PROCESS FOR THE PREPARATION OF HYDROGENCONTAINING GASES

METHODE DE PREPARATION DE GAZ A COMPOSANTE D'HYDROGENE

English Abstract

ABSTRACT
A PROCESS FOR THE PREPARATION OF HYDROGEN-
CONTAINING GASES.
In a process for preparing hydrogen-containing gases,
including ammonia synthesis gas, from hydrocarbons by
desulfurization, primary and secondary reforming, shift
conversion in two steps, removal of CO2 and methanation,
the first step of the shift conversion is carried out with
a catalyst consisting of copper oxide, zinc oxide and
chromium oxide at a steam to dry gas ratio below 0.5,
preferably 0.3-0.5, at 10-50 atm. abs. and 190-400°C,
preferably 200-360°C; and second step of the shift
conversion with a catalyst of copper oxide, zinc oxide and
aluminium oxide at an inlet temperature of 160-195°C,
preferably 175-195°C, at the same time being at least the
highest of the temperatures (T1+ 10)°C and (T2 + 10)°C where
T1 is the dew point and T2 the equilibrium temperature for
the reaction <IMG>.
One avoids the carbide formation occurring when using
the conventional iron catalysts in the first step of the
shift conversion; and avoids the difficulties encountered
with conventional copper-containing low-temperature shift
catalysts in the first shift step. At the low temperature
in second shift step one obtains a low reaction rate for
methanol-formation (which involves a loss of energy), and
avoids with the particular catalyst the large amounts of
catalyst that is needed in the second shift step of known
shift conversions when conventional shift catalysts are
used.

Claims

14
Patent Claims
1. In a process for the preparation of hydrogen-
containing gases from hydrocarbons by desulfurizing the
starting material, subjecting the desulfurized gas to
primary and secondary reforming, converting carbon monoxide
in the reformed gas by the shift process
<IMG> (1)
in two steps, removing CO2 from the shifted gas and
subjecting the resulting gas to methanation, the improvement
of
(a) carrying out the first step of the shift process in
the presence of a catalyst consisting of copper oxide, zinc
oxide and chromium oxide while using a feed gas wherein the
steam dry gas ratio is below 0.5, the pressure being 10
to 50 atmospheres absolute and the temperature 190 to 400°C,
and
(b) carrying out the second step of the shift process
on the exit gas from the first step of the shift process,
in the presence of a catalyst consisting of copper oxide,
zinc oxide and aluminum oxide, at an inlet temperature of
160 to 195°C, which inlet temperature at the same time
fulfils the condition of being at least the highest of the
two temperatures (T1 + 10)°C and (T2 + 10)°C, where T1 is
the dew point under the reaction conditions actually
prevailing and T2 is the equilibrium temperature for the
reaction
<IMG> (6)
under the reaction conditions actually prevailing.
2. The process claimed in claim 1, in which the gas
mixture used in the first step of the shift process has
a steam to dry gas ratio of 0.3 to 0.5.

3. The process claimed in claim 2, in which the
first step of the shift process is carried out at a
temperature of 200-360°C.
4. The process claimed in claim 1, in which the
second step of the shift process is carried out at an
inlet temperature of 175 to 195°C.
5. The process claimed in claim 3, in which the
catalyst used in the first step of the shift process has
the composition 15-70% by atoms of Cu as copper oxide,
20-60% by atoms of Zn as zinc oxide and 15-50% by atoms
of Cr as chromium oxide, the percentages by atoms being
calculated solely on the metal contents of the catalyst.
6. A process according to claim 5, in which the
catalyst used in the first step of the shift process has
the composition 20-40% by atoms of Cu as copper oxide,
30-40% by atoms of Zn as zinc oxide and 20-50% by atoms
of Cr as chromium oxide, the percentages by atoms being
calculated solely on the metal contents of the catalyst.
7. A process according to claim 4, in which the
catalyst used in the second step of the shift process
has the composition 25-60% by atoms of Cu as copper
oxide, 25-45% by atoms of Zn as zinc oxide and 15-30% by
atoms of Al as aluminum oxide, the percentages by atoms
being calculated solely on the metal contents of the
catalyst.

