Refining

Refining

Refineries use molecular sieve units to treat various gas and liquid hydrocarbon streams requiring purification to meet product specifications, prevent equipment corrosion and damage, protect catalyst units and prevent freezing in the plant. Processes where the molecular sieve units are found include isomerization units (C4 and C5), hydrogen (PSA and TSA, for catalytic units, general use, recycle streams), alkylation units (HF alkylation), recovery streams (LERU, offgas) and iso-normal separation (C4 and C5).

For general purpose drying, a 3A or 4A sieve is often used depending upon the type and level of olefins in the stream. 5A or 13X molecular sieves are often used for stream purification where removal of sulfur, oxygenates, CO2 and CO is required.

Zeochem’s 13X molecular sieve is typically used to remove sulfur and oxygenate impurities. Isomerization units, which convert straight-chain hydrocarbons to branched-chain hydrocarbons that have higher octane value for gasoline blending or for feed to alkylation units, require an upstream molecular sieve for catalyst protection.


Applications

Hydrogen streams are in high demand in most refineries, and although hydrogen is generated in areas of the plant such as catalytic reforming, the demand often outstrips the supply. Hydrotreating and especially deep hydrotreating uses a significant amount of hydrogen, making the drying and purification of produced, recovery and recycle hydrogen an important part of maintaining the hydrogen supply. Hydrogen purification includes pressure swing as well as temperature swing units.

Pressure swing adsorption units consist of several vessels with multiple layers of adsorbents. Typical layers may include activated carbon, activated alumina, silica gel and molecular sieve, which is normally the top bed layer. Zeochem’s family of 5A and 13X molecular sieves are often used for the removal of carbon monoxide and carbon dioxide. Nitrogen and other contaminants can also be removed if needed. The resulting hydrogen product is very pure, often to 4 or 5 significant figures.

Thermal swing adsorption units are typically used on recycle, recovery and various other normally small sources of hydrogen; molecular sieve 3A, 4A and 13X are often used. Zeochem Z3-06 and Z4-04 are used when dehydration is the main goal and performance objectives are met.

Custom Solutions

Let our technical service team and its decades of experience guide you through product selection and customize a technical design with proper vessel sizing, regeneration flow and operational sequencing. Our team will help you craft the best adsorbent solution for your isomerization, alkylation, hydrogen, LERU and offgas needs. Our experienced team and quality products will help you run reliably, efficiently and predictably.

Related Products

13X

Type 13X offers enhanced adsorption properties and the ability to remove impurities too large to be adsorbed by the type A zeolites.

3A

3A is made by ion-exchanging the sodium in type 4A zeolite with potassium. The 3A molecular sieve will exclude most molecules except water, making it very selective.

4A

4A is the sodium form of the type A zeolite molecular sieve and is widely used as a general purpose drying agent. Under certain conditions, it can also be used for removal of ammonia, alcohols, carbon dioxide, H2S and other specific molecules.

5A

5A is the calcium-exchanged form of the type A zeolite molecular sieve and is primarily used for removing carbon dioxide, carbon monoxide, alcohols and other oxygenates, hydrogen sulfide, methyl and ethyl mercaptans, and others.

Activated Alumina

Activated alumina adsorbent is an effective bed-topping layer used as a protective layer, at the inlet of gas-phase molecular sieve beds.


Frequently Asked Questions

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. Also, the liquid can cause accelerated 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 help 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 higher risk of regeneration refluxing and laminar flow.

Ordinarily the largest 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 lastsfor approximately 15-30 minutes after 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 anupset occur where a water spike or slug of water hits the bed, a much more pronounced temperature rise can result.

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 should there be weather or unexpected delays that occur.

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 dryconditions, theunit can be started up on regular feed at 50% flow rate whilesimultaneously startinga 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 cycles 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, andaging 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 flowrates 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 wintertime 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 in order to maintain positive pressure to avoid air ingress. Note that any beds in a regeneration heating cycle should be completed prior to shutdown in order to avoid repeating the regeneration heating step from the beginning.

Bead dusting and breakupis a common cause 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 orthe 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 eventsin which liquids slam into the bed can cause sieve breakup, especially for repeated occurrence 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 breakdown 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, anddust and particulate matter that collects in the bed.