CONTACT SULFURIC ACID PLANT
INTRODUCTION:
Sulfuric acid (oil of vitriol, H2SO4) is a colorless, oily liquid, dense, highly reactive, and miscible with water in all proportions. Heat is evolved when concentrated sulfuric acid is mixed with water and, as a safety precaution, the acid should be poured into the water rather than water poured into the acid.
Anhydrous, 100% sulfuric acid, is a colorless, odorless, heavy, oily liquid (boiling point: 338°C with decomposition to 98.3% sulfuric acid and sulfur trioxide). Oleum is excess sulfur trioxide dissolved in sulfuric acid. For example, 20% oleum is a 20% sulfur trioxide–80% sulfuric acid mix. Sulfuric acid will dissolve most metals and the concentrated acid oxidizes, dehydrates, or sulfonates most organic compounds, sometimes causing charring.
The contact process has evolved to become the method of choice for sulfuric acid manufacture because of the ability of the process to produce stronger acid.
PROCESS DESCRIPTION:
When sulfur is used as a raw material, as is the case in this plant, the production of Sulfuric Acid consists first of burning the sulfur (S) to form sulfur dioxide (SO2), second of combining the sulfur dioxide with oxygen (O2) to form sulfur trioxide (SO3), and third of combining the sulfur trioxide with water (H2O) to form a solution containing approx. 98% sulfuric acid (H2SO4). The reactions in chemical shorthand are:
1. S + O2 ----------> SO2
2. SO2 + ½O2 ----------> SO3
3. SO3 + H2O ----------> H2SO4
As stated above, the first step in the process is the combustion of sulfur. The sulfur is melted to eliminate the moisture in it and so far as predictable to free it from any solid impurities. If not eliminated, moisture leads to corrosion, difficulties in the process and often also to loss of sulfuric acid fumes from the exit stack of the plant. Some of the solid impurities settle out of the sulfur in the melter. The portion which escapes is caught in the plant equipment and gradually increases the pressure drop so that finally the blower can not force enough air through the plant to maintain the desired production. The molten sulfur is pumped to a sulfur burner where the air supplied by the blower burns it to sulfur dioxide.
Before entering the burner, the air passes through a drying tower where it is scrubbed with 98-99% sulfuric acid to remove the water vapors always present in it. As in the case of sulfur, the moisture in air, if not eliminated, leads to process difficulties.
The pre-dried air entering the burner contains approx. 21% oxygen (O2), and 79% nitrogen (N2). In the sulfur burner, only part of the oxygen from the air is used in burning the sulfur. The composition of the gas leaving the burner varies according to the proportion of air and sulfur used. For example if the gas leaving the burner contains 8% SO2 by volume, there will be approx. 13% oxygen and 79% nitrogen in the gas. The sum of the volume percentages of SO2 and oxygen in the gas leaving the burner is always approx. 21%, the same as the percentage of oxygen in air supplied to the burner.
The burning of sulfur evolves a large amount of heat which materially raises the temperature of the burner gas. This temperature reached is in proportion to the gas strength (that is, the % SO2 in the gas) and also depends on the temperature of the sulfur and air entering the burner as well as on the amount of heat lost from the sulfur burner shell by radiation.
In the second step of the process, the SO2 in the gas is made to combine with some of the remaining oxygen to form SO3. For this step, the gas is wanted at a lower temperature that it left the burner; therefore it is necessary to cool the gas leaving the sulfur burner. The cooling is accomplished by passing it through a waste heat boiler, which is installed at the exit of the sulfur burner. The sulfur melter is not highly efficient in the removal of ash and hence, the gas from the burner contains traces of dust. Dust might also come from the brick lining of the burner or from other equipment. After being cooled, the gas is passed through a hot gas filter to remove as much dust as practicable.
This filter is provided because it is easier to clean it when necessary than to clean the entire plant which would otherwise become contaminated.
The SO2 is converted to SO3 in the converter. The converter contains several layer of Vanadium Pentaoxide catalyst, which causes the chemical reaction to take place at an enormously higher rate than would be the case if no catalyst were used. The catalyst is not itself affected or used up. As the SO2 is converted to SO3, additional heat is evolved and therefore the gas temperature increases. The SO2 can be only partly converted to SO3 if the temperature becomes too high. Therefore, after passing through the first layer of the catalyst, the gas is taken out of the converter, cooled in a tubular steel intercooler and returned to the converter to pass through the second and third layer of catalyst is accomplished by the addition of cool dry air directly into the gas stream. This gas is thoroughly mixed by passing through an external loop duct before entering the next layer of catalyst.
Upon leaving the converter, the gas passes through another tubular cooler and is cooled to the proper temperature for good absorption of the SO3 in the absorption tower.
The SO3 made in the converter, even though adequately cooled, will not combine directly with water, must be combined indirectly by absorbing it in 98-99% sulfuric acid. Under this condition, the SO3 readily unites with the water in the acid. This operation is carried out in the absorbing tower where the SO3 gases are scrubbed with 98-99% acid just as the air was scrubbed in the drying tower.
The 98-99% acid supplied to the two towers is weakened in the drying tower by the water vapor removed from the air, while in the absorbing tower it is strengthened by the SO3 absorbed. There is never enough moisture in the air to supply all the water required for combination with the SO3 to form H2SO4. Hence the acid flowing from the bottom of the drying tower after mixing with that from the absorbing tower is at a higher strength (about 0.3-0.5%) than it was when it entered the top of the two towers. Before it is again pumped to the top of the tower, it is diluted with water to the desired strength for efficient absorption of the SO3. This water is added at the pump tank.
