WO2000040324A1

WO2000040324A1 - Process for separation of ammonia gas and a solid adsorbent composition

Info

Publication number: WO2000040324A1
Application number: PCT/FI1999/001088
Authority: WO
Inventors: Jarkko Helminen; Joni Helenius; Erkki Paatero; Ilkka Turunen
Original assignee: Kemira Agro Oy
Current assignee: Yara Suomi Oy
Priority date: 1998-12-31
Filing date: 1999-12-29
Publication date: 2000-07-13
Anticipated expiration: 2001-06-30
Also published as: EP1146949A1; FI111245B; FI982849A0; AU2111600A; NO20013275L; NO20013275D0; FI982849L

Abstract

The present invention is a process for ammonia gas separation from gas mixtures by a solid copper (I)-containing adsorbent. The process enables an effective and selective separation of ammonia gas from gas mixtures. Additionally it enables the efficient release of adsorbed ammonia by regeneration as well as the ammonia recovery as a pure gas for reuse. The present invention also comprises the adsorbent composition for use in selective adsorption of ammonia and the manufacturing of the adsorbent.

Description

PROCESS FOR SEPARATION OF AMMONIA GAS AND A SOLID ADSORBENT COMPOSITION

TECHNICAL FIELD

The present invention relates to a novel adsorbent composition for use in selective adsorption of ammonia, the manufacture of the ammonia selective adsorbent composition and a separation process employing the same. More specifically, the present invention relates to a copper(I) containing adsorbent composition having a high selectivity for ammonia and also having highly improved adsorption capacity for ammonia as well as having improved regeneration properties, and a process for producing the same. More specifically, the present invention relates to an ammonia separation process employing a specially prepared adsorbent composition to effectively separate and recover ammonia from a gas mixture of ammonia and at least one other gas selected from the group consisting of air, nitrogen, carbon dioxide, methane, hydrogen, argon, helium, ethane, and propane as well as solvent vapors commonly known as VOC compounds, in an efficient manner using a copper(I) containing adsorbent composition having a high adsorption capacity for ammonia.

BACKGROUND OF THE INVENTION

Ammonia (NH₃) is widely used in chemical industiy as a raw material for the synthesis of various chemicals including fertilizers, melamine and urea. Almost all ammonia utilizing chemical processes produce also ammonia containing gas mixtures, in which it is necessary to separate ammonia gas from other gases. Additionally, the ammonia manufacturing processes, e.g. Haber-Bosch process, produce ammonia containing gas mixtures, which are essential to separate and recover because of process competitiveness.

Firstly, the ammonia separation is conventionally accomplished by cryogenic separation in which a gas mixture is liquefied by cooling and the resulting liquid mixture is then subjected to distillation at a low temperature to obtain each gas component separately. However, the cryogenic separation is unsatisfactory, because it has the following drawbacks: (1) A large amount of electric power is required for separation.

(2) Complicated cooling and heat recovery systems have to be used.

(3) The construction costs are high.

(4) The separation of close-boiling compounds is difficult.

Secondly, ammonia is also separated from gas mixtures by wet absorption techniques known to those skilled in the art, for example, gas scrubbers. The absorption processes can be irreversible or reversible. If the ammonia gas is absorbed, e.g., in acidic solution, which converts the ammonia gas to a soluble ammonia salt in a chemical reaction, the process is typically irreversible and re-use is not possible. The reversible absorption processes also exist, where ammonia is absorbed in an aqueous solution or an organic solvent, such as polyhydric alcohols, and it is desorbed, e.g., by heating. These processes are unsatisfactory, however, because they are difficult to operate, the investment cost of apparatus is high, and the absorbing solutions are unstable in use.

Gas adsorption is a separation process, which consists of passing a gas stream through a solid adsorbent bed, so that a gas component is adsorbed and the gas component depleted effluent gas stream is obtained. Thereafter, the adsorbed gas is desorbed, in other words, regenerated from the solid adsorbent by heating and/or by depressurising. Generally, the adsorption processes can eliminate the drawbacks accompanying the ciyogenic and absoφtion separations. However, the ammonia separation and recovery by adsorption differs from other gas adsorption processes, e.g. separation of nitrogen and oxygen from air, because ammonia as a polar molecule is adsorbed very strongly onto many conventional adsorbents, e.g. zeolites, alumina, and silica gel as well as ion-exchange resins in H+, Co(II), Cu(II), Ni(II), and Zn(II) forms (S.Kamata and M.Tashiro, J.Chem.Soc.Jpn., Ind.Chem.Soc, 73, 1083(1970)). This causes that the adsorption isotherms are unfavourable for the desorption and the adsorbent regeneration is difficult. To regenerate these adsorbents completely, a very low pressure and/or a high temperature is needed. Activated carbons behave differently in the ammonia adsorption compared to the other conventional adsorbents. The ammonia adsoφtion isotherms of activated carbons are almost linear. Therefore, they are easy to regenerate even by depressurising. Unfortunately, the ammonia selectivity and the adsoφtion capacity of activated carbons are low, especially, at low pressures, and high bed volumes are needed in adsoφtion columns. It can be said the problems in the ammonia separation and recovery by adsoφtion are related to the poor selectivity, low capacity and regenerability of the conventional adsorbents.

Only few attempts have been made to develop an economical, effective, and selective adsoφtion process for the ammonia separation and recovery from gas mixtures, which are based on selective adsorbents. The previous problems have mostly been tried to be solved by different process constructions in the processes, which use the conventional ammonia adsorbents. A typical example is U.S. Pat. No. 4,537,760, which relates to the adsoφtive separation and recovery of ammonia gas in the ammonia synthesis. The ammonia is adsorbed at high pressures, namely over 100 bar, onto the conventional activated carbon, silica, alumina or zeolite beds. The regeneration is carried out by direct contact of said adsorbent with a concentrated hot stream of ammonia-containing gases at 150 °C. The high pressure is needed to achieve the high capacity for the adsorbent as well as the high temperature in the regeneration for releasing a sufficient amount of ammonia gas.

