JPS60191001A - Production of ammonia - Google Patents

Abstract

PURPOSE:To reduce the load of the primary steam reforming stage and to increase the efficiency by introducing an excess of air into the secondary steam reforming stage, and regulating the nitrogen content by mixing the reacted gas which is removed from nitrogen into the gas. CONSTITUTION:An excess of air is introduced into the secondary steam reforming stage to convert a large amt. of methane, and consequently the load of the primary steam reforming stage which is externally heated at high temps. and pressure is reduced. The cost of fuel, power, and equipment is reduced in this way, and the efficiency can be increased. After the carbon oxides in the formed gas are removed, the reacted gas from an adsorption separator wherein the nitrogen is removed along with the nonreactive gas is mixed into said formed gas to regulate the nitrogen content which is made excess due to the introduction of excess air. The nitrogen to hydrogen ratio in the gas is stoichiometrically adjusted in this way, and ammonia is produced by using the gas as the ammonia synthesis gas.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to improvements in the production process of ammonia synthesis gas.

The most common method for producing ammonia consists of the following steps.

1 Primary steam reforming reaction step CH4+H2
O→CO+3H2 2 Secondary steam reforming reaction step CH4-H2
O→CO+3H2 CH4+O→CO+2H2 3 Carbon monoxide conversion reaction step CO+H2O→CO2+H2 4 Carbon dioxide removal step 5 Carbon monoxide removal step 6 Ammonia synthesis reaction step N2+3H2→2NH3 Reaction 1 is economical in terms of steam/carbon ratio, temperature, and pressure. Under conditions where reaction 2 is considered, the reaction proceeds incompletely, and a small amount of unreacted methane is almost converted in reaction 2. That is, reaction 2 obtains a higher degree of preparation by introducing air into the hot gas from reaction step 1 and combusting some of the gas, and unreacted methane is reduced to a low level over the catalyst.

The gas produced in the steam reforming step then converts most of the carbon monoxide into hydrogen in a carbon monoxide conversion reaction step 3. The gas then passes through a carbon dioxide removal step 4, then a carbon monoxide removal step 5, and then enters an ammonia synthesis reaction step 6, where ammonia is synthesized.

The conditions for reaction 2 are usually such that the amount of air required to supply oxygen in reaction 2 introduces the amount of nitrogen required in reaction 6. That is, the amount of air introduced in reaction 2 is restricted from this point. reaction 1
, reaction 2 are both reactions for converting methane into carbon monoxide and hydrogen (finally hydrogen), and if it is possible to introduce a large amount of air in reaction 2, then more reaction takes place, and the load on reaction 1 is greatly reduced.

The reformer in which reaction 1 is carried out is usually an external heating type,
The tubes are exposed to high temperatures and pressures, and are subject to extremely harsh material conditions. Reducing the load on reaction 1 under these conditions frees the reformer tubing material from this harsh condition, and conversely allows for higher pressure reforming reactions.

If this happens, the compression power in ammonia synthesis will be significantly reduced. Further, the secondary reformer is an internal heating type and has extremely high thermal efficiency, whereas the primary reformer is an external heating type and has a considerably low thermal efficiency, so by doing so, the overall thermal efficiency is increased. That is, this method is extremely attractive. However, for this purpose, the excess nitrogen introduced must be removed somewhere.

As a method for this purpose, a method is known in which the ammonia is removed using cryogenic separation before the ammonia synthesis reaction 6.

In this method, synthesis gas is adiabatically expanded using an expander to obtain a low temperature, nitrogen is liquefied and separated to some extent, and the ratio of hydrogen and nitrogen is adjusted to an appropriate ratio for the ammonia synthesis reaction. At the same time, methane is almost completely removed, and carbon monoxide and argon are also removed to some extent.

A method has also been proposed in which the gas is treated by cryogenic separation in the ammonia synthesis loop.

(Japanese Unexamined Patent Publication No. 54-60298) The former method is an excellent method, but because the separation of carbon monoxide is not sufficient, the synthesis gas is subjected to a methanation reaction process and a water removal process using an adsorbent. Introduced into separation equipment. Therefore, process
Cost-wise, it hasn't been streamlined that much.

The latter performs similar treatment in the ammonia synthesis loop, but in order to perform cryogenic separation, ammonia must first be completely removed, and the concentration of nitrogen to be treated is higher than in the former. This is disadvantageous from a nitrogen treatment standpoint. Therefore, if these factors are taken into consideration, it cannot necessarily be said that it is an economical method.

