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Best Solution for the SO2-SO3 Conversion
Rajesh S Agrawal, Manager Production, Godrej Industries Limited. The role of the vanadium pentoxide catalyst in the Sulphonation plant is very critical. In allocating the catalyst between various stages consideration of kinetic data shows that the first two stages will not require as much as catalyst as subsequent beds, owing to decrease in the rate of reaction which occurs as conversion proceeds. The more the quantity will also reverse the rate of reaction and that cannot be noticed very easily. The Le-Chatelier Principle helps in understanding the impact by using its three factors, which govern the rate of reaction.

This paper will focus on how right quantity of catalyst impacts the various factors and efficiency of the catalyst depends on the heat removal from the converter and the residence time of the reactants during the conversion process, which leads to sizable saving in industry.

Godrej IndustriesĘ Chemical division at Valia is big hub for the oleo chemicals, which include fatty alcohol, fatty acid, Glycerin and Surfactant. Sulphonation is technique used in the manufacture of the surfactants. Sulphonation is carried out in three stages, 1st - SO3 Generation, 2nd - Reaction and 3rd - Neutralisation.

In this paper we will see that how application of Le-ChatelierĘs principle indirectly helped in improving the productivity of plant, reduced the caustic consumption in the alkali scrubber and also helped in energy conservation along with improvement on environmental front.

The SO3 generation process is just somewhat similar to the sulphuric acid manufacturing except in sulphuric acid SO3 is taken out intermediately scrubbed and remaining SO2 go back to converter bed, whereas here whole volume passed through 4 bed converter. Sulphur is melted in the sulphur melter and pumped into the furnace where dry air is available.

The sulphur is burned in presence of excess of oxygen; the outlet temperature of the converter is around 600-650??C. The SO2 generated is at 6-6.5 per cent concentration which is passed through the 4 bed catalyst converter. The typical conversion at this concentration is in the range of 97.5 to 98.5 per cent (Table 1). The SO3 gas after 4th pass is cooled down by two-shell and tube heat exchanger to get the temperature of 48-52C. This SO3 gas is filtered to remove the mist. The dilute SO3 is feed to the reactor along with organic material depending upon the product to be made. This reactor is Multi Tube Falling Film Rector supplied by M/s Balletra, Italy. The reaction of the SO3 and organic is exothermic so heat to be removed from the system by circulating the water around the reactor. The sulphonated product is acidic in nature which is neutralised with caustic lye and water, depending upon the nature of the product such liquid or paste we change the water concentration in the neutralisation.

The unreacted gases are passed through the cyclone to remove the mist and again ESP (Electrostatic Precipitator). In both the places the mist particles are removed. Unreacted SO3 and unconverted SO2 are scrubbed in the caustic scrubber. (Figure 1)

Issue lead to study the system:
• The yields in the production are poor than standard.
• Pressure drop in the conver ter bed are more.
• The caustic consumption in the caustic scrubber is also more than standard.

There are other issues related to this are chocking of the caustic scrubber, more water consumption in the scrubber, more TDS in the effluent, more power consumption etc. After preliminary study we found that we must focus on the converter which is affecting SO3 generation and which is one of the raw materials of the sulphonation reaction. The SO3 conversion taking place is at 97.5 per cent which is back calculated from the organic going to the reactor. The pressure drop to be addressed.

Steps followed to resolve the issue:
• Study the pressure drop data across the converter bed.
• Study the chemistry of the SO3 generation.
• Study the data.
• Make the action plan.
• Implementation of the action.
• Study the benefits out of the action plan.

1. Study of the pressure drop across converter: We collected the data across the whole system and found that there is major pressure in the second bed, which is seen from the Table 2.

2. Study of the SO3 generation: The SO2 to SO3 generation is exothermic reaction and this reaction takes place in presence of the V2O5 catalyst at temp 400-440C.

Le-Chatelier principle is a very useful tool to predict the equilibrium. There are mainly three factors, which disturb the equilibrium
a. Change in concentration: If concentration of the O2 or SO2 increased then reaction equilibrium will shift toward to right and SO3 generation will be more. If O2 or SO2 concentration is decreased the reaction will shift toward left, same thing will repeat for SO3 but only difference is forward reaction is exothermic and backward reaction is endothermic. b. Change in pressure: If the pressure of the system increased by decreasing the volume, the system will try to decrease the pressure, which lead to shift the equilibrium to right
c . Change in tempera ture: If the temperature of the system is increased, the system will to try to decrease this by consuming heat; the reverse reaction will be effective. The equilibrium will shift to left. Endothermic reaction will consume heat. Efficiency of the catalyst depends upon the heat removal from the converter and residence time of the reactants during conversion process.

3. Study the data: From the pressure drop data of the converter it is confirmed that there is chocking or problem inside the second bed of the reactor. At the same time we had collected the data for the catalyst and found to surprise that the catalyst loading was too high than the required for the conversion. (Table 3)

4. Action plan:
a. Open the converter bed to sieve the catalyst of 4 beds. b. Reduce the catalyst of the first and second bed.

5. Implementation of the action plan:
We opened the converter and found some chocking on the second bed. We cleaned the same. Weighted the qty of the catalyst and dust and decreased the catalyst as per requirement as shown in Table 3. "In allocating the catalyst between various stages consideration of kinetics data shows that the first two stages will not require as much as catalyst as subsequent beds, owing to the decrease in rate of reaction which occurs as conversion proceeds"1
A very interesting thing is what is happening in conver ter when the catalyst qty in the first and second bed is more, the concentration of SO2 and O2 are more so SO3 generation will be more and temp increase so according to Le-ChatelierĘs principle equilibrium will try to shift to left to maintain the temp and in this process partial reverse reaction will take place. The delta Temp of these beds from outside does not seem any major impact so we could not easily understand the reversal reaction happening inside.

6. Benefits out of the action plan: Saving take place in (Ref: Table No: 4)
a) Sulphur consumption/mt of product
b) Caustic consumption in scrubber.
c) Water consumption in scrubber.
d) Productivity increased/day

There are also some unseen improvements on the following fronts: a) SO2/SO3 going to environment reduced.(Ref: Table No:5)
b) TDS going to the ETP also reduced. (Ref: Table No:6)
c) Cleaning of the catalyst also help in reducing power consumption

7. Some more changes done in system to have more accuracy in measurement and improve the conversion:
a) Piston type sulphur pump is changed with gear pump.
b) Mass flow meter for sulphur is provided.
c) Caustic going to scrubber is provided with mass flow meter and its control is given based on outlet water pH.
d) High activity catalyst was added in the 3rd and 4th bed in small quantity to improve the conversion.

8. Observation and conclusion: From the above data it is observed that the adding more catalyst doesnĘt produce any greater percentage of SO3 in the equilibrium stage. The addition of more catalyst will help in DCDA process of sulphuric acid manufacturing. Catalyst increase the rates of both the forward and reverse reaction equally, thus they reduce the time to reach the equilibrium. Le-ChatelierĘs Principle had made remarkable saving and gave new dimensions for thought process, which had not mentioned in literature. The effect of more quantity of catalyst may impact the reaction in reverse direction, which can be revealed by practical experience.

I would like to thanks M/s Godrej Industries Limited for providing me support for carrying out some experiments to convert theoretical assumption in to results. I would also like to thanks each and every member of Corporate Technical Services team for providing me guidelines throughout the whole experiment. I would also like to have special thanks to S N Trivedi (Director- CEPL) and C L Subhedar (VP Technical- RSPL Ltd) for their technical support.