Ethanol Dehydration

It is recommended to discuss with Zeochem the proposed changes so that a system review and design simulations can be run to determine if any operating changes are necessary or recommended. Zeochem can further check to make sure that all changes fall within recommended operating guidelines, and propose alternatives if changes may be problematic.

Beads are round and smooth, strong and durable, exhibiting low dusting characteristics and potential breakage. The spherical shape results in only compressive forces, while pellets (extrusions) undergo compression as well as tension, making breakage more likely. The ends of the pellets also have angled edges, making them subject to chipping and breakage. In addition, beads naturally dense load for optimum loading density without the use of dense phase loading equipment.

When beds are adsorbing in parallel and there is more flow restriction in one vessel than the others, the flow will automatically balance between the beds to achieve an equivalent pressure drop. If it is minor, then it is ordinarily not an issue. If the flow imbalance is large enough, early breakthrough on the vessel with the high flow rate (lowest pressure drop restriction) can occur. To better balance the flow rate between the vessels, feeds must be adjusted. This is normally done by adjusting (partially closing) valves, either manual shutoff valves or the valve travel of automatic valves. It can be a trial and error process to achieve balance, so it should occur as a series of small adjustments until the operation noticeably improves. In cases where this is not possible, adjusting the cycles and/or conditions may be necessary to prevent breakthrough. When all else fails, the inlet feed rate must be reduced until breakthrough no longer occurs.

Schedule the change out well ahead of time, preferably during an already scheduled shutdown or turn around. Order and have all needed products and supplies on site well ahead of time to avoid any delays. Make sure all contractors, plant personnel, and equipment will be ready to begin the morning of the scheduled start, with any necessary orientation, training, etc., completed in advance. Follow the Zeochem guidelines and recommendations for unloading and reloading the sieve to streamline the process and avoid delays. Have contingency plans in place for weather or unexpected delays.

The water capacity is the percent by weight that the sieve adsorbs. At equilibrium, the adsorption is basically driven to completion to determine the absolute maximum amount of potential adsorption. This is most often used as a general baseline measure of the sieve’s quality and ability to adsorb water. Dynamic capacity is the working capacity that is expected from the sieve to avoid breakthrough of water in an actual process. The design simulation determines this capacity based upon the feed and regeneration stream compositions and conditions, and the water concentration, as well as the concentration of other contaminants. It involves not only the equilibrium capacity, but calculation of the mass transfer zone, effects of other contaminants, and aging of the sieve over time. All of these factors contribute to the difference between achievable water adsorption in service and the theoretical maximum water loading for the sieve.

For short-term shutdowns of less than 1 day, especially in warm climates, the vessels can be locked in under full pressure and simply restarted from the point of stoppage. In cold climates or winter conditions, 6-12 hours could be a downtime limitation due to more rapid cooling. In such cases, depressurizing or partial depressurizing of the vessels and surrounding piping and equipment would be recommended in order to avoid condensation and liquids during the shutdown. For upstream pipelines and equipment that cannot be depressurized, low point drains should be checked to make sure any collected liquids are drained off before restarting. For long-term shutdowns of more than 1 day, it is recommended to regenerate all the beds first, depressurize down to 5-10 psig while maintaining a blanket of dry inert gas in each vessel. Check the vessel pressures periodically to maintain positive pressure to avoid air ingress. Note that any beds in a regeneration heating cycle should be completed before shutdown to avoid repeating the regeneration heating step from the beginning.

Bead dusting and breakup are common causes of pressure drop increase. This is often caused by upsets such as rapid pressure swings, liquids carryover, regeneration refluxing, and flow channeling. Rapid pressure swings can cause movement of the sieve or the entire bed in some cases. When severe enough, it can lead to flow channeling in the bed and early breakthrough in addition to increased pressure drop. Liquids carryover events in which liquids slam into the bed can cause sieve breakup, especially for repeated occurrences when it is a chronic problem. Regeneration refluxing occurs when liquids rain down on the sieve bed due to cooling and condensing at the top of the vessel early in the heating cycle while simultaneouslyliquids are being vaporized and driven from the sieve bed. It creates a rolling boil that can break down the sieve over time. In general, even under the best of circumstances, the pressure drop across the sieve bed normally goes up by 1.5-2 times over the life of the sieve due to bed settling and compaction, coke and carbon buildup on the sieve, and dust and particulate matter that collects in the bed.

Common upsets include feed water concentration spikes, pressure and temperature swings, flow changes or disruptions, changes in pressure drop, and operating control issues. It is recommended to immediately call Zeochem Technical Service once the upset is discovered. After review of the details and the current operation, Zeochem will provide a technical assessment and make recommendations to return the plant to normal operation. In many cases, the unit can be lined out; however, sometimes the molecular sieve is damaged to the point where the adsorption capability is compromised. In these cases, a reload of fresh sieve may be required. Our Technical Service team can help determine if that is necessary. It is also very important to identify the source and the cause of the upset so that measures can be taken to prevent it from happening again in the future.

Make sure all bed loading materials, supplies, equipment, and manpower are accounted for and ready to go ahead of the start date. Keep the sieve as dry as possible prior to loading by storing it appropriately. In the field, stage the product for each bed so that all the materials to be loaded in a bed are present and ready to be loaded. Do not place packaging directly on the ground (keep on pallets) and cover with tarps when needed to keep packaging and the sieve dry. Maintained sealed packaging until time to load into the vessel or hopper (when applicable). Avoid loading in the rain whenever possible. Sieve readily adsorbs water from the air, and the wetter the environment, the more water the sieve will adsorb during the loading.