Oxygen: the Solution for Sulfur Recovery and BTX



BOC began developing oxygen using burners and processes for application in Claus units in the mid
scale laminar flow reactor work carried out both within BOC and by external R&D organisations. Further
information relatin...
Full furnace profiles of the different species within the furnace were measured using gas chromatography.
Corresponding te...
CH2                                        CH2

                                                   H3C         CH3

the case of BTX, the methyl radical serves as the main precursor to CS2, this being produced during the
initial decomposit...
Figure 5 Comparison of BTX destruction at 1000oC and 1100oC

The reduced reaction scheme used in the simulation above h...
CH 3                 CH3

                                M,H,CHx                  M,H,CHx                 H2O
Figure 8 Predicted contours of temperature, sulfur dioxide and hydrogen sufide
Intense combustion regions result in a rela...
Figure 9 Decomposition of xylene to toluene and subsequent formation and destruction of benzene

Figure 10 illustrates a c...
The results from the model described above provide a large amount of data, far more than is presented
here. Using this CFD...
breakthrough at these temperatures and feed concentrations.          In practise, an increase in oxygen
enrichment would b...
In spite of the on cost of oxygen, the operating costs can also be lower than that of an equivalent air-
based plant as sh...
incinerator would be greatly reduced. In these instances, the cost of oxygen can be offset against
    these operating cos...
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Sulfur Recovery BTX Destruction


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A published document explaining how modeling was used to simulate the destruction of BTX in Claus reaction furnaces.

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Sulfur Recovery BTX Destruction

  1. 1. Oxygen: the Solution for Sulfur Recovery and BTX By Jason Norman BOC Murray Hill, NJ Stephen Graville and Richard Watson BOC Sheffield, UK ABSTRACT Oxygen has been in use in sulfur recovery units for more than twenty years. Oxygen allows for flexible sulfur plant operation for expanded capacity, peak shaving and turndown with minimal capital investment; this is beneficial in assisting an industry under pressure to increase gas capacity for the power generation sector. BTX contaminates, which cause serious problems with sulfur plants, can readily be destroyed with the use of oxygen. This article reports pilot plant and simulation studies conducted to evaluate burner design features for BTX destruction. The use of oxygen has been evaluated as a cost effective means of destroying BTX compared to alternative approaches whilst providing the additional option to increase sulfur plant capacity up to 180% of design. BACKGROUND BTX (Benzene, Toluene, and Xylene) are common contaminants in Claus plant feed gases originating from associated and some natural gases. Whilst not difficult to destroy in a conventional combustion process, the fact that the associated acid gas often contains much less than 60% H2S means that the temperature in the Claus furnace is often too low for effective BTX destruction. Failure to remove the BTX, either upstream or in the reaction furnace, leads to carbon and hydrocarbon contamination in the catalyst beds with subsequent loss of activity, high pressure drop in the first catalytic bed and a need to change out catalyst on a frequent basis. Bypassing some of the process gas around the furnace to increase the furnace temperature is not possible since BTX is then passed directly to the catalyst beds. The conventional way of handling the problem within the reaction furnace, for air-based plants, is to preheat the feed gas (and air) either directly or indirectly. Indirect pre-heat is less efficient thermally and requires the use of direct-fired heaters, or gas to gas heat exchangers if an external source of heat is available. If the heat required for preheat is taken from the Claus unit itself, less steam is available for amine stripping and difficulties can arise during start-up. In either case, preheating is energy intensive and has a significant effect on the cost and the complexity of the Claus unit. Furthermore, as highlighted in a paper by Chen[1], the heat required to preheat the air and / or acid gas streams tends to increase operating costs of the plant. If preheat is to be retrofitted to an existing burner / furnace configuration, the increase in gas volume to the burner may lead to mixing and pressure drop issues. Adding natural gas to the Claus feed is more attractive from an efficiency point of view, but this can significantly increase the size and cost of the Claus unit. If the added methane is not effectively burnt it too can add to problems with carbon deposition. The reducing nature of the furnace also tends to result in higher CO and COS concentrations due to incomplete combustion of the methane if added directly to the feed acid gas. Oxygen enrichment or total replacement of the combustion air with oxygen is an elegant and cost effective solution to the BTX problem and this paper will concentrate on illustrating the benefits and potential cost savings that can be achieved. It will also describe the programme of test work carried out by BOC on it’s 4 ton per day Claus pilot plant to collect the data which forms the basis of the evaluation work. 1
