circulating inventory to maximize propylene.
Catalysts are a key for HOFCC processes. Due to crack-
ing of light straight-run naphtha, condensate naphtha, tight oils and olefinic feedstocks from other refinery and petro- chemical processes, research is developing different zeolites and catalysts for cracking these lighter components. The ZSM-5 additives are being investigated for higher gasoline se- lectivity and for producing propylene. This application may be suitable for producing more propylene in a diesel-model FCC operation.11
Other research is aimed at light-olefin cracking of larger al- pha olefins (C12+) for producing more propylene and ethylene.
There is additional catalyst development to crack C10+ and high-
er carbon numbers for increased C3 =/C
4
=. These zeolites and
catalysts apply in cracking the tail end of paraffinic naphthas in stand-alone naphtha cracking processes.12 In addition, kinetic
catalytic cracking models of light feeds based on this research are being developed to gain better understanding of yields and operating conditions for cracking these light feedstocks.13
CATALYTIC OLEFINS
The FCC process is used to produce propylene and eth- ylene from various feedstocks in specially designed catalytic cracking units. These new processes are directly competing with steam cracking of GOs and naphtha. Steam cracking is a thermal process, operating at 1,400+°F (800°C), and it is based on a free-radical-reaction mechanism for producing eth- ylene as the primary product. These FCC processes combine carbenium ion catalytic cracking with its β-scission mecha- nism with minimal thermal cracking to provide high yields of propylene with some ethylene.
TABLE 4 lists light-olefin yields for steam cracking and is
based on general industry knowledge. This table shows the typical ethylene and propylene yield in wt% for a pound of feed as it varies per a particular feedstock. The propylene/ ethylene (P/E) ratio indicates the selectivity of the cracking conditions to produce propylene.
The P/E ratios of 0.65 and 0.53 for GO and naphtha re- spectively indicate that heavier feeds produce a higher ratio of propylene to ethylene. Globally, GO steam cracking is being reduced due to the GO feedstocks being diverted to produce more diesel and other fuels to meet these higher-product de- mands. More ethane and less naphtha are being used in steam cracking due to increased natural gas production in the US.
To produce high quantities of ethylene and propylene, both thermal and catalytic cracking conditions must occur. These
units operate with reactor temperatures as high as 1,150°F. The reactor temperature is lower than steam-cracker furnace tem- peratures of 1,470°F (800°C). The regeneration temperature must be controlled to prevent excessive catalyst deactivation.
The key to this process is the catalyst, which provides both cracking (free radical and carbenium ion) mechanisms. This catalyst has the pore size distribution to ensure secondary C5–
C12 olefin cracking in the gasoline range material. A second pen-
tasil zeolite additive may not be needed, as is typical for most propylene processes. This catalyst has robust hydrothermal and attrition properties to successfully operate at these severe oper- ation conditions. Severely hydrotreated/mildly hydrocracked, high-H2-content VGO and resid feedstocks are required. TABLE 5
lists a comparison of light olefin yields based on pilot-plant data for catalytic olefins, FCC, HOFCC and a steam cracking unit all processing heavy oils.
The catalytic olefins processes produce high levels of ethyl- ene and propylene compared to the HOFCC. Thus, this pro- cess uses both thermal and catalytic cracking mechanisms to produce desired olefins. The high yields of ethylene and buta- diene are hallmarks of the purely thermal steam cracking. The propylene and butylenes are produced in the HOFCC process due to the carbenium ion mechanism in catalytic cracking. One observation is that some of the amylenes in the HOFCC process are converted in the propylene and ethylene.9
Converting catalytically low-value olefins in the C4–C8 car-
bon number in a separate riser on an existing FCCU or a stand alone design can achieve significant propylene and ethylene yields. Potential olefinic feedstocks are mixed butanes, FCC light naphtha, coker and visbreaker naphthas, naphtha steam- cracker pyrolysis gasoline, and other selectively hydrogenated raffinates from refinery and petrochemical complexes into pro- pylene and ethylene.14
Catalytically cracking paraffinic naphtha to produce light olefins, propylene and ethylene, and aromatics as a liquid by- product is competing with naphtha-steam cracking to produce propylene. The severe operating conditions with high reactor temperatures of +1,100°F mean different catalysts are required. P/E ratios of 0.7–2.4 compared to naphtha-steam cracking ra- tios of 0.55 P/E ratios are being produced.
