ARTICLE pubs.acs.org/IECR
Upgrading of FischerTropsch Waxes by Fluid Catalytic Cracking erny ‡ David Kubicka*,† and Radek C †
Research Institute of Inorganic Chemistry (VUAnCh), Department of Renewables and Environmental Technologies (UniCRE-RENTECH), Zaluzí 1, 436 70 Litvínov, Czech Republic ‡ Research Institute of Inorganic Chemistry (VUAnCh), Department of Efficient Refining Technologies (UniCRE-EFFRET), Zaluzí 1, 436 70 Litvínov, Czech Republic ABSTRACT: Synthesis gas originating from alternative feedstocks such as biomass, coal, or natural gas can become an important source of hydrocarbon-based automotive fuels in the future. FischerTropsch synthesis (FTS) affords these hydrocarbons, but upgrading of the primary FTS products is unavoidable. One option is to use the fluid catalytic cracking (FCC) process to convert FTS waxes into a wide range of products. To investigate the impact of FTS wax addition to a conventional FCC feedstock on product yields and properties, microactivity test (MAT) experiments were performed. The tests were performed at 803 K and the catalyst to feed ratio (C/F) was varied in the range 24 m/m. The feed contained 0100% of FTS wax. The addition of FTS wax resulted in increased conversion and yield of the middle distillate fraction (light cycle oil, LCO) and decreased yield of coke. Owing to the increased conversion, higher yields of propylene were obtained by catalytic cracking of feedstocks containing FTS wax. The detailed GC analysis of the products indicated enhanced properties of LCO due to the lowered content of aromatics.
1. INTRODUCTION The continuing depletion of crude oil reserves together with the increasing awareness of the impact of fossil fuels consumption on the global environment1,2 have promoted research activities focused on alternative energy resources. Alternative raw materials for production of liquid fuels that are virtually exclusively obtained by crude oil processing3 comprise natural gas, coal, and biomass. Liquid fuels form currently an indispensable energy carrier for the transportation sector owing to the existing infrastructure (comprising fuel distribution as well as fuel-driven vehicles) and their energy density. For instance, the amount of usable energy per unit weight is significantly higher (2040 times!) than in electrically driven vehicles, and liquid fuels can be also easily stored and handled.4,5 Except for vegetable oils (lipids) and sugars that can be directly converted into liquid fuels, i.e., biodiesel or renewable diesel6,7 and ethanol,8 respectively, the common route for the other raw materials (natural gas, coal, lignocellulosic biomass) to liquid fuels includes their gasification to yield synthesis gas (a mixture of CO and H2) and Fischer Tropsch synthesis (FTS) to produce hydrocarbons.913 Alternatively, synthesis gas can be converted either to methanol that is then transformed to hydrocarbons (e.g., in the methanol-togasoline (MTG) process)9,10,1416 or directly to dimethyl ether (DME)17,18 that has been proposed as an alternative diesel fuel.19 Owing to the polymerization-type kinetics of FTS the distribution of products follows the so-called AnderssonSchulz Flory (ASF) distribution. Depending on the chain-growth probability (α) a mixture of products, mainly n-alkanes having a wide range of carbon atoms in their molecules, is obtained (with the exception the two theoretical extremes, i.e., α = 0 and α = 1 that result in formation of methane and an infinite paraffin chain, respectively).13 The practical consequence of ASF distribution is that by using conventional catalysts the highest directly attainable yields of gasoline and diesel fractions are 42 and 20% (m/m), respectively.13 Hence, the achievable product distribution differs r 2011 American Chemical Society
