Operations involving heat transfer with phase change present challenges to the engineer, but successful results can provide economic and environmental benefits. An integrated solvent-recovery system in the process of electrode manufacture for lithium-ion batteries (LiB) offers one such example.
During manufacture, LiB cathode active materials are dispersed with a binder in an organic solvent — most commonly N-Methyl-2-pyrrolidone (NMP). This dispersion is coated onto both sides of a current collector — typically aluminum foil — and then passed through a convection air dryer. The heating system evaporates the NMP, leaving a dried, active layer on the foil. For this process, electronic-grade NMP is required, and its raw material cost makes it a good candidate for solvent recovery. With onsite distillation, it is possible to recover NMP product at greater than 99 percent purity, and this material can be reused. Alternatively, the solvent is spent once through, and the byproduct can be sold for purification offsite.
NMP has a key characteristic that makes it a good candidate for solvent recovery in condensing coils: its boiling point. At 395°F (202°C) for pure NMP, the organic compound is classified as a high boiler. In a condensing coil, energy is removed from the process exhaust — NMP-laden air — causing the solvent to condense out of the airstream.
This article will discuss the process, coil configurations, performance and efficiency of a condensing solvent-recovery system, and why recycling should matter for your bottom line.
To produce quality lithium-ion batteries, particulate and humidity must be controlled. Without mitigating those two components, water and impurities can have adverse effects on quality such as shorter battery life.
Oftentimes, a dry room is a necessity to provide particle filtration and humidity-level management. Many facilities operate with a humidity level as low as -40°F (-40°C) dewpoint, though some units are placed in ambient settings. For the latter, HEPA filters upstream of the solvent-recovery system are common.
High moisture levels may be bad for battery production, but they can aid in solvent recovery. If the process air is recycled, why does room humidity matter? Two horizontal openings in the dryer provide entry and exit for the webbing — the substrate for the lithium coating. These are referred to as web slots.
To prevent hazardous fugitive emissions, the dryer is operated at a slight negative pressure. As a result, a small amount of room air enters through the web slots. This infiltration air carries ambient moisture, and the total infiltration air mass must be removed for mass-balance preservation.
In the integrated process, the overall balance of process drying air is key. Both the spent drying air (exhaust) and the evaporated solvent are recovered for reuse. The hot, solvent-laden air leaves the convection air dryer and enters the solvent-recovery plenum. Temperatures can range between 176 and 248°F (80 and 120°C). Energy is removed from the system with fin-and-tube heat exchangers (coils).
The gas stream typically is composed of air, water and NMP. As the temperature is reduced, NMP approaches the dewpoint. As the temperature falls below the dewpoint, substantial energy needs to be removed to overcome the latent heat of condensation. As this energy is removed, droplets of the water/NMP mixture begin to form on the heat exchange surface. The composition of these droplets can be predicted based on the vapor liquid equilibrium (VLE) of the NMP, water, air system and the overall concentration of each component. As these droplets get larger and larger, they combine and succumb to the forces of gravity. The condensate drains from the coil fins to the bottom of the plenum housing.
Depending on the gas velocity, liquid can re-entrain into the gas stream as it exits the fins of the coil. When this occurs, a demisting device becomes necessary to prevent large carryover into a reheat coil or electric heating element further downstream. The demister’s purpose is to take very small droplets, or aerosols, and force them to coalesce and drain out of the airstream.
The condensate pools in a plenum sump and drains from the housing into a tank for storage. Later, it will be refined or disposed. The treated process air then is redirected back to the dryers for reuse. In addition, a small slip-stream — to balance the ambient air introduced through the web slots previously mentioned — is further abated by the emission-control system.
Within the solvent-recovery plenum housing, two common configurations exist: a single coil or a series of coils. The single-coil design is simple and compact. It exchanges both sensible and latent heat, but it lacks flexibility. Localized heat recovery is not an option, and the plant chiller must be able to support the coil’s duty requirement. A single-coil solution is best suited to a pilot coater setting, in which dryer exhaust air may not be recirculated.
When a plenum houses a series of coils, sensible and latent heat typically are exchanged at different heat exchangers. This design requires extra piping, is not as compact, and is more expensive. The series-coils design offers better process flexibility, however. Within this configuration, the first and last coils usually are configured in a closed-loop heat-recovery circuit, thereby reducing the burden on the process chiller and overall utility costs. (The recycled airstream is reheated before circulating back to the dryer.) A series-coils approach is suitable for a full-scale production facility or a pilot coater in which the dryer exhaust is recycled.
