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Natural Gas Conversion VIII F.B. Noronha, M. Schmal, E.F. Sousa-Aguiar (Editors) © 2007 Published by Elsevier B.V.
SNOW: Styrene from Ethane and Benzene a
a
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Domenico Sanfilippo , Guido Capone , Alberto Cipelli , Richard Pierce , b b Howard Clark , Matt Pretz a
Snamprogetti Snamprogetti S.p.A. Viale DeGasperi 16 - S. Donato Milanese, 20097, Italy The Dow Chemical Company, 2301 N. Brazosport Blvd., B-250 Freeport, TX 775413257 U.S.A. b
1. Introduction
Styrene is one of the most important monomers for the production of polymers, resins and rubbers. Styrene production exceeds 25 Million MT/y. The biggest consumer of Styrene monomer (SM) is Polystyrene (PS). Other major derivatives are expanded polystyrene (EPS), SB Latex, SB Rubber, styrene block co-polymers (eg ABS, MBS, SBS) and unsaturated polyester resins. SM is a commodity and a lower production cost is a critical factor in the value chain of its derivatives. The styrene market has been growing at a rate of 4.4% yearly for the last several decades. It is expected that worldwide capacity will expand by 5.5 million MT over the next five years In the current art there are two commercial routes for the synthesis of SM: a) The classic and largely applied EB/SM route: in a first step ethylbenzene (EB) is formed by alkylation of benzene with ethylene in the presence of acid catalyst (typically a zeolite). EB is catalytically dehydrogenated to SM in the second step. Unconverted EB is recycled back to the dehydrogenation section. C6H6 + CH2=CH2
C6H5-CH2-CH3
C6H5-CH=CH2 + H2
(1)
b) Alternatively and to a lower extent (<20% of the SM market) SM is co produced together with propylene oxide (PO/SM route). The EB synthesis is followed by the EB peroxidation with the formation of the hydroperoxide. This latter reacts in a further step with propylene forming propylene oxide (PO) and ethylbenzyl alcohol that is dehydrated to styrene in a final step. EB+O2C6H5-CHOOH-CH3+C3H6PO+C6H5-CHOH-CH3 SM+H2O
(2)
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The conventional EB/SM route utilizes benzene and ethylene as raw materials and the core is represented by the EB catalytic dehydrogenation step. The continuous effort during several decades for improving this process in the areas of reactor technology and catalyst technology has brought it to a high degree of maturity and there is today little room for further improvement. 2. The innovative SNOW technology
The SNOW technology has been jointly developed by Snamprogetti and Dow (SNOW = SN amprogetti + DOW ) and represents a technological and economical breakthrough in the Styrene Industry. The SNOW technology is innovative as concerns many factors: raw materials, reactor design, heat supply system, catalyst, feed composition. a) Raw materials and reaction scheme The SNOW complex is fed with benzene and ethane, which is dehydrogenated in the same reaction system for EB dehydrogenation to produce the stoichiometric amount of ethylene necessary for the benzene alkylation (Fig. 1). In an alternative version the SNOW Unit can be fed with ethylene and benzene, similarly to the conventional technology (Fig 2: Ethylene Option). The two Options (ethane or ethylene feed), allow the maximization of profit according to the location, price and availability of the raw materials. The Ethane Option can add value to stranded or limited use gas streams. Ethane is a significant component of Natural Gas, and is also contained in some refinery and petrochemical streams (FCC and crackers off gas, by-product of liquid feed crackers). SNOW provides an opportunity to monetize Natural Gas without the need of an associated Ethylene Project, which may be attractive in some locations. Additionally, since this Option decouples SM production from Steam Cracker it becomes possible to build a new SM unit (or retrofit an existing conventional one) in refinery/petrochemical complexes without the need of debottlenecking the Steam Cracking plant. Ethane Benzene
Ethyl Benzene Process
Separation &SSt yt yr e r ennee Purification Purification
D eEhtyhdyrl o bg en ez n eant ieo n D e h ySdercotgi oenn a t i o n
C 2 Recycle
Styrene
E B R e c y c le
Fig. 1: Conceptual block diagram of SNOW technology (Ethane Option) Ethylene
EB
Benzene
Ethyl Benzene Process
SM Ethylbenzene D e h y d r o g e n a t io n
Separation & Styrene Purification
EB R ecyc le
Fig. 2: SNOW Technology (Ethylene Option)
Styrene
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SNOW: styrene from ethane and benzene
