heavy oil processing impacts refinery and effluent

New Technologies and Alternative Feedstocks in Petrochemistry and Refining DGMK Conference October 9 – 11, 2013, Dresden, Germany

DGMK-Tagungsbericht 2013-2, ISBN 978-3-941721-32-6

Heavy Oil Processing Impacts Refinery and Effluent Treatment Operations P. Thornthwaite Nalco Champion, Northwich, Cheshire, United Kingdom Abstract Heavy oils are becoming more common in Europe. The processing of heavier (opportunity or challenge) crudes, although financially attractive, introduce additional challenges to the refiner. These challenges are similar whether they come from imported crudes or in the future possibly from shale oils (tight oils). Without a strategy for understanding and mitigating the processing issues associated with these crudes, the profit potential may be eroded by decreased equipment reliability and run length. This paper focuses on the impacts at the desalter and how to manage them effectively while reducing the risks to downstream processes. Desalters have to deal with an increased viscosity, density (lower API gravity), higher solids loading, potential conductivity issues, and asphaltene stability concerns. All these factors can lead to operational problems impacting downstream of the desalter, both on the process and the water side. The other area of focus is the effluent from the desalter which can significantly impact waste water operations. This can take the form of increased oil under-carry, solids and other contaminants originating from the crudes. Nalco Champion has experience in working with these challenging crudes, not only, Azeri, Urals and African crudes, but also the Canadian oil sands, US Shale oil, heavy South American crudes and crudes containing metal naphthenates. Best practices will be shared and an outlook on the effects of Shale oil will be given.

Introduction Today’s refining and energy market is undergoing significant volatility and transition. The nominal cost of oil has fluctuated more in the last few years than ever before while refinery utilisation rates have steadily fallen over the past several years from a figure of 90% in 2005 to 75% in 2012[8]. From a global perspective, European refiners are suffering the worst of these changes. Reduced demand, decline in gasoline exports to the US coupled with on-going clean fuels requirements and the continued tightening in environmental regulations are adding to the erosion of refining operating margins in the region. Consequently, the European refining market will continue to consolidate over the short to medium term[2] and unit availability and reliability will need to be consistently high in order to control margin attrition, regardless of crude feedstock quality. One way that refineries can achieve improved operating margins is through the processing of opportunity crudes. The accepted definition of opportunity crude is either a crude oil that is new to market with unknown or poorly understood issues, or existing crudes with known processing concerns. A discount often exists when running these challenging crudes, however, this profit potential can be eroded without strategies to assess and mitigate these processing issues[1].

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New Technologies and Alternative Feedstocks in Petrochemistry and Refining

DGMK-Tagungsbericht 2013-2

As refineries seek additional volumes of crude to ensure future capacity is met while still maintaining plant profitability, the historically lighter (sweeter) crudes are being replaced with cheaper, higher margin feedstocks that are often heavier and more difficult to desalt. Crude oils from West Africa and South America are becoming more readily available and many of these crudes contain new processing challenges; this includes higher solids levels, asphaltene instability in blends, higher viscosity and conductivity, amongst others, all of which can threaten unit availability. With the existing refinery configuration there can be significant difficulty to maintain reliability and control operating expenses when faced with these new issues. This discussion will focus on the processing of opportunity crudes and how the quality of crude blends and their impact on desalter performance can be managed.

