GB2522989A - Sorbent for halogen compounds - Google Patents

Abstract

A method for preparing a sorbent comprises the steps of: mixing together a particulate calcium silicate cement and a particulate alkali metal compound; and treating the mixture with water to hydrate the calcium silicate cement, wherein the particulate alkali metal compound may comprise a oxide, hydroxide, carbonate or hydrogen carbonate and the sorbent is shaped before and/or after the treatment with water. The method may also comprise a drying step to remove excess water at 5-50°C. The calcium silicate cement may be a Portland cement. The alkali metal is sodium and/or potassium. The particulate alkali metal compound may be present in the sorbent in the range 3-86% by weight and may be in the form of a powder with an average particle size, [D50], of 5-100 µm. The water may contain alkali metal carbonate. The mixture may further comprise a binder or a particulate support material. A process for removing a halogen compound from a fluid stream using the sorbent prepared by this method are also disclosed.

Description

Sorbent for halogen compounds This invention relates to sorbents for halogen compounds, and methods for making and using such sorbents, Sorbents for halogen compounds such as hydrogen chloride present in gaseous or liquid refinery streams may be removed using alkalised alumina or zinc oxide materials.

W09522403(A1) discloses an absorbent for hydrogen chloride in the form of granules, preferably having a particle size greater than 2mm and a BET surface area of at least 10m2/g, comprising an intimate mixture of an alumina component selected from alumina and/or hydrated alumina, an alkali component selected from sodium carbonate and/or sodium bicarbonate in weight proportions of 0.5 to 2 parts of said alkali component per part of said alumina component, and a binder, said granules containing from 5 to 20 % by weight of said binder and having an alkali component content such that, after ignition of a sample of the granules at 900 DEC C, the sample has a sodium oxide, Na20, content of at least 20% by weight.

W09939819(A1) discloses shaped absorbent units suitable for use as chloride absorbents comprising a calcined intimate mixture ofan alkali or alkaline earth, zinc and aluminium components having an alkali or alkaline earth metal to zinc atomic ratio in the range 0.Sxto 2.5x and an alkali or alkaline earth metal to aluminium atomic ratio in the range 0.Sx to 1.Sx, where xis the valency of the alkali or alkaline earth metal, and containing from 5 to 20% by weight of a binder. Preferred compositions are made from sodium carbonate or bicarbonate, basic zinc carbonate or zinc oxide and alumina or hydrated alumina.

These sorbents are highly effective but there is a need to provide alternatives.

JP58177136(A) discloses a method to prepare a fluidisable absorbent for removing acidic gases from an exhaust gas, wherein an alkaline earth metal particle comprising a Ca compound such as limestone or quick lime or a Mg compound such as magnesium hydroxide and a cement such as Portland cement or alumina cement are kneaded and hydrated while hydroxide is partially liberated. These materials require insoluble alkaline earth components as the active sorbent material, which is used in a fluidised state in exhaust gas treatment.

JP2000288369 discloses a method for producing an adsorbent for a high temperature acidic exhaust gas in which a mixture of amorphous calcium silicate hydrate and sodium aluminate is subjected to hydrothermal treatment.

We have found that alkali metal compound-containing hydrated calcium silicate cement compositions are surprisingly effective sorbents for halogen compounds.

Accordingly the invention provides a method for preparing a sorbent comprising the steps of: (I) mixing together a particulate calcium silicate cement and a particulate alkali metal compound, and (U) treating the mixture with water to hydrate the calcium silicate cement, wherein the particulate alkali metal compound comprises an oxide, hydroxide, carbonate or hydrogen carbonate and the sorbent is shaped before and/or after the treatment with water.

The invention further provides a sorbent obtainable by the method in the form of a shaped unit comprising a hydrated calcium silicate cement and an alkali metal compound, and the use of the sorbent in removing halogen compounds from fluid streams, particularly refinery fluid streams.

By "sorbent" we include absorbent and adsorbent.

By "calcium silicate cement" we mean compositions containing one or more calcium silicates that react with water to form hardened solid masses. A particularly suitable calcium silicate cement is Portland cement, which is the product of grinding Portland cement clinker (more than 90%), a limited amount of calcium sulphate (which controls the set time) and up to 5% minor constituents as allowed by various standards such as the European Standard EN 197-1.

