Separations are the underappreciated workhorse of the refining, petrochemical and specialty chemicals industries. This covers everything from crude towers used to separate crude oil into fractions for various refining technologies, filtration and centrifugation used to recover crystallized products in specialty chemicals, and distillation used for product recovery in most products that we use every day.
In 2016, Nature published an article titled ‘Seven chemical separations to change the world’. The seven identified included:
1) hydrocarbons from crude oil
2) uranium from seawater
3) alkenes from alkanes
4) greenhouse gases from emissions
5) rare-earth metals from ores
6) benzene derivatives
7) trace contaminants from water
This list covers both improvements to existing separations (1, 3, 6) and new separation needs for a more sustainable future (2, 4, 5, 7). Let’s look at three areas where advances in separation technology will be critical to get to a sustainable, low carbon future:
· Separations to enable new chemical, biological, or electrochemical technology
· The separation IS the novel technology
· Energy efficiency—boring, but effective
Separations to enable novel reactor technology: Separations are important for advancing sustainable technology because new technologies introduce new separation challenges, such as recovery of intracellular products from microbes, often referred to as Downstream Processing (DSP), separation and purification of bio-oil fractions produced from pyrolysis of waste plastic or biomass, or gas separations from electrolytic reduction of CO2.
While these separation problems are new, the solutions will likely rely on conventional unit operations like membranes, solvents, solid sorbents, and distillation.
When developing a new technology, it is important to think about the separations from the start. The figure below that shows that by integrating these steps early in technology development, we can develop a more optimized flow scheme. It is sometimes tempting to focus exclusively during early-stage R&D on maximizing yield or conversion in the reaction. This is a mistake. While it is important to understand the conditions needed to maximize yield or conversion, this is rarely the optimal overall condition. Perhaps high conversion generates more byproducts, reducing yield and increasing downstream separation costs. Perhaps we need higher pressure or temperatures to achieve that high conversion, which may create additional downstream costs. We can avoid surprises by integrating the separation system early in technology development and evaluating reaction/separation tradeoffs.
The Separation IS The Technology
Removal of CO2 from air seems crazy, and probably is. Any ‘smart’ engineer will tell you that trying to recover a gaseous component at a 400 ppm concentration from a stagnant gas at ambient pressure is a fool’s errand, but that’s what we are trying to do. As a newbie at (pre-Honeywell) UOP we were fortunate to have a visit from Nobel Laureate George Olah, when he spoke about his ideas around a methanol economy which included CO2 recovery from the air. A few of us sat around afterward pooh-poohing the silly academic who thought this was a good idea, because there was no way this could be done economically. Well, here we are nearly 25 years later, and a number of companies are trying to do just that, with billions of $ in investment flowing into this space. I think we have no choice but to pursue this route, which in addition dramatically reducing the amount of CO2 that flows into the atmosphere in the first place. A number of creative companies are exploring ways to do this with various solid sorbents or liquid solvents, innovating in ways that didn’t seem possible decades ago.
Other interesting activities where the separation is the technology involve lithium recovery from brine or seawater for battery technology, and metal organic framework (MOFs) reaching maturity as advanced sorbents for a number of applications.
Energy Efficiency—Boring, but effective
Separations require energy input, usually using heat, pressure, or mechanical energy to drive the separation and often have a yield loss. So one way to improve sustainability of technology is to improve the separation and reduce energy demand of existing separations technology. This can include:
· Heat pumps/mechanical vapor recompression to improve the energy efficiency of distillation columns
· Dividing wall distillation columns (DWCs) to replace multiple columns and reduce energy requirements
· Membrane systems as pre/post separations to support existing distillation systems
Often, we are trading off additional capital to reduce operating cost, but also save energy and along with it reduce CO2 emissions associated with those energy sources. The growing availability of cheap, renewable electricity can open up new opportunities that may not have made sense in the past.
Some guidelines when considering separations for sustainable technology:
· Don’t forget about the separation. Integration and optimization of the separation system with the reactor systems is important to achieving commercial success.
· Don’t reinvent the wheel. Often, we can use existing technology, perhaps with some modification, to accomplish the separation we need. This is often a quicker and cheaper way to scale up.
· Don’t forget about energy efficiency. Typically low hanging fruit--may require some upfront capital with the benefits of lower operating costs and a significant reduction in CO2 emitting fuel sources.