Sealed and gas-purged decanter-centrifuge systems can be effective for separating Class I, Div. 1-, or Class I, Div. 2-rated solvents and hazardous materials
Traditionally, solids and liquids have been separated by filtration or with press equipment. While these methods are effective, modern automated separation technologies, such as decanter-centrifuge systems, are excellent alternatives for faster separation and a more efficient approach.
When processing flammable, hazardous or otherwise dangerous material that can adversely impact the environment, it is important to prevent it from escaping the process stream. It is equally important to prevent atmospheric oxygen from entering the centrifuge system and creating an unsafe environment that can potentially cause an explosion or to prevent toxic materials/vapors from leaving the machine to protect operators in the area. A purged centrifuge is an inert-gas-blanketed system that forms a “technically tight” seal between the interior of the centrifuge and the environment.
Sealed centrifuges can be used safely and effectively while avoiding these types of situations. In some cases, it is important to minimize exposure to atmospheric oxygen to prevent explosions, but it may also be necessary to prevent hazardous material from escaping during processing. Some applications in which sealed, inert-gas-blanketed centrifuges are used include the following:
• Oil and industrial sludges
• Tar cleaning
• Drilling mud and emulsion processing
• Industrial and chemical waste processing
• Solvent extraction processes
• Chemical processes using flammable solvents
• Fermentation applications using flammable media
This article explains how technically sealed decanter centrifuges, blanketed with inert gas, improve efficiency and safety in dewatering and separating processes of combustible or hazardous materials.
Purging principles
There are three general approaches for inert-gas purging of process equipment, as described below.
Overpressure. This method can only be used with equipment that is designed for over-pressure. The air inside is removed by continually adding and releasing high-pressure inert gas into the sealed equipment. This step is repeated until the oxygen concentration inside the equipment measures at the desired level. Then, while the equipment is running, the internal pressure is held at a higher differential pressure than the atmospheric pressure (though this operating pressure is still much lower than that used during inertization).
Vacuum. All air inside the equipment is removed by evacuating the equipment, at which time, the vacuum is broken by flushing with inert gas. Depending on the equipment, this procedure must be repeated. Equipment designed for holding vacuum is required for this process.
Continuous-flow purging. This method is used for plants, machines, containers and equipment that are designed for high overpressure or vacuum. It works by feeding inert gas into the equipment at one point in the closed system and simultaneously releasing it at another point a distance away from the infeed. There is typically a pre-purge sequence at higher flowrates, and a blanketing sequence during normal operation of the equipment.
The centrifuge systems discussed in this article use a version of the third method (continuous-flow purging). They are built to Class I, Div. 1 or Class I, Div 2 (U.S. National Electrical Code (NEC) 500) standards. Continuous-flow purged centrifuges can also be designed for Canadian NEC 505 and are currently available for European ATEX Directive (2014/34/EU). Design requirements are subject to different standards in different countries. The focus here is on NEC 500 classifications.
It is also important to note that continuous-flow purged centrifuge systems are “technically tight,” which is different from being hermetically sealed. A hermetically sealed piece of equipment provides a zero-leakage tolerance, while the “tightness” in a technically tight system cannot be permanently ensured, due to its design, function and so on (Figure 1).
FIGURE 1. Continuous-flow purged centrifuge systems are “technically tight,” which is different from being hermetically sealed
Two protection concepts
There are two protection philosophies used for applications that require continuous-flow purging. Both can be customized to specific materials and settings.
Combustible materials. When processing flammable or potentially explosive materials, the goal is to avoid fire or explosion. Because any spark can cause combustion when oxygen is present, an inert atmosphere must be created and maintained inside the centrifuge. This is accomplished by removing oxygen from inside the centrifuge during startup and maintaining an inert atmosphere during operation. Examples of combustible media include alcohols (methanol, ethanol, isopropanol and so on) and solvents (hexane, heptane, octane, acetone, toluene and so on).
Hazardous materials. When processing hazardous materials, the goal is to protect operators as well as the surrounding environment by preventing materials from escaping the centrifuge.
Internal atmosphere for safety
Combustion requires oxygen because it results from a chemical reaction between the combustible material and air. The goal of a sealed centrifuge system is to achieve and maintain the desired limiting oxygen concentration (LOC) inside the decanter system. This is also known as the minimum oxygen concentration (MOC) and is defined as the limiting concentration of oxygen below which combustion is not possible. It is expressed in units of volume percent of oxygen (Figure 2). LOC varies with pressure and temperature as well as the type of inert gas used [1].
FIGURE 2. Minimum oxygen concentration is the limiting oxygen concentration (LOC) below which combustion is not possible
Steps for low-O2 atmosphere
Creating an internal atmosphere that meets the desired LOC requires two steps: the pre-purge and inert gas blanketing (Figures 3 and 4).
FIGURE 3. The diagram shows the pre-purge flow of a decanter centrifuge system
FIGURE 4. The blanketing purge flow is shown here for the same type of decanter centrifuge
The pre-purge step occurs before the machine is started. The interior of the centrifuge is flushed with inert gas (such as nitrogen, Figure 5) to displace the air/oxygen. This is accomplished by filling the housing until the pressure exceeds 0.290 psi and continues until the flow of inert gas has replaced the interior volume of the centrifuge multiple times over or until an oxygen sensor indicates a non-hazardous concentration of oxygen, or both. The amount of time this takes varies according to the decanter size, but is usually less than 30 min.
