Sulphuric Acid Plant
The sulphuric acid plant was constructed by Lurgi Chemie of Germany and commenced production in September 1967. The plant is capable of producing up to 240 tonnes of acid per day. The production rate, which is dependent on the demand for fertiliser, varies between 40 and 240 tonnes per day. When operational, the sulphric acid plant is a continuous process operating 24 hours per day. There is at least one planned maintenance shutdown period per year, this is typically undertaken in winter to reflect the seasonality of fertiliser demand.
The process for manufacturing sulphuric acid consists of the combustion of sulphur to produce sulphur dioxide, oxidation of the sulphur dioxide to sulphur trioxide and absorption of the sulphur trioxide in water to produce sulphuric acid.
Steps:
Solid sulphur is recovered from covered storage by front end loader and conveyed by belt conveyor to an above ground melter which is fitted with steam heated coils and an agitator. Moisture and hydrogen sulphide gas in the solid sulphur are released as the sulphur is melted. These are collected and vented to atmosphere after treatment in a scrubber. This scrubber can be operated either as a biotrickling filter, or as a caustic scrubber. The molten sulphur may be filtered to remove solid impurities and is then stored until required. Any moisture and hydrogen sulphide remaining in the liquid sulphur is vented directly to the atmosphere from the sulphur storage tank vents.
The liquid sulphur is sprayed into the sulphur furnace (operating at temperatures up to 1,000 °C) where it spontaneously ignites, providing a gas stream containing approximately 10% of sulphur dioxide by volume.
Air required for the combustion of the sulphur is dried by contact with 98.5% sulphuric acid in the drying tower. Removal of the moisture in the combustion air is necessary to minimise corrosion within parts of the plant.
The gas stream is cooled to 440 °C in a fire tube boiler, which creates high pressure steam. The steam is used to generate up to 1.7 MW of electrical power. The power is supplied to the other departments within the Works and any excess supplied to the National Grid.
The cooled gas then passes to the converter where the sulphur dioxide is converted to sulphur trioxide using vanadium pentoxide catalyst. The conversion takes place in four stages with cooling of the gas between each stage in order to increase the efficiency of the conversion. The overall conversion of the sulphur dioxide to sulphur trioxide is typically 98.7 to 99.2%.
The gas from the converter is further cooled in the economiser and then mixed with sulphuric acid in the absorbing tower. The sulphur trioxide is absorbed in the sulphuric acid increasing its concentration. The concentration is restored to 98.5% by the addition of dilution water and by blending weakened acid from the drying tower. The acid produced is transferred to the production tank and then pumped to storage. The remaining gas is discharged to the atmosphere via a 55 metre high stack, including any remaining sulphur dioxide.
Drying of the combustion air for the plant and the absorption of the sulphur trioxide generates considerable quantities of heat, which must be removed from the acid. This is achieved by using seawater in a plate heat exchanger. After passing through the plate heat exchanger, seawater is discharged to the Otago Harbour via Outfall 1. The discharge also contains blowdown from the acid plant boiler, waste water from the water treatment plant, cooling water from various pumps, stormwater from the acid plant buildings and yards and stormwater from the Ravensbourne Road catchment.
The steam raised in the acid plant is used to generate electricity. At the exhaust of the turbine the steam is collected, condensed and returned to the boiler. Seawater is used as the coolant in the condenser. After passing through the condenser the seawater is discharged to the Otago Harbour via Outfall 1.
During normal operation the sulphuric acid plant is self sufficient in energy. However, prior to starting the plant external sources of energy are required to heat the plant up to operating temperatures and to melt the sulphur. Diesel fired heaters are used to pre-heat the plant while the sulphur is melted using steam raised in a diesel fired boiler.
Various plant process parameters throughout the Acid Plant operation are continuously monitored and controlled by a computer system. Should parameters deviate from normal operating range then alarms are triggered to alert the engineer. The sulphur dioxide concentration and mass emission rate from the Acid Plant stack are logged and displayed on the control room computer at all times, as well as the ambient sulphur dioxide concentration at two locations in the suburb of Ravensbourne.