Decarbonisation Technology - November 2021

heater trips during rich fuel conditions, the refinery operated with a target level of O 2 in the stack of 5% By shifting to controlling the fuel gas flow on a mass basis with a Coriolis meter, the refinery was able to lower the target oxygen in the stack from 5% to 3.5%, resulting in $400,000.00 net present value. Green diesel production Driven by recent government subsidies and fuel credits encouraging the use of bio-based feedstock for fuel production, many refineries in the United States and a few in Europe are adding or converting to green diesel production. This new wave of biodiesel production involves the hydroprocessing esters and fatty acids (HEFA) and the isomerisation of triglycerides. While there are many similarities to traditional refining with the reaction mechanisms and process equipment involved, refineries are finding the traditional volumetric flow control and measurement practices have limitations in these newer operations. Because of the application flexibility, repeatability, and cost advantages, the majority of the flow measurement points in a traditional refinery are controlled on a volumetric flow basis using differential pressure orifice flow meters. However, for green diesel production, there are a few reasons why traditional differential pressure orifice flow meters can be limiting in such applications. Differential pressure orifice flow meters are impacted by changing process conditions and fluid properties. The meter correction factor must be calculated to determine the difference between actual process conditions and meter design conditions. The accuracy of these meters can range from 0.5-1% when compensated for temperature and pressure changes or 1-5% when uncompensated. (This is assuming that the specific gravity of the fluid stays constant.) During green diesel production, a variety of feedstocks, including tallow oil, soybean oil, rapeseed oil, etc, are blended and processed in the reactor section. Feed specific gravity into the reactor section can vary by up to 10% and viscosity tenfold depending on the process

Figure 2 Coriolis flow meters are multivariable measurement devices that directly and accurately measure mass flow regardless of changing fluid properties or process conditions.

efficiency and reduce emissions is by improving combustion control of fired heaters and boilers.

Combustion control impacts the safety, efficiency, and emissions of fired equipment. Because of process control variability, fuel gas composition variability, and safety risks, the equipment operates with a high level of excess air. While the excess air ensures complete combustion and maintains a safer operating margin, the trade-off is excess NOx emissions and higher fuel and energy costs. Typically, the fuel gas flow is controlled using a pressure or volumetric-based control scheme. However, the energy content and stoichiometric air required for combustion of fuel gas are more proportional on a mass basis than a volume basis. Therefore, controlling fuel gas flow on a

Controlling fuel gas flow on a mass basis with Coriolis meters provides more stabilised control as fuel gas composition varies

mass basis with Coriolis meters provides more stabilised control as fuel gas composition varies. The more stabilised control allows operators to lower the target oxygen levels in the stack and operate with less excess air, reduced emissions, and increased efficiency. One North American refinery controlled fuel gas on a volumetric flow basis with a differential pressure orifice meter. The variations in fuel gas composition caused control variability. The fuel gas composition range was methane from 0-56%, hydrogen from 17-72%, non-combustibles from 0-2%, and nitrogen from 0-4.8%. To prevent