The present invention relates to a process and a reactor design for ammonia production using ammonia selective membranes. More specifically, a membrane system is used which consists of one or more low temperature, high pressure am ^¬ monia selective membranes with an ammonia/H ₂ selectivity of at least 4. Such ammonia selective membranes may be mem- branes consisting of polymer-based organic materials, but they may alternatively be membranes based on inorganic ma ^¬ terials .

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

N ₂ + 3H ₂ <-> 2NH ₃ (ΔΗ = -92.4 kJ/mol) was developed around 1908 and improved to industrial scale a few years later. Since then, this method (the Haber-Bosch method) has been the predominant industrial scale method for ammonia production. The synthesis is carried 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 concentration 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 H ₂ / 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 water and carbon dioxide.

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 the process according to the invention, ammonia selec ^¬ tive membranes are used after the ammonia condenser on the feed gas to the converter. The ammonia content in the gas will typically be around 4%, and the membrane system will remove the ammonia to below 2%. Thus the conversion per pass can be increased significantly. EP 1 083 150 Al discloses a process for producing ammonia in a recycle circuit stream which comprises at least hydro ^¬ gen and nitrogen and also at least a portion of the ammonia formed as a product. The ammonia is abstracted from the re- cycle circuit stream by means of a ceramic membrane unit.

The process starts from a hydrogen and nitrogen containing feed, wherein (a) the feed is fed into a recycle circuit stream, the recycle circuit comprising at least one reactor in which ammonia is formed from hydrogen and nitrogen, and (b) the ammonia is abstracted from the recycle circuit stream leaving the reactor, after which the recycle circuit stream is fed back to the reactor. Furthermore (c) the re ^¬ cycle circuit incorporates a membrane unit, which is perme ^¬ able at least to ammonia. The recycle circuit stream is passed through the membrane unit on one side of the mem ^¬ brane and the feed is passed through the membrane unit on the other side of the membrane in such a way that at least ammonia from the recycle circuit stream is incorporated in ^¬ to the feed.

Figs. 2 and 3 of EP 1 083 150 Al show a recycle circuit op ^¬ erating at pressures and temperatures similar to those of the present invention, with a pressure differential across the ceramic membrane of between 25 and 350 bar, the recycle circuit forming the ^x high pressure side' and the feed side forming the ^x low pressure side' . The operation temperature is from -10°C to 100°C. The action of the membrane ensures that ammonia in the recycle gas diffuses through the mem ^¬ brane, thereby lowering the concentration of ammonia in the recycle circuit stream to the inlet of the reactor. This results in an increased ammonia production. According to EP 1 083 150 Al, the membrane is placed be ^¬ tween the recycle gas and the make-up gas, either before (Fig. 2) or after (Fig. 3) the first compressor. In the first case, the gas is cooled after the first compressor step and the ammonia is condensed out together with i.a. water. In the second case, the make-up gas, now containing ammonia, is sent to the second compressor step, and from there it moves on to be mixed with the exit gas from the converter to the separator. Especially this second case cannot be favourable because the make-up gas stream is about 5 times smaller than the recycle gas stream, and the partial pressure difference will soon be very small. This means that only a limited amount of the ammonia will be captured via the membrane.

The process steps and the advantages obtained according to EP 1 083 150 Al are rather similar to those of the present invention, but the EP publication only discloses use of a ceramic membrane, not a polymeric membrane or a combined polymeric/ceramic membrane. The membranes described in EP 1 083 150 Al are not described as being low temperature or high temperature membranes, nor is the ammonia/H2 selectiv ^¬ ity of these membranes described. US 4.758.250 A discloses a process for ammonia separation using polymeric ion exchange membranes and sorbents from an ammonia synthesis plant recycle loop gas. It is not clearly stated where the membrane is located in the recycle loop. In US 5.455.016 a polymeric membrane is disclosed, which is positioned before the compressor for the feed to the con ^¬ verter. There is no disclosure regarding the pressure in the recycle streams. However it is stated that recycling with a lower ammonia concentration has the effect that the productivity of the reactor in terms of ammonia conversion per pass is increased.

EP 0 293 736 A2 describes the separation of ammonia from mixtures containing ammonia and other gases, such as nitro ^¬ gen and hydrogen, using semipermeable ion exchange polymeric membranes and sorbents. The active materials in said semipermeable membranes may also be used as selective N¾ sorbents for the recovery of ammonia from such mixtures.

