December 2024
Biofuels, Alternative/Renewable Fuels
Address measurement challenges in sustainable aviation fuel production
Lipid-based conversion processes for sustainable aviation fuel are similar to fossil-based aviation fuel processes, but they have some critical differences. This article highlights the technical challenges associated with measuring and controlling biofuel-based aviation fuel processes, and provides suggestions to optimize conversion, while minimizing downtime and maintenance costs.
Fossil-based aviation fuels have historically powered most aircraft, but greenhouse gas (GHG) reduction efforts are driving an industry transition to sustainable feedstocks to provide significant carbon emissions reductions. Biojet, or sustainable aviation fuel (SAF), is a renewable replacement for fossil-based aviation fuels and is approved by ASTM International to be blended up to 50% with conventional jet fuel.
Lipid-based conversion processes for SAF are similar to fossil-based aviation fuel processes, but they have some critical differences. This article highlights the technical challenges associated with measuring and controlling biofuel-based aviation fuel processes, and provides suggestions to optimize conversion, while minimizing downtime and maintenance costs.
SAF and renewable fuel processes challenges. Ever since biofuel-based aviation fuel was approved for commercial airlines in 2011, the demand for this renewable fuel has expanded dramatically. While the current products are significantly more expensive than fossil-based alternatives, the market remains strong due to environmental credits and government subsidies. Multiple suppliers are pursuing these markets, and competition is driving down prices as producers search for better ways to optimize production and reduce operating costs.
Most of the SAF produced today comes from the lipid conversion process, which utilizes renewable feedstocks with high oil content such as used cooking oil, oil-bearing plants (e.g., rapeseed, carinata, soybeans) and tallow. These feedstocks are pretreated to eliminate contaminants that can poison reactor catalysts, and to chemically break down the feedstock into common and consistent intermediates to feed the hydrotreater and isomerization reactors.
Despite this pretreatment, these feedstocks tend to be more corrosive and can cause more operational issues than conventional fossil fuel feedstocks, with conditions varying due to feedstock availability. The SAF refining process also requires significantly more hydrogen (H2), along with elevated pressures and temperatures.
SAF hydrotreatment measurement and controls. Once pretreated, the SAF intermediates are fed into a hydrotreating process similar to that used in traditional refining. A variety of licensed processes are in use, with a generic overview provided in FIG. 1. As shown in FIG. 1, the feedstocks are mixed with H2 and first preheated, then heated further in a fuel gas fired heater before entering the hydrotreater/hydrodeoxygenation reactor.
FIG. 1. SAF plants use hydrotreating and isomerization reactors to process the biofuel lipid feedstocks and convert them to intermediates that can be processed to produce SAF.
FIG. 1 shows several points of measurement critical for plant operations. FT1 meters make key flow measurements where accurate mass flow and density are required. For these applications, a Coriolis meter (FIG. 2) offers very high reliability and accuracy for both gas and liquid measurement. Its direct measurement of both mass flow and density for liquid streams allows these meters to assess variability in feedstock streams, and these readings are then used to adjust the process to optimize conversion and performance. These mass flow measurements are also critical to extend production runs. Catalyst life tends to be shortened in biofuel service, so the ability to accurately measure and adjust optimum feed ratios of the raw materials with varying composition is critical to maximize production.
FIG. 2. Coriolis meters (left) provide mass flow and density measurements. A quad vortex metera (right) provides SIL 2oo3 voting, along with a fourth independent process flow measurement, in a single meter body.
It is also important to optimize combustion operations to maximize the overall energy efficiency of the process. Controlling the mass flow of the fuel gas to the fired heater using a Coriolis meter—instead of controlling the volumetric flow—in a cascade control loop to the outlet heater temperature can significantly improve the stability and energy efficiency of the heater.
Another key process measurement area involves the H2 feeds into the hydrotreater reactor. This is a very exothermic process, so as the combined feeds enter the catalyst beds, they tend to create very high temperatures that can run away if the flows are not tightly controlled. FT2 meters in the FIG. 1 diagram indicate critical flows that often require safety integrity level (SIL)-rated measurements. A preferred option for these applications is a quad vortex meter (FIG. 2 right). This type of meter includes four independent vortex meters built into a single body. Three of the four readings provide a 2oo3 voting SIL-3 capable measurement, while the fourth meter provides an independent flow measurement for process control.
Differential pressure and temperature measurements are also extremely important for the hydrotreater reactor (FIG. 3). Out-of-control exotherms are always a concern, and differential pressure measurements are necessary to indicate plugging and material build-up inside the catalyst beds.
FIG. 3. Critical bed temperature transmitters (left) should utilize dual sensors and detect sensor drift. Remote electronic pressure transmitters measuring catalyst bed differential pressure (dP) (right) will require gold-plated diaphragms for H2 service. Both must be SIL-2 or SIL-3 capable.
Often, the temperature transmitters are specified with dual hot backup sensors and advanced diagnostics to detect sensor drift (FIG. 3 left). To provide accurate dP measurement across large catalyst beds without injecting errors due to lengthy capillaries, the dP transmitters should be safety-rated, and will likely require electronic remote seals (FIG. 3 right) and gold-plated diaphragms to minimize H2 permeation.
Corrosion monitoring. The variable feedstocks in renewable fuel service tend to be more corrosive and can create unexpected material compatibility problems. High-temperature H2 attack and hydrogen sulfide (H2S) corrosion are two common issues that vex these processes. Most plants utilize corrosion monitors in key locations to monitor metal loss (FIG. 4). Non-intrusive wireless sensors are installed in strategic points throughout the process to provide real-time indications of corrosion attack.