Processing Issues Associated with Opportunity Crudes The deterioration in desalter performance can manifest itself in numerous areas. Opportunity crudes often exacerbate the deterioration in performance, or cause a previously satisfactory desalter operation to start to decline. The typical problems are: • Oil undercarry and higher levels of oil coated solids in desalter effluent. • Increased wastewater treatment plant loading • Solids or asphaltene stabilised emulsion (rag) layer and rag growth. • Water and/or solids carryover in desalted crude. • Desalting and dehydration performance decay. • Increased corrosion potential in crude unit overhead systems. • Higher fouling rates in the hot preheat exchanger network. • Higher furnace fouling rates • Increased energy consumption (excess water and preheat fouling). • Crude charge reduction. • Negative impacts to FCC catalyst life, coker and visbreaker unit operation, etc. Upon the introduction of a new crude, procedures must be put in place in order to assess the potential processing risks. This should also help the refinery to identify the mechanical, operational and chemical improvements they may need to consider. However, even with a sound process in place, refiners are often constrained in their efforts to process higher rates of opportunity crudes due to the performance decline at the desalter and the potential downstream impact[3]. Refiners often have to make compromises on their crude schedules in order to offset the potential risk of processing problems in downstream equipment (corrosion, fouling, increased energy usage, reduced run length, for example) by reducing the amount of opportunity crude in the slate, thereby reducing the margin gain and operating profits the opportunity crude provides. The desalting of heavier opportunity feeds are often a challenge to the refinery due to the characteristics of the crude oil themselves. As the density of the crude increases, it is typical to also see the viscosity of the crude oil increase. The impact of these characteristics can be understood by looking at how the increase in density and viscosity impact on the settling velocity of the water droplets in the desalter. This is determined by applying Stokes Law which predicts that the settling velocity of a spherical object (water droplet) in a viscous liquid (crude oil) is directly proportional to the density difference between oil and water and inversely proportional to the viscosity of the crude oil. As the density of the crude oil increases, the density differential between it and the wash water diminishes. Additionally, as density of the crude increases, the viscosity tends to increase. Therefore, the driving force for the separation of the oil and water is diminished as a result of processing crudes that are higher in density and viscosity.

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It is also common to observe that opportunity crudes can have much higher mineral solids content. Poor removal efficiencies at the desalter will result in the increased risk of forming stable emulsions at the oil and water interface with high amounts of solids carrying over with the desalted crude. The finely divided solids, commonly described as filterable solids, found in crude oils have long been known to contribute to the formation of stable emulsions at the oil and water interface. These oil coated solids migrate to the interface where they accumulate, thus preventing the water droplets in the crude oil from coalescing into larger droplets thus stabilising the emulsion band at the interface. This emulsion, if left unresolved, can grow to the extent where it will begin to impact on effluent quality and wastewater operations can be impacted due to the higher loading of oil and oil coated solids in the desalter effluent. Wide emulsion bands in the desalter can also result in reduced desalting and dehydration efficiencies which can lead to more chlorides in crude tower overheads, which can potentially form higher levels of hydrochloric acid (HCl) and increase corrosion rates. Furthermore, reduced dehydration and increased water in the desalted crude leads to increased energy consumption. Poor solids removal from the crude oil can also lead to higher fouling rates in hot preheat exchanger networks and crude unit furnaces leading to increased furnace fuel gas usage, and potentially reductions in crude charge. Further downstream, the solids can contribute to foaming in preflash vessels, accelerate fouling in the convection section of Visbreaker and coker furnaces and have negative impact on catalysts in residue FCC units. Many of the so-called opportunity crudes also tend to possess higher naphthenic acid levels. This can have a significant impact on the performance of the desalter since naphthenic acids can act to stabilize emulsions when desalter wash water sources with an elevated pH are used, such as poor quality SWS bottoms. As discussed above, wide emulsion bands in the desalter can also result to reduced desalting and dehydration efficiencies. Additionally, it has also been shown that higher naphthenic acid containing crudes can increase the hydrolysis of chlorides (calcium, magnesium, sodium) in crude unit overheads, especially vacuum tower overheads, and further increase corrosion[4]. Another negative aspect associated with naphthenic acid crudes is that they can be conductive, thus increasing transformer power consumption. As the conductivity rises, the amperage draw increases while the voltage decreases which has the effect of reducing the electrical driving force that coalesces the water droplets. In order to compensate for the increase amp draw, power consumption of the transformer increases and in extreme cases, a full reversal occurs (i.e. maximum amps, zero volts). The net effect of crude conductivity and the loss of coalescing power is that again, dehydration and desalting efficiencies will be compromised. It has also been reported that some crude blends have a higher conductivity than the individual components [6,7] so while there may be a benefit in blending crude to reduce viscosity and density this benefit may be offset by an increase in conductance. In many refineries today, crude slate changes are common and the composition of the processed blends changes significantly. Blending of incompatible crudes, such as a paraffinic condensate or very light crudes with heavier oils, can result in asphaltene incompatibility and subsequent precipitation and flocculation. This is a common problem encountered in US refineries where processing of blends with light shale / tight oils, such as Eagle Ford or Bakken, blended with heavier crudes is becoming increasingly prevalent. Asphaltenes are highly surface active and are known to stabilise emulsions at the oil / water interface. As discussed already, stabilised emulsions can have serious impact on the desalter operation and additionally destabilised asphaltenes can cause high rates of fouling in the hot preheat of the crude unit.

