Revamping crude and vacuum units to process bitumen
Revamping crude and vacuum units to process dilbit can involve extensive equipment replacement as well as major changes to the crude preheating scheme
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Bitumen has become an increasingly significant source of raw crudes for refineries, especially in North America. About 170 billion barrels of proven reserves are economically recoverable with the latest methods, as reported in 2010 for Alberta oil sands in Canada. In addition to this vast bitumen reserve, the regional market price difference between light WTI crude and Canadian bitumen-based heavy crude, for example, reached $30/bbl in April 2012. Processing bitumen crudes can be financially lucrative for refineries located in the market regions of these crudes.
The typical API gravity of bitumen is in the range of about 7° to 12°. For pipeline transportation, up to 35 vol% of diluent is added to become diluted bitumen (dilbit) with API gravity in the range of 21 to 23, which is at the high end for API gravity in typical heavy crudes. Sulphur compounds, naphthenic acid and viscosity are all high in dilbit, in addition to other undesired characteristics including high asphaltene and fine solid contents. An existing refinery will generally need to be revamped to process dilbit or crude blends with high percentages of dilbit, and the extent of the requirement for revamping depends on the targeted dilbit percentage in the crude feed and the complexity of the existing refinery. Typically, revamping crude and vacuum units is necessary for processing 100% dilbit, and revamps for these units are discussed next.
Refining scheme and dilbit capacity
Planning the revamp scope for crude and vacuum units should be developed with timely consideration of the overall refinery revamp plan which will depend on market assessment and the targeted product slates. For a given design feed capacity, higher percentages of dilbit in the crude feed increase the rates of heavy products from crude and vacuum units. For example, atmospheric residue and vacuum bottoms rates increase with higher percentages of heavy or dilbit crudes. These require capacity expansion of the vacuum unit and the downstream coking unit. Other downstream units such as hydroprocessing or even catalytic cracking units will require performance evaluation and/or modification as processing dilbit results in changes of operating parameters such as feed rates and compositions, sulphur concentration, microcarbon residue, solid contaminants, and metal contents.
The capital costs necessary for modifying downstream units affect the economics of a crude and vacuum unit revamp project to process dilbit. To identify the overall required scope for modification, evaluation of equipment performance in the crude and vacuum units, along with the impacted downstream units, will need to be completed. The associated costs of the overall refinery modification will need to be thoroughly assessed in parallel to using linear programming and simulation models as decision supporting tools for evaluating the product slate from the crude feed. Generally, the design capacity target for processing dilbit could potentially be specified in accordance with existing equipment capacities and plot space limitations to minimise capital investment and possibly maximise the associated rate of return on investment, but this minimised approach to investment will not necessarily generate maximum revenues when dilbit market prices are favourable.
Lower percentages of dilbit in the crude feed could minimise the need for material upgrade in the crude and vacuum units. Bitumen contains high levels of naphthenic acid compounds which are corrosive to carbon steel in an operating temperature range of 400-750°F (200-400°C). The rate of corrosion increases as operating temperature and naphthenic acid content become higher. Processing higher percentages of dilbit in the crude feed increases naphthenic acid content and consequently the corrosion rate. Generally, the required scope for material upgrade could potentially be reduced by lowering the percentage of dilbit in the crude feed. Additionally, to reduce sulphidic corrosion through processing conventional sour crudes, low alloy steels with chromium and/or molybdenum contents are typically used in sections of crude and vacuum units. Naphthenic corrosion rate reduces with increasing content of chromium or molybdenum in steels. Existing sections with adequate chromium content may not require replacement if the naphthenic acid content of the crude is sufficiently low at reduced percentages of dilbit in the crude feed.
Nevertheless, 317L stainless steel is typically used to replace equipment operating at temperatures higher than 400°F (200°C). Figure 1 shows areas in crude and vacuum units that typically require material upgrade. For example, processing 100% dilbit typically requires 317L tubes in both the crude and vacuum furnace heaters.
Stability of crude blends
Crude blend stability may need to be assessed when processing less than 100% dilbit crude blends. Mainly as a result of the asphaltene content of bitumen, dilbit has been reported to have stability issues with phase separation and high fouling rates. Blending dilbit with other compatible crudes reduces or minimises these stability issues and associated increases in the operating costs. If the objective of a revamp is to refine less than 100% dilbit crude blends, crudes compatible with dilbit should be considered as the blending components when these can be made available at competitive prices. Methods to characterise the stability of dilbit blends include analysis of the saturates, aromatics, resins and asphaltenes (SARA) contents of the blend, determination of insolubilty number (IN) and solubility blending number (SBN), and others such as the ASTM D7157 test method for instability determination and ASTM D2007-80 procedures for separating asphaltenes.
Crude preheating and desalting
As Figure 1 shows, the raw crude (or cold) preheat train upstream of the desalters and the hot (desalted crude) preheat train downstream of the desalters recover heat supplied through the crude and/or vacuum furnace heaters. Improved heat recovery in these trains will reduce the net energy or fuel consumption of the crude and vacuum units, and optimised design of these preheat trains enhances the economics of the refinery. Revamping crude units for processing dilbits typically includes modifications to the preheat train exchangers.
The high viscosities (at ambient temperatures more than 100 times more viscous than light crude) of dilbits cause excessive crude-side pressure drops in exchangers and significant reduction in the overall heat transfer coefficients. Replacement preheat exchangers may also be required for metallurgical upgrade to improve resistance to naphthenic acid corrosion. These replacement exchangers need to be designed with reasonably high velocities (typically 3 ft/sec shell side and 6ft/sec tube side) on the dilbit side to minimise fouling and improve overall heat transfer. High velocity design, however, results in high pressure drops which increase the discharge pressure requirement of the raw crude charge pump. For a given allowable pressure drop, special design exchangers such as helical baffle exchangers may offer relatively higher overall heat transfer coefficients and reduce the required exchanger surface areas. For existing crude units with limited plot space available for revamping the preheat trains, specially designed exchangers with high heat transfer coefficients can be evaluated.
The raw crude (cold) preheat train heats the raw crude to the required inlet temperature of the desalters, which remove salts in the raw crude. Poor desalting performance accelerates the rate of corrosion, especially in the crude overhead system. Flowing through a mixing valve, raw crude at the required temperature is mixed with desalting water, forming a water in oil emulsion. A demulsifying chemical is added to enhance the desalting process where crude salt content is extracted into the water droplets of the water in oil emulsion. The desalter’s electric field coalesces these water droplets which settle by gravity to form a brine phase. The settling rate is directly proportional to the differential density of the brine droplet and the oil emulsion and inversely proportional to the emulsion viscosity.
Due to its high specific gravity and excessive viscosity, desalting dilbit requires a longer residence time for separation, and high desalter inlet temperatures, typically in the range 280-310°F (140-155°C), mainly for reducing the viscosity effect. The requirement for a longer residence time necessitates replacement or expansion of desalters intended for conventional light crudes.
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