Knowledge for the Sulphuric Acid Industry
Sulphuric Acid on the Web
Acid Plant Database
Boiler Feed Water
Materials of Construction
DKL Engineering, Inc.
April 15, 2003
Converter Conversion Efficiency
Overall Sulphur Fixation
Plant Water Balance
The fundamental condition of the adiabatic process is the absence of heat exchange between the gas and the surrounding medium. In theory, there is no heat loss or gain to the system but in reality there will be a small amount occurring.
In an acid plant there are two principal processes that occur adiabatically; quenching of hot gases entering acid plant gas cleaning system and conversion of SO2 to SO3 in the converter.
Hot gases entering the gas cleaning system are cooled to the adiabatic saturation temperature by contact with weak acid that is sprayed into the gas. The heat given up by the gas is used to evaporate water into the gas so the gas leaves the quench tower at a lower temperature but has the same heat content as when it entered the tower due to the added water content.
In a converter catalyst bed, when SO2 is converted
to SO3, the heat of reaction is released and the
gas mixture rises in temperature. Since the converter is well
insulated, heat losses from the catalyst bed is small so the reaction occurs
Since the converter is well insulated, heat losses from the catalyst bed is small so the reaction occurs essentially adiabatically.
Steel containing substantial quantities of elements other than carbon and the commonly-accepted limited amounts of manganese, sulfur, silicon, and phosphorous. Addition of such alloying elements is usually for the purpose of increased hardness, strength or chemical resistance. The metals most commonly used for forming alloy steels are: nickel, chromium, silicon, manganese, tungsten, molybdenum and vanadium. “Low Alloy” steels are usually considered to be those containing a total of less than 5% of such added constituents.
Vanadium pentoxide does not act as a catalyst for the SO2 to SO3 reaction below a certain temperature referred to as the ignition temperature. At the auto-ignition temperature, the active part of the catalyst melts which enables it to function as a catalyst.
The auto-ignition temperature, sometimes referred to as the catalyst strike temperature, is the minimum temperature at which the catalyst promotes the reaction of SO2 to SO3. Traditional catalysts have an auto-ignition temperature in the range of 415 to 425ºC (779 to 797ºF). Cesium-promoted catalysts have a much lower auto-ignition temperature in the range of 380 to 390ºC (716 to 734ºF).
The autothermal limit of a plant is the minimum SO2 concentration at which the heat generated by the SO2 to SO3 reaction in the converter balances with the amount of heat required to heat the cold gas up to the catalyst ignition temperature. At lower SO2 concentrations, there is insufficient heat generated and the temperatures in the converter can not be maintained. The converter temperatures will begin to decrease and eventually fall below the auto-ignition temperature of the catalyst.
Converter temperatures can be maintained by the addition of heat from an outside source such as from the operation of a preheat system.
The autothermal limit of a plant is generally set by the size of the Cold or Cold Reheat Exchangers. The larger these heat exchangers, the greater the amount of heat that can be recovered to heat the cold gases entering the converter.
Named for its inventor, Antoine Baumé (1728-1804), the Baumé scale is actually two scales, one for liquids that are more dense (heavier) than water and one for liquids that are less dense (lighter) than water. For liquids that are more dense than water, the hydrometer is calibrated using the following rules:
0°Bé = distance the hydrometer sinks in pure water
15°Bé = distance the hydrometer sinks in a solution that is 15% sodium chloride (NaCl) by mass.
To convert from °Bé to specific gravity at 60°F/60°F:
Specific Gravity = 145 /( 145 - °Bé)
The Baumé scale does not directly measure the concentration of a solution. To find the concentration from a hydrometer reading, you need a table of known specific gravity values at known concentrations.
Ingot mold, with the top constricted; used in the manufacture of “capped steel,” the metal in the constriction being covered with a cap fitted into the bottleneck, which stops “rimming” action by trapping escaping gases.
The converter conversion efficiency is determined by the amount of SO2 entering the converter versus the amount of SO2 leaving the final catalyst pass.
