Sulphuric acid process

 

CEAMAG is dealing in Sulphuric Acid projects through the partnership with Daniel LEFORT, one of the most experienced Experts having numerous references in design, full engineering, procurement of key equipment and catalysts, and eventually managed the Commissioning of recent plants at several design capacities.

CEAMAG is in position to propose a full range of production capacities, from small plants till 3000 tpd under several contractual forms from Project development, Feasibility studies, Basic Engineering and EPC with contracting Partners.

Process general aspects

The raw materials used for the production of sulfuric acid are sulfur, water, and oxygen from the ambient air. The sulfuric acid manufacture involves three basic chemical reactions and associated operations.

  • Production of sulfur dioxide gas;
  • Catalytic oxidation of sulfur dioxide to produce sulfur trioxide;
  • Absorption of sulfur trioxide in concentrated sulfuric acid.

All chemical reactions are exothermic. Therefore, the process for sulfuric acid manufacture is also a heat generation process and this heat is used for the production of superheated steam.

Production of Sulfur Dioxide Gas

Sulfur kept as a liquid at a temperature around 130°C/140°C, is pumped under pressure to burners where it is sprayed into a combustion chamber to be burnt with dry air.

As dried air is used, the sulfur dioxide gas produced is clean and free of moisture. The necessity of and the means to obtain dried air are described in a further paragraph.

Air and sulfur flows are adjusted to reach approximately 11.3% SO2 in the combustion gas, which corresponds, with combustion air at 60°C, to a temperature around 1060 / 1080°C at the furnace outlet.

A waste heat boiler cools down the gas to 410°C approximately before it flows to the conversion section.

Catalytic Oxidation of Sulfur Dioxide to Sulfur Trioxide

The exothermic oxidation of sulfur dioxide with oxygen from the air to form sulfur trioxide is according to the reaction:

2 SO2 + O2 ↔ 2 SO3

and is a reversible reaction that has to be catalyzed to proceed rapidly. The catalyst used is vanadium pentoxide.

All thermodynamic laws related to chemical equilibrium are applicable to this reaction and all the following encourage the formation of SO3:

  • Increase of pressure;
  • Excess of oxygen;
  • Removal of produced sulfur trioxide.

However, an increase in temperature reduces the rate of SO3 formation. Therefore, as this reaction is exothermic, the temperature of the gas mixture rises as the conversion of sulfur dioxide to sulfur trioxide proceeds, so that the conversion is limited to the one corresponding to the equilibrium temperature.

In order to achieve the highest conversion of sulfur dioxide to sulfur trioxide, it becomes be necessary to cool the partly converted gas mixture, each time its temperature approaches the equilibrium one. After cooling, the conversion of sulfur dioxide may continue in a further pass of the converter.

The number of conversion passes and their inlet / outlet temperatures are chosen in order to give the optimum plant design.

The heat resulting from the conversion of sulfur dioxide is removed between catalyst passes and used for steam production and for reheating gases returning back from the inter-absorption section.

The proposed sulfuric acid plant includes four catalyst passes operating with inlet temperatures ranging from 400 to 450°C, the maximum temperature at each pass outlet is limited to 625°C.

The conversion of sulfur dioxide to sulfur trioxide is achieved in three catalyst passes before the interpass absorption tower and in one final pass before the final absorption tower.

Cooling of the gas between converter passes takes place in different types of heat transfer equipment installed as indicated here below:

  • Pass I outlet: Superheater for superheating the produced high-pressure steam;
  • Pass II outlet: Gas heat exchanger for reheating the gas from the interpass absorption tower;
  • Pass III outlet: Gas heat exchanger (complementary to the one above) and one economizer;

Absorption of Sulfur Trioxide Gas

The sulfur trioxide gas produced during the catalytic oxidation of sulfur dioxide has to be reacted with water in order to produce sulfuric acid.

This reaction or absorption will take place in packed towers, where the sulfur trioxide reacts with the water of a 98,5% sulfuric acid. Inside these towers, the gas passes counter-currently to a flow of 98,5% acid. Sufficient acid flow is circulated through the tower so that the absorbed sulfur trioxide does not significantly increase the circulating acid concentration, which is restored to the required value by water addition.