Description

16012-23/KP/io
Haldor Tops0e A/S, Lyngby, Denmark.
A process for the preparation of hydrogen--containing gases.
_eld of the Invention
The present invention relates to a process for the
preparation of hydrogen-containing gases and especially an
ammonia synthesis gas from hydrocarbons by desulfurization
of the starting material, primary and secondary reforming,
conversion of CO by the shift process in two steps
mentioned below, removal of CO2, and methanation.
The invention aims at the accomplishment of one of
these part-processes, viz. the conversion of carbon monoxide
by the so-called shift process:
H20 + CO ~ ` H2 + C2 ( 1 )
Background of the Inven.ion
-
A study of the course of the processes in an ammonia
or hydrogen plant based on these processes shows that
considerable savings of energy can be obtained if there is
made an alteration of the operational conditions in
comparison with those earlier employed. During recent years
such alterations have already been carried out in certain
process steps, such as the introduction of ammonia
converters having radial flow and reduced synthesis pressure,
by the introduction of a physical absorption process for
the removal of CO2 after the conversion of CO, and by the
reduction of the steam to carbon ratio at the inlet of the
primary reformer. Hereby analteration of the stearn balance
, . . ~,

5~.
in the plant takes place, and it can be shown that the
utili~ation of the energy supplied can be improve~
considerably by a further reduction of the above-mentioned
steam to carbon ratio. However, carrying out such reduction
involves problems, especially in connection with the shift
process (1).
A low steam to carbon ratio at the inlet to the primary
reformer thus causes a lower steam to dry gas ratio and thereby
a higher CO partial pressure in the shift section.
In the prior art the shift process is commonly
carried out in two steps whereby the first step is
accomplished while using an iron- and chromium-containing
catalyst at a temperature of 360-500C, a steam to dry gas
ratio of 0.5-1.2 and a pressure of 10-35 atm. abs., and the
second step is accomplished while using a copper-containing
catalyst at 200-250C.
The catalyst usually employed in the first step of
the shift process in its active form consists of Fe3O4
promoted with Cr2O3. At a high CO partial pressure, however,
Fe3O4 may be converted into iron carbides which may act as
Fischer-Tropsch catalysts, resulting in the formation of
undesired hydrocarbons.
The carbide formation may take place by various
reactions while forming various iron carbides, but the main
reaction will be
5 Fe3O4 + 32 CO ~ ` 3 Fe5C2 + 26 CO2 (2)
Because of lack of or uncer-tain thermodyna~ical data
for iron carbides the equation (2), however, is unsuitable
for equilibrium calculations.
It has been found that a good approximation to the
real facts is obtained by a calculation on the basis of the
equation
5 Fe3O4 + 32 CO ~ ' 15 Fe + 6 C + 26 CO2 (3)

The equilibrium constant K fol this re~clLon (3)
is expresse~ as follows:
26
Pco2
P p32
Data for the calculation of K for reaction (3)
may be found in thermodynamical tables (e.y. J. Barin, O.
Knacke, O.Kubaschewski: Thermodynamical properties of
inorganic substances, 1973, and supplement, 1977, Springer
Verlag, Berlin). The drawing shows log Kp calculated on
the basis of these data and plotted as a function of the
temperature.
From the drawing it can be deduced whether Fe in the
iron-containing catalyst at an actual set of interconnected
values for temperature and partial pressures Gf CO and CO2
will be present in oxide form or carbide form. Thus, iE log
Kp at a given temperature is lower than shown by the curve~,
then the stable state is carbide. If log Kp is higher, the
stable state is oxide.
Such a calculation has been carried out for a gas
the composition of which is typical in relation to the
operational conditions according to the process in question,
and the results are reported in Experiment 1 hereinafter.
As appears from the Experiment, the catalyst in the
typical case will be present in carbide form. Moreover, it
can be shown that by employing the desired low steam/dry gas
ratio one cannot bring the catalyst into oxide form
because this would require so high tempera-tures as to
destroy the catalyst because of lacking thermal stability.
As the said problems of carbide formation are
connected with the use of iron~containing catalysts, it has
been attempted to replace them by conventional Cu-containing
low -temperature shift catalysts in the first step of the
shift process. However, these catalysts do not possess
sufficient temperature stability for use in the present
process, in which there is employed temperatures up -to

5~
400 C for the sake of energy utilization.
In the second step of the two step sh L ft process one
also encounters problerns when using conventlonal low-
temperature shift catalysts.
When carrying out the second step at the usual
temperatures of 200-250C while using a feed gas having a
low steam/dry gas ratio, methanol will be formed in such
an amount that one will not obtain the intended advantages
in the conversion as regards energy. This is due to the
fact that Cu-containing low-temperature shift catalysts
also catalyze the methanol synthesis.
-

