WO2019052824A1

WO2019052824A1 - A process for ammonia production

Info

Publication number: WO2019052824A1
Application number: PCT/EP2018/073332
Authority: WO
Inventors: Peter Mølgaard Mortensen; Poul Erik HØJLUND NIELSEN
Original assignee: Haldor Topsøe A/S
Priority date: 2017-09-13
Filing date: 2018-08-30
Publication date: 2019-03-21

Abstract

In a process for ammonia production, in which an additional separation step within the ammonia loop is used, remaining ammonia in the recycle feed gas is extracted by condensation in the additional separation step and adsorbed over a swing adsorption bed placed after the separator. This way, the inlet concentration of ammonia to the converter is reduced. The swing adsorption bed can have pressure swing adsorption configuration, temperature swing adsorption configuration or purge swing adsorption configuration.

Description

A PROCESS FOR AMMONIA PRODUCTION

The present invention relates to a novel process for the production of ammonia and a process design for carrying out the process. More specifically, the invention relates to a concept where the remaining ammonia in the recycle feed gas in the ammonia loop is extracted using an additional sepa^¬ ration step. This way, the ammonia slip to the ammonia con^¬ verter is markedly reduced. Furthermore, the invention re- lates to a process design comprising a classical ammonia loop including a separator for use in the process, wherein an additional separation step is incorporated within the loop for further reduction of the ammonia concentration in the recycle gas .

The catalytic synthesis of ammonia from hydrogen and nitro^¬ gen according to the equation

N₂ + 3H₂ <-> 2NH₃ (ΔΗ = -92.4 kJ/mol) was developed around 1908 and improved to industrial scale a few years later. Since then, this method (known as the Haber-Bosch method) has been the predominant industrial scale method for ammonia production. The synthesis is car- ried out in a circulatory system commonly known as an ammonia synthesis loop. Only a fraction of the synthesis gas is converted per pass, as limited by the equilibrium concen^¬ tration of N¾ at the exit conditions of the converter. In general, the make-up gas fed to the loop will contain about 99% of nitrogen and hydrogen in a molar ratio H₂/ ₂ of around 3.0 with about 1% methane and argon besides minor amounts of other molecules, some of which may be catalyst poisons .

The catalytic activity of an ammonia catalyst may be re- duced in the presence of certain chemical compounds (poi^¬ sons) . These may be gaseous, occurring as minor components of the synthesis gas, or they may be solids introduced as impurities to the catalyst during the manufacturing pro^¬ cess.

In the case of gaseous catalyst poisons, a distinction can be made between permanent poisons (causing an irreversible loss of catalytic activity) and temporary poisons (lowering the activity only while present in the synthesis gas) . Per- manent poisons such as sulfur accumulate upon the catalyst surface and may be detected by chemical and spectroscopic analysis, while temporary poisons such as oxygen, carbon and water do not interact as strongly with the catalyst. The concept of catalyst poisoning is dealt with in chapter 8 of "Catalytic Ammonia Synthesis", edited by J.R. Jen^¬ nings, Plenum Publishing Corporation (1991).

The deactivation of the ammonia catalyst is dependent on the operating conditions (pressure and temperature) , but more significantly on the small amounts of gaseous com^¬ pounds contained in the feed gas. It is therefore impera^¬ tive that such compounds are removed from the feed gas and that a feed gas with low N¾ content to be fed to the con^¬ verter is obtained.

The average lifetime of an ammonia synthesis catalyst has increased markedly since the early days of ammonia manufac^¬ ture due to a number of process improvements. One of these is the incorporation of a secondary ammonia condensation system, in which the make-up gas together with the recycle gas is washed in liquid ammonia before entering the ammonia synthesis converter.

There are a number of industrial processes in which it is necessary to separate N¾ from mixtures of other gases.

Perhaps the largest scale separation is the removal of N¾ from the gas mixture that is present in the recycle loop of an ammonia synthesis plant. This separation is currently accomplished by refrigeration, with ammonia being condensed and removed in a liquid state. Thus, in the conventional ammonia synthesis procedure, ammonia is separated from the synthesis gas by cooling, typically to a temperature of around -28 °C which condenses the ammonia, using an ammonia cooling system. The condensed ammonia also serves as a fi^¬ nal purification of the make-up gas since it removes trace components of water, carbon dioxide, carbon monoxide and methanol, among others.

It has previously been proposed to use membranes for sepa^¬ ration of ammonia from the synthesis gas; however, a system like that will not protect the synthesis gas against trace amounts of poison, such as water and carbon dioxide coming from the methanation reactor.

In WO 2017/065924, a method for separating ammonia from a gaseous mixture including hydrogen, nitrogen and ammonia is described. In said method, ammonia is complexed with a po- rous structure containing an organoborane material to pro^¬ duce an ammonia-organoborane material complex and a gas stream. The ammonia can subsequently be separated from the complex .