The temperature of the strong acid circulated over the absorption towers increases, due to the heat of reaction and the sensible heat of the gas stream entering the towers. This heat is removed from the system via acid coolers and absorbed by the cooling water circuit.

The removal of sulfur trioxide will encourage further SO2 conversion. This is the basis of the so-called double absorption process, in which sulfur trioxide is removed by absorption from the gas after a certain quantity of sulfur trioxide has been converted. Compared to the single absorption, this process allows for a higher overall conversion efficiency (99.70%) even with inlet gases at a higher SO2 concentration (11.3%), meaning smaller gas volume and smaller equipment.

The gases leaving the third catalyst layer are sent to the intermediate absorption tower and, after having been reheated (by means of the heat of conversion) they flow back to the final conversion stage.

After final conversion of the remaining SO2, gases are sent to the final absorption tower, which is similar to the interabsorption one.

Mist eliminators installed in the top section of the absorption towers remove acid mists carried over together with gases and produced during the absorption process.

Air Drying

To avoid corrosion problems due to condensation of sulfuric acid vapor in plant equipment, conventional modern sulfuric acid plants use only dried air for sulfur burning.

Atmospheric air is received at the drying tower inlet and passes through the packing of this tower counter-currently to a flow of a strong sulfuric acid to remove water vapor from the air.

Sufficient acid flow is circulated over the tower to allow an efficient contact between air and acid. Mist eliminators installed in the top section of the drying tower remove acid mists carried over together with the air.

The heat of condensation of water vapor and the dilution heat raise the temperature of the circulating sulfuric acid over the drying tower. This energy is removed via an acid cooler and absorbed by the cooling water circuit.

Pumping and Cooling of Acid

The sulfuric acid that flows back from the three towers (DT, IPAT, and FAT) is mixed together in a common pump tank. The required process dilution water is introduced to keep the acid concentration close to 98.5%.

The resulting 98.5% acid is cooled in two (02) shell and tube type acid coolers. The first one cools the acid that will be fed to the Drying Tower, up to a temperature of 60ºC, and part of the acid to be fed to the Intermediate Absorption Tower. As the feed temperature of this tower is 80°C, the acid stream @ 60°C is mixed with the acid from the 060-TN-608 compartment so that the final temperature prior to tower feeding is 80°C. The second shell and tube type heat exchanger cools the acid to be fed to the final absorption tower up to 80°C.

The level of the pump tank is kept constant by the means of a control valve through which the produced acid is sent to the storage, after having been cooled down to 40°C approximately in a plate cooler.

Plant Description

Sulfur Unloading and Storage

The unloading system and the sulfur pile are located in the sulfur warehouse or in the outdoor sulfur area. The sulfur is discharged into the Sulfur Unloading Hopper T110.

From this hopper, sulfur is extracted by 03 (three) vibrating Sulfur Extractors W110 A/B/C onto a horizontal belt Sulfur Conveyor M110.

Two other Sulfur Conveyors M111 and M112 are used to unload sulfur onto the pile. The conveyor M112 is equipped with a mobile chute.

Air exhaust is achieved from the vibrating extractor area by means of the Dedusting Fan K110.

Sump pumps are foreseen to evacuate the possible liquid effluents containing sulfur particles from the hopper bottom section and at ground level near the pile. These effluents will be collected in the Sump Unloading Area Sumps T111 and T112 and sent to the chemical treatment system by the vertical Sulfur Unloading Area Sump Pumps P111 and P112.

Sulfur Melting and Filtration

The solid sulfur is transferred from the storage pile to the Sulfur Melting Feed Hopper T120 by a payloader.

From the hopper T120, the sulfur is transferred by the Sulfur Melting Feed Conveyor M120 to the Sulfur Melting Tank T122.

Addition of lime for neutralization of the sulfur acidity takes place after the Hydrated Lime Feed Hopper T121.