AROMATICS
The HOFCC produces high yields of light olefins, resulting in reduced gasoline yields with very high aromatic composi- tions. TABLE 6 summarizes characteristics of high-aromatic
gasoline against other gasolines produced from steam cracking and continuous catalytic reforming (CCR) reformate.
TABLE 6. Concentration ranges of aromatics in gasolines11
SC pyrolysis gasoline Reformate, low-severity Reformate, high-severity Conventional FCC gasoline HOFCC gasoline
Vol% Benzene 30–40 2–6 9–12 0.5–1.5 2–5 Toluene 15–20 15–19 22–28 5–10 12–18 Xylenes, EB 5–10 16–22 22–28 2–12 22–30 C9+ aromatics 5–10 25–35 16–30 12–18 32–40 Total 65–70 60–75 75–90 20–40 60–80
84NOVEMBER 2014 | HydrocarbonProcessing.com
Petrochemicals
The HOFCC naphtha has been characterized as high-sulfur reformate. The HOFCC gasoline, as shown, is high in BTX and would require additional refining extraction and treating if the BTX is to be recovered as a petrochemical feedstock. Hydrotreat- ing the 160°F plus-naphtha would remove most of the sulfur, and the raffinates could be recycled to the FCCU or sent to a reformer.
There are discussions in the industry for further increasing xylene production in the HOFCC gasoline. At this stage, there is limited flexibility in increasing HOFCC xylenes. Increasing catalyst RE content will only slightly increase aromatics through hydrogen-transfer reactions, which is detrimental to light-ole- fins production from ZSM-5.16 The key for producing xylenes
is to maximize the conversion for propylene, which will concen- trate the aromatics in the naphtha fraction, as shown in TABLE 6.
Benzene content in HOFCC is in the 2 vol%–5 vol% range
compared to the 0.5 vol%–1.5 vol% in conventional FCC gas- oline that is becoming a concern in gasoline-blending pools to meet the current 1 vol% specification found in many coun- tries. In the US, the specification is 0.62 vol%, which makes benzene-reduction technology a must. Alkylating the benzene with ethylene may be the most cost-effective way of handling this problem. If the benzene is recovered for BTX production, then it becomes an asset rather than a concern.
Benzene production is very feedstock dependent. With
higher aromaticity, more benzene and total aromatics are pro- duced. Higher conversions will produce more benzene and to- tal aromatics in addition to concentrating them in the naphtha fraction. Benzene can increase due to cyclohexane dehydro- genation or alky benzene dealkylation. Higher RE-exchanged zeolites provide higher hydrogen transfer, and moderate zeolite-to-matrix ratios that favor benzene production. Tolu- ene production is not as affected by reactor temperatures as benzene formation as shown by an increase in the benzene-to- toluene ratio. In addition, high zeolite/matrix zeolite catalysts tend to suppress additional toluene formation.15, 17
A comparison of BTX composition in the light naphtha prod- uct from a catalytic ethylene and propylene unit, an HOFCCU, and a steam cracker—all processing heavy oil—is shown in
TABLE 7. Pilot-plant data is the source. The steam cracking feed-
stock is lighter, resulting in more benzene, and, as shown pre- viously, high ethylene and butadiene yields are due to thermal cracking reactions. However, the catalytic olefins process does show acceptable BTX, especially xylenes, for petrochemicals.
Options. A refinery that has a continuous catalytic reformer
and a high-olefin FCCU can produce large amounts of C2 to C4
olefins and BTX. Middle distillates can be sold as valuable die- sel, and the bottom of the barrel can be coked or hydrotreated and fed to cracking processes depending on the market needs. If a steam cracker for ethylene production is included, then C2+ re-
covery vs. the typical C3
+ recovery of refined products provides a
much more versatile and profitable refining platform. The FCC process will continue to play a central role in future refineries due to its ability to process a wide range of feedstocks, greatly reduce heavy fuel production, and make a very wide range of products including transportation fuels and petrochemicals.
LITERATURE CITED
1 Letzsch. W. S. and C. Dean, “How to make anything with a catalytic cracker,”
Hydrocarbon Processing, July 2014.
