Process for the distillation of Fisher-Tropsch products

A process for the distillation of a hydrocarbon mixture, which mixture has been prepared by a Fischer-Tropsch synthesis, comprises supplying the hydrocarbon mixture to a wiped film evaporator and recovering a first fraction having a low boiling point range and a narrow melting point range as the light product from the evaporator and recovering a second fraction having a high boiling point range and a wider melting point range as the heavy product of the evaporator. The process is particularly suitable for the distillation of hydrocarbons having a carbon number of C.sub.20 or greater.

FIELD OF THE INVENTION 
The present invention relates to a process for the distillation of waxes, 
in particular hydrocarbon waxes, such as paraffin waxes produced by a 
Fischer-Tropsch synthesis. 
BACKGROUND OF THE INVENTION 
It is well known in the art that hydrocarbons may be prepared by means of 
the Fischer-Tropsch synthesis, in which a mixture of carbon monoxide and 
hydrogen is contacted at elevated temperature and pressure with a suitable 
catalyst. Recently, it has been found that certain catalysts active in the 
Fischer-Tropsch synthesis are highly selective in the preparation of 
hydrocarbons having high molecular weights, in particular paraffinic 
hydrocarbons. Fischer-Tropsch catalysts comprising cobalt as the 
catalytically active metal have been found to be particularly selective in 
the preparation of the aforementioned paraffinic hydrocarbons. 
In the preparation of commercial products from the hydrocarbon effluent of 
the Fischer-Tropsch synthesis, it is most convenient to separate and 
refine the components of the effluent by applying distillation. The lower 
molecular weight components of the hydrocarbon effluent may be separated 
and refined by means of conventional distillation techniques operated at 
atmospheric or super-atmospheric pressures. However, the higher molecular 
weight components of the hydrocarbon effluent may be subject to thermal 
degradation at the high temperatures encountered in conventional 
atmospheric distillation operations. To avoid such degradation occurring, 
it is necessary to apply vacuum distillation techniques in the separation 
and refining of these higher molecular weight components. 
A variety of vacuum distillation techniques and apparatus are known for 
separating and refining thermally sensitive materials, for example the 
operation under vacuum of conventional distillation columns as practiced 
in the conventional refining of crude oil. In addition, a number of 
specific vacuum distillation techniques have been developed, for example 
the short-path vacuum distillation techniques. 
More recently, the use of a specific distillation apparatus, namely the 
wiped film evaporator, has been proposed for use in refining the thermally 
sensitive, high molecular weight hydrocarbons recovered from conventional 
crude oil refining operations. Thus, U.S. Pat. No. 5,032,249 discloses a 
process in which a petroleum wax, in particular a heavy intermediate 
petroleum wax, is separated into two fractions in a wiped film evaporator 
to provide a lower boiling fraction having a narrow melting range and a 
higher boiling fraction having a wider melting range. 
It has now been found that wiped film evaporators may be advantageously 
applied in the separation and refining of the hydrocarbon product of a 
Fischer-Tropsch synthesis, in particular the higher molecular weight 
hydrocarbon components of the product. 
SUMMARY OF THE INVENTION 
The present invention therefore provides a process for the distillation of 
a hydrocarbon mixture, which mixture has been prepared by a 
Fischer-Tropsch synthesis, which process comprises supplying the 
hydrocarbon mixture to a wiped film evaporator and recovering a first 
fraction having a low boiling point range and a narrow melting point range 
as the light product from the evaporator and recovering a second fraction 
having a high boiling point range and a wider melting point range as the 
heavy product of the evaporator. 
DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The process of the present invention provides the advantage that the 
hydrocarbon products of the Fischer-Tropsch synthesis may be separated and 
refined without substantial degradation of the product occurring, the 
products of which require little or no finishing treatment to meet the 
standards required of commercial hydrocarbon products. In particular, it 
has been found possible to prepare hydrocarbon wax products achieving 
excellent standards of color and odor. The hydrocarbon mixture for use as 
feed for the process of the present invention is prepared by means of the 
Fischer-Tropsch synthesis. In the Fischer-Tropsch synthesis, a mixture of 
carbon monoxide and hydrogen is contacted with a suitable synthesis 
catalyst at elevated temperature and pressures. Typical operating 
conditions for the synthesis are temperatures in the range of from about 
125.degree. C. to about 350.degree. C., preferably from about 175.degree. 
