Thermodynamic Limitations and Side reactions

The first step in the production of commercial EB is the reversible reaction between benzene and ethylene forming EB.

\(C_{6}H_{6}+C_{2}H_{2}\Longleftrightarrow C_{6}H_{5}CH_{2}CH_{3}\)

EB can react further to form polyethylbenzenes (PEBS) such di-ethylbenzenes (DEBs) and Tri-ethylbenzenes (TEBs). An excess of benzene is typically used to shift the thermodynamic equilibrium to the right-hand side, thereby limiting the available ethylene for further alkylation to PEBs. In addition to transalkylation, isomerization and polymerization may occur producing xylenes and oligomers; these side reactions occur at negligibly low rates because modern industrial catalysts are highly selective for monoalkylation and seldom partake in these side reactions. Alkylation catalysts will be discussed later on in SECTION 2.2.2. To further increase conversion of ethylbenzene, polyethylbenzenes are transalkylated to EB via a transalkylation process:


This reaction should also be done in the presence of excess benzene to shift the thermodynamic equilibrium to the right and increase conversion of PEB. Although alkylation and transalkylation can be carried out in the same reactor, higher EB yield and purity are achieved with a separate alkylator and transalkylator, operating under different conditions optimized for the respective reactions. EB production with 2 separate reactors is optimal for the highest possible yield of EB. Due to the thermodynamic limitations as described above, benzene excess dictates, in both alkylation and transalkylation reactors, the conversion of EB1,3,4,5,6. Having too much benzene excess, however, leads to a large benzene recycle flowrate, increasing feedstock, capital, and operating costs. It is therefore important to strike a balance between the benefits gained from increased conversion and additional costs incurred due to benzene excess.

Benzene Alkylation Catalysts

Commercial production of EB is primarily achieved using two distinct routes of exothermic catalyst-aided alkylation: the traditional method makes use of aluminium chloride catalysts (Friedel-Crafts reaction) and newer methods use more contemporary synthetic zeolite catalysts.

Aluminium Chloride Catalyst

In the 1950s, approximately 40% of the global commercial EB production was done though the Friedel-crafts reaction with aluminium chloride (AlCl3) catalysts7. In addition to AlCl3, a wide range of Lewis acid catalysts, including AlBr3, FeCl3, ZrCl4, and BF3, have been used. These catalysts are extremely corrosive and toxic, capable of corroding storage and disposal containers. Additionally, the disposal of unavoidable side products such as the production of hydrogen chloride and oxidation has raised serious environmental concerns8. It is therefore imperative that AlCl3 traces in the reactor effluent streams are removed and recovered for reuse, adding complexity in both the plant design and operation procedures. In the 1980s, however, a new alkylation method using zeolite-based catalysts process was commercialized. These zeolite catalysts boast improved selectivity of EB and lower toxicity and corrosiveness, overcoming the main drawbacks of AlCl31,6,9. Zeolite catalysts are therefore considered advantageous for modern EB production processes.

Zeolite Catalysts

Over the years, many production processes involving zeolite catalysts have been developed. Each of these processes use their own variation of zeolite catalysts resulting in different operating features. The conventional production processes used today were first developed in the 1990s, including the Mobil-Badger (1990), CD TECH EB (1994), EBMax (1995), and the Lummus EBOne (1996) process. A critical analysis of these process is detailed below.

The Mobil-Badger uses catalyst ZSM-5, a medium pore modified Y zeolite, in a vapour fixed bed reactor for alkylation and transalkylation. Vapour-phase benzene alkylation in Mobil-Badger requires a high benzene/ethylene molar ratio, and high reaction temperatures, around 400oC. This results in high capital costs from larger vapour-phase reactors and high energy costs10,11. Unlike vapour phase alkylation, liquid phase alkylation reduces both the operating temperature (to 240-270oC) and the size of the reactors. Zeolite catalysts for liquid phase operations also have a longer lifetime and are more selective, decreasing the required benzene excess. These characteristics make liquid-phase alkylation more commercially favourable than vapour-phase.

EBOne and EBMax utilize liquid phase alkylation; The EBMax process performs benzene alkylation in liquid phase but transalkylation in vapour phase. EBMax uses a zeolite catalyst known as MCM-22 in the alkylation reactor and ZSM-5 in the transalkylation reactor. MCM-22 has an increased lifetime of 3 years and has a much higher ethylbenzene selectivity than Y zeolites, lowering the required feed ratio of benzene/ethylene to only 4/111. Similarly, Lummus EBOne alkylates benzene in liquid phase while transalkylation is performed in liquid phase. Lummus EBOne uses a modified beta zeolite in both alkylation (EBZ-500) and transalkylation (EBZ-100) reactors. Beta zeolites have a comparable selectivity to MCM-22 while giving a higher ethylbenzene yield than both Y and MCM-22 zeolites11,12,13. Using beta zeolites catalyst, the required feed ratio of benzene/ethylene is 4-6/1 with a lifetime of 2 years10,13 .Overall EBMax and EBOne have a comparable performance and costs. EBOne may operate both reactors in liquid phase but requires a higher benzene excess than EBMax resulting in similar reactor sizing and operating costs. The main difference is that the overall yield of EBOne is 99.6%, 0.1% higher than EBMax10, hence EBOne was the process that was chosen for the Bentham Cooperation venture.

In addition to the ethylbenzene processes above, other techniques have been used in industry such as ‘catalytic distillation’ using Y zeolites, also known as the CDTECH EB process. While CDTECH EB has a comparable performance to Lummus EBOne, can use dilute feedstocks, has increased catalyst lifetime of 6 years, and a high yield of 99.7%10, this technique has many limitations that makes a fixed bed reactor more favourable. The segregation of ethylene in vapour phase and benzene in liquid phase hinder the mass transfer of ethylene to the catalyst. This lowers the conversion of ethylene to ethylbenzene, thus requiring a large catalyst volume and complex reactor system, substantially increasing capital costs1. The characteristics of each process are summarized in Table 1.