Portland cement clinker is a hydraulic material which consists of at least two4hirds by mass of calcium silicates (3 CaO3i02 and 2 CaO3i02), the remainder consisting of aluminium-and iron-containing clinker phases and other compounds. The ratio of CaO to Si02 is not less than 2.0. The magnesium oxide content (MgO) does not exceed 5.0% by mass. Typical Portland cements comprise; Tricalcium silicate (CaO)3.Si02 25-75% wt Dicalcium silicate (CaO)2.Si02 15-73% wt Tricalcium aluminate (CaO)3.A1203 0-13% wt Tetracalcium aluminoferrite (CaO)4.A1203.Fe203 0-18% wt Calcium sulphate CaSO4.2 H20 2-10% wt All types of Portland cement may be used to prepare the sorbent. Thus ASTM C150 Types I, II, Ill, IV and type V and EN 197-1 Types I, II, Ill, IV and V, including ordinary Portland cement, Portland composite cement, blast-furnace cement, pozzolanic cement and composite cement, which may include artificial or natural pozzolans.

The particulate calcium silicate cement may have a broad particle size range. For example, Portland cements may comprise 15% by mass of particles below 5pm diameter, and 5% of particles above 45 pm, i.e. 80% by mass of the particles have a particle size between S and 45 1im. The measure of fineness usually used is the specific surface area, which is the total particle surface area of a unit mass of cement. The rate of initial reaction (up to 24 hours) of the cement on addition of water is directly proportional to the specific surface area. Typical values are 320-380 m2kg1 for general purpose cements, and 450-650 m2kg1 for"rapid hardening" cements. Such calcium silicate cements are readily commercially available.

The particulate calcium silicate cement is mixed with a particulate alkali metal compound. The particulate alkali metal compound may be a sodium compound, a potassium compound or a mixture of these. The particulate alkali metal compound may be an oxide, hydroxide, carbonate or hydrogen carbonate. Alkali metal oxides, carbonates and hydrogen carbonates are preferred. Good results have been obtained simply using sodium carbonate and/or potassium carbonate.

Preferably the particulate alkali metal compound content of the mixture is in the range 5-90% by weight, more preferably 20-75% by weight, most preferably 30-60% by weight. Preferably the particulate alkali metal compound content of the sorbent, which has been treated with water, is in the range 3-86% by weight, more preferably 10-72% by weight, most preferably 15- 58% by weight.

The particulate alkali metal compound may be in the form of a powder, preferably with an average particle size, ED50], in the range 5-100 rim.

The mixture of particulate calcium silicate cement and particulate alkali metal compound may further comprise a binder, such as clays, including bentonite, sepiolite, minugel and attapulgite clays; calcium aluminate cements such as ciment fondu; and organic polymer binders such as cellulose binders, or a mixture thereof. The total amount of the binder in the sorbent may be in the range 2.5-30% by weight. The binders are desirably in the form of powders, preferably powder with a D particle size in the range 1-100 1im.

Other components may also be present in the sorbent to enhance the properties of the sorbent.

For example, the sorbent may include a particulate support material. The particulate support material may be any inert support material suitable for use in preparing sorbents. Such support materials are known and include alumina, metal-aluminate, silicon carbide, silica, titania, zirconia, alumino-silicates, zeolites, carbon, or a mixture thereof. Other supports include zinc compounds such as zinc oxide, zinc carbonate or zinc hydroxycarbonate. A support material offers a means to adapt the physical properties of the sorbent to the duty. Thus the surface area, porosity and crush strength of the sorbent may suitably be tailored to its use.

Furthermore, the presence of support particles can increase the strength and durability of the sorbent composition by acting as a diluent. Support materials are desirably oxide materials such as aluminas, titanias, zirconias, zinc oxides, silicas, zeolites and aluminosilicates, or mixtures of two or more of these. Hydrated oxides may also be used, for example alumina trihydrate or boehmite. Particularly suitable supports are aluminas and hydrated aluminas, especially alumina trihydrate. The sorbent may comprise 2.5-30% by weight in total of one or more supports. The support is desirably in the form of a powder, more preferably a powder with a D50 particle size in the range 1-100 jtm.

