Impact of Flow Configurations on Response Time and Data Quality in Real-Time, In-Line Fourier Transform Infrared (FTIR) Monitoring of Viscous Flows

Abstract

The real-time, in-line monitoring of continuous flow concentrations is widely conducted via infrared (IR) spectrometry by using a flow cell connected to a reactive flow stream. For protective purposes, the IR sensor tip is typically offset from the flow. This offset can cause the formation of a stagnant boundary layer above the sensor, especially when dealing with highviscosity fluids. As a result, the IR signal response time is often controlled by the slow diffusional exchange of fluid in the boundary layer, as confirmed via 2D computational fluid dynamics (CFD) simulations. We evaluated several flow configuration modifications in a typical IR flow cell in order to identify the changes to the flow dynamics that enable improved response times with minimal changes to the cell configuration: the use of (i) vertical flow, where the standard horizontal flow over the sensor is redirected to contact vertically with the sensor, (ii) a static mixer to create radial flow momentum above the IR sensor, and (iii) horizontal or vertical nozzles to direct the flow toward the IR sensor. The vertical flow configuration did not show any significant improvement over the standard horizontal flow configuration. However, the static mixer, horizontal nozzle, and vertical nozzle configurations all resulted in markedly improved response times and signal quality, albeit at the expense of a higher pressure drop across the flow cell. These results point toward straightforward, user-accessible modifications of in-line IR flow cells that result in significant improvements in signal stability and acquisition times.

Infrared Spectroscopy as Process Analytics to Identify and Quantify Grignard Reagents

“…In commercial process IR spectrometers with an incorporated flow cell, the tip of the probe is offset from the flow path by e.g., 1 mm to protect the probe . In general, to obtain consistent data when monitoring the increase and decrease of reaction components by using an IR probe, a continuous and well-mixed flow around the probe is necessary . A backpressure regulator, placed after the flow cell, can prevent drainage of the flow cell …”

“…If using a stirred reactor, the probe should be placed outside the vortex introduced by the impeller to again reduce spectral noises and to protect the probe from damage. In commercial process IR spectrometers with an incorporated flow cell, the tip of the probe is offset from the flow path by e.g., 1 mm to protect the probe . In general, to obtain consistent data when monitoring the increase and decrease of reaction components by using an IR probe, a continuous and well-mixed flow around the probe is necessary .…”