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Improving plant operation ­between turnarounds

Summary

In this article, K. A. Pillai, Deputy General Manager of Fact Engineering and Design Organisation (FEDO), Cochin, India, reviews some of the practical problems ­encountered in various process steps during the operation of sulphuric acid plants.

Abstract

The main process steps in the manufacture of sulphuric acid by the DCDA (double contact double absorption) route using solid sulphur are well known and can be summarized as follows:

  • Melting the solid sulphur with steam coils, followed by filtration, if required, to obtain clean sulphur containing less than 10 ppm ash;
  • Burning the molten sulphur in air to produce gas containing sulphur dioxide;
  • Cooling the gas in a waste heat boiler system which produces superheated or saturated steam at conditions fixed, based on requirements;
  • Catalytic oxidation of nearly 95-97% SO2 to SO3 in three consecutive passes of converter containing vanadium pentoxide catalyst with intercooling of gas in between. The exothermic heat of reaction is utilized to produce steam in the waste heat boiler system and to reheat the gas going to it in the fourth pass from the intermediate absorber.
  • Absorption of SO3 formed in the gas stream in 98.4% acid in an intermediate absorption tower producing sulphuric acid.
  • Catalytic oxidation of the remaining SO2 to SO3 in the fourth pass of the converter to get a conversion efficiency of more than 99.7%. The exothermic heat is also utilized as in the previous passes.
  • Absorption of the SO3 formed in the fourth pass of the converter in the final absorption tower to produce sulphuric acid.
  • A low grade heat recovery system can be considered instead of the intermediate absorber, which will produce additional steam at low pressure.

In the remainder of this article, typical problems in the manufacture of sulphuric acid will be outlined together with solutions that have been effectively adopted to remedy them.

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Understanding your Claus reaction furnace

Summary

Measuring and monitoring what is happening within the Claus reaction furnace is a priority when fine tuning your SRU front end furnace for better performance. Other important considerations include burner design, oxygen enrichment and operating control. Lisa Connock reports.

Abstract

The conversion of hydrogen sulphide to elemental sulphur by the modified Claus process is attained in two stages. In the first stage, the thermal section, H2S is converted to elemental sulphur at high temperatures. The partial oxidation of H2S to SO2 is highly exothermic and is not limited by equilibrium:

H2S + 3/2O2 -> SO2 + H2O

The unburned H2S in the acid gas reacts with the sulphur dioxide to form elemental sulphur vapour:

2H2S + SO2 -> 3/2 S2 + 2H2O

This reaction is endothermic and is limited by equilibrium.

In the second stage, the catalytic section, the overall conversion of H2S to sulphur is increased in a series of conversion steps (typically three catalytic reactors) by reaction of the generated SO2 and unreacted H2S over fixed beds of an alumina or titania Claus catalysts at much lower temperatures. A sulphur condenser is provided to condense and separate the sulphur formed in each conversion stage.

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PNS gains global momentum

Summary

Over the past two decades, sulphur management in crop production has become more prevalent worldwide. The increasing incidence of sulphur deficiencies, due to reduced sulphur dioxide emissions, the predominant use of high-analysis fertilizers, and intensified agricultural production, has become an important factor in most agricultural systems that must be addressed to sustain and increase agricultural production. D. L. Messick and M. X. Fan of The Sulphur Institute take a global look at the international agricultural requirements for Plant Nutrient Sulphur.*

Abstract

In 1995, 9.1 million tons of sulphur fertilizers were applied to soils worldwide, 73% of which was in the form of ammonium sulphate and single superphosphate. These products will continue to be important as soil sulphur sources, particularly where their sulphur benefit is recognized; however, newer products are gaining market share as technological developments allow the fertilizer industry to commercialize innovative sulphur materials. Currently, there is an estimated annual market potential worldwide for an additional 7.3 million tons of plant nutrient sulphur. If food production continues to grow at recent growth rates, and sulphur applications do not change, the market opportunity for sulphur fertilizers is estimated to grow at an average annual rate of 3.7% to 10.5 million tons in 2007.

 

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S8 after Y2K?

Summary

J. B. Hyne cannot resist taking a forecasting 'shot' at the first decades of the new millennium, especially after having had the opportunity to preside over a session at Sulphur 98 which included some of the industries best-known forecasters.

Abstract

This last year of the Millennium will be a year of Centennial Reviews, Best Of and Worst Of the Century Awards and endless Forecasts of What Will Be in the future. Those who have attempted to forecast the future of sulphur in years gone by have usually wished that they hadn’t. But the temptation to take a shot at, at least, the first decade or two of the next Millennium, is irresistible.

This is especially true when I have just had the honour to preside over a session at the annual British Sulphur Conference, “Sulphur ’98”, in Tucs-on, Arizona (Nov.’98). At this session, three of the current best known “forecasters”, Mike Kitto of British Sulphur Consultants, Gerry d’Aquin of Con-Sul Inc., and Don Messick of The Sulphur Institute gave their views and together discussed the various supply, demand and out-of-balance factors that we have become used to. Although there were disagreements among these Oracles, there was a surprising harmony to the sounds that emerged. I hesitate to call it music, because it certainly lacked that kind of attraction for the audience. A Dirge, perhaps.

In this “View from the Chair” I will combine many of their contributions with those of my own in answering the question “S8 after Y2K?” ... What about elemental sulphur after the year two thousand?

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Better control of acid cooling

Summary

Recent developments in the acid cooling technology for sulphuric acid plants have focused on tailoring equipment to specific customer needs and improving fabrication procedures and control systems using the latest state of the art techniques and equipment.

Abstract

The acid cooling system in a sulphuric acid plant plays an important role in determining the efficiency and safe operation of the plant. The most appropriate selection of a particular cooling system depends on several factors such as heat duty, acid temperature, acid con­centration, pressure level, the availability and quality of the cooling water, as well as the cost of water in relation to the cost of energy.

The introduction of the Kvaerner Chemetics anodically protected sulphuric acid cooler in 1970, then known as the CIL cooler, revolutionized the task of acid cooling. This was followed in about 1980 by another significant milestone – the introduction of special stainless steels, for example Sandvik SX, SARAMET and 1.4575).

Cast iron cascade coolers, the standard equipment for many decades, are now virtually obsolete in modern acid plants and have been replaced predominantly by shell and tube coolers with or without anodic protection and plate-type coolers. Even in existing plants cast iron coolers are in most cases being replaced by better alternatives.

The use of standard stainless spiral coolers, tank coil coolers etc. is limited to special applications. Air coolers are used in sulphuric acid plants when cooling water is not available in adequate quantity or at reasonable cost.

The choice of cooler is dictated by process conditions, economics and customer preference. Material selection is made based on the quality of the cooling water to be used and in particular its chloride content.

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Firing up tea

Summary

Following the systematic environmental clean-up throughout the world and the massive reduction in industrial sulphur emissions there has been renewed interest and development in sulphur fertilizers to compensate for loss of industrial sulphur falling on the landscape to the benefit of crop growth and nutrition.

Abstract

Sulphur has been called the fourth major element after nitrogen, phos­phorous and potassium (NPK). It was also the ‘forgotten’ ­element as free sulphur various sources – industrial sulphur from factory and power station chimneys and ‘on the backs’ of other fertilizer materials as non-costed ‘rider impurities’ – were usually sufficient to satisfy most crop needs.

The situation in tea is special for this crop requires substantial sulphur fertilization to attain maximum yield and bring out the finest flavour, the basis on which it is successfully sold.

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