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DKL Engineering, Inc.
Handbook of Sulphuric Acid Manufacturing
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Preface
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Definitions
April 15, 2003
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.
Alloy Steel
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.
Capped Steel
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.
Converter Conversion Efficiency
The converter conversion efficiency is determined by the amount of SO2 entering the converter versus the amount of SO2 leaving the final catalyst pass.
Converter
Conversion = (SO2
Entering Converter) – (SO2 Exiting Converter)
x 100%
(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.
Grain Size
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.
Killed Steel
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.
Rimmed Steel
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 - American Petroleum Institute
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).
ASTM - American Society for Testing and Materials
