Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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This application is related to copending Canadian
application 233,307, filed August 12, 1975.
One very useful source of energy is a product known
as synthesis gas. This is an equimolar mixture of carbon ~
monoxide and hydrogen and is produced from petrochemical
raw materials. More specifically, any hydrocarbons such as -
propane, gasoline, liquefied gas, natural gas, and including
crude oil, heavy oil, tar and asphalt are reacted with hy- -
drogen under conditions creating incomplete combustion.
Usually, these hydrocarbons are reacted with oxygen in an
amount
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which is insufficient for complete combustion in an open
flame and in the absence of a catalyst. The reaction is
generally carried out at temperatures of approximately
1300 C. and at pressures up to 90 bar. It has been known
that pressures can go as high as 160 bar at least in pilot
plants.
The products of this reaction include hydro-
gen sulfide, carbon monoxide, methane, carbon dioxide and
hydrogen, all of which are thermodynamically stable under
the reaction conditions. Carbon black, although not an
equilibrium partner under these conditions, is unavoidably
formed. When natural gas is reacted, a minor amount of
carbon black is produced. In the case of residual oils,
the quantity of carbon black will normally amount to ap-
proximately 2 to 4% of the carbon content.
When producing synthesis gas from petro-
chemical raw materials by processes carried out on a com-
mercial scale, the hydrocarbons and oxygen are separately
preheated and introduced into the reactor through one or
more burners. The burners may be water-cooled and they
permit rapid and intimate mixing of the reactants. The hot
gas leaving the reactor (having a temperature of 1400 to
1500 C.) is cooled in a waste heat boiler. Thereafter the
carbon black is removed from the raw gas by scrubbing with
water. Then the pre-purified gas is desulfurized and, if
necessary or desired, passed to a carbon monoxide convert-
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er. Finally, carbon dioxide, hydrogen sulfide and carbon~xysulfide are largely removed in the gas scrubbing unit
downstream of the converter. In certain cases, e.g. if the
gas is to be used for synthesis of ammonia, a more complete
purification is necessary.
Carbon black can be removed from the raw
synthesis gas without any difficulty by simple scrubbing
with water. However, problems are encountered when attempt-
ing to further utilize the carbon black, especially when
returning it into the process, which is desirable for rea-
sons of economy.
In one known process, the total carbon
black is returned into the reactor in a closed system. To
this end, the carbon black-containing water is mixed in a
decanter with naphtha, i.e. a gasoline fraction having a
boiling range of about 70 to 112 C. This has the result
that the total carbon black passes over into the naphtha
phase. After having separated the two phases, water which
is free from carbon black is withdrawn from the decanter.
The carbon black-containing naphtha is mixed with part of
the feed oil and distilled off as the overhead fraction in
a distilling column. The oil with the total carbon black
remains as bottoms in the column. After being mixed with
the remaining feed oil, it is passed to the preheater and
thence into the reactor where it is completely gasified.
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In another known process, small amounts of feed
oil are admixed with the carbon black-containing water.
The carbon black passes over into the feed oil phase and,
with an appropriately high concentration, forms semi-solid
carbon black-oil pellets. The clarified water is separated
from the black-oil pellets by means of screens. The clari-
fied water is returned into the process and the carbon
black-oil pellets are ground to a slurry with the addition
of further feed oil and either passed to a combustion unit
or returned into the process as gasification agent.
Both of the known processes of carbon black sepa-
ration have substantial disadvantages. The separation of
carbon black by means of large amounts of naphtha causes
the substantial expense of distillation for the recovery of
the naphtha, and the separation of carbon black with small
amounts of feed oil results in a not inconsiderable pollu-
tion of the environment by vapors containing noxious mater-
ials. In addition, there are technical difficulties as well.
Accordingly, it is an object of this invention to
provide a process which overcomes the difficulties mention-
ed above and permits the treatment and processing of carbon
black-containing effluent waters in a manner which is satis-
factory from both the technical and economical point of
view.
In accordance with the invention, carbon black is
separated from water (especially from water obtained when
scrubbing the synthesis gas produced by partial oxidation
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of hydrocarbons) by treatment with hydrocarbons. In this
process, the carbon black is separated from a liquid contain-
ing it by adding 20 to 100 parts of a liquid hydrocarbon per
part of carbon black to the liquid to form a first mixture~
whereby the carbon black passes into the hydrocarbon phase.
This first mixture is passed into a settling zone and the
hydrocarbon and carbon black are separated from the liquid
by permitting the hydrocarbon and carbon black to form a
layer upon the surface of the liquid. The layer is then
passed through a disk centrifuge having a plurality of
conical disks. The bulk of the hydrocarbons is separated
from the resultant suspension of carbon black in hydrocarbon -~
in the disk centrifuge and the remaining carbon black-
hydrocarbon mixture is, if desired, distilled in known man-
ner. The carbon black remaining after distillation may be
used in usual manner. For example, it may be introduced
into the synthesis gas reactor after being mixed with feed
oil. However, the carbon black-hydrocarbon suspension may
also be used to recover dry carbon black.
