IL191284A - Process for eliminating hydrogen sulfide and odors from wastewater systems - Google Patents

Abstract

Use of doped and catalytically active moulded activated carbons as exhaust air filters for wastewater systems for removing hydrogen sulphide and odours.

Description

191284 ί?·τι I 453584 τηκ PROCESS FOR ELIMINATING HYDROGEN SULPHIDE AND ODORS FROM WASTEWATER SYSTEMS The invention relates to a process for eliminating hydrogen sulphide and odors from wastewater systems using activated carbon.

Activated carbon is very well suitable for separating non-polar substances, like for example butane and toluene. Here, high loading capacities of up to 50 %by weight can be achieved with very highly activated carbons (surface about 1200 m2/g). Temperatures at room temperature are sufficient to effect this physical adsorption.

Conventional activated carbons, however, can only adsorb polar gases, like sulfur dioxide, formaldehyde, ammonia, and hydrogen sulphide, to a low extent. Due to that, activated carbons are impregnated for the separation of polar substances. The impregnation is a process, in which the adsorbents are treated with special reagents. The reagents applied to the pores of the activated carbon may then react with the polar gases, and are thus chemisorbed at the impregnated inner surface of the activated carbon. The reaction time for that is significantly longer than for the physical adsorption and amounts to several seconds. This process may e.g. include catalytic reactions.

\ To remove hydrogen sulphide from gas flows, the activated carbon can be impregnated with alkaline compounds (e.g. with KOH, NaOH, etc.). The hydrogen sulphide then reacts in the form of a neutralization reaction with the alkaline component applied to the inner surface. U.S. Patent 5,024,682 discloses the use of activated carbon impregnated with NaOH or KOH for removing H2S from oxygen-containing gas flows.

Special impregnations for removing hydrogen sulphide from air and gas flows are known.

Significant is the Desorex method (K. Storp, DECHEMA Monographie 64 (1970)). In this method, the activated carbon is impregnated from a hot, aqueous solution with potassium carbonate (K2C03) for the catalytic oxidation of H2S. Here, maintaining temperatures of around 50 °C, hydrogen sulphide is catalytically oxidized into sulfate in the pores of the activated carbon.

H2S + K2C03 (in activated carbon) + 2 02 → K2S04 + C02 + H20 The disadvantage of each impregnation is the associated reduction of the physical adsorption capacity of the initial carbons. The impregnation reagent partially covers the surface, and thus the effective pore volume is reduced or even blocked. Impregnation tests at formed activated carbons on a charcoal basis have shown that the iodine number of the initial carbon is reduced from 1058 mg/g by the application of K2C03 onto the pore system of the activated carbon by 14 %, if the carbon contains 7.5 % by weight of K2C03 following the impregnation. The adsorption capacity, measured as the CTC number, is reduced by the K2C03 impregnation compared to the initial carbon by about 10 % by weight. This indicates, that the partial blockage of the pores - in particular the micro-pores - is caused by the impregnation reagent. Part of the micro-pores or adsorption pores, respectively, therefore is no longer available for the actual adsorption process. Therefore, e.g. in gas protection, very highly activated, finely pored basic carbons (specific surface: 1200 and 1500 m /g) are used as starting materials for the impregnation.

Therefore, it would be of central significance to not affect the access to the catalytic centers in the micro-pores, i.e. to avoid the disadvantages of the impregnation in regards to blockage and/or constriction of the adsorption pores (micro-pores), but to effectively utilize the advantages of the "- chemisorption or the catalytically acting substances, respectively, in the activated carbon. In the o . . ( ? °^er Patent Application 10 2004 033 561.3-44, therefore another strategy than that of subsequently impregnating activated carbons was suggested. In the patent application, the manufacture of a very finely-pored activated carbon, i.e. micro-porous activated carbon, is described, which beside the high micro-porosity also has catalytically active K2C03 centers in the carbon structure, without those effecting the blockage/constriction of pores - in particular of micro-pores. The approach for manufacturing such carbons lies in the special K2C03-doping of activated carbons, whereat the doping substance is added to the starting materials (carbon carrier and/or binding agent), preferably into the carbon carrier, in advance already. Thus it is also guaranteed, that during the manufacture of the activated carbon (carbonization, water-steam 5 activation) the K2C03 is finally present uniformly in the carbon structure of the highly micro- porous activated carbon.

