Flow theory: vapour flow control

In our daily life, we are surrounded by vapours. We inhale them, and we can sense their presence through sight or touch. Another example is water vapour as part of the air that surrounds us in our homes, which is noticeable if the air encounters a colder surface, such as a window. The contained water vapour may condense on the windowpane and become visible and touchable as a liquid.

In industrial processes and laboratories, vapours have a useful role. So what are vapours, where are they used, and how can they be delivered in a controlled way to these processes?

What is a vapour?

A vapour is very similar to a gas — which is a fundamental state of matter, just as solids and liquids are. Gases and vapours consist of separate molecules that move as free particles. However, there is an essential difference between a gas and a vapour. If a compound is a liquid at room temperature (around 20°C) and normal pressure (1 atmosphere), then we call the ‘gaseous’ form of that compound a vapour. That is why we call the gaseous form of water a vapour, whereas the gaseous form of oxygen is a gas, as oxygen is still a gas at ambient conditions.

How to generate a vapour?

When raising the temperature or lowering the pressure, a liquid can evaporate and convert into a vapour. On a molecular level, at any temperature at the liquid surface there are always molecules with enough velocity to leave the liquid. So, above a liquid there is always a vapour of the same liquid. Evaporation occurs at any temperature, and not just at the liquid’s boiling temperature. This boiling temperature is just a definition: it is a point at which the vapour pressure of the liquid equals the ambient pressure.

Playing with temperature and pressure are two ways to control the vapour pressure. A third way to control the vapour pressure — actually reducing it — is by diluting the vapour, for example by adding an inert gas such as nitrogen to the vapour.

Why use vapours?

There are circumstances in which you may want to add a vapour to a process in a controlled manner. For example, consider fuel cells — PEMFC — of which the electrolytes need to be in a hydrated state (humidified) to maintain a high conductivity and, hence, optimal performance. Alternatively, you might want to supply accurate water vapour concentrations for the calibration and certification of humidity sensors, in order for these devices to display the correct humidity values.

Another type of vapour delivery is in the controlled supply of metalorganic vapours to a reactor. These vapour compounds act as precursors in a chemical vapour deposition reaction to deposit a thin layer on an object, for example to deposit semiconducting thin films. Accurate vapour supply is necessary here, to precisely control the layer growth — even on complex-shaped objects — and to avoid spilling expensive metalorganic precursors.

Traditional vapour flow control using bubblers

A traditional way to deliver a vapour to a client’s process is by using a bubbler system. Here, a gas flow is bubbled through a heated vessel filled with a liquid compound. This carrier gas flow becomes entirely or partly saturated with the compound vapour, and this vapour flow is further guided by the carrier gas to the client’s process. Although this is quite a simple set-up which can be used versatilely, there are a few drawbacks. Small changes in process conditions may give large variations in vapour flow, rendering it a relatively inaccurate delivery technique with a poor long-term stability. Since the vapour pressure largely relies on the vessel temperature, a slight change in temperature will result in a rather large deviation in vapour pressure and thus vapour flow. In addition, the total pressure and the carrier gas flow rate need to be stable to give a stable vapour flow. This vapour flow solution strongly relies on temperature and pressure.

Improved vapour flow solution via Controlled Evaporation & Mixing

One way to overcome the above hurdles is by making use of a CEM (Controlled Evaporation & Mixing) system for vapour delivery. In this vapour system, a gas mass flow controller (such as an EL-FLOW Select model) provides an accurately controlled carrier gas flow rate, whereas a liquid flow meter (such as a mini CORI-FLOW or LIQUI-FLOW) measures the flow of the liquid to be evaporated, for example drawn from a room temperature pressurised liquid vessel. The liquid control valve, functioning also as a three-way actuator, is tasked with blending tiny liquid droplets with the carrier gas. The mix fluid enters a heated, temperature-controlled mixing and evaporation chamber where the liquid fully evaporates immediately, and a homogenous vapour/gas mixture is generated.

A complete CEM system can be monitored with an external readout/control unit, including power supply or with a PC by using Bronkhorst software like Bronkhorst Flowsuite for operation of the CEM-system components. A CEM system is a straightforward vapour delivery module that outperforms a bubbler in many ways. The process is independent of pressure and temperature because gas and liquid flows are controlled using mass flow controllers, enabling precise control of the molar ratio due to the precision and repeatability of the mass flow instruments. This accurate mass flow control provides significant stability of vapour output from the evaporator. The direct injection of liquid flow into the carrier gas stream enables better dispersion of liquid molecules in the gas and significantly reduces response time.

The evaporation chamber is designed with an internal geometric shape that generates turbulence, thereby enhancing the uniformity of the gas-liquid mixture. Additionally, it serves to heat the fine liquid droplets introduced into the gas stream, facilitating their evaporation. The mass flow rate of liquid can be easily determined using the online Fluidat on the Net application. Additionally, the relative humidity (%RH) and molar concentration can also be calculated, thus facilitating the utilisation of Bronkhorst’s CEM evaporation system.

Vapour delivery using CEM systems occurs typically in applications such as surface treatment, the food and beverage industry, pharmaceutical applications, materials study and testing, and environmental research.

For vapour flow applications, avoid condensation

A vapour can be converted back into a liquid relatively easily by increasing the pressure or decreasing the temperature — something that will never work with a dry gas. One significant challenge in working with vapours is preventing condensation, as it can result in droplets that may disrupt or harm the ongoing process.

So, without altering the system pressure, keep the temperature beyond the CEM always higher than the temperature inside the CEM and higher than the dew point temperature. Or alternatively, without changing the temperature, keep the pressure beyond the CEM always lower than the pressure inside the CEM.

Bronkhorst is represented by AMS Instrumentatation & Calibration Pty Ltd in Australia and Streat Control (NZ) in New Zealand.

Image credit: iStock.com/ThomasVogel

Originally published here.