ABSTRACT

The dangers posed by hydrogen sulphide (H2S) are well-known to oil companies as are the potential risks to the end user if it is not removed from prod-ucts at the refinery or blend plant producing them.

In the case of marine fuels, this has led to the in-troduction of a new fuel standard limiting liquid phase H2S levels in marine fuels. On July 1st 2012, the 4th Edition of the International Organisation for Standardisation (ISO) 8217 for H2S in marine distillate and marine residual fuels will be imple-mented. This specifies a limit of 2 mg/kg of liquid phase H2S by test method IP 570.

What is not as well known is that H2S levels in untreated bitumen can be much higher and that without the proper measurement and control pro-cedures being put in place the risk of unnecessary exposure to this toxic gas can be dangerously high. To date, most bitumen producers have focused on establishing safe handling procedures aimed at minimising the risk of plant and tank farm person-nel’s exposure to the gas rather than reducing the concentration of the H2S present.

The main advantage of reducing H2S levels by chemical means is that not only are the exposure risks to refinery personnel minimised, but also exposure risks to end users who often don’t have systems in place to detect or deal with H2S release. E.g. personal H2S alarms and site H2S monitors.

HYDROGEN SULPHIDE CONCERNS

The dangers posed by H2S both inside refineries and potentially in the products that are produced have been known for many years.

To minimise the risk of exposure to what is a very toxic gas, regulatory bodies have set exposure limits which are regularly reviewed. Limits vary from one body to the next, but some key limits and recommendations are listed:

• UK Short Term Exposure Limit (STEL): 10 ppm (15 mins)

• United Kingdom (UK) and European Union (EU) Long Term Exposure Limit (LTEL): 7 ppm; 8-hour TWA = 5 ppm

• World Health Organisation (WHO) guidelines: 150 µg/m3 (24 hours)

• US Occupational Safety & Health Administra-tion (OSHA): 20 ppm H2S ceiling and a 50 ppm peak which should not be exceeded for more than 10 minutes

• US National Institute for Occupational Safety & Health: Recommend that exposure is limited to a maximum of 10 ppm

• American Conference of Governmental In-dustrial Hygienists (ACGIH): Have recently lowered the recommended threshold limit value Threshold Limit Value (TLV) from 10 ppm to 1 ppm and the short term exposure limit from 15 ppm to 5 ppm

Inside refineries, the main concerns centre on tank farm operation and road tanker loading where historically, high levels of H2S have been detected. In storage tanks, this problem can in part be man-aged by venting tank outlet lines back to fuel gas systems.

At road tanker loading facilities where drivers in some countries frequently stand on top of tanks during the loading process, some refiners have minimised the risk of exposure to H2S by fitting extraction collars around tanker fill lines with the extracted toxic off gas being routed to the fuel

By Tony O'Brien, Ron Sharpe and Simon Crozier, Nalco

Hydrogen Sulphide in Bitumen:Minimisation of Exposure Risks during Production and at End User Facilities

An Ecolab Company

Reprint R-1078E

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gas system. This provides protection to tanker drivers however it does not ad-dress problems that can arise when the bitumen is offloaded at the user’s facility. This is of particular concern where the bitumen is used in the manufacture of carpet tiles and other flooring which is done in relatively confined spaces.

While natural malodours originating from bitumen can lead to complaints from local authorities and residents living close to storage and handling fa-cilities, the presence of H2S can further exacerbate the problem. In the USA, because of health concerns many states limit H2S levels in air to a few parts per billion. For example, Minnesota and Cali-fornia have state H2S regulations, which are 50 ppb for not more than half an hour, and not more than two occurrences per year, and 30 ppb for not more than half an hour for not more than two occurrences in a 5-day period.

Other than safety and environmental concerns from a refiner’s perspective the presence of H2S in bitumen can have financial implications. Firstly if it leads to a delay in the release of bitumen while venting/weathering operations take place, and secondly where it limits the type of crude oil they can process.

Concentrations of H2S in vacuum residue tower bottoms (typically the principal bitumen base feedstock) are heavily influenced by the types of organo-sulphur compounds present in the crude oils from which they are derived. A lack of flex-ibility in the ability of a refinery to process low cost or specific crude oils can significantly impact its profitability.

GENERATION OF H2S

On bitumen production plants H2S is typically generated by the cracking of organo-sulphur com-pounds at the bottom of the Vacuum Distillation Unit (VDU) column and in the rundown line to tankage. In the case of Polymer Modified Bitumen (PMB) plants this occurs during the sulphidation process.

In the manufacture of PMB the point in the process when a scavenger is injected into the mix is criti-cal. If added before the sulphidation step, much of the scavenger will be consumed unnecessarily by the reactions occurring during the sulphidation process. When added after the sulphidation process sufficient scavenger can be added to react with any H2S present in the PMB (Figure 1).

