CS258273B1 - Process for producing ammonia - Google Patents

Abstract

The solution relates to the production of ammonia from a synthesis nitrogen-hydrogen mixture, in which hydrogen is obtained by gasifying a hydrocarbon raw material with oxygen and steam, after which it is mixed with pure nitrogen to form a synthesis mixture, which leads to the synthesis of ammonia, consisting in the hydrocarbon raw material being gasified with technical oxygen containing at most 1.25 mol%, preferably 0.25 to 0.8 mol%, of argon.

Description

The invention relates to the production of ammonia from a synthesis nitrogen-hydrogen mixture, which is obtained by mixing technically pure hydrogen and nitrogen, the hydrogen being obtained by gasification of a hydrocarbon feedstock with an oxygen-vapor mixture.

Oxygen for gasification is usually obtained by rectification of liquefied air, while nitrogen is separated and about 3 mol% of argon usually remains in the oxygen. When producing hydrogen by gasifying oil residues with oxygen and steam, argon remains in the hydrogen in an amount of 0.41 to 1 mol%, which is reflected in the values of 0.31 to 0.75 mol% in the synthesis nitrogen-hydrogen mixture.

Argon is an inert gas which, together with methane, originating from the gasification of hydrocarbon feedstock in the production of hydrogen, which also has the character of an inert gas under the synthesis conditions, accumulates in the ammonia synthesis circulation gas, causing a part of the circulation gas to be vented and replaced with fresh hydrogen. The amount of circulation gas to be vented is directly proportional to the amount of inert gases present in the fresh synthesis nitrogen-hydrogen mixture, which thus directly affects the specific consumption of hydrogen and nitrogen for ammonia production.

To reduce the consumption of hydrogen and nitrogen for ammonia production, the discharged circulation gas from ammonia production is therefore additionally processed in order to separate the accumulated inert components and hydrogen, possibly nitrogen, so that they can be returned to the synthesis. Several methods are known for this purpose, e.g. cryogenic using low temperatures, separation of components on molecular sieves or semi-permeable membranes or finally additional conversion of hydrogen and nitrogen from the discharged circulation gas in a separate ammonia synthesis under deteriorated reaction conditions.

All these known methods have the disadvantage that they require the construction of new facilities and additional energy expenditure to utilize part of the hydrogen and nitrogen from the discharged circulating gases from ammonia synthesis. They are used especially when, in addition to nitrogen and hydrogen, some other components forming inert gases are obtained from the circulating gas, which compensate for these additional costs.

A method for producing ammonia has now been found which newly solves the problem of the quality of the circulating gas for ammonia synthesis. A method for producing ammonia from hydrogen and nitrogen, in which hydrogen is obtained by gasifying a hydrocarbon feedstock with oxygen and steam, after which it is mixed with pure nitrogen to form a synthesis mixture, which is used for ammonia synthesis and circulated as a circulating gas, consists according to the invention in that the hydrocarbon feedstock for hydrogen production is gasified with technical oxygen containing at most 1.25 mol % argon, preferably up to 0.8 mol % argon.

Under normal conditions of rectification of liquefied air, oxygen containing 1.5 to 3 mol % argon is usually obtained. The rectification conditions are mainly determined by the requirement of an optimal energy regime of rectification, with less attention being paid to the quality of oxygen, which is sufficient for the desired purposes.

However, in the ammonia production method according to the invention, the improvement in oxygen quality will translate into savings in the following technologies, namely in the production of hydrogen and nitrogen and in the production of ammonia, by reducing the consumption of oxygen for the gasification of hydrocarbon fractions for the production of hydrogen, thereby reducing the energy consumption for its production and compression, and by reducing the consumption of hydrogen and nitrogen for the production of ammonia, thus reducing the energy consumption for the production of hydrogen and nitrogen and their compression.

Another benefit will be the reduction in the amount of discharged circulation gas, which is usually used for heating purposes, which is uneconomical in terms of the components it contains, especially hydrogen. The lower hydrogen consumption will have a positive effect on the reduction of the amount of hydrocarbon fractions, e.g. fuel oil, for gasification.

As regards improving the oxygen quality to an argon content of no more than 1.25 mol %, this can be achieved in the existing plant in several ways, e.g. by increasing the reflux ratio in the overhead product of the rectification columns, reducing the oxygen recovery from air and increasing the hydraulic load on the columns. Although this will somewhat reduce the economic efficiency of the rectification of liquefied air, the savings in subsequent technological processes will significantly exceed these losses. Improving the oxygen quality can also be achieved by reconstructing the rectification columns or building a new plant. In this case, it is possible to reduce the argon content in oxygen to up to 0.25 mol %.

