Integrating CPT with existing OCT units
Lummus Technology’s Olefins Conversion Technology (OCT) has achieved industry acceptance, as evidenced by the 25 units under licence, representing almost 6 MMKTA of propylene capacity.
Robert J Gartside, James M Hildreth, Shaun McGovern, Lummus Technology
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This technology has achieved its success by taking advantage of the lower-cost C4 streams produced by either steam cracking or FCC and subsequently upgrading them to higher-value olefins.
Lummus has developed a unique addition to its olefins technology portfolio called Comonomer Production Technology (CPT). This technology upgrades mixed C4 streams to 1-butene and or 1-hexene, both valuable polyethylene/polypropylene co-monomers, and can be applied on a standalone basis or integrated with an OCT unit.
This article will describe the CPT process and its operation in a unique campaign mode that allows for the low-cost integration of CPT with an OCT unit while producing comonomer at a capacity viable for the captive market.
Lummus Technology’s Olefins Conversion Technology (OCT) has received widespread acceptance over the past five years as the lowest capital and energy cost option for meeting the increased demand for propylene. By utilising the lower-valued C4 streams obtained from either steam crackers or refineries to produce higher- valued propylene, OCT provides significant product flexibility for operators.
The classic OCT unit utilises the metathesis reaction between ethylene and 2-butene to form two propylene molecules. The flowsheet is shown as Figure 1. Fresh Raffinate II plus recycle C4s are mixed with ethylene. The mixture is then fed to a treater to remove impurities such as oxygenates, carbonyls, mercaptans and water that may be present in fresh feed. The treated feed is then vapourised and heated in a feed/effluent exchanger followed by a trim fired heater before going to the OCT reactor. The OCT reactor is a fixed-bed reactor where the metathesis reaction takes place. The reactor effluent consists of propylene, unreacted ethylene and butenes, and a small amount of heavy components as a result of side reactions. In addition, any paraffins present in the fresh feed pass through unreacted and must be eventually purged. The effluent is first cooled and then fractionated. In the ethylene column, unreacted ethylene is separated from propylene and heavier components and recycled. The ethylene column bottoms are then fractionated in a second column where the propylene product is taken overhead. Unconverted butenes are recycled to the reactor and a purge stream (consisting of butanes present in the feed, some butenes and C5 and heavier components( is sent offsite. This purge stream is typically recycled to a steam cracker as LPG feed.
The metathesis reaction is energy neutral, which leads to low energy consumption per unit propylene production, and the process produces ultra-high-purity propylene product since no propane component is created by the reaction.
CPT is an extension of the conventional metathesis technology for propylene. It utilises the reactions between the C4s themselves, without ethylene, to produce comonomer-grade 1-butene and/or 1-hexene with the co-production of either ethylene or propylene. This greatly expands the product flexibility from C4 olefin streams, which is one of the driving factors for the success of the metathesis technology portfolio.
The two primary OCT reactions between C4s (or auto-metathesis reactions) are:
1-Butene + 2-Butene → Propylene + 2-Pentene
1-Butene + 1-Butene → Ethylene + 3-Hexene
These reactions are the side reactions in OCT and are minimised through the use of excess ethylene. Consider the reaction between 1-butene and 2-butene to form propylene and 2-pentene: this is less selective for propylene, since two butenes form only one propylene.
The reaction for the production of 1-hexene is between two 1-butenes to form ethylene and 3-hexene. In the conventional metathesis system, the isomerisation function that converts 1-butene to 2-butene (for subsequent reaction with ethylene to produce propylene) is combined with the metathesis reactions. In CPT, these functions are separated. The key to CPT technology is the ability to produce the metathesis feed of interest by a combination of isomerisation and/or fractionation and then separately carry out the metathesis reaction to create the specific desired products. As part of CPT, Lummus has developed a highly selective double-bond isomerisation catalyst and a highly selective metathesis catalyst.
CPT for 1-butene production simply combines a double-bond isomerisation reactor system with a butene fractionation system. Raffinate 2 (with iso-butene removed to reach 1-butene specification) is fed to a combination isom unit plus fractionation system (Figure 2). Unlike conventional separation-only systems, the 2-butene is converted to 1-butene and the normal butene utilisation is dramatically increased. The double-bond isomerisation catalyst has no skeletal isomerisation activity, so 1-butene specification is maintained and there is no metathesis activity.
The production of 1-hexene from Raffinate 2 involves two more steps. After producing 1-butene, the second step is the metathesis of the 1-butene with itself to form ethylene and 3-hexene over a catalyst system that minimises any isomerisation of the 1-butene to 2-butene. The auto-metathesis effluent is sent to a depentaniser where high-purity 3-hexene is recovered (for the third step) and by-products ethylene and propylene are sent to recovery.
The third and final step is the isomerisation of the 3-hexene to 1-hexene, with purification, in a superfractionator where high-purity 1-hexene is produced overhead. This step uses the identical catalyst used for the 1 butene isomerisation.
High-purity 1-butene and 1-hexene can be co-produced simply by taking a 1-butene product stream. The technology has been fully demonstrated at semi-commercial scale. Using feed rates of over 1 MT/hr, all three steps have been successfully demonstrated in co-operation with our partner Sinopec. This unit ran for over 8000 hours and met all process goals. In addition, the product 1-hexene was used to produce commercial hexene comonomer LLDPE.
CPT for 1-hexene production can be compared to another on-purpose hexene technology: ethylene trimerisation. CPT uses a relatively low-cost feed and produces valuable by-products. Trimerisation starts with expensive ethylene feed. The trimerisation process utilises a loop reactor system and a rather complicated chromium-based homogeneous catalyst system. The residual chromium must be continually removed from the system and disposed.
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