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Addressing waste water treatment plant (WWTP) challenges is a critical part of opportunity crude processing. Historically, the WWTP has been expected to handle any variation in loading whilst still achieving effluent discharge permit requirements. As refiners look to process more opportunity crudes, maintaining good WWTP operation is becoming challenging as the primary and secondary treatment processes are pushed beyond their intended capacity [1]. Increased loading to the WWTP from poor desalter operation can usually be handled by anticipating the change in load and maintaining good operational practices. Failure to do so will lead to poor removal efficiencies, poor biomass settling (known as sludge bulking) and eventually breach of effluent permits. Critical aspects of good WWTP operation are as follows:

• Primary Treatment: Oil and Solids removal is working to design, scrapers and solids removal from the API separator and DAF are maintained in working order, the DAF air pressure and mixing is adequate and the correct emulsion breaking program is in place.

• Secondary Treatment: The biological treatment plant has adequate oxygen supply

to deal with increased COD and amine loading. The resulting increase in biomass growth is anticipated with optimised nutrient addition and sludge age control strategies.

• Process monitoring and control: systems should be put in place to adequately

monitor the unit when being run under a higher load so that issues can be identified quickly and mitigation steps put place when performance indicators deteriorate

• Contingency Planning: Have back-up strategies, like metal removal,

bioaugmentation or antifoam products on site to help control the situation if above best practices are not adequate for this particular crude or treatment plant design.

However, it is important to remember that by maintaining good control of the desalter operation and effluent brine quality, the loading on the WWTP will be reduced. Additionally, reducing oil undercarry will also impact favourably on the slop make of the refinery and this is beneficial since the reprocessing of slop streams themselves can lead to deterioration in desalter performance. All of the issues arising from poor desalter operation can have an enormous impact on the overall efficiency and operating costs of the refinery. Therefore, refiners must balance the margin benefits that opportunity crudes can provide against the potential negatives associated with that particular crude. If the issues are not addressed at the desalter, problems can manifest themselves in further downstream units. An area of focus for Nalco Champion has been the improvement of opportunity crude emulsion breaking, particularly for heavy and high solids crudes. As a result, new emulsion breakers and enhanced solids removal aids have been formulated for use in desalter systems that run heavy opportunity crudes. The performance gain from the application of these new desalting chemistries, in conjunction with the optimisation of mechanical and operation parameters of the desalter operation, can help to successfully reduce and even mitigate the risks of opportunity crude processing.

Managing the Risks: A Holistic Approach to Desalter Management When looking to optimise the desalter performance, the role of the chemical supplier, such as Nalco Champion, is not merely supplying a new emulsion breaker chemical for evaluation. When considering chemical treatment programs, a holistic approach should be adopted

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considering the mechanical, operational and chemical aspects of the operation. The key element at the start of this process is conducting a thorough audit of the process unit, from the tank farm through to the atmospheric tower overheads and waste water treatment operation. By undertaking this audit, any mechanical and operational constraints that prevent the process meeting the key performance indicators (KPI’s) are identified and an improvement plan can be identified and prioritised in agreement with the refinery. Additionally, prior to commencing a trial of a new treatment program, detailed benchmarking and assessment of the current desalter performance must be undertaken in order to confirm the positive performance gains that occur after the transition to a new desalter management programme. Upon the commencement of the chemical transition and new desalter management programme, the Nalco Champion approach follows a three step process:

• Step one: observe the initial impact in desalter performance from the change in demulsifier chemistry only over a short time frame of the first 24 hours.

