DESCRIPTION OF THE INVENTION
[0001] The filter feed rate and liquid/solid ratio of solvent dewaxed oil are improved by
conducting the lube oil solvent dewaxing process under conditions of agitation in
indirect chilling means while employing dewaxing aids. By practicing such dewaxing
aid assisted indirect chilling lube oil solvent dewaxing under agitated conditions,
the quantity of dewaxing aid employed can be minimal, on the order of 0.01 to 1.0
weight percent active ingredient, based on waxy oil feed. The indirect chiller apparatus
makes use of paddles, which preferably extend substantially the length of the chiller
apparatus, to produce agitation in the chiller. These paddles must not touch the interior
wall of the chiller since to do so results in scraping, which is detrimental to filter
rate and liquid/solid performance. In the present invention the paddles are at least
0.05 inch to 5 inches away from the internal wall of the chiller, i.e., a gap of 0.05
to 5 inches exists between the tip of the paddle and the internal chiller wall. This
gap is preferably 0.1 to 2 inches, most preferably 0.25 to 1 inch. Of course, the
larger gaps can exist only in those units which have a correspondingly larger internal
diameter. This gap, therefore, ranges from between about 0.5 to 40% of the chiller
internal diameter, preferably about 1 to about 40% of the chiller internal diameter,
most preferably about 2 to about 20% of the chiller internal diameter. These paddles
are rotated so as to exhibit a tip velocity of between about 25 to 400 ft./min., preferably
between about 50 to 250 ft./min., most prefer ably 50 to 100 ft./min., which for
the purposes of the specification is deemed to be tangential fluid velocity in the
region of crystallization. For example, when employing a chiller having 5.25 inch
internal diameter, paddles which exhibit a paddle to chiller wall gap of about 0.5
inch are rotated at speeds between about 50 to 200 rpm. Preferably, in such 5.25 inch
system paddle speeds of about 100 rpm are employed, as operation in this agitation
region produces the best results when one considers both feed filter rate and liquid/solid
ratio as a total entity. These rotational speeds correspond to fluid velocities of
between about 50 to 250 feet/minute in the crystallization region near the chilling
surface, where the flow generated by a rotating paddle is mainly tangential. This
additional agitation component, when added to the longitudinal flow due to fluid
velocity through the vessel will produce an excellent environment for good heat transfer,
optimum dewaxing aid-wax interaction and the formation of dense, spherical crystal
structures. The paddle rotational speed needed to achieve a tangential fluid velocity
of between about 25 to 400 feet/minute, preferably between about 50 to 250 feet/minute,
by means of a rotating agitator means in an indirect chiller with the agitator not
touching the walls of the chiller can be easily determined by practitioners for any
size apparatus. The determination of the range of distance (i.e., the size of the
gap) between the agitator blade tip and interior wall needed to insure meeting this
fluid velocity in the region of crystallization near the chiller wall within a given
range of tip velocities regardless of sizing of chiller equipment is similarly within
the skill of the practitioner with this teaching of the importance of this tangential
fluid velocity of between about 25 to 400 feet/minute preferably between about 50
to 250 feet/minute, most preferably between about 50 to 100 feet/minute before him.
[0002] The waxy oil feeds which may be effectively dewaxed employing the process of the
present invention include any waxy hydrocarbon lube oil feedstream, preferably a
waxy petroleum lubricating oil, speciality oil, turbine oil, white oils, refrigerator
oils, etc. These waxy hydrocarbon oil feedstocks can come from any natural or synthetic
source, including coal oils, tar sand oils, shale oils, etc. The natural petroleum
oil stocks can be any naphthenic or paraffinic oil, such as oils obtained from Aramco,
Kuwait, the Panhandle, North Lousiana, Western Canada, South Texas, Tia Juana, North
Sea, North Slope, etc. Typically, the waxy hydrocarbon lube oil feedstock will have
a boiling range within the broad range of about 500°F to about 1,300°F. The preferred
oil stocks are the lubricating oil and speciality oil fractions boiling within the
range of 550°F to 1,200°F (atmospheric).