U.S. Pat. No. 4,758,250 discloses ammonia separation processes, which utilize anion-exchange polymer adsorbents. It is, however, well known that the anion- exchange polymers are expensive and unstable at elevated temperature. Therefore, it is impractical and uneconomical to use the anion-exchange polymers in the ammonia adsoφtion processes.

EP-570 835 suggests the use of Fe(II), Fe(III), Co(II), Ni(II), Cr(II), Cr(III), Mn(II), Zn(II), Cu(I) or Cu(II) salts supported on an amoφhous oxide, such as Si0₂, A1 0₃, or silica-aluminas, for the selective separation of ammonia in a gas or liquid phase. These adsorbents are prepared directly by impregnation using an aqueous solution of the mentioned salts. The method presented in EP-570 835 is applicable only for water-soluble salts, e.g., Cu(II) salts. The preparation of Cu(I) adsorbents is impossible by the previous method: (1) the solubility of Cu(I) in water is scarce, namely, about 10^'3 M, and (2) Cu(I) salts will disproportionate in water to Cu(II) form. Evidently, the preparation presented in EP-570 835 produces Cu(II) adsorbents, even by using Cu(I) salts as raw materials.

This invention overcomes the above drawbacks by providing a novel adsorbent, which has an unexpectedly high ammonia selectivity and capacity, but is still easy to regenerate as will be described in greater detail below. SUMMARY OF THE INVENTION

In a first aspect, the present invention provides a process for the separation of ammonia from a gas mixture with a solid copper(I) containing adsorbent composition, in which copper predominantly is in oxidation state I. The process comprises contacting a gas mixture with a solid adsorbent composition prepared as described hereinafter. The contacting of the gas mixture and the solid adsorbent composition occurs under conditions such that the ammonia is adsorbed onto the solid adsorbent composition. The gas mixture contains ammonia and at least one other gas preferably selected from the group consisting of air, nitrogen, carbon dioxide, water, methane, hydrogen, argon, helium, ethane, and propane.

In accordance with the present invention the separating and recovering of ammonia from offgases in melamine preparation is possible. A melamine plant is not needed to be built near a urea plant.

If the invention is applied for the separation of ammonia from a NH₃/C0₂ mixture from a melamine reactor, the preferred temperature and pressure are such that carbamate formation is prevented. The temperature can for example be within the range of 100-250 °C and the pressure, depending on possible pretreatments, can for example be within the range of 1-200 bar.

Preferably, the ammonia separation can be carried out by techniques, which are per se known in the art, such as the pressure swing adsoφtion (PSA) or vacuum swing adsoφtion (VSA) process.

In a referred embodiment the process of the invention for separating ammonia from a gas mixture, which is passed through one or more copper(I) containing adsorbent beds, comprising the steps of:

1) adsorbing ammonia from said gas mixture in said adsorbent bed(s),

2) desorbing said adsorbent bed(s) after adsoφtion a) by purging said adsorbent bed(s) with a product, and bl) by depressurising said adsorbent bed(s) to recover said ammonia, and/or b2) by heating said adsorbent bed(s) to recover said ammonia, and

3) repressurising said adsorbent bed(s) to the pressure of adsoφtion. Preferably, said adsorbent bed(s) is/are purged with ammonia in step 2(a). In the above repressurising step (3) the repressurising can be performed by passing the feed gas or a non-adsorbing or weakly-adsorbing gas into the adsorbent bed(s).

The pressure of the desoφtion step can e.g. be one fifth or less, preferably one tenth or less than the pressure of the adsoφtion step.

In a second aspect, the present invention provides high capacity, selective and easily regenerable copper(I) containing ammonia adsorbent compositions.

In a third aspect, the present invention provides methods for the preparation of high capacity, selective and easily regenerable copper(I) containing ammonia adsorbent compositions. The said adsorbent compositions comprise copper(I) bound to a solid support. The said adsorbent may also be the solid mixture of copper(I) compound and an other compound. The solid adsorbent compositions can be prepared by three methods. Firstly, an inorganic or an organic support selected from the group consisting of activated carbon, polymer, amoφhous oxide or crystalline material is impregnated with a solution of a cupric compound. Thereafter, the copper(II)-impregnated support is heated at an elevated temperature under inert, oxidative or reductive atmosphere. If necessary, after heating the copper(II)-impregnated support is reduced under reductive atmosphere such that the copper(II) compound is converted to copper(I). Secondly, the solid adsorbent compositions can be prepared by heating a solid mixture containing a cuprous or cupric compound and an inorganic or organic support, and if necessary, reducing the copper(II) compound to copper(I). Thirdly, the solid adsorbent compositions can be prepared by ion-exchanging an inorganic or an organic support with a soluble cupric solution. Thereafter, the copper(II)-exchanged support is heated at an elevated temperature and, if necessary, reduced to copper(I) form.

BRIEF DESCRIPTION OF THE DRAWINGS

Fig. 1 shows the adsoφtion isotherm of a CuCl/Si0₂ adsorbent of the present invention,

Fig. 2 shows the adsoφtion isotherm of a commercial 5 A zeolite adsorbent,

Fig. 3 A shows the adsoφtion equilibrium isotherms of a Cu(I)Cl/Al₂0₃ adsorbent of the present invention and a Cu(II)Cl₂/Si0₂ adsorbent, at 383 °K, Fig. 3B shows the adsoφtion equilibrium isotherms of the same adsorbents as in Fig. 3A at 423 °K,

Fig. 4 shows the adsoφtion equilibrium isotherms of a Cu(I)Cl/Al₂0₃ adsorbent of the present invention at high pressure,

Fig. 5 shows the adsoφtion equilibrium isotherms of a Cu(I) adsorbent of the present invention, and

Fig. 6 shows the C0 adsoφtion on a Cu(I)Cl/Al₂0 adsorbent of the present invention and a Cu(II)Cl₂/Si0₂ adsorbent.

