present invention the first and second 
mixing zones, each comprising a plurality of mixing stages, are disposed 
in a common vessel. The solvent added to both mixing zones preferably is 
the same. When the feed stream is a slack wax, the solvent preferably is 
selected from the group consisting of methyl ethyl ketone, methyl isobutyl 
ketone, acetone, toluene, ethylene dichloride, methylene chloride and 
mixtures thereof. The solvent added to the second mixing zone, typically 
comprising at least 30 wt% of the total solvent added, preferably is added 
at a temperature substantially the same as that of the slurry passing from 
the first mixing zone into the second mixing zone. The temperature of the 
solvent added to the second mixing zone preferably is at least about 
15.degree. C. higher, more preferably at least about 35.degree. C. higher, 
than the temperature of the solvent added to the first mixing zone. The 
temperature of the solvent added to the second mixing zone preferably in 
not less than the temperature of the slurry entering the second mixing 
zone.

DETAILED DESCRIPTION OF THE INVENTION 
Referring to the FIGURE, a preferred embodiment for practicing the present 
invention is shown. In the FIGURE, all valves, piping, pumps, 
instrumentation and other equipment not essential for an understanding of 
this invention have been deleted for clarity. A feed stream, such as a 
slack wax stream, is shown entering the top of crystallizer vessel 10 
through line 12. Vessel 10 comprises a plurality of mixing zones, such as 
first mixing zone 20 and second mixing zone 30. Although first mixing zone 
20 and second mixing zone 30 are shown located in the same vessel 10, each 
zone also may be located in one or more separate vessels. Zones 20, 30 
each comprise one or more separate mixing stages, such as stages 22 a-e, 
32 a-e, respectively. In this embodiment, tower 10 has a central shaft 42 
which communicates with drive means 40 and with impeller means 44 disposed 
in each stage 22 a-e, 32 a-e. Stages 22 a-e, 32 a-e are shown having fresh 
solvent inlets 24 a-e, 34 a-e, respectively, extending from manifolds 26, 
36, respectively. Line 38, extending from the base of vessel 10, 
transports the slurry exiting from zone 30 to a separation zone 50. Zone 
50 may comprise any equipment reasonably adapted to separate the products 
being processed. In a lube oil-wax separation process, separation zone 50 
preferably comprises a rotary filter means, although other separating 
equipment also may prove satisfactory. The slurry in line 38 preferably is 
contacted in separation zone 50 with additional solvent entering through 
line 64 to facilitate the oil-wax separation. The wax fraction comprising 
crystalline wax and solvent, is separated and is removed via line 52 while 
the lube oil fraction comprising lube oil, low melting point wax and 
solvent exits zone 50 through line 54. 
A critical element of the present invention is the addition of solvent to 
first mixing zone 20 through manifold 26 and inlets 24 a-e at a 
temperature than the temperature of the solvent added to second mixing 
zone 30 through manifold 36 and inlets 34 a-e. In the embodiment shown 
this may be accomplished by passing a fraction of the solvent in line 60 
through an additional refrigeration zone, such as zone 62, before the 
solvent enters manifold 26. In first mixing zone 20 the relatively cold 
solvent operates to cool the feed stream thereby crystallizing at least 
one crystallizable component from the feed stream. The relatively warm 
solvent added to second mixing zone 30 through line 60, manifold 36 and 
inlets 34 a-e operates to dissolve certain of the low melting crystals and 
to remove residual liquid from the remaining crystals. As used herein, the 
term "crystallizable component" means a component which forms crystals at 
the temperature of the solvent utilized, while the term 
"non-crystallizable component" means a component which is not crystallized 
at the temperature of the solvent utilized. 
