Nov-2022

Electric process heaters help decarbonise petrochemical refining

Petrochemical processes have traditionally been heated with fossil fuels for a number of reasons, but the pressure is building to mitigate carbon dioxide emissions and advance long-term decarbonization goals.

Michael Jones
WATLOW

Article Summary

Engineers are exploring the potential of electric process heaters to enable all-electric processes, but they often approach the idea with many questions… and maybe a misconception or two.

That’s to be expected. Fossil fuel heated processes leave big shoes to fill when it comes to thermal processes, so electric process heaters often raise two big questions:
How big can an electric heater be?

What is required to maintain proper control of a large electric heater?
The technology behind process heaters has changed dramatically in the last 10 years, and so the answers to these questions have changed in that time. This is good news for the industry, as electric heaters must be able to provide the same–or improved–performance that petrochemical engineers have come to expect from fossil fuel powered heaters, if we are to make progress towards true decarbonization.

Can Electric Process Heaters Keep Pace With Fossil Fuel-Burning Heaters? The Size Question
To even consider replacing fossil fuel-burning heaters, we need to have a clear understanding of the current capabilities of electric process heaters. For example, replacement does not make sense to begin with if electric heaters do not come with the needed size and power required to heat processes that currently depend on fossil fuels. Many of those processes would require larger electric heaters well above the well-known one megawatt (MW) variety; for example:
- Feed/Product Exchanger, 2 MW (or larger)
- Dehydration Inlet Preheater, 3 MW (or larger)
- Heater Treater, 4 MW (or larger)
- Molecular Sieve Regen, 6 MW (or larger)
- Waste Gas Heater, 6 MW (or larger)
- Once-Thru Steam Generator, 20 MW (or larger)
- Crude Heater, 28 to 200 MW (or larger)
- Hot Oil Heater, 32 MW (or larger)
- LNG Vaporizer, 34 MW (or larger)
- Thermal Oxidizer, 54 MW (or larger)
- Hot Water Heater, 56 MW (or larger)
- FCC Heaters, 150 to 200 MW (or larger)

Fortunately, heaters do exist that can handle these power requirements. In terms of physical size, 60-inch NPS tubesheets are highly attainable. Nothing prevents even greater size; the technology exists to produce electric heat exchangers of physical size comparable to a shell and tube heat exchanger. An electric heat exchanger of the same physical size (as a shell and tube) will have a larger available heating duty, due to the constant heat flux technology.

With equipment of this size, a single vessel can have two heat exchanger bundles. Such a setup can produce a single process vessel with a 15-MW duty rating or more. The few suppliers providing electric heaters at this scale can raise or lower the duty rating as technical requirements dictate. This kind of size and power presents a viable alternative for operations currently fired by fossil fuels.
- Besides reducing the use of fossil fuels, electric heaters and heat exchangers have other well-documented advantages as well:
- Less thermal lag: Temperature is controlled through direct application of electricity.
- Safer operation: No fossil fuels to burn or combust.
- Smaller overall footprint: Constant heat flux capability results in a smaller footprint as compared to the non-constant heat flux in shell and tube heat exchangers.

What Is Required to Maintain Proper Control of a Large Electric Heater? The Control Question
Simply put, most engineers have never seen electric process heaters and heat exchangers of these sizes or capabilities. So, naturally, some of the most common questions about larger process heaters have to do with control. What additional elements are needed to ramp up the heater? How does that affect the existing electrical system? How is temperature monitored and controlled? What additional safety precautions are needed?

Just because larger electric heaters have not traditionally been used to heat all processes in the petrochemical industry does not mean that the technology is theoretical or untested. Far from it: Field-proven power switching devices have been in use for low voltage electric process heaters and electric medium voltage motors in a number of industries for years, and the ability to control voltage is well established.

PLCs and similar power management systems bring heaters online in ways that do not cause problems for other devices connected to the same power source. The heater and controller are part of one closed-loop system, which streamlines integration and yields more control over the entire system. This technology’s tried and true nature in other applications reduces the risk for petrochemical process heating.

New Technologies Driving the Energy Transition in Thermal
With these two common questions now answered, it is worth taking a closer look at some of the technologies that make electric process heaters and heat exchangers promising candidates for replacing more traditional heaters.

HELIMAX™
HELIMAX™, with its Continuous Helical Flow (CHF) Technology™, is playing a critical role in making large electric process heaters more robust and economical. With CHF, the baffles with the heater do not exist as discrete elements, but rather as a single continuous spiral winding around the interior of the shell side of the heater. This further forces the flow to be rotational and helical, resulting in an even better heat transfer coefficient per unit pressure drop compared with more traditional segmental baffles.

Heaters with Continuous Helical Flow Technology, including Watlow’s HELIMAX, do not have dead zones, or areas with insufficient flow. Because there are no disruptions to flow, there is dramatically less likelihood that hot spots can develop. Without hot spots or dead zones where material can collect and adhere, the potential for greatly reduced fouling rates can be realized. The improved heat transfer compared to legacy technologies also means that these heaters can have a much smaller footprint, thus allowing the process machinery to make a much more efficient use of space.

A single removable HELIMAX bundle can supply up to five megawattsof power duty range, even with a smaller footprint, than fuel-based heat exchangers. Combine this efficiency with the reduced need for maintenance to address coking, and you have a product that increases productivity even as it contributes to decarbonization efforts. This ultra- efficient heat exchanger also has a reputation for lasting longer and avoiding system failure because it is not as susceptible to fouling.