European Journal of Scientific Research ISSN 1450-216X Vol.39 No.2 (2010), pp.289-295 © EuroJournals Publishing, Inc. 2010 http://www.eurojournals.com/ejsr.htm
Regeneration and Activity Test of Spent Zinc Oxide Hydrogen Sulfide Removal Catalyst Karim.H.Hassan Department of Chemistry, College of Science, University of Diyala, Baquba, Iraq Abstract
Two methods were experimented to regenerate the spent ZnO catalyst. The first one involved combustion by air while the second one involved steam treating of catalyst. The results abstracted from the first method showed that it is not suitable and reliable in regeneration of the catalyst because of the formation of zinc sulphate which cause blockage of the catalyst pores and hence reduce its activity. The steam regeneration method seem to be very practical and promising as the catalyst regenerated have a good activity which approached 97 %, in addition, the physical and chemical properties of the regenerated catalysts was very close and similar to the commercial new one Keywords: Regeneration, Spent zinc oxide, Hydrogen sulphide, Catalyst, Activity
1. Introduction The natural gas composed of light hydrocarbons in which methane is the major one. As far as the sulfur compounds are concerned hydrogen sulfide is the important one with carbonyl sulfide, carbon disulfide and mercaptans. The chemical composition of an Iraqi natural gas reported in Basraha refinery, 1990 is given in table (1). Table 1:
Gas Vo1%
Chemical composition composition of Khor Al-Zubair natural gas, H2S in ppm
N2 0.95
CO2 2.63
C1 73.82
C2 10.95
C3 8.44 8 .44
iC4 0.68
nC4 1.34
iC5 0.25
nC5 0.41
C6+ 0.53
H2S 0.62
Hydrogen sulfide has unpleasant odor and corrosive, Cadena,F and R.W.peters (1988) in addition of being poison for most industrial catalysts, so it has to be removed from industrial gases and the methods used to do so can be classified as old method by Iijima, Masaki , Kobayashi , Kazuto, Morita, Kazuhiro, 2004 and new methods by Susan, 1989, the first one includes the phosphate method, iron oxide method and sodium hydroxide method and the second one used activated carbon adsorption, molecular sieve and zinc oxide. The activated carbon was used to adsorb or separate hydrogen sulfide and other gases from natural gas in a fixed bed reactor where two or three reactor were connected some of them for adsorption of gases and the other for regeneration of the deactivated bed. Owing to increase of the heavy hydrocarbon gases in the natural gas in recent years which cause to decrease the life time of the activated carbon as adsorbent and required high temperature for regeneration, the use of activated carbon process became limited. Molecular sieve method is very limited and not widely used. º C
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Zinc oxide method is the most important one. It is used to remove the traces amount hydrogen sulfide being present in the gas mixture that enter the primary and secondary steam reforming units and the secondary CO to CO 2 converter i.e. it acts as a guard to prevent all the catalysts used in fertilizer industry from poisoning with sulfur present in the feed. The natural gas or naphtha feed were heated to 285-400 º C and then passed through one or more bed of ZnO in the form of tabletes, spheres, or extrudates where the function of the catalyst depends on the chemical reaction: ZnO + H2S ZnS + H2O (1) Usually zinc oxide catalyst is used as a mixture of ZnO and alumina as a binder in addition to some filler. In general 90 % wt of ZnO is quiet acceptable. On using naphtha as feed the sulfur being reduced to 0.02 ppm whereas with natural gas as a feed the sulfur can be removed to a level of 0.1 ppm. At ordinary condition and with out side interferences the adsorption capacity can be theoretically approach 41.7 Kg H 2S/100Kg catalyst or 39.5 Kg of S if it contain 100 % ZnO. The catalyst loose its activity suddenly when the H 2S concentration is high enough as a result of the formation of ZnS layer on the external surface and then filling the pores and preventing the next coming H2S to pass and react with new ZnO. Most sulfur removal reactors consist of two beds of catalysts the first one is CoMo catalyst that convert the sulfur compounds by hydrogen to H 2S, 'Man Haimour, Ribhi El-Bishtawi and Abdulrakib Ail-Wahbi (2005) and Elvers, Barbara et al. (1989) and