This reconstruction or construction of a new facility is justifiable provided that the produced oxygen will be used mainly for oxygen steam cracking of hydrocarbon feedstock and with the aim of producing hydrogen intended for the production of ammonia.

The process for producing ammonia according to the invention is further described in an example which compares the current state of the art with the process according to the invention and demonstrates the effects achieved.

Example

To produce 1 ton of ammonia, the following parameters are used in a normal technological mode:

Composition of oxygen:

oxygen content: argon content: nitrogen content:

96.75% mol. 1.75 » mol. 1.50% inol.

By oxy-fuel gasification using this technical oxygen, hydrogen with the following composition was produced:

Composition of hydrogen:

mixture

hydrogen content: 97.96% mol. argon content: 0.47 mol % methane content: 0.77 4 mol. nitrogen content: 0.80 i mol. By mixing this hydrogen with nitrogen Nitrogen composition: nitrogen content: 99% hydrogen content: 1% Composition obtained synthetic mixtures: nitrogen content: 24.77% mo. hydrogen content: 74.29% mol. methane content: 0.58 mol% slang content: 0.36 mol %

When passing this mixture through a synthesis loop and producing 1 t of ammonia, the following results were found:

consumption of synthetic mixture:

of which:

hydrogen consumption: nitrogen consumption:

729.05 m

062.78 m ^J 666.27 m ³ amount of gases released: 83.12 m

Composition of discharged gases mol. %:

nitrogen content: 18.08% mol hydrogen content: 54.20% mol methane content: 14.75% mol argon content: 10.65% mol ammonia content: 2.32% mol

After adjusting the technological regime in oxygen production, which led to a reduction in the argon content, the technological regime in the monitored values was as follows:

Case 1 Case 2 Case 3

oxygen composition (mol %) oxygen content 97.35 97.70 98.13 argon content 1.35 1.20 1.12 nitrogen content 1.30 1.10 0.75

hydrogen composition (mol. S)

hydrogen content 98.06 98.11 98.11 argon content 0.37 0.33 0.30 methane content 0.77 0.79 0.78 nitrogen content 0.80 0.80 0.80 composition of the synthesis mixture (mol. %) nitrogen content 24.79 24.79 24.80 hydrogen content 74.35 74.35 74.38 methane content 0.58 0.60 0.59 argon content 0.28 0.26 0.23

The nitrogen for creating the synthesis mixture had the same composition as in the normal technological mode.

When passing the synthesis mixture through the synthesis loop and for the production of 1 t of ammonia, the following results were found:

Case 1 Case 2 Case 3 consumption of synthesis mixture (m^)

720.12

715.74

715.60 of which:

hydrogen consumption 2 055.48 2,051.90 2,051.79 nitrogen consumption 664.64 663.84 663.81 amount of gases released: 73.83 69.29 69.11 composition of released gases (mol. %) nitrogen content 18.08 18.08 18.08 hydrogen content 54.20 54.20 54.20 methane content 16.24 16.80 17.40 argon content 9.16 8.60 8.00 ammonia concentration 2.32 2.32 2.32

These savings were achieved across the entire production section from the oxygen production node to the synthesis loop (compared to normal technology) - with a daily production of 1,000 tons of ammonia.

Case 1

Case 2

Case 3

electricity saving (kWh/day) 4,538 6,636 6,959 fuel oil savings (t/day) 2.5 3.82 3.86 Gas calorific value loss (GJ/day) 74.7 119.7 105.8 loss of electrical energy at the oxygen plant (for the production of higher quality oxygen) (kWh/day) 4,459 4,976 5,317 savings on raw material and energy costs for ammonia production (%) 0.23 0.36 0.38

The total savings also include savings for steam and cooling water. This item is not reported separately in physical units here.

The share of savings from total savings for raw materials and energy is related to the total costs of the entire production line from the oxygen plant to the synthesis loop.

Case 2 and case 3 differ little, as in the synthesis mixture, in addition to a decrease in argon content, there is a slight increase in methane compared to case 1.

Claims (1)

object of the invention A process for producing ammonia from hydrogen and nitrogen, wherein hydrogen is obtained by gasifying a hydrocarbonaceous feedstock with oxygen and steam, followed by mixing with pure nitrogen to a synthesis mixture which leads to synthesis and circulates as a circulating gas, characterized in that the hydrocarbonaceous feedstock for production. The hydrogen is gasified with technical oxygen containing not more than 1.25 mol%, preferably 0.25 to 0.8 mol%, of argon.