On-Line Monitoring of Chord Distributions in Liquid–Liquid Dispersions and Suspension Polymerizations by Using the Focused Beam Reflectance Measurement Technique Israel Bernardo Poblete, 1 Carlos Alberto Castor, 2 Marcio Nele, 1 Jos  e Carlos Pinto 2 1 Escola de Qu  ımica, Universidade Federal do Rio de Janeiro, Cidade Universit  aria, Rio de Janeiro 21941-909, Brazil 2 Programa de Engenharia Qu  ımica/COPPE, Universidade Federal do Rio de Janeiro, Cidade Universit  aria, Rio de Janeiro 21941-972, Brazil The on-line monitoring of the droplet/particle size distri- butions is very important to ensure the quality and applic- ability of various products in heterogeneous systems. For this reason, the main objective of the present work was to study the usage of the focused beam reflectance mea- surement (FBRM) technique for monitoring of liquid–liquid dispersions (styrene dispersion in aqueous solutions) and suspension polymerization of styrene. To do better under- stand the FBRM technique in these systems, the effects of surfactant concentrations, agitation speed and ambient light were evaluated during the in-line monitoring of aver- age chord lengths and chord-length distributions (CLD) at different operation conditions in batch experiments. In addition, a preliminary investigation of the optimal probe position was conducted in the polymerization experi- ments. It is shown that the FBRM technique is sensitive to variations of particle sizes in the characteristic ranges of particle diameters of typical styrene suspension poly- merizations, being useful for monitoring and also control applications that require the on-line characterization of CLD in real time in liquid–liquid dispersions and polymer- ization systems. POLYM. ENG. SCI., 56:309–318, 2016. V C 2015 Society of Plastics Engineers INTRODUCTION Several techniques have been developed since the second half of the twentieth century to study the bubbles/droplets/parti- cle size distributions [1–3] in heterogeneous systems (gas–liq- uid, liquid–liquid and solid–liquid) to understand and control the evolution of these distributions in chemical processes [4]. These techniques are often based on laser spectroscopy, capillary spec- troscopy [5], laser scattering [6], near-infrared spectroscopy [7], and backscattered lighting [8]. The droplet size distribution of a liquid–liquid dispersion results from a dynamic balance between breakage and coales- cence rates of droplets. These phenomena are affected by many intrinsic physico-chemical properties, e.g., the viscosity of dis- persed and continuous phases and the interfacial tension, and also by distinct operation conditions, e.g., the dispersed phase holdup, the impeller speed and the reaction temperature. In most suspension polymerizations, the particle size distribu- tions and average particle sizes of the final product constitute major quality control variables, which can be controlled through proper manipulation of breakage and coalescence rates in the reaction medium. Fine particle fractions are usually undesirable because they can cause clogging of filter screens and centrifuge tubing, lead to significant losses of the final polymer product and cause occupational and environmental problems when sus- pended in the air and in the water. Coarse particle fractions are also undesirable because they can cause processing problems, due to slow melting during extrusion and injection molding, leading to production of off-spec polymer pieces. Many publications have shown that the mean sizes of sus- pended droplets/particles and the respective size distributions are sensitive to distinct operation variables in liquid–liquid dis- persions and suspension polymerization, including the concen- tration of the suspending agent, the holdup of the dispersed phase, the impeller geometry, the agitation speed, the reactor geometry, the geometry of baffles, among others [9–11]. All these factors can affect, for example, the nature of the flow, the dissipation of mechanical energy and the rates of breakage and coalescence inside the process vessel. Particle/droplet size measurement techniques are based on a variety of principles, such as visual or microscopic observation [12–14], light scattering [15], focused beam reflectance mea- surement (FBRM) [16], ultrasound extinction [17], passage through sieve openings [18], sedimentation rate [17], Brownian motion [19], electrical resistance [20] and three-fold dynamic optical back-reflection measurement (3D ORM) [21]. Each tech- nique yields a characteristic size or a size distribution of equiva- lent spheres, which—for non-spherical particles—is dependent on the measurement principle. According to Merkus [22], proper classification of all techniques is difficult due to the huge differ- ences between the techniques and modern instrumental develop- ments. Furthermore, these distinct measurement techniques require distinct sample preparation schemes, which can lead to significant time delays during monitoring of liquid dispersions. When the main objective is measuring and characterizing the average particle size and the shape of size distributions, techni- ques such as laser diffraction and/or the electrical sensing zone method may not be suited. In these cases, one can apply the FBRM [16, 23], particle vision and measurement (PVM) [24] and 3D ORM techniques [25]. All these techniques allow for in situ data acquisition, useful to monitor the evolution of particle/ droplets populations in dynamic processes. FBRM provides the chord-length distribution (CLD) of the particle/droplet popula- tion, whereas PVM produces images of part of the particle Correspondence to: C.A. Castor, e-mail: carlosche@peq.coppe.ufrj.br Contract grant sponsor: CNPq (Conselho Nacional de Desenvolvimento Cient  ıfico e Tecnol ogico, Brazil). DOI 10.1002/pen.24256 Published online in Wiley Online Library (wileyonlinelibrary.com). V C 2015 Society of Plastics Engineers POLYMER ENGINEERING AND SCIENCE—2016