C. to about 250.degree. C., and a pressure in the range of from about 5 to 
about 100 bar, preferable from about 10 to about 50 bar. 
Suitable catalyst compositions for use in the Fischer-Tropsch synthesis are 
well known in the art. Catalytically active metals for use in the 
Fischer-Tropsch catalysts are typically selected from Group VIII of the 
Periodic Table of the Elements. In particular, metals selected from iron, 
cobalt, nickel and ruthenium have been found to be particularly suitable 
for preparing high molecular weight hydrocarbons. Cobalt-containing 
catalysts have been found to be especially suitable for use in the 
preparation of high molecular weight paraffinic hydrocarbons in high 
yields. 
The catalytically active metal is preferably supported on a porous carrier. 
The porous carrier may be selected from any of the suitable refractory 
metal oxides or silicates or combinations thereof known in the art. 
Particular examples of preferred porous carriers include silica, alumina, 
titania and mixtures thereof. Silica is a particularly preferred carrier 
material. 
The amount of catalytically active metal on the carrier is preferably in 
the range of from about 3 to about 10 parts by weight per 100 parts by 
weight of carrier material, more preferable from about 10 to about 80 
parts by weight. 
Catalysts for use in the Fischer-Tropsch synthesis may also comprise one or 
more co-catalysts or promoters. Suitable promoters include oxides of 
metals selected from Groups IIA, IIIB, IVB, VB and VIB of the Periodic 
Table of the Elements, or the actinides or the lanthanides. Oxides of 
metals from Group IVB of the Periodic Table, in particular titanium and 
zirconium, are preferred. Suitable metal promoters include metals of Group 
VIII of the Periodic Table, with platinum and palladium being preferred. 
The amount of promoter present in the catalyst is in the order of from 
about 0.1 to about 150 parts by weight per 100 parts by weight of the 
carrier material. 
Processes for preparing suitable Fischer-Tropsch catalysts are known in the 
art, for example as described in European Patent Applications Publication 
Nos. EP 0,104,672, EP 0,110,449, EP 0,127,220, EP 0,167,215, EP 0,180,269, 
EP 0,221,598 and EP 0,428,223. 
The hydrocarbon mixture produced by the Fischer-Tropsch synthesis comprises 
a wide range of hydrocarbon components having a wide range of molecular 
weights. The lower molecular weight or lighter components may be removed 
from the mixture by conventional distillation techniques prior to the 
mixture being used as feed in the process of the present invention. The 
process of the present invention has been found to be particularly 
suitable for the separation and refining of Fischer-Tropsch hydrocarbon 
products having a carbon number of C.sub.20 and above. The process has 
been found to be especially suited to the refining of substantially 
paraffinic hydrocarbons having carbon numbers in this range. It is to be 
noted that paraffinic hydrocarbons having carbon numbers of C.sub.20 or 
greater and produced by the Fischer-Tropsch synthesis are present as 
solids under conditions of ambient temperature and pressure. 
The wiped film evaporators (also referred to as agitated thin-film 
evaporators) for use in distillation process of the present invention are 
known in the art and are available commercially. In this respect, for a 
general discussion of the principle of operation of these evaporators 
reference is made to "Agitated Thin-Film Evaporators: A Three Part 
Report", Parts 1 to 3; A. B. Mutzenburg, N. Parker and R. Fischer; 
Chemical Engineering, Sep. 13, 1965. 
Typically, wiped film evaporators comprise a generally cylindrical 
evaporating vessel. The vessel may be either vertical or horizontal, with 
vertically arranged vessels being preferred. The evaporator further 
comprises a rotor mounted within the cylindrical evaporating vessel and 
provided with a number of wiper blades. A motor is provided to drive the 
rotor. The rotor is arranged within the cylindrical evaporating vessel so 
that, upon rotation by the motor, the wiper blades are caused to move over 
the inner surface of the cylindrical vessel. The wiper blades may contact 
the inner surface of the cylindrical vessel. Alternatively, a gap or 
clearance may be left between the tips of the wiper blades and the inner 
surface of the cylindrical vessel. 
