Petrochemicals

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.

A liquid slug can slam into the bed at high velocity, moving, displacing, and even crushing sieve beads. The liquid coats the sieve, slowing mass transfer, which leads to poor adsorption of water and other contaminants, and adds more load to the regeneration step. Additionally, the liquid can accelerate coking during heating. To minimize coking, it is recommended to ramp heat at 100°F/hr (55.5°C/hr) when there is time to do so and in some cases an additional cool purge step for 30-60 minutes prior to heating is also recommended to help remove and strip out liquids prior to heating.

In general, higher sieve adsorption capacity is favored by lower temperature and higher pressure. This also helps to lower the feed water concentration for water saturated applications. There is a balance to be maintained though in order to avoid approaching the hydrocarbon dewpoint in vapor phase systems. It is recommended to maintain operation at 10-20°F (5.5-11°C) above the dewpoint in order to avoid potential two-phase flow. Mixed phase is always to be avoided given that adsorption and working capacity are negatively affected and can be unpredictable when this occurs. As a result, operation should always be 100% vapor phase or 100% liquid phase. For regeneration, lower pressure is favored for minimized flow rate and better turbulence, and lower temperature is favored for optimum sieve life. There are practical limitations and the heating temperature typically falls within a given range depending upon the type of sieve being used and the application details. Temperatures that are too low are too inefficient and may not remove enough contaminant; temperatures that are too high will cause accelerated coking and can cause decomposition of stream components. Pressure typically cannot be too low due to excessive velocity in the up flow direction that will cause bead movement; pressures that are too high require additional flow or time, and a higher risk of regeneration refluxing and laminar flow.

Ordinarily, the most significant temperature swing occurs when a freshly regenerated sieve bed is placed back online. Although the bed has been cooled, it is often several degrees above the inlet feed temperature. As a result, a temperature bump of 15-20°F(8.3-11°C) often occurs and lasts for approximately 15-30 minutes after the feed has been reintroduced to the bed. In addition, the adsorption process is exothermic, giving off heat. Normally, the amount of contaminant being adsorbed is small enough to generate an increase in the product stream temperature of only2-4°F (1.1-2.2°C). Should an upset occur where a water spike or slug of water hits the bed, a much more pronounced temperature rise can result.

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.

When possible, first regenerate the sieve beds to ensure dryness. Normally, a short 70-80% heating cycle is sufficient for this purpose, given the sieve will not be as wet as during normal operation. If initial regeneration is not possible or if the sieve is loaded under sufficiently dry conditions, the unit can be started up on regular feed at 50% flow rate while simultaneously starting a regeneration cycle of 1 of the beds. As soon as the regeneration is completed, switch beds and start regeneration of another bed. Once all the beds have been regenerated, ramp up the feed rate to full design rate and adjust the cycle times to the technical recommendations.

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.

Typically, there are some options that allow continued operation in the plant until a change out can occur. Adjustments to the cycle times, regeneration and feed conditions, and flow rates are sometimes available and can be made. The last thing considered is a reduction in the feed flow rate once all else has been done and further adjustment may be necessary. Zeochem can help with recommendations and a prioritized plan of action.

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.