THE DEVELOPMENT OF A SOLID SULPHUR BURNER AS AN ALTERNATIVE FOR THE PRODUCTION OF SO 2 FOR USE IN A SULPHURIC ACID PLANTS

Transcription

1 THE DEVELOPMENT OF A SOLID SULPHUR BURNER AS AN ALTERNATIVE FOR THE PRODUCTION OF SO 2 FOR USE IN A SULPHURIC ACID PLANTS Werner Vorster TENOVA MINING & MINERALS 58 Emerald Parkway Road Greenstone Hill, 1609 Johannesburg South Africa Prepared for 37th Annual International Phosphate Fertilizer & Sulfuric Acid Technology Conference American Institute of Chemical Engineers Central Florida Section American Institute of Chemical Engineers, 4798 S Florida Ave # 253 Lakeland, FL USA April 2013

2 TABLE OF CONTENTS ABSTRACT... 1 INTRODUCTION... 1 Current practise overview... 1 Problems with current practise... 2 DESIGN, DEVELOPMENT AND FABRICATION... 6 TEST WORK RESULTS... 9 WHAT ABOUT GAS CLEANING? ECONOMIC COMPARISON CURRENT STATUS AND FUTURE WORK CONCLUSION... 13

3 ABSTRACT Typical sulphur burning acid plants (not having a ready supply of liquid sulphur from a refinery or elsewhere) use dry sulphur in prills or pastilles to produce sulphuric acid. The dry prills are melted in a melting circuit using low pressure steam prior to combustion in the sulphur furnace. A typical sulphur melting plant, especially in rather remote areas with less skilled operators, is a very unpleasant and dirty section of the plant with sulphur and SO 2 fumes, sulphur foaming, severe corrosion and, depending on the source of the sulphur, possible H 2 S emissions which present an SHE risk. Tenova Minerals (previously Bateman) developed a concept whereby solid sulphur is directly combusted and to this end did some FEM modelling of flow through the burner to mathematically determine the residence time. A small pilot unit was constructed and tested at low rates. This paper provides an overview of the results obtained as well as the current status of the project. INTRODUCTION Current practise overview Sulphur burning sulphuric acid plants, by definition, uses sulphur as a feedstock into the sulphuric acid facilities. Depending on the location of the plant and availability of the sulphur, the sulphur can be in liquid form (for those close to refineries) or in remote locations depending on sulphur imports, in prill or pastille form : Figure 1 : Sulphur prills and pastilles used as feedstock (photos from Google) For plants using solid sulphur as feedstock the first step is to melt and filter the sulphur to produce a neutral, clean liquid sulphur for combustion in the sulphur furnace. Sulphur usually contains approximately 400 ppm ash but the addition of lime and pre-coat increases the particulate content to in excess of 2,000 ppm. Although the melting process is easy with no major chemical, thermodynamic or kinematic hurdles to overcome, it generally is not a pleasant environment to work in. Basically, a melting circuit comprises the following main components and is shown schematically in Figure 2. Bulk sulphur storage (preferably under roof) Melting and neutralisation Precoat and Filtration Storage 1

4 Figure 2 : Schematic of a melting circuit up to the sulphur furnace (NOT ALL PROCESS STEPS SHOWN) The melting process is simple and well understood, having been implemented for many years. Solid sulphur melts at approximately 120 C (248 F). The melting is accomplished in concrete pits or above ground, carbon steel, brick-lined tanks with carbon steel heating coils. Saturated stream at approximately 6 bar and 158 C (316 F and 87psi) is used as energy source. Neutralisation (required as a result of free acid formation in the sulphur during transport and storage) is usually accomplished with hydrated lime. Gypsum is removed in the filtration stage with other impurities (ash, dust etc) present in the bulk sulphur. A polishing stage may be necessary depending on the particulate content and filtration rates required. Pre-coating of the filters improves filtration but adds an additional cost and complexity to the operation. The filter cake from the filtration stage is a source of a continuous loss of sulphur (filter cake contains approximately 60% sulphur). Once the sulphur has been melted, neutralised and filtered it is stored in large storage vessels prior to pumping to the sulphur furnace for combustion. Problems with current practise At the face of it, the process is simple, well understood and without problems. However, in reality there are many pitfalls, some of which are listed and briefly described below. Sulphur temperature control Sulphur has the peculiar property that the viscosity changes dramatically with temperature. At approximately 158 C (316 F) the viscosity increases as shown in Figure 3. 2

