JPH0317761B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention is a process for producing hydrogen-containing gases, and in particular ammonia synthesis gas, from hydrocarbons by desulfurization of the starting materials, primary and secondary reforming, and a two-step conversion process as described below. Concerning methods by conversion of CO, removal of CO ₂ and methanation.

The present invention aims to achieve one of these partial processes, the so-called conversion process: H ₂ O + COH ₂ + CO ₂ (1).

A study of the course of the process in ammonia or hydrogen plants based on these processes shows that considerable energy savings can be achieved if the operating conditions are changed compared to those previously adopted. In recent years, such changes have led to the introduction of ammonia conversion equipment in some processes, for example with radial flow and reduced synthetic pressure, to the introduction of physical absorption processes for the removal of _CO2 after conversion of CO2. and the steam to carbon ratio at the inlet of the primary reformer.
This is already being done by reducing the carbon ratio. This results in a change in the water vapor balance within the plant, and it is shown that the utilization of the supplied energy can be considerably improved by further reducing the water vapor to carbon ratio mentioned above. However, such a reduction is problematic, especially in connection with conversion process 1.

That is, a low steam to carbon ratio at the inlet to the primary reformer causes a low steam to dry gas ratio and therefore a high CO partial pressure in the conversion section.

In the prior art, the conversion process is generally carried out in two stages, where the first stage uses an iron- and chromium-containing catalyst at a temperature of 360-500°C, a steam-to-dry gas ratio of 0.5-1.2 and a pressure of 10-35 absolute atm. It was done in
The second stage is carried out at 200-250°C using a copper-containing catalyst.

The active form of the catalyst commonly used in the first stage of the conversion process consists of Fe ₃ O ₄ with Cr ₂ O ₃ as a cocatalyst. However, at high CO partial pressures, Fe ₃ O ₄ converts to iron carbide, which acts as a Fischer-Tropsch catalyst leading to the formation of undesirable hydrocarbons.

Carbide formation can occur in a variety of reactions, and different iron carbides are formed, but the main reactions will be as follows.

5Fe ₃ O ₄ +32CO3Fe ₅ C ₂ +26CO ₂ (2) However, equation (2) is not suitable for equilibrium calculations because the thermodynamic data for iron carbide is missing or inaccurate.

It has been found that a good approximation to the correct fact can be obtained by calculations based on the formula 5Fe ₃ O ₄ +32CO15Fe+6C+26CO ₂ (3).

The equilibrium constant K _p of this reaction (3) is expressed as follows.

K _p = p ²⁶ / _CO2 / p ³² / _CO The data for calculating K _p for the reaction (3) can be found in thermodynamic data tables (e.g. J. Barin, O. Knacke, O.
Kubachewski: Thermodynamical properties
of inorganic substonces, 1973, and supplements,
1977, Springer Publishing, Berlin). The attached figure is calculated based on these data,
Shows log K _p plotted as a function of temperature.

From the figure _, it can be seen that the
It can be inferred whether Fe is present in the form of an oxide or a carbide. That is, if the log K _p at a given temperature is lower than shown on the curve, the stable state is carbide. If log K _p is high, the stable state is an oxide.

Such calculations were performed for typical gases with respect to the operating conditions whose composition follows the process in question. The results are described in Experiment 1 below.

As seen from experimentation, the catalyst in a typical case will be present in carbide form. Furthermore, it will be appreciated that by adopting the desired low water vapor/dry gas ratio it is not possible to convert the catalyst to the oxide form. This is because it requires high temperatures which can destroy the catalyst due to its lack of thermal stability.

Since the aforementioned problem of carbide formation is associated with iron-containing catalysts, attempts were made to replace it with conventional Cu-containing low temperature conversion catalysts in the first stage of the conversion process. However, this catalyst does not have sufficient temperature stability to be used in the process, where temperatures up to 400° C. are used for efficient energy utilization.

Problems are also encountered when using conventional low temperature conversion catalysts in the second stage of a two-stage conversion process.

When carrying out the second stage at a typical temperature of 200-250°C with a feed gas having a low water vapor/dry gas ratio, methanol is produced in such quantities that it is not possible to obtain the intended advantage in conversion in terms of energy. It will probably be formed. This is due to the fact that Cu-containing low temperature conversion catalysts also catalyze methanol synthesis.

At relatively high temperatures, the amount of methanol formed will be decisive for the equilibrium of methanol synthesis in which the reactions involved are: CO + 2H ₂ CH ₃ OH (4) CO ₂ + 3H ₂ CH ₃ OH + H ₂ O (5) CO + H ₂ OCO ₂ + H ₂ (1) At relatively low temperatures, on the other hand, the amount of methanol depends on the kinetic conditions, since the reaction rate of methanol synthesis decreases with decreasing temperature more rapidly than the reaction rate of the conversion process. It will depend.