In a preferred embodiment, the sorbent consists essentially of hydrated calcium silicate cement and sodium carbonate (Na2CO3) and/or potassium carbonate (K2C03).

The mixture comprising the particulate calcium silicate cement and particulate alkali metal compound before and/or after shaping, is treated with water to hydrate the calcium silicate cement. The treatment results in a chemical transformation and hardening of the cement. The transformation is complex, but is believed to include the following reactions; (1) 2Ca3SiO5 + 6H20 3 Ca38i207.3H20 + 3 Ca(OH) (2) 2Ca2SiO4 + 4H20 3 Ca3 3Si2O73.3.3H20 + 0.7 Ca(OH)2 Without wishing to be bound by theory, a benefit of using calcium silicate cement therefore appears to arise from the ability of the calcium hydroxide hydration product, in addition to the alkali metal compound, to trap halogen compounds such as hydrogen chloride.

Shaping of the sorbent may be by casting into moulds, pelleting, extruding or granulating.

Hence, castings of the sorbent may be made by forming a slurry comprising the calcium silicate cement and alkali metal compound in water and pouring it into moulds of predetermined shape to harden. Alternatively the sorbent may be cast as slabs, which after hardening, may be crushed and sieved to give particles of the sorbent according to a desired particle size range. It will be understood that in this case, although the hardening of the cement largely takes place after shaping, in forming the sluriy the treatment with water is before shaping.

Sorbent pellets may be shaped by moulding a powder composition comprising the calcium silicate cement and alkali metal compound, generally containing a material such as graphite or magnesium stearate as a moulding aid, in suitably sized moulds, e.g. as in conventional tableting operation. The pellets may then be subsequently treated with water.

Alternatively, sorbent extrudates may be formed by forcing a suitable composition comprising the calcium silicate cement and alkali metal compound and often a little water and/or a moulding aid as indicated above, through a die followed by cutting the material emerging from the die into short lengths. For example extrudates may be made using a pellet mill of the type used for pelleting animal feedstuffs, wherein the mixture to be pelleted is charged to a rotating perforate cylinder through the perforations of which the mixture is forced by a bar or roller within the cylinder: the resulting extruded mixture is cut from the surface of the rotating cylinder by a doctor knife positioned to give extruded pellets of the desired length. The amount of water used in extrusion may be sufficient to cause bulk hydration of the calcium silicate cement.

Where this is not the case, the extrudates may be subsequently treated with water.

Alternatively, sorbent granules may be formed by mixing a powder composition comprising the calcium silicate cement and alkali metal compound with a little water, insufficient to form a slurry, and then causing the composition to agglomerate into roughly spherical, but generally irregular, granules in a granulator. The amount of water used in granulation is generally insufficient to cause bulk hydration of the calcium silicate cement. Therefore granulate sorbents will typically require a subsequent, treatment with water.

The pellets, extrudates or granules preferably have a length and width in the range ito 25 mm, with an aspect ratio (longest dimension divided by shortest dimension) «= 4.

The different shaping methods have an effect on the surface area, porosity and pore structure within the shaped articles and in turn this often has a significant effect on the sorption characteristics and on the bulk density. Thus beds of sorbents in the form of moulded pellets may exhibit a relatively broad absorption front, whereas beds of granulated agglomerates can have a much sharper absorption front: this enables a closer approach to be made to the theoretical absorption capacity. On the other hand, agglomerates generally have lower bulk densities than tableted compositions. It is preferred to make the sorbent in the form of granules and thus a preferred shaping method involves granulating a mixture of particulate calcium silicate cement, particulate alkali metal compound and any other components in a granulator.

The amount of water used to granulate the mixture may be in the range 100-500 mI/kg of mixture, depending upon the composition. Granules with a diameter in the range 1-15mm may be formed. Granules of diameter 1-5mm are preferred for most duties.

Where the shaped sorbent is subjected to a water treatment, this may be performed by spraying the shapes with water or dipping the shapes in water, or a combination of both.

