How not to kill the molecular sieve

FROM THE JULY ISSUE: Like all processes in an ethanol plant, careful attention is required in ethanol dehydration units to care for the molecular sieve.
By Mark J. Binns | June 13, 2017

Molecular sieves are a critical part of the commercial ethanol production process. They allow ethanol to be dehydrated past 95 percent purity to nearly 99.9 percent, which is critical to meet the specifications for use as a fuel additive. Like all processes in an ethanol plant, careful attention is required in ethanol dehydration units to care for the molecular sieve (also referred to as mole sieve beads, desiccant or zeolite). Proper preventive maintenance, care during the actual operating process and awareness of what can cause long-term damage to the beads and vessel, or bed, are crucial. Proactively monitoring and maintaining sieve beads is imperative to ensure a long, efficient lifespan of up to 10 years. 

Understanding the dehydration process and following a few simple rules can extend the lifespan of molecular sieve beads. Talk to a sieve professional—we’ve seen it all.

Don’t Wet the Bed
Great care should be taken to ensure the process stream reaches and remains in vapor phase when flowing into the vessel and during operation. If the vapor condenses back to liquid phase, it can severely affect the mass transfer dynamics of water into and out of the vessel, which leads to a reduced working capacity and possible damage to the beads. When a process stream in liquid phase enters an ethanol dehydration bed, water can form a layer around each bead, essentially coating it with liquefied water because of water’s cohesive properties. Beads coated in liquid water slow or completely prevent the adsorption of vapor phase impurities (water) from the desired, pure ethanol product stream. 

To prevent the occurrence of liquid phase molecules during any point of the dehydration process, it is critical to achieve and maintain a feed stream at maximum pressure with 50 degrees Fahrenheit of superheat—50 F over the condensation temperature. The optimum superheat of 50 F offers a temperature high enough to prevent vapor from returning to liquid phase, while remaining low enough to not significantly reduce working capacity of the sieve beads inside the vessel. Working capacity directly correlates with operating temperature, as beads in high heat have a lower working capacity, thus too much heat during operation is not desirable either. Properly insulated vessels and pipes are also important to avoid cold spots or uncontrolled changes in temperature of the feed stream, especially during cold, winter months.

Don’t Bounce the Bed
Proper velocity control inside the bed must be carefully monitored to ensure that the fluidization velocity is not exceeded, as specified by each unique system, to avoid Levi's expansion. Fluidization velocity is a function of bead size, flow rate, vapor density, vessel pressure and feed stream temperature. Levi's expansion occurs when the beads are lifted and suspended in the air on a cushion of vapor, a process known as fluidization. When fluidized, the beads can grind against one another, resulting in high attrition, dusting and possible cracking. In turn, cracked beads can lead to channeling of the mass transfer zone through the bed, causing an uneven, early breakthrough, a negative feedback loop and repeated damage each cycle. These sudden changes in pressure, caused by sticking valves or improper pressurization of the vessel, can cause instantaneous fluidization, or the literal bouncing of sieve beads inside of the vessel, sometimes referred to as popcorning.

Consult with a sieve professional for assistance to better understand the capabilities of unique ethanol dehydration units. Routinely check valves to confirm proper function, hold team meetings, encourage continuing education on proper pressurizing conditions and teach the benefits of preventative maintenance to increase efficiency and successful operation.

Don’t Exceed Critical Velocity
Avoiding critical velocity is necessary to prevent damage to dehydration units and the sieve beads inside. Each piece of equipment in a system has a limit—a maximum pressure that can be handled without causing damage. When the vapor rate for a plant's unique configuration is too high, vapor velocity can exceed critical velocity levels, which causes a sound resembling screaming or high-pitched whistling. Once critical velocity is exceeded, beads can shatter and crack, causing excess dust, an increased need for top-offs in lateral units and jeopardization of overall working capacity, and could eventually require a full change-out to restore productivity.

Pay Attention to pH
A three-angstrom (3A) molecular sieve, such as the EthaDry offered by Hengye Inc., is specifically designed to dehydrate ethanol, with crystal pore openings measuring about three angstroms in diameter. This sieve is ideal for ethanol production because water molecules measure about 2.8 angstroms, while ethanol molecules are about 3.6 angstroms. Water can pass into 3A crystals and be trapped, while ethanol molecules are too large for adsorption and bounce off the crystals.

If molecular sieve beads are exposed to pH that is too high, it can induce an ion exchange, changing 3A sieve crystals into 4A or larger sieve crystals, ultimately allowing ethanol molecules to be adsorbed alongside the water, decreasing capacity.
If exposed to a feed stream with pH too low, sieve beads will dissolve and fuse into clumps. An ideal feed stream should maintain a pH between 4.5 and 9.0  to prevent any ion exchanges or bead fusion. As a part of preventative maintenance, operators should pay extra attention to ensure clean-in-place chemicals, such as sulfuric acid or caustic soda (sodium hydroxide), are thoroughly washed away and not allowed to pass into sieve vessels.

Avoid Contamination
Low molecular weight carbohydrates, in the form of water soluble sugars, and fusel oils can cause significant changes to working capacity in ethanol dehydration units. The carbohydrates come from water soluble sugars that remain in the process stream along with other contaminates, in micellar compounds, which are passed into the beds. These contaminants attach to the outside of molecular sieve beads in a process referred to as coking, creating a layer of coke, or burned carbohydrates. Coke begins to appear as dark spots and, over time, can create a complete layer around the surface of the beads, ultimately turning the beads black. The layer of coke prevents vapor from entering the microchannels within each bead that allow water to be adsorbed by the molecular sieve crystals, thus causing a drastic decrease in working capacity and loss of efficiency.

The easiest method of reducing hydrocarbons—sugars, fusels and other contaminants—is to install demister pads or coalescing filters between the vaporizer and the dehydration unit. These filters essentially are packed layers of steel wool that trap contaminants and clean the vapor stream as it passes through. After installing these filters, preventative maintenance teams should check the drain on the bottom of the filter and replace them as needed to significantly reduce the occurrence of coke and preserve the working capacity and mass transfer rate within the vessels.

Proper care for the molecular sieve in the dehydration unit is an important part of the ethanol production process. Operators should ensure that the process stream reaches and remains in vapor phase, carefully monitor pressure changes, avoid reaching critical velocity, and prevent strong acids and bases, or contaminants that can cause coking, from entering the vessels. Any of these occurrences could cause damage to sieve beads, leading to decreased working capacity, issues with mass transfer rate and eventually total failure of the unit. Educating operators—including training on what not to do—and establishing regular preventative maintenance can increase the lifespan and efficiency of molecular sieve, creating long-term savings and increased profitability.

Author: Mark Binns
Technical Business Director, Hengye Inc.