ARTICLE
Pyrolysis of Cracked Gasoline into Olefins: I. Design and Construction of a Cold Model for Circulating Type Reactor J. Freitez,† R. Galiasso,‡ Y. Gonzalez,† and J. Rodrıguez*,† †
Universidad Nacional Experimental Politecnica “Antonio Jose de Sucre” Vicerrectorado Barquisimeto, Centro de Investigacion de Procesos (CENIPRO), Barquisimeto, Venezuela ‡ School of Chemical, Biological, and Materials Engineering, University of Oklahoma, Norman, Oklahoma 73109, United States ABSTRACT: A new concept of reactor—a prototype of transported-loop reactor that reduces residence time for the cracking of a light naphtha stream—enhances commercial, low-value, light streams with an alternative process to applying these fractions to light olefins (ethylene and propylene). In particular, a prototype of a transported-loop reactor was developed to reduce the resistance time in the cracking of a light naphtha stream to produce olefins. In particular, a prototype of a transported-loop reactor has been developed to reduce resistance time in the cracking of a light naphtha stream to produce olefins. It is currently used to study the gassolid fluid-dynamic. The novel device includes a uniflow-type reactor and a partial gasification downer riser circulating reactor. The novelty lies in converting the light naphtha at a very high temperature and ultrashort residence time by contacting a hot solid with the naphtha to immediately separate the olefins to avoid further reactions. The solid is then partially gasified to provide autothermal operation of the reactors. The design of the equipment provides information about the gas/solid contact to develop several prototypes of the pyrolysis and the carbon gasification reactor. The cold model will derive suitable hydrodynamic relationships and performance constraints for high solid loading not available in the literature. The main result obtained is the need to provide an initial high solid contact to ensure the heat transfer and then proceed to separate the gas and solid. The other result is the effect of the hydrocyclone in the clustering behavior of the circulation reactor. Knowledge of the fluid dynamic in both reactors and kinetic information already developed will facilitate design and scale-up of a new process for cracked naphtha valorization.
1. INTRODUCTION Reaction engineering methodology to develop new processes is well described in the literature.1 Innovation is driven by the need for a new or improved product produced from existing or different feeds using commercially available or new technology. The combination of expected breakthroughs in these three factors (feed, process and product) generates a particular process design strategy, but all are based on the following three main stages: visualization, conceptual and basic design, and detailed engineering for construction. The valorization of light cracked naphtha streams (light delay coker and fluid catalytic cracking naphthas) is used worldwide in refineries. In addition, there is an important petrochemical market for light olefins (ethylene and propylene). Both interests can be served by developing a new process that produces olefins from cracked naphtha. Naphtha formed from Catalytic Cracking and delay Coker has higher olefin content than that of straight-run naphtha. The pyrolysis and steam cracking of these feeds produce different product distribution and more coke formation than straight-run naphtha. Significant valorization could be gained from creating high-value light olefins from these feedstocks. To achieve these objectives, available technologies such as steam-cracking or fluid catalytic cracking provide limited yields in olefins, produce a high proportion of low-quality aromatic fraction, and form a large portion of coke when operated with cracked feeds.2 Therefore, they present a short operational cycle length, operational problems, and large financial investment. Research and development r 2010 American Chemical Society
in catalytic cracking of hydrocarbons to improve the selectivity of cracking technology, including naphtha to light olefins, started in the late 1970s. Despite numerous claims that catalytic cracking of cracked naphtha improved the thermal process, no commercial application has been implemented. One method is the use of acidic catalysts such as Ag-mordenite/Al2O32 that give low ethylene but high propylene and aromatics yields at moderate temperatures. The acid catalyst follows the ion carbonion mechanisms that form dimers, which are then cracked into olefins. Other options are the basic catalysts, such as KVOx/corundum,3 in present of steam at high temperature, conditions that approach a free radicals chain