[0003] Solvent dewaxing, as the name implies, employs a solvent to both dilute the oil and
as an aid in wax precipitation. This solvent can be selected from any of the known,
readily available solvents. Representative of such solvents are the aliphatic ketones
having from 3 to 6 carbon atoms, such as acetone, methylethyl ketone (MEK), methylisobutyl
ketone (MIBK), and mixtures thereof; the lower molecular weight, normally gaseous,
liquefied hydrocarbons, such as methane, ethane, propane, butane, propylene, butylene,
etc. or mixtures of solvents typically identified and described as autoreferigerative
solvents with the aforementioned ketones, such as propylene, MEK, mixtures of the
aforementioned ketones with C₆-C₁₀ aromatic hydrocarbons, such as MEK/toluene. Further,
halogenated hydrocarbons of lower molecular weight, such as C₁-C₄ halogenated hydrocarbons,
may be employed, as exemplified by C₁-C₄ chlorinated hydrocarbons (i.e., dichloromethane,
dichloroethane, tetrachloropropane, etc.) may also be employed. Ethers may also be
employed as dewaxing solvents, such as methyl tertbutyl ether (MTBE). The preferred
solvents are mixtures of MEK/MIBK, MEK/toluene, propylene/acetone.
[0004] The process will be conducted under typical solvent dewaxing conditions, including
chilling to a wax filtration temperature in whatever range satisfies the requirements
of the practitioner and is compatible with the oil being employed. Typically, waxy
oils are chilled to a temperature about 10 to 30°F below the finally desired oil cloud
point temperature. Consequently, if an oil with cloud point of about -10°F is desired,
the oil will be chilled to a temperature, called the wax filtration temperature, of
between about -20° to -40°F, depending on the oil and solvent employed. Chilling will
be at a relatively uniform rate so as to minimize the effects of shock chilling. Consequently,
chilling rates on the order of 1 to 20°F/minute will be employed.
[0005] The waxy lube oil feed is introduced at a temperature above its cloud point into
an indirect chilling apparatus, such as a double tube chiller. The space between the
inner and outer pipes is the space through which a chilling medium is passed. In such
indirect chilling apparatus the oil, with or without solvent predilution, preferably
with solvent predilution, is introduced into the central pipe in which an agitated
environment is maintained by means of a centrally rotating shaft upon which are located
paddles, blades, turbine plates, propellers, etc., in other words, means suited for
establishing a level of agitation in the central pipe. As previously stated, agitation
can be achieved by the use of propellers, blades or paddles which effectively sweep
the cold exchanger wall surfaces and which are uniformly distributed to sweep at least
10% of the wall, preferably about 50% of the wall surface, and most preferably at
least 90% of the wall surface. A typical design would be similar to a conventional
scraped surface exchanger where the scraper blades are moved away from the wall providing
greater than 90% sweeping of the cold exchanger surfaces. These agitator means, i.e.,
paddles, blades, propellers, etc., are sized so as not to touch the interior wall
of the chiller. Touching would result in a scraping action, which has been found to
be detrimental to filter rate and liquids/solids ratios on increasing the speed of
rotation of the scraper, as compared to the present invention where increased agitation
levels in combination with the use of added dewaxing aid has been found to very favorably
affect feed filter rate and liquid/solids ratios. In this manner, the contents present
in the central pipe are indirectly chilled by the chilling medium present in the inner
and outer pipes. The feed may be introduced into the central pipe undiluted with dewaxing
solvent, the dewaxing solvent being added incrementally along the length of the chiller.
