The pathway to process scale-up

Translating chemical processes from laboratory to industrial scales is a critical aspect of chemical engineering. At the journal, we view scale-up as a complex path whose significance depends not only on the demonstrated scale, but also the underlying science.

Chemical process scale-up is a cornerstone of chemical engineering that serves as a bridge between laboratory-scale discoveries and industrial-scale production. Linking these scales is critical to ensuring that innovative processes and materials can be implemented efficiently and responsibly at scales meaningful to society. Today, given the many rapidly approaching and societally pressing targets (such as those set forth to mitigate climate change), developments in scale-up science and advanced demonstrations of these strategies are urgently needed at a much faster pace.

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The simplest form of process scale-up is the regime in which the relation between process and product scale is linear: that is, if you require a larger amount of product, you simply build a larger reactor (all else being equal). While this may hold true over a select range of sizes, more often, scaling introduces non-ideal behavior (nonlinearities) that necessitates tailored approaches. The size range at which there is a departure from the ideal linear case depends on the system: there is no one-size-fits-all strategy for scale-up science. What works in a laboratory setting may not be cost effective at much larger scales. Heat transfer and/or mixing dynamics might change with scale, potentially leading to inefficiencies or safety issues that were not present otherwise. By-products such as waste generation are amplified at larger scales, and environmental, regulatory and quality constraints may play a decisive role in process design.

The science of scale-up encompasses the strategies used to navigate the differences between often controlled or simplified laboratory testing conditions and the complexities of industrial environments. Our May issue featured two Articles that propose separate but complementary strategies necessary to expeditiously link these scales: (pre-)pilot-scale studies and digital twins. In the former, Jinhee Lee and co-workers demonstrate a pre-pilot-scale immobilized enzyme reactor fed live off-gas from a steel mill to produce high-purity formate powder (Nat. Chem. Eng. 1, 354–364; 2024). In the latter Article, Eric Lees and co-workers employ kinetic continuum modeling to develop an effective digital twin of a CO₂ electrolyzer that can be used across scales (Nat. Chem. Eng. 1, 340–353; 2024).

In this issue of Nature Chemical Engineering, our cover Article by Bradie Crandall and co-workers showcases a pre-pilot-scale two-step tandem electrolyzer that converts CO to multi-carbon products, acetate and ethylene, at a kilowatt scale. The authors built on smaller-scale studies to design and operate a 1,000-cm² CO electrolyzer at 0.71 kW and a 500-cm² CO₂ electrolyzer at 0.40 kW. The kilowatt-scale CO electrolyzer stack operated stably at a current of 300 A over 125 h, resulting in 98 liters of 1.2 M acetate at a 96% purity.

As highlighted in an accompanying News & Views article by Huiyue Liu and co-workers, this tandem electrolysis demonstration marks an important step towards industrial implementation. Beyond demonstrating stable performance of their tandem electrolyzers, Crandall and co-workers quantified the impact of common feed gas impurities, including CO₂, O₂, SO₂ and NO, on a CO electrolyzer stack, revealing the need to consider upstream feed purification or catalysts that are tolerant to such contaminants if the feed is to be directly industrial flue gas. The authors also conducted a detailed techno-economic analysis to identify key target areas for improvement and present a pathway to achieving economic viability at scale.

At Nature Chemical Engineering, we are very interested in all aspects of scale-up science. Our view of scale-up science includes studies on systematic process analysis (such as economic and environmental impact); digital twin development; design and optimization through modularity, retrofitting, integration and/or intensification; pre-pilot- and pilot-scale demonstrations; use of realistic feedstocks; setting key performance indicators across scales; and risk assessment and mitigation.

At the journal, we view scale-up as a path function. The final demonstrated scale is important, but to us, the salient engineering science lies in the path traveled and the thinking used to overcome relevant nonlinearities. Raising awareness of the fundamental questions underlying scale-up is crucial in guiding the intended process design — a mindset that is most effectively applied early and often throughout the design process. This proactive approach brings promise to practice, improving the likelihood that innovative laboratory-scale processes translate effectively into impactful industrial operations within a relevant timeframe.