A catalyst used in propylene polymerization is usually of high activity, and therefore should be added into a polymerization reactor in a stable, continuous, and tiny flow. Hence, it further requires that a catalyst suspension should be dispersed as homogeneously as it can be. Otherwise, locally rapid polymerization reaction, incompatibility between heat release and heat removal during reaction, and agglomeration on the reactor would occur, thus affecting stability of production. Therefore, one of the key technologies in a propylene polymerization process is to allow the catalyst to be sufficiently dispersed so as to feed it into the reactor in a stable, continuous, and tiny flow.
CN 1927440A provides a method of feeding a slurry of catalyst in a stable and homogeneous flow at an accurate quantity, comprising the step of feeding the slurry through a catalyst tank, a six-way valve, a quantitative tube, and a circulatory system of nitrogen in a feeding device. The method has the benefit of stable feeding, with decreased amount of sediment and reduced variation of concentration during transportation. The major problem of the method lies in that static aggregation of solid particles during preparation of the slurry of catalyst would result in the phenomenon of inhomogeneity of the slurry of catalyst. Moreover, when this method is used in industrial production, revamping of the process apparatus is largely required.
CN 102218277A provides a method for preparing a slurry with fragile particles and a viscous liquid. In this method, the phenomenon of particle breakage during dispersion and preparation of the slurry is eliminated. However, due to a density difference and other property differences between solid particles and a liquid, the slurry is insufficiently stable, and the method fails to provide any special technology that can solve the problem of inhomogeneous dispersion caused by particle aggregation.
Currently, three methods are used for dispersing a catalyst in industrial propylene gas phase polymerization. In the first method, a solid particle catalyst and liquid propylene are mixed on site of an apparatus (e.g., a Novolen propylene polymerization apparatus). According to the feeding mode of this method, a solid catalyst bucket is connected to a catalyst storage tank of a polypropylene production apparatus. The solid catalyst is fed into the catalyst storage tank through a pipe and a valve under the function of gravity. The catalyst storage tank is then added with liquid propylene to prepare a catalyst slurry of a certain concentration, which is fed into a polymerization reactor in a controlled flow. This method is beneficial in that the raw material propylene is used as a dispersing medium of the catalyst, without introducing any other component. Nevertheless, there exist the following problems in this method. At the outset, the work of catalyst preparation is rather labor intensive, with high operative risks when the catalyst bucket is lifted by a worker. Next, liquid propylene is of low viscosity and the catalyst particles settle easily, which causes the concentration of the slurry to be inhomogeneous with large fluctuation. Consequently, the amount of the catalyst entering the reactor becomes instable, causing abnormity in the reactor. Moreover, the solid catalyst component cannot be readily dispersed in liquid phase propylene in a homogeneous manner, with the phenomenon of particle aggregation and agglomeration, causing unstable amounts of the catalyst entering the reactor. As a result, violent reaction occurs locally in the reactor, generating lump materials which affect stability of the production. In formulating the solid catalyst component and liquid propylene, in order to reduce the phenomenon of agglomeration, enhanced stirring can be used. However, the catalyst particles are fragile, and would be inevitably broken under too large shearing forces, which would further affect the polymerization process. In the second method, the catalyst in solid particles and mineral oil are respectively added into a catalyst storage tank and dispersed on site of an apparatus (e.g., a Novolen propylene polymerization apparatus), and then the resulting slurry is directed into the propylene polymerization reactor. The formulation of this method is similar to that of the first method, wherein a solid catalyst bucket needs to be connected to a catalyst tank, followed by addition of mineral oil. After a catalyst slurry is formulated, it is fed into a polymerization reactor at a controlled flow and impact with propylene therein to initiate a polymerization reaction. As a benefit of this method, mineral oil with a certain viscosity is selected to formulate a catalyst slurry with good stability. However, this method is also inevitably labor intensive, with large operative risks as in the case of the first method. Furthermore, the second method has the following major problems. First, since the catalyst is required to be added into the polymerization reactor at a low concentration, a large amount of mineral oil is necessary, which increases the costs. Second, a high viscosity of the mineral oil prevents the solid catalyst particles from being readily dispersed in a homogeneous manner in a short time. When the catalyst is added into the reactor, inhomogeneous heat release and large fluctuation of temperature may occur, which affects stability of the production. In the third dispersion method, a slurry of catalyst in mineral oil from a supplier is directly used to catalyze propylene polymerization. The slurry is transported to the site of an apparatus and fed into a catalyst tank (e.g., an INNOVENE propylene polymerization apparatus) for direct use, without having to establish a separate catalyst dispersing system. Nonetheless, this method is inapplicable to some gas phase polymerization processes which require a low concentration of feeding materials, such as Novelen propylene polymerization. Furthermore, a high content of mineral oil used in this method also causes the same problems as those existing in the second method.
The industrial production practice and existing literature reports at present still reveal a need to develop a method of dispersing a solid catalyst component used in propylene polymerization more homogeneously, so as to transport the catalyst component to a polymerization reactor in a stable, continuous, and homogeneous manner, thus enabling more stable and longer operation of the reaction system and improvement of polymer properties.
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