April 2018

Process Optimization

Enhancing refinery profitability with a novel offgas conversion technology

The refining industry is challenged every day to optimize product slates that comply with market demand, product specifications, environmental regulations and refining-petrochemical integration, all while remaining profitable.

Aronson, A., Zaziski, D., Siluria Technologies; Carter, D., Wood Group

The refining industry is challenged every day to optimize product slates that comply with market demand, product specifications, environmental regulations and refining-petrochemical integration, all while remaining profitable. Typically, this means running less-expensive, heavier crudes with higher severity processing, resulting in increased refinery fuel gas production. The ability of refiners to effectively manage fuel gas production has a direct impact on operational flexibility and profitability.

Refinery offgas recovery

Refiners long on fuel gas often alter refinery operations to meet fuel gas constraints, flare fuel gas or sell it at a discount. Some refinery fuel gas streams contain significant quantities of valuable light olefins (ethylene and propylene), hydrogen and natural gas liquids (NGL), typically from fluid catalytic crackers (FCC) and delayed coking units (DCU).

Past approaches for extracting value from the fuel gas fall within two categories:

  1. Using recovery systems to extract and purify the valuable components
  2. Simply using the stream as fuel to generate onsite heat or power.

Recovery systems can offer greater potential uplift in value; however, they come with several technical, operational and financial hurdles. These systems involve complex, capital and energy-intensive compression, front-end gas treatment and cryogenic fractionation. Installation of these systems can require significant downtime, which negatively impacts refinery operations. Furthermore, the refiner must enter new markets and establish buyers and supply chain solutions for these logistically challenging products. As a result, many refiners opt to simply use gas as fuel, and value it for its Btu (energy value) by burning it for power within the refinery operation. Rather than burning this offgas for heat or power, what if there was a way to turn it into a valuable commodity?

Chemical conversion of refinery offgas

An attractive and alternative approach is to use one or more chemical reactions to convert the fuel gas components into high-value liquids that are already produced and sold by the refinery. This approach provides a much simpler, lower-cost and more-effective solution for maximizing the value of offgas streams. While such conversion processes are technically feasible, their implementation has been limited, in part, due to complications with low component concentrations, as well as the need for extensive contaminant removal, both of which adversely impact economic conversion.

Offgas conversion technology

Recently, a new proprietary process technology has been developed that selectively converts dilute concentrations of light olefins to a high-octane, low-sulfur gasoline blendstock and LPG.a The process uses robust, commercially proven catalysts that have been engineered to directly process the stream, without requiring extensive contaminant treatment.

The process also meets important needs that most refiners have for the adoption of a new technology: small footprint, easy installation and limited operational disruption. Utilizing a compact, modular, skid-mounted design, most of the system can be procured and assembled offsite. Site preparation, primarily foundation work and utility tie-ins, can occur concurrently with minimal impact on refinery operations. Final onsite installation of the modules can then be done during a routine maintenance turnaround, providing less interference and downtime when compared with a typical FCCU revamp project.

A block diagram of the offgas conversion process is shown in FIG. 1. Offgas from the refinery FCCU and/or DCU, typically downstream of the amine unit, can be bypassed from the fuel gas system and redirected to the conversion system. After compression, simple guard beds are provided for selective contaminate removal (primarily sulfur-containing organics, metals and basic nitrogen species), which affect product quality and/or downstream process units. Other common impurities in the offgas, such as oxygen, carbon dioxide (CO2), nitrogen oxide (NOx) and water, remain in the stream since the conversion catalyst and downstream units are not impacted.

FIG. 1. Block flow diagram of offgas conversion process scheme.

After pretreatment, the compressed feedstream enters the reactor section, which contains a proprietary catalyst and a multi-reactor system. The catalyst is based on a commercially proven catalyst system that has been modified to selectively oligomerize dilute light olefins streams to longer hydrocarbon chains in the gasoline range. Typical of this class of catalyst, the catalyst will deactivate during operation due to coking until conversion of light olefins falls below a target level. Full catalyst activity and gasoline selectivity can then be fully restored by simple, in-situ regeneration for coke removal via dilute air combustion.

Based on capacity requirements, the reactor system consists of at least three adiabatic fixed-bed reactors in parallel. The reactor set is run in a cyclic or swing mode, similar to a cyclic catalytic reformer, to ensure continuous operation. In the typical operating configuration, one reactor is offline in catalyst regeneration mode, while the other reactors simultaneously operate with catalyst at various (staggered) stages of deactivation. Staggered reactors help maintain high conversion and consistent product composition. As a catalyst bed nears the end of activity, identified by olefin breakthrough, the reactor is switched out to regeneration mode, while the freshly regenerated reactor is returned to operation.