The invention is based on the object to create a method for eliminating hydrogen sulphide and odoursfrom wastewater systems, in which the disadvantages of impregnated activated carbons, i.e. the reduction of the physical adsorption capacity of the initial carbons, are avoided. 0 According to the invention, this is achieved by using doped and catalytically-active formed activated carbons as exhaust air filters.

In the present invention, micro-porous, K2C03-doped formed activated carbons are used for removing H2S from gas flows. These formed activated carbons contain catalytically-active K2C03 centers in the carbon structure for the reaction with hydrogen sulphide. The adsorption 5 pores are not constricted or blocked by the doping substance, in particular when the doping substance is simultaneously used as a catalytic component for the hydrogen sulphide conversion and as an oxidation catalyst for the carbon carrier for the formation of the pore system, as is the case with K2C03.

In a further advantageous embodiment of the invention it is intended to use K2C03/Mn02-doped 0 formed activated carbons for removing H2S.

The addition of the above-stated doping substances, the addition of binding agents, and the manufacture of the shaped bodies, carbonizing agent and activating agent are described in the *- older application 10 2004 033 561.3-44, and therefore will not be explained further here.

Activated carbons doped with K2C03 were deployed and tested - also under field conditions -5 for the catalytic removal of hydrogen sulphide as an exhaust air filter for wastewater systems, e.g. in pumping stations for the reduction of H2S emissions below the odor threshold of 0.15 mg/m (0.10 ppm) under ambient conditions (>15 °C) without additional auxiliary means and/or energy.

Current developments for manufacturing activated carbon with an increased catalytic activity for 0 acidic components (S02, H2S) go into another direction and comprise the inclusion (thus doping) of carbon carriers with nitrogen-containing compounds. From U.S. Patent Nos. 5,353,370; ,356,849, and 5,444,031, e.g. methods are known, which start with hard coal, which is subjected to a low-temperature carbonization and oxidation, followed by contacting it with a low amount of a nitrogen-containing compound, like e.g. urea. The actual carbonization and water-steam activation take place following this pre-treatment only. As the field of application for the catalytically effective activated carbons thus manufactured, the catalytic conversion of H2S, SOx, and ΝΟχ is stated. The catalytic activity in that has to be attributed to the nitrogen content in the activated carbon. The surface chemistry of the carbon matrix by the inclusion of heteroatoms, like e.g. nitrogen, therefore plays an important role (comp. the publication: A. Bagreev et al., Carbon 42 (2004) 469).

The Patent WO 2004/052497 A2 discloses the manufacture of filter elements of activated carbon with metal oxides and a binding agent. For that, however, activated carbon is mixed with the metal oxide ("blending of activated carbon") and provided with a binding agent. The mass obtained that way is subsequently formed and calcined, and then used as a filter element for removing H2S.

In the publication of Z. Liu et al., Fuel 79 (2000) 1991, iron doping of tar as pellets with ferrocene with subsequent carbonization and water-steam activation is described. Here the influence of iron or the resulting iron compounds (Fe, Fe203, Fe3C), respectively, as oxidation catalyst for the water-steam activation is described.

The method according to the invention shows considerable advantages. The formed activated carbon used for this - with specific surfaces of more than 900 m2/g (BET surface), with iodine numbers of more than 1000 mg/g, and with a micro-pore volume of more than 0.38 ml/g -homogenously contains the catalytically-active component K2C03 or K2C03 together with Mn02 in the porous, carbon-containing matrix. In this manner, the potassium carbonate fulfils two functions: on the one hand, it serves as an oxidation catalyst for the conversion of the carbon in the activation, forming the pore system with a high micro-pore portion (>0.38 ml/g), and on the other hand, the K2C03 acts as the catalytic component for removing hydrogen sulphide from gas flows. It turned out, that the addition of Mn02 likewise acts as the active component for removing hydrogen sulphide. In that, it is irrelevant, whether the primarily added Mn02 is chemically changed during the further process steps (carbonization, activation) (e.g. by reduction into MnO).

