On-line conversion monitoring through ultrasound velocity measurements in bulk styrene polymerization in a recycle reactor - Part I: Experimental validation

Ultrasound propagates through an elastic medium at a rate or speed named th e ultrasound propagation velocity (UPV). This velocity is determined by the rate at which the wave energy is transmitted, inversely related to the squ are root of the density and compressibility of the medium. This technique i s based on the increase of UPV with rising elasticity of the medium due to polymerization. As monomer is transformed to polymer, both the density and adiabatic compressibility change, making the UPV closely related to monomer conversion. Longitudinal waves are commonly employed in ultrasonic applica tions since they are easily generated and detected. The application of the UPV technique to follow the composition evolution of the solution during th e polymerization reaction in a recycle tubular reactor is presented. The se nsor can be directly inserted in the tubular reactor and does not require a sampling circuit. This makes its use particularly simple and suitable for laboratory and industrial purposes. Another application of this technique i s the direct measure of the residence time distribution (RTD), which affect s both monomer conversion and polymer molecular weight distribution. RTD me asurements can also be used to detect any problem of fouling or plugging du ring the polymerization reaction and to follow the efficiency of the reacto r clean up during the shut-down procedure of continuous polymerization proc esses. Experimental validation of this technique during a bulk styrene poly merization is presented in this paper. Technical parameters such as tempera ture, pressure and flow speed, as well as medium parameters such as monomer conversion and polymer molecular weight directly influence the UPV value. The quantitative modeling of the UPV change during polymerization in order to calculate monomer conversion will be discussed in detail in a further pa per (part II).