Where the alkali metal compound is water-soluble, the water treatment of the shaped sorbent preferably uses a minimum amount of water and techniques to minimise leaching of the alkali metal compound from the sorbent. For example, the shaped units may be treated by an incipient wetness technique whereby the amount of water added is sufficient to fill the pores of the sorbent. The amount of water should preferably be sufficient to fully hydrate the calcium silicates present in the cement.

The water used to treat prepare the sorbent may be mains water, demineralised water or water recovered from suitable industrial processes. It may be desirable to use water containing an alkali metal compound, such as an alkali metal carbonate. For example water containing dissolved alkali metal compound such as a carbonate at a concentration from «= 1% wt upto saturation may be used. Using water containing alkali metal compound such as a carbonate may allow for improved control of the alkali metal compound content of the sorbent.

The water treatment results in a hardening or setting of the cement in the sorbent as the calcium silicate hydration reactions proceed. The water treatment is preferably performed at a temperature in the range 1-95°C, more preferably 5-50°C. The treatment may be continued for a period (a setting period) to harden the cement and generate the reactive sorbent. The setting period may be from 0.1 to »=100 hours depending upon the properties of the mixture and amount of water used.

The sorbent comprises water as the hydration product of the calcium silicate cement. Water may also be present in a hydration product of the alkali metal compound, and further as excess water adsorbed within the pore structure of the sorbent. The amount of water in the sorbent may be in the range 4-50% by weight, preferably 10-45% by weight, more preferably 15-45% by weight on the sorbent. Where excess water in an amount beyond incipient wetness of the sorbent has been used, the excess water is preferably separated and the sorbent subjected to a drying step. Such a drying step removes free water within the sorbent but should be performed without substantial dehydration of the formed hydrated calcium silicate. We have found that the degree of hydration in the sorbent should be kept as high as possible. Therefore preferably the drying step is performed at 5-50°C, most preferably 5-35°C. The drying step may be performed at atmospheric pressure or under vacuum.

The sorbent thus obtainable by the method is in the form of a shaped unit comprising a hydrated calcium silicate cement and an alkali metal compound selected from an oxide, hydroxide, carbonate or hydrogen carbonate.

The invention includes a process for removing halogen compounds from a fluid stream by contacting the fluid stream with the sorbent.

The sorbents may be used absorb halogen compounds from gas streams to avoid corrosion problems during subsequent processing of the gas stream and/or to avoid poisoning of downstream catalysts. The fluid stream may be a hydrogen gas stream comprising preferably »=50% vol hydrogen, more preferably »= 80% vol hydrogen, most preferably »=90% vol hydrogen.

Alternatively, the fluid stream may be a synthesis gas stream comprising hydrogen, carbon monoxide and carbon dioxide. In some cases, the fluid stream may be a gas stream comprising a hydrocarbon such as a natural gas or a refinery off-gas, containing, for example, one or more hydrocarbons such as methane, ethane, propanes, or butanes and especially one or more alkenes such as ethane, propene and butenes. Alkynes may also be present. The hydrocarbon content may be in the range 0.1-100% vol, but is preferably in the range 0.5-20% vol. Alternatively the fluid stream may be a liquid hydrocarbon stream. Such streams include liquid natural gas, natural gas liquids, condensates, LPG, kerosene, cracked naphtha and diesel fuels.

The halogen compounds that may be removed by the sorbent may be bromine, chlorine, fluorine, or iodine compounds but more commonly are chlorine compounds. In particular the sorbent may be used to capture hydrogen chloride, hydrogen bromide, hydrogen iodide and hydrogen fluoride. Organic halide compounds such as haloalkanes, including chloromethanes, chloroethanes, chloropropanes and chlorobutanes, as well as other longer chain chloroalkanes may also be removed from fluid streams using the sorbent. Preferably the halogen compound is hydrogen chloride.

The amount of halogen compounds may vary depending upon the fluid stream but the present process is particularly effective where the hydrogen halide content of the process fluid fed to the sorbent is in the range 0.1-20 ppm.