DETAILED DESCRIPTION OF THE INVENTION

The present invention is a process for selective ammonia separation from gas mixtures containing ammonia and at least one other component, which uses novel copper(I) containing compositions, in which copper predominantly is in oxidation state I. The process comprises contacting a gas mixture with a solid adsorbent composition prepared as described hereinafter. The contacting of the gas mixture and the solid adsorbent composition occurs under conditions such that the ammonia is adsorbed onto the solid adsorbent composition. The gas mixture contains ammonia and at least one other gas selected from the group consisting of air, nitrogen, carbon dioxide, water, methane, hydrogen, argon, helium, ethane, and propane.

In its broadest sense, this invention is directed to the preparation and use of copper(I) containing compositions that have a high selectivity and capacity as well as are easily regenerable for ammonia separation by adsoφtion. The adsorbent compositions can be used in pressure-swing adsoφtion (PSA), vacuum-swing adsoφtion (VSA) and temperature swing adsoφtion (TSA) types of processes. Actually, all these processes are methods, how to operate the adsoφtion separation of a desired gas from a gas mixture. They do not deteπnine whether the separation is possible or not. The adsorbent determines this. The PSA process comprises adsorbing the desired gas onto an adsorbent at a high pressure (above 1.013 bar) and then, desorbing the adsorbed gas by depressurising. In the industrial application of the PSA process, a plurality of columns packed with the adsorbent are installed and in each adsoφtion column, a series of operations of pressurising- adsoφtion-purging-depressurising is repeated so as to effect a continuous separation and recovery of the product gas. The VSA process is similar as the PSA process except the adsoφtion step can be carried out at 1.013 bar or above, and the depressurising is always performed by evacuating. In the TSA process the desired gas is first adsorbed onto an adsorbent at a low temperature and then, by raising the temperature of the adsoφtion system, the adsorbed gas is desorbed. It is possible enhance the regeneration by utilizing the heat released at the adsoφtion step. The utilized heat can remain inside the bed in the adsoφtion step, or it can be removed from the bed in the adsoφtion step and transferred back in the regeneration step. In the previous processes, the regeneration can also be carried out by purging using a non-adsorbing or weakly-adsorbing gas. It is also possible to enhance the prevailing regeneration mechanism by an additional purging and/or heating. Additionally, the combinations of the previous processes are possible, such as VTSA, PTSA, and the like. The adsorbent composition according this invention is also feasible to use in rotating wheel type adsoφtion processes.

Preferably, the adsoφtion separation by the adsorbent composition according this invention can be carried out by the PSA or VSA processes. The gas mixture is passed through one or more adsorbent beds in a sequence of steps comprising: (1) adsorbing ammonia from the gas mixture in the said adsorbent bed composition, (2) desorbing the said adsorbent bed after adsoφtion, (a) by purging the adsorbent bed with product, i.e., ammonia, and (b) by depressurising the adsorbent bed to recover ammonia, and (3) repressurising the adsorbent bed to the adsoφtion pressure e.g. by passing a non-adsorbing or weakly-adsorbing gas into the adsorbent bed.

These adsorbent compositions not only have a high adsoφtion capacity for ammonia, but also have an ability to remove that at very low partial pressures of the ammonia. Most of them are capable of reducing the ammonia content in a gas mixture to as low as 10 ppm by volume or even lower.

In this invention the form of the adsorbent is not critical. For example, the adsorbent may be of any of granular, spherical and particulate forms. Furthermore, the adsorbent may be of any other form than the above mentioned forms, e.g. honeycomb, structured packing, membrane or massive form prepared by the molding or pelletising of the adsorbent having a granular, spherical or particulate form.

As the support used to prepare the adsorbents of this invention, a fairly large class of solid materials may be utilized. However, it is desirable that the surface area of those materials used as a support is greater than 100 m² g^"1. Non-limiting examples of those materials that can be used as the support for the adsorbent composition of the present invention are as follows: natural or synthetic zeolites, silica gel, silica, alumina, activated alumina, silica-alumina gel, porous aluminium phosphate, clay minerals, such as montmorillonite, titania, charcoal, activated carbon, polymer, such as cation exchange resin, organic-inorganic hybrid composition, such as sulphonated polystyrene grafted silica and the like. Preferred zeolites include, e.g., zeolite A, zeolite X, zeolite Y, dealuminated Y zeolite, zeolite omega, zeolite ZSM, mordenite, silicalite, and their mixtures. The cations present in zeolites can be NH ions, H⁺ ions, Na⁺ ions, K⁺ ions, Ca²⁺ ions, Mg²⁺ ions, Cu⁺ ions, Cu²⁺ ions, Ag⁺ ions, Fe²⁺ ions, Fe³⁺ ions, and combinations of thereof.

The solid adsorbent composition employed in the process of this invention comprises a cuprous compound or mixture of thereof supported on the above described support. Cuprous compound can be introduced onto the support by techniques known to those skilled in the art, e.g., impregnation, ion-exchange or thermal dispersion. These techniques are illustrated in detail hereinafter.

The starting zeolite, silica-alumina, silica, silica gel, activated alumina, alumina, clay mineral, polymer, hybrid, charcoal and activated carbon support of this invention are available commercially or can be synthesised according to procedures well documented in the art.

