Two-Stage Propylene Oxidation Catalysts for Acrylic Acid Purity
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Solution Overview
Problem
The production of acrylic acid via heterogeneously catalyzed partial oxidation of propylene is hindered by impurities in crude propylene, particularly cyclopropane, which isomers to propylene and forms propionic acid, making it difficult to separate from acrylic acid using thermal processes, leading to contamination and increased complexity in purification.
Innovation Solution
A two-stage process using multimetal oxide catalysts with Mo, Fe, and Bi in the first stage and Mo and V in the second stage, with controlled molar ratios of reactants and catalysts, to achieve high propylene conversion and selectivity for acrylic acid, while managing cyclopropane content to minimize its conversion to propionic acid.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If crude propylene containing cyclopropane impurity is used as feedstock, then the productivity of acrylic acid production is improved, but the purity of acrylic acid product deteriorates due to propionic acid contamination
Solution Approach 1:
The patent divides the oxidation process into two distinct reaction stages with different catalysts and operating conditions. The first stage uses a Mo-Fe-Bi catalyst at lower temperature (300-350°C) to selectively oxidize propylene to acrolein with high selectivity, while the second stage uses a Mo-V catalyst at higher temperature (350-400°C) to oxidize acrolein to acrylic acid. This segmentation allows the first stage to minimize cyclopropane isomerization to propylene (which would form propionic acid), thereby improving product purity while maintaining productivity.
2Productivity
If the reaction temperature is increased to improve propylene conversion, then the productivity increases, but the selectivity to acrylic acid deteriorates due to increased cyclopropane conversion to propionic acid
Solution Approach 1:
The patent segments the oxidation process into two temperature zones: the first stage operates at 300-350°C with a Mo-Fe-Bi catalyst to achieve high propylene conversion to acrolein while suppressing cyclopropane isomerization, and the second stage operates at 350-400°C with a Mo-V catalyst to complete the oxidation to acrylic acid. This temperature segmentation resolves the contradiction by performing high-temperature reactions only after cyclopropane has been minimized in the first stage.
Solution Approach 2:
The patent changes multiple parameters including temperature, catalyst composition, and oxygen-to-propylene molar ratio across the two stages. In the first stage, the oxygen-to-propylene ratio is maintained at 0.5-1.5 to optimize acrolein formation while suppressing side reactions. In the second stage, the ratio is adjusted to 0.3-0.8 to optimize acrylic acid formation. These parameter changes allow high conversion and selectivity to be achieved simultaneously.
3Device complexity
If a single-stage oxidation process is used to simplify the process, then the device complexity is reduced, but the manufacturing precision deteriorates due to inability to control cyclopropane conversion
Solution Approach 1:
The patent uses two separate reactor units with different catalysts and operating conditions to segment the oxidation process. The first reactor contains a Mo-Fe-Bi catalyst optimized for propylene-to-acrolein conversion, while the second reactor contains a Mo-V catalyst optimized for acrolein-to-acrylic acid conversion. This segmentation enables precise control over cyclopropane conversion and acrylic acid selectivity that cannot be achieved in a single-stage process.
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 achieves high propylene conversion (>90%) and selectivity for acrylic acid (>70%) while keeping cyclopropane conversion to propionic acid low, simplifying the separation of acrylic acid and reducing contamination, thus improving the efficiency and purity of the acrylic acid production.
Implementation Method 1
a first reaction zone, a reaction gas starting mixture 1 containing propylene and molecular oxygen as reactants... through at least a first catalyst bed, the catalysts of which have at least one multimetal oxide containing Mo, Fe and Bi as the active material
Implementation Method 2
partial gas phase oxidation of propylene to form acrylic acid... first in a first reaction stage at elevated temperature through at least a first catalyst bed
Data Source
AI summary
The invention relates to a method for the heterogeneously catalysed partial gas phase oxidation of propylene to form acrylic acid in the presence of propane as an inert diluent gas. According to said method, cyclopropane as an impurity is largely avoided in the reaction gas starting mixture and raw propylene is used as a propylene source.