ABSORBENTS

Abstract: Absorbent for acidic gases such as hydrogen chloride in the form of granules, preferably having a particle size greater than 2 mm and a BET surface area of at least 10 m<2>/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 DEG C, the sample has a sodium oxide, Na2O, content of at least 20 % by weight.

Description: Absorbents This invention relates to absorbents, and to a process for their manufacture, in particular to absorbents suitable for removing contaminant acidic gases such as hydrogen chloride from gas streams Such absorbents are typically employed as a bed through which the gas stream to be treated is continuously passed the contaminant hydrogen chloride is absorbed by the absoroent so that the effluent gas nas a relatively iow nydrogen cnloπde content After a penoα of time the absorbent becomes loaded with absorbed hydrogen chloride until the absorbent bed is unable to reduce the hydrogen chloride content to an acceptable level typically it is desired to have an effluent gas containing less than a specified amount e g 0 1 ppm by volume of hydrogen chloride When the effluent gas contains an unacceptable Drooortion of hydrogen chloride "breaκ-through" is said to have occurred It is normally found that when Dreak-through has occurred the chloride content of the bed is somewhat less than the theoretical maximum thus while samples taken from oed inlet region may have a chlorine content equal to or near the theoretical maximum samples taken tram the bed outlet region are liable to have a chlorine content significantly below the theoretical maximum One type of known aDsoroent for removing contaminant acidic gases such as hydrogen chloride from gas streams comprises alumina granules that have been impregnated with sodium nyαroxiαe or carbonate and then calcineα at a high temperature at for example above 500°C Calcination at such temperatures causes at least some of the sodium hydroxide or carbonate to react with the alumina to form soαium aiummate Such absorbent granules in which some of the sodium is present as sodium aluminate as a result of the reaction Detween the sodium carbonate and alumina, typically have a total sodium content (expressed as sodium oxide. Na-O) of the order of 12-14% by weight as measured on a sample that has been ignited at 900°C These absorbents exhibit acceptable activity for the absorption of hydrogen chloride at relatively high temperatures for example of the order of 350°C, and "break-through" (giving a product gas containing more than 0 1 ppm by volume of hydrogen chloride) typically occurs when the absorbent has a total chlorine content of about 15-17% by weight However the activity of such absorbents at low temperatures, eg below about 150°C, is limited and the chlorine content of the bed when break-through occurs may be relatively low For example at ambient temperature, break-through (0 1 ppm HCI) may occur when the beα has an average chlorine content of only about 8-9% by weight It has been proposed in GB 1433762 to employ as an absorbent for acidic gases activated alumina granules of a high surface area that has been impregnated with a solution of sodium carbonate followed by evaporation of the water and heating at 120°C In PCT publication WO 93/22049 it has been proposed to make similar absorbents by impregnating alumina with an alkali metal salt of an organic acid such as acetic acid followed by drying and calcination at 300-550°C This latter reference suggests that absorbent granules could be made by an agglomeration tecnnique involving feeding alumina powαer to a rotating pan and spraying an aqueous solution of the alkali metal compound onto the alumina powder in the rotating pan By this technique it is suggested that the amount of alkali that could be incorporated is such that, expressed as alkali metal oxide, the alkali content of the absorbent is up to 15% by weight of the alumina By sucn an impregnation metnoσ it is not practical to proαuce absorbents containing more than about 15% by weight alkali expressed as alkali metal oxide By utilising an alternative production process we have found that it is possible to achieve absorbents having a much higher alkali content and hence a greater chloride capacity Such absorbents nave significantly imDroveα low terriDerature perrormance Accordingly