One particularly useful application of the present invention is in the 
processing of a slack wax stream from a lube oil process. The slack wax, 
typically comprises about 60 wt.% or more wax with the remainder generally 
comprising lube oil. The slack wax preferably is passed through a 
multi-stage contacting vessel, such as vessel 10, where the solvent added 
to stages 22 a-e of first mixing zone 20 through line 60, refrigeration 
zone 62, manifold 26 and inlets 24 a-e operates to gradually cool the 
slack wax thereby promoting the desired wax crystal growth. The 
wax-oil-solvent slurry then passes into second mixing zone 30 having 
stages 32 a-e. The solvent added to stages 32 a-e through line 60, 
manifold 36, and inlets 34 a-e operates largely to dissolve low melting 
point wax compounds and remove entrapped lube oil from the remaining wax 
crystals. The slurry thereafter may be transferred to separation zone 50, 
such as a rotary filter means, where the wax fraction may be separated 
from the lube oil fraction by methods well-known in the art. The wax 
fraction, primarily comprising crystalline wax and solvent, may be removed 
from separation zone 50 through line 52 for further separation of the 
crystalline wax from the solvent (not shown). Typically, this is 
accomplished in a distillation zone. Similarly, the lube oil fraction, 
primarily comprising lube oil, low melting point wax and solvent, may be 
removed from separation zone 50 through line 54 for further separation of 
the lube oil and low melting point wax from the solvent. The lube oil and 
low melting point wax, which commonly are referred to as foots oil, also 
frequently are separated from the solvent in a distillation zone. 
Vessels substantially similar to vessel 10 previously have been used for 
slack wax processing. It may be possible to modify an existing contacting 
vessel wherein all the solvent is added at substantially the same 
temperature, to the present design wherein the solvent is added at a 
plurality of temperatures to the mixing zones. The following examples 
demonstrate that a conventional contacting vessel, modified generally as 
shown in the FIGURE, may produce a wax product having a significantly 
lower residual oil content than that achieved by a conventional process at 
the same overall solvent addition rate. In these examples a one stage 
laboratory crystallizer six inches in diameter and three inches high was 
used in batchwise operation to simulate operation of a fourteen stage 
continuous contacting vessel. Solvent was added incrementally to the feed 
and mixed for a predetermined time at the appropriate temperature to 
simulate the dilution and mixing which occurs at each particular stage in 
a continuous contacting vessel. 
The feed used in these tests was a slack wax from a 600 Neutral feedstock 
having 30 wt.% oil content. Certain properties of this slack wax are 
presented in Table 1 below. 
TABLE 1 
Properties of Slack Wax Tested 
Oil Content--30 wt.% of SAE 30 grade oil 
Viscosity--8.5 cps @ 100.degree. C. 
ASTM Congealing Point--64.degree. C. 
Specific Gravity--0.8 @ 80.degree. C. 
Comparative tests were run in which the feed rate to vessel 10 was 350 
cc/min. The feed was prediluted with 0.5 v/v of a solvent comprising equal 
volumes of methyl ethyl ketone and methyl isobutyl ketone. The agitator 
tip speed was maintained at 305 cm/sec. In all tests the slurry exited 
from vessel 10 through line 38 at 25.degree. C. 
In a conventional cold solvent addition test where vessel 10 comprised a 
single mixing zone, such as first mixing zone 20, the temperature of the 
slack wax was reduced substantially uniformly from 57.degree. C. to 
10.degree. C. at an average chilling rate of 1.7.degree. C./min. The 
solvent added to vessel 10 was maintained at a temperature of -13.degree. 
C. for all solvent additions. 
In another comparative test substantially similar to that for the cold 
solvent addition, but using a relatively warm solvent, feed entering at a 
temperature of 57.degree. C. was reduced substantially uniformly to an 
outlet temperature of 25.degree. C. at an average cooling rate of 
1.7.degree. C./min by the addition of solvent at a temperature of 
9.degree. C. Varying amounts of wash solvent were used in the subsequent 
processing of the slurry from the crystallizer. 