5 Figure 3 : Sulphur viscosity temperature dependence ( Proper agitation is required for the melting process to ensure good heat transfer. Increases in temperature (and pressure) of the steam, can cause the sulphur to become extremely viscous and impossible to pump or agitate. Similarly, in the jacketed sulphur transfer lines, temperature control is important to ensure the sulphur does not solidify or approach the point of minimum viscosity too closely. Should the steam supply to the melting circuit fail for a prolonged period of time, there is a danger that the sulphur in the tanks and pits will solidify. This would either need to be remelted (unless the steam coils have been damaged) or physically broken out and removed. Moisture Moisture content in the sulphur feed has three main effects on the sulphur melting circuit. Firstly, it greatly increases the energy consumption of the melting process as additional water now needs to be evaporated. The energy required to take the temperature of solid sulphur from 30 C to that of liquid sulphur at 140 C is 155 kj/kg S. An additional 26 kj/kg S is required for the evaporation of every additional 1% water in the sulphur feed. This can significantly affect the steam consumption in the melting plant. Secondly, sulphur with a moisture content above 4% frequently causes frothing at the sulphurair interface which could affect instruments, cause blockages etc. An example of solidified froth on a radar instrument is shown in Figure 4. In addition, moisture in the sulphur, especially if there are fines present in the feed, cause scale build up on conveyors, chutes and feeders (Figure 5). 3

6 Figure 4 : Solidified sulphur froth on radar level instrument Figure 5 : Sulphur scale built up Ash, gypsum and dirt Residual ash and dirt from transport and storage have to be removed prior to combustion. If this is not removed properly, the solid impurities will be caught in the first catalyst bed of the converter causing an increased pressured drop. This will decrease the performance of the plant. In addition the waste heat boiler is affected by reducing the overall heat transfer thereby decreasing the acid plant capacity. In addition, neutralisation by-products need to be removed from the sulphur. These impurities are removed in the sulphur filters. However, the resultant filter cake still contains up to 60% sulphur which is not easily recovered and generally disposed of as a waste product. 4

7 Piping congestion Due to the number of pipes (steam, condensate, air, sulphur) that is required for the sulphur melting circuit, the areas surrounding the equipment is generally extremely congested as shown in Figure 6. This makes maintenance especially challenging and has implications when equipment has to be replaced when it reaches end of life or if there was some major failure. Figure 6 : Sulphur melting circuit piping Corrosion As a result of the residual acid in the sulphur formed during shipping and storage on site (especially in wet climates) and more often than not, incorrect neutralisation practise or measurement of acidity in the sulphur, corrosion is a problem in the melting circuit. H 2 S H 2 S will be present in the sulphur as the bulk of sulphur supplied originates from Claus plants. The H 2 S can form a polymeric complex with sulphur as the sulphur vapour condenses : H 2 S + S 8 H 2 S x Higher temperatures and pressures favour H 2 S x whereas lower temperatures favour the formation of H 2 S. Thus : during shipment the equilibrium shifts to H 2 S which may be released and is hazardous. The polymeric H 2 S x complex also has an effect on the viscosity of liquid sulphur. There will also be traces of SO 2 present in Claus plant sulphur. H 2 S is lethal at concentrations above 500 ppm. The USA OSHA exposure limit for H 2 S for 8 hours per day, 5 days per week is 15 ppm. The presence of H 2 S in the sulphur should not be under-estimated and at least considered and addressed during the HAZOP. Work environment The melting circuit is probably the area where most operator and maintenance personnel will be used in the acid plant. The environment is not pleasant to work in : dirty (usually), smelly (SO 2, H 2 S), hot (steam, liquid sulphur) and congested (shown in Figure 7). 5