~ 3~ ~
At higher temperatures the equilibriurn of the
methanol synthesis, the relevant reactions of which are
2 ` CH30H ( )
C2 + 3H2 ~ ` CH 0~ H 0 (5)
C0 + H20 ~ C02 + H2 (1)
will be decisive for the amount of methanol formed. At lower
temperatures the amount of methanol, on the other hand,
depends upon kinetic conditions since the reaction rate of
the methanol synthesis decreases faster with decreasing
temperature than the reaction rate of the shift process.
It has therefore also been attempted to carry out
the second step of the shift process at lower temperatures.
However, hereby a further problem arises because the lower
activity in consequence of the lower temperature calls for
the use of extremely high volumes of catalyst in order to
obtain the desired degree of C0-conversion. An increased
content of C0 in the exit gas is undesired because more
hydrogen is lost thereby in the subsequent methanation
process.
Summary of the Invention
It has now been found that it is possible to avoid
the said problems in the firs-t as well as the second step
of the shift process by using certain conditions of
operation and catalysts rendered optimum thereto.
Accordingly, the invention relates to an improved
process for the preparation of hydrogen-containing gases
and especially an ammonia synthesis gas frorn hydrocarbons
as starting material by desulfurizing the starting material,
subjecting the desulfurized material to primary and
secondary reforming, converting the carbon monoxide
contained in the reformed gas into hydrogen and carbon
dioxide by the abovementioned shift process (1) in two steps,
removing C02 from the shifted gas and methanating the gas.
According to the present invention the process is
characterized in that (a) the first step of the shift process

is carried out in the presence of a catalyst consisting of
copper oxide, zinc oxide and chrornium oxide while using
a feed gas having a steam to dry gas ratio below 0.5,
preferably of 0.3 to 0.5, at a pressure of 10 to 50 atrn.
abs. and a temperature of 190 to 400C, preferably 200 to
360C, whereas (b) the second step of the shift process is
carried out in the presence of a catalyst consisting of
copper oxide, zinc oxide and aluminum oxide,, at an inlet
temperature of 160 to 195C, preferably 175 to 195C, said
inlet temperature being at the same time at least the
highest of the two temperatures (Tl -~ 10)C and (T2 + 10)C,
where Tl is the dew point under the reaction conditions
actually prevailing and T2 the equilibrium temperature for
the reaction
ZnO + CO2 ~ - ` ZnCO3 (6)
under the reaction conditions actually prevailing.
The pressure during the second step of the shift
process normally will be the same as that during the first
step or because of a natural pressure drop of little below
-that, i.e. normally about 10 to 50 atm. abs.
Detailed Description of the Invention
According to the invention -the catalyst employed in
the first step of the shift process may have the
composition
15-70, preferably 20-40 % by atoms Cu in the form
of copper oxide,
20-60, preferably 30-40 % by atoms Zn in the form
of zinc oxide,
]5-50, preferably 20-50 % by atoms Cr in the form
of chromium oxide,
wherein the percentages by atons are calculated solely on the
metal contents and the oxygen content is not taken into
account.

s~ ~
As to the ranges of the various components of the
catalyst to use in the first step of the shi~t process
according to the invention, it should be emphaslzed that
catalysts having a composition within the broa~er ranges
stated (15-70% at. Cu, 20-~0~ at. Zn, 15-50% at. Cr) are
very well suitable for use according to the invention;
whereas the preferred range of 20-40% at. Cu, 30-40% at.
Zn and 20-50% at. Cr represents catalystshaving particularly
advantageous properties with respect to thermostability and
catalytic activity.
According to the invention the catalyst employed in
the second step may have the composition
25-60 % by atoms Cu in the form of copper oxide
25-45 % by atoms Zn in the form of zinc oxide
15-30 ~ by atoms Al in the form of aluminum oxide,
wherein the percentages by atoms are calculated in -the same
manner.
The catalyst employed according -to the invention in
the second step of the shift process is marked by a high
activity and high selectivity for the shift reaction.
The lower limit stated for the inlet temperature
in the second step of the shift process according
to the invention accordingly is not determined out of
consideration for the activity but on the contrary limited
by the said two parameters, viz. the steam pressure~ P
and the carbon dioxide pressure, Pco . The reason for
this is that one should avoid condensation of water in -the
inner parts of the catalyst bodies because it would prevent
the adn,ission of the reacting gases to the active catalyst
surface; and also avoid the formation of Cu or Zn carbonates
because formation of carbonates besides deactivation may
involve bursting of the catalys-t particles.
To ensure a reasonable safety margin it is prescribed
according to the invention tc use inlet temperatures a-t
least 10C above the dew point Tl or equilibrium temperature
T2 .
In the following the process of the invention will
be illustrated by some Experiments and Examples.