EXAMPLE 1 
In this example, substantially all the feed cooling was accomplished in the 
simulated first mixing zone 20 comprising stages 1-7. The cooling rate was 
increased to 2.9.degree. C./min by the incremental addition of solvent at 
-13.degree. C. The slurry was cooled in the first mixing zone to 
25.degree. C. To simulate the second mixing zone 30, comprising stages 
8-14, solvent subsequently was added incrementally at a temperature of 
25.degree. C. to dissolve low melting wax and remove entrapped lube oil 
from the remaining wax crystals. Varying amounts of wash solvent were used 
in the subsequent processing of the slurry from the crystallizer. 
A comparison of the data for the two temperature solvent addition process 
of the present invention where the temperature of the solvent added to the 
first mixing zone and to the second mixing zone differed by approximately 
38.degree. C., with that for conventional one temperature warm and one 
temperature cold solvent addition processes is presented in Table 2. This 
data demonstrates that the two temperature solvent addition process 
produced a wax product having a reduced oil content. For example, a 
comparison of run 2, the lowest oil content wax with conventional warm 
solvent addition, and run 8, the lowest oil content with two temperature 
solvent addition, showed that the two temperature solvent addition process 
produced a wax having only about one-sixth the oil content of the 
conventional warm solvent addition process even though approximately 30% 
more wash solvent was used in the conventional case. Thus, use of the 
present invention may reduce the residual oil content of the wax and/or 
permit the use of less wash solvent without increasing the residual oil 
content of the wax above a predetermined limit. 
TABLE 2 
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COMISON OF CONVENTIONAL AND TWO 
TEMPERATURE SOLVENT ADDITION 
Feed: Arab Light 600 Neutral 
Slack Wax; Oil Content = 30% 
Solvent: 50/50 v/v MEK/MIBK 
Conventional 
Cold Solvent 
Addition 
Conventional Warm Solvent Addition 
Two Temperature Solvent 
__________________________________________________________________________ 
Addition 
Tower Stages 14 14 14 
Solvent Temp .degree.C. 
-13(1-14) +9(1-14) -13(1-7) 
+25(8-14) 
(Stages) 
Tower Outlet .degree.C. 
10 25 25 25 
Run No. 1 2 3 4 5 6 7 8 9 10 
Slurry Dilution 
5.2 5.5 
5.2 
5.4 5.3 
5.3 
5.2 
5.3 5.3 4.8 
Wash Solvent v/v 
1.6 2.4 
1.5 
1.04 0.77 
0 2.2 
1.79 1.36 0 
Wash Time/Filter Time 
1.0 1.0 
0.71 
0.47 0.41 
0 1.0 
0.81 0.6 0 
Total Solvent v/v 
6.8 7.9 
6.7 
6.44 6.07 
5.3 
7.4 
7.09 6.66 4.8 
Performance 
Wt. % Oil in Wax 
1.3 1.2 
1.5 
3.5 8.0 
13.1 
0.2 
0.2 0.47 13.9 
Liquids/Solids w/w 
2.7 4.2 
3.5 
3.4 3.5 
2.8 
3.5 
3.3 3.1 2.6 
Wax Congealing Point .degree.C. 
67 70 70 69 69 67 70 69 69 67.5 
Wax Yield Wt. % on 
53 33.1 
34.4 
36.8 41.5 
49.5 
32.3 
32.9 34.1 50.8 
Slack Wax 
__________________________________________________________________________ 
Another comparison test was performed in which a conventional warm solvent 
addition process was compared with a two temperature solvent addition 
process. The feed, feed rate, inlet feed temperature, dilution, solvent 
composition and agitator tip speed were substantially similar to those for 
the previously described tests. In the conventional 14 stage warm solvent 
addition process the temperature of the stack wax again was reduced 
substantially uniformly from 57.degree. C. to an outlet temperature of 
25.degree. C. at a cooling rate of 1.5.degree. C./min. by the addition of 
9.degree. C. solvent. 
EXAMPLE II 
In this example substantially all the feed cooling was accomplished in a 
first mixing zone, comprising stages 1-10, using solvent at a temperature 
of approximately 9.degree. C. to simulate a cooling rate of 1.2.degree. 