b) Reactor Design Dehydrogenation of ethylbenzene to styrene and as well of ethane to ethylene is an endothermic, equilibrium-controlled reaction. Maximum conversion increases with temperature. In order to achieve reasonable economic conversion per pass, temperatures exceeding 600°C are a prerequisite. Since the equilibrium conversion of ethane and EB are significantly different for a specific set of conditions, a suitable process design allows SNOW Ethane Option to produce per pass as much ethylene as EB converted. The core of the SNOW technology is the reaction system, characterized by the use of fluidized beds with circulating catalyst, similarly to the well known and largely applied FCC technology (Fig. 3). The SNOW reactor is of Riser type, where the catalyst is fast moving upwards, entrained by the co-current hydrocarbons stream at a gas velocity of 4-20 m/s. Catalytic reactions are performed rapidly (1-5 seconds) in the riser. Temperature ranges between 590 and 700°C according to the feed type and riser level. Conventional technologies operate at EB partial pressures lower than one atmosphere (vacuum and steam dilution) to get a higher driving force and better selectivities. This is not required with SNOW. Product Gas
Flue Gas
Nitrogen
Catalyst
Fuel Air Nitrogen
Reactor
Catalyst Regenerator
Catalyst
Hydrocarbon Feed
Fig. 3: SNOW reaction section
The products leave the disengaging zone after complete removal of the catalyst with a cyclone system. The reactor effluent is processed using conventional separation technology. EB can be produced using conventional technology.
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c) Heat Supply System The catalyst exiting the reactor is conveyed to the Regenerator (fig. 3), a bubbling fluidized bed where the initial catalyst activity is restored by burning the minor carbonaceous deposits built up on the catalyst surface during the reaction time and restructuring the active sites. Air is fed to the regenerator bottom countercurrently to the down-flowing catalyst. Flue gas leaves the disengaging zone of the regenerator through a cyclones system. As already mentioned, the dehydrogenation of EB and ethane are highly endothermic: C6H5-CH2-CH3 C6H5-CH=CH2+H2
ΔHr =
28.1 kcal/mole, 264 kcal/kg
CH3-CH3
ΔHr =
32.7 kcal/mole, 1086 kcal/kg (2)
CH2=CH2 + H2
(1)
The endothermicity results in a high heat demand and requires high heat fluxes at temperatures higher than 590°C along the reaction coordinates. In the SNOW technology this heat is supplied by the heat capacity of the circulating catalyst: "hot" catalyst, heated up in the regenerator at a temperature higher than that required for reaction, goes to the reactor. The catalyst cools down in the riser reactor during the reaction and flows back "cold" to the regenerator. Indeed in order to satisfy the heat balance and generate the heat necessary to the reaction loop in the regenerator some fuel is catalytically burnt, preferentially the hydrogen rich off gas of the process is used. It increases significantly the carbon efficiency of the process because of the low CO2 emissions in the flue gas. The high surface of catalyst ensures a very efficient heat exchange directly in the process side without any intermediate wall. Of course heat is recovered from the reactor/regenerator effluents. Since the heat generation in the reaction loop takes place through catalytic combustion of fuel gas at relatively low temperature, SNOW is characterized by very low CO/NOx emissions. d) Catalytic System Design Conventional dehydrogenation technologies in the prior art have selected promoted iron oxide as a unique catalyst to be operated under low EB partial pressure and a significant dilution with steam. For the SNOW project several candidates as active phase have been considered and tested. The chosen formulation is based on promoted Gallium oxide. Gallium systems are little known in scientific and patent literature. A new chemistry and catalysis has been developed for the SNOW technology. When duly promoted, gallium catalyst performance is excellent for ethane as well as EB dehydrogenation. The catalyst is so active that it is possible to operate it with very short contact time, typical of a fast riser reactor. The selectivities to ethylene and styrene respectively are so high that no steam dilution is required. Thus the SNOW technology has the unique advantage of avoiding the steam cycle (generation, condensation, heat exchangers etc.). This
