Bimodal Chromium Catalyst for High-Density Slurry Loop Polymerization
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Solution Overview
Problem
Existing processes for producing ethylene polymers using supported chromium catalysts result in high amounts of fine particles, leading to issues like dust explosions and reactor fouling due to electrostatic charging, which complicates processing and increases operational risks.
Innovation Solution
A process involving a supported chromium catalyst with a bimodal particle size distribution, comprising two main fractions with d50 values of 15-40 μm and 45-80 μm, and a total d10 value below 20 μm, is used to increase polymer powder density while maintaining low fine content, achieved through specific preparation steps including the use of spray dried silica supports and calcination under oxidative conditions.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Quantity of substance
If smaller catalyst particles are used to produce smaller polymer particles with higher bulk density, then powder density increases, but fine particle content increases above 1.5% causing dust explosions and reactor fouling
Solution Approach 1:
The invention changes the particle size distribution parameters of the catalyst from a narrow distribution to a bimodal distribution with specific d50 values (15-40 μm and 45-80 μm) and d10 value (<20 μm). This parameter change allows the system to achieve high powder density (≥500 g/L) while maintaining fine particle content below 1.5% wt, resolving the contradiction between density and fine particle generation
Solution Approach 2:
The catalyst is designed as a composite system with bimodal particle size distribution, combining two distinct particle size fractions. This composite structure enables the polymerization process to produce polymer particles with optimized density characteristics while controlling fine particle formation through the synergistic effect of different catalyst particle sizes
2Quantity of substance
If smaller catalyst particles are used to increase polymer powder density, then bulk density improves, but electrostatic charging increases causing wall sheeting and reactor fouling
Solution Approach 1:
The invention optimizes the catalyst particle size distribution parameters to achieve a bimodal distribution where the d50 values are specifically controlled at 15-40 μm and 45-80 μm with d10 <20 μm. This parameter optimization produces polymer with high bulk density while reducing electrostatic charging effects, thereby preventing wall sheeting and reactor fouling
Solution Approach 2:
The invention converts the potential harm of fine particle formation into a benefit by carefully controlling the lower size limit (d10 <20 μm) of the bimodal distribution. This controlled fine fraction contributes to high bulk density while the presence of larger particles (d50 = 45-80 μm) reduces overall electrostatic charging, transforming the potential problem into a solution
3Productivity
If continuous discharge system is used to increase average proportion of solids in reactor, then productivity increases, but fine particle generation increases leading to processing problems
Solution Approach 1:
The invention changes the catalyst particle size distribution parameters to a bimodal pattern with d50 values of 15-40 μm and 45-80 μm and d10 <20 μm. This parameter change enables the continuous discharge system to maintain high reactor solid content (improved productivity) while producing polymer with controlled fine particle content (<1.5% wt), thus improving both productivity and manufacturing precision
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
The process successfully increases polymer powder density to 500 g/L or higher with a low fine particle content below 1.5 wt%, reducing the risk of dust explosions and reactor fouling, and maintaining high reactor density and ethylene throughput.
Implementation Method 1
polymerization in the presence of supported chromium compounds, known as Phillips catalysts
Implementation Method 2
the polymerization mix is pumped continuously through the cyclic reactor tube. Such circulation of the polymerization mix helps to approach an optimum homogeneization of the reaction mix, it improves the catalyst distribution and also prevents sedimentation of the suspended polymer
Implementation Method 3
a support material is treated in suspension with a chromium salt solution and subsequently, after removing the solvent, is calcinated in an oxygen-containing atmosphere at temperatures above 300° C.
Implementation Method 4
calcinated in an oxygen-containing atmosphere at temperatures above 300° C.
Implementation Method 5
it is well known that smaller catalyst particles produce smaller polymer particles with higher bulk density
Implementation Method 6
As smaller polymer particles are, as more they tend to electrostatic charging. Such charged particles increase the risk of operability problems during polymerization by wall sheeting and reactor fouling
Data Source
AI summary
The instant invention pertains to a process for preparing an ethylene homo- or copolymer in the presence of a supported chromium catalyst by slurry loop polymerization or copolymerization, whereby the resulting polymer powder has an increased powder density, in which the supported chromium catalyst has a chromium content of from 0.01 to 5 wt.-%, based on the element in the finished catalyst, and shows a particle size distribution measured according to ISO 13320-2009 comprising two main fractions one of which having a d50 of from 15 to 40 μm and the other having a d50 of from 45 to 80 μm said catalyst being further characterized by the fact that less than 10% wt of its catalyst particles has diameter lower than 20 μm.