Mar-2021

Accelerated catalyst screening and scale-up for aromatics alkylation

Advanced high throughput experimentation workflow enabled the development of new catalyst materials for improved stability and regenerability.

BENJAMIN MUTZ, PETER KOLB and ALEXANDER HIGELIN, hte GmbH
DOUGLAS LENZ, SABIC

Article Summary

Developing heterogeneous catalysts for petrochemical processes continues to be of major interest in industrial R&D. The main objectives in this case are higher stability and longer lifetime of the catalysts for extended and more efficient production cycles in commercial operation. We are describing an advanced high throughput approach for the accelerated development of novel catalysts for the alkylation of aromatics toward heavy products. The multi-stage project comprised screening of 150 novel materials within just 15 weeks, scale-up and testing of the most promising materials in the shape of industrial-sized pellets, as well as quality control for commercial operation. SABIC provided all catalytic materials as well as process knowledge, whereas hte performed the screening experiments based on 20 years of experience in the application of high throughput workflows.

The alkylation of aromatics is an essential process in all petrochemical refineries and can uplift margins significantly by producing high-value intermediates and products. Without going into too much detail, the catalytic process involves heterogeneously catalysed gas phase alkylation of aromatic compounds at 400-500°C. Heterogeneous catalysts generally deactivate due to a wide variety of chemical, thermal, or mechanical causes during operations. In this specific field of aromatic alkylation, coke formation, thermal degradation, and attrition of the material have the greatest impact. Coke formation can be mitigated by removing carbonaceous deposits through regeneration treatment in air, but this comes with the significant drawback of hydrothermal degradation due to hot-spot formation and the presence of CO2 and steam. Therefore, in the search for novel catalysts, one has to take into account the desire for greater activity and lower decay just as much as improved chemical and physical stability under regeneration conditions.

Initially, SABIC prepared a selection of materials by varying synthesis variables under a very broad parameter space. hte identified the most promising materials by comparing them to commercial benchmark catalysts under reference conditions in a parallel high throughput screening system. Selected materials were pelletised by hte according to the commercial process to develop realistic commercial shapes. The same high throughput screening systems were then used for final performance testing of these full-sized catalyst tablets in parallel single pellet string reactors.

Set-up
A 16-reactor fixed-bed high throughput testing system was specifically laid out for this type of reaction and used for this project (see Figure 1).1 hte’s testing systems are highly flexible and typically allow the dosing of a wide variety of gas feeds via individual mass flow controllers for different catalyst treatments such as activation, reduction, conditioning, or regeneration. A high-precision syringe pump transfers the liquid feed from a reservoir to an evaporator unit, where it is mixed with the gas feed, distributed equally over 16 positions, and steadily fed to the reactors at high temperature. Independent reactor heating blocks (4x4) allow screening at different temperatures at the same time. The stainless steel reactors are packed with the catalyst material as well as purified silicon carbide above and below the catalyst bed to ensure smooth plug flow. Reactors are available in a broad range of inner diameters for different particle sizes and can fit an inner thermocouple to record the axial temperature profile of the catalyst bed. Condensers collect the liquid products at room temperature and periodically release the samples using a custom-made automated sampling unit. All channels are operated under identical pressure conditions, which are regulated by feeding downstream nitrogen behind the condensers. An incinerator unit treats all effluent gas to mitigate toxic emissions.

Gaseous products are fully analysed within 15 minutes in a multi-detector online gas chromatography (GC) set-up specifically tuned by hte for this application. Liquid products are analysed using a separate offline GC. hte process control software allows fully automated and reliable continuous operation and monitoring of the high throughput system. Data evaluation was supported by the proprietary myhte software solution, which was specially developed for the treatment of large amounts of data produced by a high throughput set-up.¹

Material screening
Before screening different materials, the test system is always set up and validated to ensure equal feed distribution and exact temperature and pressure settings. The same amount of a reference material is then tested in each reactor to verify the reproducibility of the results.  

Figure 2 illustrates a typical reactor packing design for a material screening in a 16-fold high throughput system. Different materials are loaded into the isothermal zone of the reactor heaters either with constant mass or constant volume. Usually, one reactor is left as a blank position to determine the feed composition and to measure blind activity. Variations in the space velocity can easily be realised by applying different catalyst masses and bed lengths. Examples of designs and possibilities that can be realised in only one reactor set have been described previously.^2,3 It is recommended to always include the incumbent reference in all catalyst performance screening runs in order to determine reproducibility and to support statistical analysis of significant differences in performance.

The mass recovery is mostly determined based on the liquid product sampling during the screening (see Figure 3a). Position 5 in the illustrated example unfortunately lost a great share of the aromatic compounds due to ring opening and decomposition reactions. Nonetheless, it was possible to fully close the mass balance by taking into account the gas phase from online GC analytics (see Figure 3b).

The catalytic materials were screened using a rigorous standard operating procedure (SOP) to ensure constant and reproducible process conditions for start-up, activation, steady state, and catalyst regeneration. After activation, the performance was observed for 125 h under constant reaction conditions. Plotting the data by selectivity versus conversion (see Figure 4) allowed accumulations of data points to be classified into groups and the subsequent rationalisation of the different performance behaviour. The conversion of Material Group A is very limited and produces the desired target product at only moderate selectivity. The selectivity toward the target product slightly increases over time at constant conversion. For Material Group B, both selectivity and conversion increase over time, indicating the formation of the active phase under process conditions. The materials in group C show a run-in behaviour that comprises declining conversion but increasing selectivity. The catalysts in Group D are very active, reaching good conversion, but with only moderate selectivity toward the target product. The most promising catalysts were identified in Group E. These materials achieved the highest conversion and selectivity.

In summary, around 150 materials were screened and benchmarked in just 15 weeks, resulting in time saving by a factor of 10 compared to a conventional single-reactor system. While hte delivered performance descriptors with high accuracy and precision for all catalysts, SABIC measured and controlled all catalyst synthesis descriptors based on their confidential expertise in the synthesis of novel catalysts. This is the classic approach at hte to organise firewalls in a competitive, confidential catalyst business: only the customer is able to identify and optimise structure-performance relations. High throughput experimentation is the prerequisite for enabling the complex design of experiments in a broad parameter space of synthesis and performance test variables. Based on a high population of experiments and systematic filing of all available catalyst characterisation and performance data within one accessible relational data base, the application of advanced statistical data evaluation procedures becomes possible. Based on these capabilities, high throughput experimentation is able to identify even small incremental improvements of catalyst performance which will have a big impact on commercial performance and economic results.