The advantage of the doping substance Mn02 (or the products resulting from it during the manufacture) is the practical water-insolubility of these compounds (as opposed to K2C03). This has the advantage, that the activated carbon doped that way largely retains its catalytic properties towards hydrogen sulphide, even after flooding the activated carbon bed with water (buffer function), since the manganese oxide is not washed out from the porous activated carbon structure.

It turned out, that such doped activated carbons have a very high adsorption capacity towards hydrogen sulphide. This has to be attributed to the fact that first the hydrogen sulphide is preferably physically adsorbed in the present micro-pores, and thus is highly concentrated in the micro-pores (concentration effect). Subsequently, the H2S is oxidized at the catalytically-active component (K2C03 or K2C03/manganese oxide), predominantly in the micro-pore (sulfate formation), or converted into the metal sulphide. It is also characteristic for the H2S elimination catalysts used in the method according to the invention, that they retain their catalytic activity for H2S, even if the doping substances are subject to extreme temperature conditions (>800 °C) under the manufacturing conditions (carbonization, activation) in the carbon matrix. The easy reachability of the catalyst in the micro-pores by H2S enables a strong catalyst/adsorbate interaction. It is known, that by the physical adsorption in the pores - in particular in the micropores - adsorption heat is released. A very strong heat tone in particular occurs with carbon molecular sieves, since here the pore diameter approximately corresponds to the molecule diameter of small molecules (J. A. Allen et al. Carbon 37 (1999) 1485). It is therefore assumed that this heat tone, in particular in the micro-pores of the micro-porous formed activated carbons described here, occurs clearly. For activated carbons impregnated with K2C03 it is known that the catalytic reaction with H2S proceeds particularly well at >50 °C. The very good functioning of these formed activated carbons even at e.g. <15 °C indicates, that a local heat tone occurs with the very micro-porous and doped carbons. This local heat tone occurring in the micro-pores would accelerate the further reaction of H2S with K2C03, forming sulfate even at low ambient temperatures, due to the local temperature increase in the micro-pores (local temperature effect). This fact likewise is an advantage of the micro-porous and K2C03-doped activated carbons used here.

Furthermore, the catalysts remain active for a long time and the doped formed activated carbon has a good mechanical stability and a very low pressure loss under the flow velocities given in practice in wastewater structures, e.g. pumping stations. The doped activated carbon can be easily manufactured in large amounts.

Residual moisture in the activated carbon is of advantage for the adsorption process, since here a dissociation of H2S to SH7S2" ions, in particular in the micro-pores, is favored. This is of advantage for the reaction with manganese oxide due to the sulphide formation. In contrast to the catalytic oxidation of H2S at K2C03, no oxygen is required for the reaction of H2S with manganese oxide to the manganese sulphide. This reaction therefore takes place under anaerobe reaction conditions.

In the following, the invention will be described in more detail on the basis of examples: a) Loading tests with hydrogen sulphide For the loading tests with hydrogen sulphide at doped formed activated carbon (pellet diameter: 4 mm), a bed of activated carbon with the following dimensions was used: bed height: 5 cm, bed diameter: 7.8 cm. The filter surface therefore was 47.76 cm2 and the bed volume 238.80 cm3. A gas flow of 0.06 m3/h (60 liters/h) with 16.7 % by volume of H2S gas in air was fed. The H2S gas was produced from iron sulphide (FeS) by adding an aqueous HC1 solution. Thus the hydrogen sulphide gas was saturated with water steam. With the data stated resulted a nominal linear velocity flow of 0.35 cm/s and an average residence time of 4.3 s with a porosity of the bed (relative free gap volume) of ε = 0.30. The activated carbon was loosely filled into the adsorber and the surface then smoothed. The gas supply through the activated carbon bed was continued until the H2S detector at the exit of the activated carbon bed displayed 10 ppm of H2S. During loading, the activated carbon bed heated up. Upon reaching a temperature of 50 °C, the loading test was interrupted, the activated carbon bed cooled to room temperature, and only then the loading test with hydrogen sulphide was continued. The hydrogen sulphide capacity was determined over the duration of the gas supply - based on 20 °C and atmospheric pressure - and thus through the supplied quantity of H2S until 10 ppm of H2S were reached.

