While some diesel engines can operate on vegetable oil, the process of turning plant- and animal-based oils and fats into a practical sustainable fuel suitable for more widespread use calls for more extensive treatment.
There are major chemical differences between petroleum oil extracted from the ground, and oil extracted from soybeans or olives. For a facility trying to make renewable diesel from bio-based feedstocks, the differences are very important. The process of converting bio-based oil into something more like petroleum is the main topic of my article in the November issue of Hydrocarbon Processing, titled Reaction Control for Hydrotreatment of Bio-Based Oils in Renewable Diesel Production.
Bio-based oils are chemically different than petroleum. Primarily, the former includes carboxyl groups that make them good for cooking, but not so much for engine fuel. To make sunflower oil like petroleum, it must go through a dehydrogenation reaction, such as hydrodeoxygenation (HDO). This can be challenging.
The HDO reaction as practiced by RDU (renewable diesel unit) licensors requires high temperatures and pressures to drive catalytic action, typically 700 F (370 C) and 1,000 psi (69 bar) respectively. The process also produces carbon dioxide, carbon monoxide, and water. It is an exothermic reaction, so strict temperature control is necessary to prevent it running away. On the other hand, if the reaction is not fully complete, too much unconverted oil may remain in the product, so it can’t pass specs as true diesel.
The primary lever that operators have to control the reaction is modulating the flow of recycle oil and “treat gas” fed into the reactor. The article goes into more detail as to how this works, but for now, understand that this calls for critical flow measurements at multiple points.
Monitoring flow rates of the recycle oil and hydrogen lines calls for flow meter technology capable of withstanding high temperatures and pressure, along with a capability to measure two-phase liquid and gas flows. Since both basic process control and safety instrumented functions (SIFs) are necessary at multiple points of measurements, up to 12 flow meters are necessary in some designs so the functions can be separated, and half of these flow meters must be safety certified.
The flow meter technology that is growing rapidly for use in this application is Emerson’s Rosemount 8800 Series Vortex Flow Meter, especially in quad configurations to perform the critical SIF using 2oo3 voting for maximum false trip resistance. One facility that we work with has changed to our vortex flow meters, replacing older conventional differential pressure (DP) units due to impulse line clogging problems they were experiencing. Vortex designs are able to tolerate dirty flows, as are common in this application, along with the high temperatures and pressures involved.
Visit the Renewable Biofuels Production pages at Emerson.com. You can also connect and interact with other engineers in the Downstream Hydrocarbon and Chemical Processing Groups at the Emerson Exchange 365 community.