The conveyor M120 is driven by an electric motor with a frequency variator and has a walkway on both sides.

The intensive Sulfur Melting Tank T122 is equipped with the Sulfur Melting Tank Heaters E122 A/B/C/D. These heaters are dismountable heating coils fed with 7 bar g steam. Steam and condensate pipes to and from the coils are protected by a sleeve at the air / liquid sulfur interface.

A Sulfur Melting Agitator A122 is provided in the center of the tank T122 with its shaft protected with a sleeve at the air / liquid sulfur interface.

The tank T122 is lined with acid proof bricks at the air / liquid sulfur interface and its cover is made of several removable concrete slabs, one over each coil. The lower part of the tank is conical and equipped with a drain valve for solids removal.

After melting, the raw molten sulfur overflows into the Liquid Sulfur Pit T123. The sulfur pit is a cylindrical type, located above ground, and equipped with a central Sulfur Pit Agitator A123 of turbine type with removable blades. From the T123 pit, the vertical pump P123 sends the molten sulfur to the Sulfur Filter S123 of leaf type.

The precoat is prepared in the Precoat Preparation Pit T124 using filtered molten sulfur. The T124 pit is also cylindrical, located above ground and equipped with a central mixer, A124 of turbine type with removable blades. From the pit T1234, the precoat is pumped to the leaf filter S123 by means of the Precoat Pump P124.

The molten sulfur pits T123 and T124 are equipped with dismountable heating coils fed with 4 bar g steam.

The sulfur belt conveyor and the lime injection device are stopped when:

  • High level is reached in the unfiltered molten sulfur tank;
  • The melting agitator stops.

The filtered molten sulfur leaving the filter is stored in the Liquid Sulfur Storage Tank T125, which is also equipped with dismountable heating coils E125 A/B/C/D/E/F fed with 4 bar g steam.

The Filtered Sulfur Pump P125 A or B sends the sulfur to the burners. These pumps are a vertical immersed centrifugal type and installed on top of the tank T125. There is one pump operating and one as standby.

Sulfur Furnace and Associated Equipment

The filtered liquid sulfur is fed by the pump P125 A or B to the 03 (three) Sulfur Burners X131 A/B/C installed on the front end of the Sulfur Furnace F130. The F130 furnace is a horizontal cylindrical steel vessel internally protected with fire bricks and insulation bricks and provided with internal baffles to improve gas mixing.

The pressure of the liquid sulfur at burner inlets is in the range of 10 to 15 bar g. This high level of sulfur pressure provides good sulfur pulverization ensuring complete combustion.

The sulfur burning equipment is of a proven design suitable for continuous sustained trouble-free operation. The burner equipment is provided with air cooling arrangements. The burners are steam jacketed.

Fire bricks are 60% Al2O3 up to the second baffle and 42 / 44% Al2O3 after the second baffle, able to withstand a working temperature of about 1100oC. The internal baffles ensure a good mixing between combustion air and sulfur, avoiding the risk of unburned sulfur carry over.

The furnace F130 temperature is recorded and an alarm warns the operator and stops sulfur pump P 125 A/B in case a preset limit value (1200°C) is exceeded.

The necessary combustion air is delivered by the motor-driven Air Blower K130 located upstream of the Drying Tower S141. The required air flow being adjusted with a valve located on blower suction side or variable speed motor.

The furnace F130 and the converter R130 are preheated by diesel oil combustion in the Startup Burner X130

SO2 to SO3 Catalytic Conversion System

The gas at 1060°C / 1080°C leaving the furnace passes through the Waste Heat Boiler G161 where it is cooled down to 410°C. The adjustment of the gas temperature at the outlet of the furnace is obtained by the hot gas by-pass of the boiler.

The conversion from SO2 to SO3 is carried out in the Catalytic Converter R130. The converter has its first pass located at the top.

The SO2 gas enters into the first catalyst pass at a temperature of around 410°C and leaves it at around 610°C.

At the outlet of the first pass of catalyst, the gas is cooled from 610°C to 445°C in the Steam Superheater E164. A by-pass on gas side is used for temperature adjustment of the gas at Pass II inlet.

