On-Line Calorimetry in the Ethylene Coordination Polymerization
- PDF / 231,766 Bytes
- 6 Pages / 432 x 648 pts Page_size
- 48 Downloads / 187 Views
On-Line Calorimetry in the Ethylene Coordination Polymerization José R. Infante-Martínez, Enrique Saldívar-Guerra, Odilia Pérez-Camacho, Víctor ComparánPadilla, Maricela García-Zamora Centro de Investigación en Química Aplicada, Blvd. Enrique Reyna #140, Saltillo, Coahuila, 25294 México ABSTRACT The kinetic performance of metallocene type catalysts as well as their instantaneous activity is determined on line by two independent methods in the semi-batch polymerization of ethylene via metallocenes. On the basis of first-principles, both methods are described and guidelines for their implementation at a laboratory scale reactor are offered. Polymerization tests were conducted with two heterogenized metallocene catalysts showing that the direct method (based on ethylene flow measurement) and also the calorimetric method (based on energy balances) reported equivalent high quality information. The calorimetric method here developed can be readily used by the chemical practitioner as the notions and tools required for its implantation are easily grasped. It is noted that the calorimetric method has the advantage of requiring a low cost instrumentation (only thermocouples) whereas the direct method needs a relatively more sophisticated equipment (mass flow meter). INTRODUCTION The activity of a Ziegler-Natta type catalyst, defined as the polyethylene weight produced per mol of catalyst and per unit time, is an important technological variable that expresses the performance of a catalyst in the operation of an industrial plant of polyethylene. Frequently, mainly in industrial practice, mean activity values are used when comparing different catalysts, however this information is not enough, because one same mean activity value can result from widely different kinetic behavior. For instance, a multi-site catalyst could have the same mean activity as a single site catalyst [1]. Instantaneous activity values are a better choice to characterize a given catalyst. These are determined experimentally in low scale polymerization tests that report the instantaneous polymerization rate (kinetic curve). The evolution of the polymerization rate, rp, can be considered the fingerprint of the catalytic system. Additionally to its value as a catalyst characterization means, a kinetic curve can guide the modelling studies, required for a deep understanding of the catalytic medium [2,3]. Usually, the metallocene (or Ziegler-Natta) catalyst behavior study is performed in a semibatch stirred tank reactor provided with a heat transfer jacket, operating at the 0.2 L to 2 L scale of operation. The pressure is maintained constant by means of a pressure regulator. In this way, the addition of ethylene is continuous and governed by the reaction requirements. The measurement of this flow directly provides the instantaneous polymerization rate. The reactor temperature (or jacket temperature) is automatically controlled. In the simplest case, the jacket temperature is maintained constant by means of a controlled temperature circulator connected to the j
Data Loading...
