Monitoring against process problems
Careful monitoring for preheat exchanger fouling and H2S release during unloading is vital when tight oil blends are processed.
Emerson Process Management
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The refining industry has changed over the past few years with an ample supply of opportunity crude oils available. Opportunity crudes have been around for many years, but only until recently has the abundance of these discounted crudes changed the behaviour of refiners to shift away from their usual feedstocks.
Tight oil, sometimes referred to as light tight oil or LTO, is also an opportunity crude oil from shales or other low permeability formations. Although most of the news about tight oil production is in the United States, tight oil is not unique to this region and is found throughout the world; see Figure 1 with the map of basins with assessed shale oil and shale gas formations, as of September 2015 (US Energy Information Administration, EIA). Production from tight oil formations requires the same hydraulic fracturing and often uses the same horizontal well technology employed in the production of shale gas.
Crude variability challenges
The main challenges with processing opportunity crudes include: crude blending to match the refiner’s configuration and processing capabilities, crude switch disturbances, fouling and accelerated fouling from incompatible crude blends, corrosion, and energy balancing across the crude unit preheat exchangers. In addition to the above mentioned issues, light tight oils also have challenges typically related to H2S (treated with amine-based H2S scavengers), paraffin waxes, significant quantities of filterable solids, variability in API gravity from the same source, and catalyst performance related to cold flow properties.
Heat exchanger fouling is one of the biggest challenges in refinery operations. Many refiners still use a spreadsheet with monthly calculations, typically based on incomplete data, to evaluate heat exchanger condition, with manual checks on individual bundles just prior to a turnaround to determine if cleaning is required. The traditional approach to monitor heat exchanger fouling through spreadsheets with manual entry of temperatures and pressures was usually sufficient before the increase in crude blending from opportunity crudes such as tight oil. However, some crude oil blends are not compatible, leading to unanticipated accelerated fouling.
Because tight oils tend to be lighter, they need to be blended with other crude oils to get the right balance for best utilisation of existing downstream units. Having a more consistent feed to the crude unit also allows for the opportunity to optimise operation. If light tight oil feeds are not blended, the lighter oil can bottleneck the crude overhead and downstream naphtha processing units, and limit production for bottom of the barrel processing. Some refiners are blending more than two crudes to get the right balance of feed qualities, which creates unknown issues with crude incompatibilities. When crudes are incompatible, accelerated fouling occurs in the crude unit preheat exchanger train due to asphaltene precipitation. Accelerated fouling can lead to additional energy costs with the crude unit fired heater, limited throughput when the fired heater becomes duty limited, or earlier shutdown for heat exchanger cleaning. All these negatively impact the profitability of the refinery. Traditional manual heat exchanger fouling monitoring with limited data and Excel spreadsheets does not always catch which crude blends are incompatible, thus the same condition for accelerated fouling can be repeated in the future. It should also be noted that the percentage of crude oils blended will have an impact on crude incompatibilities. For example, an 80-20 blend with 20% tight oil may not be enough to see accelerated fouling, whereas a 70-30 blend may be unstable and have additional unwanted fouling.
For the US, the EIA expanded its monthly reporting of crude oil production with new data on API gravity. What was so interesting was that, for the first nine months of 2015, most (50.8%) of the crude oil produced in the Lower 48 states were light oils with an API gravity above 40 degrees (see Figure 2). The largest share of production was in the 40.1 to 45 degree API gravity range. Production increases over the past several years in the Bakken, Permian Basin, and Eagle Ford formations account for almost all recent growth in US crude oil output. These low permeability (tight oil) formations are producing mostly light crude oils.
For the US Gulf Coast, there was a major investment in refineries to process heavier and sour crude oils from sources like Venezuela and Canada. This upgrade investment in refineries was done prior to technology advancements and the shale boom that has taken place in the United States over the past several years. Unfortunately, tight oil is not heavy or sour, so this creates a mismatch in crude oil properties required for refineries upgraded to handle heavier and sour crudes.
Another noticeable change is the crude unit cold section preheat exchangers prior to the desalter. When operating with one crude oil or a stable crude oil blend, fouling occurs primarily in the hot section downstream of the desalter and not in the cold section. Because of this, the cold section exchangers typically have minimal process measurements like temperatures and pressures in and out of each bundle for monitoring heat exchanger fouling. But tight oils have paraffin waxes and significant quantities of filterable solids (as much as 200 lb [90 kg] per thousand barrels) that are fouling the cold section heat exchangers. Refiners are now beginning to monitor these heat exchangers more closely and work with both automation and chemical companies to mitigate abnormal and accelerated fouling.
Crude incombatability and fouling
Today, refiners are also installing WirelessHART (IEC 62591) temperature and pressure measurements around all crude unit preheat exchanger bundles and implementing predictive analytic software applications to monitor and analyse heat exchanger performance and minimise energy and capacity losses. Improvements in online monitoring and analysis enable refineries to better understand accelerated fouling due to crude incompatibilities and identify which tube bundles require cleaning. Fouling across the bundles is not linear, so determining which bundle is fouled and needs cleaning can be difficult to determine without all the process measurements like temperature, flow, and differential pressure.
For example, a Gulf Coast refiner that was one of the first to use tight oil experienced severe and unexpected fouling in the crude unit preheat exchangers. Unfortunately for the refinery, it was forced to shut down to clean the excessive fouled heat exchangers, resulting in lost production and additional cleaning costs. The refiner has since added online temperature and pressure measurements on all heat exchanger bundles, implemented software to analyse fouling, and the crude unit process engineer reviews fouling every day. They also have a better understanding on the percentage of different crudes blended to determine what crude blends are incompatible, resulting in accelerated fouling; this information is forwarded to the schedulers and planners. Although tempting to buy any discounted opportunity crude on the market, now crude oil purchasers also utilise crude incompatibility information along with crude properties and price to determine ‘compatible’ feedstocks for the refinery.
Even with knowledge about crude blend incompatibilities, a refiner may still experience accelerated fouling issues when the supply chain is disrupted. For example, a crude shipment may be delayed due to severe weather in the Gulf, thus a refinery will run with what crude is available on site, which may not be the preferred crude blend.
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