Method for pretreating feedstocks for coal hydrogenation

A method for pretreating a coal hydrogenation feedstock with preheated hydrogen containing a hydrogenation gas under high pressure and at elevated temperature, in a liquid phase slurry system, is disclosed. In this process a mixture of a slurry of finely ground coal and a slurry oil are fed to a preheater before being subjected to a hydrogenation and liquefaction reaction in a cascade of reactors at a pressure of from 100 to 40 bars and a temperature of 420.degree. to 490.degree. C. The reaction products are fed to a hot separator. In the process of the invention, prior to its preheating, a first partial stream of the hydrogenation gas, referred to as the slurry gas, is added to the mixture at process pressure. A second partial stream of the hydrogenation gas is heated by indirect heat exchange with a gaseous hot separator product in a gas heat exchanger. The mixture of slurry and slurry gas is preheated through indirect heat exchange in at least one heat exchanger downstream from the first gas heat exchanger, through which flows the hot separator head product after passing through the first heat exchanger. The heated second partial stream of hydrogenation gas is then added to the preheated mixture of slurry and slurry gas. This process provides improved heat transfer during the preheating of the coal-oil slurry and the mixture in a slurry with a hydrogenation gas.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The invention relates to a method for pretreating feedstocks for coal 
hydrogenation with a preheated hydrogen containing hydrogenation gas. 
2. Discussion of the Background 
It is known in coal hydrogenation processes that the total required 
quantity of hydrogenation gas is heated with the coal-oil slurry. It is 
also known that a part of the hydrogenation gas is preheated separately 
and added to the coal-oil slurry before the preheater and another part of 
the hydrogenation gas is preheated separately and added downstream from 
the preheater to the coal-oil slurry, prior to entering the hydrogenation 
reactor (cf. EP-OS No. 0 083 830). 
The hydrogen-containing hydrogenation gas is made up of two parts; a first 
part is the circulating gas remaining after separation of the coal 
liquefaction products and a second part is fresh hydrogen added to make up 
for hydrogen consumption (cf. "Die katalytische Druckhydrierung von 
Kohlen, Teeren und Mineraloelen" (The catalytic hydrogenation under 
pressure of coals, tars and mineral oils), Springer-Verlag 
Berlin/Gottingen/Heidelberg 1950, p. 36). 
It is also known that the coal-oil slurry of finely ground coal and the 
slurry oil, which is a recycle distillate stream from the operation of a 
coal liquefaction process, undergoes a swelling stage during heating. 
Depending on the type of coal, the type of slurry oil and the 
pretreatment, the swelling of these mixtures takes place within a 
temperature range of about 280.degree. to 390.degree. C. Appropriate means 
in which the swelling of the slurry takes place can be provided for 
upstream from the preheater, said means being an expanded pipe or a 
bottle-shaped receptacle (cf. DRP No. 715 988). The heat transfer in the 
vertical heat exchanger pipes of the preheater, on the outside of which 
flows the heating gas longitudinally and on the inside of which flows the 
slurry, is a critical step in all coal liquefaction processes. 
When heating the three-phase mixture of the coaloil slurry in the presence 
of the hydrogen-containing hydrogenation gas there is a risk of 
sedimentation of the solid components in the heat exchanger pipers of the 
preheater. In addition, the suspension can evaporate to dryness through 
the evaporation of the low-boiling components of the slurry oil. 
As a result of the swelling process, a great increase in viscosity occurs 
in the section between the initial heat exchanger(s) for the slurry and 
the hydrogenation gas mixture and the preheater. The increase in viscosity 
can cause a considerable pressure drop in the absence of special 
precautions. This loss of pressure would have to be compensated by 
conveying means as pumps and the like. 
SUMMARY OF THE INVENTION 
Accordingly, one object of this invention is to improve the heat transfer 
during the preheating of the coal-oil slurry and the mixtures of the 
slurry with the hydrogenation gas. 
It is another object of this invention to provide a method for improving 
the heat transfer during heating of the slurry and of the mixtures of the 
slurry with the hydrogenation gas. 
It is another object of this invention to provide a method to keep the 
coal-oil slurry from sedimentation and drying out, particularly in the 
preheater section. 
It is another object of this invention to provide a method which reduces 
the energy consumption for the operation of the conveying means, in 
particular slurry pumps and gas compressors. 
Therefore a method is provided for pretreating feed stock for coal 
hydrogenation which satisfies all of the above objects, and other objects 
which will become apparent from the description of the invention given 
hereinbelow. The method of the invention is a method for pretreating 
feedstocks for coal hydrogenation with preheated hydrogen containing a 
hydrogenation gas under high pressure and at an elevated temperature, in a 
liquid phase system. If needed, a catalyst may be added. In this process a 
slurry of finely ground coal and a slurry oil, preferably originating from 
the coal liquefaction process, is fed to a preheater. It is subsequently 
subjected to a hydrogenation and liquefaction reaction in a cascade of 
reactors at a pressure of from 100 to 400 bars and at a temperature of 
from 420.degree. to 490.degree. C., where the reaction products are fed to 
a hot separator which is a vapor/liquid separation system for the reaction 
products. 