MEASUREMENT OF H2S

The prime risks posed by H2S relate to high gas levels in the vapour phase in and around storage tanks and in road tankers during loading. For this reason historically it was considered that vapour phase measurement was the most applicable when determining whether or not a batch of bitumen was safe to transport. This however, is not the case. H2S in the liquid phase and in the vapour above tanks exist in equilibrium. Consequently, if for example H2S vapour in a tank was lowered by purging, H2S present in the liquid phase would relatively quickly migrate from the liquid to the vapour phase establishing a new equilibrium. For this reason Nalco would advocate liquid phase over vapour phase measurements as a test standard.

Physical factors affecting H2S liberation from the liquid phase include:

• Temperature – Hot hydrocarbons release more H2S

• Viscosity – Viscous hydrocarbons trap and re-lease H2S more slowly than less viscous liquids

• Mixing – Agitation causes H2S to be more quickly released from the bulk liquid

• Weathering – H2S levels decrease through sparging and venting through open hatches

• Hydrocarbon Composition – H2S solubility varies with hydrocarbon composition

Test data has shown that 1 mg/kg of H2S dissolved in the bulk liquid phase can produce between 5 – 500 mg/kg in the vapour phase. Consequently, by lowering H2S levels in the liquid phase you can mi-nimise, but not totally remove toxic exposure risks.

Although some oil companies have their own internal standards there is no industry agreed limit for H2S when it comes to bitumen and PMB supply. Part of the reason for this is that there is

Figure 1 – Impact of Scavenger Addition on Levels of H2S

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no industry agreed standard H2S measurement technique with individual oil companies either not measuring at all or using their own in-house methods for either determining H2S in the liquid or vapour phases.

The situation is very similar to that seen several years ago with respect to the supply of marine distillate and marine residual fuels. Then suppli-ers were either simply not measuring H2S levels or using different measurement techniques many of which gave significantly different answers. It took protracted discussions between refiners, fuel oil testing laboratories and end users before a wet chemistry technique was agreed (IP 399).

Recently, a new rapid liquid phase extraction method has been developed to determine the concentration of H2S in fuel oils. IP 570 was first published in 2009 and utilises an instrument de-

veloped by SetaAnalytics (Figure 2). It requires Suppliers looking to conform to the new ISO 8217-2010 standards to limit liquid phase H2S levels to a maximum of 2 mg/kg.

IP 570 has a sample viscosity limit of 3000 cst at 50°C therefore H2S concentrations in viscous materials such as vacuum or visbroken residue cannot be measured.

To analyse viscous material such as bitumen, Nalco have produced a simple analyser with no sample viscosity limitation (Figure 3).

The method involves dissolving a known weight of bitumen or PMB in an aromatic solvent in an aluminium test cell. This cell is heated and mixed whilst sparging with nitrogen. Any H2S evolved is detected using a Dräger tube and recorded in mg/kg of H2S in the liquid phase. Depending on the solvent used, the instrument is capable of heating samples up to normal bitumen storage tempera-tures of between 160 and 180°C.

Figure 2 – IP570 Instrumentation with VPP for H2S Measurement

Methods to determine the concentration of H2S in the vapour phase are also still commonly used in the industry. For example, variants of ASTM D 5705 (sometimes termed can tests) are utilised to measure not only H2S in residual fuel oils but also much heavier streams such as vacuum residue and bitumen.

Refineries often employ their own vapour phase methods which have been developed in house. These include a method whereby a specific volume of H2S-containing vapour is displaced from above a test sample using a neutral oil and passed over an indicating tube.

For the future, Nalco would propose under the aus-pice of Eurasphalt and Eurobitume that bitumen producers and end users work on the development of an agreed industry method for liquid phase de-termination of H2S in bitumen.

MITIGATION OF H2S

Where H2S contamination has been identified as an issue refiners have several options available to them. While we have listed several below we ap-preciate that for cost and operational reasons many may not be considered practical. Options include:

Figure 3 – Nalco Dilute and Purge Analyser for H2S Measurement

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• Reviewing crude oil purchase selection – because of the organo-sulphur compounds they contain, some crude oils are known to generate signifi-cant amounts of H2S when processed through VDUs.

• Lowering and better controlling VDU column bottom temperatures – although bottom temper-atures are adjusted to maximise product yields and the quality of the final residue lowering bottom temperature by just a few degrees can significantly reduce the amount of H2S gener-ated in the residue (Figure 4).

• Weathering of tanks – This option simply involves allowing the tank to stand for a few days allowing H2S to slowly escape through tank vents. While the process can be speeded up by air or nitrogen sparging there are obvious potential safety issues if venting to atmosphere rather than for example into a fuel gas system. The other disadvantage of this technique is that all you in effect to do is to set up a new equi-librium between H2S in the liquid and vapour phases something which could subsequently be affected by factors such as temperature and viscosity changes and turbulence caused by mix-ing or turbulence produced during tank transfer operations.

