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Sulfuric Acid Production by Contact Process

The Contact Process is a procedure that produces sulphuric acid involving three major steps. The steps are divided into 1) Production of sulphur dioxide from the original source or a sulphur ore; 2) The sulphur dioxide is combined with oxygen via a catalytic process using vanadium oxide as catalyst to produce sulfur trioxide; then 3) finally sulfur trioxide after converted into fuming sulphur dioxide is reacted with water to produce the concentrated sulfuric acid. In the process conditions such as temperature, pressure, and the catalyst are crucial to achieve efficient production of the desired product.

Before discussing further the influence of these factors let us examine the processes involved in the production. Outlined below are the complex process with the corresponding chemical reactions and the resulting chemical compounds Stage1: Sulphur dioxide production The initial production of sulphur oxide can be achieved by two means such as heating or burning the sulphur ore. The burning process is achieved through the presence of excess air and the chemical reaction is illustrated as follows: S(s) + O2(g) ?

SO2(g) In the other hand the heating process, usually done for metallic ores, also involving the presence of oxygen can be achieved with the following chemical equation: 4Fe¬S(s) + 11O2(g) ? 2Fe2O3(s) + 8SO2(g) The crucial point in this production stage is the presence of excess air to ensure the proper oxidation of the sulphur. Stage 2: Sulphur trioxide production The process involve an exothermic reaction that is reversible wherein sulphur dioxide is reacted further with oxygen gas to produce sulphur trioxide

The reaction process can achieve an efficient equilibrium condition at about 400-450o C in the presence of vanadium oxide (V2O5) catalyst that hastens the process speed. Stage 3: The final sulphuric acid production step The process is not a simple combining of water to sulphur trioxide since it is highly exothermic and uncontrollable that doing this would create a fume of sulphuric acid. Instead, the sulphur trioxide is first dissolved in concentrated sulphuric acid producing fuming sulphuric acid or oleum. The chemical reaction involved is as follows:

Now the final oleum can then be mixed safely with water to produce concentrated sulphuric acid at the same time recovering the initial sulphuric acid used in the production of fuming sulphuric acid. The chemical process is illustrated as: Control conditions The control conditions mentioned above are very critical in the second stage of the process involving the conversion of sulphur dioxide to sulphur trioxide since the process is reversible. Without the controls we end up having the mixture of the reactants and the resulting product at the same time without achieving optimum sulphur trioxide result..

Oxygen The mixture of sulphur dioxide and oxygen going into the reactor is in equal proportions by volume. Avogadro’s Law says that equal volumes of gases at the same temperature and pressure contain equal numbers of molecules. That means that the gases are going into the reactor in the ratio of 1 molecule of sulphur dioxide to 1 of oxygen. However the reaction equation requires only half the amount of oxygen for every molecule of sulphur which means an excess of oxygen relative to the proportions demanded by the equation.

The process is made to ensure the maximum conversion of the initial sulphur compound to sulphur trioxide. Excess oxygen may not harm the process in fact it would even facilitate increase in the yield, and besides oxygen is cheap. However using more amount of oxygen may harm the process as being reducing production time as more oxygen is involved. The temperature Since we need to have a maximum conversion to the sulphur trioxide, the process should see to it that the reaction should proceed forward. By favoring forward reaction in the equilibrium state more of the desired compound is produced.

By principle this will be achieved by lowering the process temperature. Since the process is exothermic the system will respond by moving the position of equilibrium to counteract this in other words by producing more heat to attain the equilibrium conditions. In order to get as maximum possible sulphur trioxide in the equilibrium mixture, you need as low a temperature as possible. The temperature range 400 – 450°C is a the best known temperature that can produce a fairly high proportion of sulphur trioxide in the equilibrium mixture, but in an optimum time.

The pressure By direct observation in the equation there are three molecules on the left-hand side of the equation, but only 2 on the right. Based on Le Chatelier’s Principle, if you increase the pressure the system will favor the reaction which produces fewer molecules. That will cause the pressure to fall again. In order to get the optimum amount of sulphur trioxide in the equilibrium mixture, you need as high a pressure as possible. High pressures also increase the rate of the reaction.

However, in the common manufacture of the product the reaction is done at pressures close to atmospheric pressure. This is because even at these relatively low pressures, there is a 99. 5% conversion of sulphur dioxide into sulphur trioxide. This small increase derived from tha application of increased pressure is not cost effective. The catalyst The catalyst has no effect whatsoever on the position of the equilibrium. Adding a catalyst doesn’t produce any greater percentage of sulphur trioxide in the equilibrium mixture. Its only function is to speed up the reaction.

In the absence of a catalyst the reaction is so slow that virtually no reaction happens in any sensible time. The catalyst ensures that the reaction is fast enough for a dynamic equilibrium to be set up within the very short time that the gases are actually in the reactor.


Clark, J. (2002). The Contact Process. Retrieved on February 2, 2008 from http://www. chemguide. co. uk/physical/equilibria/contact. html Ryan, L. (n. d). The Manufacture of Sulphuric Acid. Retrieved on February 2, 2008 from http://www. patana. ac. th/parents/curriculum/chemistry/units/ LR1702. html

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