|  | EBMax (Ethylbenzene)
In the early 1970's, aluminum chloride technology was the preferred means for the production of ethylbenzene (EB). However, environmental concerns and the high maintenance costs associated with this technology created a market for alternative processes. In response, Mobil and the Badger Technology Center of Raytheon Engineers & Constructors developed two processes:
- EBMax, a liquid-phase process, based on MCM-22 and TRANS-4, proprietary zeolite molecular sieve catalysts developed by Mobil.
- The 3rd Generation Ethylbenzene Process, a vapor-phase process based on ZSM-5, a proprietary zeolite molecular sieve catalyst also developed by Mobil.
ExxonMobil is the world's leader and has a strong tradition in zeolite catalysts. It started research in zeolite catalysts for EB manufacture in the 1960's and turned to Badger for the commercialization effort. The first commercial plant started up in 1980, with the vapor-phase catalyst. EBMax was commercialized in 1995.
- Features of EBMax
- Process description
Features of EBMax
Yields
Process yields are close to stoichiometric and superior to those of any other commercially demonstrated technology. This is demonstrated by the low production of residue. This feature results in a significant reduction in production costs.
Superior Product Purity
The levels of C8 and heavier impurities as well as non-aromatics are very low because of the use of highly selective catalysts. By proven design techniques, product purity may be further adjusted to suit the requirements of the subsequent use of the EB. The xylene content of the ethylbenzene product is less than 5ppm wt.
Feedstock Flexibility
The process is able to utilize, in an efficient manner, ethylene feedstocks with concentrations of 80% vol through polymer-grade.
Capital Cost
The high activity and selectivity of the catalysts result in substantial cost savings in both the alkylation and transalkylation sections. This is a direct result of smaller catalyst charges and reduced benzene recycle and processing requirements. This applies to both new units and upgrades/expansions, in particular conversions of aluminum chloride catalyzed units.
Low Energy Consumption
In the twenty years of process improvement, energy consumption has been greatly reduced due to design changes and integration of unit operations. Steam generation, tailored for the downstream styrene monomer plant or other units, recovers essentially all the heat of reaction and energy input. The energy input is low due to the low benzene-to-ethylene ratio and the high selectivity of the catalysts.
Catalysts
High activity, selectivity and stability are characteristics of these ExxonMobil catalysts. They are regenerable by simply burning the coke off the catalyst. The demonstrated long cycle lengths and resultant long on-stream times provide distinct operating advantages.
Upgrades/Expansions
Technology improvements make this process ideal for economical upgrades/expansions of existing facilities based on either aluminum chloride or earlier zeolite catalysts, with maximum reuse of existing equipment.
Operation
The simplicity of the process, and moderate temperature and pressures used, and the non-corrosive process streams make the plant easy to operate.
Maintenance
Since all process streams are non-corrosive, carbon steel is the principal material of construction. The non-corrosive nature of the process, together with its mechanical simplicity and the long length of the catalyst cycle, makes the plant easier to maintain and results in a high on-stream factor.
Environmental Impact
The catalysts are environmentally inert, non-toxic and non-corrosive. A small amount of residue is the only liquid waste stream. Vent gases are treated to an appropriate standard.
Process Description
The process consists of three major subsystems:
- An alkylation section, where ethylene is reacted with benzene;
- A secondary reactor system, where polyethylbenzenes (PEBs) are converted to EB, higher alkylbenzenes are dealkylated; and
- A distillation section, where unreacted benzene, PEBs, and heavier compounds are separated from the alkylation and secondary reactor effluents to produce EB of high purity.
In the alkylation section, ethylene is reacted with benzene in a liquid-filled reactor that is equipped with multiple fixed-catalyst beds. The proprietary ExxonMobil catalyst used in this section, MCM-22, is highly active and highly selective to monoalkylation and operates at a low benzene-to-ethylene ratio. A portion of the total ethylene feed is injected upstream of each bed. The benzene feed is heated to reaction temperature prior to being introduced to the first reactor bed. The heat of reaction is used to generate steam.
TRANS-4, the liquid-phase transalkylation step, uses ExxonMobil's very active TRANS-4 catalyst, which is highly active and selective in the transalkylation of PEBs to EB at a low benzene-to-PEB ratio. The PEBs recovered in the distillation section, together with benzene, are preheated prior to being fed to the transalkylation reactor. The effluent is combined with the alkylation effluent and fed to the benzene column. The high activity and selectivity of ExxonMobil's proprietary catalysts, together with the low benzene-to-ethylene and -PEB ratios, result in substantial savings in operating and capital costs.
The distillation section consists of three columns, each of which can generate steam in its overhead system. The benzene column separates untreated benzene from the reactor effluent. Distillate from the benzene column is recycled to the reactor system, and its bottoms feed the second distillation column, where the EB product is recovered as liquid distillate. The bottoms from the second column feed the PEB column, which recovers recyclable PEBs for conversion to EB in the transalkylation reactor. Bottoms from the PEB column, which consist primarily of diphenyl compounds, are withdrawn from the process as a residue stream.
Nearly all of the heat of reaction and energy input to the process is recovered through steam generation. Rejection of heat to cooling water occurs only in vent condensers and in coolers on streams flowing to off-site storage.
|
|