  2. 2. BOC SURE™ PROCESSES BOC began developing oxygen using burners and processes for application in Claus units in the mid 80’s. Much of this work was aimed initially at the refining Claus market, but its applicability to gas recovery Claus was always recognised. BOC offers a complete range of options from simple enrichment, where oxygen is added to the combustion air before it reaches the burner, through burner replacement, which allows higher enrichment levels and mixes oxygen directly with feed gas at the burner tip, to pure oxygen using processes. In the latter case, BOC’s Double Combustion process allows pure oxygen use even on high combustible content refinery feeds. Three plants now operate successfully with pure oxygen. Since the feed streams derived from most gas recovery operations (incl. POX) contain less than 60% H2S by volume (often much less) pure oxygen can be used without process modifications such as Double Combustion. The parts of BOC’s SURE™ portfolio most applicable to gas operation therefore are the SURE™ burner design and related technologies that can have a significant effect on BTX destruction. BOC SURE™ BURNERS Burner development is regarded as a core competency within BOC. Our experience has been built up over many decades and in a wide range of user industries. The tip-mixed design was chosen because of its safety and flexibility and because it generally has a much lower pressure drop than the pre-mixed alternative. As the name implies, oxygen (and air in the case of lower levels of enrichment) does not mix with the acid gas until they leave the burner tip. Keeping the feed gas, oxygen (and air) apart in this way, facilitates the creation of zones of different temperature and stoichiometry within the flame. In this way, the flame is used as a chemical reactor and the burner is staged to promote the beneficial reactions that aid the destruction of unwanted contaminants such as BTX. It should be noted that, within the flame, localised temperatures in excess of 2000oC might exist. This is achieved without any risk to the furnace refractory or any other part of the Claus plant. Since the BTX destruction reactions are kinetically limited, higher localised temperatures result in much greater destruction rates. Burner-furnace matching is of major importance and in order to evaluate this fully, BOC has developed a three-dimensional kinetic computational fluid dynamics (CFD) model of the Claus furnace. This model has been fully validated with data from BOC’s 4TPD pilot plant, from commercial installations using SURE™ technology and from small-scale laminar flow reactor work where applicable. BOC’s burner and process technology has been in commercial use for approximately 12 years and has proved highly successful. Burners are designed and produced to exacting standards in order to meet the requirements for oxygen use and to satisfy the design standards of the user industry. The burners have been developed to ensure optimum performance no matter what the feed composition may be. These designs have been obtained after extensive research examining the key parameters associated with good contaminant destruction and burner / furnace operation within the refinery and gas plant industries. To do this, a purpose built Claus burner development facility was constructed, a brief description of which follows. BOC Burner development facility The burner development facility comprises a commercial-scale 4TPD Claus pilot plant with one thermal and one catalytic stage (see Figure 1). Based on the former Courtaulds site in Trafford Park, Manchester, UK, this unit was operated for a period of three years. The plant was equipped with sophisticated in-furnace sampling and gas temperature measuring devices (see Figure 2), some of which were developed by BOC specifically for this application. The plant was able to simulate virtually any feed stream including high ammonia and BTX contaminated options. The programme looked at the performance of all SURETM burner designs and the complete range of oxygen use up to 100%. The work on the pilot facility was supplemented by additional small- 2
  3. 3. scale laminar flow reactor work carried out both within BOC and by external R&D organisations. Further information relating to the facility is available from Graville et al. [2-5]. Realising that the burner requirements may be different between gas plant and refinery operations, a programme of work specifically geared to examine BTX destruction was undertaken. This consisted of a range of operating conditions that included: varying BTX and H2S concentrations in the feed gas to the burner varying levels of oxygen enrichment natural gas addition as an alternative and in combination with oxygen enrichment varying the burner design and modes of operation Figure 1 BOC SURE™ burner development facility Figure 2 Gas sampling from within the reaction furnace 3
