Petrochemical plants use various olefin and other hydrocarbon feed streams to produce a variety of products, many of which are polymer based. The lower olefins, ethylene, and propylene, are by far the most widely used feedstocks in the petrochemicals industry. Ethylene is the primary feedstock used to fulfill most of the worldwide demand for polyethylene, ethylene oxide, ethylene dichloride and styrene. Propylene is the primary feedstock used for the production of polypropylene, propylene oxide, acrylonitrile, cumene and acrylic acid. Over 50% of the ethylene and propylene produced worldwide are used for the production of polymers.

Impurities in produced ethylene, propylene and other streams can include water, oxygenates and sulfur compounds that can negatively affect plant performance, damage equipment and poison the catalysts used in downstream reaction processes. Zeochem’s molecular sieves with 3A type zeolites are used primarily to remove water in these applications while 13X type zeolites and other specialty adsorbents are used to remove oxygenate and sulfur impurities to low levels, ensuring the reliable, efficient, predictable performance of the plant.

Applications requiring molecular sieves include ethylene plants producing cracked gas from various hydrocarbon feedstocks. Additional applications include drying of ethylene and propylene feed streams in polymer plants, drying of ethylene and propylene stored in underground caverns, and various small specialty applications requiring very clean olefinic feed streams.


The most common application requiring molecular sieves is ethylene plants producing cracked gas from various feedstocks, including ethane, propane, butane, LPG, naphtha, heavier hydrocarbons, gas oil and at times various mixtures of some of these feeds. The feed is cracked using steam in high-temperature tube furnaces, producing ethylene, other unsaturated as well as saturated hydrocarbons, hydrogen, carbon monoxide and some heavier hydrocarbons. The stream is then quenched with water and caustic washed in several stages to remove contaminants. The resulting purified cracked gas stream is water saturated and must then be dried with a molecular sieve before entering the cold section of the plant.

Our 3A type molecular sieves are commonly utilized for dehydration of cracked gas, ethylene product and propylene product. These products are made from a high exchanged potassium form of the A type zeolite and have the smallest crystal pore openings that exclude ethylene and propylene from entering the crystals and being adsorbed. Our 3A product line has standard grades for the less demanding applications, as well as specialty molecular sieves which offer enhanced capacity, low coking characteristics that provide significantly more adsorption capacity. This allows for more throughput or longer adsorption cycle times while being coking resistant – which results in slower degradation over time – and increased service life. The 3A products have a normal regeneration temperature of 450°F (232°C), with a typical range of 400–500°F (204–260°C).

Some petrochemical applications require various levels of protection from water and other impurities to ensure high-level catalyst performance over their expected lifetimes. Often a guard bed is used to provide protection against otherwise unexpected contaminants.

In cases requiring water removal from olefin-containing streams, Zeochem molecular sieves are the recommended choice to avoid adsorption of olefins and the resulting temperature rise that can occur when high levels of olefins are present in the feed stream. In cases where oxygenates or sulfur compounds need to be removed, our 5A type zeolite molecular sieves are a consideration in select applications when removal of small oxygenates such as methanol or sulfur compounds (e.g., H2S, methyl mercaptan or ethyl mercaptan) is required. 13X type zeolite molecular sieves are recommended when removal of larger sulfur species is required. They also remove water, alcohols and other oxygenates up to a kinetic diameter of approximately 10 angstroms. The 5A and 13X products are best suited for olefin-free or feeds with low olefin concentration. Consult with our technical team to determine if the use of our products in these applications will allow you to operate without additional precautions and process steps such as purging and preloading.

Highly olefinic feeds require preloading of the feed to avoid temperature spiking due to the heat of adsorption of the olefins. The preloading step may require a slow bleed of the olefinic product to minimize temperature spiking with the feed stream. This can often be accomplished with a 10–20% blend rate during the final hour or two of cooling. Call our technical team for guidance on your specific feed stream and adsorption unit.

Various petrochemical product streams require drying in a regenerated system or may include a guard bed to ensure proper dryness to meet final specifications. For olefin-containing streams, Zeochem's 3A molecular sieves are the recommended choice to avoid adsorption of olefins and the resulting temperature rise that can occur. Our 4A products can be utilized when olefins are not present or are present at very low levels. Likewise, the 13X molecular sieves can be used for enhanced dehydration performance and when co-adsorption of other components in the feed stream is not an issue.

Olefin products are often stored in underground storage caverns. While in storage, water is picked up by the stored olefins and must be removed when the product is removed from the cavern. Once again, 3A molecular sieves are recommended to avoid adsorption of olefins and the resulting temperature rise that can occur.

In petrochemical applications requiring treating of olefinic feed streams for the removal of oxygenates, sulfur compounds and other impurities, 5A and 13X type zeolite molecular sieves may be a consideration in specific applications. 5A molecular sieves can remove the smaller oxygenates and sulfur compounds while the 13X type zeolite molecular sieves are recommended for removal of larger sulfur species.

Preloading of the sieve bed with olefin product is required to avoid temperature spiking due to the heat of adsorption of the olefins. The preloading step may require a slow bleed of the olefinic product to minimize temperature spiking with the feed stream. This can often be accomplished with a 10–20% blend rate during the final hour or two of cooling. Similar precautions are needed when first starting up units with fresh sieve.

Ethylbenzene/styrene monomer production plants use molecular sieve pre-treater units to protect catalyst units. Feed streams, consisting of ethylene and benzene, contain ppm level impurities such as water, oxygenates and nitrogen compounds that are removed by molecular sieve. Ethylbenzene is formed from these feed streams in an alkylation unit. Ethylbenzene can further undergo dehydrogenation to form styrene. The alkylation catalyst life is limited by the impurities in the feed streams, and the nitrogen contaminants often include N-Formylmorpholine (NFM) and N-Methylpyrrolidone (NMP), which can be the most difficult to remove. Upstream molecular sieve treaters adsorb these impurities to ppb levels, creating purified benzene and ethylene feed streams that allow for increased catalyst life.

Zeochem's Z4-04 and Z10-03 molecular sieves are commonly utilized for this application. The setup typically consists of a split bed arrangement with Z4-04 primarily used for water removal and Z10-03 used for removal of the larger impurity compounds, or a complete bed of Z10-03 for removal of all impurities. The bed is regenerated infrequently (1 or 2 times per year) or not at all. Regeneration is only semi-effective due to the reactive nature and elevated boiling point of some of the compounds present, making removal prior to cracking and coking more difficult.

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Custom Solutions

Contact our experienced technical service team to discuss your feed stream composition and targeted impurities for removal. Let us recommend the best molecular sieve for your needs.

Related Products

Zeochem offers a diverse line of molecular sieve products ideally suited to purification needs across a wide range of petrochemical operations. These 3A, 4A, 5A and 13X products are specialized for reliable operation in each of their various dehydration, catalyst protection and general treating applications.


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


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 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 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.

Deuterium Used as Tracer or Marker

Zeochem offers deuterium oxide in different isotopic qualities, which are used in the tracer and marker fields for food, beverage, agriculture and hydrology industries.

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.

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. In order 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, adjustment of the cycles and/or conditions may be possible 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 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.