8 Figure 7 : Sulphur melting circuit work environment Based on the wide range of factors and difficulties outlined above, the question must be asked whether there is a better way to produce SO 2 to reduce risk and complexity as well as to improve the operating environment. DESIGN, DEVELOPMENT AND FABRICATION Tenova believes that there are significant opportunities to optimise the current practise by eliminating and replacing equipment and simplifying the overall process by reducing the number of process steps. The process optimisation involves direct combustion of solid sulphur to form SO 2 without the need for melting. The proposed system is shown in Figure 8. 6

9 Figure 8 : Proposed system for combustion of solid sulphur Solid sulphur, in prill or pastille form (at the moment) will be pneumatically fed to the bottom of a vertical burner chamber where it combusts to form SO 2. The motion of the gas in the vessel will be helically upward as a result of the burner and secondary air inlet and tangential gas outlet. The main reason for the required flow pattern of the gas is to increase turbulence and residency time. Hot gas flows through a water tube boiler and subsequently to the hot gas filter for the removal of particulates prior to entering the converter. A 3D model was developed of a vertical reaction vessel with central, bottom air and sulphur inlet and tangential secondary air inlets doubling as start-up heaters. Once the 3D model was completed the flow of a typical 2-4mm spherical solid sulphur particle through the burner was evaluated (as described below). The original intent was for a total residence time in the vessel of around 1.5s which is a typical design figure for liquid sulphur furnaces and selected as the first test case for the solid burner. The specific approach adopted for the analysis of the sulphur combustion reactor was as follows: A solid model of the combustion reactor was imported from AUTOCAD through Solid Edge into ANSYS. The solid model was geometrically cleaned of unnecessary parts to prepare a fluid volume model for STAR-CD. Meshing of the volume was done in ANSYS with tetrahedral cells and the numerical model exported to STAR-CD. Inlet- and pressure boundaries and constant fluid properties of air were applied to the air-only analysis (Figure 9). No heat transfer and temperature distributions due to combustion were modelled. The oxidation reaction was not included in the evaluation. Sulphur particle density, diameter, flow rate and inlet position and distribution were applied to the coupled air- and sulphur particle numerical model. A steady-state solution was obtained in both analyses that represent no timedependant flow phenomena in the sulphur combustion reactor. 7

10 Figure 9 : Model of the solid combustion chamber Sulphur The finite element modelling revealed that design had achieved its two most important criteria (Figure 10) : Turbulent, helical flow through the reactor Residence time (of un-combusted particle) of 1.5s Figure 10 : Coupled air- and sulphur particle distribution / streamlines in the sulphur combustion reactor Having achieved these results, it was decided to build a small scale unit to test the hypothesis in practise without further modelling. 8

11 Figure 11 : Test reactor TEST WORK RESULTS A small reactor and feed system was built, shown in Figure 11. SO 2 outlet (310 SS) Sulp Bi Sulphur Burner Limestone scrubber (purely for off gas treatment at labscale tests) LPG burners for start-up Sulphur Inlet The intent of the test reactor was to prove the concept only. The initial runs were performed at approximately 10% of the design throughput and viewed through a sight-glass on top of the reactor. After start-up, some leakage of SO 2 did occur through the site glass which was not tightened and sealed properly. That was quickly resolved and testing resumed. Because the intent of the reactor was only to demonstrate the concept, the scrubber was only designed to quench the off gas and remove limited concentrations of SO 2 from the discharge stream. Therefore, as the feed rate of sulphur increased, gas slippage started 9