Conversion = (SO2
Entering Converter) – (SO2 Exiting Converter)
(SO2 Entering Converter)
The SO2 entering the converter will not be the same as the SO2 at the inlet of a metallurgical plant due to the absorption of SO2 in the weak acid.
The SO2 exiting the converter will not necessarily be the same as the SO2 in the stack due to the problem of SO2 transfer in the acid system.
Average diameter of grains in the metal under consideration, or alternatively, the number of grains per unit area. Since increase in grain size is paralleled by lower ductility and impact resistance, the question of general grain size is of great significance. The addition of certain metals affects grain size, for example vanadium and aluminum tend to give steel a fine grain. The ASTM has set up a grain size standard for steels, and the McQuaid-Ehn Test has been developed as a method of measurement.
Isentropic compression refers to the reversible adiabatic compression process. Isentropic work (head) is the work required to compress a unit mass of gas in an isentropic compression process from the inlet pressure and temperature to the discharge pressure. The isentropic efficiency is the ratio of the isentropic work to the work required for the compression process.
The term “killed” indicates that the steel has been sufficiently deoxidized to quiet the molten metal when poured into the ingot mold. The general practice is to use aluminum ferrosilicon or manganese as deoxidizing agents. A properly killed steel is more uniform as to analysis and is comparatively free from aging. However, for the same carbon and manganese content Killed Steel is harder than Rimmed Steel. In general all steels above 0.25% carbon are killed, also all forging grades, structural steels from 0.15% to 0.25% carbon and some special steels in the low carbon range. Most steels below 0.15% carbon are rimmed steel.
Mechanical efficiency relates to the mechanical losses of the blower components including gearbox, gears, coupling, etc.
The overall sulphur fixation for a plant must take into consideration the total SO2 entering the plant and the total SO2 leaving the plant. SO2 leaves the plant primarily in the stack but will also be present in the weak acid effluent and product acid. When all sources and emissions of SO2 are considered, the overall sulphur fixation of the plant can be determined.
Acid is made up of SO3 and water. In a sulphur burning plant, a small amount of water comes in with the atmospheric air required for sulphur combustion. The majority of water is added as dilution water in the absorber system. In a metallurgical or regeneration acid plant, considerably more water enters the contact section of the acid plant with the gas exiting the gas cleaning system. If there is too much water in the gas or the SO2 concentration is low, the proportion of water to SO3 that is produced may be too high to produce the acid at the desired concentration.
The plant water balance is the point at which the incoming water meets the exact requirements for producing acid at the desired strength. If the plant is not in water balance, the desired acid concentration cannot be maintained and will decrease.
Polytropic compression is a reversible compression process between the compressor inlet and discharge condition which follows a path such that, between any two points on the path, the ratio of the reversible work input to the enthalpy rise is constant. Polytropic work (head) is the reversible work required to compress a unit mass of the gas in polytropic compression process. Thus, the polytropic efficiency is the ratio of the polytropic work to the actual work required for the compression process.
A reaction is said to be reversible when both reactions (forward and backward) proceed continually until a state of dynamic equilibrium is reached. At equilibrium, the opposing forward and backward reactions proceed with equal speed.
The rate or "speed" of a chemical reaction is defined as the quantity reacting per unit time, usually per unit volume.
An example of a reversible reaction is the reaction of SO2 with O2 to form SO3.
Low-carbon steel in which incomplete deoxidation permits the metal to remain liquid at the top of the ingot, resulting in the formation of a bottom and side rim of considerable thickness. The rim is of somewhat purer composition than the original metal poured. If the rimming action is stopped shortly after pouring of the ingot is completed, the metal is known as capped steel. Most steels below 0.15% carbon are rimmed steels. For the same carbon and manganese content rimmed steel is softer than killed steel.
The gas entering a Drying Tower in a metallurgical or acid regeneration plant will contain SO2 which will be absorbed into the strong acid. The amount of SO2 absorbed into the acid is dependent on the acid concentration and temperature. Drying acid that crossflows to the absorber system transfers the absorbed SO2 to that system. When the acid is circulated over the absorber tower it is contacted with a gas that contains very little SO2. Since the vapour pressure of SO2 in the gas is less than the equilibrium vapour pressure of SO2 above the acid, stripping of the SO2 from the acid will occur. If this occurs in a Final Absorbing Tower, then the SO2 that is stripped goes directly up the stack as added SO2 emissions.