In the method provided by this invention, a part of the gas after ammonia has been synthesized and separated is treated with an adsorption separation device (if a small amount of ammonia remains in the gas, ammonia and nitrogen), nitrogen and non-reacted The ratio of hydrogen and nitrogen in the synthesis gas is controlled by almost completely separating the synthetic gases (namely methane and argon) and controlling the amount of this treatment.

When separating synthesized ammonia from synthesis gas,
Depending on the separation method, (1) cases where separation is incomplete, and cases where (
2) In some cases, it is almost perfect.

(1) In conventional ammonia synthesis methods, the former is incomplete in all cases. That is, in this case, by cooling the reacted synthesis gas, ammonia in the gas is liquefied and separated. Therefore, in the synthesis gas, 1 to several %, usually 2 to
Approximately 3% ammonia remains.

(2) In cases where the separation is almost complete, ammonia in synthesis gas can be almost completely separated by adsorption or absorption methods, although this does not seem to have been practiced industrially yet. be.

For an example of the absorption method using water, see JP-A-54-6029.
9.

An example of a method for separating most of the ammonia from synthesis gas by an adsorption method is described in Japanese Patent Application No. 58-155548, an invention by the inventor of the present invention. Not limited to that invention, when ammonia is separated by an adsorption method, the density of ammonia in the separated synthesis gas becomes very low.

In contrast, in the case of the absorption method using water, the concentration of ammonia is slightly higher, but it is at a much lower level than in the case of liquefaction separation (1).

In these two cases, the gas to be separated by adsorption (1) is a small amount of ammonia, nitrogen, methane, and argon, and (2) is nitrogen, methane, and argon, and each is treated differently. .

Treatment methods in case (1) include a method in which ammonia is separated by absorption with water and then nitrogen, methane, and argon are separated, and a method in which these are simultaneously adsorbed and separated using an adsorption device.

The former method has no problems in terms of process, but it is actually a troublesome method. Since the latter method involves mixing ammonia and other gases at low pressure, it is difficult to separate and recover ammonia from this, both in terms of process and equipment.

In the case of (2), that is, when the gases to be separated are only nitrogen, methane, and argon, and ammonia is present in an extremely small amount, the pressure fluctuation adsorption separation method can be suitably applied to this separation. Nitrogen, methane, and argon have slightly weaker adsorption power, but
If all of these are adsorbed, they can be adsorbed and separated without any problem by a commonly used pressure fluctuation adsorption separation method. Moreover, since the adsorption force is not so strong, desorption is easy.

In the case of (1), if ammonia can be removed by appropriate pretreatment, then the pressure fluctuation adsorption separation method can be applied as in the case of (2).

As a method for this pretreatment, a method of selectively adsorbing and separating ammonia using 3A type zeolite is suitable. Type 3A zeolite has pores with a diameter of 3 Å (angstroms), and molecules other than hydrogen, helium, neon, water, and ammonia cannot enter the pores. Therefore, in other cases ammonia is selectively adsorbed and other components are not. That is, ammonia is selectively adsorbed by an adsorption device using type 3A zeolite. On the other hand, in this case, it is better not to reduce the pressure at all, or even if the pressure is reduced, it should be kept at a pressure that would liquefy ammonia gas at room temperature, so that it is convenient to obtain the liquid solution.

(Since a considerable amount of non-condensable gas enters, it is better to set the liquefaction pressure slightly higher.) In this case, the appropriate desorption method is to heat the adsorption bed to release ammonia. As the heat source, it is desirable to appropriately utilize waste heat such as outflow gas from the ammonia synthesis apparatus.

In this way, the synthesis gas from which ammonia has been separated is
Whether the ammonia separation is incomplete or complete,
The gas composition can be suitably adjusted using the adsorption separation device.

Next, a specific example will be explained in more detail.

FIG. 1 shows a flow sheet for adsorption separation of ammonia from synthesis gas containing a small amount of ammonia.

The adsorbents 1a, b, and c are filled with 3A type zeolite, and heat transfer tubes for pressurization and cooling are installed in the bed.

The processing gas is introduced into the adsorbent in the adsorption process from 21,
Ammonia is adsorbed and separated and flows out from 22. Once the adsorbent is saturated, it is switched to desorption, heated to release ammonia, then cooled and recycled back to the adsorption process.

The desorbed gas is sent to a liquefier by purge gas, where it is liquefied and separated. The reaction gas discharged from the ammonia synthesis apparatus is suitable as the heating medium for this desorption.

Next, FIG. 2 shows a flow sheet of an example of a pressure fluctuation adsorption separation apparatus for separating gases containing almost no ammonia (nitrogen, methane, and argon), and FIG. 3 shows its cycle diagram.

