Oct-2024

Separation technology addresses challenges in emerging technologies

How a proven technology has been repurposed to offer a versatile solution for separating biochar from hydrocarbon vapours and ash from flue gases.

Kusume Srinivasa Rao, Ramkumar Ramanathan and Todd Foshee
Shell Catalysts & Technologies

Article Summary

As the energy transition accelerates, innovative processes based on pyrolysis, hydropyrolysis, and gasification, among others, are being developed to process lower-carbon and circular feeds, such as biomass and plastic waste. A key challenge in these processes is the effective removal of by-products like biochar from hydrocarbon vapours and ash from gasification flue gases. Addressing these challenges is crucial for improving the efficiency of these emerging technologies.

To tackle these challenges, Shell Catalysts & Technologies explored applying the Shell third-stage separator (TSS) beyond its traditional role in refining. For more than 60 years, it has been used to reduce particulate emissions to below 50 mg/Nm³ from fluid catalytic cracking (FCC) unit operations and protect downstream equipment from potential damage. Its research suggests that the Shell TSS offers significant advantages over conventional technologies, like cyclones and candle filters, when used in pyrolysis, hydropyrolysis, and gasification. These advantages include lower costs and, because it has no moving parts, increased reliability and lower maintenance requirements.

The potential of the Shell TSS lies in its adaptability from its original function in FCC to these newer applications. In pyrolysis and hydropyrolysis, it can efficiently separate biochar from reactor vapours, while in gasification, it can remove ash from flue gases. This use of proven technology demonstrates the Shell TSS’s ability to address contemporary challenges in renewable energy production and waste processing, offering a promising solution for modern energy processes.

Separating solids, such as biochar or ash, from gas streams
With energy companies striving to reduce the carbon footprint of the products they sell, processes like pyrolysis, hydropyrolysis, and gasification are becoming increasingly important. These processes can efficiently convert lower-​carbon and circular feedstocks, such as biomass and plastic waste, into valuable fuels and chemicals while minimising greenhouse gas (GHG) emissions and improving overall sustainability.
Pyrolysis and hydropyrolysis are both thermal decomposition processes that convert biomass or plastic waste into hydrocarbons by heating the material in the absence of oxygen. A key by-product of these processes is biochar, a low-value byproduct. Efficient separation of biochar from these vapours is crucial for producing high-quality transportation fuels, such as gasoline and gasoils.

Similarly, gasification transforms feedstocks into syngas by reacting with them a controlled amount of oxygen and steam at high temperatures. One of the challenges in this process is the removal of ash from the flue gas to prevent equipment fouling, ensure efficient operation, and meet emissions standards.

Evaluating available separation technologies
Several technologies, including single cyclones, two-stage cyclones, and candle filters – and now, the Shell TSS – are available for separating biochar from hydrocarbon vapours and ash from flue gas.

A single cyclone can effectively remove the bulk of solids for FCC catalyst particles. However, it is less effective for removing biochar and ash particles, which have a lower particle density and can break into smaller particle sizes more readily in the cyclones than FCC catalyst.

For improved separation, a two-stage cyclone system can be used. This system consists of a primary cyclone that handles the bulk of the separation, followed by a secondary cyclone to capture residual particles. Two-stage cyclones used for removing FCC catalyst particles from reactor or regenerator effluent can have a high overall efficiency with a combined d50 cutpoint of around 15 µm. The d50 cutpoint is defined as the solid particle size at which 50% of the particles get captured and separated.

However, like the single cyclone, biochar and ash particles have a lower particle density and can break into smaller particle sizes more readily in the cyclones than FCC catalyst. In addition, proper design of the two-stage cyclone in biochar and ash service is crucial, including a loop seal between the primary and secondary cyclone diplegs to prevent secondary cyclone dipleg issues that could impact system efficiency.

A candle filter is an option to replace the two-stage cyclone system, as it has the highest separation efficiency. Such a system can remove almost all the biochar from the vapours. However, it is susceptible to coking, which can lead to reliability issues and higher operational costs compared to other methods.

Another option is to install a Shell TSS downstream from a single cyclone. In pyrolysis and hydropyrolysis applications, modelling suggests that a single cyclone can separate the bulk of the biochar material. However, having the single cyclone outlet directed to a Shell TSS allows for even better separation efficiency than having a second-stage cyclone since the Shell TSS has a d50 cutpoint of 2 µm. As the Shell TSS and the primary cyclone operate independently of each other, their separation efficiencies are unaffected.

The main advantages that the primary cyclone and Shell TSS system have over other options include:
· Higher efficiency and less complexity than a two-stage cyclone system. The Shell TSS has a much lower d50 cutpoint compared to a secondary cyclone, which allows for better separation efficiency. Also, the two-stage cyclone requires that the primary dipleg be connected to the secondary dipleg due to low solids flux in the secondary. This, in turn, requires that a loop seal be installed in the primary dipleg. However, loop seals are not proven in commercial units, and their proper function relies on correct design and aeration, which makes two-stage cyclones with a loop seal more complicated than a primary cyclone with a Shell TSS.
· Lower Capex and Opex required for the downstream guard bed (solids trap) than with a two-stage cyclone system, owing to less solids carryover into the downstream equipment.
· Higher reliability than a candle filter system. The risk of swirl tubes plugging with coke is lower because the vapours pass through the free space between the vanes, as opposed to mass transfer through pores in the candle filter. Additionally, candle filters represent a more expensive alternative in terms of both Capex and Opex.

Shell’s cyclone and TSS technologies have been commercially proven over the last 70 years, achieving high performance in FCC units. Recent modelling efforts and tests using a cyclone followed by a TSS in biochar service have shown good performance.