As noted hereinbefore, cuprous ions are an essential component of the solid adsorbent composition in this invention. Impregnation, thermal dispersion and ion- exchange, described hereinafter, may be employed to introduce copper into the support. In the case of ion-exchange and impregnation, typically, a water-soluble cupric compound is employed because cuprous compounds are not sufficiently soluble or stable, especially in water. It is obvious that if a cupric compound is impregnated or ion-exchanged into the support, then the reduction of copper(II) is required to obtain copper(I), the more desirable form of copper.

Preparing adsorbent composition by ion-exchange

The term "ion-exchange" is taken to mean a technique whereby metal ions, specifically copper ions in this case, actually replace a portion or essentially all of the cations of the zeolite, ion-exchange resin or sulphonated polystyrene grafted silica. Ion-exchange can be carried out by stirring or slurrying the zeolite or the ion-exchange resin or the sulphonated polystyrene grafted silica with an excess of a solution containing a soluble copper(II) compound. However, the ion-exchange can also be carried out continuously in a column. In both cases the concentration of solutions will vary depending upon the desired degree of ion-exchange, but typically range from about 0.01 M to about 10 M. Heating may also be beneficial in some cases to enhance the ion-exchange.

The ion-exchanged adsorbent is dried at a temperature in the range from about 50 °C to about 120 °C to remove excess and adsorbed solvent. Alternatively, before drying the ion-exchanged adsorbent can be washed with water to remove nonbounded compounds. After drying, the ion-exchanged adsorbent is heated at a temperature in the range from about 80 °C to about 800 °C under inert, oxidative or reductive atmosphere and, if necessary, reduced to convert a portion of the copper(II) to copper(I) form. The reduction can be carried out with any reducing agent and reductive atmosphere that is capable of this conversion. Non-limiting examples of oxidative and inert atmospheres are air and nitrogen, respectively. Non-limiting examples of reducing agents and reductive atmospheres include hydrogen, carbon monoxide, and ammonia, nitrogen, helium, argon or mixture of thereof. The reduction temperature ranges from about 80 ° to about 500 °C. For the ion-exchange resins and sulphonated polystyrene grafted silica are applied the temperatures below 120 °C as well as for the zeolites the whole range is applicable.

The soluble cupric compound may be cupric halides, cupric oxide, cupric carboxylates, cupric basic salts, copper(II) amine complex salts, cupric sulphate, and cupric nitrate or mixture of thereof. Preferably, the cupric compound is cupric halide, cupric sulphate or cupric nitrate. The amount of copper in the form of copper compound in the adsorbent compositions according this invention can be 1 to 95 w-%.

Preparing adsorbent composition by solid-solid, liquid-solid, and vapour-solid method

As a second method of preparing the adsorbent composition of this invention, a thermal dispersion may be employed. For the preparation of the adsorbent composition according to the invention, a mixture containing the copper compound and the above described support is used. In a preferred embodiment of the invention the mixture is prepared simply by mixing a solid powder form of the copper compound with the solid support. The mixture can also be obtained by adding to the solid support a solution or suspension of the copper compound in a suitable solvent and thereafter removing the solvent from the resultant mixture by heating and/or pumping. Many cuprous compounds and cupric compounds or their mixtures can be used as the copper compound. Non-limiting examples of the copper compound which can be suitably utilized in the practice of this invention include, e.g., cuprous halides, such as cuprous chloride, cuprous bromide, cuprous fluoride, and cuprous iodide; cuprous oxide; cuprous carboxylates, such as cuprous carbonate, cuprous formate, and cuprous acetate; cupric halides, such as cupric chloride, cupric bromide, cupric iodide, and cupric fluoride; cupric oxide; cupric carboxylates, such as cupric acetate and cupric formate; cupric basic salts, such as basic copper(II) carbonate, basic copper(II) acetate and basic copper(II) phosphate; and copper(II) amine complex salts, such as hexamine coρper(II) chloride as well as cupric sulphate, cupric nitrate, and the like. A mixture of compounds is also acceptable. Preferably, the copper compound is cuprous or cupric halide or oxide. More preferably, the copper compound is cuprous chloride or cupric chloride.

As the support used to prepare the adsorbents of this invention, a fairly large class of solid materials may be utilized. However, it is desirable that the surface area of those materials used as a support is greater than 100 m² g^"1. Non-limiting examples of those materials that can be used as the support for the adsorbent composition of the present invention are as follows: natural or synthetic zeolites, silica gel, silica, alumina, activated alumina, silica-alumina gel, porous aluminium phosphate, clay minerals, titania, charcoal, activated carbon and the like. Preferred zeolites include, e.g., zeolite A, zeolite X, zeolite Y, dealuminated Y zeolite, zeolite omega, zeolite ZSM, mordenite, silicalite, and their mixtures. The cations present in zeolites can be NH }⁺ ions, H⁺ ions, Na⁺ ions, K⁺ ions, Ca ⁺ ions, Mg²⁺ ions, Cu⁺ ions, Cu²⁺ ions, Ag⁺ ions, Fe²⁺ ions, Fe ⁺ ions, and combinations of thereof.

In the above described mixture containing the copper compound and the support, the amount of copper in the form of the copper compound is preferably from 1 to 200 w-%, more preferably from 5 to 90 w-%.

The above prepared mixture containing the copper compound and the support is subjected to heating. The heating temperature determines, whether the preparing the adsorbent composition is the solid-solid, liquid-solid or vapor-solid method. If the heating temperature is below the melting temperature of the copper compound, the preparing takes place by the solid-solid method. As before, if the heating temperature is below or above the vaporising temperature of the copper compound, the preparing is conducted by the liquid-solid method or the vapour-solid method, respectively. In all cases the support is stayed as a solid form. Generally, the heating temperature is greater than 150 °C, but lower than the decomposition temperature of the compound. In the practice of the present invention, the heating step is preferably performed at a temperature, which ranges from about 200 °C to about 800 °C. The time needed for heating ranges from about 1 minute to about 100 hours, preferably from about 10 minutes to about 50 hours. During the heating the copper compound and the support can be mixed continuously, occasionally or they are not mixed at all. The mixing can be carried out pneumatically or mechanically, e.g., in the rotating oven or drum.