The table shows the measuring results of the loading tests. It can be seen, that the H2S capacity increases with increasing K2C03 doping. With doping of 8.7 % by weight of K2C03, the H2S capacity amounts to 10.3 g/100 g of activated carbon with an iodine number of 1170 mg/g. If the doping is increased to 20.8 % by weight of K2C03, then - with approximately the same iodine number of 1200 mg/g - the H2S load also increases to 19.0 g/100 g of activated carbon. This clearly shows the influence of K2C03 with approximately the same surface (iodine number) on the H2S capacity. Furthermore, Mn02 may also be used as a practically water-insoluble doping substance for H2S separation. Mn02 (doping 5.8 % by weight in the activating agent) was used together with K2C03 (4.4 % by weight in the activating agent) as the doping reagent. The H2S loading test shows, that the H2S capacity amounts to 14.4 g/100 g of activated carbon. This load is higher than the capacity of activated carbon doped with 5.8 % by weight of K2C03 (without Mn02), which shows a clearly lower capacity of only 4.7 g/100 g. Thus Mn02 also may be used as the doping reagent for H2S adsorption, even if Mn02 might be chemically changed during the manufacturing process (carbonization, activation) (e.g. reduced to MnO, which also is practically water-insoluble). The carbon doped with K2C03 and Mn02 was washed with water for removing the water-soluble K2C03. The water-treated sample, however, showed a H2S capacity of 10.7 g/100 g of activated carbon, and is therefore very well suitable for the H2S removal.

Table: Loading tests with H2S at doped formed activated carbon manufacturing process were not considered; TSample with 4.4 % by weight of K2C03 and 5.8 % by weight of Mn02, washed with water (for removing K2C03), and H2S capacity determined b) The use of the filters under field conditions will now be explained in more detail on the basis of the figures. In the figures: Fig. 1 shows the hydrogen sulphide concentration in a wastewater pumping station and the hydrogen sulphide concentration following the installation of the filter downstream of the filter; Fig. 2 shows the hydrogen sulphide concentration in the pumping station with and without a filter at low temperatures; Fig. 3 shows the hydrogen sulphide concentration in the pumping station with and without a filter at medium temperatures; Fig. 4 shows the pressure loss through different activated carbon beds as a function of the nominal linear velocity flow.

A formed activated carbon doped with 5.8 % by weight of K2C03 (pellet diameter 4 mm, vibration density 482 g/1) was used for tests in the wastewater pumping station for reducing the hydrogen sulphide concentration in the gas flow (wastewater exhaust air). The specific surface (BET surface according to DIN 66131) of the doped carbon was 950 m /g and the iodine number 1077 mg/g. The micro-pore volume was determined from the nitrogen adsorption at 77 K according to an evaluation of Horvath and Kawazoe (DIN 66 135, Part 4) and amounts to 0.41 ml/g.

An activated carbon bed with the dimensions (bed height: 4.6 cm, surface: 65.5 cm χ 35.5 cm) was used. The volume flow of the supplied hydrogen sulphide-containing gas flow (wastewater exhaust air) was 400 liters/h on average (volume flow of a pumping cycle in the time between the pumping operations). From this, a velocity of flow of w = 0.048 cm/s and an average residence time of τ■= 28.8 s is calculated in the activated carbon filter with a porosity of the bed of ε = 0.30. This residence time is by far sufficient to completely let the chemisorption processes (here the conversion of hydrogen sulphide at the K2C03) take place, since chemisorption processes though may basically take place slower - compared to the physical adsorption - but are still completed within approximately 10 s.