FIGURE 5. Nitrogen is a common choice for inert-gas blanketing applications. The requirements for nitrogen are shown her
After purging, the inside of the centrifuge is “blanketed” with the inert gas, forming a technically tight seal. In this step, the system is held at a slight overpressure. The unit’s control system also maintains a higher pressure at the seals on both ends of the decanter. This results in a constant differential pressure to the atmosphere, which prevents the outside atmosphere from penetrating the inside.
Importantly, the inert-gas supply system must be equipped with sufficient redundancy to guarantee the continuous availability of inert gas during centrifuge downtime, in case of any failure.
Blanketing system operation
A standard mathematical formula is used to calculate the amount of gas required to displace the volume of the decanter. Once an adequate volume has been displaced and safe operating conditions are reached, the system is considered to be in an operational state (that is, inert). During operation, the differential pressures must be maintained to ensure safety. This is done with an overpressure sealing and monitoring system consisting of approximately five flowmeters, two flow switches and eight pressure sensors (Figure 6).
FIGURE 6. Differential pressures are maintained with an overpressure sealing and monitoring system
Specialized mechanical shaft seals prevent the surrounding atmosphere from penetrating into the interior of the centrifuge. They also prevent gases from escaping the inside as long as steady overpressure is maintained.
A seal consisting of several sealing rings separates the interior of the centrifuge from the outside atmosphere. By using multiple sealing elements, including additional seals on shaft feedthroughs and housings, a technically tight seal is created. This also reduces the amount of inert gas used, since the seals help ensure minimal loss throughout the system.
Inert gas is fed between the sealing rings. It flows through narrow seal openings, both into the interior and to the atmosphere. This is only possible if the pressure at the seal face is higher than the pressure in the decanter housing (>0.290 psi) and the outside atmosphere. The differential pressure (0.725–1.45 psi) between the housing and the gas feed point is monitored and controlled. Seals are pressurized to keeping oxygen from entering the system.
Managing O2 concentrations
Internal oxygen concentrations can be managed in two ways: with oxygen sensors and with a continuous-flow purge system (Figure 7). Oxygen sensors monitor the oxygen concentration inside the centrifuge housing. An advantage to using these sensors is the continuous monitoring they offer, especially if the differential pressure between the internal housing and the atmosphere is less than 0.290 psi.
FIGURE 7.Internal oxygen concentrations can be managed in two ways: with oxygen sensors and with a continuous-flow purge system
Oxygen sensors provide direct information about the risk factor of the oxygen concentration. In contrast, a continuous-flow purge system is indirect and relies on pressure-sensor setpoints. As long as the overpressure on the seals or housing is maintained, no oxygen from the environment can penetrate the system and the oxygen concentration is held below the critical level. This is done with flowmeters and pressure gages.
However, since continuous-flow systems rely on pressure sensor setpoints, applications using lower differential pressures may be susceptible to erroneous system shutdowns. In these cases, oxygen sensors may provide more sensitive monitoring of this danger zone.
Both methods ensure safe operation if they are properly planned, installed and maintained. Unfortunately, the weak point with an oxygen sensor is the sensor itself, which requires programmable logic controller (PLC) programming, calibration, periodic replacement and maintenance. Overall, continuous-flow purging offers better net positive usability when compared with oxygen monitoring.
Safe and efficient separation
Using an inert gas to form a tightly sealed or purged centrifuge is an excellent solution for a variety of applications, including explosion protection in the chemical, pharmaceutical, petrochemical or biodiesel industries; and environmental protection from hazardous materials, such as those generated during tar cleaning.
Once commissioned, a leak-tightness test schedule should be created and performed regularly to verify the tightness of the purge unit. Routine maintenance and testing points include threaded connections, manometers, meters, flowmeters and hoses. Supply lines also must be checked at regular intervals for damage, clogs and tightness (Figure 8).
FIGURE 8. Routine maintenance and testing points include threaded connections, manometers, meters, flowmeters and hoses
Centrifuges are extremely efficient, with higher throughput than filters or presses and they operate continuously, instead of in batches as with filters or presses. It is possible to separate two phases, three phases or a very light emulsion with a purged centrifuge. Many systems are also fully automated, which reduces operator involvement overseeing the machine.
Edited by Scott Jenkins
References
1. Green, D. and Southard, M., “Perry’s Chemical Engineers’ Handbook,” 9th ed., McGraw-Hill Professional Publishing, 2018.
2. National Fire Protection Association (NFPA) Standards Council, National Electric Code NFPA 70, 2023 edition, nfpa.org.
Acknowledgements
All images in this article appear courtesy of Flottweg Separation Technology Inc.
Author
Lee Betkowski has been the biotech, chemical, pharmaceutical (BCP) industry manager for Flottweg Separation Technology Inc. (10700 Toebben Drive, Independence, KY 41051; Email: [email protected]) for 13 years. Prior to joining Flottweg, he worked for Krauss Maffei (pusher, peeler centrifuges, dryers), Rosenmund (filter dryers) and De Dietrich (glass-lined steel equipment). Betkowski has nearly 30 years of experience in the biotechnology, chemical and pharmaceutical industries, working predominantly with centrifuge products and technology.