Experiment 1 shows the first steE) of the shi~t
process carried out in conventional manner.
Experiment 2 shows both steps of the shift process,
the first step carried out in the same manner as the process
of -the invention and the second step in conventiorlal manner.
Examples 1 to 4 show both of the steps of the shift
process carried out by the process of the invention.
Experiment 1
Reforming of a natural gas containing 0 33~ 2'
3.91% N2, 83.50% CH4, 9.31% C2H6, 2.83% C2E-~8 and 0.12%
C~Hlo was carried out after the addition of aqueous steam
to a steam to carbon ratio of 2.5. After the primary reformer
a certain amount of air is added. At the outlet from the
secondary reformer, where the pressure is 31 atm. abs., the
gas composition is:
H2:38.95 % by vol.
N2:17.23 % by vol.
CO:10.89 % by vol.
CO2:4~38 % by vol.
Ar:0.20 % by vol.
CH4:0.22 % by vol.
H2O:28.13 % by vol.
The gas thereafter is conveyed to the shift section
where CO conversion is carried out by the shift process (1).
The first step in the shift process is carried out at an
inlet temperature of 360C while using a conventional iron
oxide-chromium oxide catalyst having a chromium content of
about 8 % by atoms,calculated solely on the metal con-tents.
The adiabatic temperature increase during the
passage of the first step provides an outlet temperature
of 444C corresponding to 717K. At this temperature the
shift process (1) will have gone to equilibrium.
The gas composition after the high-temperature shift
reactor, where the pressure is 30 atm. abs. in the
absence of other reactions will be
H2 46.44 % by vol.
N217.23 % by vol.

CO:3.40 % by vol.
CO2:11.86 % by vol.
Ar:0.20 % by vol.
CH4:0.22 ~ by vol.
~220.65 ~ by vol.
However, the prerequisite that other reactions do
not take place is erroneous. From Pco = 3.558 atm. abs.
and Pco = 1.020 atm. abs. a calculation of the equilibrium
constant K for the reaction (3) gives the result:
26
Kp = 32 = 1.15 . 10
and from this
log Kp = 14.06.
By comparison with the drawing it is seen that the
catalyst is present in carbide form. Laboratory experiments
accordingly have shown that hydrocarbon formation takes
place. ~nder the above assumptions the laboratory experiments
thus show the formation of
0.5-0.7~ by vol. of CH4
0.1-0.15~ by vol. of C2H4 and C2H6
0.05% by vol. of C3H6 and C3H8
and minor amounts of higher hydrocarbons, alcohols and other
oxygen-containing organic compounds. I~ appears from this
that conventional high-temperature shift catalysts are
useless at the gas compositions employed according to the
invention.
Experiment 2
____________
One proceeds as in Experiment 1 with the exception
that there is used an inlet temperature of 209C in the
first step of the shift process and a catalyst in
accordance with the invention containing 20 ~ by atoms Cu,
30 % by atoms Zn and 50 ~ by atoms Cr, all as oxides, the
atomic percentages calculated solely on the metal contents.

The adiabatic temperature incrcase ~urin~l the p.l~ic~ e of
first step gives an out~et temperature of 321 C. At a
pressure of 30 atm. abs. there is hereby obtainc~d an e~it
gas having the following composition:
248.60 % by vol.
~2:17.23 % by vol.
Co:1.24 % by vol.
CO2:14.03 % by vol.
Ar:0.20 % by vol.
CH40.22 % by vol.
H2O:18.48 % by vol.
As there are no carbide problems in this process,
one proceeds to the second s-tep of the shift process.
This step is carried out while using the gas obtained
above at an inlet temperature of 200C and while using a
conventional low-temperature shift catalyst consisting of
30 % by atoms Cu, S0 % by atoms Zn and 20 % by atoms Al in the
form of oxides, the percentages stated being calculated
solely on the metal contents. The adiabatic temperature
increase during the passage of second step is about 12C.
At a pressure of 30 atm. abs. there is hereby obtained an
exit gas having the following composition:
H249.17 % by vol.
N2:17.30 % by vol.
CO:0.24 % by vol.
CO2:14.88 % by vol.
Ar:0.20 % by vol.
CH40.22 % by vol.
H2O:17.77 % by vol.
CH30H:0.22 ~ by vol.
Under these conditions there is thus formed methanol
in undesired amounts. In an ammonia plant where 1000 tons
of ammonia are produced per day, there will at the sarne
time be produced about 13 tons of methanol per day, which
represents an unacceptable energy loss.