Finally, US 6.065.306 discloses a method of producing a pu ^¬ rified, pressurized ammonia stream from a feed stream com- prising ammonia, moisture and other impurities. The method is more focused on treatment of the ammonia exiting the converter than on the removal of residual ammonia from the recycle gas . The previously suggested applications of polymer-based am ^¬ monia selective membranes have mainly looked at the ammonia membrane as an alternative to the removal of ammonia based on condensation, which is used today. However, this is a dubious way of using the membrane. First of all, the bene- fit of final purification of the make-up gas by washing it in liquid ammonia is lost and, secondly, the membrane sys ^¬ tem will be very large. The system proposed with the pre ^¬ sent invention includes the best of the previous practice along with a new membrane system.

The present invention is based on the idea that the mem ^¬ brane system is to be placed at a point after cooling and condensing of the ammonia, thereby both removing impurities from the gas and obtaining a feed gas with low N¾ content to be fed to the converter. In connection with the present invention, possible membrane types to separate the residual N¾ from N ₂ , H ₂ , CH ₄ and Ar in the recycle gas of an ammonia plant have been investi ^¬ gated. The purpose was to be able to increase the ammonia production in the synthesis loop. Basically there are two types of membranes: i) inorganic materials, such as MgCl ₂ and zeolites, and ii) organic polymers. The inorganic mate ^¬ rials are recommended for high temperature applications, whereas the polymers are suitable for low temperature ap ^¬ plications. In the present invention, the target tempera- ture is 0 to 35°C.

Previous investigations within the field of gas phase N¾ removal from other gases have been very sparse. Some of the attempts have been directed to incorporating the inorganic membranes right at the exit of the ammonia synthesis reac ^¬ tor to overcome the thermodynamic equilibrium limitations. However, these attempts could not identify a suitable mem ^¬ brane with the right permeance selectivity to N¾ at tem ^¬ peratures above 300 °C with the mixed gas feeds. As one of the best cases with inorganic membranes, an ammonia/H ₂ per ^¬ meance ratio of 30 was obtained at 50°C with a silica mem ^¬ brane that had 0.35 nm pores.

Organic polymer based membranes have shown higher permeance selectivities . Thus, an ammonia/H ₂ permeance selectivity of 90 at 25°C obtained with block-copolymer membranes has been reported. One of the best cases presented is a polyvinyl- ammonium thiocyanate membrane having an ammonia/H ₂ perme ^¬ ance selectivity of 1500 at 25°C. The mechanism of this membrane type was investigated in J. Am. Chem. Soc. 113, 742-749 (1991) . This investigation also found that Nafion® (perfluorosulfonic acid) membranes could have a selectivity of 500 at 25°C.

The idea underlying the present invention has been to find out whether any membrane technologies could be implemented in the recycle line of an ammonia converter. It has not been considered to separate the ammonia at the reactor ex ^¬ it, but rather to purify the recycle gas from residual am ^¬ monia, thereby being able to increase the reactor yield. The most critical factors when designing a low temperature, high pressure membrane for ammonia separation are:

- Membrane permeance: This depends on the gas composition, the temperature, the pressure and the physical characteris- tics of the membrane.

- Membrane stability: For the purposes of the invention, stability against high pressure is very important. This can be achieved by depositing the membrane polymer on a porous support and then topping the membrane surface with a non ^¬ selective and very permeable polymer layer to prevent cracking or pinhole formation on the selective membrane layer . - Membrane selectivity: Especially the ammonia/H ₂ selectiv ^¬ ity should minimum be 4. Thus, the present invention relates to a process for ammo ^¬ nia production by separating residual N¾ from N ₂ , H ₂ , CH ₄ and Ar in the recycle gas of an ammonia plant in a membrane system, wherein the separation is carried out in the recycle line of an ammonia converter with the membrane system located at a point after cooling and condensing of the ammonia, there ^¬ by both removing impurities from the gas and obtaining a feed gas with low N¾ content to be fed to the converter, wherein the membrane system consists of one or more low temperature, high pressure ammonia selective membranes with an ammonia/H ₂ selectivity of at least 4, said ammonia se- lective membranes consisting of polymer-based organic mate ^¬ rials, and wherein N¾ on the permeate side is removed by pumping or with the aid of a sweep gas.