There are times, however, when a coil in a single or series configuration proves to be too large. In such cases, the application may be better served with two coils operating in parallel. Doing so can potentially increase the efficiency of the coil by providing better heat transfer fluid distribution from the supply headers to the tubes.
Performance and Efficiency
Successes in condensation-type solvent-recovery systems are tied to heat transfer, so it is important to audit the system’s energy balance periodically to validate its effectiveness. Even when a system is operating at design conditions, downstream process samples may report higher concentrations than those predicted by vapor-liquid equilibrium (VLE) models. Causes of such deviations are many, but there are a few things to consider.
Improper coil design can lead to deviations:
- Were the coils sized appropriately for the application?
- Are the coil tubes too long, and the heat transfer fluid flow too slow?
- Are the heat transfer circuits configured correctly?
Contaminants also can contribute to deviations:
- Are there components outside of water and NMP inside the process air? Acids may be present and can corrode coil materials over time, inhibiting heat transfer and reducing efficiencies.
- Contaminants and products of corrosion can reduce the quality of the recovered solvent, thereby lowering its value.
Clogged coils can lead to deviations:
- Is the process air particulate free? Coils should only be used in clean process streams. Particulate can, and will, impede performance as airflow becomes obstructed.
- Are the control setpoints appropriate for your process, or is there a deviation? In low temperature applications in which water is present in the gas stream, the condensate can freeze and obstruct airflow.
Finally, keep in mind that transient conditions can lead to a premature diagnosis of deviations. Consider:
- The heat transfer fluid control loop may not have stabilized, causing oscillations in fin-side air temperature. Remember, VLE is temperature dependent, and a swing in either direction will affect the solvent concentration.
- Record and react only when the system has achieved a true steady state.
Reduce Costs by Recycling Process Air
In processes like convection drying, venting the process gas to atmosphere is not only costly but also unnecessary. By treating the exhaust gas with a solvent-recovery system, the airstream can be recirculated. In doing so, it is possible to recover approximately 50 to 70 percent of the energy that would otherwise be wasted. For example, if the exhaust air from the dryer is 212°F (100°C), 90 to 95 percent of this air can be returned to the dryer between 158 and 172°F (70 and 78°C). Of course, heating the dryer feed air with the exhaust is an option, but the plant’s ambient requirements must not be overlooked.
It may seem obvious, but all lost air from within the plant must be replaced. If the process is not located in a dry room, filtration can occur within the process stream itself, but the air is likely treated through the plant’s HVAC. The costs associated with a complete process stream evacuation and retreatment can add up fast as that air is replaced. Alternatively, more than 90 percent of the treated process air can be recycled back to the dryer.
As previously discussed, all infiltration air (less than 10 percent of the process flow) must be exhausted to atmosphere when the airstream is recycled. Even though the dryer exhaust has already been treated by the solvent-recovery system, it must be further abated prior to venting. The capital and operating expenditures associated with the required emission-control components are lower with a smaller flow. Air emission permitting also benefits from the reduced exhaust airflow because the VOC mass flow rate is reduced.
It should be noted that other technologies such as scrubbers can remove solvent from a process stream. In the case of LiB battery production, however, the collected liquid has a much lower NMP purity (70 to 90 percent). In a condensate-recovery system, the collected liquid typically is 95 to 98 percent pure NMP. This high concentration makes further purification less expensive and, therefore, the collected solvent has a higher value.
In conclusion, battery production is about making quality batteries — not solvent recovery. When engineered appropriately, facilities can minimize operating costs and capitalize on their investment with a solvent-recovery system.
This article focused on NMP recovery from lithium-ion batteries production. However, there are many processes in which solvents are evaporated. If the chemical properties and process align well with condensation, a solvent-recovery system can leverage VLE to reduce the impact of these unavoidable production costs.
The system must be designed carefully to maximize performance and avoid issues such as fouling or liquid re-entrainment, however. It also may be desirable to test the commercial conditions in a pilot system to alleviate any doubts about success for the application. Under the right circumstances — and if executed correctly — the system will pay for itself in a short time and help the plant achieve production efficiency.