Attempts have therefore also been made to carry out the second stage of the conversion process at relatively low temperatures. But that raises another problem. This is because the relatively low activity as a result of the relatively low temperature requires the use of extremely large volumes of catalyst to obtain the desired degree of CO conversion. Addition of CO content in the exit gas is undesirable as it increases hydrogen losses in the subsequent methanation process.

The inventors have found that it is possible to solve such problems in the first and second stages of the conversion process by using certain operating conditions and catalysts that optimize them.

Accordingly, the present invention provides an improved process for making hydrogen-containing gases and in particular ammonia synthesis gas from hydrocarbons as starting materials, comprising: desulfurization of the starting materials;
The desulfurized material is subjected to primary and secondary reforming, and the two-step conversion process (1) described above converts carbon monoxide contained in the reformed gas into hydrogen and dioxide. It relates to a process by converting to carbon, removing CO ₂ from the converted gas and methanating the gas.
According to the invention, the process comprises: (a) the first step of the conversion process has a water vapor to dry gas ratio of less than or equal to 0.5, preferably between 0.3 and 0.5, in the presence of a catalyst consisting of copper oxide, zinc oxide and chromium oxide; With supply gas, 10
Preferably 190-400℃ at a pressure of ~50 atmospheres absolute
(b) the second stage of the conversion process is carried out at a temperature of 200 to 360°C, in the presence of a catalyst consisting of copper oxide, zinc oxide and aluminum oxide;
~195 °C, preferably at an inlet temperature of 175-195 °C, provided that at the same time the inlet temperature is at least two temperatures (T ₁ +10) °C and (T ₂ +10) °C (where T ₁ actually dominates). is the dew point under the reaction conditions that are
T ₂ is the equilibrium temperature of the reaction Z _o O+CO ₂ Z _o CO ₃ (6) under the reaction conditions that prevail in practice. ) is characterized by being greater than or equal to the highest of

The pressure during the second stage of the conversion process is usually the same as that during the first stage or slightly lower due to natural pressure drop, i.e. usually about 0-50
It would be absolute atmospheric pressure.

According to the present invention, the catalyst employed in the first stage of the conversion process can have the following composition: 15 to 70 preferably 20 to 40 atomic % in the form of copper oxide
Cu, 20-60 preferably 30-40 atom% in the form of zinc oxide
Zn, 15-50 preferably 20-50 atomic% Cr in the form of chromium oxide.

The atomic % here is calculated based only on the metal content and does not take into account the oxygen content.

Regarding the range of the various components of the catalyst to be used in the first stage of the conversion process according to the invention, the wide ranges mentioned (15-70 at.% Cu, 20-60 at.% Zn, 15-20 at.% It must be emphasized that catalysts with a composition within Cr) are very well suited for use according to the invention; while 20-40 at.
A preferred range of Cr represents a catalyst with particularly advantageous properties with respect to thermal stability and catalytic activity.

According to the invention, the catalyst used in the second stage is:
It can have the following composition: 25-60 atomic % Cu in the form of copper oxide, 25-45 atomic % Zn in the form of zinc oxide, 15-30 atomic % Al in the form of aluminum oxide.

Here, atomic % is calculated in the same manner as described above.

The catalysts used according to the invention in the second stage of the conversion process are characterized by high activity and high selectivity for the conversion reaction.

The lower limit stated for the inlet temperature in the second stage of the conversion process according to the invention is not determined by activity considerations, but is determined by the two parameters mentioned above, namely the water vapor pressure P _H2O and the carbon dioxide pressure P _CO2
limited by. The reason for this is that condensation of water inside the catalyst body can prevent the reaction gas from reaching the active catalyst surface and must be avoided; and the formation of Cu or Zn carbides can lead to deactivation. Apart from this, it may cause the catalyst particles to explode, so this must be avoided.

In order to guarantee a reasonable margin of safety, according to the invention, the dew point T ₁ or the equilibrium temperature T ₂ is at least 10
It is prescribed to use an inlet temperature above °C.

Below, the process of the invention is illustrated by some experiments and examples.

Experiment 1 represents the first stage of the conversion process, which is carried out in a conventional manner.

Experiment 2 shows two stages of the conversion process, the first stage being carried out in the same manner as the process of the invention and the second stage being carried out in a conventional manner.

Examples 1-4 illustrate two stages of the conversion process carried out by the process of the present invention.

