Valve Solutions for Biofuels
by Jari Kirmanen, Product Manager Neles, Metso’s Automation Business Line
Biofuel production is growing very fast. The main drivers for growth are greenhouse gases and the need to reduce the use of conventional crude oil. In addition, the EU has set a target of 10% for the share of biofuels in overall road transportation consumption by 2020. The two main biofuel products are bioethanol (usually produced by sugar fermentation) and biodiesel (made from vegetable oils). Bioethanol production is already considerable - in the USA and Brazil for example, while biodiesel production is increasing in the EU.
Bioethanol and biodiesel
Bioethanol production is based on a fermentation process using corn, wheat or sugar cane as the raw material. Biodiesel is traditionally produced by the transesterification of vegetable oils, such as rape. The 2nd and 3rd generation biodiesel processes, such as Neste Oil's NExBTL or Choren's Sun Diesel, have recently appeared on the market. These processes are more complex than transesterification, but increase the high value added products (biodiesel and glycerin) and they avoid the production of low end products, such as fatty acids. In contrast to bioethanol, biodiesel is normally used as such in engines without the need for any engine changes.
Valve solutions and challenges
Most valve applications in biofuel production are at low or moderate pressures and low temperatures. Because production units have had relatively small capacities (e.g. Neste biodiesel unit in Porvoo 170 000 ton/year), control valve sizes are also relatively small – typically 2-3 inches. Modern biodiesel production is more challenging for control valves than that of bioethanol, because high pressures and temperatures may be needed in biodiesel conversion and product-upgrade phases. However, both processes require safe, reliable and accurate control, which means high-performance control valves.
Environmental issues have led oil companies to pay increased attention to emissions from control valves. This has made rotary valves more attractive, because they typically have lower gland-emissions than globe valves. Rising stem packings tend to wear more quickly than rotary stems, because their linear movement transports process media into the packing area. Spring-loaded packings in rotary valves (Fig 3) provide an easy, maintenance-free solution to the control of fugitive emissions.
Soft- and metal-seated technology is utilized for common control valve applications in biofuels. However, metal-seated control valves are more rugged and last longer, especially if the flow media contain impurities, which may require the use of non-clogging valve designs. Valves may 'stick' or become completely clogged, if dirt lodges in the valve trim, bearing or stem area. Valve seat and body may also suffer erosion. In such conditions, metal-seated rotary valves are the most reliable. In extreme cases, dust-proof seats will prevent media build-up behind the seat area.
When a control valve’s downstream pressure is lower than the liquid vapour pressure, some liquid is vaporized and remains downstream of the valve, thus the flow downstream is partly liquid and partly vapour - a phenomenon called flashing. It cannot be avoided by valve design, but is addressed by re-considering both valve and piping design. Flashing may cause mechanical damage, e.g. erosion or vibration, if the velocity is too high. An eccentric plug valve is a reliable solution for moderate-pressure flashing, because the flow stream does not hit the valve body or downstream pipe wall (see Fig 4).
Cavitation and noise
Cavitation is a two-phase phenomenon that occurs in the liquid flow through the control valve under certain conditions. Initially, the liquid pressure decreases below vapour pressure to create vapour bubbles. In phase two, the vapour bubbles collapse rapidly, creating pressure shocks as the pressure rises above the vapour pressure. If process design cannot eliminate cavitation, valves with anti-cavitation trims are usually utilized.
When a high pressure-differential is released in a control valve, noise and vibration may be generated. Noise is generated mainly by turbulence and shock waves produced in choked flow conditions. To reduce noise, multi-stage noise reduction trims are used – e.g., the Neles segment valve with Q-trim, which provides high capacity combined with good cavitation resistance and noise reduction, in a manner no sliding stem valve can match (see Fig 5).
A typical dryer system consists of 2 or more columns packed with molecular sieve material (Zeolites). While the wet or sour stream is processed in one column, the other is regenerating. It is the molecular sieving valves that sequentially switch from adsorption to regeneration.
The operating speed of these valves is not demanding, but they must withstand frequent cycling while maintaining bi-directional tightness. Particulates and temperature fluctuation during sequential operation present an additional challenge. Metal-seated and special soft-seated butterfly and ball valves are typically used in this very demanding application.
Because the operation of this system is critical for the process, valve performance should be monitored. Intelligent products, such as Neles SwitchGuard, an intelligent valve controller for automated on/off-valves (Fig 6), can be utilized to detect valve operation automatically.
Intelligent, modern on-line diagnostics
Valves must be serviced regularly to keep processes efficient and maintain performance throughout their life cycle. Servicing valves before necessary is a possible answer, but expensive and time-consuming. Waiting until valves fail and cause an un-scheduled shutdown can also be costly. Ideally, only those valves that require maintenance should be serviced during a shutdown, but this requires valve diagnostics and/or a monitoring program.
On-line diagnostics can monitor valve performance while the process is running, indicating any decrease in valve performance and warning the user before failure causes excessive process variability or a shutdown. The most efficient predictive maintenance uses valve controllers, which store results in their memory and send warnings and alarms based on performance limits stored in their memory. No additional manpower is needed to analyse these results continuously, because the controller together with advanced asset management software measures valve performance automatically.
On-line diagnostics can be utilized for all valve functions from control to on/off, even for safety valves. Together the utilization of inherently reliable valves and of intelligent devices can raise valve application reliability to a significantly higher level than before.
METSO, 0870 606 1478
Published in Valve User Magazine Issue 9
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