2 Pinho, A., et al., “Double Riser FCC: An Opportunity for the Petrochemical Industry,” 2006 NPRA Annual Meeting, March 2006, Paper AM-06-13. 3 “Milos Shell’s Ultimate Flexible FCC Technology in Delivering Diesel/
Propylene,” 2008 NPRA Annual Meeting, San Diego, March 9–11, 2008. 4 Golden, S. et. al, “Catalyst changes, downstream improvements increase FCC
propylene yields,” Oil & Gas Journal, Oct. 4, 2004.
5 Kapur, S. and R. Anil, “Catalytic Routes to Olefins Shaping the Integrated Complex Configuration,” AIChE National Meeting, New Orleans, April 2008. 6 Couch, K. A., et al., “FCC Propylene Production—Closing the Market Gap by
Leveraging Existing Assets,” 2007 NPRA Annual Meeting, San Antonio, Texas, March 2007, Paper AM-07-63.
7 Lambert, O., et al., “HS-FCC for propylene: Concept to commercial operation,”
Petroleum Technology Quarter, 1Q, 2014.
8 “Evolution of resid to propylene Axens,” Technip S&W Axens 10th FCC Forum, May 2013.
9 Swaty, E., et al., “Catalytic pyrolysis process (CPP) and it integration with a refinery and petrochemical plant,” PetroTech, 2003.
10 Xhao, X. and T. Roberie, “ZSM-5 Additive in Fluid Catalytic Cracking, Effect of Additive Leveland Temperature on Light olefins and Gasoline Olefins,” Industrial
& Engineering Chemistry, 1999.
11 Buchana, et al., “Gasoline selective ZSM-5 FCC additives; effects of crystal size, SiO2/Al2O3. Steaming and other treatments on ZSM-5 diffusivity and selectivity in cracking of hexene/octene feed,” Applied Catalysts, 2001.
12 Le Van, M., et al., “Catalytic Cracking of Heavy Olefins into Propylene Ethylene and Other Light Olefins,” Catalyst Letter, March 4, 2009.
13 Longstaff, D., “Development of Comprehensive Naphtha Catalytic Cracking Kinetic Model,” Energy & Fuels, American Chemistry Society, 2012.
14 Len, A. S. and T. Pavone, “An alternative option for producing light olefins,”
Petroleum Technology Quarter, Winter 2004.
15 Dean, C. F., “Naphtha catalytic cracking for propylene production,” Petroleum
Technology Quarter, Processing Shale Feedstocks, 2013.
16Petroleum Technology Quarter, 4Q, 2013,” p. 6.
17 Yatsu, et al., “Benzene Levels in Fluid Catalytic Cracking Gasoline,” Chapter 3,
Fluid Catalytic Cracking, Vol. III, 1994.
WARREN S. LETZSCH has 46 years of experience in petroleum refining including petroleum catalysts, refining, and engineering and design. His positions have included R&D, technical service and sales, which have led to senior management positions in sales, marketing, and technology development and oversight. He was one of the developers of the Technip/Axens R2R process, and has authored over 80 technical papers. Mr. Letzsch holds eight patents in the field of fluid catalytic cracking. He was the FCC/DCC program manager at Stone & Webster for 10 years and is now a senior refining consultant for Technip, as well as a private consultant to the refining industry.
CHRISTOPHER DEAN is an independent process engineering consultant with over 37 years in the worldwide refining business with an emphasis on high olefin fluid catalytic cracking (HOFCC) with petrochemical integration. He is the founder and principal consultant for High Olefins FCC Technology Services LLC. His worldwide refining background includes the development and commercialization of the High Severity-FCC Process, the development of several integrated refinery and petrochemical projects, catalyst technical service, process engineering, design and unit operations on a variety of refinery units. He has published or presented over 30 papers and has been issued two patents on FCC gasoline desulfurization and has three other FCC pending process patents.
TABLE 7. BTX process comparison9
Process Catalytic olefi ns FCC HOFCC Steam cracking Feedstock 70% VGO + 30% VTB 85% VGO + 15% VTB AGO Reaction temperature 1,150°F (620°C) 1,010°F (545°C) 1,470°F (800°C) C6–C8s in naphtha, wt% Benzene 4.6 1.57 37.75 Toluene 16.56 5.69 14.85 Xylene 23.73 9.96 2.92 Styrene 1.09 — 3.55
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