Fluidized Bed Chromium Catalyst Activation for Exotherm Control
Find Innovative SolutionsGenerate Solutions
Solution Overview
Problem
Commercial activation of supported chromium catalysts results in low conversion to Cr(VI) and inefficient production of high melt index polymers with high film gel content, accompanied by significant exotherms that can damage the catalyst and activation vessel.
Innovation Solution
A fluidized bed process involving controlled cycles of oxidizing and inert gases at specific temperature ranges and durations to activate chromium pre-catalysts, minimizing exotherms and emissions, producing catalysts with high Cr(VI) content and improved melt index potential.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If supported chromium pre-catalysts are exposed to oxygen-containing atmosphere at elevated temperatures for activation, then chromium is converted to hexavalent state (Cr(VI)), but the conversion efficiency is low and significant exotherms occur that can damage the catalyst and activation vessel
Solution Approach 1:
The patent applies periodic action by cycling between oxidizing gas (oxygen-containing) and inert gas atmospheres during catalyst activation. The process involves repeated cycles where the pre-catalyst is exposed to oxidizing conditions to convert Cr(III) to Cr(VI), then switched to inert gas to control exotherms and prevent catalyst damage. This periodic switching enables efficient conversion while maintaining catalyst integrity and avoiding runaway exothermic reactions.
Solution Approach 2:
The patent utilizes inert atmosphere (nitrogen or other inert gases) to control the activation process. By switching to inert gas during and between oxidation cycles, the process prevents uncontrolled exotherms, protects the catalyst from overheating and damage, and allows safe handling of the highly reactive Cr(VI) species formed during activation. The inert atmosphere serves as a thermal and chemical buffer.
2Productivity
If traditional activation methods are used, then chromium catalysts are activated, but the resulting catalyst cannot efficiently produce high melt index polymers with low film gel content
Solution Approach 1:
The patent applies parameter changes by optimizing multiple activation conditions: temperature ranges (200-400°C for oxidation cycles, up to 600°C for final calcination), oxygen concentration (2-21% O2 in nitrogen), cycle durations (1-60 minutes per cycle), and number of cycles (5-50 cycles). These parameter optimizations produce Cr(VI) catalysts with specific properties that enable efficient production of high melt index polymers with low film gel content, achieving both high productivity and precise quality control.
3Quantity of substance
If large quantities of supported chromium catalysts are activated, then production scale is increased, but conversion to Cr(VI) becomes relatively low
Solution Approach 1:
The periodic cycling between oxidizing and inert gases enables efficient activation of large quantities of catalyst. The repeated oxidation-inert cycles ensure uniform conversion throughout the catalyst bed, preventing localized overheating and maintaining consistent Cr(VI) formation across large batches. This approach scales effectively while maintaining high conversion rates.
Solution Approach 2:
The process incorporates feedback control by monitoring temperature during activation cycles and adjusting the oxidizing/inert gas cycling accordingly. Temperature sensors detect exotherm development, and the control system modulates the oxidation cycles to maintain optimal conversion rates while preventing runaway reactions. This feedback mechanism ensures consistent high conversion rates regardless of catalyst quantity.
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 yields activated chromium catalysts capable of producing ethylene-based polymers with low film gel levels, high melt index, and broad molecular weight distribution, enhancing extrusion processability without significant exothermic events or emissions.
Implementation Method 1
exposing a pre-catalyst in a fluidized bed vessel to feed cycles of a second oxidizing gas and a second inert gas to convert at least a portion of lower valence chromium to an oxidation state of +6 (hexavalent chromium)
Implementation Method 2
heating the pre-catalyst, while introducing a third inert gas or a third oxidizing gas into the fluidized bed vessel, to a temperature T4 in a range from 1000° F. to 1600° F.
Implementation Method 3
heating the pre-catalyst... to a temperature T4... and holding the pre-catalyst at T4
Data Source
AI summary
Processes for producing activated chromium catalysts such as chromium/silica catalysts and titanated chromium/silica catalysts are disclosed, and these processes utilize a multistep process involving exposure to inert and oxidizing atmospheres at specific temperature conditions. The resulting activated chromium catalysts have unexpectedly high melt index potential and can produce ethylene-based polymers with lower gel counts in addition to higher melt indices. Related activation systems are provided in which the fluidizing gas entering the fluidized bed vessel can be adjusted between an inert gas, an oxygen-containing gas, or a mixture of the inert gas and the oxygen-containing gas to minimize or prevent exotherms.