The impact of volume on precision process control
29 October 2020
Frequently Asked Question: How to minimize dead volume or internal volume in your process?
By Mitch Spronck
In many applications, the concepts of “dead volume” and “internal volume” play an important role. Especially the question of how to keep these values to a minimum is asked frequently. The terms ‘’dead volume’’ and ‘’internal volume’’ are sometimes used interchangeably, but actually have very different meanings. In this blog, we will explain what the difference is between the 2 and what implications dead volume and internal volume have on processes. Obviously, we will also share some expertise on how to address such implications!
The figure shows a cross-section of an example process, depicting the swept and dead volume in the example instrument. The inlet of the instrument is on the left and the outlet on the right. The flow path will thus be from left to right. The plug at the bottom creates a dead volume in which fluids come to a halt.
Internal volume vs. Dead volume
Internal volume generally means: The volume that is occupied in a certain space by the process fluid. It can be defined as the sum of the “swept volume” and the “dead volume”. For example, if we take a 3-way fitting, with one side plugged and the others open to flow, we can very easily determine what the swept volume and dead volume is. Indeed, the swept volume is the volume through which the fluid moves (flow path) and the dead volume is the volume that is outside of the flow path and essentially acts as a tank or space in which the fluid is immobile. This space is also referred to as “dead leg”. This is where confusion occurs, as the term dead volume is sometimes incorrectly used to describe the internal volume.
What is the impact of dead and internal volume on process control?
As one can imagine, stagnant fluids can be a problem in certain applications. One of the most common examples are fluids that contain microbes. If these fluids come to a halt and stay in the dead volume without being cleaned or removed, they may cause problems as they may produce harmful substances.
This issue is commonly seen in the food and beverage industry (and other hygiene-sensitive industries). For these specific applications, the Equilibar SDO and FDO valves have been developed. These valves have no dead legs and therefore no dead volume. Moreover, these regulators have been designed with CIP and SIP requirements in mind and are therefore very easy to clean, maximizing the process performance. You can read more about the product characteristics in this introduction blog.
Dead volume is of significant importance in chromatography; it can lead to peak broadening, which compromises peak resolution. There are many other instances in which dead volume can pose a problem. Examples are the build-up of debris and particles; or standstill of a solution, potentially resulting in gelling.
How Equilibar ® regulators solve or prevent issues related to dead volume
To prevent issues in the controlled process, the back pressure regulators in the Equilibar Research Series don’t have dead-legs.
Additionally, the FDO series regulators were specifically designed to avoid dead volume. The FDO series regulators can handle pressures up to 10 barg and are specific for (bio)pharma and food industries, containing only USP Class VI certified materials and being electropolished.
For applications in which these strict requirements aren’t required, use of the NLD Series regulators is a good option. Instead of having a body and a cap, which is the standard for all Research and GS Series regulators, the NLD units have 3 parts: a body, a top cap and a bottom cap. Because of this design, no machining ports are necessary and the units are devoid of dead-legs or dead volume.
Challenges and solutions for varying situations around internal volume
Internal volume on the other hand, can be a critical factor for completely different reasons. In processes requiring precise measurements or use small volumes, for example, it is essential to have the lowest amount of internal volume possible. A great example of this are microfluidics and related fields of application. Often, the used volumes are so low that adding a new component with relatively large internal volume can compromise the precision of the measurement or process.
Another application that can benefit from small internal volumes is dosing. This specifically applies to situations in which very small amounts have to be dosed and precision is required; when this is the case, a smaller internal volume will lead to more precise dosing.
Yet another application in which internal volume can play an important role is catalytic research. These processes often contain very small amounts of (a) certain catalyst (particles), which makes it important to keep the internal volume – and thus the volume in which particles can collide – small, in order to increase the probability of effective collisions and with that the speed of reaction.
In applications or processes which require low internal volume, the LVF Equilibar back pressure regulator is ideal. It has a staggeringly low internal volume of only 0,23 ml(!) and, in addition, is very compact (not much larger than a 2 euro coin) for set-up in small spaces. Additionally, Equilibar back pressure regulators can be fitted with almost any fitting. For small internal volumes, 1/16’’ HPLC fittings are often used, which can easily be incorporated into several types of regulators.
Want to know more?
If you want to learn more about our products for use in your application and how these help increase the precision, reliability and reproducability of your process, please do not hesitate to contact our engineers – you can also start a conversation right now by opening the green chat widget in the low right corner of the page. We are happy to advise you to help you maximize the performance of your system. Should you wish to explore more options for your process on your own, have a look at our product configurator to play around with potential process parameters and available product features.
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