Example 1
_ _ _ . _ _ _
In the First step of the shiFt prc,cess there is use(l
as in Experimert 2 arl inlet temperature c,f 209C ~nc~ a
catalyst in accordance with the invention, consisting of
copper oxide, zinc oxide and chromium oxide having the same
contents of the metals as in Experiment 2, i.e. 20%by atoms
Cu, 30% by atoms Zn and 50% by atoms Cr, all calcula-ted solely
on the metal contents. The ad1abatic temperature increase
during the first step as in Experiment 2 is to an outlet
temperature of 321C and at the pressure of 30 atm. abs.
there is obtained an exit gas having the same composition
as stated in Experiment 2, i.e. 48.60% by vol. of H2,
17.23% of N2, 1.24~ of CO, 14.03% of CO2, 0.20% of Ar,
0.22~ of CH4 and 18.48% of H2O.
This gas is conducted to the second step of the
shift process where the inlet temperature is 175C and -the
catalyst is in accordance with the inve~tion, having the
composition 60 % by atoms Cu, 25 % by atoms Zn and 15 % by atoms
Al, all calculated solely on the me-tal con-tents. The
adiabatic temperature increase by -the passage of the second shift
step is about 13C and at a pressure of 30 atm. abs. there
is obtained an exit gas having the following composition:
H2:49.61 % by vol .
N2:17.25 % by vol .
CO:0.15 % by vol.
C215.08 % by vol.
Ar:0.20 ~ by vol.
CH4:0.22 % by vol.
H2O:17.45 ~ by vol.
CH3OH:0.04 % by vol .
At these conditions, which are in accordance with
the invention, the formation of methanol thus is extremely
limited and per 1000 tons of ammonia a day only corresponds
to about 2 tons of methanol a day, which is acceptable.
Moreover, the CO content in the exit gas is almost halved
compared to the content according to Experiment 2.

;?~
12
Example 2
___ _____
The two steps of the shi~t process are carried out
in the manner explained in Exarnple 1, with the only
exception that the catalyst used in the first step of the
S shift process has the composition 15% by atoms of Cu, 35%
by atoms of Zn and 50% by atoms of Cr, all in the form of
oxides, the percentages being calculated solely on the metal
contents. The exit gas obtained has practically the same
composition as that in Example 1.
Example 3
_________
The two steps of the shift process are carried out
in the manner explained in Example 1, with the only
exception that the catalyst used in thè first step of the
shift process has the composition 25~ by atoms of Cu, 60%
by atoms of Zn and 15% by atoms of Cr, all in the form of
oxides, the percentages being calculated solely on the metal
contents. The exit gas obtained has prac-tically the same
composition as that in Example 1.
Example 4
_______
The two steps of the shift process are carriecl out
in the manner explained in Example 1, with the only
exception that the catalyst used in the first step of the
shift process has the composition 62% by atoms of Cu, 20~
by atoms of Zn and 18% by atoms of Cr, all in the form of
oxides, the percentages being calculated solely on the metal
contents. The exit gas obtained has practically the same
composition as that in Example 1.
The inlet temperature in the second step of the
Examples is within the temperature range prescribed according
to the invention. As one can calculate a CO2 partial
pressure of 4.209 atm. abs. for the gas employed,
corresponding to an equilibrium temperature in reaction (6)
of 164C, and a steam partial pressure of 5.54~ a-tm. abs.,
corresponding to a dew point of 155C, the lowest usable inlet

13
temperature according to the lnvention ~s 174 C. Moreover,
the temperature is below 195C which is st,ated herelrlheFor~
as the highest temperature in the second step of the shlEt
process.
Beyond the advantages apperaring from what, has bc~en
said herein, it should be added that by the process of the
invention one removes a source of sulfur poisoning of -the
catalyst in the second step because the use of sulfur-
containing iron catalysts is avoided.