the second one is ZnO bed where H 2S is being adsorbed on. Catalysts deactivation are governed by several factors and the three most important are poisoning, fouling and sintering of the catalysts. Using a huge amount of catalysts with very high costs in industry necessitates studying and developing methods for regeneration. Most catalysts regeneration processes fall into two categorizes which are the oxidative regeneration and non oxidative regeneration as classified by Furimsky.E and F.E.Massoth, (1993).The oxidative one involves treatment or exposing the waste catalyst to oxygen diluted by N 2 and / or steam, CO 2, N2O, O3. The non oxidative one involves treatment of the waste catalyst with selective solvents that are capable of removing the contaminants from the surface of the catalysts with out effecting the distribution and the percent of the active centers and the active materials of the catalysts. Regenerability of ZnO discussed by Susan (1989) is restricted by loss of surface area at high regeneration temperatures and formation of zinc sulfate at low regeneration temperatures. Also, during sulfidation , reduction of ZnO followed by vaporization of elemental zinc can take place at temperatures below 600 º C. Of important works on the regeneration of the ZnO catalysts was done by Susan and Copeland.R,J, M. Cesario, D. Feinberg, B. Windecker and J. Yang] by preparing a regenerable Zinc oxide -Titanium dioxide sorbents for high temperature H 2S removal from fuel gases. Sulfidationregeneration cycles of ZnO- and CaO containing sorbents were conducted to examine the hydrogen sulfide removal performance by Sophia E.A. Evangelos , and Vasalos.A, Iacovos (1995). Regeneration of sulfided metal oxide absorbent in an absorber is conducted using as diluents for oxygen a portion of Claus plant gas-in-process withdrawn upstream of the absorber. The result is a reduction in the size of the absorber and/or a reduction in the rate of return of regeneration effluent to a Claus plant during regeneration of such absorbent, Mcgalliar.L.Russell (1988). ZnO was Prepared from zinc nitrate by precipitation and the performance and kinetic studies were done by Karim. H.Hassan , Z. A-A Khammas and A. M. Rahman (2008) where activation energy and enthalpy of activation were calculated. The aim of the present investigation is to establish a new, practical method to regenerate the spent ZnO catalyst and test its activity.
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2. Materials and methods 2.1. Materials
Spent zinc oxide catalyst used in Baji fertilizer factory in Iraq, CoMo hydrodesulfuization catalyst, barium chloride, cadmium sulphate, hydrochloric acid, Nitrogen and hydrogen gases. 2.2. Chemical Analysis 2.2.1. Hydrogen Sulfide in the Gas Mixture A measured volume of the gas containing the H 2S was passed into a solution of cadmium sulfate where yellow precipitate CdS is formed. The precipitate thus formed is treated with dilute hydrochloric acid to liberate the hydrogen sulfide which is then analyzed by iodometric titration . 2.2.2. Zinc Sulfide Zinc sulfide in spent catalyst is calculated from hydrogen sulfide being determined by iodometric titration described in IP standards for Petroleum and Its Products (1976) and Vogel.A.I (1978) after being liberated from the spent catalyst by dilute HCl. 2.2.3. Zinc Sulfate A weight catalyst sample is dissolved in dilute hydrochloric acid and the sulfate is precipitated as barium sulfate by reacting the solution with barium chloride solution. The precipitate is filtered, washed, dried and then calcined in a furnace at 850ºC to evaluate the weight of BaSO 4 from which ZnSO4 can be calculated. 2.3. Regeneration of Spent Catalyst
Two procedures were tested to regenerate the spent catalyst, the combustion by air and treatment with steam. 2.3.1. Combustion with Air Method The apparatus used in regeneration the catalyst is explained by Karim et al. (2008) and shown in fig.1.It composed of a quartz tube reactor of 175 mm diameter and 350 mm length in which the catalyst was load; it is inserted in a heat controlled tubular furnace. The spent catalyst is placed as a bed in the reactor with a current of air passed over with flow rate of 5 liter/hour and the amount of ZnS in the catalyst is determined to ensure the conversion of ZnS to ZnO, the temperatures range were 500 to 800ºC.