In this process the total hydrogenation gas requirement for the coal 
liquefaction reaction is split into two streams. A first partial stream of 
the hydrogenation gas, designated as slurry gas and prior to its 
preheating, is mixed with slurry brought to the process pressure. The 
second partial stream of the hydrogenation gas is heated by heat exchange 
with the gaseous hot separator head product in a first heat exchanger 
having a gas heat exchanger configuration. The mixture of slurry and 
slurry gas is preheated by heat exchange in one or several heat exchangers 
located downstream from the gas heat exchanger, through which the hot 
separator head product passes after passing the first gas heat exchanger. 
The said second partial stream of the hydrogenation gas thus heated is 
then added to the preheated mix of slurry and slurry gas.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
In the present process, the said second partial stream of the hydrogenation 
gas which is heated by heat exchange with the gaseous hot separator head 
product and added to the preheated mixture of slurry and slurry gas 
preferably constitutes 50 to 80% by volume of the total quantity of 
hydrogenation gas required. 
An advantageous embodiment provides that the said second partial stream of 
the hydrogenation gas which has been heated in the first gas heat 
exchanger is added to the mixture of slurry and slurry gas only after the 
mixture has passed through the preheater section. 
Another embodiment provides that the said second partial stream of the 
hydrogenation gas which is passed through the said first gas heat 
exchanger and further heated in a furnace is added to the slurry 
downstream of the preheater. Yet a further partial stream can be split 
from this hydrogenation gas heated in the furnace and be added to the 
slurry prior to entering the preheater. The heating of the said second 
partial stream of the hydrogenation gas is in another embodiment effected 
separately in the furnace. 
As discussed above, in the present process the total required quantity of 
hydrogenation gas is split into two partial streams. The first partial 
stream is added to the slurry that has been brought up to process pressure 
but not yet heated in a heat exchanger. The second partial stream is first 
heated by heat exchange with the gaseous hot separator head product in the 
said first gas heat exchanger and/or separately in the said furnace. By 
feeding the second partial stream of the hydrogenation gas to the mixture 
of slurry and slurry gas which is preheated in one or several heat 
exchangers, the desired advantageous bubble flow characteristics are 
obtained in all the heat exchangers except the gas heat exchanger. This 
provides improved heat transfer and suppression of sedimentation in the 
slurry suspension. 
It is also possible to split the flow of the hydrogen-containing 
hydrogenation gas in such a manner that fresh hydrogen is added as the 
slurry gas to the slurry. The remaining quantity of the total quantity of 
fresh hydrogen to be added is admixed to the recycled portions of 
hydrogenation gas. 
A higher partial hydrogen pressure in the slurry gas is thereby achieved 
and the compressor for circulating the hydrogenation gas can be designed 
with a correspondingly smaller capacity. Smaller heat exchange tubes 
("hairpin tubes") can also be provided in the preheater for the 
slurry-hydrogenation gas mixture. 
Swelling is severely accelerated by admixing the hydrogenation gas that has 
been preheated to such high temperatures as 350.degree. to 550.degree. C. 
to the slurry and by providing a swelling section limited to a defined 
space. For this, the swelling section can be designed as a section in 
which the effective cross-section is enlarged. 
The head product of the hot separator initially passed through the gas heat 
exchanger for heating the said second partial stream of the hydrogenation 
gas is fed subsequently to the slurry preheaters, where, in counterflow to 
the slurry, the hot separator head product is further cooled by heat 
exchange. Here, the slurry containing only a part, preferably 20 to 50% by 
volume, of the quantity of hydrogenation gas (slurry gas) is preheated in 
the heat exchangers. 
The evaporation in the slurry preheaters is thereby reduced by 30 to 60%, 
whereby evaporation to dryness in only locally limited zones is avoided. 
Because of the higher content of unevaporated liquid product, a better 
dissolution of the coal is also achieved. In the reaction part of the 
system, i.e., in the actual hydrogenation reactors, the entire required 
quantity of hydrogenation gas is available after the remaining quantity of 
50 to 80% by volume of the second partial stream of hydrogenation gas is 
added. Here the hydrogenation gas also assumes the function to effectively 
transfer to the hot separator the resulting vaporizable reaction products. 
In the subject process, a part of the capacity of the preheater is 
substituted by the gas heat exchanger to which the head product of the hot 
separator has been contacted, or by the furnace. In this connection it is 
of great importance that the gas heat exchanger or the furnace for heating 
the said partial stream of the hydrogenation gas require only about one 
tenth of the total exchange area of a heat exchanger or preheater, 
respectively, for preheating the slurry which constitutes a multiphased 
system. 