This added solvent should be at about the same or a slightly lower temperature than
the waxy feed at the point of introduction. Alternatively, and preferably, the waxy
feed is pre-diluted with from 1 to 10 volumes of solvent prior to being introduced
into the central pipe, more preferably 2 to 5 volumes. This predilution alternative
is the preferred mode of operation. The agitation means within scraped surface chiller
devices must not be in its "as designed" configuration, that is, the scraper blades
must not be touching the interior walls of the chiller pipe, but must be adjusted
so as not to be in contact with the wall, but just sweep past the interior wall at
some specific distance from the wall. This configuration is necessary as it eliminates
the possibility of the precipitated wax being phsyically crushed by the blades as
the blades sweep past the interior wall, as compared to the configuration in which
the blades are in physical contact with the interior chiller walls. It is also advantageous
to move the blades away from the interior wall as this facilitates increasing the
rotational speed of the sweeper blade assembly without the need to overcome blad/wall
friction or drag.
DESCRIPTION OF THE FIGURES
[0006]
Figure 1 shows the improvement in dewaxing aid performance (at similar dose levels)
when a gapped agitator operates at a high agitation tip speed, as compared to a standard
scraper also operating at high rotational speeds.
Figure 2 shows that increasing agitator speed along which, produces some benefit,
is not as significant as when agitator speed is increased when a dewaxing aid is
used.
BACKGROUND OF THE INVENTION
[0007] In the past, wax precipitation was conducted under conditions of low or no agitation.
This procedure was followed since it was believed that precipitation under conditions
of high agitation would result in the formation of fine wax particles which could
clog the liquid-solid separators. The typical wax precipitation technique employed
scraped surface chillers. In such a unit, a waxy oil and a dewaxing solvent are premixed
at a temperature sufficient to effect complete solution of the oil and wax. If necessary,
the waxy oil is heated (either prior to or after addition of solvent) to insure complete
solution of the wax contained therein.
[0008] The solution is then indirectly cooled at a uniformly slow cooling rate, e.g., 1°F
to 5°F per minute, under conditions which avoid substantial agitation of the solution
during precipitation of the wax. Because of fouling of the exchanger wall in the indirectly
cooled heat exchangers due to wax deposition on the exchanger surface, scrapers are
employed to remove the wax. However, because of the physical crushing of the wax crystals
formed on the chiller wall by the action of the scrapers, non-uniform crystal growth
occurs which results in slow filtration rates and large amounts of occluded oil in
the wax.
[0009] The DILCHILL® (DILCHILL is a registered service mark of Exxon Research and Engineering
Company) process was developed so as to overcome the inherent limitations and disadvantages
of scraped surface chilling dewaxing. In the DILCHILL process, cooling is accomplished
in a staged tower. The waxy oil is removed through the tower, while cold solvent is
injected along the tower directly into a plurality of the stages (either some or all
of the stages have cold solvent injected into them). The cold solvent injection is
accompanied by a high degree of agitation in at least a portion of the stages containing
waxy oil and solvent so as to insure substantially instantaneous mixing of the cold
solvent and waxy oil. Chilling is conducted to a temperature of between about 0
F and 50°F. A substantial portion of the wax is precipitated from the waxy oil under
these conditions of cold solvent injection and high agitation. The DILCHILL process
is described in greater detail in U. S. Patent No. 3,773,650.
[0010] A modification of the DILCHILL process is presented in U. S. Patent No. 3,775,288.
In the modified DILCHILL process, cooling by means of cold solvent injection and
high agitation is conducted to a temperature greater than the temperature at which
the wax is separated from the oil, i.e., the wax separation temperature, but generally
less than about 40°F above said separation temperature, and preferably less than about
35°F above said separation temperature, thereby precipitating at least a portion
of the wax from the waxy oil. This oil-solvent-wax slurry is then withdrawn from the
DILCHILL chilling zone and introduced into a second chilling zone wherein it is cooled
to the wax separation temperature, thereby precipitating a further portion of the
wax from the waxy oil. Cooling rates in this zone are in the range of 5°F-20°F per
minute.
