NL8105202A - METHOD FOR PREPARING HYDROGENIC GASES - Google Patents

Description

Br / Bl / lh / 24

Process for the preparation of hydrogen-containing gases.

The invention relates to a process for the preparation of hydrogen-containing gases and in particular an aramonia synthesis gas from hydrocarbons by desulphurisation of the starting material, primary and secondary reforming, conversion of CO according to the two-stage shifting process, and as stated below, removal of CO2 and methanation.

The object of the invention is to perform one of these sub-processes, namely the conversion of carbon monoxide according to the so-called shifting process: H „0 4-CO -51 H„ + C0 „(1) 2 x- 2 2

An examination of the progress of the processes in an ammonia or hydrogen plant based on these processes shows that significant energy savings can be achieved if a change in operating conditions is made compared to the previously applied operating conditions. In recent years, such changes have already been made at certain process steps, such as the introduction of radial flow ammonia converters with reduced synthesis pressure, by introducing a physical absorption process for the removal of CO 2 after the conversion of CO 2 and by lowering the steam to carbon ratio to the primary reformer feed. This results in a change in the steam balance in the factory and it can be shown that the consumption of the supplied energy can be considerably improved by a further reduction of the above-mentioned ratio of steam to carbon. However, the introduction of such a reduction creates problems, particularly with regard to the shifting process (1).

For example, a low ratio of steam to 8105202 X 4 '

V

-2- carbon to the feed of the primary reformer has a lower ratio of steam to drying gas and therefore a higher partial CO pressure in the shift department.

According to the prior art, the shifting process is usually carried out in two stages, the first stage taking place using an iron and chromium containing catalyst at a temperature of 360-500 ° C, a steam to drying gas ratio of 0, 5-1.2 and an absolute pressure of 10-35 atm, and the second stage mit-10 is conducted using a copper containing catalyst at 200-250 ° C.

The catalyst, usually used in the first stage of the shifting process, consists in its active form of Fe 3 O 4 with Cr 2 O 3 as promoter. However, at a high partial CO pressure, Fe 2 O 3 can be converted to iron carbides which can act as Fischer-Tropsch catalysts, which would lead to the formation of un

", vV

desired hydrocarbons.

The carbide formation can take place according to various reactions to form various iron carbides, although the main reaction will be the following: 5 Fe304 + 32 CO ....... n 3 Fe ^ + 26 CO2 (2) 25 '. however, the lack of or with uncertain thermomechanical data for iron carbides, equation (2) is not suitable for equilibrium calculations.

It has been found that a good approximation of reality is obtained by a calculation based on. 30 of the equation 5 Fe304 + 32 CO - * 15 Fe + 6 C + 26 C02 (3)

The equilibrium constant K for this reaction (3)

P

35 is performed as follows: P26 co9

P p32 CO

8105202 -3-

Data for calculating pre-reaction (3) can be found in the thermodynamic tables (e.g., J. Barin, 0. Knacke, 0. Kubachewski: Thermo-dynamical properties of inorganic substances, 1973, and 5 supplement, 1977, Springer Verlag , Berlin). In the drawing, log K, as calculated on the basis of this data, is plotted as a function of the temperature.

From the drawing it can be deduced whether Fe in the iron-containing catalyst will be present in the oxide form or carbide form at an actual combination of interdependent values for temperature and partial CO and CO 2 pressures. So, if log K is at a given

P

temperature is lower than indicated by the curve, the stable state is the carbide. If log K is higher,

P

15, the stable state is the oxide.

Such a calculation has been made for a gas, the composition of which is specific depending on the operating conditions of the respective process, the results of which are stated below in test 1.

As is apparent from the test, the catalyst will be present in the carbide form in this particular case. Moreover, it can be shown that the desired low ratio of steam to dry gas cannot be used to bring the catalyst into the oxide form because it would require so high temperatures that the catalyst would be destroyed due to the lack of thermal stability.

