Process for low temperature carbonization of hydrogenation residues

The residues obtained in the hydrogenation of oil, especially heavy oil, or of coal are subjected to low temperature carbonization in a drum, preferably a rotary drum, at temperatures between approximately 400.degree. C. and approximately 600.degree. C., by means of a carbonization gas after the separation of the condensable portions and heating to temperatures between approximately 600.degree. C. and approximately 950.degree. C. which is introduced into the low temperature carbonization drum. The gas is heated to temperatures between approximately 600.degree. C. and approximately 950.degree. C. indirectly by flue gases arising from the combustion of oil or gas, for example, of excess carbonization gas. The residue to be carbonized at low temperature is introduced into the hot gas in a finely dispersed state and preferably atomized.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to an improved process for the low temperature 
carbonization of residues produced by the hydrogenation of oil, especially 
heavy oil, or of coal. More specifically, these residues, produced during 
the hydrogenation process, are subjected to low temperature carbonization 
in a drum, preferably a rotary drum, at temperatures between 400.degree. 
C. and 600.degree. C., whereby a carbonization gas is introduced, after 
the separation of the condensable portions and heating to temperatures 
between 600.degree. C. and 950.degree. C., into the low temperature 
carbonization drum and the low temperature carbonization of the residues 
is thereby effected. 
2. Description of the Prior Art 
In the hydrogenation of coal and mineral oils, especially of distillation 
residues of mineral oils as well as of heavy and very heavy oils at 
temperatures of 400.degree. C.-520.degree. C. and pressures of 100-700 
bars in the liquid phase, the hydrogenation product contains, in addition 
to the desired products that are volatile at the reaction temperature, 
nonvolatile residues, such as asphaltenes, catalysts and unreacted carbon 
in coal hydrogenation and/or coal or coke as catalyst supports in oil 
hydrogenation. The residues are separated from the gaseous product in one 
or more hot separators and must be reprocessed. In the coal hydrogenation 
process used in Germany until the end of World War II, residues, 
optionally after increasing the solids content by centrifuging, were 
introduced into a low temperature carbonization plant, in which 
nonvolatile portions were decomposed into gas, oil and coke. 
A typical low temperature carbonization plant for the hydrogenation 
residues consisted of a slightly inclined rotary drum type kiln externally 
heated by gas burners, into which the pasty residue, preheated to 
400.degree. C.-450.degree. C., was introduced and subjected to low 
temperature carbonization at temperatures of approximately 550.degree. 
C.-600.degree. C. (cf. W. Kroenig "Die katalytische Druckhydrierung von 
Kohlen, Teeren und Mineraloelen," Berlin/Goettingen/Heidelberg 1950, 
especially pages 44-45 and 188-189 as well as M. Hoering and E. E. Donath 
in "Ullmanns Enzyklopaedie der technischen Chemie," third edition, volume 
10, Munich/Berlin 1958, especially pages 518-519.) To avoid coke deposits 
building up on the inside of the walls of the drum, the latter contained 
steel balls that removed deposits from the walls as the drum revolved by 
attrition. About 100 kg of steam per ton of feedstock were introduced into 
the drum to act as a sweep-through gas. 
In this previous and known process, the specific throughput of the low 
temperature carbonization drum was essentially determined by the heat 
introduced from the outside per square meter of drum surface. By 
preheating the residue to be carbonized to a temperature as close as 
possible to the cracking temperature, the heated drum surface could be 
correspondingly reduced or the drum throughput increased. However, this 
method of increasing throughput was limited by the onset of the cracking 
reaction during the heating period, if the preheating temperature became 
too high. 
Since the contents of the low temperature carbonization drums were heated 
externally, through the walls of the drums, this process required a great 
deal of energy and therefore was not very economical. Moreover, the 
annoyance caused by the noise of the steel balls falling inside the drum 
was considerable. Furthermore, purification of the foul water resulting 
from condensation of the sweep-through steam was very expensive. 
Therefore, it was generally believed that such low temperature 
carbonization drums or processes would not be used in the future (cf. 
Winnacker-Kuechler, "Chemische Technologie," Munich/Vienna 1981, Volume 5, 
page 457). 
SUMMARY OF THE INVENTION 
In accordance with the present invention, it has surprisingly been 
discovered that the low temperature carbonization of residues produced 
during the hydrogenation of coal and mineral oils, especially heavy oils, 
can be economically and efficiently effected by subjecting these residues 
to a circulating carbonization gas, heated to temperatures between 
approximately 600.degree. C. and approximately 950.degree. C. in a drum at 
temperatures within the range of approximately 400.degree. C. to 
approximately 600.degree. C. 
Futhermore, the process of the present invention has proved to be highly 
efficient because, among other reasons, the carbonization gas serves not 
only as a heat transfer medium but also as a sweep-through gas or as a 
fuel gas after the separation of its condensable portions. 
Still further in accordance with the present invention, deposits on the 
walls of the low temperature carbonization drum may be reduced by 
injecting the feedstock, i.e., hydrogenation residue, in a finely 
dispersed form into the carbonization gas stream. 