[0011] This modification is practiced so as to avoid employing the large volumes of cold
solvent which would otherwise be necessary to reduce the temperature of the oil-solvent-wax
slurry all the way down to the wax separation temperature. In this embodiment, the
second chilling zone may incorporate any conventional cooling process, such as scraped
surface chilling, autorefrigeration and the like; however, scraped surface chilling
is preferred. In scraped surface chillers the partially cooled oil-solvent-wax slurry
is indirectly cooled to the wax separation temperature without the addition of more
solvent. The scrapers are used to remove any wax which adheres to the walls of the
chillers. A disadvantage of the scraped surface chiller in this embodiment is the
same as that encountered when employing scraped surface chillers as the sole cooling
unit. The scrapers physically crush the wax crystals formed on the chiller wall, thereby
reducing the wax filtration rates and increasing the amounts of occluded oil in the
wax.
[0012] U. S. Patent No. 4,140,620 to Paulett described an incremental dilution dewaxing
process wherein a lubricating oil stock, at a temperature above its cloud point, is
cooled in a cooling zone with vigorous agitation to a temperature below its cloud
point and then further cooled with minimum agitation and incremental solvent addition
to its final temperature, followed by filtration for the removal of wax. Rapid stirring
is provided during the early part of the cooling period. The cooling zone is described
as being a conventional, double wall heat exchanger provided with means for agitating
the oil during cooling by more rapid rotation of the scrapers. The base oil stock
is diluted with solvent during the initial period of agitated chilling. The major
portion of the solvent is added to the system after the initial wax crystals have
formed, i.e., after the temperature of the oil base stock, with or without dilution,
has reached a temperature slightly below the cloud point of the waxy petroleum fraction.
From the figure in the patent it is seen that the cooling zone comprises a double
wall chiller wherein the waxy oil feed is introduced into the inner zone with cold
filtrate supplied to the outer jacket of the chiller, with increased agitation being
provided by increased rotational speed of the scrapers.
[0013] It is clear that the bulk of the solvent is added after the initial high agitation
cooling and before or during the low or no agitation final cooling steps.
[0014] The concept of using dewaxing aids in solvent dewaxing processes has also been addressed
in numerous patents.
[0015] U. S. Patent No. 4,354,003 and U. S. Patent No. 4,422,924, incorporated herein by
reference, teach a dewaxing aid and that solvent dewaxing is improved by using said
dewaxing aid, which dewaxing aid comprises a mixture of (a) an alpha-olefin polymer
having an average molecular weight of from about 10,000 to 1,000,000 and a wide molecular
weight distribution exceeding the range of from about 10,000 to 1,000,000, but falling
within the range of from about 2,000 to 3,000,000 wherein the alpha-olefin polymer
is a homo-polymer made up of a C₁₀ to C₂₅ alpha olefin monomer or is a copolymer
made up of a monomer mixture comprising more than 50 weight percent of at least two
C₁₀ to C₂₅ alpha olefin monomers, having a melt index greater than 1.8 g/10 min.;
and (b) an olefin vinyl acetate copolymer having a vinyl acetate content of from
about 15 to 40 weight percent an an average molecular weight of from about 50,000
to 1,000,000 and a melt index greater than 2 g/10 min.
[0016] In U. S. Patent No. 4,339,610, incorporated herein by reference, it is taught that
solvent dewaxing is improved by using a polymeric dewaxing aid which comprises a condensation
product of naphthalene and chlorinated wax, having an average molecular weight ranging
from about 20,000 to 500,000 and a molecular weight distribution exceeding the range
of from about 10,000 to 1,000,000.
[0017] In U. S. Patent No. 4,451,353, incorporated herein by reference, it is taught that
solvent dewaxing waxy hydrocarbon oil distillates is improved by using a dewaxing
aid which comprises a mixture of (a) polyalkyl acrylate having alkyl group side chain
lenghts of from 10-26 carbons (excluding branching); and (b) an n-alkyl methacrylate
polymer having alkyl group side chain lengths of from 10-20 carbons, excluding branching.
Component (a) typically has a number average molecular weight of from 3,000 to 500,000,
while component (b) has a number average molecular weight of from 5,000 to 200,000.
The combination (a) plus (b) may be employed in a weight ratio within the range from
about 1/100 to 100/1 at an aid dose level ranging from about 0.01 to 1.0 weight percent
active ingredient.