In use, the sorbent may be placed in a sorption vessel and the fluid stream containing the halogen compound is passed through it. The process may be operated at inlet temperatures in the range 10-400°C, but preferably in the range 5-1 00°C, more preferably 5-50°C and at pressures in the range 1-100 bar abs. Desirably, the sorbent is placed in the vessel as one or more fixed beds according to known methods. More than one bed may be employed and the beds may be the same or different in composition. For example, the sorbent prepared by the present invention may be used to capture hydrogen halides in combination with other sorbents that capture organic halogen compounds to provide a total halogen compound removal system.

The invention is further described by reference to the following Examples.

Example 1.

Anhydrous sodium carbonate, as received, was mixed with ordinary Portland cement (OPC) powder and water, cast into a tray, and left to set for 17 hours at room temperature (ca. 20°C).

The cast sorbent was then ground up using a pestle and mortar and sieved to a size fraction of 1-2 mm. The example was repeated using anhydrous potassium carbonate.

Comparative examples were prepared using calcium carbonate and magnesium carbonate. A further comparative example was made with no metal carbonate added to the Portland cement.

The amounts of each component used were as follows; Sample No. Carbonate Amount (a) OPC (a) H,O (ci) (a) Na2003 463.75 94.40 306.40 (b) K2C03 172.50 231.00 261.00 Comparative I CaCO3 125.00 97.50 109.75 Comparative II MgCO3 105.00 109.60 120.55 Comparative Ill None 0 200.26 74.70 The sorbents were tested using a chloride saturation test: 1 % HCI in a hydrogen carrier gas was passed over 5 g of sorbent in a 19 mm id glass reactor at room temperature and atmospheric pressure. The gas was passed over the sorbent at a flow rate of 45 lIhr for a period of 17 hrs.

The chloride content of the discharged samples was measured by coulometric analysis using silver electrodes in a Sherwood model 926 chloride analyser. The method used was as follows; a weighed amount of ground sample was boiled with nitric acid to dissolve all acid soluble chloride. The solution was then made up to a standard volume with demineralised water. 0.5 ml of this solution was added to an acid buffer and placed in a Sherwood 926 analyzer. The analyzer automatically titrated the chloride ions by passing a known constant current between two silver electrodes which provides a constant generation of silver ions.

These silver ions combine with the chloride in the sample and form insoluble silver chloride.

When all the chloride has been precipitated as silver chloride, the free silver ions generated by the electrodes are no longer consumed and therefore the solution conductivity changes. The change is detected by the sensing electrodes and the readout is stopped, displaying the results directly in milligrams of chloride per litre or milligrams % salt. The amount of chloride in the sample is calculated from the readout.

The saturated chloride content of each of the discharped samples is given in the table below.

Sample No. Cl content %WIw Sorbent (a) 35.2 Sorbent (b) 23.1 Comparative I 9.3 Comparative II 14.5 Comparative III 11.0 The chloride saturation test was also performed on the parent Portland cement material without metal carbonate or a hydration step, resulting in a chloride pickup of <1 wt.%. In comparison, the chloride removal capacity of Comparative III (11.0 wt.%) clearly demonstrates that the chemical transformation occurring during the hydration step is facilitating chloride removal.

Furthermore heating the hydrated Portland cement (Comparative Ill without metal carbonate) in an oven at a temperature of 105 °C for 24 hours resulted in a chloride pick up of just 4 wt.% in the chloride saturation test. Thus, heating the sorbent to a temperature above the dehydration temperature of the cement resulted in reduced effectiveness.

Despite sample sorbent (b) having a lower level of metal carbonate (26 wt.% K2C03) than Comparative I (38 wt.% CaCO3) and Comparative 11(31 wt.% MgCO3), it demonstrated much higher HCI removal capacity. Further, sorbent (a), which had a much higher level of metal carbonate (54 wt.% Na2CO3) than the other samples, demonstrated proportionately higher HCI removal capacity. The chloride content of sorbent (a) was more than double that of Comparative I and Comparative II, despite having less than twice the level of metal carbonate.

These results indicate that the sorbents containing alkali metal compounds are significantly superior to those containing the alkaline earth compounds in capturing HCI.

Example 2

Sorbent (C). 480.4 g of Ordinary Portland cement was added to a powder mixer along with 120.6 g of Na2CO3 and mixed thoroughly for 5 minutes. 1009 of the resulting mixture was removed and 120 g of demineralised water was added to the remaining mixture until it formed a paste. 609 of the removed powder mixture was added to the paste to turn it into granules.