Still further in accordance with the present invention, the efficiency of 
the process of the present invention is increased by heating the 
carbonization gas which is introduced into the low temperature 
carbonization drum by passage through a heat exchanger whereby heat is 
transferred to the carbonization gas from flue gases produced during the 
combustion of oil or gas, e.g., carbonization gas that is not needed as 
recycled gas for recirculation through the carbonization drum. 
These and other aspects of the invention will become clear to those skilled 
in the art upon the reading and understanding of the specification.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Surprisingly, it has now been found that the disadvantages of drum low 
temperature carbonization can be avoided if the drum contents, i.e. the 
low temperature carbonization material, are heated directly by a fuel gas 
heated to temperatures between approximately 600.degree. C. and 
approximately 950.degree. C. The gas resulting from the carbonization of 
the drum contents (i.e. carbonization gas) serves as a fuel gas after the 
removal of its condensable portions. Furthermore the fuel gas has a higher 
temperature than the formerly used steam. Thus, it acts not only as a 
sweep-through gas as did the formerly used steam but also as fuel for the 
low temperature carbonization of the hydrogenation residue. 
The carbonization gas, serving as a heat transfer medium, is circulated 
between the low temperature carbonization drum, a condenser and a heat 
exchanger. Condensation occurs in the usual manner, however the process is 
simpler than the former process due to the absence of steam. This also 
reduces the environmental problems which were associated with water 
disposal for the former process. In addition, the condensation stage is 
smaller and the coolant consumption is reduced. For the reheating of the 
recycle gas, the heat generated in the burning of the carbonization gas 
that is not needed as recycle gas can be used. This heat is transferred to 
the recycle gas by a heat exchanger. 
Compared to the former process with preheating of the feedstock to be 
carbonized, the process of the invention makes possible a specific 50% 
increase in throughput of the feedstock for the same drum size and has 
also a lower specific fuel gas requirement. Direct heating by the recycle 
gas enables the drum size to be reduced for a specific throughput. 
Since in the proposed process the low temperature carbonization takes place 
mainly in the space enclosed within the drum rather than at the drum wall 
in contrast to the former process, the tendency to form coke desposits on 
the wall is reduced. The tendency for deposition can be further 
counteracted by entraining the feedstock in a finely dispersed form, for 
example by atomisation, into the heating gas stream. Therefore, the use of 
steel balls in the drum can be abandoned. However, it can be advantageous 
to entrain additional solid matter together with the feedstock, for 
example fine coke particles that were produced earlier during the course 
of the low temperature carbonization, as seed to initiate the formation of 
coke particles. 
As previously described, the heat required for the low temperature 
carbonization of the hydrogenation residues is introduced into the low 
temperature carbonization drum for the most part by the circulating 
carbonization gas. The heating of the drum shell therefore could be 
dispensed with; however, it has proved to be useful to supply to the drum 
shell the heat being lost by radiation. This can be accomplished by 
external heating, for example with excess flue gas. 
A non-limiting embodiment according to the process of the invention is 
explained below in conjunction with the accompanying drawing. 
The feedstock, e.g. hydrogenation residue, 6 is atomized into the rotary 
drum 1, for example, at a temperature of approximately 370.degree. C., 
where the low temperature carbonization takes place. The drum, which is 
supported by rollers, is inclined and is driven by a motor. The heat 
required for low temperature carbonization is supplied directly by 
recycled carbonization gas 7, for example at a temperature of 600.degree. 
C.-950.degree. C. In the embodiment of the process illustrated in FIG. 1, 
the feedstock 6 and fuel gas 7 enter the drum in parallel flow directions. 
However they can also enter the drum countercurrent to each other. The 
coke 12 obtained by the low temperature carbonization is discharged from 
the drum by a suitable device. 
The carbonization gases 11 are carried to the condenser 2 in which the low 
temperature carbonization oil 13 is separated. The uncondensed portions of 
the carbonization gas are removed by blower 3. Part of the gas stream 14 
is transported for further processing and the other part 7 is recycled by 
way of the heat exchanger 4 to the rotary drum 1. The flue gases 15 
produced in the combustion chamber 5 by burning of fuel gas 10, especially 
excess carbonization gases 14 not needed as recycle gas, with air 9, are 
cooled in the heat exchanger 4 against the recycled carbonization gas 7, 
whereby the heat required for direct heating of the drum contents is 
obtained. 
External heating of the drum by flue gases 15 produced by burning of fuel 
gas 10, especially excess carbonization gas 14, with air 9 in the 
combustion chamber 5 serves to compensate for heat losses to the 
surroundings as well as to start up the process. 
Solid matter 8 may be added into the rotary drum, especially limestone 
and/or coke, to bind harmful substances during low temperature 
carbonization and to promote coke separation. 
While the invention has been described and illustrated above with reference 
to a specific preferred embodiment thereof, those skilled in the art will 
appreciate that various changes, modifications and substitutions can be 
made therein without departure from the spirit of the invention. For 
example, temperature ranges other than the preferred range as set forth 
hereinabove may be applicable as a consequence of the nature of the 
various components employed in the process and such other expected 
variations or differences in results are contemplated in accordance with 
the practices of the present invention. It is intended, therefore, that 
the invention be limited only by the scope of the claims which follow.