In general, the heating can be carried out in a suitable reducing atmosphere, such as carbon monoxide, hydrogen, acetylene, ethene, ammonia, nitrogen, helium, argon or mixture of thereof. Most preferable reducing atmosphere is carbon monoxide, hydrogen and ethene.

If the mixture of the copper compound and the support consists of only the support and the cuprous compounds and mixtures of thereof, the heating can be carried out in a suitable inert atmosphere or in vacuum. The suitable inert atmosphere is nitrogen, methane, argon, helium, carbon dioxide, or a mixture of thereof. For those mixtures containing a cupric compound, the heating can also be carried out simply in air, in an inert atmosphere or in vacuum.

If relatively large part of the copper compound contained in the composition is in a cupric form, a separate reduction is needed. The reduction of these compositions can be conducted by means of any known reduction process in the art, e.g., by heating at temperatures greater than 100 °C at a reductive atmosphere, such as carbon monoxide, hydrogen, acetylene, ethene, ammonia, nitrogen, helium, argon or mixture of thereof.

Preparing adsorbent composition by impregnation

Copper may be introduced into the above described support via impregnation. Those skilled in the art will know that "impregnation" refers to a technique whereby a metal compound, which means in this invention a soluble copper(II) compound, is deposited on the surface and throughout the pore structure of the support, but predominantly on the surface. Impregnation can be effected by dipping the support into an excess of a solution of a copper compound, such as the chloride, nitrate, acetate or sulphate. Preferably, more precise control is achieved by a technique called "dry impregnation" or "impregnation to incipient wetness". In this method the support is sprayed with a quantity of the copper(II) solution corresponding to the total known pore volume, or slightly less. The impregnation can also be carried out by using dispersing agents, such as carboxylic acids and sugars.

After the starting support is impregnated with a copper(II) compound, the support is usually dried at a temperature ranging from about 50 °C to about 120 °C to remove excess and adsorbed solvent. At this stage the copper(II)-impregnated support essentially comprises a solid mixture containing a copper(II) compound and a support. Thereafter, the dried, impregnated support is heated under inert or oxidative atmosphere. The heating temperature ranges from about 80 °C to about 800 °C. Preferably, the heating temperature is 200 °C to 500 °C. The needed time ranges from about 1 hour to about 24 hours. Alternatively, the heating can be carried out in a suitable reducing atmosphere, such as carbon monoxide, hydrogen, acetylene, ethene, ammonia, and a combination of thereof. Most preferable reducing atmosphere is carbon monoxide, hydrogen and ethene.

After heating, if necessary, the impregnated copper(II) carrier is reduced to copper(I). Any reducing agent which can accomplish the reduction efficiently is acceptable, including hydrogen, ammonia, olefinic hydrocarbons, such as butene, propylene, and butadiene; alcohols, such as propanol, and aldehydes, nitrogen, helium, argon or mixture of thereof. Preferably, carbon monoxide, hydrogen, ammonia and ethene is used. The reducing conditions are found to be specific to the reducing agent. After the reduction the copper(I) adsorbent is usually stripped under nitrogen gas at a temperature from about 100 °C to about 250 °C, for a time ranging from about 30 minutes to about 12 hours.

The soluble cupric compound may be cupric halides, cupric oxide, cupric carboxylates, cupric basic salts, copper(II) amine complex salts, cupric sulphate, and cupric nitrate or mixture of thereof. Preferably, the cupric compound is cupric halide, cupric sulphate or cupric nitrate. The amount of copper in the form of copper compound in the adsorbent compositions prepared via impregnation can be 1 to 95 w-%.

The following illustrative examples are representative of the process and adsorbent composition of this invention, but are not intended to be limiting thereof. Reference Example 1

Copper(II) adsorbent composition, namely Cu(II)Cl₂/Si0 was prepared by mixing 20.2 g of pulverised CuCl₂ • 2 H₂0 with 80.1 g of silica gel (mean pore size 60 A and particle size 0.2-0.5 mm). The mixture was heated under nitrogen gas flow at 280 °C for 23 hours. After heating, it was found that the CuCl₂ salt was evenly spread at the surface of silica gel.

Example 1

15.4 g of pulverised cuprous chloride was mixed with 30.0 g silica gel (mean pore size 60 A and particle size 0.063-0.2 mm). The mixture was heated under nitrogen gas flow at 200 °C for 19 hours. After cooling the CuCl/Si0₂ adsorbent composition was pulverised and pressed into 2x8 mm pellets. These were crushed and particles greater than 0.23 mm were recovered by sieving.

Example 2

Copper(I) adsorbent composition, namely Cu(I)Cl/Al₂0₃ was prepared by mixing 20.8 g of pulverised CuCl with 59.2 g of alumina. The mixture was heated under nitrogen gas flow at 570 °C for 23 hours. After heating, it was found that the CuCl salt was evenly spread at the surface of alumina.

Example 3

Coρper(I) adsorbent composition was prepared by mixing 20 g of dried 5A zeolite with 20 g of 1 M Cu(N0₃)₂* 3 H₂0 solution in a beaker. This mixture was dried in a heating chamber at 110 °C for 4 hours. Subsequently, the mixture was transferred into Nabertherm oven for calcination at 275 °C for 24 h. After cooling, the adsorbent composition was reduced with hydrogen in a pressure reactor at 7 bar for 8 hours. The Cu(I) content was measured to be 4.5 wt-%.

Experiments

In order to evaluate the adsorbent compositions of the present invention following experiments were carried out.