the present invention provides aosorbent granules comprising an intimate mixture of an alumina component selected from alumina and/or hydrated alumina an alkali component seJected from sodium carbonate and/or sodium bicarbonate in weight proportions of 0 5 to 2 parts of saiα alkali component per pan 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 samDle of the granules at 900°C. the sample has a sodium oxide, Na 2 0, content of at least 20% by weight The aDsorbent granules preferably nave an average size of at least about 2 mm and preferably at least about 3 mm as a bed of smaller granules is liable to present an unacceptable resistance to flow of gas therethrougn Thus an unacceptably high pressure drop is experienced upon passage of the gas througn a bed of small granules The granules preferably have a BET surface area of at least 10 nrr/g The binder may be a suitable hydraulic cement, such as calcium alummate cement Alternatively, and preferably, the binder comprises a clay, for example an acicular clay such as attapulgite or sepiolite The absorbents of the present invention may be made Dy granulating a mixture of alumina or a hydrated alumina such as alumina tπhydrate, sodium carbonate, or, preferably, bicarbonate, and the binder, in the requisite proportions and calcining the resultant mixture at a moderate temperature, iβ below about 350°C, and preferably in the range 105-300 β C At such temperatures, it is believed that little or no decomposition of aluminium tπhydrate occurs but the sodium bicarbonate is converted to sodium carbonate X-ray diffraction examination shows the sodium present only as a sodium carbonate phase little or no sodium alummate is formed By the term granulation we mean mixing the powdered ingredients, i e alumina and/or hydrated alumina, sodium carbonate anα/or bicarbonate, and binder, with a little wetting agent such as water, in an amount that is insufficient to form a slurry, and forming the resultant mixture into aggregates, generally of approximate spherical configuration Such granulation techniques are well known in the art. It is preferred to employ alumina tπhyαrate. rather than alumina, since granulation of compositions containing a high proportion of sodium carbonate or bicarbonate and alumina tends 5 to present difficulties. The use of sodium carbonate, in place of sodium bicarbonate, may result in absorbents of lower surface area. Therefore we prefer to use a mixture of alumina trihydrate and soαium bicarbonate in a weight ratio in the range 1 * 2 to 2.1. In order to make agglomerates of adequate strength it is desirable to employ the ingredients in a finely divided form Typically the ingredients have an average particle size in the range 10 1-20 μm, preferably in the range 5-10 μm. The invention is illustrated by the following Examples Example 1 Sodium bicarbonate (8 kg), alumina trihydrate (8 kg) and attapulgite clay (1.6 kg), each in finely divided powder form having an average panicle size in the range 5-10 μm, were dry mixed ■" 5 Part of the mixture (about 2 kg) was charged to a Hobart mixer of 25 I caDacity and stirred therein at a speed of about 60 rpm (1 Hz). Water was slowly added while stirring until the mixture adhered to form small balls or agglomerates. Further amounts of the powder mixture and water were gradually added until all the powder mixture had formed into agglomerates. The agglomerates were then sieved to reject agglomerates having a size Delow about 3 mm or above 2Q about 5 mm The remaining agglomerates were dried in air in an oven at 115°C for 4 hours A sample (Sample A) of the dried agglomerates was tested as set out below The temperature of the oven was then increaseo to 200 β C at a rate of 10°C/mιn. and maintained at 200°C for 3 hours to effect calcination, and then a samDle (Sample B) of the calcined agglomerates was also tested 5 Both samples of agglomerates had a bulk density of about 0 8 g/ml and a BET surface area of about 14 nr/g Analysis of samples of the agglomerates after ignition at 900°C showed a sodium oxide. Na.O, content of over 30% by weight. Samples A and B were then tested for their hydrogen chloπde absorption characteristics by passing methane containing about 1 % by volume of hydrogen chloride at atmospheric pressure 0 and about 20°C down through a vertical bed of the