Knowledge for the Sulphuric Acid Industry
Sulphuric Acid on the Web
Acid Plant Database
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Materials of Construction
DKL Engineering, Inc.
May 2, 2003
The acid regeneration or spent acid recovery process is designed to regenerate or recover sulphuric acid from a variety of spent acids containing a varying degree of contaminants.
A typical sulphuric acid regeneration plant consists of three major components:
1. Regeneration furnace for the decomposition of spent acid
2. Gas cleaning system
3. Contact sulphuric acid plant
The basic process is the decomposition of H2SO2 in a furnace operating at 1000 to 1200ºC. The decomposition reaction is an endothermic reaction so heat must be provided for the reaction to occur. Hydrocarbons that are present in the spent acid provide part of the fuel to maintain furnace operating temperature with the remaining portion being supplied by burning fuel such as natural gas, No. 2 fuel oil, etc.
A regeneration furnace is generally a horizontal refractory lined furnace designed to decompose H2SO2 into SO2 and H2O. The key design parameter is the residence time which should be sufficient to allow the decomposition reaction to go to completion. Typically, a three (3) second residence time is provided.
Spent acid is typically spray into the furnace at one end using a two-fluid atomizing type spray nozzle with compressed air as the atomizing fluid. At the same end of the furnace, fuel burners provide the additional heat input to maintain the decomposition reaction and furnace operating temperature.
Sulphur can also be burned in the regeneration furnace to supplement acid production. Sulphur burned in the furnace will reduce the quantity of fuel required since it is an exothermic reaction.
Combustion air required for the process can be enriched with oxygen to varying degrees all the way to 100% oxygen. The use of oxygen enrichment reduces the amount of gaseous inerts (i.e. nitrogen) that must be carried through the downstream equipment. In an existing plant this has the effect of increasing the throughput of spent acid without the need to increase the size of downstream equipment. In a new plant the overall size of equipment can be reduced. The use of oxygen enrichment also increases the SO2 concentration providing for a smaller downstream sulphuric acid plant.
Regeneration furnaces are typically operated with a 2% oxygen content in the gas exit the furnace. The hot gases leaving the furnace enter a waste heat boiler where high pressure steam is generated for process use or power generation. The gases are cooled to about 350-375ºC before entering the gas cleaning section of the plant.
Preconcentration of the waste acid feed reduces the amount of water that must be handle in the furnace. The result is a reduction in the size of furnace and the amount of fuel burned. The degree to which the feed can be concentrated is dependent on the concentration of solubles in the waste acid. The waste acid can be concentrated to the point where these solubles begin to precipitate out of solution.
NOx is a general term used to describe nitrogen oxide (NO) and/or nitrogen dioxide (NO2). NOx is generally classified as either fuel NOx or thermal NOx depending on how it is formed. Fuel NOx results from the oxidation of fuel bound nitrogen while thermal NOx is created by the fixation of nitrogen in the combustion air at high temperatures.
The formation of fuel NOx is dependent on the nitrogen content of the fuel, total excess air/oxygen and relative distribution of primary and secondary combustion air/oxygen. Formation of thermal NOx is affected by the oxygen concentration, temperature, pressure and residence time.
Some general parameters regarding the formation of thermal NOx are:
Thermal NOx production rate doubles for every 90 K temperature increase beyond 2200 K (1927°C, 3500°F)
Above 1538°C (2800°F) significant quantities of thermal NOx is generated
Thermal NOx increases exponentially (~ 4th power) with absolute temperature
Thermal NOx increases roughly with the square of the available oxygen