As an adsorption/separation device for such a gas, a commonly used pressure fluctuation adsorption/separation device can of course be used. However, it should be noted that hydrogen recovery is particularly important when incorporated into such processes. In this respect, the device shown in this example has a high recovery rate despite its simplicity.

The adsorbent is of three bed type, 1a, b, and c. (The adsorbent 1a will be mainly described in terms of its cycle.) Processing gas flows into the adsorbent from 21. The gas adsorbs most of the adsorbed components, nitrogen, methane, and argon, ie, is released from 22 as almost pure hydrogen. When the adsorbent becomes saturated and the adsorbed components begin to leak, the flow into the adsorbent 1a is reduced to half of the normal flow rate, and at the same time, the flow of process gas into 1b is started at half the normal flow rate.

By cutting the flow into the adsorbent in half in this way,
There is less unused adsorbent at the time of switching to the desorption process.

At this time, a small amount of adsorbed components leak from 1a, but the concentration of the leaked adsorbed components is half of that of 1a and 1b as a whole, and this does not particularly affect ammonia synthesis, so there is no problem.

The time during which these two beds are used in parallel can be changed after determining in advance how much leakage of adsorbed components is allowed. Therefore, this process is completed in a short time, and 1a is used for the vacuum desorption process, and 1a is used for the vacuum desorption process.
In b, the flow rate becomes the full amount and the adsorption process is continued.

The reduced pressure gas from the adsorbent 1a is first made to flow into the reduced pressure gas tank 2, and when both pressures become approximately equal, it is made to flow into the adsorbent 1c which is in the repressurization process. When the pressures of 1a and 1c become approximately equal, next, the reduced pressure gas is allowed to flow into 1c from tank 2 until the pressures of 1c and 2 become equal. Next, the replacement gas from the replacement gas tank 4 is flowed into 1a from the inlet side to expel the void gas inside the body, and the purge gas tank 6
to flow into. By this gas replacement, most of the hydrogen in the bed is recovered.

Inside this purge gas tank, there is a honeycomb,
Alternatively, the gas is filled with a material such as pipes arranged in the axial direction that prevents mixing in the radial direction, so that the composition of the inflowing gas is maintained as it is and is stored. Therefore, the gas from the entrance to the back of this tank has a high concentration of hydrogen, and the gas toward the entrance has a low concentration of hydrogen.

Once the gas exchange is complete, the bed is then further depressurized.

Most of the gas that flows out here contains hydrogen, but nitrogen, argon, and other gases that have weak adsorption power flow out.

Since this outflow gas is suitable as a replacement gas, it is received in the replacement gas receiving tank 3, compressed by the replacement gas compression wall 5, stored in the replacement gas tank 4, and used for replacement.

The adsorbent 1a is further reduced in pressure, and the desorbed gas is passed through the exhaust gas line 2.
It flows out from 7. When the pressure in the bed approaches almost the minimum level, the gas stored in the purge gas tank 6 is then introduced into the bed through the purge line 25, and the desorbed gas in the bed is expelled into the exhaust gas line 27. As mentioned earlier, the purge gas tank is filled with gas where the concentration of hydrogen gas is low near the inlet, and where the concentration of hydrogen gas is high toward the back. Therefore, the flow into the exhaust gas line is stopped at an appropriate point in this purge step, the gas with high hydrogen concentration is retained in the adsorption bed, and the repressurization step is started.

Next, the reduced pressure gas from 1b and then the reduced pressure gas from the reduced pressure gas tank 2 are introduced into the adsorbent 1a.

Thereafter, hydrogen gas, which is a product gas, is made to flow into 1a from the hydrogen gas tank 7 through the filling line 24.

The hydrogen gas tank 7 is placed as a buffer tank so that the flow rate of the treated gas line 22 does not fluctuate too much.

The adsorbent used in this adsorption device is primarily adsorbed with nitrogen, so it is desirable to use an adsorbent with a large adsorption capacity for nitrogen, such as zeolite.

Depending on the scale of the device, the adsorption device may be of a four-bed type or an adsorption device with more beds based on a cycle based on substantially the same idea.

This adsorption device achieves a high hydrogen recovery rate through a relatively simple process, but the reasons for this are: (1) When switching adsorbents, two beds are used in parallel for a short period of time, allowing unused adsorbents to be used. (2) performing gas replacement in the middle of depressurization and recovering the hydrogen-containing gas; and (3) using the recovered gas as a purge gas to effectively desorb the adsorbent. .

