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Publication > Issue > Articles

Why do waste heat boilers fail?

Summary

D. Martens and M. Porter of Porter McGuffie, Inc. and L. Stern of Stern Treating & Sulphur Recovery Consulting, Inc. examine the main causes of waste heat boiler failures in Claus sulphur recovery units and discuss what lessons can be learned from past WHB failures.

Abstract

Why do waste heat boilers in sulphur plants fail? We can look to the auto racing industry to find the answer. Engine size in Formula 1 race cars has continually reduced over the years as engineers have found ways to make the cars go faster and faster with a given engine size. On its super speedways, NASCAR places restrictor plates below the carburettor to limit engine output. Similar efforts take place in almost all types of racing. Why? Because the limiting factor is the ability of the driver to react quickly enough to safely control the car. Waste heat boilers fail because we try to operate them at levels beyond which we can adequately design and safely control them to provide reliable operation. Typically waste heat boilers (WHBs) fail due to three factors: excessive temperature, excessive mass flux rate and excessive water-side fouling1. Similar to the racing industry, the sulphur industry has increased temperatures and mass flux (process flow) rates to obtain greater unit capacity. However, this push has exceeded reasonable bounds, to the extent that reliability of a unit can be and has been compromised. To maintain acceptable discharge environmental criteria, often the sulphur recovery units (SRUs) must operate with significant variance in acid gas flow rates – variances that are not controllable by the SRU operators. Water-side fouling is potentially affected by these same parameters and becomes a significant factor for reduced reliability. Keywords: tube fouling, mass flux rate, metal temperature, corrosion, root cause analysis, inspection

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Sour water stripping - Part 3: WWT technology

Summary

N.A. Hatcher, C.E. Jones and R.H. Weiland of Optimized Gas Treating conclude this three-part series on sour water stripping with a case study that uses a mass transfer rate-based simulation to examine two of the important factors that determine the performance of a 2-Stage Chevron WWT (waste water treatment) unit. The purpose of such a plant is to produce relatively pure ammonia and hydrogen sulphide products rather than just combusting the ammonia in the Claus furnace. It is argued that the performance of the whole plant is controlled almost entirely by the operation of the hydrogen sulphide stripper.

Abstract

Sour water is generally classified as either phenolic or non-phenolic. Non-phenolic water, also called HDS water because it is produced by hydro-treating in hydrodesulphurisation or HDS units in refineries, contains almost exclusively ammonia, hydrogen sulphide, and possibly a trace of carbon dioxide. Part 1 of this series1 addressed sources of non-phenolic sour water, sour water chemistry, phase equilibrium in sour water systems, and the removal of contaminants in sour water strippers (SWS). Phenolic (or more broadly, non-HDS) water typically contains heat stable salts (HSSs) and HCN, although phenols and caustic may also be present, depending on how the water has been previously used in the refinery. In Part 2, the stripping of phenolic water is discussed2. In the present article, attention is turned to the WWT (waste water treatment) technology originally developed by Chevron and recently acquired by Bechtel. This is generally referenced in the literature as the Chevron WWT process. WWT technology is a two-stage sour water stripping process whose objective is to separate the hydrogen sulphide and ammonia components of sour water into two separate streams, each relatively free from the other component. Historically there were problems with the handling of ammonia in a SRU, although today these problems can be overcome by using a high enough temperature and a long enough residence time in the SRU furnace to completely destroy ammonia. Nevertheless, when high-nitrogen, high sulphur (heavy) crudes are processed, the amounts of ammonia produced in a refinery can be high enough to make it worth considering ammonia as a salable product rather than just routing it to the SRU furnace. Removing ammonia from the Claus plant unloads the Claus plant hydraulically, reduces front-end air requirement, and allows for a thermodynamically higher sulphur recovery. Keywords: Chevron, WWT, ammonia stripper, external reflux, ammonia purification, H2S stripper, scrubbing, simulation, ProTreat

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Improving Claus TGU performance

Summary

Reactor performance in a Claus tail gas treating unit (TGU) is a critical parameter in achieving TGU environmental performance. Conversion of sulphur species to H2S is a function of catalyst activity, reactor space velocity and temperature. M. Huffmaster and F. Maldonado assess the impact of these principal variables on both catalyst bed design and performance.