Since the problems of carbide formation mentioned have been associated with the use of iron-containing catalysts, it has been attempted to replace them with conventional low temperature Cu-containing shift catalysts in the first stage of the shift process.

However, these catalysts do not have sufficient thermal stability for use in the present process using temperatures up to 400 ° C for energy utilization.

Problems are also encountered in the second stage of the two-stage shifting process when conventional low temperature shift catalysts are used.

When the second stage is carried out at the usual temperatures of 200-250 ° C / while an add-on. conducted. gas with a low steam to dry gas ratio is used, methanol will be formed in such an amount that the intended benefits in energy conversion will not be attained. This is due to the fact that the low temperature Cu-containing shift catalysts also catalyze the methanol synthesis.

At higher temperatures, the balance of the. methanol synthesis, for which relevant reactions are considered CO + 2H2 - >> CH30H (4) CO2 + 3H2 - ^ CH30H + H20 (5) CO + h2o - ^ co2 + h2 (1) are decisive for the amount of methanol formed. At lower temperatures, however, the amount of methanol depends on the kinetic conditions, because the reaction rate of the methanol synthesis decreases faster with decreasing temperature than the reaction rate of the shifting process.

Therefore, efforts have also been made to perform the second stage of the shifting process at lower temperatures. This presents a further problem, because the lower activity due to the lower temperature makes it necessary to use extremely large volumes of catalyst • to achieve the desired CO conversion rate. An increased CO content in the exhaust gas 30 is undesirable because more hydrogen is lost as a result of the next methanation process.

According to the invention it has now been found that it is possible to avoid these problems in the first and also in the second stage of the shifting process by applying certain operating conditions and catalysts, which are optimally chosen for this purpose.

The invention therefore relates to an improved process for the preparation of hydrogen-containing gases 8105202: -5- and in particular an ammonia synthesis gas from hydrocarbon ale starting material by desulfurization of the starting material, subjecting the desulfurized material and a primary and secondary reforming, conversion of the carbon monoxide present in the reforming gas to hydrogen and carbon dioxide according to the above-mentioned shifting process (1) in two steps, removal of CO2 from the shifting gas and methanation of the gas. According to the invention, the method is characterized in that (a) the first stage of the shifting process is carried out in the presence of a catalyst consisting of copper oxide, zinc oxide and chromium oxide, and using a supplied gas with a ratio of steam to dry gas of less than 0.5, preferably from 0.3 to 0.5. Under an absolute pressure of 10-50 atm and at a temperature of 190-400 ° C, preferably 20Q-360 ° C, while (b) the second stage of the shifting process is carried out in the presence of a catalyst consisting of copper oxide , zinc oxide and aluminum oxide, at a supply temperature of 160-195 ° C, preferably 175-195 ° C, which supply temperature is at the same time at least the highest temperature of the two temperatures (T ^ f 10) ° C and (T2 + 10) ° C, where the descent point is under the actual prevailing reaction conditions and T2 is the equilibrium temperature for the reaction ZnO + CO2 - ZnCO 3 (6) under the actual prevailing reaction conditions.

The pressure during the second stage of the shifting process will normally be the same as that during the first stage or, due to a naturally occurring pressure drop, a slightly lower pressure, ie normally an absolute pressure of about 10-50 atm.

According to the invention, the catalyst used in the first stage of the shifting process can have the following composition.

15-70, preferably 20-40 atomic% Cu in the form of copper oxide, 20-60, preferably 30r40 atomic% Zn in the form of zinc oxide, 8105202i '. It.

-6 “15-50, preferably 20-50 atomic% Cr in the form of chromium oxide, the atomic percentages being calculated on only the metal4 contents and the oxygen content not being taken into account.

Regarding the limits of the amounts of the deformed components of the catalyst to be used in the first stage of the shifting process of the invention, it should be emphasized that catalysts having a composition within the. The broader ranges mentioned (15-70 atom% Cu, 20-60 atom! Zn, 15-50 atom! Cr) are very well suited for the use according to the invention, while the preferred range of 20-40 atom! Cu, 30-40 atom! Zn and 20-50 atom! Cr indicates catalysts which indicate particularly favorable properties, which have particularly favorable properties in terms of thermal stability and catalytic activity.

