Reaction control for hydrotreatment of bio-based oils in renewable diesel production
Over the past few years, the carbon footprint of the transportation industry has drawn attention as an area where much can be done to reduce greenhouse gas emissions.IP: 22.214.171.124
Over the past few years, the carbon footprint of the transportation industry has drawn attention as an area where much can be done to reduce greenhouse gas (GHG) emissions. This can be seen every day with the increase of electric cars and light trucks displacing gasoline-powered vehicles. However, this increase is not as practical for larger trucks and railway locomotives. A growing alternative for these applications is renewable diesel, manufactured from various bio-feedstocks rather than traditional petroleum sources. This reduces carbon footprint without changes to vehicles or infrastructure.
To clarify terminology, renewable diesel—also known as green diesel—is chemically identical and therefore fully interchangeable with traditional diesel, so it can be used in any application with no engine modifications or maintenance side effects. Conversely, biodiesel has critical differences with traditional diesel, thus limiting its use. The terms are easy to confuse—this article will focus on renewable diesel.
Renewable diesel can be manufactured from a wide variety of feedstocks. Today, oil seeds are a major source, but the use of various crop wastes and other bio-based sources is growing, as these have less impact on conventional food supplies. Consequently, renewable diesel producers must be flexible in their processing capability as the types of feedstocks change based on costs and availability.
Production is routinely separated into two stages: pretreatment and hydrotreating. Pretreatment takes the raw feedstock and does whatever is necessary to create an intermediate suitable for hydrotreating, and this process must handle anything from vegetable oil to condemned meat and animal fats, since these are all potential feedstocks. This article will concentrate on the process and safety requirements of the hydrotreating stage of a specific facility located in the southwestern U.S.
Hazards of hydrodeoxygenation (HDO) reaction
At first glance, renewable diesel production looks similar to conventional refining; however, it has unique complexities. Even when a renewable diesel unit (RDU) begins with an intermediate feedstock from a pretreater, achieving the same chemical composition as the petroleum-based product requires extra effort. The basic problem is that plant- and animal-derived oils are different than petroleum. To make them the same, a dehydrogenation reaction is necessary to remove a carboxyl group from the fatty acids. Three reaction pathways can accomplish this:
Selecting among these three is a topic for another discussion. Here, the focus will be on the HDO reaction as it is used in the actual RDU examined in this work. The HDO reaction (FIG. 1) as practiced by RDU licensors requires high temperatures and pressures to drive catalytic action, typically 370°C (700°F) and 69 bar (1,000 psi), respectively. The process also produces carbon dioxide (CO2), carbon monoxide (CO) and water. The reaction is exothermic, so strict temperature control is necessary to prevent it running away. Conversely, if the reaction is not fully complete, too much unconverted oil may remain in the product, so it cannot pass specifications as true diesel.
One of the mechanisms used to control the reaction rate is feeding the recycled oil and “treat gas” [a mix of recycled hydrogen (H2) and fresh make-up H2] into the reactor. This helps reduce overall reactivity and tends to slow the process without reducing temperature or pressure. Such control is necessary because the incoming feedstock is highly variable and can change overall reactivity in short order.
Consequently, monitoring and controlling the recycled oil and H2 purge flows are critical to general process control and safety. As the process diagram in FIG. 1 shows, six locations—three H2 and three oil—feed the reactor, and each must be monitored.
Measuring critical flows
Monitoring flowrates of the recycled oil and H2 lines calls for flowmeter technology capable of withstanding high temperatures and pressures, along with a capability to measure two-phased liquid and gas flows. Since both basic process control and safety instrumented functions (SIFs) are necessary at multiple points of measurements, up to 12 flowmeters are necessary in some designs so the functions can be separated, and half of these flowmeters must be safety certified.
Plant personnel for the RDU examined in this article selected conventional single-hole orifice plates with differential pressure (DP) transmitters mounted remotely and connected via long impulse lines for all installations. This seemed a logical choice as conventional orifice plates are a very common approach in chemical processing applications, and with some basic selection evaluation, it was a simple matter to identify units that met all the specifications. These types of orifice plates generally work well in these types of applications, but there were extenuating factors in this case.
First, a RDU is not a clean process, particularly in the earliest steps. Since feedstocks are often waste products, all manner of debris and contaminants can show up at any time. Small particulates may form deposits in problematic areas—this was the case in the long impulse lines necessary for the orifice plate installation, especially on the recycled oil side. When impulse lines are clogged, even partially, response time is slowed; if the blockage is large enough, it can disrupt the reading drastically.
Second, a single flowmeter was used for the SIF on all six lines. This met the basic safety instrumented system (SIS) requirement but left the process open to false trips. When considered along with the clogging effects just mentioned, it was clear that process interruptions caused by flow trips were a threat to uninterrupted operation and costs related to unplanned shutdowns. This left the plant’s reliability team looking for alternatives.
One suggestion was installing multiple flowmeters for the SIF using a two-out-of-three voting scheme to reduce the likelihood of false trips; however, each meter would suffer from the common cause of clogged impulse lines, providing little or no improvement.
A better flow-measuring approach
In recent years, vortex flowmeters have profited from advances in sensor and transmitter electronics that have improved their capabilities with higher reliability, and with mounting convenience for safety applications. To understand how they work, it is helpful to consider the vortex mechanism. When a shedder bar (FIG. 2) is inserted across the inside of a pipe, such that it is centered and perpendicular to the flow, fluid must pass the bar on both sides. This creates alternating vortices—described as the von Kármán effect—and the frequency of each vortex is proportional to the fluid velocity.
Integrated into the shedder bar is a small sensing element called a flexure. It oscillates back-and-forth, displaced by the vortices, and a piezoelectric sensor converts the mechanical motion into an electrical signal. The flowmeter’s transmitter translates this into a volumetric flowrate. Since the bar has a narrow profile, the pressure drop is very low, and there are no other obstructions or impulse lines, providing a high tolerance for dirty fluids.
The velocity of the fluid is directly proportional to the frequency of the vortices generated by the shedder bar within the meter body. Volume flowrate (vol/sec) = vortex frequency (pulses/sec)/K-factor (pulses/vol), where the K-factor is a calibration constant determined in a flow lab by measuring the pulses generated divided by one unit of volumetric flow. This K-factor constant is accurate over a large operating range of flow patterns and is independent of fluid type.
This design is well-suited to continuous processes since the sensor is fully sealed inside the spool, eliminating leak points and allowing the transmitter to be separated from the sensor, if necessary, without a process shutdown. The configuration also has a unique capability to mount two transmitters on a single shedder bar (FIG. 3), so one sensor can provide two independent readings in the same location. This has proven reliable enough for safety certification to safety integrated level (SIL) 3.
In applications where a two-out-of-three voting scheme is necessary, a quad assembly with two shedder bars is also very practical and compact, with three safety-certified units and a fourth for process control use. This approach provides a very high degree of protection with minimal risk of false trips.
Solving the problem
The facility replaced all conventional single-hole orifice plates with DP transmitters in this application with six quad vortex assemblies, protecting the critical measurement and control points. This increased the reliability of the feed measurements by eliminating impulse lines that are prone to plugging, while providing two-out-of-three voting. The accurate and safe control of the HDO reactor also future-proofs operations for processing variable feedstocks. Changing to this approach has been so effective that the licensing firm working with the process owner now recommends this method for its other RDUs. HP