Experiment 1 0.33% O ₂ , 3.1% N ₂ , 83.50% CH ₄ , 9.31
Reforming of natural gas containing % C ₂ H ₆ , 2.83% C ₃ H ₈ and 0.12% C ₄ H ₁₀ is carried out after adding steam to a steam to carbon ratio of 2.5. After the primary reformer, a certain amount of air is added. At the outlet of the secondary reformer, where the pressure is 31 atmospheres absolute, the gas composition is: H ₂ 38.95 Volume % N ₂ 17.23 〃 CO 10.89 〃 CO ₂ 4.38 〃 Ar 0.20 〃 CH ₂ 0.22 〃 H ₂ O 28.13 〃 The gas is then conveyed to the conversion section where CO conversion is carried out by conversion process (1).
The first stage of the conversion process is at an inlet temperature of 360°C;
It is carried out using a conventional iron oxide chromium monoxide catalyst having a chromium content of about 8 atom %, calculated on the basis of metal content alone.

The adiabatic temperature rise during the first stage passage is 444
Gives an outlet temperature of °C (717〓). The conversion process (1) probably reaches equilibrium at this temperature.

In the absence of other reactions, the gas composition after the high temperature conversion reactor where the pressure is 30 atmospheres absolute would be: H ₂ 46.44 % by volume N ₂ 17.23 〃 CO 3.40 〃 CO ₂ 11.86 〃 Ar 0.20 〃 CH ₄ 0.22 〃 H ₂ O 20.65 〃 However, the assumption that no other reactions occur is incorrect. From P _CO2 = 3.558 absolute pressure and P _CO = 1.020 absolute pressure, the equilibrium constant of reaction (3) can be calculated as K _p = p ²⁶ / _CO2 / p ³² / _CO = 1.15 x 10 ¹⁴ and from this log K _p = 14.06. Give results.

Comparison with the figure shows that the catalyst exists in carbide form. Laboratory experiments have therefore shown that hydrocarbon formation occurs. That is, under the assumption that
Laboratory experiments included 0.5-0.7 vol% _CH4 , 0.1-0.15 vol% _C2H4 and _{C2H6 ,} 0.05 vol% _{C3H6 and C3H8} and small amounts _of higher hydrocarbons _. , indicating the formation of alcohols and other oxygen-containing organic compounds. It is clear from this that conventional high temperature conversion catalysts are superfluous in the gas composition used according to the invention.

Experiment 2 An inlet temperature of 209 °C was used in the first stage of the conversion process, and 20 at.% Cu, 30 at.%
of Zn and 50 at.% of Cr, all as oxides (at.% calculated based on metal content only).
Experiment 1 is carried out as in Experiment 1, except that a catalyst according to the invention is used. The adiabatic temperature increase during the first stage passage gives an exit temperature of 321°C. At a pressure of 30 atmospheres absolute, an exit gas with the following composition is obtained: H ₂ 48.60 vol. % N ₂ 17.23 〃 CO 1.24 vol. % CO ₂ 14.03 〃 Ar 0.20 〃 CH ₄ 0.22 〃 H ₂ O 18.48 〃 In this process , there is no carbide problem, so proceed to the second stage of the conversion process.

This step uses the gas obtained above at an inlet temperature of 200 °C and contains 30 atomic % Cu, 50 atomic % Zn and 20 atomic % Al, all in oxide form (atomic % is metal content ) carried out using conventional low temperature conversion catalysts. The adiabatic temperature rise during the passage of the second stage is approximately 12°C.
At a pressure of 30 atmospheres absolute, an exit gas with the following composition is obtained: H ₂ 49.17 % by volume N ₂ 17.30 〃 CO 0.24 〃 CO ₂ 14.88 〃 Ar 0.20 〃 CH ₄ 0.22 % by volume H ₂ O 17.77 〃 CH ₃ OH 0.22 That is, under these conditions methanol is present in undesirable amounts. In an ammonia plant where 1000 tons of ammonia are produced per day, 13 tons/day of methanol are simultaneously produced, which represents an unacceptable energy loss.

Example 1 In the first stage of the conversion process, an inlet temperature of 209° C. and the same metal content as in Experiment 2, namely 20 at.% Cu, 30 at.% Zn and 50 at.% Cr (all metals) were used. A catalyst according to the invention is used which consists of copper oxide, zinc oxide and chromium oxide with a content of The adiabatic temperature increase during the first stage gave an outlet temperature of 321 °C as in experiment 2, and at a pressure of 30 atm absolute the same composition as stated in experiment 2, i.e. H ₂ 48.60 vol% N ₂ 17.23 〃 An outlet gas containing CO 1.24 〃 CO ₂ 14.03 〃 Ar 0.20 〃 CH ₄ 0.22 〃 H ₂ O 18.48 〃 is obtained.