According to the invention, the catalyst used in the second step can have the following composition.

25-60 atom! Cu in the form of copper oxide 20 25-45 atom! Zn in the form of zinc oxide 15-30 atom! Already in the form of aluminum oxide, with the atomic percentages calculated in the same way.

The catalyst used in the second stage of the shifting process according to the invention is distinguished by a high activity and a high selectivity for the shifting reaction.

The lower limit, which is stated for the supply temperature in the second stage of the shifting process according to the invention, is therefore not determined on the basis of the activity, it is, in contrast, limited by two parameters, namely the steam pressure, ρ „Λ, and the 1 carbon dioxide pressure, The reason for this is that one should avoid a condensation of water in the interior parts of the catalyst body, as this would restrict the access of the reacting gases to the surface of the active catalyst. hinder; and should also avoid the formation of Cu or Zn carbonates, because the formation of carbonates apart from deactivation, bursts of 8 1 0 5 2 02. _____ -7- can entail the catalyst particles.

In order to ensure a reasonable safety margin, according to the invention, prescribed supply temperatures are used which are at least 10 ° C above the descent point 5 or the equilibrium temperature T2.

The method according to the invention will be elucidated below on the basis of a few tests and examples.

Test 1 describes the first step of the shifting process, which is carried out in a conventional manner.

Test 2 describes both steps of the shifting process, wherein the first step is carried out in the same manner as in the method according to the invention and the second step in the usual manner.

Examples I-I7 describe both steps of the shifting process in accordance with the method of the invention.

Run 1 Reforming a natural gas containing 0.033% O2, 3.91% N2, 83.50% CH4, 9.31% C2H6, 2.83% CgHg, and 0.12% C4H10 was added after adding wet steam conducted at a steam to carbon ratio of 2.5. After the primary reformer, a certain amount of air is added.

25 At the outlet of the secondary reformer, where the absolute pressure is 31 atm. the gas composition is as follows: H 2: 38.95% by volume N 2: 17.23% by volume CO: 10.89% by volume 30 CO 2 -: 4.38% by volume

Ar: 0.20% by volume CH. : 0.22 vol.% 4 H 2 O: 28.13 vol.%

The gas is then transported to the shifting department 35, where the CO conversion is carried out according to the shifting process (1). The first stage of the shifting process is carried out at a feed temperature of 360 ° C using a conventional iron oxide chromium oxide catalyst with a chromium content of about 8 atomic%, based only on the metal contents.

The adiabatic rise in temperature during the passage through the first stage provides a discharge temperature of 444 ° C, which corresponds to 717 ° K. At this temperature, the shift process (1) to equilibrium will have progressed.

The gas composition after the high temperature shift reactor, where the absolute pressure is 10 atm, in the absence of other reactions will be H 2: 46, 44 vol% N 2: 17.23 vol% CO: 3.40 vol.% 15 CO2: 11.86 vol.%

Ar: 0.20 vol.% CII4: 0.22 vol.% HO: 20.65 vol.%

A first requirement, however, is that other reactions do not accidentally occur. Based on P = 3.558 atm.abs. and P ^ q = 1,020 atm.abs. a calculation of the equilibrium constant K for the reaction (3) gives the full

P

the following result 26 25 ^ 2 14 "K = = 1.15.1014 p p32

CO

from which follows K = 14.06.

P -.

Comparison with the drawing shows that the catalyst is present in the carbide form. Laboratory tests have therefore shown that hydrocarbon formation occurs. Thus, under the above assumptions, the laboratory tests show the formation of 0.5-0.7 vol% CH4 35 0.1-0.15 vol% C2H4 and C2H6 0.05 vol% C ^ Hg and C ^ Hg and minor amounts of higher hydrocarbons, alcohols and other oxygenated organic compounds.