As the gas flows through the second pass, its temperature rises from 445°C to 522°C approximately.

After the second pass, the gas is cooled down again to 445°C by means of the Hot Gas Heat Exchanger E131 (tube side), a by-pass with a butterfly valve being used for temperature adjustment of the gas at Pass III inlet.

The gas then crosses the third pass of catalyst and the conversion raises its temperature from 445°C to 472°C approximately.

After the third pass of catalyst, the gas is cooled down to 284°C approximately by means of the Cold Gas Heat Exchanger E132 (shell side), then down to 150°C in the Third Pass Economizer E161, before entering into the Interpass Absorption Tower S142. After this interpass absorption, the gases are reheated to 425°C approximately through the two gas heat exchangers E132 (tube side) and E131 (shell side) prior to being fed to the fourth pass of the converter.

As the gas flows through the fourth pass, its temperature rises from 425°C to 450°C approximately.

After Pass IV the gas is cooled in the economizers E162 and E163 down to 135°C before reaching the Final Absorption Tower S143.

The adjustment of the temperatures of the gas circuits is ensured by the means of:

  • Gas by-pass of the boiler;
  • Gas by-pass of the superheater;
  • Bypasses of the gas heat exchangers.

The valves provided for these bypasses are equipped with servomotors actuated from the control room.

Manholes and access ladders are provided above and below each catalyst pass, with necessary platforms and walkways. Pressure taps and thermocouples are also provided. The four layers of catalyst are disposed on stainless steel meshes, which are laid on the supporting cast iron grids.

Before start-up of the unit, the sulfur furnace brickwork and the catalyst are preheated using a diesel-oil or natural gas Startup Burner X130 installed temporarily on the front end of the Sulfur Furnace F130. The catalyst heating-up operation will typically extend over a minimum period of 36 hours approximately, not including the time required for furnace heating to 900°C / 1000°C. The Catalytic Converter R130 heating-up operation requires some care as, due to the carbon steel construction of this large vessel, a temperature differential in excess of 120°C must be avoided between both sides of a separation plate, slowing down slightly, in the beginning, the heating-up of the Pass I.

Drying and Absorption Section

Except for the mist eliminators, the three drying and absorption towers are of very similar design.

The acid distribution system, of the same troughs / downcomers type for all towers, is made from high-grade alloyed steel, having an excellent resistance to corrosion by hot and concentrated sulfuric acid.

Each tower is filled with 3" acid proof ceramic saddles.

The Drying Tower S141 is equipped with a mesh pad demister made of Alloy 20 or Alloy 20 and Teflon.

The Interpass Absorbing Tower S142 is equipped with a high-efficiency candle type mist eliminator.

The Final Absorbing Tower S143 is also equipped with a candle type mist eliminator, which is sufficient to achieve the required performance for the release of the gas into the atmosphere.

The acid mist eliminators of the tower S142 and the tower S143 are suspended for easy access and maintenance in a safe manner.

The Acid Circulation Tank T141 has two compartments. The first compartment allows the circulation of acid to the drying and intermediate absorption towers, through the Interpass Absorption Circulation Pump P141. The second compartment allows the acid circulation to the Final Absorption Tower, by means of the vertical centrifugal pump P142.

The acid coolers are shell and tubes type with anodic protection or shell and tubes with special material.

The Interpass Absorption Acid Cooler E141 is used for the Drying Tower and the Final Absorption Acid Cooler E142 is common to the two absorption towers.

One horizontal centrifugal reclaim pump, with one installed stand-by, P143 A/B sends the produced acid to the storage tanks T151 A/B through the Product Acid Cooler E143. These pumps can also be used during shutdowns if drainage of the plant to the storage is required.

An Absorption Area Sump T142 equipped with a vertical immersed reclaim pump P144 is foreseen for evacuation of the possible effluents.

Steam Generation

The waste heat recovery system is designed to use excess heat produced by sulfur burning and sulfur dioxide / trioxide conversion for steam generation.

The produced steam is used to produce energy via a turbogenerator set.