  4. 4. Full furnace profiles of the different species within the furnace were measured using gas chromatography. Corresponding temperature measurements were taken with in situ thermocouples protruding into the furnace flow area, refractory thermocouples, sacrificial thermocouples and suction pyrometry. This data was then used to help develop and validate a kinetic CFD model that is described below. Using this model, burner / furnace matching can be performed and process optimisation techniques examined and implemented. Claus reaction furnace models Many models used in the design of Claus units rely upon equilibrium assumptions to represent the chemistry occurring within the reaction furnace. This is adequate if the species being examined are not kinetically limited and one can assume the reactants to be perfectly mixed. In instances where kinetic limitations predominate, such as ammonia and BTX destruction, the equilibrium assumption is erroneous and will often predict far greater destruction levels than actually occurs within the furnace. Some models try to overcome this through empirical relationships taken from commercial plant however, due to the large number of variables between different plants, this too can give misleading results. An alternative to relying purely on equilibrium assumptions is to incorporate kinetic expressions for those species whose reactions are deemed to be kinetically limited within the reaction furnace. Unfortunately, kinetics alone are not sufficient to obtain an accurate picture of the reaction furnace chemistry. In many processes, especially combustion, turbulence and mixing play an important role in the overall reaction pathways undertaken. Therefore, both the kinetics and mixing properties need to be combined and solved simultaneously. BOC has, over the last twelve years, developed a 3D model of the reaction furnace and associated chemistry [6]. The model uses a modified computational fluid dynamics code (CFD). Using this code, the chemistry, thermodynamics, fluid flow, turbulence and radiation can be solved simultaneously. The solution process is complex and requires relatively powerful computers to obtain a solution in a timely manner. Having obtained a solution however, a great deal of information can be extracted from the model. One caveat with this type of model is that the results are only as good as the various sub-models used to describe the Claus process. To this end, a great deal of effort has been expended ensuring that these are accurate; the sub-models for the furnace chemistry are, by necessity, particularly complex. As far as BTX is concerned, those reactions associated with its destruction have appended an already complex set of chemical reactions used to represent the sulfur chemistry. BTX chemistry relevant to this work is discussed below. BTX chemistry The reaction pathways associated with the aromatic components within the reaction furnace have been the issues of some debate within the industry. Since oxygen is present, the assumption that the hydrocarbons are oxidised to CO2 is often thought to prevail. However, experience shows that invariably the oxygen requirement is less than stoichiometric for the amount of hydrocarbon present leading to incomplete or partial oxidation. Furthermore, in the flame region of the furnace, the H2, H2S and reactive flame radicals are competing for the available oxygen. As such, the relatively large aromatic molecules with associated large bond energies tend to fair badly in the battle for oxygen resulting in a different set of reactions occurring to those of hydrocarbon oxidation. An investigation was therefore conducted examining the possible reaction pathways for these higher hydrocarbon species. Experiments detailing higher hydrocarbon decomposition reaction rates are well documented [7-10] and various reaction pathways have been proposed. Figure 3 illustrates an initial decomposition step for p- xylene derived from Hippler et al.[8]. In this scheme, the initial decomposition of p-xylene is initiated through collision with another molecule (M) within the system. This either leads to a methyl group being removed from the ring or, more likely, the removal of an H atom. Once initiated, toluene is eventually formed from 4-methylphenyl reactions. In a similar manner, toluene thermally decomposes to benzene [9]. 4