[0018] In U. S. Patent No. 4,460,453, incorporated herein by reference, it is taught that
solvent dewaxing of bright stock is improved by using a dewaxing aid comprising (a)
a poly di-alkyl fumarate/vinyl acetate copolymer in combination with (b) poly alkyl
(meth-) acrylate polymers. Component (a) has a number average molecular weight of
about 1,000 to 100,000 and possesses alkyl side chains of from C₁₆-C₂₄⁺ in length
excluding branching with a pendant side chain length of predominantly (>50%) C₂₀.
The polyalkyl (meth-) acrylate is the polymeric ester of aliphatic alcohols having
groups 10 to 20 carbons in length, excluding branching (pendant side chain length
predominantly (>50%) C₁₄ and lower) and acrylic or (meth-) acrylic acid (preferably
methacrylic acid) and possesses a number average molecular weight of at least 5,000.
The solvent dewaxing processes benefited by the use of the present combination are
those using standard, normally liquid solvents, such as C₃ to C₆ ketones, aromatic
hydrocarbons, halogenated hydrocarbons and mixtures thereof. The waxy hydrocarbon
oil dewaxed is a bright stock. The dewaxing aid combination may be used at a dose
level of from about 0.005 to 5 weight percent active ingredient at a component A/B
ratio of about 4/1 to 1/100.
[0019] U. S. Patent No. 4,439,308, incorporated herein by reference, teaches solvent dewaxing
Bright stock waxy raffinate oil employing a dewaxing aid, which dewaxing aid is a
mixture of (a) poly-di-alkyl fumarate/vinyl acetate copolymers; and (b) a wax-naphthalene
condensate. The dewaxing aid mixture is a mixture of (a) poly-di-alkyl fumarate/vinyl
acetate copolymer, having a number average molecular weight of about 1,000 to 100,000,
preferably 5,000 to greater, possessing alkyl side chains of from C₁₆-C₂₄⁺ in length
(excluding branching) with an average pendant side chain length of predominantly (>50%)
C₂₀; and (b) a wax-naphthalene condensation product having a number average molecular
weight of about 1,000 and greater. The combination (a) plus (b) may be employed in
a weight ratio within the range from about 45/55 to 1/100, preferably about 1/3, and
an aid dose level ranging from about 0.005 to 2.0 weight percent, preferably 0.01
to 0.2 weight percent active ingredient.
[0020] U. S. Patent No. 4,461,698, incorporated herein by reference, teaches solvent dewaxing
processes for dewaxing waxy hydrocarbon oil distillates employing a dewaxing aid,
which dewaxing aid is a mixture of (a) a poly-dialkylfumarate/vinyl acetate copolymer;
and (b) a wax-naphthalene condensation product. Component (a) has pendant alkyl side
chain groups of from 16 to 30 carbon atoms in length (excluding branching) with an
average pendant side chain carbon length of predominantly (>50%) C₂₂. Component (a9)
has a number average molecular weight of from about 1,000 to 100,000, preferably
greater than about 5,000. Component (b) has a number average molecular weight of at
least about 1,000. The combination (a) and (b) may be employed in a weight ratio A/B
within the range of from about 1/10 to 20/1 and at an aid dose level ranging from
about 0.005 to 2.0 weight percent.
[0021] In each of the above cases directed to practicing solvent dewaxing employing a dewaxing
aid the agitation conditions are either undefined or agitation was provided by employing
rotating scrapers (as in a scraped surface chiller) which were rotating at about either
12 rpm or 28 rpm.
[0022] U. S. Patent No. 4,441,987 describes dewaxing using an agitated chiller to chill
sovlent/oil-wax slurries to effect dewaxing. Agitation expressed in terms of Reynolds
numbers is on the order of 1,000 to 1,000,000. The patent is silent as to the use
of any dewaxing aid.
[0023] U. S. Patent No. 3,393,144 describes a dewaxing process which is benefited by using
an added dewaxing aid in a high agitation, indirect chilling environment for fuels
dewaxing. Agitation is on the order of 250 to 1,000 rpm and is recited in the Examples.