12 occurring resulting in SO 2 emissions to the environment forcing the experiment to be stopped without evaluating the system throughput. Key findings of the test work program are summarised below. The theoretical model of particle flow in terms of helical motion of the particles moving through the reactor was confirmed. Complete combustion of the particles prior to discharging from the reactor was achieved Very little adhesion of sulphur to the sides of the reactor occurred. Little or no adhesion of the sulphur to the inlet of the reactor occurred which indicated that the secondary inlet air fulfils its purpose of cooling the central air inlet The original intent of tying into an existing plant for continuous testing (under site conditions) at maximum rate has not happened as the partners that originally expressed an interest, withdrew from the project. No further test work after the proof of concept has been conducted as of yet. WHAT ABOUT GAS CLEANING? The feed gas stream to the converter need to be free of dust and particulate matter which will collect on the first bed of the catalyst. In the direct oxidation of the sulphur to sulphur dioxide where the melting-filtration step has been removed, impurities present in the sulphur will now report directly to the gas stream. Together with Sulphurnet (with whom Tenova has a cooperation agreement), Tenova has conceptualised that the hot off-gas from the combustion reactor will pass through a water tube boiler and subsequently to a LCK filter to remove the solids (Figure 12). The operational principle is that the gas flows through a venturi into the separator cell comprising closely spaced lamella. Solids impinge on the plates with ever decreasing gas flow rate and increasing solids content. The concentrated solids is removed from the main gas stream with a small bypass gas stream (~1%-10% of the gas flow rate) into a solids removal vessel where the solids settle out and the gas is returned to the main gas stream. Low pressure drop and high removal efficiencies (Figure 13) are possible with this type of filter. Added to the wide range of materials of construction this offers a unique solution to the removal of solids from the gas stream. 10

13 Figure 12 : Diagram of a LCK Filter Figure 13 : LCK filter removal efficiencies 11

14 ECONOMIC COMPARISON In order to compare the installed cost for a conventional sulphur burning plant with a melting circuit and the new design with the solid sulphur burner, a cost estimate was used for a 300 t/d sulphur burning acid plant. Considering only the mechanical and bulk equipment costs (Direct Fields Costs) the costs were split into the following components : Sulphur melting (mechanical only) Sulphur furnace Gas cleaning (where applicable) Acid Plant mechanicals Plant piping E&I, Refractory, Structural Steel To estimate the cost for the Solid Sulphur Burner Circuit, the approach below was adopted : The melting circuit cost was removed A LCK filter price was added The boiler estimate is already included in the acid plant equipment The remainder of the acid plant mechanical equipment stays the same Piping, E&I+Other was adjusted as follows : o piping about 50% less complex as there is significantly less sulphur piping, steam piping etc o approximately 25% reduction in the total for E&I, refractory and structural steel The effect of the reduced plant footprint on civil cost was not considered at this time. Based on these figures and assumptions, the solid sulphur burner plant (SSB) with LCK filter will be approximately 5% cheaper than the standard sulphur burning acid plant. Not a huge reduction in capital cost due to the significant cost contribution of the remainder of the plant and the cost of the gas filter. However, the benefits of the SSB system vs liquid sulphur is around operating conditions and costs : Less intricate design Reduced steam consumption i.e. more energy efficient Much reduced system complexity Reduced electricity consumption The bottom line is that the gas cleaning won t significantly increase costs and could lead to Capex reduction. The benefits on the Opex side is difficult to quantify at present but you are as a minimum looking at 50% less electric motors and ~5-10% more steam production (no steam required for melting). 12

15 CURRENT STATUS AND FUTURE WORK Currently (2013) work is planned to continue the testing in the laboratory after the installation of a scrubber unit to allow for testing at design throughputs in a laboratory environment. The existing scrubber will be used as a quench vessel only to reduce the gas temperature prior to a full sized scrubber. A LCK filter will be installed to test the efficiency of the solid removal. Once the scrubber and filter is in place, continuous, full flow rate tests can be conducted to investigate operability and highlight areas of further development. In parallel it is planned to combine the finite element analysis of the flow characteristics of the solid particles in the reactor with the oxidation reaction as a shrinking particle model to simulate the oxidation of the solid sulphur particles. In addition to further testing using pure sulphur it is planned to test the unit using a variety of sulphur sources inclusive of discarded waste sulphur filter cake. CONCLUSION Tenova developed a concept for the combustion of solid sulphur for the formation of sulphur dioxide. Preliminary tests show that the concept is viable and may even offer a slight capital cost benefit. However, the major benefits of this unit will be more on the operational side as the circuit design is less intricate, steam consumption is reduced, the system less complex and a reduction in electrical power consumption. The application of this technology to a variety of sulphur sources could prove beneficial to the smaller operators. Further work remains prior to the commercialisation of the unit. 13