More specifically, on the permeate side of the membrane, N¾ is removed either by pumping to ensure a constant and low pressure of N¾ or with a sweep gas consisting of ei ^¬ ther CO ₂ , N ₂ , H ₂ 0, ethane, propane, butane, pentane or any combination of these.

The invention further relates to a reactor design for ammonia production, said design comprising at least one ammonia converter (AC), a heat exchanger (Hex2), a loop waste heat boiler (w) , a loop boiler feedwater preheater (bfw) , a compressor (C) and a membrane system (MS) . The reactor design according to the invention is shown in the figure, which illustrates a part of the ammonia synthe ^¬ sis loop. The membrane system would most suitably be locat ^¬ ed at a low temperature part of the ammonia synthesis loop before the gas arrives to the compressor (C) and the heat exchanger (Hex2) . At this point the gas is cold after having passed a first heat exchanger (Hexl) . However, it is preferably located after the compressor, which condenses the ammonia in the gas before the gas enters the heat ex- changer (Hex2) . The figure shows this latter possible loca ^¬ tion of the membrane system (MS) .

Organic polymer-based membranes can be used at low tempera ^¬ tures, which provide somewhat higher permeance selectivi- ties of N¾ in comparison to the inorganic membranes. The higher permeability of N¾ in polymers results from its solubility in salts and ionic polymers. 2 and ¾, which are less polar, are less soluble. A potential problem when using these polymer membranes could be their stability un- der high pressures (180-190 bar) . However, this problem can be circumvented by depositing the polymer onto a mi- croporous support with large pores to avoid mass transfer limitations. Given the flow and sweep gas rates in a mem ^¬ brane cell and knowing the permeabilities of the gases in the gas mixture, it is possible to calculate the surface area and hence the thickness of the membrane needed for a certain application. An example of such calculation can be found in J. Membrane Sci. 104, 19-26 (1995). The invention is illustrated in more detail in the follow ^¬ ing examples. However, the invention is not limited to these specific examples. Example 1

In an ammonia plant, the feed gas to the converter is com- pressed in a recirculation compressor to a pressure of 18.6 MPa and a temperature of 40 °C. The ammonia content is typi ^¬ cally in the order of 4 vol% corresponding to a partial pressure of 0.75 MPa. The feed gas is introduced into a membrane unit in which the ammonia is removed down to a partial pressure of 0.2 MPa corresponding to 1 vol% in the feed gas. The ammonia from the membrane is cooled and compressed and then sent to the product ammonia tank.

The resulting feed gas is more reactive, and thus the inlet temperature to the converter can be lowered by 10-20 °C. The higher conversion per pass will also result in a smaller flow, which may be used either to be able to use smaller converters or alternatively to reduce the synthesis pres ^¬ sure .

Example 2 A residual recycle gas contains about 4% N¾, 66% ¾, 22%

N ₂ , 5.4% CH ₄ and 2.6% Ar . Before arriving to the compressor (C) , the gas has a temperature between 0 and 35°C under a pressure of 186 bar. After the compressor, the gas passes the membrane system (MS) , and a heat exchanger (Hex) heats the gas to 232 °C. Then the gas enters the ammonia converter (AC) , where the exit composition from the converter is found to be 17.8% NH ₃ , 53.4% H ₂ , 17.8% N ₂ , 6.3% CH ₄ and 2.9% Ar . The exit gas is then cooled to condense the ammonia, and the residual gas is recycled into the converter.

Applying the same assumptions as used in J. Membrane Sci. 104, 19-26 (1995) regarding membrane area and space time needed to remove a certain amount of ammonia with a given membrane system, the following results are obtained:

Based on the flow diagram shown in the figure and assuming that the ammonia concentration is to be decreased from 4% to 1% with a membrane having an ammonia permeance of 0.01 m/s, a feed gas of 233.3 Nm ³ /s at a total pressure of 186 bar and with a permeate pressure of 1 bar N¾, a membrane area of 46000 m ² would be the result. Assuming a Nafion® membrane of 150 m2/g, this corresponds to 306 g of Nafion® deposited on a porous carrier. Further assuming a thickness of 36 ym, the diameter of the potential membrane can be calculated to be 236 cm (93") . This is a much higher diame ^¬ ter in comparison to the usual piping diameter (20") used for an ammonia plant with a production around 2200 MTPD.

However, if the membrane permeance is higher, the necessary membrane diameter can be decreased.