In operation, the hydrocarbon mixture to be separated or refined is fed, 
supplied or subjected to the evaporator and forms a thin film over the 
inner surface of the cylindrical vessel. The film is heated, typically by 
means of indirect heat exchange with a heating medium through the wall of 
the cylindrical vessel, for example steam. The action of the wiper blades 
in passing over the surface is to agitate the film of hydrocarbons, 
resulting in turbulence in the film, which in turn improves heat and mass 
transfer. In addition, the wiper blades ensure an even distribution of the 
hydrocarbons over the inner surface of the vessel and prevent channelling 
of the liquid as it passes across the surface. Under the action of the 
wiper blades and the heating, the lighter components of the hydrocarbon 
mixture are caused to evaporate. 
The light product is removed from the evaporator as a vapor and is 
subsequently condensed. Condensing is conveniently effected by indirect 
heat exchange with a cooling medium, for example water. The condenser may 
be separate from the evaporator vessel or may be located within the 
vessel. In the latter case, the vessel will comprise a first evaporating 
section in which the rotor and wiper blades are arranged and a second 
condensing section in which the condenser is housed. If desired, a 
separating section may be disposed between the evaporating section and the 
condensing section to allow removal of any liquid droplets entrained in 
the vapor prior to condensing. 
The heavy product is removed from the evaporator as a liquid leaving the 
inner surface of the cylindrical vessel. The wiped film evaporator is 
operated under a vacuum. Suitable pumps for the generation and maintenance 
of the vacuum are well known in the art. Typical examples of suitable 
pumps include steam ejector pumps and diffusion vacuum pumps. 
During the operation of the process of the present invention, the 
hydrocarbon mixture to be separated is first heated to a temperature 
sufficient to soften and melt the mixture, if necessary, and to reduce the 
viscosity of the mixture, thereby allowing it flow. The hydrocarbon 
mixture is then introduced into the evaporator to form a thin film on the 
inner surface of the cylindrical evaporator vessel. The operating 
pressures for the wiped film evaporators will vary according to the 
precise hydrocarbon feedstock. Typical operating pressures are in the 
range of from about 0.02 to about 10 millibar absolute (mbara), more 
preferable from about 0.05 to about 7.5 mbara. Operating temperatures for 
the wiped film evaporator will also depend upon the particular feedstock 
being processed. Typically, operating temperatures of the wiped film 
evaporator will be in the range of from about 100.degree. C. to about 
350.degree. C., more preferably in the range of from about 150.degree. C. 
to about 300.degree. C. The residence time of the wax within the 
evaporator is relatively very low, compared with that of a conventional 
vacuum distillation apparatus. Typical residence times range from about 20 
seconds to about 5 minutes, depending upon the feedstock and the design of 
evaporator being employed. It is important, however, that the operating 
temperature is not so high as to lead to a substantial degree of thermal 
degradation or cracking of the hydrocarbon mixture being processed at the 
particular residence time and that the operating conditions of temperature 
and pressure are selected to ensure that such high temperatures are not 
required. 
The process of the present invention is applied in the separation of the 
hydrocarbon mixture feed into a light fraction having a narrow melting 
point range and a heavy fraction having a wider melting point range. 
Generally, a given fraction or grade of hydrocarbon will comprise 
hydrocarbons having a range of melting points and boiling points. 
Accordingly, it is usual to assign a melting point range to a fraction, 
which range extends from the melting point of the lightest or lowest 
melting component to the heaviest or highest melting component. Suitable 
methods for determining the melting point range of a hydrocarbon fraction 
are well known in the art and described, for example, in The Chemistry and 
Technology of Waxes by A. H. Warth, Second Edition, Reinhold Publishing 
Corporation, at pages 602 to 605. 
The process of the present invention may be used to distill hydrocarbon 
mixtures prepared by Fischer-Tropsch synthesis and comprising compounds 
having carbon numbers ranging, for example, from C.sub.18 to C.sub.40 and 
even higher. Such a hydrocarbon mixture substantially consisting of 
paraffinic hydrocarbons and comprising C.sub.18 to C.sub.40+ compounds 
typically has a melting point range of from about 28.degree. C. to in 
excess of 83.degree. C. A typical light product from the wiped film 
evaporator may comprise compounds in the range of from C.sub.18 to 
C.sub.20, having a melting point range of from about 28.degree. C. to 
about 38.degree. C. and a boiling point range of from about 315.degree. C. 
to about 345.degree. C. (under atmospheric conditions of pressure). 