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292
Figure 1: Flow diagram of laboratory hydrodesulphurization and regeneration units 1-feed tank for naphtha 2-Ball valve 3-Pump 4-One way valve 5-Heater 6-Pressure gauge 7-Pressure control valve 8-Heaters 9-Temp. thermocouples 10- Reactor
10
tubular furnace
reactor
8 0utlet gas 1
heater To heating control
To temperature control
Thermocouple 9 9
6
2 3
7
4
5 H2
2.3.2. Steam Regeneration Method The spent catalyst is placed as a bed in the tubular reactor. The steam generated from water and a nitrogen gas was mixed with a ratio of 2:1 and passed inside the reactor at flow rate of 5 liter/hour, the produced gases from reaction were sent to a set of traps that contain cadmium sulfate solution to trap the hydrogen sulfide formed. The experiment run until the generation of hydrogen sulfide ceased. 2.4. Catalyst characterization
The metal content of the catalysts were determined by standard atomic absorption methods. The Bulk density was determined by placing a weight catalyst in a graduated cylinder then shakes it by vibrator and read the volume of it and then calculate the density i.e. it is the packing or load density of the reactor. The particle density is measure by weighting a symmetrical extrudate and calculate its volume. It is carried out for several particles and takes the average. Pore volumes and skeletal density was determined by the liquid impregnation methods described by Richardson (1982) while porosity is calculated theoretically from densities values. Hardness was determined with the ERWEKA.TBH28 hardness meter while crushing value was determined by placing 25 gm of the sample under pressure of 18 kN for 20 minutes and measure the amount of powder that pass through 53 micrometer mesh as a percent relative to the total weight. 2.5. Catalyst Testing
Activities of the catalysts for the adsorption of H 2S were determined using a plant that consists of two units the first one for generation of the hydrogen sulfide by the hydrodesulphurization (HDS) of naphtha that contain 0.07 % wt of sulfur using a CoMo catalyst and the second one for the adsorption of the H 2S on ZnO catalyst. The schematic representation of the plant is well described by Karim et.al. and shown in fig.1.Test conditions were reaction temperature 340º C , pressure, 25 bar and flow rate of naphtha was adjusted so that the gas leaving HDS unit contains 0.00277 gm H 2S /liter by maintaining the flow rate of the gas leaving the HDS unit at 10.8 liter / hour or 0.028 gm H 2S /hour.
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3. Results and Discussion 3.1. Catalyst Regenerated by Air Combustion Method
The physical and chemical properties of the ZnO catalysts regenerated at temperatures of 550-800 º C are given in table (2). From comparison one can observe that they differ from the commercial one. There is a large difference in crushing strength values, hardness and densities. The theoretical amount of H 2S that should be adsorbed by the catalyst was calculated by considering that the catalyst bed used was 15 gm of 90 % ZnO i.e 13.5 gm of ZnO Since 1 mole of ZnO (81 gm) adsorbed 1 mole of H 2S (34 gm), the 13.5 gm must adsorbed theoretically 5.66 gm of H2S which means that the catalyst can operate for 202 hours with a rate of H 2S passing of 0.028 gm H2S /hour before saturation. The H2S adsorption test conducted using 15 gm catalyst showed H 2S appearance as the yellow precipitate of CdS forms after 80 hours which is very low if compared with the theoretical time for complete saturation calculated above as 202 hours. This indicates the low activity of the regenerated catalyst calculated to be 40% from (80/202) x 100, this lead us to think about analysis of the catalysts components to know the reason. The chemical analysis indicates the oxidation of the ZnS being present in the catalyst to ZnO with the formation of ZnSO 4 when the oxygen is excess and this was confirmed by the results of Susan (1989) and our results given in table (2). Since ZnSO 4 is relatively large molecule compared with ZnS and might cause a blockage of the internal small pores in the catalyst structure and hence preventing the H 2S from reaching and continuing its reaction with the other parts of the catalyst and then reduce its activity. 3.2. Catalyst Regenerated by Steam Method
The adsorption of H 2S by ZnO is usually occurs at low temperature and even at room temperature leading to the formation of ZnS as in equation one. Regeneration necessitates conversion of ZnS to ZnO via reversing the reaction and this seem to be the idea of using steam in stead of air to do so ZnS + H2O ZnO + H2S (2) Thermodynamically the reaction was expected to be temperature dependent and require a relatively high temperature and severe conditions. Regeneration experiments was tested using a temperature of 500 º C to 800 º C which show that the H 2S begin to release from the catalyst at about 500 º C and the rate increased with temperature but can not have a reasonable value below 800 º C as the rate below this temperature is low. Choosing the temperature was made to avoid cracking and damage in catalysts structure at a temperature higher than 800 º C. Table (3) shows the physical and chemical properties of the regenerated ZnO catalyst. From comparison one can observe that they are quiet similar to the commercial catalyst with slight differences in the values of some physical properties. Testing the adsorption of H 2S by passing the gas over 15 gm catalyst was continued for a period of 150 hours with continuous monitoring of the exit H 2S by a solution of cadmium sulfate which show no indication about its appearance at all as the yellow precipitate of CdS did not form. For 150 hours operation on the catalyst with rate of H 2S of 0.028 gm / hour as calculated above, the mount of H 2S adsorbed on the catalyst should be 4.2 gm. Chemical analysis of this used catalyst for sulfur showed that it consists of 78 % wt of ZnS which means that the catalyst adsorbed 4.10 gm of H2S during 150 hr operation, a value that is very close to the theoretical one and mean that the catalyst is still active. The activity for adsorption of H 2S was calculated by using 5 gm of regenerated and commercial catalysts and measures the amount of H 2S required to saturate the catalyst and hence release H 2S which is monitored by cadmium sulfate. The regenerated catalysts was saturated after 65 hours and the commercial one after 67 hours, a values corresponds to 1.82 gm, H 2S and 1.88 gm, H2S for the regenerated and commercial catalysts respectively. From these results, the activity of the regenerated catalyst was calculated as 97 %.