Absorption of the SO3 and of the water vapor, the addition of water in the pump tank and the sensible heat in the SO3 gas materially raise the acid temperature. In order that the acid entering the tower may be at best temperature for efficient absorption, and drying, it is cooled after leaving the pump tank. The best temperature is usually between 50°C (122°F) and 80°C (176°F).
Due to the continuous formation of 98-99% sulfuric acid, the volume of the acid in the circulation system increases in proportion to the amount of acid produced. Enough acid is continuously taken away from the pump tank to keep a constant level in the pump tank. The acid taken away is the production of the plant.
When the plant first starts operation, the acid will be dark for about ten days after which time it should be clear. If product acid remains for any length of time in a steel storage tank, however, the iron contents increases and the amount of the increase depends on the length of time and the temperature of the acid.
EQUIPMENT DESCRIPTION:
1. Blower:
A single stage positive displacement type gas compressor. It is connected to an electric motor by a belt drive.
2. Sulfur Melter:
A steam pressure of at least 50 and preferably 100lb/in2 is desirable for melting sulfur. Best results will usually be obtained by maintaining the molten sulfur in the melter at approx. 130-135°C (266-275°F). Higher temperature entering the burner is sometime beneficial but never in excess of 149°C (300°F). Materially higher or lower temperatures than those stated, should not be used because of the increased viscosity of molten sulfur.
Most efficient settling of ash from the molten sulfur is obtained by keeping the melter substantially full and at a uniform level at all times.
3. Hot Gas Filter:
The function of hot gas filter is to remove ash and dust from the gas stream ahead of the converter.
The sulfur commonly used in the sulfuric acid manufacture contains a small amount of ash. In addition, a small amount of dirt may be picked up in shipping and handling. The sulfur settling pit is not 100% efficient in removing this ash and dirt. Hence, at all times, there are minute amounts of ash entering the sulfur burner along with the molten sulfur.
While the hot gas filter also is not 100% efficient, yet if it were not provided, most of the ash and dust would be carried into the converter and caught by the catalyst. This would increase the pressure drop through the converter. If enough of the catalyst surface becomes covered, conversion efficiency decreases. The increased pressure drop and reduced conversion efficiency would necessitate cleaning of the catalyst by screening. Use of filter lessens the frequency of cleaning the catalyst.
Description: The hot gas filter forms the bottom section of the converter. The filter consists of a 12” layer of 1/8”-3/8” crushed firebrick supported on a cast iron grid covered with a layer of 1/12”-1” quartz pebbles. A covering layer of pebbles is used over the filter medium to reduce the possibility of its disturbance by the incoming gas.
4. Converter:
The converter consists of 04 layers of Monsanto Vanadium Pentaoxide catalyst with provision for cooling of the gases between each layer. Therefore it is usually referred to as a “four-pass” converter.
The inlet temperature to each pass should not be allowed to fall below 400°C (752°F) at any time during operation, because conversion stops completely at low temperature. The inlet temperature of the 1st pass is regulated by the boiler by-pass damper. To raise the inlet temperature, open this damper, to lower the temperature, close this damper. All adjustment on this damper should be made with caution, as a small change has an immediate effect on the temperature at the inlet to the hot gas filter which in turn will gradually affect the temperature at the inlet to the converter.
The outlet temperature of the 1st pass should not be allowed to run continuously at more than 610°C (1130°F) as higher temperatures may damage the interior castings. Materially higher temperatures may even damage the catalyst. The boiler outlet damper should always be wide open except when putting the plant into operation. Both dampers must never be closed at the same time unless the plant is shut-down, as this would completely block the flow of the gas.
Fourth Pass: If the maximum possible amount of conversion has been achieved in the other passes, and if the plant is not operated at too high an overload, the inlet temperature which will result in maximum conversion efficiency will probably be of the order of 4230-440°C (806-824°F). The optimum inlet temperature will probably will depend on gas strength, production rate and the amount of conversion which has been achieved in the other passes. The inlet temperature to the 4th pass is regulated by cool air as addition in exactly the same manner as 3rd pass.
It is essential that most of the conversion be achieved in the initial passes. If an excessive amount of conversion is left to be done by the 4th pass, temperature rise in this pass will be greater and the gas will leave it at an excessively higher temperature. The higher the temperature at which the gas finally leaves the 4th pass, the lower the overall conversion which can be attained.
When the converter is operated properly and not in excess of rated capacity, the temperature rise in the 4th pass usually will not exceed 5°C (9°F).
5. Intercooler:
The duty of the intercooler is to cool the gases from the 1st converter pass to the proper temperature for admission to the 2nd converter pass.
The gas flues which connect the converter to the intercooler are provided with an intercooler by-pass valve and an intercooler shut-off valve.
The inlet temperature to the 2nd pass is regulated by the intercooler by-pass valve. To raise the inlet temperature to the 2nd pass, open this valve.
The shut-off valve is normally open during operations and is closed only when starting up the plant.
If the by-pass valve does not pass enough gas to give the desired temperature, there is no objection even while operating normally, to partly closing the shut-off valve. However, when this is done, the by-pass valve should be wide open. For example, such a procedure might be necessary during a heavy rain, when the intercooler may cool the gas more than required.
The intercooler should be checked for acid condensate by opening the small drain valve. Normally, the condensate should be found at this point because it operates above the dew point of acid. A small amount of condensate may be expected during start-ups and shut-downs when temperatures are lower than normal, and should be kept drain.