Adsoφtion isotherm of the adsorbent composition CuCl/Si0₂ prepared in example 1 was measured volumetrically at 25 °C by the pressure-volume method using a glass apparatus. The adsoφtion isotherm is shown in Fig. 1. The adsoφtion isotherm form of the CuCl/Si0 adsorbent composition is suφrisingly advantageous. Firstly, the isotherm is approximately linear, especially, at pressures of above 30 000 Pa, which enables the easy regeneration of ammonia from the adsorbent composition as well as the use of the adsorbent composition in the separation of high ammonia concentrations. Secondly, the CuCl/Si0 adsorbent composition has a high ammonia capacity, which enables the smaller size columns in separation processes and, thus, decreases the investment cost of processes.

Experiment 2

Adsoφtion isotherms were measured also for a commercial 5A zeolite adsorbent at 25 °C by the same method as previously. The results are presented only for a comparison to illustrate the favourableness of the adsorbent compositions according to the present invention. The adsoφtion isotherm of commercial 5A zeolite adsorbent at 25 °C is shown in Fig. 2. The adsoφtion isotherm forms of zeolites as well as of γ-Al₂0₃ are extremely favourable for adsoφtion, namely irreversible, which means the regeneration of these adsorbents is difficult enforcing to employ low vacuum and/or high temperature.

Experiment 3

The intention of the following experiment is to illustrate the benefits of the present invention compared to a process, which uses a commercial 5A zeolite adsorbent. Table 1 shows three successive adsoφtion-regeneration cycles for the 5A zeolite. The last adsoφtion step 4 was performed only to find out the adsorbent bed capacity after regeneration step 3. The adsoφtion-regeneration cycle 1 corresponded to a process, where 10.3% ammonia in air mixture was fed to the adsorbent bed until an ammonia breakthrough occurred. Immediately after the breakthrough the feed was stopped and the adsorbent bed was purged by a cocurrent 100% ammonia gas feed to remove a diluting air from the bed. Finally, the adsorbent bed was regenerated by heating at 125 °C and by evacuating at 11 mbar for 35 minutes with a countercurrent purge air flow of 10.8 mL min^"1. Table 1 shows that the breakthrough capacity in the adsoφtion step 1 was 3.55 mmol NH₃ g^"1, the total capacity was 7.34 mmol NH₃ g^"1 determined by 100% ammonia feed, and the recovered ammonia amount in the regeneration 1 was 84.5%. After the adsoφtion-regeneration cycle 1 the adsorbent bed was saturated directly by 100% ammonia gas feed without passing 10.3% ammonia into a column at the beginning as above. This was not required, since the total bed capacity is same in both cases. In the adsoφtion-regeneration cycle 2 the regeneration was carried out at 12 mbar for 5 minutes, which released 43.2% of ammoma gas. The regeneration step 3 was made as the step 1 except the regeneration time was 5 minutes. The recovery of ammonia was 69.2%.

Table 1 Ammonia capacities, procedures of adsoφtion and regeneration steps and ammonia recoveries in regenerations with a commercial 5A zeolite determined by breakthrough experiments in an adsoφtion column (i.d. 19 mm) containing 10.5 g adsorbent. In the adsoφtion step 1 10.3% ammonia-air mixture (NH₃ flow 43.2 mL min^"1 and air flow 378.2 mL min^*1) was fed into the bed until the breakthrough occurred After that the bed was saturated directly to the equilibrium by 100% ammoma gas feed (NH₃ flow 194.5 mL min^"1). In the adsoφtion steps 2 to 4 only 100% ammonia feed was used.

Cycle Procedures Ammonia capacity (q), or recovery (R)

1 Adsoφtion step 41 6 °C, 10 3% to breakthrough, 100% q=3 55 mmol g^"1 to equilibrium (breakthrough) q=7 34 mmol g^"1 (total)

1 Regeneration step 125 0 °C+11 mbar+10 8 ΠIL_A,, min^"1, R-84 5%

35 min

2 Adsorption step 39 9 °C, 100% q=6 20 mmol g^"1

2 Regeneration step 12 mbar, 5 min R=43 2%

3 Adsorption step 40 3 °C, 100% q=3 17 mmol g^"1

3 Regeneration step 124 3 °C+14 mbar+10 8 mL^_r min^"1, R= 69 2%

5 min

4 Adsorption step 40 0 °C, 100% q=5 08 mmol g^"1

Table 2 shows eight successive adsoφtion-regeneration cycles carried out by the adsorbent composition, CuCl/Si0 which was prepared in the same manner as in example 1. In the adsoφtion step 1 a gas mixture containing 10.3% ammoma and 89.7% air by volume was passed through the column The feed was stopped after an ammonia breakthrough point was achieved. The breakthrough capacity of the adsorbent bed was 2,40 mmol NH g^"1. After the adsoφtion step the regeneration was performed at 10 mbar for 5 minutes, which released 25.3% of ammonia. In the adsoφtion step of the adsoφtion-regeneration cycle 2 10.3% ammonia mixture was fed in the same way as in the step 1 until the breakthrough occurred. The regeneration was carried out by heating at 126 °C and by evacuating at 10 mbar with 10.8 mL min^"1 purge air flow. This released 87.5% of ammonia from the adsorbent bed. At the beginning of the adsoφtion step 3 the adsorbent bed was saturated by 10.3% ammonia gas mixture to the breakthrough point followed by the saturation of 100% ammoma to the equilibrium. The total capacity of the CuCl/Si0₂ adsorbent composition was considerably higher than 5A zeolite with 100% ammonia feed, namely 11.4 mmol NH₃ g^"1. In the regeneration step 3 the adsorbent bed was evacuated at 10 mbar for 5 minutes, and, suφrisingly the recovery of ammonia was 10.1% compared to the value of 43.2% of the 5A zeolite in the corresponding experiment. Combined regeneration by heating at 125 °C and by evacuating at 10 mbar with 10.8 mL min^"1 purge air flow was studied in the regeneration steps 4 and 7. The ammonia recoveiy of the step 4 and 7 was 92.1% and 99.1%, respectively. These values are considerably higher than the 5 A zeolite in the corresponding experiment, namely 69.2%. The regeneration step 5 purified the adsorbent bed completely by heating at 125 °C and by high volume air flow. Finally, the total bed capacity in the adsoφtion step 8 was 11.66 mmol NH₃ g^"1. This confirms that the adsorbent composition according the present invention is stable in several successive adsoφtion-regeneration cycles.