  5. 5. CH2 CH2 H3C CH3 + H H2 + CH3 M H CH 3 CH 3 M M CH 2 CH3 H3C CH3 CH3 M CH2C 6H4CH2 + H + M + CH 3 CH4 + H3C CH 3 CH3 CH 3 M CH3 CH2 H, M + CH3 Figure 3 Thermal decomposition of p-xylene through 4-methylphenyl to toluene Examination of the various reaction enthalpies of the dissociation channels for the larger hydrocarbon molecules, shows that xylene tends to have lower reaction enthalpies than toluene (Table 1). Toluene, in turn, has lower reaction enthalpies than those associated with the relatively stable benzene ring. As a consequence, once sufficient energy is supplied to decompose xylene, benzene tends to be formed relatively quickly; in the experimental programme of work undertaken, in furnace sampling rarely showed any species other than benzene downstream of the main flame reaction front. The deactivation potential varies with different hydrocarbon components; In catalyst deactivation experiments, Cravier et al.[11] found that xylene and toluene were particularly efficient at deactivating Claus catalysts with xylene being capable of significant catalyst deactivation within a matter of hours. In commercial units where catalyst deactivation occurs over a relative long time, the main contributor is likely to be benzene [12]. Where extremely rapid deactivation of the catalyst occurs, toluene and xylene would be the main contributors however, such events would normally be associated with ‘non-standard’ operation. Parent molecule Reaction products ∆H298/kJmol-1 p-xylene p-methyl-benzyl + H 353.2 C7H7+ CH3 423.8 toluene benzyl + H 356.1 phenyl + CH3 426.4 benzene phenyl + H 464.2 Table 1 Reaction enthalpies for aromatic hydrocarbon dissociation channels Using fundamental chemical kinetic packages, the break-up of the large aromatic rings and the associated kinetics have been investigated. Using this information, sensitivity studies examining the key reaction pathways were conducted and a series of key reaction steps determined. Before these could be incorporated into the CFD model however, the inclusion and interaction with sulfur chemistry was required. This work has therefore examined and included CS2, S2, H2 and COS chemistry. COS and CS2 The chemistry associated with CS2 and COS alone is particularly complex as is illustrated in Figure 4. In this case, the main source of CS2 comes from reaction of methane with sulfur species in the reactor. In 5
  6. 6. the case of BTX, the methyl radical serves as the main precursor to CS2, this being produced during the initial decomposition of the larger aromatic structures. COS + CO + SO2 S2 CH4 + 2S2 CS2 + CO2 2COS CO + 0.5S2 H2O SO2 H2S COS + H2S COS + S2O CO2 H2O SO2 COS + H2O CO2 + H 2S CO2 + S2O CO2 + S3 CS2 S3 CO2 S3 CO2 + H 2 CO + H 2O H2 + 0.5S2 COS + S3 3S2 Figure 4 Reaction pathways for COS and CS2 chemistry (adapted from Clark et al.[13,14]) In addition to the main stable species that are shown in the above scheme, many flame-generated radicals are also present and these play a key role in the overall formation and destruction of each stable species. Such radicals include H, SH, S, OH, CH, CH2, CH3 and SO. To include the effect of all species, a large kinetic model containing over 250 reactions was used. This large scheme was checked with experimental work available within the literature and external and internal bench scale experiments. For complexity reasons, it is prohibitive to model all of the reactions present in the fundamental kinetic model within the CFD model. Further studies determining the key reaction pathways were repeated and a simplified model of the reaction chemistry suitable for use within the CFD model was derived. Using this global kinetic model, a comparison of BTX destruction at two different temperatures is shown in Figure 5. Here, the top graph represents the system at 1000oC whereas the lower represents the same system at 1100oC. Xylene initially present reacts to form toluene and benzene. The toluene curve (inverted triangles) shows an initial formation peak and subsequent destruction to benzene and other products. Benzene formed is destroyed albeit at a slower rate than toluene. This figure clearly illustrates that a 100oC change in operating temperature has a marked effect on the rates of BTX destruction; benzene and toluene are still present after 2s residence time. This sensitivity to temperature and residence time illustrates why Claus furnace models relying on equilibrium assumptions often have errors in their BTX representation. 6
  7. 7. Figure 5 Comparison of BTX destruction at 1000oC and 1100oC The reduced reaction scheme used in the simulation above has been incorporated into the CFD model to enable the mixing and fluid dynamics to be coupled with the chemistry. An overview of this is shown in Figure 6. 7
  8. 8. CH 3 CH3 M,H,CHx M,H,CHx H2O CO + H 2 CH 3 Sulphur chemistry model Hy drocarbon S2 H2O, SO2 Pool CS2 COS Figure 6 BTX chemistry and tie-in to existing CFD sulfur model Results from the BOC burner test facility, described earlier, have been used to validate the BTX CFD model. The overall technique adopted ensures that the chemical models used are validated at each step from fundamental level through to the global reaction pathways incorporated in the CFD. The next section describes some of the CFD validation work whilst illustrating the importance of good burner design and the effect this can have on BTX destruction. Validation of the CFD model Figure 7 Illustration of 3D section of reaction furnace showing burner, hot flame region and inner refractory temperature variation A full 3D simulation of the BOC burner development facility reaction furnace was performed for a gas stream comprising 40% H2S, 59.3% CO2 and 0.7% xylene firing through a burner using 100% oxygen as the oxidant. The inside furnace refractory temperature, hot gas zone and burner location are illustrated in Figure 7. For this particular firing configuration, acid gas passes through and around the acid gas burner. Pure oxygen, fed through the central regions of the burner reacts rapidly with the combustible species liberating thermal energy. With the BTX chemistry and sulfur sub-models in place, the predicted temperature, H2S and SO2 variation within the furnace obtained are shown in Figure 8. The figures here represent a slice through the middle of the reaction furnace. 8
