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Liquid SO2 Plant: A Comparative Analysis of Technologies Offered by USA and India
Sulphur dioxide (SO2) is the key chemical for the production of sulphonating agents and it is produced in the world by a largest tonnage compared to any other chemical. Currently liquid Sulphur dioxide demand has increased particularly its uses in producing speciality chemicals for Petroleum refining and pharmaceuticals. A case study of the latest state technology to produce this key chemical is presented in this paper. A comparative analysis is made between technologies offered by India and USA.

Sulphur dioxide (SO2) has been a key chemical as a starting point of many chemical products inclusive of Sulphuric Acid, liquid sulphur di-oxide, oleums, liquid sulphur tri-oxide, etc, Recently the requirement of liquid sulphur di-oxide has increased due to production of speciality chemicals in the production of petroleum refining and other products. With the uncertainty of the prices of oil, the field has become very competitive.

Hence, technologies established over several decades have to be revalued for its capital costs, utility costs, manpower requirements, maintenance, and environment and safety considerations. It is also the starting step for the cold process of manufacture of sulphuric acid, liquid sulphur trioxide, oleum under high pressure conversion with zero emission of sulphur dioxide. A techno-economic comparison of various technologies for the production of liquid sulphur dioxide was presented by the author at the International Conference sponsored by British Sulphur at Berlin (Germany) in October 28-31, 20121 There was a need for innovation to produce liquid sulphur dioxide which will be requiring lower capital costs, raw material and utility costs and maintenance costs with safety and environment considerations. This was done simultaneously by NEAT in India and Late Dr Browder (Ex-Monsanto) in USA. NEATÊs process was presented at British Sulfur Conference at Calgary, Alberta (Canada) during 17-20 October 19992

This paper highlights the case study of plant constructed by NEAT on LSTK basis for M/s. Atul Products Ltd in Ankleshwar (Gujarat) during 2003-2004.

Comparison between technology offered by USA and indigenously developed by NEAT: Our client had obtained by paying USD 25,000 to the technology supplier from USA, a design package for eight TPD plant.

On receiving NEATÊs offer a comparison was prepared as follows and order was placed with NEAT. (See Table 1)

Capacity, raw material, utility consumption, product quality: The capacity for the plant currently under construction in Saudi Arabia is 4000 TPY. The guarantees provided are as follows:- 1. Raw Materials and Chemical Consumption Consumption of Raw material per MT of liquid SO2
• L iquid Sulphur : 0.175 MT
• L iquid Sulphur Trioxide : 0.841 MT

2. Utility Consumption
Consumption of utilities per MT of liquid SO2
• P ower : 30 KWH
• S team @ 3 - 5 Kg/cm2 : 0.3 MT
• Water for cooling tower makeup : 3 m3

3. Product Quality
4. Productivity 500 Kgs Liquid SO2/hr

Process description with safety and environment considerations: The process developed by NEAT is based on following:

It is interesting to note that a highly exothermic nature of sulphur oxidation in a furnace at about 1000oC can be carried out at reasonably low temperatures 50-110oC in a pressurised reactor. This is possible by reaction.

Since the free energy change is large and negative, the reaction is almost instantaneous. In addition, the reaction generates one additional mole in gaseous form. Thus the pressure of the reactor builds up. If there is a stoichiometric addition of liquid sulphur trioxide (30-40oC) and molten sulphur (135-140oC), under a pressure of 8-10 Kg/cm2, then the sulphur dioxide (after cleaning) formed (98-99 per cent) can be liquefied at room temperature. The exothermic heat removal of about 15625 kCal per ton of sulphur dioxide produced is achieved by circulating cold water through the jacket.

Since the reaction is instantaneous and complete, the reactor volume is very small. However, oleum 25 per cent is used as a carrier to provide uniform mixing of liquid sulphur trioxide and liquid sulphur. Agitation with proper mechanical seal is provided to ensure complete reaction. The increase in molecules of SO2 by fifty percent enables buildup of pressure without use of compressor. Figure 1 gives condensation temperature of pure sulphur dioxide at different pressures. This indicates that if a pressure of 7 to 8 kg/cm2 is maintained in the system, the liquefaction can be done at ambient temperature using water at 35-40oC, cooling is required at the rate of 1.12 million kCal/ton of product.

A block diagram for liquid SO2 plant in Figure 2 describes the various stages involved.

In view of the fact that the conversion of Sulphur Trioxide and Sulphur to Sulphur dioxide is stoichiometric, there is no gaseous or liquid effluent.

In view of the fact that the conversion of Sulphur Trioxide and Sulphur to Sulphur dioxide is stoichiometric, there is no gaseous or liquid effluent.

The plant is provided with rupture disc and safety valves for transferring the gases from the various equipmentÊs in the plant to header which is connected to a catalytic converter of the adjoining Sulphuric Acid plant. Over the past ten years, the first plant is running and there has been never an occasion of safety hazard. Similar, is the case for the subsequent three plants supplied by NEAT.

Proving of guarantees: Each plant requires a 72 hours run to prove the guarantees as per the agreement between NEAT and client. Accordingly trials were conducted and client was given due satisfaction of proving the guarantees.

Economics: The liquid Sulphur dioxide produced from NEATÊs process has been found to the most economical compared to any other alternative process available today. A detailed comparison in this regard has been made by the author in the paper presented at Berlin, Germany1.

The end use of sulphur dioxide in the production of key chemicals in merchant market applications is diverse. It is used to make other chemicals such as bisulphides, metabisulphites, thiosulphides, sulphites, hydrosulphites and sulphates.

SO2 can be used to produce dimethyl sulphoxide or thionyl chloride. In pulp and paper, it is used directly in sulphite pulping, in the in-situ production of sodium hydrosulphite, as a reductant in the production of chlorine dioxide from sodium chlorate, and to remove surplus hydrogen peroxide in the bleaching process. In food and agriculture, SO2 finds use in corn processing to remove the kernel hull for making high fructose corn syrup and ethanol. It also finds use as a sterilant, preservative and bleach in certain food and beverage products. In water treatment it is used as a chlorine scavenger, reducing free chlorine in waste water treatment plant discharges. In metallurgical processing, SO2 is used in the purification of certain elements from their ores, for the recovery of certain elements from mixtures of other materials, and increasingly in the reduction of cyanides in the leachate from gold mining.

In pollution control, SO2 is used to rescue hexavalent chromium ions to the more innocent trivalent form for easier disposal. See Table 2.

Identical plants supplied by NEAT in India and Abroad: Apart from eight TPD liquid SO2 plant supplied by NEAT at ` 1.75 crores in 2003-2004 to M/s. Atul Products Ltd, Ankleshwar (Gujarat). NEAT has given identical plants to M/s Shree Sulphuric Acid Pvt Ltd at Ankleshwar (two units) and one unit to M/s Nath Industries at Vapi, both in the state of Gujarat.

Currently NEAT is providing basic engineering for a 4000 TPY liquid SO2 plant for Addar Group of Industries at Riyadh (Kingdom of Saudi Arabia).

Conclusion: An attempt is made in this paper a case study of technology to produce liquid sulphur dioxide, highlighting the comparative analysis of Indigenous versus USA state of art. The paper also gives practical demonstration of plants already installed and operating for past ten years.