Nov-2022

High throughput experimentation meets chlorine chemistry

A novel high throughput unit designed for the accelerated testing of HCl oxidation catalysts under industrially relevant conditions.

Enrico Lorenz, Moritz Dahlinger, Tobias Zimmermann and Jean-Claude Adelbrecht, hte GmbH
Markus Frietsch, Gasmet Technologies GmbH

Article Summary

Chlorine is one of the most abundant commodity chemicals in the world. About 85% of all pharmaceuticals and more than 50% of chemicals are derivatives from the chlorine value chain.¹ Two of the largest chlorine consumers are vinyl chloride synthesis (PVC monomer) and the production of isocyanates (methylenediphenyl/toluene diisocyanate [TDI/MDI]). However, the chlorination of organic compounds is always adversely affected by hydrochloric acid (HCl), which is formed as a byproduct. 

While HCl is essentially recycled within the PVC value chain to a major extent, isocyanate production is flooding the market with byproduct HCl, at strong growth rates. If excess HCl cannot be recycled or valorised directly via muriatic acid, it needs to be disposed of by neutralisation or deep welling, which is expensive and detrimental to the environment. Chlorine recovery is the more desirable option from an economic point of view since it enhances the overall process efficiency and reduces the dependency on fluctuations in the chlorine market price and availability. The catalytic oxidation of HCl via the Deacon process is an attractive way to restore chlorine since it is much more energy-saving than the state-of-the-art electrochemical processes.²

Since the current solutions for the Deacon processes involve active but expensive Ru catalysts, the search for cheaper alternatives based on Cu or Cr is of commercial interest.³-⁵ However, even if promising catalyst candidates are developed, suitable laboratory test protocols are strongly limited in the parameter space, scale, or runtime due to the corrosiveness of the chemistry. We have developed a high throughput lab-scale technology that is able to test 16 catalysts in parallel under industrially relevant conditions to address the challenges of chlorine chemistry.

Experimental
Unit design and operation
Standard laboratory equipment is usually assembled with stainless steel components. Therefore, it suffers from severe corrosion by contact with strong acidic components like HCl or chlorine, especially in a wet gas environment. Although advanced alloys with a high Ni content significantly reduce corrosion, they do not avoid it completely. A few polymer-based materials like PTFE or glass are chemically resistant but prone to breakage when operating under high temperature or pressure. The contrast of chemical versus mechanical resistance is one of the core issues that must be overcome for the high throughput testing of chlorine chemistry, especially when approaching industrially relevant testing protocols.

Most of the state-of-the-art testing equipment operates single stage. There are only a few high throughput approaches for catalyst deactivation research. However, they work without online analytics. Furthermore, experimental programmes are often limited to mild conditions (high O₂ or N₂ dilution) and low catalyst mass or runtime.⁶ We have designed a tailored 16-fold high throughput unit to accelerate the assessment of activity, selectivity, and decay for catalysts in the field of chlorine chemistry at industrially relevant conditions. It can process corrosive gases at a high hydrogen chloride intake and extended runtime that also includes off-gas treatment on a kg scale. Figure 1 shows a simplified scheme of the 16-fold high throughput unit.

HCl, pure oxygen, internal standard, and nitrogen are dosed by Coriolis and mass flow meters and distributed at equal flow into 16 channels by a controlled pressure drop. The entire upstream section is made of stainless steel parts, and the feed gas was dried to work absolutely water free. Up to 16 catalysts can be screened in parallel, operating at reaction temperatures up to 410°C and pressures of 4 barg. Reactors are made of quartz glass with a maximum catalyst volume of 1 ml (operation at +/-2K temperature deviation).
Depending on the material type, up to 2g of Deacon catalyst can be loaded per position. Online temperature profiles can be measured using a movable thermocouple placed inside a quartz thermowell. The reactor pressure is controlled by a customised membrane pressure controller. The reactor effluent can be diluted with an inert gas to prevent condensation. Subsequently, one of the channels can be selected for analysis.

The composition of the product stream is analysed by an online-FTIR developed by Gasmet Technologies. This device allows the measurement of water and HCl in a nitrogen matrix by means of robust detection even at volume per cent concentration levels above 50% HCl at a maximum sampling frequency of 1 data point/second.

This analytical method provides an efficient alternative to conventional gravimetric titration, where a poor performance differentiation between several catalysts is obtained. The entire downstream section was designed by a modular combination of polymer, alloys, and coated materials, combined in a way that temperature and elevated pressure can be run, even in a wet gas environment.

To comply with environmental standards, HCl and chlorine need to be fully neutralised. This is achieved in a multi-stage scrubbing unit equipped with temperature and pH control. Two CSTR trains can be run in alternating operation, each capable of removing several kilograms of chlorine, to reach a runtime of up to several months, depending on the amount of HCl fed to the unit.

From a technical point of view, experimental protocols for the Deacon reaction require, on the one hand, the right equipment; on the other hand, a lot of operational experience is also necessary to prevent severe corrosion and improve overall unit availability. The most critical corrosion issues are briefly investigated, as follows.

When processing iron or iron-rich alloys (Fe >5% w/w), the feed needs to be dried carefully since water impurities above the ppb range can form liquid clusters, resulting in severe liquid phase corrosion. This especially concerns the tube wall, where the gas velocity is zero and local cold spots may occur. Heat loss during wall contact and water condensation cannot essentially be excluded. Once a liquid phase is formed, either oxygen or HCl can diffuse inside and accelerate the redox reaction to form iron hydroxide and thereby cause serious damage to the equipment.

However, even when alloys are operated dry, iron-containing alloys can be chlorinated or oxychlorinated when operating them at a temperature range above 160°C to 204°C, according to equations (2) and (3).⁷ HCl reacts in a gas phase reaction to form a metal halide and hydrogen. Furthermore, some metal halides are volatile, especially FeCl3, which sublimates at 120°C, contaminating the downstream unit equipment:

Fe + 2HCl   «   FeClâ‚‚ + Hâ‚‚                                    (2)        

FeClâ‚‚ + HCl  «  FeCl₃ + 0.5 Hâ‚‚                             (3)

Various alloy metals (such as chromium) are prone to form volatile salts that can move through the unit, depending on the surrounding conditions. Once the metal halide deposits pollute the downstream section of the unit, the major issue is their strong hygroscopic nature (deliquescence). The formation of water as a product during the Deacon reaction cannot be circumvented. If water is flowing through those substances, it will be strongly absorbed until a liquid phase is formed that consecutively acts as a corrosion hot spot. The affected part of the unit must be cleaned or replaced to remove hygroscopic deposits.

Plate design and experimental conditions
The Deacon reaction was performed using three standard catalyst systems known by the art, and it is shown in the scheme of the reactor packing design in Figure 2. Each single reactor packing consists of an inlet and outlet zone filled with corundum and a catalyst zone, which was placed in between, separated by glass wool layers. For a more comprehensive description, the reactors are divided into triplets. Different residence times for each catalyst have been realised by filling different catalyst amounts (triplet 1, 2, 3), including one reactor per material equipped with a quartz thermowell to record online temperature profiles.