Table 2 Ammonia capacities, procedures of adsoφtion and regeneration cycles and ammonia recoveries in regeneration with the adsorbent composition, which has been prepared in the same manner as example 1. These have been determined by breakthrough experiments in an adsoφtion column

(i.d. 19 mm) containing 2.9 g adsorbent. In the adsoφtion steps 1 and 2

10.3%) ammonia-air mixture (NH₃ flow 21.7 mL min^"1 and air flow 189.7 mL min^"1) was fed into the bed until the breakthrough occurred. After that the feed was stopped and the bed was regenerated. In the beginning of the adsoφtion step 3 10.3% ammonia gas mixture was fed into the bed until the breakthrough occurred followed by the saturation to the equilibrium by 100% ammonia (NH₃ flow 195.2 mL min^"1). In the adsoφtion steps 4 to 8 the bed was saturated directly to the equilibrium by 100% ammonia gas feed. Cycle Procedures Ammonia capacity (q) or recovery (R)

1 Adsorption step 39.3 °C, 10.3% to breakthrough q=2.40 mmol g^'1

(breakthrough)

1 Regeneration step 10 mbar, 5 min R=25.3%

2 Adsorption step 39.5 °C, 10.3% to breakthrough q=0.61 mmol g^"1

(breakthrough)

2 Regeneration step 125.9 °C+10 mbar+10.8 mL_A-_r min^"1, R=87.5%

5 min

3 Adsorption step 40.1 °C, 10.3% to breakthrough, q=2.10 mmol g^"1

100%) to equilibrium (breakthrough) q=l 1.4 mmol g^"1 (total)

3 Regeneration step 10 mbar, 5 min R=70.7%

4 Adsorption step 39.5 °C, 100% q=8.32 mmol g^"1

4 Regeneration step 126.3 °C+10 mbar+10.8 mLA-r min^"1, R=92.1% 5 min

5 Adsorption step 40.0 °C, 100% q=10.84 mmol g^"1

5 Regeneration step 125.0 °C+high volume purge R= 100%

6 Adsorption step 39.8 °C, 100% q= 11.77 mmol g^"1

6 Regeneration step 1 1 mbar, 5 min R=65.7%

7 Adsorption step 39.4 °C, 100% q=7.73 mmol g^"1

7 Regeneration step 126.1 °C+12 mbar+10.8 mL^ min^"1, R=99.1% 5 min

8 Adsorption step 40.1 °C, 100% q^~l 1.66 mmol g^"1

Experiment 4

Ammonia adsoφtion equilibria were determined volumetrically for the Cu(I)Cl/Al 0₃ and Cu(II)Cl /Si0₂ adsorbent compositions, which were prepared in Example 2 and Reference Example 1, respectively. Fig. 3A shows the adsoφtion equilibrium isotherms at 383 °K and Fig 3B shows the adsoφtion equilibrium isotherms at 423 °K for the Cu(I) and Cu(II) adsorbents compositions.

At high temperatures, the Cu(I) adsorbent composition adsorbs clearly more ammonia than the Cu(II) adsorbent composition. Since the adsorbed amount of carbon dioxide is nil on the Cu(I) adsorbent compositions and the ammonia capacity of Cu(I) adsorbent composition is higher than that of the Cu(II) adsorbent composition, the productivity of Cu(I) adsorbent composition will be higher in a large-scale ammonia separation process. The productivity can be defined as the amount of produced pure ammonia gas in an hour per the amount of adsorbent composition.

Experiment 5

Ammonia adsoφtion equilibria at high pressure were determined volumetrically for the Cu(I)Cl/Al₂0₃ adsorbent composition which was prepared in Example 2. The adsoφtion equilibrium isotherm is shown in Fig 4 which shows that the used adsoφtion composition is feasible at high pressure conditions, as well.

Experiment 6

The adsoφtion equilibrium isotherm of ammonia on the Cu(I) adsorbent composition prepared in Example 3 was determined volumetrically at 25 °C. The adsoφtion equilibrium isotherm is shown in Fig. 5.

Experiment 7

The ammonia gas diffusivities were determined by batch uptake experiments for the Cu(I)Cl/Al 0₃ and Cu(II)Cl₂/Si0₂ adsorbent compositions which were prepared in Example 2 and Reference Example 1, respectively. The diffusivity for the Cu(I)Cl/Al₂0₃ was found to be 6.04- 10^"9 mV¹ and for Cu(II)Cl₂/Si0₂ it was

Q 7 1

0.27T0^" m s^" . Thus it can be concluded that ammonia gas is adsorbed 20 times faster onto the Cu(I) adsorbent composition compared to the Cu(II) adsorbent composition.

Experiment 8

To compare the selectivity of the Cu(I)Cl/Al₂0₃ adsorbent composition prepared in Example 2 and the Cu(II)Cl₂/Si0₂ adsorbent composition prepared in Reference Example 1 for the separation of NH₃ and C0₂ gases, the single-gas adsoφtion equilibrium isotherms were measured volumetrically with an experimental accuracy of about 0.1 mmol g^'1. Fig. 6 shows the adsorbed amount of C0₂ on Cu(I) and Cu(II) adsorbent compositions. The adsorbed amount of C0₂ on Cu(I) adsorbent composition is very little or nothing within the range of experimental error whereas the adsorbed amount of C0₂ on Cu(II) adsorbent is clearly evident. These results combined with those from Experiment 4 (showing that NH₃ is adsorbed on both Cu(I) and Cu(II) adsorbents) show clearly that Cu(I) adsorbent composition has a higher ammonia selectivity compared to Cu(II) adsorbent composition which is likely to adsorb C0₂, as well.