• Use of odourants – Nalco does not recommend the use of odourants to mask the smell of H2S. While H2S at low concentrations does have a foul smell, detecting its presence by odour serves to warn people coming into contact with the gas of the dangers.

• Use of absorbents – Nalco have no experience in this area, but believe that these products function by “absorbing” H2S in water/mineral oil mists. Our concern is that over time H2S ab-sorbed into the mist will eventually be released to atmosphere.

Figure 4 – Impact of Vacuum Column Bottom Temperature on H2S Generation

• Utilisation of H2S scavengers – Although H2S scavengers have been used for many years to minimise H2S evolution from distillates and heavy fuel oil it was until a few years ago rarely used to treat bitumen. Following a better under-standing of the dangers posed from H2S release from bitumen and PMB and in some cases from customer complaints the use of scavengers have grown rapidly in the past 3 years. Something we see continuing as tighter local and regional restrictions on emissions are applied.

H2S SCAVENGER CHEMISTRIES

While several chemical treatment options exist, selection of a suitable programme is essential if the application is not going to give rise to other issues. The types of chemical treatment options available include the use of:

• Formaldehyde-based products – These prod-ucts while effective and inexpensive pose serious handling and storage issues. Many refiners are very reluctant to use such materials.

• Complex Proprietary Amines – Although widely and successfully used by Nalco and other companies to treat distillates and fuel oil we would advise against the use of triazines and similar molecules to scavenge H2S from bitumen. The principal concern being that at high VDU residue rundown and tank storage temperatures the triazines and activators could breakdown.

• Iron carboxylates – Nalco patented the use of an iron carboxylate based bitumen H2S scaven-ger 20 years ago. Since then it has become the most widely used chemistry to treat bitumen and PMB. The principal advantages over previ-ously used additives are:

➢ It can be injected at temperatures up to 200°C without any problem

➢ The principal reaction product, iron sulphide is heat stable and a natural contaminant of bitumen

• Zinc Carboxylates – Nalco has more recently developed what we see as a unique product that is proving to be much more cost effective than the iron based alternatives. The novel manufacturing process which is going through the patent process allows us to produce a highly active easily pumpable product.

Like iron carboxylate based scavengers the zinc sulphide reaction product is completely heat stable at tank storage temperatures.

H2S SCAVENGER APPLICATION

Nalco recommend the use of a patented purpose-designed quill installed in the rundown line to improve mixing efficiency (Figure 5) and/or the installation of a static mixer.

Figure 5 – Nalguard quill

DOSAGE REQUIREMENTS

The basic stoichiometry of the reaction between scavenger product and hydrogen sulphide will de-termine the minimum ration of scavenger product required, but this is only realised with very good mixing and sufficient time for complete reaction. For Nalco zinc based scavenger the theoretical minimum ratio would be around 12 ppm of product per ppm of H2S in the liquid phase. In practice this can only really be determined during field evalua-tions as requirements would be impacted by factors such as mixing efficiency, contact times and process temperatures, which will vary from plant to plant.

PROGRAMME OPTIMISATION

The supply of an effective product does not in itself guarantee the success of any treatment chemical programme. Of equal importance is effective appli-cation and control of chemical dosing and reaction to the results of treatment.

The site-specific product dosage ratio requirement (Scavenger : H2S) needs to be determined based on analysis of the H2S concentration in the untreated run-down and treated bitumen tank samples. In carrying out this work due allowance needs to be made for residence time within the system and any existing tank bitumen volume and its H2S content. In practice this can complicate assessment of this reaction ratio considerably and result in a true assessment of the ratio taking many days or even weeks. Whilst the ultimate sample of importance is the tank sample, in determining the effective scavenger ratio it is probably better to sample the treated bitumen as close to the tank inlet as pos-sible using inert, sealable sample containers and subject these samples to storage at tank conditions, in a controlled temperature oven before determin-ing the hydrogen sulphide content.

Having established the “effective” reaction ratio, it is then important that the chemical injection rate is varied to maintain this ratio if the base loading of hydrogen sulphide in the run-down bi-tumen varies. In our experience the base loading can vary significantly over relatively short time frames (Figure 6) and it may be necessary at some plants to test untreated blend components up to three times per day to effectively control scavenger dosage rates. This level of analysis and control can represent a significant increase over the refiners current practice but is critical to the performance of the programme where hydrogen sulphide loading levels vary in this way. Longer term the service provider should work with the refiner to identify key process and feed component variables that might be controlled to reduce both the level and variation in hydrogen sulphide content.

Figure 6 – Samples from a vacuum residue rundown at a European refinery

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