In the Examples feed filter rate improves on going to higher impeller speeds (rpm).
PRESENT INVENTION
[0024] In the present invention it has been discovered that feed filter rate is maximized
in a solvent dewaxing environment by practicing the dewaxing in the presence of a
dewaxing aid in indirect chilling means under conditions of agitation. From between
about 0.01 to 1.0 weight percent dewaxing aid active ingredient may be employed, preferably
between about 0.01 to 0.5 weight percent dewaxing aid active ingredient based on waxy
oil feed. Any of the dewaxing aids common in the trade may be employed, such as those
listed in the above-identified patents, as well as in U. S. Patent No. 3,475,321 and
U. S. Patent No. 3,479,278 to Henselman, et al. From 1 to 10 V/V dilution solvent
are employed, preferably with at least part of the solvent being added to the waxy
feed prior to the feed being introduced to the dewaxing zone. From .2 to 2 volumes
of solvent per volume of waxy feed is preferably used as the predilution solvent.
Total solvent employed (predilution plus dilution) is preferably in the 1 to 8 V/V
range.
[0025] The dewaxing environment is an agitated environment. Agitation can be achieved,
for example, by employing indirect chillers within which is a rotating shaft upon
which are affixed paddles or blades extending substantially the entire internal length
of the chiller unit, rotating at a paddle tip velocity of about 25 to 400 ft./min.
The system for practicing this invention uses rotating paddles having a diameter only
slightly smaller than the chilling vessel, the gap between the paddle tip and the
chiller internal wall being from about 0.5 to about 40% of the chiller internal diameter,
preferably 1 to 15% of the chiller internal diameter, most preferably about 2% to
about 20% of the chiller internal diameter. This configuration imparts additional
agitation, which is mainly radial and tangential close to the chiller wall in the
critical region where crystallization is initiated. The data indicate a sensitivity
and critical agitation limitation of about 50 to about 250 feet per minute fluid velocity
near the chilling surface. With this present teaching before him, the practitioner
can easily determine how to achieve such levels of fluid velocity for any size apparatus.
For example, in a 12" vessel with an 11" agitator a tip speed of 100 ft./min. would
require an agitator speed or 35 rpm.
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[0026] Indirect chilling devices are benefited most by the practice of the present invention.
Practicing the present invention with direct chilling dewaxing procedures does not
result in the same, if any, benefit.
[0027] A single number, called the activity parameter, is suggested as a convenient measure
of crystallization performance. This number, which is the feed filter rate in m³/m²
day divided by the wax cake liquids/solids, increases in magnitude as dewaxing performance
improves. Typical activity parameters are summarized below in Table II for a 600 neutral
feedstock dewaxed under lab-simulated surface chilling (comparing scraped surface
chilling and agitated surface [blades not touching the walls] conditions and DILCHILL
conditions.
TABLE I
Feed
[0028] (600N oil) Typical Properties
Feed Cloud, °C 51
Dist. Range, °C 383 to 582
Viscosity cSt @ 100°C 9.56
R. I. @ 75°C 1.4596
API Gravity 28.8
Dewaxing Aid
Component 1 - (25 Wt.% as received)
[0029] A dialkylfumarate vinyl acetate copolyer made from a mixture of 70% C₂₂, 15% C₂₀
and 15% C₁₈ alcohols.
Molecular Weight (by GPC)
[0030] Weight Average 8,600
Number Average 61,000
Compoent 2 - (75 Wt.% as received)
[0031] Acryloid 144 (Rohm and Haas Company)
A polyalkyl methacrylate polymer made from a mixture of 4% <C₁₂, 7% C₁₄, 39% C₁₆,
45% C₁₆, 45% C₁₈, 5% >C₂₀ alcohols.
Molecular Weight (by GPC)
[0032] Weight Average 33,000
Number Average 206,000
Predilution Conditions:
[0033] 2.4 v/v 40/60 MEK/MIBK @ 60°C; Filter @ -10°C.
