Maximising interfacial gas-liquid area with Scalable Agitated Baffle Reactor (SABRe)

Many processes use gas-liquid phase boundaries in reactors, from lab to production scale.

When assessing and designing a process, in particular processes when intrinsic chemistry is much quicker than mass transfer across the phase interface, the observed process rates are often limited by the gas-liquid interfacial surface area.

The interfacial surface area is fundamental for improved reaction efficiency to maximise contact between the gas and liquid, meaning it is a common metric used to compare reactors and estimate process performance.

What determined the rate of gas-liquid reactors?

The rate of gas dissolution is proportional to the concentration difference: how much gas could be dissolved and how much is dissolved. The proportionality coefficient, k_La is the mass-transfer coefficient.

Gas saturation is fixed in practice dependant on temperature and pressure, or the existing gas concentration (often determined by the process requirements). What you could do is to use the reactor with the highest k_La to maximise the performance of the process. Doubling the k_La value, you could double the product throughput.

For fast reactions (such as CO₂ absorption by an alkali), the rate is often determined by the interfacial surface area.

Data for the batch reactor

There is plenty of data accumulated for batch processes. These include know-how on operating and servicing batch reactors and plenty of research papers that cover all aspects of batch.

A few examples (picture) show several papers with correlations that describe how the interfacial area depends on operating parameters.

The key parameters that affect the gas-liquid interfacial area are:

• Specific mixing power (in W/L), the mixing force introduced by agitators.
• Superficial gas velocity (in m/s), the velocity gas would have travelled being in reactor alone.

Rapid mixing agitators and high-velocity gas streams substantially increase the gas-liquid area, often resulting in fast processes.

Predictions and correlations

Lopes de Figueiredo and Calderbank [1] suggest the simplest correlation to study and predict the gas-liquid area:

, where p  is the agitation mixing power (in W), V_L is the liquid volume (in m³) and Q_g is the superficial gas velocity (in m/s). The resulting superficial area in (m²/m³) is obtained from values that are easy to estimate.

The liquid volume is incredibly straight forward to afind out, a first approximation is the reactor volume. We can focus on surface mixing as the mixing power is strongest at the gas-liquid interface. In practice, a good estimation could be obtained from the equation: , where  is the fluid density (in kg/m³),  is the impeller rotation rate (in s^-1),  is the impeller diameter (in m), and  is the power number (well tabulated for various impeller types, typically around 3 to 5). Substituting these values, we estimate the mixing power. The superficial gas velocity is another simple parameter to calculate – we divide the total volumetric gas flow rate introduced by the apparent reactor cross-section area.

Experimental measurements of the gas-liquid interfacial area

aWe have studied the gas-liquid area in the SABRe system using a conventional method of studying CO₂ absorption by a NaOH solution. The experiment is simple – we introduce CO₂ gas and measure the unabsorbed gas. Doing some calculations described in our paper^[2], we determine the interfacial area.

We could compare the obtained data and the data expected by the correlation, yet correcting them for (i) increased process temperature that decreases CO₂ solubility compared to collated data, and (ii) large concentration we used that results in high viscosity and rapid reaction rates. Both factors require the correlated areas to be a factor of 0.36 lower.

The comparison of the predicted and measured data show excellent agreement; within ±25%, highlighting the reliability of the SABRe system. It is an excellent agreement between the 1978 paper on batch reactor when applied to SABRe.

The SABRE system provides large gas-liquid interfacial area predictable up to 5,000L reactor volume to maximise the reaction throughput and selectivity.