8105202 -9-

This shows that conventional high temperature displacement catalysts are useless in the gas compositions used in the present invention.

5 Trial 2

The procedure is as in Test 1 except that a feed temperature of 290 ° C is used in the first stage of the shifting process and a catalyst of the invention is used which contains 20 atomic percent Cu, 30 atom. % Zn and 50 atomic% Cr, all in the form of oxides, which atomic ages are only calculated for the metal contents. The adiabatic rise in temperature during the completion of the first stage yields a discharge temperature of 321 ° C. Under an absolute pressure of 30 atm. a discharged gas is obtained with the following composition: H2: 48.60% by volume N2: 17.23% by volume. CO: 1.24 vol.% 20 CO2: 14.03 vol.%

Ar: 0.20 vol.% CH4 - 0.22 vol.% H 2 O: 18.48 vol.%

Since no carbide problems occur in this process, the second step of the shifting process is continued -

This step is performed using the gas obtained in the manner described above at a feed temperature of 200 ° C and using a conventional low temperature shift catalyst consisting of 30 atom% Cu, 50 atom% Zn and 20 atomic% Al in the form of oxides, which percentages are calculated based on the metal contents only. The adiabatic increase in temperature during walking. 35 of the second stage is about 12 ° C. Under an absolute pressure of 30 atm-, a discharged gas is obtained with the following composition: 8105202 * - V - -10 - H2: 49.17% by volume N2: 17.30% by volume G0 0.24 vol. % CO2: 14.88 vol.% 5 Ar: 0.20 vol.% CH4: 0.22 vol.% H 2 O: 17.77 vol.% CH 3 OH: 0.22 vol.%

Thus, under these conditions, methanol becomes. 10 unwanted quantities formed. In a. ammonia refrigeration, in which 1000 tons of ammonia are prepared per day, will counteract. approximately 13 tons of methanol are formed at a time, which means an unacceptable energy loss.

Example I

In the first stage of the shifting process, as in test 2, a feed temperature of 209 ° C is used and a catalyst according to the invention is used, which consists of copper oxide, zinc oxide and chromium oxide with the same metal contents as in test 2, ie 20 atom % Cu, 30 atom% Zn and 50 atom% Cr, all calculated only on the metal contents. The adiabatic rise in temperature during the first stage leads, as in test 2, to a discharge temperature of 321 ° C, while under an absolute pressure of 30 atm. a vented gas is obtained with the same composition as stated in test 2, ie 48.60 vol.% H2, 17.23 vol.% N2, 1.24 vol.% CO, 14.03 vol.% CO2, 0.20 vol.% Ar, 0.22 vol.% CH2 and 18.48 vol.% H 2 O,

This gas is passed to the second stage of the shifting process, the feed temperature being 175 ° C and the catalyst according to the invention being used consisting of 60 atom% Cu, 25 atom% Zn and 15 atom%

Al, all calculated only on the metal contents. The adiabatic rise in temperature by passing through the second stage is about 13 ° C and under an absolute pressure of 30 atm. discharged gas of the following composition is obtained: H2: 49.61 vol.% N2: 17.25 vol.% - 8105202 1 -11-CO: 0.15 yol.%

CO2: 15.08 vol. S

Ar: 0.20 vol.% CH4: 0.22 vol.% H 2 O: 17.45 vol.% CH 3 OH: 0.04 vol.%

Thus, under these conditions, which are in accordance with the present invention, the formation of methanol is greatly limited, and a daily production of 1000 tons of ammonia corresponds only to about 2 tons of methanol per day, which is acceptable. Moreover, the CO content in the discharged gas is almost halved compared to the content according to test 2.

Example II

The two stages of the shifting process are carried out in the manner described in Example I, with the only exception that the catalyst used in the first stage of the shifting process has a composition with 15 atom% Cu, 35 atom% Zn and 50 atom % Cr, all 20 in the form of oxides, which percentages are only calculated on the metal contents. The resulting exhaust gas has almost the same composition as that of Example I.