Abstract

Claus tail gas treating units (TGUs) are built for a specific purpose – increasing the overall sulphur recovery of the Claus TGU sulphur recovery complex to about 99.9% from about 96% achieved on a Claus plant alone. Their sole purpose and economic justification is reducing sulphur emissions, which improves overall environmental quality. In the reductive tail gas process addressed herein, achieving good performance requires high conversion of sulphur compounds to H2S in the reactor. Achieving good performance therefore requires setting reactor operating conditions based upon understanding the influence of key operating variables affecting the catalyst bed. The key parameters affecting performance of the catalyst bed are catalyst kinetic properties, temperature, and tail gas loading/space velocity. l Catalyst kinetic property is determined by its manufacture, activation and aging. l Temperature affects catalytic activity and thermodynamic equilibrium, limiting conversion. l Reactor loading directly impacts space velocity, which controls conversion. Keywords: Keywords: tail gas unit, catalyst, reactor modelling, carbonyl sulphide, kinetics, space velocity, COS hydrolysis

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Hydrogen gas incidents

Summary

The ever-popular Annual Clearwater Convention of the AIChE Central Florida Section was the 38th such meeting, attracting full houses during the workshop and formal sessions. Mark Evans reports on the discussions at the sulphuric acid workshop.

Abstract

The 17th Annual Sulphuric Acid Workshop was chaired this year by Rick Davis of Davis & Associates Consulting and Joshua Every of Mosaic. The theme this year was Hydrogen Gas Incidents. A series of hydrogen incidents have occurred in the sulphuric acid industry worldwide. The goal of the workshop was to highlight the underlying causes of these incidents and provide suggestions for mitigation and prevention. Many of these incidents have a common thread: knowing the potential causes will assist sulphuric acid plant operators, maintenance personnel, engineers and designers to minimise the risks of these incidents. The session comprised five presentations that discussed the events leading to the incident, the effect and the action taken. Corrosion is a constant in sulphuric acid production and on metal surfaces, and this leads to hydrogen formation. Hydrogen can explode at very low temperature levels. An industry work group has been formed to look at the underlying causes of hydrogen incidents at sulphuric acid plants, with the objective of establishing a code of practice. One common pattern has been identified in the incidents that have been reported, namely a “sudden pressure gradient”. “No-one wants to talk about these,” Rick Davis observed, and it is believed that hydrogen incidents have been under-reported in the industry. Keywords: corrosion, hydrogen formation, explosion limits

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Better suphuric acid plant performance and reliability

Summary

High performance equipment designs for sulphuric acid plants together with improved technical services and operating training are key contributors to achieving better plant performance, increased reliability and longer equipment life.

Abstract

Over the past few decades there has been a continuous and gradual improvement in the technology for sulphuric acid manufacture. Key developments of the contact sulphuric acid technologies include: low emissions plant designs with double absorption and high performance catalysts, low pressure drop acid tower packing, anodically protected acid coolers, stainless steel SO2 converters, radial flow gas heat exchangers with hot sweep, acid distributors, improved equipment configurations and high-silicon alloy steel equipment for acid service. Sulphuric acid catalyst The same basic vanadium pentoxide catalyst used in the late 1920s and 1930s is still used today, with 5-7% vanadium for upper bed catalyst and 7-9% vanadium used in lower bed catalyst. The vanadium/potassium/sodium sulphates are supported in the pores of ceramic or silica base (diatomaceous earth). The original catalyst was an extrudate about 1/4" (6 mm) diameter by about 1/2" (13 mm) long. The major changes to modern catalyst, are the catalyst shape to reduce pressure drop and increase ash/dust capacity, and in some cases, the addition of caesium to reduce the ignition temperature. Currently all three major catalyst manufacturers market a ribbed ring catalyst (Fig. 1). Essentially all sulphuric acid plants around the world use ribbed ring catalyst to increase dust holding capacity, reduce clean pressure drop, and reduce pressure drop build-up (increases) over time. Ribbed ring catalysts have reduced the energy consumption of the main blower and increased the time between turnarounds (shutdown for maintenance). Turnaround intervals have increased from once per year to once every other year, or even to once every three years. Keywords: converter, sulphuric acid catalyst, acid tower, acid cooler, equipment, hydrogen, operator training, simulator

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Hybrid – the future of sulphuric acid plants

Summary

The concept of hybrid sulphuric acid plants is a bit of a novelty today. However, the technology does have a number of compelling value advantages that could propel hybrid plants into the mainstream of the future. N. Porter, E. Vera-Castañeda and S. Puricelli of MECS compare hybrid plants (single absorption + SolvR™ technology) with conventional double absorption plants.

Abstract

MECS, Inc. (MECS) is a worldwide leader in the licensing of sulphuric acid plants and has been in the sulphuric acid business for almost 100 years. Over the years, MECS has introduced a progression of innovations such as Brink® mist eliminators, ZeCor® stainless steel, HRS™ and most recently, the new regenerative SO2 scrubbing technology called SolvR™. MECS® SolvR™ technology The MECS® SolvR™ regenerative scrubbing technology captures sulphur dioxide (SO2) from waste gas and recycles it back to the acid plant for additional acid production. The SolvR™ flow sheet closely resembles that of a classical regenerative absorption system and is shown in Fig. 1. Keywords: MECS, SolvR, regenerative scrubbing, energy integration, MAXENE, debottleneck, SO2 emissions

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The crunch is approaching

Summary

From January 1st 2015, the fuel that ship operators use in the Emission Control Areas (ECAs) off Europe and North America must contain a maximum weight of 0.1% sulphur. With just three months until the deadline, Sulphur looks at the prospects for the marine fuel mix in the coming years.