  9. 9. Figure 8 Predicted contours of temperature, sulfur dioxide and hydrogen sufide Intense combustion regions result in a relatively short flame with peak temperatures around 1900oC. In this figure, the red end of the scale depicts higher temperatures and concentrations. Oxidation of the sulfur species initially liberates SO2 the profiles of which follow maximum temperature relatively closely. The hottest region of the flame consumes virtually all of the H2S, that which is not oxidised tends to be dissociated to H2 and associated species. Downstream of the main reaction zone, the temperature drops fairly quickly to give bulk gas temperatures of around 1050oC, which agree well with experimental observations. Using the model, it is possible to confirm that refractory temperatures throughout the furnace remain below the material temperature limits. This is not as great a concern for gas plant feed streams owing to the low H2S concentrations and high CO2 levels in the feed. In this example, the high temperature region of the flame is shrouded by a staged feed gas stream, which ensures the refractory temperature remains well within material design limits. In Figure 9, xylene, toluene and benzene concentrations are depicted together for the same case. The initial break-up of the xylene molecules leads to some formation of toluene part of which subsequently forms benzene. As far as kinetics are concerned, the xylene destruction is faster than toluene and benzene, complete destruction occurring before the first sampling port of the test facility. Benzene takes longer to be fully removed owing to the stability of the benzene ring. The scale in this figure is not common but serves merely to illustrate the high and low concentration regions within the furnace. In this example, the high temperatures associated with the flame region give very high BTX destruction rates. Even though xylene is depicted to pass around some of the flame, there is sufficient temperature, mixing and time for the key reactions to proceed. Even with a relatively low bulk gas temperature of 1050oC, the high levels of feed xylene (7000ppmv) are completely destroyed using pure oxygen whilst ensuring refractory temperatures remain well with design limitations. 9
  10. 10. Figure 9 Decomposition of xylene to toluene and subsequent formation and destruction of benzene Figure 10 illustrates a comparison between predicted and measured benzene concentrations for this case. Benzene is illustrated here since it was the only component measured within the furnace at the sampling locations. The burner is located at the left end of the figure. Considering the complexity of the BTX chemistry and sampling and analysis errors, the model gives good agreement with measured values. The lower plot, depicting the predicted values, is slightly conservative with respect to benzene destruction, which is favourable for design purposes. The figure illustrates that complete BTX destruction is obtained. Figure 10 Comparison between measured (top) and predicted (bottom) levels of benzene (ppmv) 10
  11. 11. The results from the model described above provide a large amount of data, far more than is presented here. Using this CFD model different burner designs and operating conditions have been examined and the effects and dependencies these have, within the Claus reaction furnace, have been determined. With a validated model, this can be done without necessarily having to build and test each burner design. The next section illustrates an example where the CFD model is used to examine some of the differences between general oxygen enrichment and specific oxygen enrichment through a purpose-built burner. Oxygen enrichment options At levels below 28%v/v, oxygen addition to the reaction furnace can be performed through either general enrichment, lancing or a purpose built burner. Above 28%v/v, oxygen compatibility requires different materials to be used and purpose built oxygen equipment is normally supplied. Oxygen requirement is normally determined through process modelling of the type discussed previously. Due to the nature of this analysis tool, only general enrichment can be considered. Using the CFD model however, different burner designs and firing configurations can be examined and the best solution determined. In this example, the same burner is fired in two different modes. The feed stream is the same as the previous example i.e. 40% H2S, 59.3% CO2 and 0.7% xylene. In this case however, oxygen enrichment to 28%v/v is utilised as opposed to 100% in the previous example. In dropping from 100% oxygen to 28%, the difference is made up with nitrogen from air and as a consequence, the furnace operating temperature will drop