The problem of SO2 transfer in the acid system can be addressed by the use of a cross-flow acid stripper which strips the SO2 from the drying acid system cross-flow using atmospheric air. This air is returned to the inlet of the Drying Tower.
In a double absorption plant, the cross flow acid can added to the acid entering the inlet of the Interpass Absorbing Tower. The SO2 will be stripped back into the gas stream by the lean SO2 gas stream entering the Interpass Absorbing Tower.
A final method of dealing with SO2 transfer is to design and operate the plant at a higher converter conversion efficiency to compensate for the amount of SO2 being transferred in the acid system so the stack emissions remain within specification.
API 650 Welded Steel Tanks for Oil Storage
Covers material, design, fabrication, erection and testing requirements for aboveground, vertical, cylindrical, closed- and open-top, welded steel storage tanks in various sizes and capacities. Applies to tanks with internal pressures approximating atmospheric pressure, but higher pressure is permitted when additional requirements are met. This standard applies only to tanks whose entire bottoms are uniformly supported and in non-refrigerated service with maximum operating temperature or 90°C (200°F).
A36/A36M-01 Standard Specification for Carbon Structural Steel
This specification covers carbon steel shapes, plates, and bars of structural quality for use in riveted, bolted, or welded construction of bridges and buildings, and for general structural purposes.
A131/A131M-01 Standard Specification for Structural Steel for Ships
This specification covers structural steel shapes, plates, bars, and rivets intended primarily for use in ship construction. Material under this specification is available in the following categories:
Ordinary Strength—Grades A, B, D, DS, CS, and E with a specified minimum yield point of 34 ksi [235 MPa], and
Higher Strength— Grades AH, DH, and EH with specified minimum yield points of either 46 ksi [315 MPa], 51 ksi [350 MPa], or 57 ksi [390 MPa].
A178/A178M-95(2000) Standard Specification for Electric-Resistance-Welded Carbon Steel and Carbon-Manganese Steel Boiler and Superheater Tubes
This specification covers minimum-wall-thickness, electric-resistance-welded tubes made of carbon steel and carbon-manganese steel intended for use as boiler tubes, boiler flues, superheater flues, and safe ends.
Note 1--Type C and D tubes are not suitable for safe-ending for forge welding.
The tubing sizes and thicknesses usually furnished to this specification are 1/2 to 5 in. [12.7 to 127 mm] in outside diameter and 0.035 to 0.360 in. [0.9 to 9.1 mm], inclusive, in minimum wall thickness. Tubing having other dimensions may be furnished, provided such tubes comply with all other requirements of this specification.
A240/A240M-02 Standard Specification for Chromium and Chromium-Nickel Stainless Steel Plate, Sheet, and Strip for Pressure Vessels and for General Applications
This specification covers chromium, chromium-nickel, and chromium-manganese-nickel stainless steel plate, sheet, and strip for pressure vessels and for general applications.
A249/A249M-02 Standard Specification for Welded Austenitic Steel Boiler, Superheater, Heat-Exchanger, and Condenser Tubes
This specification covers nominal-wall-thickness welded tubes made from the austenitic steels listed in , with various grades intended for such use as boiler, superheater, heat exchanger, or condenser tubes.
Grades TP304H, TP309H, TP309HCb, TP310H, TP310HCb, TP316H, TP321H, TP347H, and TP348H are modifications of Grades TP304, TP309S, TP309Cb, TP310S, TP310Cb, TP316, TP321, TP347, and TP348, and are intended for high-temperature service such as for superheaters and reheaters.
The tubing sizes and thicknesses usually furnished to this specification are 1/8 in. [3.2 mm] in inside diameter to 5 in. [127 mm] in outside diameter and 0.015 to 0.320 in. [0.4 to 8.1 mm], inclusive, in wall thickness. Tubing having other dimensions may be furnished, provided such tubes comply with all other requirements of this specification.
