Controlled Evaporation Mixing, A New Method for Vapor Stream Generation
KEYWORDS
Vapor Generation, Controlled Evaporation Mixing, Vapor Stream, Gas Flow, Liquid Flow, Reference Gas
ABSTRACT
This new technology is used to generate low flow, precise, and repeatable vapor streams for use as reference gases, automating sampling systems, and more. This technology is an improvement on traditional vapor generation technologies and can reduce loss due to inaccurate testing processes.
PRESENTATION
Written for and published at the Analyzer Technology Conference 2024 in Galveston, Texas.
INTRODUCTION
A new technology is being implemented in numerous applications across the globe that offers high precision and control for generating vapor streams. This technology, called Controlled Evaporation Mixing (CEM), combines a low flowrate gas mass flow controller (MFC), a low flowrate liquid MFC, and the CEM device that atomizes the liquid into a carrier gas stream then passes it through a heating chamber, and a PLC to control the process components. Heat tracing control is used to maintain vapor phase downstream of the outlet. As a result, this system provides unprecedented control of vapor concentrations, flowrates, and temperatures when compared to traditional laboratory bubblers. This technology offers high accuracy, high precision, and datalogging capabilities to improve repeatability and performance reliability. Unlike traditional bubblers, its performance is not affected by outside variables such as liquid level, atmospheric temperature and pressure, thus it provides the same performance and operation year-round, regardless of altitude and weather.
COMPARING CONTROLLED EVAPORATION TO BUBBLER SYSTEMS
Although bubblers can produce vapor, they are affected by the level of liquid in the container, as well as the atmospheric pressure and temperature. They are not easy to start and stop and are inherently imprecise because it is not possible to know the liquid content of the vapor while flowing. In applications that require precise vapor deposition, users must turn to additional measurement and control devices, such as vapor MFCs, which increases the complexity and cost of using bubbler systems.
Compared to a more conventional bubbler system, a CEM system offers a more direct approach for generating and controlling vapor streams. The controlled evaporation method is very straightforward and, theoretically, any concentration can be made in seconds with high accuracy and repeatability. Moreover, it’s possible to adjust a relative humidity between five and ninety-five percent(1).
OPERATING PRINCIPLE OF A BUBBLER SYSTEM
Small concentrations of moist air can be created using a bubbler system. This conventional method requires optimal pressure and temperature control of the bubbler system. A complete bubbler level measurement system therefore consists of a source of compressed gas, a gas flow restrictor, sensing tube, and pressure controller, as seen in Figure I. The pressure controller adjusts the back pressure to provide output to a controller, which calculates the liquid level. The liquid content of the vapor fully depends on the theoretical calculation of the degree of saturation of the gas flowing through the liquid and the accuracy of pressure and temperature control(1).
Although this is a simple setup which can be used widely, there are a few drawbacks. Small changes in process conditions may give large variations in vapor flow, rendering it an inaccurate delivery technique with poor long-term stability. Since the vapor pressure relies on vessel temperature, a slight change in temperature will result in a significant deviation in vapor pressure and thus vapor flow. In addition, the carrier gas flowrate and pressure need to be controlled to give a stable vapor flow. With this conventional approach it is difficult to achieve a specific air moisture content(2).
ADVANTAGES OF A CONTROLLED EVAPORATION SYSTEM
In a controlled evaporation system, a gas mass flow provides an accurately controlled carrier gas flowrate, while a liquid mass flow controller regulates the amount of liquid to be evaporated; e.g., drawn from a room temperature, pressurized liquid vessel. Subsequently, a mixing control valve allows tiny droplets of the liquid flow to be atomized into the carrier gas flow in a special mixing valve assembly, which enters a heated, temperature-controlled evaporation chamber. Here the liquid fully evaporates, and a vapor mixture is generated. A complete CEM system, as seen in Figure 2, also contains a readout/control unit, including power supply, for operation of the CEM-system components(2).
A CEM system is a vapor delivery module that outperforms a bubbler in many ways. Since temperature distribution inside the evaporation chamber is more controlled, vapor supply that is being carried along by the carrier gas is much more accurate and reproducible. This results in a stable vapor delivery rate. In traditional bubbler systems, atmospheric pressure and temperature changes can cause the vapor outlet to sputter and create an inaccurate and unreliable vapor flow. Because a CEM uses mass flow controllers for both the liquid and carrier gas, it is virtually temperature and pressure independent since those values are controlled by the instrumentation, thus improving performance and ensuring accuracy and repeatability regardless of atmospheric conditions(2). Moisture content is determined by the liquid mass flow controller and carrier gas flow ratio can be adjusted with the gas MFC. On top of the CEM, a mixing valve sprays the liquid into a ring of gas jets atomizing the mix. Because of the relatively low pressure ratio of the liquid mist to gas flow, the water can evaporate at a lower temperature in the spiralized heater tube at the outlet of the mixing valve(1).
