Petrochemicals

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.

Applications

Cracked Gas Drying / Ethylene Product

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).

Catalyst Protection

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.

Product Stream Drying

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.

Olefin Treating

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.

EB / SM (Ethylbenzene / Styrene Monomer)

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.

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.

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.

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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.

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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.

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13X

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

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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.