In Fig. 1 , the hydrogen sulphide concentration in a wastewater pumping station is represented in ppm. The average hydrogen sulphide concentration in the pumping station (without filter) amounts to 13.5 ppm over a period of 49.5 h with concentration peaks of up to approx. 220 ppm at an average temperature of 19.3 °C. Following the installation of the activated carbon filter, the hydrogen sulphide concentration was clearly reduced. The average H2S concentration then only amounted to 0.08 ppm (measured downstream of the filter) over a period of 79 hours with peaks up to a maximum of 16 ppm at an average temperature of 21.2 °C.

The measurement of the concentration took place using a gas tester, without changing the ambient conditions within the pumping station. The representation clearly indicates, that the partially noxious concentrations of the H2S can be eliminated using the doped, micro-porous activated carbon with 5.8 % by weight of K2C03 with an average residence time of approx. 29 s, since the average hydrogen sulphide concentration of 0.08 ppm lies below the odor threshold of 0.10 ppm (or 0.15 mg m3), and concentration peaks are nearly completely reduced and thus buffered off.

Fig. 2 shows the hydrogen sulphide concentration in the pumping station with and without a filter system at relatively low temperatures. Until then, the filter had been in use for half a year and the water content (moisture) of the activated carbon was very high and amounted to approx. 35 % by weight. The average hydrogen sulphide concentration in the pumping station amounted to 3.97 ppm over a period of 149 hours with concentration peaks at 75, 141, 164, 130, 184, 113, and 175 ppm at an average temperature of only 13.5 °C. Downstream of the filter, an average hydrogen sulphide concentration of 1.48 ppm was measured over a period of 149 hours, with concentration peaks at 13, 26, and 19 ppm at an average temperature of only 13.5 °C. This clearly shows that even at relatively low temperatures (average temperature 13.5 °C) and with high moisture (approx. 35 % by weight of water) in the formed activated carbon, the concentration peaks of the hydrogen sulphide are buffered off by the activated carbon filter.

Fig. 3 shows the hydrogen sulphide concentration in ppm in a wastewater pumping station with and without a filter system at an average temperature of 17.7 °C. Until then, the filter had been in use for one year. The average hydrogen sulphide concentration in the pumping station amounted to 29.2 ppm over a period of 168.5 hours with many concentration peaks of around 200 ppm. Thus the average hydrogen sulphide concentration lay above the MAK value of 10 ppm.

Downstream of the filter, an average hydrogen sulphide concentration of only 0.0126 ppm was measured over a period of 168.5 hours. The concentration peaks was below 10 ppm. Thus the average hydrogen sulphide concentration downstream of the filter was below the odor threshold of 0.1 ppm. The concentration peaks were practically completely buffered off.

Due to the special form of the high-quality, doped formed active carbon, the pressure loss in the activated carbon bed is very low. Fig. 4 shows the pressure loss in mbar/m through activated carbon beds of formed activated carbon of various diameters (2 and 4 mm) as a function of the nominal linear velocity flow in m/s. With the calculated nominal linear velocity flow of 0.0005 m/s (see above), the pressure loss within the bed (bed height: 4.6 cm) only amounts to 0.037 μbar and therefore is practically negligible. 1 Abstract PROCESS FOR ELIMINATING HYDROGEN SULPHIDE AND ODORS FROM WASTEWATER SYSTEMS Use of doped and catalytically-active formed activated carbons as exhaust air filters for wastewater systems for removing hydrogen sulphide and odors. 191284/2

Claims (2)

1. A process for eliminating hydrogen sulfide and odors in wastewater systems by using doped and catalytically active molded activated carbons as exhaust air filters, characterized in that the molded activated carbon used is doped with the component K2C03 or with the component K2C03/Mn02.

2. The process according to claim 1, characterized in that the microporous molded activated carbon used comprises the component Mn02 as water- insoluble component For the Applicants Maty Bapzam Patent Attorney