Anti-fouling additives to improve heavy oil processing

A continuous pilot testing evaluation shows how magnesium sulphonated-based nano-additive is highly effective in mitigating and reducing upgrader/refinery fouling'

Darius Remesat, University of Calgary - Edward Maharajh, Energy Technical Resources
Youssef Elgahawy, Suncor Energy - Justin Martin, Western Research Institute

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Article Summary

Additives have been developed to disperse fouling-generating molecules, such as asphaltenes and inorganics, and mitigate fouling that poses operational concerns and production upsets to upgraders and refineries across the world. A comparative analysis of different dispersive additives in a continuous pilot testing environment is provided to determine the effectiveness of these additives in mitigating or reducing the fouling in two specific areas of concern: intermediate piping, vessels, and exchangers in bitumen upgraders; and the kerosene pumparound zone. Results from larger-scale continuous pilot testing vs limited batch lab testing give operators confidence when deciding if or how to incorporate an additive programme into their facility to improve reliability and/or debottleneck operations. Notably, a nano-additive magnesium sulphonate base appears to be effective on numerous types of fouling mechanisms experienced at heavy oil upgraders/refiners.

Fouling in upgraders and refineries challenges operability and profitability through long-term accumulation that can reduce throughput or, in severe cases, lead to operational upsets that trigger unplanned outages. Chemistry has been developed that can play a role in improving and/or maintaining performance, with the inclusion of nanoparticles in formulations stretching the applicability of additives.

However, offerings from additive suppliers are based on proprietary experiential data supported by lab-scale testing of the problematic fluid and/or foulant, which does not address the specific operating scenario of concern to the operator. To improve confidence in the claimed benefits of the additive, continuous flow testing at operating conditions was carried out at the Western Research Institute (WRI) in Laramie, Wyoming.

Though not at a commercial scale, the test facility operates at elevated temperatures and pressures, with flow capabilities in both the laminar and turbulent regimes for extended periods to provide insight into additive performance closer to actual operations. Various additives were tested in different environments to inform both additive vendors and operators and support decision-making when choosing fouling mitigation strategies for towers, heat exchangers, reactors, and piping in upgraders/refineries.

Coke/foulant formation
Foulants can be generated from various mechanisms, and combinations of accumulated foulant can be found in many locations within an upgrader/refinery. Some typical fouling mechanisms and locations are shown in Table 1.

Additives are formulated to address a specific fouling  mechanism and thus different foulants typically require different formulations (concentration and type of additive).  The challenge is identifying the appropriate fouling mechanism and creating an additive that addresses the foulant.

Continuous pilot testing
Suncor Energy, an integrated energy firm, owns and operates bitumen upgrading and refining operations that include atmospheric and vacuum separation, coking, hydrocracking, and hydrotreating units. Various additives have been investigated and tested to varied degrees of success. To improve the selection and utility of additives, a low-cost and agile pilot plant testing set-up was created at the WRI. The purpose of continuous pilot testing is to create conditions similar to field operations to replicate fouling conditions and determine the impact of additives on mitigating and suppressing the amount of fouling in various situations, such as an empty pipe, heat exchanger, reactor, and in trayed/packed tower columns.

Pilot testing methodology
WRI developed a continuous closed flow loop pilot operating at 5-10 b/d under representative process conditions up to 300 psig and 780°F (416ºC). In one set-up, shown in Figure 1, the target crude material is introduced into the flow loop and passed through an electric heater. The flow, pressure, and temperature are set to the desired conditions, and the material is circulated. Pressure indicators are used to gauge any increase in foulant material. The pilot is operated until a noticeable pressure drop is witnessed in the pipe segment. Depending on the material and conditions, the pilot can be operated until the pipe is fully plugged, if desired.

For additive testing, a fresh batch of crude is injected and operated at the same conditions but with the additive in place. Different concentrations can be tested. The difference in time to become plugged or reach a certain level of fouling can be compared with the base case operation with no additive. A variation in testing is to inject the additive part-way through the foulant generation step to investigate if the additive (type and concentration) can both mitigate fouling and remove previously generated foulant (a mid-run option).
For the testing of fouling in distillation towers (trays and grid provided by a third-party vendor), a 12in ID pipe with cartridge-style trays and/or grid is installed in place of a storage vessel. Figure 1 shows two trays installed along with three foulant collection points for the distillation internal evaluations.

Pilot testing for fouling in a distillation tower set-up is typically divided into three main stages:
•    Evaluate the effectiveness and impacts of different additives on fouling reduction and mitigation. The main objective of this stage is to determine the best additive
•    Determine the minimum required dosage of the best additive to mitigate fouling and improve performance during commercial operations
•    Determine the impact of different internal tower configurations and designs (installing grid/packing instead of trays) without additive injection.

Once the crude (in this example, coker kerosene pumparound material) is added and fills the testing loop, flow circulation is initiated. Then, the temperature of the system is increased to a final target temperature of 600-610°F (315-321ºC) on the top tray. A system pressure of 47-60 psi is maintained by utilising a pressure control valve on the storage vessel.

For the coker kerosene pumparound material, WRI performed eight separate tests with the same duration and operating conditions for the three stages of pilot testing, with fresh material charges for each test. The amount of fouling and severity was qualitatively characterised by opening the distillation tower between and after the operation to visually inspect for fouling.

Quantitatively, the mass of foulant material collected and the pressure differential across the distillation tower were used as tools to quantify the fouling severity. Three foulant collection locations were identified and are labelled on the distillation vessel in Figure 1. Furthermore, the results from testing heaters, piping, and distillation fouling configurations are discussed in the following sections.

Immersion heater
An electric immersion heater in a pool of topped Athabasca bitumen was tested in a once-through configuration at WRI. After 80 test runs, without the benefit of an additive, the maximum sustainable temperature was around 755°F (402ºC). When the temperature was above 755°F (402ºC), toluene insolubles in the outlet accelerated until the temperature hit a maximum before dropping due to foulant build-up on the immersion heater.

The fouling mechanism was heavy hydrocarbon thermal coking. Figure 2 shows a test run where the immersion heater was pushed above 755°F (402ºC) to test the benefit of adding the nano-additive, a high-temperature dispersant comprised of magnesium oxide (MgO) nanoparticles dispersed in magnesium sulphonate (MgSO3H). 300 ppm of the additive was injected into the feed once the heater had been fouled and the performance had deteriorated.

The additive immediately improved the operation, effectively ‘cleaning’ the foulant off the immersion heater and allowing the immersion heater temperature to reach a new maximum by preventing any new foulant from forming. Conversion of the crude increased by 2-3 wt%, improving the overall performance of the operation.
Subsequent tests determined that the minimum amount of nano-additive required to maintain the new maximum temperature and improved performance was 100 ppm. For commercial-scale operations, this can likely be in the range of 75-125 ppm to gain the same benefits.

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