A283/A283M-00 Standard Specification for Low and Intermediate Tensile Strength Carbon Steel Plates
This specification covers four grades (A, B, C, and D) of carbon steel plates of structural quality for general application.
A285/A285M-01 Standard Specification for Pressure Vessel Plates, Carbon Steel, Low- and Intermediate-Tensile Strength
This specification covers carbon steel plates of low- and intermediate-tensile strengths which may be made by killed, semi-killed, capped, or rimmed steel practices at the producer's option. These plates are intended for fusion-welded pressure vessels. Plates under this specification are available in three grades (A, B and C) having different strength levels.
A515/A515M-01 Standard Specification for Pressure Vessel Plates, Carbon Steel, for Intermediate- and Higher-Temperature Service
This specification covers carbon steel plates intended primarily for service in welded pressure vessels where improved notch toughness is important. Plates under this specification are available in four grades (55, 60, 65, and 70) having different strength levels.
A516/A516M-01 Standard Specification for Pressure Vessel Plates, Carbon Steel, for Moderate- and Lower-Temperature Service
This specification covers carbon steel plates intended primarily for service in welded pressure vessels where improved notch toughness is important.
A573/A573M-00a Standard Specification for Structural Carbon Steel Plates of Improved Toughness
This specification covers structural quality carbon-manganese-silicon steel plates in three tensile strength ranges intended primarily for service at atmospheric temperatures where improved notch toughness is important. Plates covered by this specification are limited to a maximum thickness of 1.5 in. [40 mm].
A662/A662M-01 Standard Specification for Pressure Vessel Plates, Carbon-Manganese-Silicon Steel, for Moderate and Lower Temperature ServiceC267 Standard Test Methods for Chemical Resistance of Mortars, Grouts, and Monolithic Surfacings and Polymer Concretes
This specification covers three grades of carbon-manganese-silicon steel plates intended primarily for service in welded pressure vessels where improved low temperature notch toughness is important. The maximum thickness of plates is limited only by the capacity of the composition to meet the specified mechanical property requirements; however, current practice normally limits the maximum thickness of plates furnished under this specification to 2 in. [50 mm]. Grades A, B, and C comply substantially with the requirements of ISO Pressure Vessel Steels P9, P15, and P18, respectively.
These test methods are intended to evaluate the chemical resistance of resin, silica, silicate, sulfur, and hydraulic materials, grouts, monolithic surfacings, and polymer concretes under anticipated service conditions. These test methods provide for the determination of changes in the following properties of the test specimens and test medium after exposure of the specimens to the medium:
Weight of specimen,
Appearance of specimen,
Appearance of test medium, and
Compressive strength of specimens.
Test Method A outlines the testing procedure generally used for systems containing aggregate less than 0.0625 in. (1.6 mm) in size. Test Method B covers the testing procedure generally used for systems containing aggregate from 0.0625 to 0.4 in. (1.6 to 1.0 mm) in size. Test Method C is used for systems containing aggregate larger than 0.4 in.
C307 Standard Test Method for Tensile Strength of Chemical-Resistant Mortar, Grouts, and Monolithic SurfacingsThis test method covers the determination of tensile strength of cured chemical-resistant materials in the form of molded briquets. These materials include mortars, brick and tile grouts, machinery grouts, and monolithic surfacings. These materials shall be based on resin, silicate, silica, or sulfur binders.C308 Standard Test Methods for Working, Initial Setting, and Service Strength Setting Times of Chemical-Resistant Resin MortarsThese test methods cover the determination of the working, setting and service strength setting times of chemical-resistant resin mortars.
C321 Standard Test Method for Bond Strength of Chemical-Resistant Mortars
This test method covers the determination of the bond strength between chemical-resistant mortars and chemical-resistant brick.
C397-00 Standard Practice for Use of Chemically Setting Chemical-Resistant Silicate and Silica Mortars
This practice provides detailed information for the proper storage, mixing, and use of chemically setting silicate and silica mortars for bonding chemical-resistant brick or tile in order to obtain the optimum chemical resistance and physical strength of the mortar. This practice does not apply to the air-setting type of silicate and silica chemical-resistant mortars. Chemical-resistant brick or tile conforming to Specifications C279, C410, or C980 are considered satisfactory for use with these mortars.
