To encourage “Energy Conservation & Environment Protection” in the industrial sector PCRA has been continuously upgrading the energy conservation knowledge thru energy audits and sharing the experience of energy efficiency improvement and achievement of industries. Based on these information, developing case studies for dissemination of information to various industrial groups and organization. The case study detailed below is one of the modification carried out by Shriram Alkali & Chemicals at Bharuch Gujarat which is being reproduced for the benefit of Alkali & Chemical sector.

Introduction

Shriram Alkali & Chemicals (SAC) is a 62500 tonnes per annum Chlor Alkali Plant situated at Jhagadia (Distt. Bharuch, Gujarat) based on state of the art memberane cell technology and integrated with energy efficient captive co-generation plants with 5 nos. DG sets of 3X6 MW and 2X3 MW each. The unit is manufacturing caustic soda lye, caustic soda flakes, chlorine, hydrochloric acid & hydrogen since March 1996 and generating power for its captive use.

Background

Chlor Alkali Plant of M/s Shriram Alkali & Chemicals is based on energy efficient memberane cell technology process. The process starts by dissolving common salt (i.e. sodium chloride) into water. The salt solution is known as brine. The optimum temperature of brine is 600C for optimum performance of Ion Exchange columns where the impurities such as Ca, Mg are removed in ppb (parts per billion) levels.

A through analysis of heat balance has revealed that we can eliminate the brine heating requirement by modification and better control of vacuum in dechlorination tower where chlorine is removed from brine by vacuum.

Earlier Process System

There was only a manual valve in the steam line going to dechlorination ejector as per design of technology supplier M/s Asahi, Japan. This valve was kept fully open. The absolute pressure in the dechlorination tower was remaining in the range of 0.32-0.34 kg/cm2.

Due to higher vacuum, the brine temperature at outlet of dechlorination was remaining in the range of 72-740C (saturation temperation of brine at 0.32-0.34 kg/cm2) 52-540C brine temperature was obtained at inlet of Ion exchange columns as there was a drop of 200C in brine circuit as per block diagram.

Therefore, to maintain the brine temperature of 600C at the inlet of Ion exchange columns, the brine was heated from 72-740C to 800C in a plate type heat exchanger at return brine tank outlet.

At low load operation when the waste steam was not available this heating was done by using steam generated in FO based package boiler.

The furnace oil consumption was 210 MT/annum.

Modification

Now a pressure control valve is installed in steam line going to dechlorination tower for auto control of pressure. The absolute pressure is increased from 0.32-0.34 to 0.4 kg/cm2 corresponding to 81⁰C brine temperature at dechlorination outlet as per block diagram.

The brine temperature is 60⁰C without any heating. Now there is no need to run furnace oil based packaged boiler.

The furnace oil consumption in now zero.

Conclusion

Provision of an auto control valve in a vacuum system for dechlorination has eliminated the negative impacts of both higher vacuum (i.e. if the vacuum is high, the brine will get cooled to its equivalent saturation temperature resulting in higher heating load / energy consumption) as well as lower vacuum (i.e. if the vacuum is low, the efficiency of dechlorination will reduce resulting in higher sodium sulphite consumption in removing the chlorine).

The result is a saving of Rs.21.7 lac / annum with an investment of Rs.0.5lac and the payback period work out to be less than one month.