The granules were sieved to a size fraction of 1-2mm and then left for 16 hours at room temperature (20°C) to set.

Sorbent (d). The above granulation procedure was repeated with a 50/50 wfw mix of Na2CO3 and cement. In total 600 g of the Na2CO2lcement combination was mixed with 1609 of water to form a granular material. The granules were sieved to a size fraction of 1-2 mm and then left for 16 hours at room temperature (20°C) to set.

The resulting granules were assessed for chloride removal capability using the chloride saturation test.

Samples of the granules were further treated with sufficient water to just exceed the water uptake capacity. In each case, the treated granules were left for 64 hours at room temperature to set. The treated granular sorbents were then re-assessed for chloride removal capability using the chloride saturation test. The results are presented in the table below.

Sample Water treatment Cl content (wt%) Sorbent (c) no 3.9 Sorbent (c) yes 8.7 Sorbent (d) no 4.2 Sorbent (d) yes 32.6 It is apparent from the resultant chloride capacities before and after additional water treatment that the degree of hydration of samples is an important factor in determining chloride removal capacity.

In comparison, a sorbent prepared according to W095122403 had a Cl content of 25.7% wt following the chloride saturation test, and a sorbent prepared according to W099139819 had a Cl content of 33.1% wt following the chloride saturation test. Thus the sorbent according to the present invention is able to match or surpass the absorbency of the prior art sorbents.

Claims (7)

1. Claims.1. A method for preparing a sorbent comprising the steps of: (i) mixing together a particulate calcium silicate cement and a particulate alkali metal compound, and (H) treating the mixture with water to hydrate the calcium silicate cement, wherein the particulate alkali metal compound comprises an oxide, hydroxide, carbonate or hydrogen carbonate and the sorbent is shaped before and/or after the treatment with water.
2. 2. A method according to claim I wherein the calcium silicate cement is a Portland cement.
3. 3. A method according to claim 1 or claim 2 wherein the alkali metal is Na and/or K.
4. 4. A method according to any one of claims Ito 3 wherein the particulate alkali metal compound is sodium carbonate and/or potassium carbonate.
5. 5. A method according to any one of claims 1 to 4 wherein the amount of particulate alkali metal compound in the sorbent is in the range 3-86% by weight.
6. 6. A method according to any one of claims 1 to 5 wherein the particulate alkali metal compound is in the form of a powderwith an average particle size, [D50], in the range 5-im.
7. 7. A method according to any one of claims Ito 6 wherein the mixture of particulate calcium silicate cement and particulate alkali metal compound further comprises a binder.6. A method according to any one of claims ito 7 wherein the mixture of particulate calcium silicate cement and particulate alkali metal compound further comprises a particulate support material.9. A method according to any one of claims ito 8 wherein the sorbent is shaped by casting a slurry of the mixture of particulate calcium silicate cement and particulate alkali metal compound into moulds, orby pelleting, extruding orgranulating the mixture.10. A method according to claim 9, further comprising treating the resulting casting, pellet, extrudate or granule with water.11. A method according to any one of claims 1 to 10 wherein the water contains an alkali metal compound.12. A method according to any one of claims Ito 11 further comprising a drying step to remove excess water at 5-50°C, preferably 5-35°C.13. A sorbent obtained by the method of any one of claims 1 to 12, in the form of a shaped unit comprising a hydrated calcium silicate cement and an alkali metal compound selected from an oxide, hydroxide, carbonate or hydrogen carbonate.14. A process for removing a halogen compound from a fluid stream by contacting the fluid stream with a sorbent according to claim 13 or as prepared according to the method of any one of claims 1 to 12.15. A process according to claim 14 wherein the fluid stream is a hydrogen gas stream comprising »=50% vol hydrogen.16. A process according to claim 14 wherein the fluid stream is a synthesis gas.17. A process according to claim 14 wherein the fluid stream is a liquid hydrocarbon stream.18. A process according to any one of claims 14 to 17 wherein the halogen compound is hydrogen chloride.