Example III

The two stages of the shifting process are carried out in the manner described in Example I, with the only exception that the catalyst used in the first stage of the shifting process has a composition with 25 atom% Cu, 60 atom% Zn and 15 atom% Cr, all in the form of oxides, which percentages are only calculated on the metal contents. The resulting exhaust gas has almost the same composition as that of Example I.

Example IV

The two steps of the shifting process are carried out in the manner described in Example I, with the only exception that the catalyst used in the first step of the shifting process has a composition with 62 atomic% Cu, 20 atomic% Zn and 18 atomic% Cr, all - 8105202-12 - in the form of oxides, which percentages are calculated only on the metal contents. The resulting exhaust gas has almost the same composition as that of Example I.

The supply temperature in the second stage of the 5 examples is within the temperature range prescribed according to the invention. Since the partial gas pressure of 4,209 atm.abs is used for the gas used. can calculate, which corresponds to an equilibrium temperature at the reaction (6) of 164 ° C, and a partial steam pressure of 5.544 10 atm.abs., which corresponds to a drop point of 155 ° C, which is the lowest supply temperature, according to the invention can be used, 174 ° C. In addition, the temperature is below 195 ° C, which is stated above as the highest temperature for the second stage of the shifting process-

In addition to the benefits mentioned above, it should be added that by. the process of the invention removes a source of sulfur poisoning from the second stage catalyst because 20. the use of sulfur-containing iron catalysts is avoided.

8105202

Claims (7)

1. Process for the preparation of hydrogen-containing gases, in particular an ammonia synthesis gas, from hydrocarbons by desulfurization of the starting material, primary and secondary reforming, conversion of carbon monoxide 5 by carrying out the shifting process h 2 O + CO --H 2 + CO 2 (1) in two pedaling, removal of CO2 and manification,. characterized in that (a) the first stage of the shifting process is carried out in the presence of a catalyst 10 consisting of copper oxide, zinc oxide and chromium oxide, using a fed gas with a steam to dry gas ratio of less than 0 , 5, under an absolute pressure of 10-50 atm and at a temperature of 190-400 ° C, and (b) the second stage of the shifting process is carried out in the presence of a catalyst consisting of copper oxide, zinc oxide and aluminum oxide, at a supply temperature of 160-195 ° C, which at the same time fulfills the condition that it is at least the higher of the two temperatures (T ^ + 10) ° C and (T2 + 10) ° C, with the Is 20 point and T2 is the equilibrium temperature for the reaction ZnO + CO2 - ZnCO3 (6) both under the actual reaction conditions. 2. Process according to claim 1, characterized in that a gas mixture is used, which has a steam to dry gas ratio of 0.3 to 0.5. Method according to claim 1 or 2, characterized in that the first stage of the shifting process is carried out at a temperature of 200 to 350 ° C. 4. A method according to any one of claims 1-3 · 30, characterized in that the second stage of the shifting process is carried out at an inlet temperature of 175-195 ° C. 5. Process according to any one of claims 1-4, characterized in that in the first step of the shifting process a catalyst is used with a composition of 15-70 atom% Cu as copper oxide, 20-60 atom% Zn as zinc oxide and 15-50 atomic% Cr as chromium oxide, which atomic percentages are calculated only on the metal contents of the catalyst. 6. Process according to claim 5 ', characterized in that in the first stage of the shifting process a catalyst is used, which has a composition with 20-40 atom% Cu as copper oxide, 30-40 atom% Zn as zinc oxide and 20-50 atomic% Cr as chromium oxide, the atomic percentages being calculated only on the metal contents of the catalyst. 10 Process according to any one of claims 1 to 6, characterized in that in the second stage of the shifting process a catalyst is used which has a composition with 25-60 atomic% Cu as copper oxide, 25-45 atom% Zn. as zinc oxide and 15-30 atomic% Al as aluminum oxide, the atomic percentages being calculated only on the metal contents of the catalyst. 8105202