Incremental Dilution Conditions:
[0034] .6 v/v @ 60°C
.7 v/v @ 35°C
.7 v/v @ 5°C
.4 v/v @ -10°C
Filter @ -10°C
[0035] Chilling was performed using externally chilled equipment. The waxy oil-solvent-dewaxing
aid slurry was formed from the cloud point (about 60°C) down to the filter temperature
(about -10°C). External bath temperature was about 10°C below the slurry temperature.
Volumes of solvent (as indicated above) were added during the chilling as the slurry
reached the recited temperature using the external chilling. Final external bath temperature
was about -20°C.
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[0036] The data show that both the total predilution and increased agitation contribute
to the significant improvements in dewaxing aid activity at low doses with agitation
being the prime variable. This suggests that the controlling limits are the magnitude
of the turbulence produced in the system, a function of average slurry viscosity,
slurry flow rate and agitator speed.
[0037] Additional experiments were carried out under total solvent predilution conditions
(described above) using 2.4 v/v on feed of a 40/60 V/V mixture of MEK/MIBK. The slurry
was chilled at about 3°C per minute using either a rotating scraper or rotating paddle.
The scraper simulates a plant spring loaded scraper, whereas the paddle simulates
the same equipment configuration with the scrapers removed from the wall to produce
about a 1/2" scraper tip-to-wall gap.
[0038] Filtration performance is significantly improved for the agitated surface chilling
unit when dewaxing aid is used as shown by the comparison data above and in Tables
III and IV. The best overall performance considering both feed filter rate and liquids/solids
in agitated chiller apparatus is achieved at 0.5% dewaxing aid at 100 rpm, which for
the 5.25 inch internal diameter indirect chiller corresponds to a paddle tip-fluid
velocity of 111 ft./min. (relative activity parameter 14.6). However, at 400 rpm,which
corresponds to a paddle tip-fluid velocity of 445 ft./min., while extremely low liquids/solids
of 1.2 is demonstrated, feed filter rate declines. The ability to reduce the dewaxing
aid dosage to 0.1 weight percent and maintain relative activity parameter above 10
are shown, compared to the maximum activity para meter of 8.5 at 1.0 weight percent
aid dosage using conventional scraped surface equipment (see Table II, 25 rpm at 1.0
weight percent aid dose).
[0039] Figure 1 demonstrates the significant improvements in activity parameters for the
paddle over the scraper operating at similar rpm in identical systems employing the
data from Tables III and IV.
[0040] The advantages of this invention are, therefore, seen to be:
1. Low dewaxing aid dosages;
2. Low apparent slurry viscosities;
3. High feed filter rates;
4. Dense spherical wax structures with an absence of fines;
5. Low cake liquids/solids;
6. Reduced solvent volumes; and
7. Higher dewaxed oil yields.
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[0041] Comparison of the data clearly demonstrates the positive performance advantages of
paddle agitation versus the negative effects produced by increased scraper agitation.
With paddle agitation at the low dewaxing aid level feed filter rate peaks at between
100 and 150 feet per minute tip speed and at the high aid level at between 50 and
100 feet per minute. In contrast, when scraper-induced agitation is employed filter
rates gradually decrease to below base case levels. With the paddle the liquids/solids
ratio levels out to levels below 2 at agitator speeds of between 100 and 150 feet
per minute. With the scraper liquids/solids show an increase at the high aid level
about 150 feet per minute and absolute values significantly higher than the paddle
cases. Activity parameters (feed filter rate divided by the liquids/solids ratio)
demonstrate the overall significant performance improvements caused by optimized dewaxing
aid/wax interaction with peaks in the 100 to 250 feet per minute tip speed range when
paddle agitation is used. Increasing scraper speed over this range produces negative
results, especially at high aid dosage.
[0042] The effect of the size of the gap between the paddle and the internal chiller wall
is presentedin Table V. From the data it is seen that the presence of a gap significantly
increases the performance of the dewaxing system. The 600N oil used in the runs reported
in Table V had the following properties:
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