mechanism. SINOPEC Research Institute of Petroleum Processing (RIPP) in China uses proprietary zeolitic catalysts to increase the yields of light olefins when processing heavy feedstocks4 in a fluid catalytic cracking type unit. UOP and Total Petrochemicals Research announced the OCP process,5 which is a catalytic conversion of C4 raffinates and olefinic naphthas originating from Steam Crackers, FCC, Coker, and Visbreaker units. The SUPERFLEX process6 for light olefins production is licensed by Lyondell Chemical Company. The process uses a fluidized catalytic reactor system with a proprietary catalyst to convert Special Issue: IMCCRE 2010 Received: May 4, 2010 Accepted: October 25, 2010 Revised: October 10, 2010 Published: December 15, 2010 2726
dx.doi.org/10.1021/ie100982q | Ind. Eng. Chem. Res. 2011, 50, 2726–2735
Industrial & Engineering Chemistry Research low-value, olefin-rich feedstocks preferably into carbon C4 to C8, and valuable propylene and ethylene products. The fluidized reactor system, very similar to a normal fluid catalytic cracking (FCC) unit, is composed of a riser reactor, regenerator, preheating system, and fuel gas system. Deng et al. 20027 studied a downer-type catalytic reactor (deep catalytic cracking), and concluded that this kind of reactor represents an improvement in selectivity with respect to the riser type conventionally used in FCC. Song et al. (2005),8 provided a fundamental explanation why the downer with short residence time and high-relation solid-gas produces better selectivity to olefin. UOP in 20079 introduced and demonstrated the use of millisecond technology for the fluid catalytic cracking of heavy hydrocarbon. The mechanism of reactions that takes place during thermal cracking of cracked naphtha is not very well-known. Joo and Park (2001)10 and Froment (2004),11 among others, proposed a free radical reaction mechanism base on a first stage, where free radicals are produced by the splitting of hydrocarbons at their weakest bonds, followed by the abstraction of hydrogen and isomerization of the long chain of new free radicals that propagate to form aromatics and polymerize into coke. Radicals higher than or equal to C4 stabilize themselves by splitting into their final products, and only the radicals lower than C4 are active radicals in the reactions. The chain ending occurs through association and/or disproportionation of radicals. The basic studies indicated that, by shortening the residence time, the polymerization and condensation can be reduced. Coke is formed and deposited at the walls of the pyrolysis reactor (coil) and at the walls of the quench cooler, usually called the transfer line heat exchanger. Both types of deposits are produced in greater quantities when cracked, rather than straight-run naphtha is used, limiting even more the time on stream. The simulation of coke formation is very difficult to model because the thermal and fluid dynamic conditions in the reactor cannot be reproduced using laboratory equipment. Despite that limitation, Kopinke et al. (1993)12 proposed that the coke deposition occurs through a mechanism in which that tar droplets are condensed and start to grow by reacting with active species from gas phase, reactions that might be catalyzed by metals particle. Liu et al (2009)2 investigated the pyrolysis performance of catalytic cracking and coker naphtha on different carriers, and an active catalyst was investigated in a confined fluidized bed reactor. For pyrolysis of cracking naphtha on quartz grain, the yield of total light olefins increased with the enhancement of temperature, while it varied slightly with increasing residence time and steam/ oil weight ratio. The optimal reaction temperature was around 700 °C, while the maximum yield of olefins was obtained for a WHSV of 5 h-1. The selectivity of total light olefins of coker naphtha was better than that of catalytic cracking naphtha under the optimal reaction conditions.
2. METHODOLOGY On the basis of the previous information about kinetics, thermodynamic, reactions rates, catalysts, and types of reactors the following target were selected: (1) increase the yield of olefins; (2) increase the ratio propylene/ethylene; (3) reduce the coke and aromatics production; (4) improve the heat integration; and (5) improve the cycle length and capital cost of a steamcracking process of cracked naphtha. To achieve that the new process characteristic would:
ARTICLE
Table 1. Typical Product Expected Using Coker Naphtha, Ni-La/Dolomite Solid and Steam/HC: 2a steam crack
steam crack
composition (wt %)
feed
600 °C - 4 s
700 °C - 1 s
diolefins ethylene
1.5
2.3 17
3.5 20
propylene
0.2
8
14
paraffins
80
33
26
coke