Next, an example will be described in which this invention is combined with the invention related to a method for separating ammonia by adsorption method by the same inventor. The invention includes three inventive methods,
Since the synthesis gas from which ammonia has been separated is generally the same in all methods, only the method of the first invention will be described.

FIG. 4 shows an example in which the present invention is combined with the first method.

The high-temperature reaction gas flowing out from the ammonia synthesis device 12 flows into the adsorbents 14c and 14d in the desorption process of the adsorption separation device 14, and adsorbs the ammonia adsorbed there, increasing the concentration of ammonia and reducing the amount of ammonia. It flows into the liquefaction device 16. Here, most of the ammonia is liquefied, and the gas then flows into the adsorption beds 14a and 14b in the adsorption stage of the adsorption separator 14, where the ammonia is almost completely adsorbed and separated and flows out of the adsorption separator. The synthesis gas compressor 11 circulates the ammonia synthesizer.

The desorption process mainly takes place in the first adsorption bed (in the figure) 14c.
Desorption is performed in the next adsorption bed 14d, and the adsorption bed is preheated with this desorption gas.

Further, in the adsorption step, adsorption is performed in the first adsorbent 14a, and the bed heated by desorption is cooled in the next adsorbent 14b.

Once these adsorbents reach saturation, they are sequentially switched to the next step.

A portion 21 of the synthesis gas that has flowed out of the adsorbent 14a flows into the pressure fluctuation adsorption separation device 17, where nitrogen, methane,
Most of the argon is adsorbed and separated.

The processed gas 22 is immediately transferred to the synthesis feed gas 31.
, flows into the synthesis gas compressor 11 from 32, and is sent to the synthesis apparatus.

The synthesis feed gas from 31 is almost in a nitrogen-excess state, but as the process gas consisting almost only of hydrogen is synthesized from 22, the hydrogen/nitrogen ratio is corrected to the stoichiometric ratio, and the synthesis gas from 32 is will be sent to Modification of the ratio is easily accomplished by controlling the amount of gas 22 processed. Simultaneously with this ratio correction, methane and argon are also effectively removed, keeping the inert gas concentration in the synthesis loop at a low level.

Since the pressure drop in the pressure fluctuation adsorption separation device can be kept small, there are no problems with inserting this adsorption device into this part.

Next, the balance of a 1000 ton/day ammonia plant according to the present invention shown in FIG. 4 is shown.

Preconditions Excess rate of nitrogen in raw gas 40% Hydrogen recovery rate of pressure fluctuation adsorption separation device 95% Ammonia concentration at inlet of ammonia synthesis device 0% 〃 Outlet 〃 1
0% Synthesis gas compressor outlet pressure 55kg/cm2g (31)
etc. indicate the lines in Figure 4.

Raw material gas (31) (Reference) Current method vol% Nm3/hr vol% H2 66.23 84.170 73.94N2 3
0.88 39.244 24.65CH4 2.41
3.069 1.09Ar 0.47 600 0.
32 Total 127.083 Adsorption device line gas amount (21) Processing gas (22) vo
l% Nm3/hr 〃 H2 69.6 36.252 34.439N2 2
3.2 12.084 292CH4 6.0 3.1
25 56 Ar 1.2 625 25 Total 52.086 34.812 Synthesis gas compressor inlet Recycle (32) Nm3/hr (33)〃 H2 118.609 307.652N2 39.5
36 102.551 CH4 3.125 26.522 Ar 625 5.304 Total 161.895 442.029 Synthesizer inlet (35) Outlet (36) vol% Nm3/hr H2 70.58 426.261 343.908N
2 23.58 142.087 114.636CH
4 4.91 29.647 29.647Ar 0.
98 5.929 5.929NH3 54.902 Total 603.924 549.020 As you can see from the above balance, even though the methane concentration of the raw material gas is 2.4%, the inert gas concentration at the inlet of the synthesizer is only a few. %
It has stopped at .

Thus, by using the method of the invention it is possible, on the one hand, to increase this pressure by reducing the load on the reformer and, on the other hand, to synthesize ammonia at low pressure.

The advantages of this treatment method are as follows.

1. The amount of nitrogen removed in this treatment step must be the same as the excess nitrogen in the feed gas.

Therefore, the amount of gas treated in this treatment step varies depending on the excess ratio of nitrogen in the raw material gas, but is approximately 5% of the amount of raw material gas.
Becomes 0% or less. (As shown in the above example, when the nitrogen excess rate is 40%, the amount of processed gas is 41% of the amount of raw material gas.) When processing before synthesis by cryogenic separation, naturally the entire amount of raw material gas must be treated. There must be. Even when processing in a synthesis loop, nitrogen cannot be removed to such a large extent due to the cooling temperature. (If the pressure drop is increased in the expander, a lower temperature can be obtained and the amount of nitrogen removed will also be increased, but the power required to restore the pressure will be increased.) In other words, in this case as well, there is no big difference from the case of processing before synthesis. Therefore, the amount of gas to be processed in this adsorption separation method is significantly smaller than that in cryogenic separation.