Boiler feed water (BFW) for steam production and produced steam are subsequently heated up during the following steps:

  • BFW preheating in the Third Pass Economizers E161 (cooling of gas leaving cold heat exchanger 060-TQ-503 from shell side) and E162 and E163 (cooling of gas leaving the fourth pass of catalyst);
  • Evaporation in the Waste Heat Boiler G161 (cooling of gas leaving Sulfur Furnace F130);
  • Superheating of produced steam in the Steam Superheater E164 (cooling the gas leaving the first pass of catalyst).

The boiler feed water flow coming from the feed water tank, by means of Boiler Feed Water Pumps P181 A/B, feeds the economizers E163, E161, and E162 in series.

Outlet gas temperature from E163 to FAT is kept at 135°C using the by-pass on the water side of this economizer.

In the same way, outlet gas temperature from E161 to IPAT is kept at 150°C using the by-pass on the water side of this economizer.

Boiler feed water is leaving the economizer E162 partly vaporized and flows to the Waste Heat Boiler G161.

Saturated steam leaving the boiler drum is superheated in the superheater E164 to 410°C approximately, in view of having 400°C at the turbine inlet.

Temperature prior to the inlet of the turbine is kept steady by possible desuperheating of the superheated steam with water at the outlet of the superheater E164.

Economizers E162 and E163 are installed in one common casing in a superposed arrangement.

Waste Heat Boiler G161 is a fire tube type of single pass design with water / steam drum and natural circulation. Boiler tube bundle is installed horizontally. Unit constituted of a furnace and a waste heat boiler is supported by one fixed and one sliding saddle.

Boiler steam drum is provided with primary steam baffles, secondary mesh type steam purifier, internal feedwater distribution pipe, continuous blowdown and conditioning chemical feed pipe.

Superheater E164 is of horizontal finned tube type with horizontal gas flow design. One tube bundle is made of Cr alloy steel tube with 11% Cr fins. One tube bundle is made of SA 106 B tube with carbon steel fin. The unit is supported on one fixed and one sliding saddle.

Most of the generated superheated steam is used to feed the turbine of the generator.

The rest of the steam is depressurized and desuperheated to feed the 7 bar g network. Steam extraction from the turbine feeds the 3 bar g network.

Cogeneration System

Power is generated from high-pressure steam (40.0 bar g and 390°C) produced by the heat recovery of the sulfur combustion and from the recovery from the exothermic catalytic conversion.

The main characteristics of the Cogeneration Package G171 are as follows:

  • Equipment: Turbogenerator, type synchronous
  • Turbine: Condensing turbine
  • Inlet steam: 40.0 bar g and 390°C (superheated)
  • Turbine exhaust conditions: 0.15 bar a and 54°C

The superheated high-pressure steam is expanded in the Steam Turbine X171. The turbine is of condensing type, with one Low-Pressure Steam extraction. The exhausted steam at 0.15 bar a is condensed in the Vacuum Condenser E171, shell and tubes type.

The Generator X172 is of the three-phase synchronous type and will feed a high tension distribution panel. This panel supplies electrical power to the sulfuric acid plant substations. Part of the generated power will be exported.

The condensates are removed from the condenser E171 by the Steam Condensate Pump P171 A/B and sent to the BFW Deaerator V181.

 

Typical figures
Turndown ratio Sulfuric Acid Plants are normally running 24 hours per day with during 18 months/24 months according to the actual overall pressure drop. After this period it is considered a shut-down for catalyst screening and general maintenance.
Product specification  
Concentration: 98,5% ± 0.5% (w/w)
SO2: 40 ppm maximum
Ignition residues: 300 ppm maximum
Iron: 20 ppm maximum
Temperature: 40°C maximum
Gaseous Effluents Specification  
Sulfur Dioxide (SO2): 2.0 kg/metric ton of produced acid @ 100%. This SO2 emission level corresponds to a global conversion efficiency (from SO2 to SO3) of 99.70%.
Acid mist and SO3: 0.075 kg (as H2SO4)/ metric ton of produced acid @ 100%