FIG. 4. Wireless corrosion and erosion sensorsb can be placed in strategic areas throughout the process to detect and monitor corrosion. Their wireless design allows them to be added or moved easily to meet service needs.
Wireless sensors can be easily and inexpensively added or moved to focus on problem areas as they develop, and installation costs are much lower than with wired alternatives.
Gas analysis for process control and emissions monitoring. H2 is a key component of any hydrotreating process, but biofuel feedstocks demand more H2 to complete the conversion process. Monitoring the strength and purity of make-up and recycled H2 flow streams is critical to maintain high conversion rates. Analyzers in this service must be durable and flexible to provide reliable and accurate measurements (FIG. 5). Key features for these analyzers are the ability to scan for multiple components, and to handle multiple sample streams with a single analyzer. For example, the recycle stream gas analyzer should measure H2 purity as well as H2S, the latter value critical for process control.
FIG. 5. Dependable and accurate analyzer and chromatograph measurements are a mainstay for this industry. The gas analyzerc (left) and the emissions monitoring systemd (right) can monitor multiple gases. The gas chromatographe (middle) measures multiple components in up to six streams.
Industrial gas analyzers can measure up to six separate sample streams and can scan for a broad range of components. Continuous emissions monitoring system (CEMS) analyzers are also critical to measure final stack emissions, especially due to the environmental focus on GHG emissions created by these plants. The CEMS analyzer should be able to measure carbon dioxide, nitrous oxides, sulfur dioxide and oxygen.
Heat exchanger monitoring. There are many heat exchangers across a typical renewable fuel plant, and most are subject to plugging and fouling due to the waxy nature of the feedstocks. Active measurement of incoming and exiting flows, temperatures and pressures provides staff with the information needed to optimize heat transfer, and to monitor for fouling or plugging in real time (FIG. 6).
FIG. 6. Measurement of flows, temperatures and pressures around heat exchangers provide real-time indications of fouling and allow optimization of heat transfer.
Permanent installations can utilize orifice or vortex meters, and temperature transmitters with thermowells. Non-invasive installations utilizing wireless transmitters, clamp-on ultrasonic flowmeters and/or surface mount thermal sensors allow a facility to monitor troublesome heat exchangers without expending significant capital expense. For these installations, the authors’ company’s non-intrusive ultrasonic flowmetersf can measure two flows simultaneously and enable cost-effective retrofitting on existing assets without process disruption, and wireless high-density temperature instruments can measure up to four readings with one device, further reducing installation costs.
Simplifying regulatory reporting. The renewable fuels industry's expansion is significantly tied to government subsidies, which comes with stringent data reporting requirements for compliance. The tax incentives and penalties vary depending on global regions and local government policies. These regulations almost always involve a specific methodology for reporting data, and often require verification of the accuracy of measurement data and an audit trail.
SAF producers are required to gather data related to custody chain documentation, invoices and transportation. Using automated, smart level and metering instrumentation that can provide highly accurate measurements independent of fluid properties becomes critical. Also, instruments with onboard diagnostics make it easier for operators to dependably meter fluid transfers and to report receipts, usage and shipments to obtain government subsidies and comply with regulations (FIG. 7).
FIG. 7. Validation and verification of metering using smart onboard diagnostics are critical to provide accurate reporting of fuel or feedstock transfers in compliance with commercial transactions and regulations.
Case study. Finland-based oil refining company Neste commissioned a very large conversion of its Singapore refinery to expand the production of renewable fuels and aviation fuel from biofuel feedstocks. The refinery is one of the largest global producers of SAF, which reduces GHG emissions by 80% over the fuel’s lifecycle compared to fossil fuel-based jet fuel.
Neste partnered with the authors’ company to take full advantage of its line of advanced instrumentation and process control software to digitize and fully optimize refining operations and minimize unexpected downtime. The plant applied the authors’ company’s automation system and softwareg to control production for efficient performance and to deliver on-demand remote access to data and analytics. Advanced systems, software, analytics and mobility tools established the foundation to digitally transform operations by turning relevant data into new operational insights and actionable information that empower better decision-making. Much of this data is supplied by a variety of different instruments and analyzers, many of which have been mentioned in this article.
Takeaway. Renewable fuel process measurement and control demands the careful selection of instrumentation and control strategies. When properly chosen and utilized, the right instruments can result in dramatic improvements to unit operations and increase the time between maintenance outages. While the biofuel feedstock conversion process is similar to typical hydrotreating, it has specific measurement and control challenges that make reliable operation more difficult.
If a company is considering an upgrade to SAF or renewable fuels production, it is important to fully understand the process and corrosion challenges, and to carefully evaluate instrument options. To address these needs, it can be helpful to partner with a capable automation vendor that offers a broad range of solutions to help end users find the right instruments to suit the application, process requirements and budget.
NOTES
a Rosemount™ 8800 Quad Vortex flowmeter
b Rosemount™ Wireless Permasense sensors
c Rosemount™ X-STREAM Enhanced XEFD Continuous Gas Analyzer
d Rosemount™ XE10 Continuous Emissions Monitoring System
e Rosemount™ 700XA Gas Chromatograph
f Emerson’s Flexim™ flowmeters
g Emerson Delta V
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