Experiment 9

The intention of the following experiment is to illustrate the benefits of the present invention coupled with a melamine process.

Several hundreds of successive adsoφtion-regeneration cycles were carried out in a high-pressure and high-temperature adsoφtion bench-scale system of four columns by the adsorbent composition, which was prepared similarly to Example 1. The type of separation process was a pressure swing adsoφtion, and it involved the following steps:

(1) pressurisation with the feed gas i.e. a gas mixture from the melamine process containing 50% ammonia and 50% carbon dioxide by volume at 200 °C; (2) adsoφtion with feed gas at 100 bar;

(3) cocurrent purge with a pure ammonia product from step (4);

(4) countercurrent depressurisation at 1 bar.

Then the cyclic process was continued from step 1 to step 4. After 300 cycles, the average values for ammonia recovery and purity were 90-95% and 99.5-99.9%, respectively.

This experiment confirms that the adsorbent composition according the present invention is stable in a continuous pressure swing adsoφtion process, and enables the separation and recovery of ammonia from ammonia-carbon dioxide gas mixture.

The foregoing general discussion, Examples and Experiments are intended to be illustrative of the present invention, and they are not to be considered limiting. Other variations within the spirit and scope of this invention are possible and will present themselves to those skilled in the art.

Claims

1. A process for separating ammonia from a gas mixture with a solid copper(I) containing adsorbent composition.

2. The process according to claim 1, wherein said gas mixture contains ammonia and at least one other gas selected from the group consisting of air, carbon dioxide, water, nitrogen, hydrogen, argon, helium, methane, ethane and propane.

3. The process according to claim 1 or 2, wherein said gas mixture consists essentially of ammonia and other gases released from melamine production.

4. The process according to any of claims 1-4 for separating ammonia from a gas mixture, which is passed through one or more copper(I) containing adsorbent beds, comprising the steps of:

1) adsorbing ammonia from said gas mixture in said adsorbent bed(s),

3) repressurising said adsorbent bed(s) to the pressure of adsoφtion.

5. The process according to claim 5, wherein the pressure of the desoφtion step is one fifth or less, preferably one tenth or less than the pressure of the adsoφtion step.

6. A solid adsorbent composition to be used in separation of ammonia from a gas mixture, said composition comprising an organic or inorganic support and copper predominantly in oxidation state I.

7. The adsorbent composition according to claim 7, wherein the inorganic support is selected from the group consisting of natural and synthetic zeolites, silica gel, silica, alumina, activated alumina, silica-alumina gel, porous aluminium phosphate, clay minerals, and titania.

8. The adsorbent composition according to claim 7, wherein the organic support is selected from the group consisting of activated carbon, charcoal, ion-exchange resins, such as a macroporous cation-exchange resin, unfunctionalised polymers, such as a polystyrene-divinylbenzene resin, and organic-inorganic hybrid compositions, such as a sulphonated polystyrene grafted silica.

9. The adsorbent composition according to any of claims 7-9, comprising a cuprous compound selected from the group consisting of cuprous halides, cuprous oxide, cuprous carboxylates and mixtures thereof.

10. The adsorbent composition according to claim 10, wherein the cuprous compound is cuprous chloride.

11. A method for the preparation of a solid adsorbent composition as defined in any of claims 7-11 comprising the steps of:

a) impregnating an inorganic or organic support with a solution of a soluble cupric compound,

b) heating the copper(II)-impregnated support at a temperature within the range of 200-800 °C under inert, oxidative or reductive atmosphere, and if necessary

c) reducing the copper(II)-impregnated support under such conditions that the copper(II) compound is converted to copper(I), said conditions including a temperature within the range of 80-500 °C and an atmosphere comprising hydrogen, carbon monoxide, ammonia, unsaturated hydrocarbon, nitrogen, helium, argon or a mixture thereof.

12. A method for the preparation of a solid adsorbent composition as defined in any of claims 7-11 comprising heating a solid mixture containing a cupric or cuprous compound and an inorganic or organic support at a temperature within the range of 150-800 °C under inert, oxidative or reductive atmosphere, and if necessary, reducing the copper(II) compound to copper(I).

13. A method for the preparation of a solid adsorbent composition as defined in any of claims 7-11 comprising the steps of:

a) ion-exchanging an inorganic or organic support with a solution of a soluble cupric compound,

b) heating the copper(II)-exchanged support at a temperature within the range of 80-800 °C under inert, oxidative or reductive atmosphere, and if necessary c) reducing the copper(II)-exchanged support under such conditions that the copper(II) compound is converted to copper(I), said conditions including a temperature within the range of 80-500 °C and an atmosphere comprising hydrogen, carbon monoxide, ammonia, unsaturated hydrocarbon, nitrogen, helium, argon or a mixture thereof.

14. The method according to any of claims 12-14, wherein the cupric compound is selected from the group consisting of cupric halides, cupric oxide, cupric carboxylates, cupric basic salts, copper(II) amine complex salts, cupric sulphate, cupric nitrate and mixtures thereof.

15. The method according to claim 15, wherein the cupric compound is cupric chloride, cupric sulphate or cupric nitrate.

16. The method according to claim 13, wherein the cuprous compound is selected from the group consisting of cuprous halides, cuprous oxide, cuprous carboxylates and mixtures thereof.

17. The method according to claim 17, wherein the cuprous compound is cuprous chloride.