C./min. To reach the desired outlet temperature of 25.degree. C. in 10 
stages without using an excessive amount of solvent, auxiliary jacket 
chilling of the slurry was utilized. Additional solvent was added to a 
second mixing zone comprising stages 11-14, at substantially the same 
temperature as the second mixing zone slurry inlet temperature, 25.degree. 
C. 
Table 3 presents comparative data on this conventional warm solvent 
addition process, and the two temperature solvent addition process. From a 
comparison of the data in Table 3 it can be seen that the two temperature 
deoiling process, where the temperature difference between the solvent 
added to the first and second mixing zones differed by approximately 
15.degree. C., also produced a wax having a significantly lower oil 
content, even though less solvent had been used. 
TABLE 3 
______________________________________ 
COMISON OF CONVENTIONAL AND 
TWO TEMPERATURE SOLVENT ADDITION 
Feed: Arab Light 600 Neutral 
Slack Wax; Oil Content = 30% 
Solvent: 50/50 v/v MEK/MIBK 
Two 
Temperature 
Conventional Warm 
Solvent 
Solvent Addition 
Addition 
______________________________________ 
Tower Stages 14 14 
Solvent Temp. .degree.C. (Stages) 
+9(1-14) +9(1-10) 
+25(11-14) 
Tower Outlet .degree.C. 
25 25 
Run No. 11 12 
Slurry Dilution 5.8 5.3 
Wash Solvent v/v 
1.0 1.0 
Wash Time/Filter Time 
0.5 0.8 
Total Solvent v/v 
6.8 6.3 
Performance 
Wt. % Oil in Wax 
5.0 2.1 
Liquids/Solids w/w 
3.46 2.3 
Wax Congealing Point, .degree.C. 
69 69 
Wax Yield Wt. % on 
36.7 33.4 
Slack Wax 
______________________________________ 
In the examples presented above, the temperature of the solvent added to 
the second mixing zone was substantially the same temperature as the 
slurry entering the second mixing zone. While it is not critical to the 
successful practice of this invention that the solvent and slurry added to 
the second mixing zone be at substantially the same temperature, 
frequently this will be the preferred method of operation, particularly if 
the solvent added to at least one of the zones requires some 
refrigeration. If the solvent utilized in the first mixing zone must be 
refrigerated to produce the desired cooling of the feed, addition of 
solvent to the second mixing zone at a significantly higher temperature 
than the slurry entering the second mixing zone would not be energy 
efficient, but would deoil the wax crystals. Conversely, addition of the 
solvent to the second mixing zone at a significantly lower temperature 
than that of the slurry entering the second mixing zone would promote 
additional crystallization and inhibit the desired removal of oil from the 
wax crystals. Since some variations may occur in the feed or solvent flow 
rates and/or temperatures, it may be desirable in some operations to add 
solvent to the second mixing zone at a slightly higher temperature than 
the normal temperature of the slurry entering the second mixing zone. This 
would assure that temperature and/or flow rate variations do not result in 
further crystallization of the slurry in the second mixing zone by the 
addition to the second mixing zone of solvent at a lower temperature that 
the slurry. The temperature of the solvent added to the second mixing zone 
preferably should be maintained to more than about 5.degree. C. above the 
average temperature of the slurry entering the second mixing zone. 
From the data of Tables 2 and 3 it can be seen that two temperature solvent 
addition permits a significant reduction in the oil content of the wax 
over one temperature solvent addition for similar solvent addition rates 
in similar crystallizer vessels. 
The specific solvent temperatures to be utilized in each zone will be 
dependent upon many factors including the following: lube oil content of 
the wax feed stream; solvent addition rate; desired residual lube oil 
content in product wax stream; available solvent cooling capacity; and 
desired final wax product congealing point or melting point. 
While the present invention has been described with particular reference to 
a specific multi-stage vessel which comprised the first and second mixing 
zones, it is clear that this invention also could be practiced utilizing 
other multi-stage vessel designs, or utilizing a plurality of mixing zones 
in separate vessels.