US 2003/0172809 Al belonging to the Applicant relates to ammonia synthesis loops containing gases, which do not re^¬ act and would accumulate if they were not purged out. Ammo^¬ nia in a purge gas is recovered by an adsorption agent that operates at the full synthesis loop pressure. The adsorp^¬ tion agent is chosen in such a way that the ammonia can be removed again by passing a gas comprising hydrogen and nitrogen through it at the same elevated pressure as the loop pressure. This enables the adsorption agent to be regener- ated by fresh synthesis gas coming from the synthesis gas compressor just before this gas enters the synthesis loop. Thereby, the regeneration requires an absolute minimum of energy consumption and equipment. US 4.172.885 discloses an ammonia preparation process whereby ammonia is removed from a purge stream from the synthesis loop, thereby providing a purge stream containing less than about 0.1 vol% ammonia which can be removed using a separation membrane.

Also in US 4.180.553 a process is disclosed, in which ammo^¬ nia is removed from a purge stream from the synthesis loop using a permeation membrane. Finally, GB 2 145 702 A describes a process for the synthe^¬ sis of ammonia, where an ammonia synthesis gas comprising a mixture of fresh feed gas and recycled reaction product gas is pressurized in a synthesis gas compressor and then fed to an ammonia synthesis reactor. The effluent from the am^¬ monia synthesis reactor is then passed to an adsorption separation unit which can be of several different types, i.a. thermal swing adsorption and pressure swing adsorption. The effluent may also be sent to a liquefier before being sent to the adsorption separation unit. From the liquefier, a gas is passed through the adsorption separation unit, and ammonia is adsorbed in the unit. The remainder of the gas is then recycled to the synthesis gas compressor.

In this GB document, impurities of the make-up gas are not condensed in the ammonia wash. Consequently, the make-up gas of the GB document needs to be completely free of any poisons in the configuration.

The present invention specifically utilizes that the combi^¬ nation of flash separation and swing adsorption (SA) has the combined benefits of removal of impurities and thorough recovery of ammonia.

In a classical ammonia loop, the make-up gas is added to the cold product gas before entering the flash separator, in which ammonia is separated at a high pressure and a tem^¬ perature of around 0 C. This leaves around 4 ~6 ammonia in the recycle gas, which is heated and sent to the ammonia converter .

According to the idea underlying the present invention, an extra separation step is incorporated within the ammonia loop for further reduction of the ammonia concentration in the recycle gas. This extra separation step is based on an SA-like configuration. Especially the purge swing adsorption principle is useful to maintain the gas at a fixed pressure . Swing adsorption is a gas separation technology which is known in various embodiments, such as pressure swing ad^¬ sorption, temperature swing adsorption, purge swing adsorption and partial pressure purge swing adsorption. Pressure swing adsorption-like configurations for ammonia separation purposes have been widely discussed in the art. However, the concept has typically been used as substitute for ammonia flash separation. Consequently, the ammonia is delivered at a low pressure, and the benefit is lost be- cause the product ammonia needs compression downstream.

It has now turned out that incorporating an extra separa^¬ tion step in the ammonia loop leads to unexpected benefits. Thus, the present invention concerns a process for ammonia production, in which an additional separation step within the ammonia loop is used, wherein remaining ammonia in the recycle feed gas is

- extracted by condensation in the additional separation step and

- adsorbed over a swing adsorption bed placed after the separator, thereby both - reducing the inlet concentration of ammonia to the converter, and

- removing any impurities.

The swing adsorption bed may be a pressure swing adsorption configuration, a temperature swing adsorption configuration or a purge swing adsorption configuration. In the following, the invention is described in more detail with reference to the figures, where

Fig. 1 illustrates the basic principle of the invention, Fig. 2 shows a swing adsorption (SA)-like configuration, which is described in more detail in "Configuration 1" below,

Fig. 3 shows another swing adsorption (SA)-like configura- tion, which is described in more detail in "Configuration 2 " below, and

Fig. 4 and Fig. 5 illustrate the increase in the ammonia production and the impact on the loop pressure, respec- tively, with reference to the example.

Thus, the basic principle of the invention is illustrated in Fig. 1. The key feature is that ammonia from the recycle gas is recycled back to the flash separator, whereby the converter is bypassed. This way, the flash separator will function as a primary separator and the adsorption bed will act as a polisher. The inlet concentration of ammonia to the converter is thereby reduced, and this will increase the rate of reaction in the first part of the catalyst bed.