BUILDING THE SYSTEM
This technology can be assembled in-house with existing components or engineered and prebuilt by a system manufacturer. The choice of gas and liquid MFC depends on the desired flowrates and output composition of the vapor stream. A critical piece of the system is the PLC and engineered program that brings these separate components into a unified system. The system can be intended for use in tabletop applications such as in laboratories, thus it is mobile, or it can be rack-mounted and integrated into a production line.
GAS MASS FLOW CONTROLLER
Typically, a thermal MFC will be used to measure and control the gas flow input for the vapor generation system. A thermal MFC is a common choice as it can handle many types of gases and gas blends. They do not require correction for changes in gas: temperature, pressure, or density, and they are extremely accurate, especially when measuring low flowrates, and are no longer regarded as high cost. However, mass flow is often expressed in volumetric units and the mass is converted to volume under provided conditions. Because Heat Capacity (Cp) and Thermal Conductivity (Lambda) affect the conversion factor, changes in pressure and density have an impact(3).
For this case, the thermal MFC uses the bypass principle. The mass flow is measured in a bypass of the main flow channel – instead of the channel itself, also known as the “shunt” principle of the Laminar Flow Element (LFE) as seen in Figure 3. Thin plates, which are used to create the LFE, allow the measurement of very small (less than 1 mln/min) as well as medium (20 ln/min) flowrates. The bypass is a capillary tube with an internal diameter of 0.2 mm, and this small size allows for fast response times and low energy requirements, as only a small tube volume needs to be heated(3).
LIQUID MASS FLOW CONTROLLER
The choice of liquid MFC also depends on the outcome required by the process and application. An MFC that relies on the Coriolis principle is the best choice because it can handle liquid blends or changing chemistry. Mass flowrate and density are measured independently of each other using the same device (4).
CONTROLLED EVAPORTION – VAPORIZER AND MIXING CHAMBER
The vaporizer is the key component of the CEM system, which the rest of the system is designed around. The mixing valve atomizes the liquid and adds it to the carrier gas, creating an aerosol vapor. To monitor the heat exchanger internal temperature, the vaporizer incorporates a thermocouple. An internal safety switch prevents heat exchanger overheat by cutting off power when the temperature exceeds setpoint(5).
PLC WITH OPERATING PROGRAM
To control the liquid and gas supply flows and the CEM temperature, a PLC is required, also providing a user interface. The PLC will also provide power for the instrumentation and can communicate using analog or digital protocol. It can also interface with outside controls systems via Modbus or Ethernet via an RJ485 port(6).
Programs can be written in-house or complete, ready-to-use systems can purchased from an engineering company such as the PSC Controlled Evaporation System for Vapor Generation from Process Solutions Corp. These complete systems include a user-friendly, color touchscreen interface for easy vapor generation control. This CEM system arrives complete with all instrumentation required and the program is already installed on the controller, allowing users to immediately begin operating. The program includes easy control of flowrates, setpoints, temperature, stop/start, and control of the concentration and composition of the vapor stream. Input / Output is also conveniently available on the control unit to allow users to easily connect various flow meters and flow controllers, change carrier gas, and change process liquids. This system includes an internal temperature safety switch to prevent the vaporizer from burn-out, heat tracing to ensure the output maintains vapor phase, data logging capabilities for process tracking and to improve repeatability(7).
The obvious benefits of using a preprogrammed unit are the relatively short lead time and cost savings. Delivery and commission can be achieved within a matter of weeks versus the time it would take to write a custom program.
TEMPERATURE SETPOINTS AND HEAT TRACING
The temperature setpoint can be calculated for the CEM using specialized software. These calculations ensure that the heater will evaporate the liquid and that the vapor temperature at the outlet will still be high enough to prevent condensation. Further down the line, however, tubing length, process and ambient conditions can cause the vapor temperature to drop to a point where condensation might occur. This is where heat tracing is critical(6).