from the previous example. The two firing options considered are: 1. GENERAL: General enrichment of the air fed to burner to a level of 28%v/v oxygen 2. SPECIFIC: Pure oxygen feed to the burner gun with separate air being fed to an air annulus. The quantity of oxygen used is the same as case 1 thus giving the 28%v/v overall oxygen levels. Figure 11 Comparison of temperature profiles for the two firing configurations. Top = general enrichment. Bottom = Pure oxygen addition to same level of overall enrichment Figure 11 illustrates that, with the SPECIFIC mode of operation, the maximum gas temperature is higher than the GENERAL case by approximately 300oC. As a consequence of this, the maximum rates of BTX destruction are higher. Bulk gas temperatures on the other hand, are virtually the same. Figure 12 depicts the xylene destruction for the two configurations. The general enrichment case obtains approximately 81% destruction of the feed xylene whereas the specific enrichment case achieves 94% destruction. Although in each case, xylene breakthrough would occur, the concentrations are less with the purpose-built burner. Bulk gas temperatures are ~980oC and one would normally expect some HC 11
  12. 12. breakthrough at these temperatures and feed concentrations. In practise, an increase in oxygen enrichment would be sufficient to destroy all of the BTX. Through using CFD modelling and pilot plant testing, BOC has been able to design SURE™ burners that are optimised for BTX destruction. As such, the amount of required oxygen to achieve complete destruction is less than is required for general enrichment. In a similar manner, a purpose built burner offers the same benefits over oxygen lancing for higher levels of oxygen enrichment. The decision as to whether to use oxygen or not should not therefore be based solely on the oxygen requirement predicted by industry standard process simulations since these only account for the general enrichment option. With correctly design equipment, oxygen usage can be less and a reduction in operating costs can be gained. Figure 12 Comparison of xylene concentrations for the two different firing configurations The economics of oxygen use Oxygen use within Claus units treating gas plant acid gases offer two main advantages. The first is the ability to completely destroy BTX without the need for natural gas addition or preheat. The second is the opportunity to increase the plant throughput due to the removal of diluent nitrogen associated with the air stream. The benefits of this can either be realised through debottlenecking existing plants or reduced plant size for grass roots units. Since the concentrations of H2S are relatively low in gas plants, higher levels of oxygen enrichment are achievable within the single furnace without furnace overheating. For materials reasons, oxygen enrichment above 28% v/v would require dedicated oxygen equipment in these instances. In cases where the acid gas is reasonably strong, in excess of 70% H2S, alternative technologies, such as BOC’s Double Combustion, can be used to control the higher associated temperatures. For grass-roots installations of equivalent throughput, there are capital, space and complexity savings to be made with a pure oxygen system over an air-based unit. This is mainly as a result of the removal of air associated nitrogen from the system; an oxygen-based plant treating a 40% H2S feed would be approximately 55% of the normal size of an equivalent air based unit. In cases where additional fuel gas and air are required for BTX destruction purposes, the size of the air-based unit is necessarily larger and the oxygen-based unit can be significantly less than 50% of the air-based plant. In addition to the above- mentioned savings, the main air blower would not be required since the air separation plant would provide oxygen already compressed to the required pressure. A small packaged air blower could be used for start-up conditions. The use of oxygen also enables catalyst savings to be made owing to the smaller sized units. This is of additional benefit in plants using proprietary catalysts. 12
  13. 13. In spite of the on cost of oxygen, the operating costs can also be lower than that of an equivalent air- based plant as shown in Table 2. Three options for achieving the required temperature of 1100oC have been evaluated: preheat acid gas and air up to 500oC add fuel gas into feed to reaction furnace pure oxygen feed Net requirement Preheat Fuel gas Oxygen Fuel gas 100% 95% 25% (Heating/RGG/incinerator) Electrical 100% 123% 16% (Air fans) Net production Preheat Fuel gas Oxygen Steam LP 3.5 Barg 100% 122% 83% (Condensers/coolers) Steam MP 23 Barg 100% 98% 65% (WHB/reheaters) Table 2 Operating costs for different process options The fuel gas and power requirements are very much larger for the fuel gas and preheat processes when compared to oxygen usage. The flip side of the coin is that there is a net increase in steam production for the preheat and fuel gas processes. However, the value of the steam will be limited by the cost of providing boiler feed water (typically BFW costs are a third of the value for steam