Since the separation comprises both condensation and ad- sorption, the separator will capture all impurities (such as H₂0, CH₃OH, CO, CO₂ etc.) that may be present in the make-up gas, so that they are kept from reaching the cata^¬ lyst, thereby securing a long catalyst life in the ammonia loop. The subsequent adsorption bed will then remove the excess ammonia from the separator process gas and thereby enable a more effective operation of the ammonia converter. Regeneration of the adsorption bed is subsequently done by purge gas treatment at elevated pressure, so that no extra compressor work is needed.

The lowered ammonia concentration will additionally estab^¬ lish potential for (a) a lower loop pressure, (b) a higher ammonia production per converter pass, (c) an ammonia converter of smaller size or a combination of all three fea- tures.

The internal ammonia recycle according to the invention can be done in two different ways, as will be illustrated in the following through description of two different configu- rations:

Configuration 1

A swing adsorption (SA)-like configuration as the one shown in Fig. 2 can be considered. In this configuration, ammonia in the recycle gas from the flash separator is adsorbed over an adsorption bed, which is placed more or less di^¬ rectly after the flash separator where the temperature is as low as possible. The resulting gas from the adsorption bed is sent to compression, heat exchange and other unit operations typically used, before it enters the ammonia converter .

When the adsorption bed has been sufficiently saturated, it is switched to the make-up gas which will act as a purge gas. This gas does not contain any ammonia, and the ammonia in the adsorption bed will therefore desorb due to the dif^¬ ference in partial pressure. As the make-up gas is already pressurized, it can be mixed as usual into the product gas from the ammonia converter, and the ammonia will therefore remain pressurized during the entire swing adsorption (SA) process .

When the ammonia has been sufficiently desorbed from the adsorption bed, the bed can be changed back to adsorption mode from the recycle gas. Several adsorption beds can be used in parallel to obtain a consistent flow of ammonia through the PSA-like configuration.

Configuration 2

In another configuration, shown in Fig. 3, the principle can be run in another swing adsorption-like configuration. In this case, ammonia is still adsorbed cold from the recy^¬ cle gas. However, to desorb the ammonia, the hot product gas from the ammonia converter is used. This product gas will typically have a temperature of around 450°C. As the adsorption is done at 0°C and the desorption is done at 450°C, there will be a large driving force available for the desorption despite the fact that the purge gas already contains around 18-20 "6 3.ΓΠ.ΓΠ.ΟΠ Ϊ cL ·

The adsorbent to be used in both configuration 1 and configuration 2 should be ammonia selective over hydrogen and nitrogen. Suitable adsorbent candidates for this purpose are zeolites, active carbon, Nafion™ or similar compounds.

The benefits of the reduced ammonia inlet concentration to the ammonia converter can be gathered as either a reduced loop pressure, a larger production of ammonia per converter pass or a smaller reactor volume.

The invention is illustrated further by the example which follows .

Example

In an S-300 type converter of a fixed size, operated with a pressure of 180 kg/cm², the inlet concentration of ammonia is decreased from 4% in the base case down to a concentra^¬ tion approaching 0%. Ideally, the inlet concentration is lowered to 0%, but any concentration reduction has a posi^¬ tive effect. This means that an ammonia slip of e.g. 1% is still advantageous.

This way, a significant increase in the ammonia production is obtained. More specifically, by lowering the inlet con^¬ centration to 0%, a linear increase of the production to 2734 MTPD from 2102 MTPD (at an inlet concentration of 4%) is obtained, corresponding to a 30% increase, as can be seen in Fig . 4.

Similarly, the same base case was used to investigate the impact on the loop pressure. This time, the production was fixed, which gave the impact shown in Fig. 5. By lowering the inlet concentration of ammonia to 0%, the pressure was reduced from 180 to 116.7 kg/cm², i.e. by 35%.

Claims

Claims :

1. A process for ammonia production, in which an additional separation step within the ammonia loop is used, wherein remaining ammonia in the recycle feed gas is

- extracted by condensation in the additional separation step and

- adsorbed over a swing adsorption bed placed after the separator, thereby both

- reducing the inlet concentration of ammonia to the converter and

- removing any impurities.

2. Process according to claim 1, wherein the swing adsorption bed is a pressure swing adsorption configuration

3. Process according to claim 1, wherein the swing adsorption bed is a temperature swing adsorption configura^¬ tion.

4. Process according to claim 1, wherein the swing adsorption bed is a purge swing adsorption configuration.

5. A process design comprising a classical ammonia loop including a separator for use in the process according to any of the claims 1-3, wherein an additional separation step is incorporated within the loop for further reduction of the ammonia concentration in the recycle gas.

6. Process design according to claim 5, wherein the additional separation step has a pressure swing-like configu ration .

7. Reactor design according to claim 5, wherein the additional separation step has a temperature swing-like con^¬ figuration .

8. Reactor design according to claim 5, wherein the additional separation step has a purge swing-like configura^¬ tion.