Abstract

The International Maritime Organisation’s rule revisions on permitted sulphur content in marine fuels have been the biggest shake-up in sulphur fuels regulations for some years, and promise to cause headaches for refiners and ship operators alike. While the first stage of the tightening, from a global cap of 4.5% sulphur by weight to 3.5% in 2012, was met with very little problem, as most bunker fuels were already beneath that level anyway, the next drop, down to 0.1% sulphur in fuel for designated emission control areas (ECAs) from January next year, looks set to be far more problematic. It could mark the beginning of the end for heavy duty fuel oils as a marine fuel, especially problematic for refiners as these have traditionally been a ‘sink’ for high sulphur refinery bottoms, and many refiners have been busy installing upgrading capacity over the past couple of years to deal with a potentially changed product slate. Keywords: HFO, LSHFO, LSFO, MGO, LNG, methanol, gasoil

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H2S - how much is too much?

Summary

The American Conference of Governmental Industrial Hygienists' (ACGIH) decision to lower its recommended maximum permissible exposure limits for hydrogen sulphide from 10ppm to 1ppm for a time weighted average continues to cause some disagreement among safety professionals.

Abstract

Hydrogen sulphide is the leading cause of death by gas inhalation, and according to a review of United States Bureau of Labor Statistics (USBLS) Census of Fatal Occupational Injuries (CFOI) for occupational deaths, responsible for 52 deaths in the period 1993-1999, in 21% of cases of mutliple fatalities. The US Occupational Health and Safety Administration (OSHA) sets the maximum permissible exposure limits at 50ppm for a 10 minute period and 20ppm on a long-term (eight hours), time weighted average (TWA). However, it is widely recognised that these limits probably underestimate the damage that exposure to H2S can do, and levels from 10-50ppm are known to cause headaches and dizziness, nausea and vomiting, coughing and breathing difficulty. The standard guidance issued by the American Conference of Governmental Industrial Hygienists (ACGIH) therefore used to set a 10ppm 8-hour TWA for long term exposure and a 15ppm peak exposure limit. These limits have become widely accepted throughout industries which deal with H2S, and built into many state and local standards. For example, the Canadian province of Alberta sets these values as maximum permissible exposures. The US National Institute for Occupational Safety and Health also lists 10ppm as a maximum recommended exposure level, and OSHA sets a maximum 10ppm TWA for areas like shipyards and construction sites. Keywords: ANSI, NFPA, ASSE, monitoring, Z390

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Demand for sulphur dioxide

Summary

Beyond its use in the manufacture of sulphuric acid, sulphur dioxide also has many industrial uses, especially in the food, paper, pharmaceutical and refining industries.

Abstract

Sulfur dioxide, SO2, is mainly an intermediate in the sulphur industry; in sulphur burning acid plants, it is the first step from combustion of elemental sulphur, and thereafter is further oxidised to SO3 and converted to oleum. It is also, of course, a major by-product of the roasting of sulphide ores in the metals industry, from which sulphuric acid can be produced from pyrites or copper or nickel sulphides. Its generation and release to air by burning sulphur-based compounds has had well-documented deleterious effects on the environment (‘acid rain’) and human health, and avoiding its accidental release has become the rationale for ever-tightening restrictions on sulphur levels in fuels. Keywords: Hydrosulphite, refining, captive, merchant

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Mishraq Sulphur Purification Facility

Summary

Uday Parekh, Doug Houston and Jeff Hutsell from Devco USA, David Singleton, Robin Strickland and Jolly Jatinder of Crescent Technology, Inc., and Mohammed Alayaan Atteyeh of Al-Hawarth Est, Jordan discuss the design, commissioning and construction of the first new sulphur processing plant for many years, for the Mishraq State Sulphur Company in Iraq.

Abstract

The Mishraq native sulphur deposit lies in northern Iraq, about 45 km southeast of Mosul on the west bank of the Tigris River, at its confluence with the Great Zab River. This deposit is the largest known occurrence of stratiform bioepigenetic sulphur, containing at least 100 million tonnes of elemental sulphur, and is part of the Mesopotamian Depression, which may be the largest reserve of elemental sulphur in the world.1 The Mishraq Sulphur State Company (MSSC), an Iraqi Ministry of Industry and Mineral Company, was established in 1969 and began production of sulphur from these deposits using the Frasch process in the early 1970s. The sulphur mined from these deposits contains a significant amount of hydrocarbons, and must be purified to meet commercial specifications. This is also necessary for the effective use of the sulphur in downstream applications, primarily the manufacture of sulphuric acid for phosphate fertilizer production. Keywords: Submerged combustion, SCD, Frasch, carsul

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