Flow control for optimising growth of microorganisms
Today, more than ever, enzymes and microorganisms are being used to increase sustainable production. This is particularly true in industries such as pharmaceuticals and (bio)chemicals. In these and other industries, researchers within universities, R&D organisations and other institutes, as well as within industry want to know under which conditions these biological cells grow. While it is essential and important to know how and under what conditions they grow faster, researchers need the full story. That means they are just as interested in what makes them grow slower or abnormally. It is also essential to learn about the influence of nutrients or additives to understand the underlying biological processes.
For research organisations the accurate measurement and control of low flows of liquids and gases is often required. What is low flow? The real values will be application dependent but might be as low as 0.014 to 0.7 mL/min of N2 to around 600 kg/h in liquid applications.
Bronkhorst excels in this arena and their customers are many and varied. In the area of biological cell growth, for one recent application the organisation sought to learn more about the conditions under which a population of microorganisms will grow. Flow control was used to give an accurate and steady flow of aqueous liquid while they varied an array of other parameters.
It was essential for them to investigate under which conditions a population of microorganisms can grow. For this they would vary one typical parameter, whilst other parameters, such as temperature, pressure and nutrient concentration, needed to be kept constant.
For this recent application, a research organisation contacted Bronkhorst. They had an application where they were struggling to stabilize the low flow in an aqueous stream — in this case their range was 30 to 200 mL/min. They had two reactors that needed to be kept in balance. They had tried to find a balance but were regularly emptying one of the reactors. To that end, their desire was that the liquid levels of two reactor vessels containing these microorganisms needed to be accurately and repeatably kept at a stable, constant value using flow control.
After consideration, Bronkhorst supplied two of their liquid mass flow instruments. In this case it was their mini CORI-FLOW series. The CORI-FLOW series uses the Coriolis effect which was first postulated as an explanation of the deflection of flowing air moving in a rotating system. In fact, the Coriolis effect is a mass inertia effect. A Coriolis-based mass flow meter is particularly suitable when you want to measure the mass flow of varying or unknown gas or liquid mixtures or for measuring supercritical gases. Besides measuring direct mass flows which eliminates inaccuracies due to the physical properties of the fluid, these devices are highly accurate and have a high repeatability. The Coriolis flow meter is the ultimate flexible, reliable and extremely accurate flow meter.
In this application, each CORI-FLOW was inserted in the circulation system in between the reactor vessels, with the aim to provide a continuous flow of aqueous liquid.
The main reactor was approximately 1 litre and the researchers allowed the micro-organisms to grow in the reactor within an aqueous environment. Regular sampling of the main reactor gave them information of the number of cells and the cell growth rate. The researchers also identified temperature as an important parameter. Too low temperatures will hold back the microorganisms and slow or stop them from growing, and too high temperatures are detrimental to the longevity of the microorganisms themselves.
For this sophisticated application, the liquid mass flow instrument with a control valve provides a signal to a control unit. That control unit is ‘in charge’ of a pump. The pump speeds or slows in response to the flow measurement and the control action, making for a very precise flow in this line. From there the liquid then flows from the main reactor to a second reactor. In this case the second reactor is much smaller than the main reactor and has a volume of about 200 mL. Using the same methodology, the fluid is again moved via the direct control pump scenario, described above, back to the main reactor. What is now set up is a continuous circulation, in which the flow is very steady. The process continues day and night for as long as the research requires.
While it sounds like smooth sailing, a further complication was identified. The microorganisms in this experiment were approximately 3 μm diameter. That provided a further challenge as all the microorganisms needed to be kept alive and they needed to be in perfect health (without any damage) during the process of circulation. For this Bronkhorst advised the researchers to use peristaltic pumps in their process to keep their microorganisms fit and healthy.
The Control Unit/s and setpoints were run within the research organisation’s systems; however, it was further determined that, in this application both the flow controller and pump combinations would best have the same capacities. This simplified the operation and helped the levels in both reactors remain at the same, stable value.
Bronkhorst’s flow meter range includes:
Thermal Mass Flow meters & controllers for gases and liquids
Coriolis Mass Flow Meters & Controllers for gases and liquids
Ultrasonic Meters for liquids
Pressure Controllers for gases and liquids
Control Valves and Control Electronica and accessories
In this case the correct flow meter was a Coriolis-based mass flow meter. This technology is particularly suitable when you want to measure the mass flow of varying or unknown gas or liquid mixtures or for measuring supercritical gases. The fundamental theory for a Coriolis meter is direct mass flow measurement. There are no estimations or assumptions or inaccuracies due to the physical properties of the fluid. A CORI-FLOW from Bronkhorst is thus highly accurate and they have high repeatability. For many users, the Coriolis flow meter is the ultimate flexible, reliable and extremely accurate flow meter.
The original article was published as an Application Note by Bronkhorst High-Tech B.V.