production) and the demand for a steam supply on site. The major operating benefit using pure oxygen is seen in fuel gas (natural gas) savings. Obviously the cost of fuel gas determines the relative savings in operating costs achieved. In order to provide the oxygen for BTX destruction, an air separation unit (ASU) has been assumed as the supply. If the power requirements of the ASU to produce the required oxygen can be met with integrated power generation, then the cost of oxygen supply can be reduced by approximately 50% further. In order to do this, an equivalent natural gas requirement of 25% of the preheat case, would be required as a fuel source. For a grass-roots system, both capital and operating cost savings can lead to oxygen being an economically attractive alternative to other technologies. In the instance of retrofitted oxygen supply to destroy BTX, the economics become more dependent upon local gas prices and the local cost of oxygen which in turn is dependent upon demand requirements. Conclusions • An extensive programme of work has been conducted aimed at improving the understanding of Claus plant operation and the controlling parameters for BTX destruction. A greater understanding of the chemistry associated with BTX and sulfur has been achieved and this work has resulted in a kinetic model that has been incorporated into a customised CFD package. Through extensive testing over a range of different operating conditions, the model has been validated on a commercial scale pilot facility. • In conjunction with pilot plant experiments, modelling work has examined and optimised SURE™ burner designs specifically for BTX destruction. The design of burners enables more efficient contaminant destruction than can be achieved using general enrichment and lancing techniques to the same level of enrichment. Consequentially, operating costs can be less when SURE™ equipment is used. • Converting a gas plant sulfur recovery unit to oxygen can provide an additional 80% increase in capacity at a marginal capital cost. The added benefits of more reliable operation with good contaminant destruction can also be attained. • In addition, to the capital savings, a significant reduction in energy consumption can be achieved. Virtually all the electrical power cost would be incorporated in the oxygen cost (for over the fence supply). Additional energy would not be required for pre-heating and the fuel gas supply for the 13
  14. 14. incinerator would be greatly reduced. In these instances, the cost of oxygen can be offset against these operating cost savings making oxygen enrichment an attractive alternative to other technologies. References 1. Chen, J.K. ‘Processing Lean Acid Gas in Sulfur Plants’, In proceedings of Brimstone Sulfur Recovery Conference, Canmore, Canada, 2001. 2. Graville, S.R. and Watson, D. ‘BOC Burner Development Technology’, In: Proceedings of the Brimstone Sulfur Recovery Symposium, Vail, US, 1997. 3. Graville, S.R. and Watson, D. ‘Optimising the use of Oxygen in Claus Plants’, In: Proceedings of the Brimstone Sulfur Recovery Symposium, Canmore, Canada, 2001. 4. Graville, S.R., Norman, J.S. and Watson, D. ‘Claus Plant Reaction Furnace: Misconceptions’, In: Proceedings of the Brimstone Sulfur Recovery Symposium, Vail, US, 1998. 5. Graville, S.R., Norman, J.S. and Watson, D. ‘Contaminant destruction using the BOC SURETM burner’, In: Proceedings of the Brimstone Sulfur Recovery Symposium, Vail, US, 1999. 6. Norman, J. S. and Watson, R. W. ‘ Claus Reaction Furnace Modelling’ Sulfur, pp43-51, August 1999. 7. Kern, R.D., Wu, C.H., Skinner, G.B., Rao, V.S., Keifer, J.H., Towers, J.A. and Mizerka, L.J. ‘Collaborative shock tube studies of benzene pyrolysis’, In: Twentieth Symposium (International) on Combustion, The Combustion Institute, pp789-797, 1984. 8. Hippler, H., Seisel, S. and Troe, J. ‘Pyrolysis of p-xylene and of 4-methylbenzyl radicals’ In: Twenty Fifth Symposium (International) on Combustion, The Combustion Institute, pp875-882, 1994. 9. Brouwer, L., Muller-Markgraf, W. and Troe, J. ‘Identification of primary reaction products in the thermal decomposition of aromatic hydrocarbons’ In: Twentieth Symposium (International) on Combustion, The Combustion Institute, pp799-806, 1984. 10. Muller-Markgraf, W. and Troe, J. ‘Shock wave study of benzyl UV absorption spectra: Revised toluene and benzyle decomposition rates’, In: Twenty-first Symposium (International) on Combustion, The Combustion Institute, pp815-823, 1986. 11. Crevier, P.P, Clark, P.D., Dowling, N. I. and Huang, M. ‘Quantifying the effect of individual aromatic contaminants on a Claus Catalyst’, In: Saudi Aramco Journal of Technology, pp46-54, 2001. 12. Klint, B. ‘Hydrocarbon destruction in the Claus SRU reaction furnace’, In: Proceedings of the Laurance Reid Gas Conditioning Conference, Norman, Oklahoma, US, Feb. 2000. 13. Clark, P.D., Dowling, N.I. and Huang, M. ‘Mechanisms of CO and COS formation in the Claus furnace’, ASRL quarterly bulletin, July-Sept. 1999. 14. Clark, P.D., Dowling, N.I., Huang, M., Cooper, J. and Butlin, G. ‘The chemistry of sulfur recovery by the Claus process’, ASRL quarterly bulletin, Oct-Dec. 1998. 14