• Step two: evaluation of the desalter KPI’s compared to the agreed baseline and

before any further optimisation steps. This requires regular sampling and testing to determine any changes in the dehydration and desalting efficiencies, solids removal rates, tri-line appearance, emulsion layer thickness and effluent quality. During this time the desalter achieves a “new” steady state condition.

• Step three: performance improvements are assessed through optimising the mechanical and operational aspects (non-chemical levers). Following a pre-approved trial protocol with the refiner, the chemical supplier and refinery systematically evaluates areas such as mix valve settings, wash water rates and injection locations (balance between before cold preheat and the mix valve), interface level, mud wash procedures and potentially changes to the electrical settings of the transformer.

The changes are carried out one at a time and the regular sampling regime continues in order to evaluate the impact of these changes, positive or negative, prior to further optimisation steps. In this process, the key consideration is that any of the changes made to the desalter operation should not negatively impact on the desalter or the downstream operation. Any observable improvements have to be “real” and in order to prove that the optimisation step has delivered a benefit, a statistical verification process is used to assess the data (after the optimisation step) against the agreed pre-trial benchmark using a two sample t-test. If the result of this t-test shows a p-value of <0.005, it demonstrates that the improvement in results is statistically significant [5]. By adhering to these optimisation protocols, together with utilising a best practice desalting programme, a higher level of desalter performance can be achieved and maintained, even when processing challenging crude slates. This can provide the refinery with increased operational flexibility, allowing the increase in the opportunity charge rate whilst minimising the risks both to the desalter operation and downstream process units. In addition to providing improved performance when processing opportunity crude slates, the adoption of an holistic approach encompassing the mechanical, operation and chemical aspects has also delivered performance benefits on stressed systems. Examples of stressed systems include those where the desalter is operating over its design capacity, where there are low wash water rates or poor quality wash water is utilised.

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This article has discussed common desalting problems associated with processing opportunity crudes and the potential solutions provided by Nalco Champion. The right chemistry applied correctly can offer substantial improvements to the desalter operation. However, it is also vital to audit the process unit in order to have a thorough understanding of the desalting system and its processing limitations. Armed with this information, refiners can also address the uncovered gaps since the performance can be improved further through the optimisation of the mechanical and operational levers. It is through the optimisation of the all the Mechanical, Operational and Chemical aspects that makes a true best practice desalting programme. The case histories discussed in this paper have covered some of the real benefits realised by implementing a best practice desalting program coupled with mechanical and operational changes. These are able to provide statistically significant improvements to the desalter operation when processing opportunity crudes, providing a steady operation and limiting downstream impacts. This allows the refiner to maximise the amount of opportunity crudes processed on the unit, realising the margin benefit without it being eroded due to decreased downstream equipment reliability and run length.

References 1. Mason, B., Scattergood, G., and Gardenhire, J., “Handling the Challenge”, Hydrocarbon

Engineering, 2008 2. Weafer, C., Uralsib Capital, “The Shape of Global Energy Markets”, October 2009

3. Mason, B., Lordo, S., “Opportunities, Problems and Solutions”, Hydrocarbon Engineering,

Mach 2010.

4. Claesen, C., Thornthwaite, P., Lordo, S., “Changing Crude Oil Quality and Refinery Corrosion Inhibition”, EUROCORR 2009.

5. Minitab Technical Support Document, “Interpreting and Calculating p-values”. Retrieved

from www.minitab.com

6. Potter, A.C., “Crude Oil Conductivity”, presentation made to COQA Meeting, February 2007

7. Jones, Henry B., Heimbaugh, William A., “Ten Practical Tips From An Electrical Desalter

Supplier For Desalting Heavy Crudes”, AIChE Spring National Meeting, March 2000

8. Doshi, Viren, “Refining Perspectives for the European Region”, GTF Webinar on Refining: Perspectives from Around the Globe, 27 June 2013.

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heavy oil processing impacts refinery and effluent

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