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So we can conclude that it is possible to use steam heated at 800 º C to regenerate the spent ZnO catalyst and renew and refresh its characteristics and use it again with a good activity. Table 2:
Properties of regenerated ZnO catalyst using air combustion method at 500-800ºC.
Properties
Spent
Form Color Size (mm) Bulk density,gm/cm3 Porosity Pore volume cm3 Hardness, N Crushing value % wt ZnO % ZnS % ZnSO4 % Al2O3% gm H2S/gm catalyst
Table 3:
Extrudates Grey 4x5 1.45 0.15 /gm 9 8.0 0.0 91.51 0.0 8.49 0.0
500 º C same grey 4x5 1.211 0.11 0.10 9.5 8.0 20.7 67.5 1.8 10 0.09
Regenerated at 600 º C 700 º C same same white yellow 4x5 4x5 1.215 1.225 0.13 0.14 0.11 0.12 10 11 7.5 7.5 27.9 36 55.8 37.8 6.3 16.2 10 10 0.13 0.09
800 º C same yellow 4x5 1.230 0.12 0.15 12 6.5 40.5 26.1 23.4 10 0.05
Commercial Extrudates white 4-5 x 5-7 1.18 0.28 0.14 10 8.5 90.0 0.0 0.0 10.0 37.73
0.23
Properties of regenerated ZnO catalysts using steam method at 800ºC.
Properties Form Color Size (mm) Bulk density, gm/cm 3 Particle density, gm/cm 3 Skeletal density, gm/cm 3 Porosity Pore volume, cm3 Hardness, N Crushing value, % ZnO, % ZnS % ZnSO4 % Al2O3, % Zn, % Al, % gm H2S/gm catalyst Activity , %
Spent Extrudates Grey 4x5 1.45 2.40 3.33 0.15 /gm 9 8.0 0.0 91.51 0.0 8.49 72.22 5.29 0.0 0.0
Catalysts Regenerated Extrudates White to yellow 4x5 1.22 2.23 3.23 0.31 0.10 8 9.5 87.3 2.7 0.0 10.0 71.86 5.29 36.60 97
Commercial Extrudates White 4-5 x 5-7 1.18 2.20 3.25 0.28 0.25 10 8.5 90 0.0 0.0 10 72.22 5.29 37.73 100
0.23
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References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12]
[13] [14]
Basraha refinery, 1990, Internal report, Southern Refineries Company, Iraq Cadena,F and R.W.Peters,1988, Evaluation of chemical oxidizer for hydrogen sulfide control, J Water Poll Control Fed 60, 7 , pp1259-1263 Copeland.R.J, M. Cesario, D. Feinberg, B. Windecker and J. Yang, TDA Research Inc, Wheat Ridge, Colorado 80033. Elvers, Barbara et al, 1989, Ullmann s Encyclopedia of Industrial Chemistry, Volume A12, page 192. Furimsky.E and F.E.Massoth, 1993, Catalysis Today, 17, 4, pp 537-659. rd IP standards for Petroleum and Its Products", 1976, The institute of Petroleum, England , 3 Edition. Karim. H.Hassan , Z. A-A Khammas and A. M. Rahman, 2008, Al-Khwarizmi Engineering Journal, Vol.4, No.3, pp74-84. Iijima.M , Kobayashi. K and Morita, K, 04/27/2004, United States Patent 6726852 , Application Number: 09/801720. 'Man Haimour, Ribhi El-Bishtawi and Abdulrakib Ail-Wahbi, 2005, Desalination, 181, pp145152. Mcgalliar.L.Russell ,1988, United States Patent 4769229, (Broken Arrow, OK) , Application Number: 06/912023. Richardson.J.T , 1982, "Principles of Catalyst Developments", University of Houston. Sophia.C , Evangelos.E.A ,and Vasalos.A, Iacovos ,1995, Publisher: American Chemical Society, Publication Name: Environmental Science & Technology, Subject: Science and technology, ISSN: 0013-936X, Susan, L, J.Kandaswami. and F.S.Maria, 1989, Ind.Eng.Chem.Res, 28, 535 th Vogel,A.I,1978, "Vogel's Textbook of Quantitative Inorganic Analysis", 4 Edition, Longman Scientific & Technical, London.