In another typical process scenario, a hydrocarbon feed substantially 
consisting of paraffinic hydrocarbons and comprising compounds having 
carbon numbers in the range of from C.sub.21 to C.sub.40 and higher, 
having a melting point range of from about 40.degree. C. to in excess of 
83.degree. C. and a boiling point range of from about 355.degree. C. to in 
excess of 390.degree. C. (under atmospheric pressure), may be distilled in 
a wiped film evaporator to yield a light product comprising compounds in 
the range of from C.sub.21 to C.sub.27, having a melting point range of 
from about 40.degree. C. to about 60.degree. C. and a boiling point range 
of from about 355.degree. C. to 442.degree. C. (under atmospheric 
pressure). Other light fractions having different ranges of melting points 
and boiling points may also be prepared. 
The hydrocarbon mixture may be refined into a range of fractions in a 
number of stages, with one or more wiped film evaporators being employed 
in each stage. 
The hydrocarbon products of the process of this invention may be subjected 
to conventional finishing processes known in the art to yield a 
commercially acceptable material. Such finishing processes include mild 
hydrotreating or hydrofinishing. Hydrofinishing processes remove any 
oxygenates, olefins or aromatic hydrocarbons which may be present in the 
hydrocarbon fractions leaving the wiped film evaporator. Suitable 
hydrofinishing processes, in particular catalytic hydrofinishing 
processes, are well known in the art. However, it has been found that the 
process of the present invention allows hydrocarbon mixtures prepared by 
Fischer-Tropsch synthesis processes to be distilled into the required 
fractions or grades without significant degradation or cracking of the 
feed occurring. This in turn results in significantly less or even no 
finishing treatments being needed to meet the requirements of, for 
example, color and odor of the finished hydrocarbon fraction.

The process of the present invention will be further described, by way of 
illustration only, in the following example. 
EXAMPLE 
Hydrocarbon Preparation 
A hydrocarbon mixture was prepared by means of the Fischer-Tropsch 
synthesis using the following method: 
A cobalt/zirconium/silica catalyst was prepared following the procedure 
described in European Patent Application publication No. 0, 428,223. The 
catalyst was loaded into a reaction vessel and reduced by contacting the 
catalyst with a mixture of hydrogen and nitrogen at a temperature of 
250.degree. C., a pressure of 5 bar and a gas hourly space velocity of 
from 500 to 600 Nl/l/h. The activated catalyst was then contacted with a 
mixture of carbon monoxide and hydrogen having a hydrogen/carbon monoxide 
ratio of 1.1 at a gas inlet pressure of from 37 to 39 bar, a temperature 
of from 210.degree. to 220.degree. C. and a gas hourly space velocity of 
from 1110 to 1130 Nl/l/h. The product of the reaction was a mixture of 
substantially paraffinic hydrocarbons. 
The hydrocarbon fraction was subjected to a conventional distillation to 
remove the C.sub.20 -components, leaving a C.sub.21 + hydrocarbon mixture. 
Distillation of Hydrocarbon Mixture 
The C.sub.2 + hydrocarbon mixture produced as hereinbefore described was 
fed to a vertical wiped film evaporator of the SAMVAC type. The wiped film 
evaporator was operated at a temperature of between 190.degree. and 
195.degree. C. and a pressure of 0.1 mbara. The vapor leaving the wiped 
film evaporator was collected and condensed to yield a light hydrocarbon 
fraction. The liquid leaving the wiped film evaporator was collected as a 
heavy fraction. The carbon number distribution of the light and heavy 
fractions recovered is given in Table 1. 
TABLE 1 
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Light Heavy 
Fraction 
Fraction 
(wt %) (wt %) 
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C.sub.20 - 0.2 0.0 
C.sub.21 to C.sub.27 
67.3 1.7 
C.sub.28 to C.sub.40 
32.5 39.2 
C.sub.40 + trace 59.1 
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The light hydrocarbon fraction recovered had a Saybolt color of greater 
than +30, a penetration value (at 25.degree. C.) of 34 and a melting point 
of 53.degree. C. The light hydrocarbon fraction did not require further 
finishing treatment.