This gas is directed to the second stage of the conversion process. Therein, the inlet temperature is 175°C and the catalyst is 60 at.% Cu, 25 at.% Zn and
It has a composition of 15 atomic percent Al (all calculations based on metal content only). The adiabatic temperature increase due to passage through the second conversion stage is approximately 13 °C, and at 30 atm absolute pressure an exit gas is obtained with the following composition: H ₂ 49.61 % by volume N ₂ 17.25 〃 CO 0.15 〃 CO ₂ 15.08 〃 Ar 0.20 Volume % CH ₄ 0.22 〃 H ₂ O 17.45 〃 CH ₃ OH 0.04 〃 Thus, in these conditions according to the invention methanol formation is extremely limited, 1000 tons/day of ammonia results in only about 2 tons/day of methanol. , which is acceptable. Furthermore, the CO content in the outlet gas is almost halved compared to the content according to experiment 2.

Example 2 The two stages of the conversion process are carried out as in Example 1, except that the catalyst used in the first stage of the conversion process is 15 at.% Cu, 35 at.% Zn and 50 at.% Cr. (Percentage values are calculated based on metal content only.) in oxide form. The resulting outlet gas has virtually the same composition as that of Example 1.

Example 3 The two stages of the conversion process are carried out as in Example 1, except that the catalyst used in the first stage of the conversion process is 25 at.% Cu, 60 at.% Zn and 15 at.% Cr. Contains all in oxide form (percentages are calculated based on metal content only).
The resulting outlet gas has virtually the same composition as that of Example 1.

Example 4 The two stages of the conversion process are carried out as in Example 1, except that the catalyst used in the first stage of the conversion stage is 62 at.% Cu, 20 at.% Zn and 18 at.% of Cr, all in oxide form (percentage values are calculated based on metal content only). The resulting outlet gas has virtually the same composition as that of Example 1.

The inlet temperature in the second stage of each embodiment is within the temperature range defined according to the present invention. of the gas used corresponding to the equilibrium temperature of reaction (6) of 164℃.
When the partial pressure of CO ₂ of 4.209 atm absolute and the partial pressure of water vapor of 5.544 atm absolute corresponding to a dew point of 155 °C are calculated,
According to the invention, the lowest usable inlet temperature is 174
It is ℃. Furthermore, this temperature is 195
below ℃.

In addition to the obvious advantages mentioned herein, the process of the present invention avoids the use of sulfur-containing iron catalysts, thereby eliminating sulfur sources that poison the catalyst in the second stage. should be added.

[Brief explanation of the drawing]

The figure shows the calculated equilibrium constant K _p of reaction (3) as log
This is a graph plotting K _p against temperature (〓).

Claims (1)

[Claims] 1. Desulfurization of the starting material, primary and secondary reforming, conversion of carbon monoxide by carrying out the conversion process H ₂ O + COH ₂ + CO ₂ (1) in two stages, removal of CO ₂ and A process for producing hydrogen-containing gas from hydrocarbons by methanation, in which: (a) the first step of the conversion process is in the presence of a catalyst consisting of copper oxide, zinc oxide and chromium oxide;
190-400 at a pressure of 10-50 atm absolute using a feed gas with a water vapor to dry gas ratio of 0.5 or less
(b) the second stage of the conversion process is carried out at an inlet temperature of 160-195°C in the presence of a catalyst consisting of copper oxide, zinc oxide and aluminum oxide, and at the same time the inlet temperature is at least two temperatures (T ₁ +10) °C and (T ₂ +10) °C, where T ₁ is the dew point under the actually prevailing reaction conditions;
T ₂ is the equilibrium temperature of the reaction Z _o O+CO ₂ Z _o CO ₃ (6) under the reaction conditions that prevail in practice. ). 2 In the first stage of the conversion process, 15 to 70 atomic % Cu as copper oxide and 20 atomic % as zinc oxide.
~60 atom% Zn and 15-50 as chromium oxide
2. A process according to claim 1, using a catalyst having a composition of atomic % Cr (percentage values are calculated based only on the metal content of the catalyst). 3 In the second stage of the conversion process, 25 to 60 atomic % Cu as copper oxide and 25 as zinc oxide
~45 atomic% Zn and aluminum oxide as
3. A process according to claim 1, using a catalyst having a composition of 15 to 30 atomic % Al (percentages calculated based solely on the metal content of the catalyst).