C399 Standard Practice for Use of Chemical-Resistant Resin Mortars
This practice provides information on the handling and proper use of chemical-resistant resin mortars such as those covered in Specification C395. Resin mortars and grouts are differentiated as follows: resin grouts are applied to the joints, generally 1/4 in. (6 mm) wide, after the brick or tile are set in place
C515-95(2001) Standard Specification for Chemical-Resistant Ceramic Tower Packings
This specification covers fired ceramic shapes formed from naturally occurring clays and from compounded bodies that are used as packing in tower installations. These ceramic units are designed primarily for use in process equipment for the chemical or allied industries.
The physical and chemical properties that affect quality of packing materials are covered in this specification. Properties that affect actual operational efficiency or characteristics of processing towers are not covered.
C531 Standard Test Method for Linear Shrinkage and Coefficient of Thermal Expansion of Chemical-Resistant Mortars, Grouts, Monolithic Surfacings, and Polymer Concretes
This test method covers the measurement of the linear shrinkage during setting and curing and the coefficient of thermal expansion materials. A bar of square cross-section is cast to a prescribed length. The change in length is calculated and shown in percent. The change in length at a specific elevated temperature is measured and used to calculate the coefficient of thermal expansion. This test method is limited to materials with aggregate size of 0.25 in. (6 mm) or less.
C533-95(2001) Standard Specification for Calcium Silicate Block and Pipe Thermal Insulation
This specification covers calcium silicate block and pipe thermal insulation for use on surfaces with temperatures between 27 to 927°C (80 and 1700°F), unless otherwise agreed upon between the manufacturer and the purchaser.
C579 Standard Test Methods for Compressive Strength of Chemical-Resistant Mortars, Grouts, Monolithic Surfacings and Polymer Concretes
These test methods cover the determination of the compressive strength of chemical-resistant mortars, grouts, monolithic surfacings, and polymer concretes. These materials may be based on resin, silicate, silica, or sulfur binders. Method A outlines the testing procedure generally used for systems containing aggregate less than 0.0625 in. (1.6 mm) in size. Method B covers the testing procedure generally used for systems containing aggregate from 0.0625 to 0.4 in. (1.6 to 10 mm) in size. Method C is used for systems containing aggregate larger than 0.4 in. (1.6 mm). These test methods provide two different methods for controlling the testing rate.
C612 Standard Specification for Mineral Fiber Block and Board Thermal Insulation
This specification covers the classification, composition, dimension, and physical properties of mineral fiber (rock, slag, or glass) semirigid and rigid board insulation for the use on cooled surfaces and on heated surfaces up to 982°C (1800°F). For specific applications, the maximum and minimum temperature limits shall be agreed upon between the supplier and the purchaser.
C795 Standard Specification for Thermal Insulation for Use in Contact with Austenitic Stainless Steel
This specification covers non-metallic thermal insulation for use in contact with austenitic stainless steel piping and equipment. In addition to meeting the requirements specified in their individual material specifications, issued under the jurisdiction of ASTM Committee C-16, these insulations must pass the preproduction test requirements of Test Method C692, for stress corrosion effects on austenitic stainless steel, and the confirming quality control, chemical requirements, when tested in accordance with the Test Methods C871. These thermal insulations may be either homogeneous or non-homogeneous and either organic or inorganic.
RP0302-2002 (recommended practice)
Selection and Application of a Coating System to Interior Surfaces of New and Used Rail Tank Cars in Molten Sulfur Service
RP0391-2001 (recommended practice)
Materials for the Handling and Storage of Concentrated (90 to 100%) Sulfuric Acid at Ambient Temperatures
RP0590-96 (recommended practice)
Recommended Practice for Prevention, Detection, and Correction of Deaerator Cracking
RP0592-2001 (recommended practice)
Application of a Coating System to Interior Surfaces of New and Used Rail Tank Cars in Concentrated (90 to 98%) Sulfuric Acid Service
Corrosion Control of Ductile and Cast Iron Pipe