The separate preheating of one part of the hydrogenation gas permits a 
better heat utilization of the heat capacity of the head product of the 
hot separator, and makes possible a simplified preheater design. The 
layout and the operation of the preheater for heating the slurry are 
critical for the operation of a coal liquefaction facility. 
The partial stream of the hydrogenation gas is heated in the gas heat 
exchanger to 350.degree. to 480.degree. C., maximum 500.degree. C., in 
counterflow with the head product of the hot separator, or in the 
additional furnace to temperatures of between 350.degree. and 550.degree. 
C. and depending on operational requirements and on operating conditions 
of the preheater can be split into a partial stream admixed to the slurry 
at an upstream location from the preheater and a partial stream is 
admixxed to the slurry downstream from the preheater. 
For further recovery from the head product of the hot separator, the slurry 
made of finely ground coal and a slurry oil is brought into pressure, 
mixed with the slurry gas and is contacted with the head product of the 
hot separator by indirect heat exchange in consecutively and in a 
downstream position from the said gas heat exchanger arranged heat 
exchangers, in counterflow. All types of coals which can be economically 
hydrogenated can be used, e.g., typical gasflame coal of the Ruhr area can 
be used. The slurry mixed with the slurry gas and, if appropriate, with 
the remaining hydrogenation gas, reaches a temperature of about 
400.degree. C. after passing through the heat exchangers, and a 
temperature of about 470.degree. C. after passing through the downstream 
preheater. The head product of the hot separator which has cooled down 
after passing through the heat exchangers, is passed on in the usual 
manner for further reprocessing. 
DESCRIPTION OF THE DRAWINGS 
Reference is now made to the drawings, wherein like reference numerals 
designate identical or corresponding parts throughout the several views. 
After being mixed with the eventually preheated quantity of hydrogenation 
gas designated as slurry gas and the slurry is passed via conduit 1 
through one or more consecutively arranged heat exchangers 2 as well as 
through preheater 4. If appropriate it is subsequently passed through an 
expansion section 3. The slurry enters the first hydrogenation reaction 
via conduit 5. The gaseous head product of the hot separator passes 
through a gas heat exchanger 6, in which the second partial stream of 
hydrogenation gas carried via conduit 7 is heated to a temperature of from 
350.degree. to 480.degree. C., maximum of 500.degree. C. The head product 
of the hot separator, then at a lower temperature, is passed through the 
slurry preheaters 2 and, if appropriate, a slurry gas preheater not 
illustrated in the drawings. The part of the hydrogenation gas carried in 
conduit 7 can be heated further by furnace 8 in a variant of this method. 
If necessary, a part of the slurry gas needed as quench gas is separated. 
This part of the slurry gas is carried by an appropriate conduit to the 
reactor and the preheater. 
A light and middle coal oil is obtained from the cooled head product of the 
hot separator, via fractionated condensation and, if appropriate, 
expansion and distillation in a known manner. This is used for further 
processing steps as hydrogenating and reforming in a vapor-phase 
hydrogenation which is consecutive to the liquid phase hydrogenation. 
Alternatively the light and middle coal oil constituents are directly, 
without pressure decrease and atmospheric distillation, subjected to 
refining into liquid heating oils and transportation fuels. The heavy oil 
components separated from the head product of the hot separator are 
generally recycled as part of the slurry oil. 
More specifically, according to FIG. 1, slurry under process pressure is 
combined with the slurry gas by way of conduit 1. To heat further the 
remaining partial stream of hydrogenating gas carried in conduit 7 and 
heated in gas heat exchanger 6, a furnace 8 is provided. The thus heated 
hydrogenation gas flow is split and added to the mixture of slurry and 
slurry gas in one part before entering the preheater 4 and in the other 
part downstream from preheater 4. The heated reaction mixture is carried 
by conduit 5 to the first hydrogenation reactor. 
FIG. 2 shows an embodiment of the method in which the flow of hydrogenation 
gas carried via conduit 7 and gas heat exchanger 6 is fed to the slurry in 
swelling section 3. 
According to FIG. 3, the flow of hydrogenation gas heated in gas heat 
exchanger 6 is added to the slurry in one part in swelling section 3 and 
in the other part downstream from the preheater 4. 
Within the framework of the embodiment of this method shown in FIG. 1, it 
is also advantageous to replace for the purpose of heating up the hydrogen 
gas altogether the gas heat exchanger 6 by the furnace 8. In this case, 
the hydrogenation gas carried in conduit 7 can be heated to, e.g., 
550.degree. C. 
Obviously, numerous modifications and variations of the present invention 
are possible in light of the above teachings. It is therefore to be 
understood that within the scope of the appended claims, the invention may 
be practiced otherwise than as specifically described herein.