It was assumed heretofore when separating carbon black
fr~m scrubbing water that it is necessary for the complete
extraction of the carbon black from the aqueous phase to
use liquid hydrocarbons which are free from carbon black.
I~ this manner, it was desired above all to avoid having very
fine carbon black particles remain suspended in the water.
Therefore, the hydrocarbons have been separated from the
hydrocarbon-black suspensions by distillation. Moreover,
the extent and the effect of the mechanical separation of
part of the suspension liquid on the economy of the overall
process were underestimated.
Surprisingly, it has been found that it is not nec-
essary for treating and processing carbon black-containing
water to use hydrocarbons which have been thoroughly purified
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from small carbon black particles. Rather, it is possible
to use hydrocarbons which do contain small carbon black
particles. Therefore, it is feasible to separate mechanic-
ally the bulk of the hydrocarbons loaded with carbon black
after they have been used for the separation of carbon
black from water. In accordance with the invention, this
is effected with the use of disk centrifuges which ensure
sufficient separation of the liquid and solid phases. It
is possible in this manner to separate 80 to 90% of the
naphtha contained in the suspension.
The naphtha having been separated mechanically
can be directly re-used for separating carbon black from
suspensions of carbon black and water. From the remaining
residue having a concentration of carbon black of 10 to 15%
(as compared with about 2% in the original carbon black-
hydrocarbon suspension) the carbon black-containing naphtha
may, after addition of feed oil in known manner, be dis-
tilled off except for small traces. However, a substan-
tially lower expenditure of energy is required to enable
it to be used again as a suspending agent. This naphtha
is also used again for separating carbon black from sus-
pensions of carbon black and water. The distillation resi-
due is used as feed product in the synthesis gas reactor.
The process according to the invention will now
be described with reference to the flow sheet attached
hereto and described hereinafter in greater detail.
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Oxygen and fuel oil are passed through lines 1
and 2 and into a gasification reactor 3. The synthesis gas
consisting of carbon monoxide and hydrogen and containing
carbon black is freed from this carbon black by means of
water in a carbon black scrubber 4. The s~nthesis gas pas-
ses out of the system to further use through line 5. The
mixture of carbon black and water is passed through line 6
and into a decanter vessel 7 into which naphtha is simul- :
taneously introduced through line 8. Separation of phases
takes place in the decanter vessel 7. The naphtha/carbon .
black phase leaves the decanter vessel 7 through line 9
while the clarified water is withdrawn through line 11 and
returned as recycle water into the carbon black scrubber 4.
Excess water leaves tl-le process through line 12. The
naphtha-carbon black mixture emerging through line 9 from
the decanter vessel is introduced into a disk centrifuge 10
in which separation into a phase enriched with carbon black
and a clarified phase takes place. The clarified phase
emerging through line 13 (naphtha having a residual content
of carbon black of about 0.3%) is returned into the de-
canter vessel 7. The concentrated phase having a carbon
black content of 10 to 15% and leaving the Separator 10
through line 14 is mixed with fuel oil in a mixer 15 and
passed through line 16 and into the ~asification reactor 3.
The mixture of naphtha and feed oil withdrawn from the mix-
er 15 may be subsequently distilled to recover pure naphtha.
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Example
The test was performed with a disk centrifuge
which had the following characteristics:
disk diameter 160 mm
number of disks 37
distance between the disks 0.5 mm
diameter of the separation
layer approx. 125 mm
rate of revolutions9500 revolutions
per minute
centrifugal acceleration 5000-6000 acceleration
due to gravity
300 liter per hour of a mixture coming from the carbon
black separation consisting of naphtha, and carbon black
(with a carbon black charge of 20 gram carbon black per
liter naphtha) and residual water with a temperature of
80 C were introduced into the disk centrifuge by pipe 9.
During the test, which lasted two hours, the quantity per
time unit of the introduced liquid (290 to 300 liter per
hour) and the quantity per time unit of the clarified li-
quid from pipe 13 (215-225 liter per hour) were measured
continuously. The quantity of concentrated carbon black
sludge was determined by collecting in a receiver and found
to be 75 liter per hour on an average. Samples of the
sludge and the clarified liquid were collected regularly at
15 min. intervals and analyzed. The analysis of all sam-
ples showed that the concentration of solids in the carbon
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black sludge was in the range of 70 to 85 gram carbon black
per liter naphtha. A residual amount of solid between 0.2
and 0.8 gram carbon black per liter naphtha was found in
the clarified liquid. The mechanically separated naphtha
could be reintroduced immediately to the separation of car-
bon black from the carbon black-water suspension.
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