Finally, a separation section is used to recover the liquid hydrocarbon products. The products are a high-octane, ultra-low-sulfur gasoline blendstock stream (TABLE 1), an LPG coproduct and clean gas that is returned to the refinery fuel gas system. Based on the refinery’s needs, the final characteristics of the gasoline blendstock, such as octane and product grade, can be adjusted through control of the reactor operating conditions.

Refinery case study

To quantify the economic and operational benefits of the offgas conversion technology, several case studies within a US Gulf Coast refinery are examined using a linear programing (LP) model.

The configuration of the refinery is shown in FIG. 2. The refinery processes 200 Mbpd of crude oil, with a product slate of gasoline (87 octane and 91 octane), diesel fuel, LPG, coke and sulfur. An equal split of heavy and light crude is selected as a typical feed for this benchmark refinery.

FIG. 2. Configuration of the refinery examined in the case study.
FIG. 2. Configuration of the refinery examined in the case study.

 

In the low-sulfur, reformulated gasoline world, refiners have been relegated to producing blendstocks instead of finished gasolines. For simplicity, the cases prepared were based on producing blendstocks that can be used for conventional gasoline finishing or reformulated oxygenate blending to meet the needs at the pump for regular unleaded, mid-grade and premium-grade finished gasolines.

The base case is configured with an 80-Mbpd nominal feed FCCU and a 50-Mbpd nominal feed DCU, with offgas directed to the fuel gas system. Two operating modes of the FCCU are considered, a gasoline production mode and an olefin mode. In the gasoline mode, the operating conditions of the FCCU are selected to boost the output of gasoline, whereas the olefin mode production is shifted to produce additional propylene using ZSM-5 catalyst additives in the FCCU. Additional ethylene is produced in the olefin mode and serves as additional feedstock for liquid fuel production using the offgas conversion process. The offgas composition for each mode is displayed in TABLE 2. As expected, the offgas in the FCCU olefin mode has overall higher concentrations of small-chain olefins.

The offgas conversion technology is included in the refinery configuration (see blue colored block in FIG. 2) by re-directing the FCCU and DCU offgas from the fuel gas system to the conversion unit. The output products, approximately 750 bpd–1.5 Mbpd gasoline blendstock and LPG, are combined with the existing refinery product streams. The energy content extracted from the offgas is replaced with additional natural gas, as needed, to maintain the fuel gas system balance.

The LP model was used to investigate the effect of integration of the offgas conversion technology into the refinery process flow. The objective function of the LP model is optimized by maximizing the refinery net operating margin. TABLE 3 shows feedstock and product pricing assumptions used in the analysis, which reflect present approximate market conditions.

Case study results

Side-by-side economic comparisons are made between the base refinery case and those in which the offgas conversion unit is included (TABLE 4), as obtained from the LP model. The results show that the offgas conversion unit expands product revenue by approximately 1%. This is partially offset by a modest increase to feedstock and operating cost, largely driven from the purchase of additional low-cost natural gas to replace the missing olefins in the fuel gas system. On a net operating margin basis, the refinery profits are enhanced by $29 MM/yr–$34 MM/yr, depending on the operating mode of the FCCU.

To evaluate the economic viability of the offgas conversion technology, a simple payout time for this investment is calculated. The total installed cost (TIC) of the conversion unit is estimated at $35 MM for the gasoline mode and $49 MM for the olefin mode system. Costs are based on applying scaled factors from a Class 4 capital estimate and include TIC for inside battery limits (ISBL), a 25% allowance for outside battery limits (OSBL) considerations, and an overall contingency factor of 10%.

Relating this investment to the net operating margin, a payout time of 1.2 yr–1.4 yr is derived. This demonstrates the attractiveness of the technology since a refiner’s criterion for economic viability is typically a simple payout time of less than 2 yr–3 yr.

Takeaway

The refining industry has been searching for an economic means of recovering the value of light olefins in the fuel gas for many years. The offgas conversion process described herein provides an alternative for these dilute olefins, which otherwise are burned as fuel. The conversion process presents refiners with an attractive and unique option to convert the under-valued components into high-value liquids that are already produced and sold by the refinery. The process also offers refiners additional flexibility in the way the FFCU can be operated to further enhance operating margins. 

NOTES

     a Refers to Wood and Siluria Technologies’ Modus process technology.

ACKNOWLEDGEMENTS

The authors would like to recognize Pete Czerpak, Senior Process Engineer with Siluria Technologies, and Dr. Jimmy Vajifdar, Process Technology Specialist with Wood, for their contributions for the simulation, figures and LP modeling results presented in the work.

The Authors

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