Since the amount of gas to be processed is small, the cost of the equipment is also low, and the loss of hydrogen is also reduced.

2 Generally, the pressure drop within an adsorption separation device is smaller than that in cryogenic separation. In particular, as mentioned above, since the amount of gas is small, it is possible to design the pressure drop to be small.

Therefore, it is easy to combine the treated gas with the feed gas at the inlet of the syngas compressor, resulting in a small increase in power for gas treatment.

3 In this adsorption, not only nitrogen but also methane and argon are almost completely adsorbed and separated, so the concentrations of methane and argon in the synthesis loop become low. Therefore, a high concentration of methane in the raw material gas can be tolerated, and a high leakage of methane at the outlet of the secondary reformer can be tolerated.

This selectively has the effect of increasing the amount of air fed to the secondary reformer, with very favorable results such as increasing the operating pressure in the reformer, lowering the operating temperature, and reducing the steam/carbon ratio. It is possible to make it happen.

4 The treated gas is mixed with the raw material gas which is in a state of excess nitrogen, and the excess is corrected before being sent to the ammonia synthesis process, so there is no adverse effect due to excess nitrogen.
It is easy to control the hydrogen/nitrogen ratio.

5. When the process gas contains ammonia, its separation can be achieved by a combination of an adsorption device and a cooling liquefaction device, eliminating the need for absorption with water and simplifying the process.

[Brief explanation of drawings]

Figure 1: Flow sheet for an absorption separator that pre-processes synthesis gas containing a small amount of ammonia Figure 2: Flow sheet for a pressure fluctuation adsorption separator that processes synthesis gas that contains almost no ammonia Figure 3: Same as above Cycle diagram Figure 4: Adsorption Flow sheets 1a, b, c of an example in which a pressure fluctuation adsorption separation device is combined with a synthesis device that separates ammonia by a method. Gas tank 11 Synthesis gas compressor 12 Ammonia synthesis device 14a, b, c, d Adsorbent 16 Ammonia liquefaction device 17 Pressure fluctuation adsorption separation device 21 Processed gas inlet 22 Processed gas outlet 23 Decompression gas line 24 Re-decompression filling gas line 25 Purge gas Line 26 Displacement gas line 27 Exhaust gas line 31 Raw gas inlet 32 Synthesis gas compressor inlet 33 Circulating gas inlet 34 Synthesis gas compressor outlet 35 Ammonia synthesizer inlet 36 Ammonia synthesizer outlet

Claims (2)

[Claims] (1) (a) subjecting the hydrocarbon feedstock to primary steam reforming under pressure; (b) introducing air into the gas obtained in step (a);
For secondary steam reforming, (c) carbon monoxide in the obtained gas is catalytically subjected to a carbon monoxide conversion reaction, and (d) carbon oxides are then removed to form nitrogen- and hydrogen-containing producing ammonia synthesis gas; (e) reacting the synthesis gas to produce ammonia; and separating the ammonia from the reacted synthesis gas by an ammonia separator; In an ammonia production method consisting of each step of treating a portion of the gas and discarding non-reactive gases present in the gas, (I) air equivalent to the nitrogen stoichiometrically required for ammonia synthesis; (II) Nitrogen, along with non-reactive gases in the gas to be treated in step (f), is almost completely removed by an adsorptive separation device, The processed gas is sent to the synthesis step (e) together with the gas that has flowed out of step (d), and by controlling the amount of gas to be processed, the ratio of hydrogen and nitrogen in ammonia synthesis can be adjusted to their stoichiometric ratio. A method for producing ammonia characterized by control. (2) If the separation of ammonia in step (e) is incomplete, as a treatment in steps (f) and (II),
First, ammonia is mainly adsorbed and separated using an adsorption separation device using type 3A zeolite as an adsorbent, and then nitrogen and nitrogen are separated using a pressure fluctuation adsorption separation device using an adsorbent other than type 3A zeolite. A process for producing ammonia according to claim 1, wherein non-reactive gases are almost completely removed. (3) If the separation of ammonia in step (e) is almost complete, the treatment in steps (f) and (II) is to use a pressure swing adsorption separation device to almost completely remove nitrogen and non-reactive gases. A method for producing ammonia according to claim 1, wherein the ammonia is removed.