PRESSURE TANK
The liquid will need to arrive pressurized since the MFC will be operating the control valve at the top of the CEM. Typically, a pump with a recirculating pressure loop is employed, allowing the CEM system to draw off pressurized liquid as needed(5). However, keep in mind that some pumps have a large internal volume, which will lengthen the start-up time of the system. In addition, some pump types, such as gear pumps, can cause cavitation, which introduces gas bubbles, which is exactly what needs to be prevented(5). If direct pressurization of the liquid with a gas is inevitable, other measures can be taken to keep gas dissolution at a minimum. Gas with a low solubility, such as helium, can be used to pressurize the liquid. Keeping the pressure on the liquid as low as possible will decrease gas dissolution, and relieving the gas pressure from the liquid when the CEM is not in use will also help remove or prevent additional gas from dissolving(6).
The liquid vessel should be large enough to provide a stable flow between refills. Purging or flushing the fluid system can consume a large amount of liquid, so this should be considered when selecting a suitable vessel size(5). Table I, below, gives an indication of the liquid consumption, based on different flowrates:
BENEFITS OF USING A CEM SYSTEM FOR VAPOR GENERATION
IMPROVING RELIABILITY THROUGH AUTOMATION
Using a CEM system reduces manual labor while increasing efficiency and repeatability. They are useful for automating sampling systems and can eliminate potential operator error and ensure results are accurate. This saves time and increases efficiency by potentially avoiding shutdowns or losses in reliability due to inaccurate measurement. Small discrepancies in measurement can lead to off specification product, which is significant when applied to large volumes. By automating sampling systems, test results be provided more quickly, and results become more reliable.
EXTREMELY REPEATABLE PROCESS
The performance of bubbler systems is highly dependent on atmospheric conditions, which can decrease consistency between batches if not carefully monitored. This batch consistency is critical for many manufacturers, such as photovoltaic solar cells manufacturers, which require multiple layers to be deposited in tightly controlled conditions through a sequence of process chambers. The fluctuation of temperature and pressure through different seasons or weather conditions will require process conditions to be adjusted to prevent sputtering and allow the bubbler system to achieve consistent performance. The repeatability offered by a CEM system improves consistency between batches regardless of atmospheric conditions. This allows manufacturers to produce a consistent product year-round without needing to frequently adjust process conditions.
VERIFICATION OF VAPOR DEPOSITION
Measuring vapor delivery via bubblers are modeled with constant temperature, a known flow of carrier gas, and relies on the assumptions that the carrier gas does not react with the liquid or dissolve in significant amounts. These variables remaining constant is neither guaranteed nor likely to occur in real life applications. Therefore, the actual vapor mass delivered cannot be accurately calculated for a bubbler system and can only be estimated. Unlike bubbler systems, a CEM system delivers an exact vapor mass using simple inputs through the controller and highly accurate flow instrumentation. This accuracy makes CEM systems ideal for life-critical applications, such as space suit testing, which requires an extremely reliable process to ensure the safety of end users. Furthermore, a quick response time allows the CEM system to replicate real life conditions of humans inside a space suit such as respiration, perspiration, etc.
CONCLUSION
Using a CEM system in place of a bubbler improves precision and increases control in vapor generation processes. The integration of highly accurate liquid and gas MFCs with a CEM vaporizing device provides unprecedented control of vapor concentrations, flowrates, and temperatures when compared to traditional laboratory bubblers. Performance predictability, reliability, and repeatability are improved through using high accuracy and high precision instrumentation, which also add datalogging capabilities that increase process measurability. Unlike with traditional bubblers, outside variables such as atmospheric conditions and gas dissolution does not affect the accuracy of a CEM system, so these systems provide consistent performance and operation year-round, regardless of the season or weather.
REFERENCES
- “Bubblers Versus Controlled Evaporation and Mixing Systems for Moist Air Applications”, Bronkhorst, February 2024.
- “Vapor Flow Control” Bronkhorst, January 2019.
- “A Fully Optimised Thermal Mass Flow Instrument with the Help of Big-Data”, Bronkhorst, June 2017.
- “Coriolis Flow Meters: How Do They Work?”, Bronkhorst, March 2018.
- “Controlled Evaporator and Mixer (CEM)”, Bronkhorst, January 2023.
- “PSC Controlled Evaporation System, Vapor Generation”, Process Solutions Corp., April 2022.
- “User Manual for XL7 & XL7 Prime”, Horner Automation Group, September 2022.
- “Flow control in solar cell production”, Bronkhorst, August 2020.
AUTHORS
Kolten Burkes; Tony Shekari; Mark Niemczyk