pellets or granules of height 12 cm and height to diameter ratio of 5 at a space velocity of approximatelt 700 h '1 The time taken before the hydrogen chloride content of the exit gas reached 1 ppm by volume was determined and is quoted in the following table as the "break-through time to 1 ppm HCI". The granules were then carefully discharged from the bed and divided into 6 portions corresponding to 2 cm bands of the bed 35 depth. Each portion was analysed for the chloride content. The results are shown in the following table: Sample A Sample B Break-through time to 1 ppm HCI (hours) ey 2 13 1 /4 Chloride content (wt%) Portion 1 (top) 19.8 23.0 Portion 2 15 2 24 3 Portion 3 9 1 24.3 Portion 4 2 9 23.7 Portion 5 0 5 16 7 Portion 6 (bottom) 0 1 4 9 Example 2 A test proceαure similar to that of Example 1 (except that the space velocity was approximately 750 h ' and "breakthrough" was considered to occur when the hydrogen cnlonde content of the exit gas reached 10 ppm bv volume) was applied to the following sorbent materials * C Granules of particle size within the range 3 to 5 mm having a bulk density of about 0 9 g/ml and a BET surface area of about 16 g/m 2 made by the procedure of Example 1 using 50 parts by weight of sodium bicarbonate 50 parts by weight of alumina trihydrate and 10 parts by weight of attapulgite clay with the drying and calcining being effected in a single stage in a rotary drier at 145°C Analysis of a sample of the granules that had been ignited at 900°C showed a sodium oxide Na,0, content of about 30% by weight D Commercially available alumina granules of about 3 mm size impregnated with sodium carbonate and calcined at above 500°C to give granules of bulk density about 0 75 g/ml and a BET surface area of about 113 m * 7g which, after ignition at ΘOO'-C. had a sodium oxide Na : 0. content of about 14% by weight E Commercially available activated alumina granules of about 3 mm size having a bulk density of 0 83 g/ml and a BET surface area of about 300 m 2 /g. In the following table the "breakthrough time" (to 10 ppm HCI) and the chloride content of the individual portions is quoted for each sorbent material In addition since an important criterion is the amount of hydrogen chloride that can be absorbed by a given volume of absorbent bed, in the table the weight of chloride (kg of CI " ) per m 3 of sorbent is calculated Sorbent C D E Break-through time to 10 ppm HCI (hours) 11 3 /, 4 TA Chloride content (wt%) Portion 1 (top) 23.9 10.5 4.5 Portion 2 24.8 9.3 5.8 Portion 3 24.9 8.5 5.1 Portion 4 21.9 7.5 3.4 Portion 5 13.1 4 2 2.9 Portion 6 (bottom) 2.4 0.9 1.0 Chloride absorbed (kg per m 3 ) 173.5 55.2 32.1 It is seen that the absorbent granules C of the invention haα a far greater capacity for cnloπde than the sorbents D or E. despite the much lower BET surface area of the absorbent granules C.

Claims: Claims 1 Absorbent granules 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 bicarponate in weignt 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°C the sample has a sodium oxide Na,0, content of at least 20% by weight 2 Absorbent granules according to claim 1 having a BET surface area of at least 10 m 2 /g. 3 Absorbent granules according to claim 1 or claim 2 wherein the binder is a hydraulic cement or a clay 4 A process for the manufacture of absorbent granules according to any one of claims 1 to 3 comprising granulating a mixture of alumina tπhyαrate. sodium carbonate or bicarbonate, and a binder, and calcining the resultant mixture at a temperature below 350°C 5 A process for tne removal of acidic gases from a gas stream comprising passing the gas stream, at a temperature below 150°C. through a bed of absorbent granules according to any one of claims 1 to 3 or granules made by a process according to claim 4

Inventors: CARNELL PETER JOHN HERBERT, DENNY PATRICK JOHN, WILLIAMS BRIAN PETER, WOODROFFE CATHY ANNE, CARNELL, PETER, JOHN, HERBERT, DENNY, PATRICK, JOHN, WILLIAMS, BRIAN, PETER, WOODROFFE, CATHY, ANNE
Applicants: CARNELL PETER JOHN HERBERT, DENNY PATRICK JOHN, ICI PLC, WILLIAMS BRIAN PETER, WOODROFFE CATHY ANNE, CARNELL, PETER, JOHN, HERBERT, DENNY, PATRICK, JOHN, IMPERIAL CHEMICAL INDUSTRIES PLC, WILLIAMS, BRIAN, PETER, WOODROFFE, CATHY, ANNE
Application year: 1995
Main classification: B01J20
IPC codes: B01D53, B01J20