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aspect is particularly important because it allows significant benefits in energy saving and CO2 emission reduction. Coke formation is very low and the catalyst circulation provides a continuous regeneration from coke build up. The commercial catalyst is a microspheroidal, attrition resistant, high heat capacity solid, synergistically designed and developed together with the reactor engineering. Catalyst PSD (Particle Size Distribution) and mechanical properties have been optimized for the use in a fluidized bed and make this catalyst more resistant than the typical FCC catalysts. The SNOW technology allows, like in the other fluidized bed processes, the continuous make up of fresh catalyst, maintaining its "equilibrium" activity stable over the time. No turndown for catalyst substitution is required and no aging from Start-Of-Run to End-of-Run conditions is present. 3. SNOW Technology development steps
Snamprogetti and Dow joined their effort for developing the SNOW technology in early 2000. The initial technology development and engineering skills of Snamprogetti [1-3], and the complementary styrene production and marketing expertise of Dow have been integrated[4-7]. R&D activity has been carried out initially in microreactors and bench scale units. Results have been validated at pilot plant level utilizing a DCR (Davison Cracking Reactor), typically adopted for FCC catalyst testing. The next step has been the construction and operation on a Dow site of a PDU (Process Demonstration Unit, fig. 8) that has allowed testing all aspects of the new technology including process yields, conversion and economics, SM product quality for derivatives application, reactor and catalyst reliability and so on. The PDU is on a scale intended to prove critical unit operations in both performance and reliability. The capacity (feed rates in the 500+ kg/hr range) is of sufficient size to minimize the risk of scale-up of critical unit operations. The PDU operates at the same conditions as a world scale unit, but has allowed enough variation around process parameters in order to optimize the process. Hydrodynamics have been modeled and demonstrated in large scale mock ups (cold flow models). Catalyst production has been successfully scaled up from laboratory formulations to commercial scale manufacture. Several tens of tons of catalyst have been produced in full scale equipment. The PDU operation has validated the catalyst performances, the scale up criteria and the whole process. 4. Final considerations
The Dow Chemical Company and Snamprogetti S.p.A. have synergistically developed a new route to produce SM from ethane and benzene (SNOW ”Ethane Option") or ethylene and benzene (SNOW ”Ethylene Option") which Dow/Snamprogetti will be uniquely offering in the market to styrene producers.
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Fig. 4: Example of economics of SNOW (Ethane/Ethylene Options) relative to conventional
The innovative SNOW process is expected to enable significant cost savings in both versions from benzene-ethane and from benzene-ethylene. Fig. 4 reports an example of the SNOW advantage versus the conventional EB/SM route. Of course the advantage may change with the plant location due to the different valorization of raw material, energy and equipment cost. The "Ethane Option" moves away from the conventional Styrene production process and raw materials, captures ethane-ethylene price differential and eliminates the need for upstream investment in ethylene production at a steam cracker or ethylene purchases and offers greater location flexibility. The "Ethylene Option" offers significant advantage in investment cost versus conventional approach. SNOW will use less energy (no dilution steam) and emit less CO2 than competing processes (no fuel gas, instead burns H2 produced in the process). Acknowledgements The mentioned authors are the iceberg tip of an impressive number of colleagues in Dow and Snamprogetti and EniTecnologie that have been working in this project around the World with competence, passion, and creativity. References: 1. F. Buonomo, D. Sanfilippo, R. Iezzi, E. Micheli USP 5,994,258 and USP 6,242,660 (1997) to Snamprogetti 2. F. Buonomo, G. Donati, E. Micheli, L. Tagliabue USP 6,031,143 (1997) to Snamprogetti 3. D. Sanfilippo, R. Iezzi USP 6,841,712 (1999) to Snamprogetti 4. I. Miracca, G. Capone USP 7,094,940 (2001) to Snamprogetti 5. D. Sanfilippo, A. Bartolini, R. Iezzi US Appl. 2004/0259727 to Snamprogetti 6. D. Sanfilippo, I. Miracca, G. Capone, V. Fantinuoli US Appl. 2005/01776016 7. W. Castor, S. Hamper, M. Pretz, S. Domke WO 2005/077867 to Dow