significantly from the current as well as expected fuel demands.20 Moreover, due to the unsuitable physicochemical properties of these fractions, such as low octane number of gasoline fraction and poor cold flow properties of diesel fraction,21 they have to be further upgraded before being used as automotive fuel components. As a result of the unsatisfactory yields and properties, upgrading of FTS products is unavoidable. Apart from upgrading of FTS distillate fractions, e.g., by hydrotreating and hydroisomerization,21 FTS waxes can be produced in high yields (at α = 0.95) and then converted into desired products by cracking technologies.13,2126 Modifications of the conventional FTS catalysts by using acid supports or acid co-catalysts (e.g., zeolites) for partial cracking and isomerization of longer paraffins to enhance the yield and improve the properties of gasoline fraction were suggested as well.2831 Among the refinery cracking technologies, i.e., thermal cracking, fluid-catalytic cracking (FCC), and hydrocracking, FCC faces the most continuing shift in fuel demand, i.e., from gasoline to diesel. Even if the production of middle distillate fraction (light cycle oil, LCO) could be increased substantially with the current feedstock composition, it has inherently poor diesel fuel properties due to its high content of aromatics. The recent attempts to selectively reduce the aromatics content by ring-opening were not yet successful.3234 At the same time, the strong demand for propylene that cannot be saturated by steam cracking has opened new opportunity for valorization of propylene produced in FCC.35,36 Hence maximizing the yield of propylene and improving the properties of LCO are highly desirable. Special Issue: CAMURE 8 and ISMR 7 Received: August 31, 2011 Accepted: November 22, 2011 Revised: November 21, 2011 Published: November 22, 2011 8849
dx.doi.org/10.1021/ie201969s | Ind. Eng. Chem. Res. 2012, 51, 8849–8857
Industrial & Engineering Chemistry Research
ARTICLE
Table 1. Feedstocks Properties
description
F100W0
F0W100
F85W15
F70W30
real FCC feedstock
wax from FTS unit
15% (m/m) F0W100 +
30% (m/m) F0W100 +
85% (m/m) F100W0
70% (m/m) F100W0
density @288 K (g/cm3)
917.4
853.2
908a
898a
sulfur (mg/kg)
17 138
0
14 500a
12 000a
Conradson CR (% (m/m))
3.2
0.05
2.5
2.0
fraction C13 (508 K)
0
3.8
0
0.8
fraction 508 643 K fraction 643 873 K
8.5 61.7
37.4 53.5
12.5 58.9
16.1 56.7
residue 873 K+
29.8
5.3
28.6
26.4
SIMDIS (% wt.)
a
Calculated value.
The objective of this work is to assess the influence of variable concentrations of FTS wax in conventional FCC feedstock on conversion and the yields of FCC products as well as on product composition and properties using FCC commercial equilibrium catalyst. The catalytic cracking was performed in a microactivity test (MAT) unit while keeping constant reaction temperature and changing the catalyst-to-feed ratio.
2. EXPERIMENTAL SECTION 2.1. Experimental Setup. The catalytic cracking of wax from FischerTropsch synthesis was carried out at an experimental unit for microactivity tests (MAT). The experimental unit was a semiautomated short contact time (SCT-MAT) unit developed, designed, and constructed by Grace Davison. The unit is based on ASTM D 3907 but it was modified to achieve better accuracy and efficiency.3739 The SCT-MAT unit consists of the three main parts: injection system, reactor, and product collection system. The injection system ensures purging of the unit with nitrogen, precise feedstock conditioning, and injection. Feedstock amount in syringe (ca. 1.75 g) is covered by a 3-mL nitrogen blanket. The nitrogen blanket can compensate for pulses during the feedstock injection and ensure that all feedstock is transported through the injection system. The reactor consists of two parts: a metal block with internal capillary for feedstock preheating and injection, and a reactor body. The body contains an internal metal core with annular fixed bed of catalyst. The catalyst is diluted by glass beads of 0.20.3 mm so its volume is always the same 8.5 mL. The reactor is placed in a three-zone electrically heated furnace. The three zones enable adjustment of the temperature profile. All reaction products are captured in a glass collection system (its volume is about 650 mL) which is cooled to 291 K. The collection system ensures withdrawing homogeneous samples of liquid and gaseous products. In a typical experiment, a mixture of activated catalyst and glass beads was loaded in the reactor body and after the unit was assembled it was purged by nitrogen for 40 min. Then the whole unit was evacuated to ensure better feedstock transport, evaporation and dispersion, smooth flow of gaseous reaction products from the catalyst bed, and better temperature profile along the catalyst bed and hydrocarbons distribution in gaseous products. After that the collection system was cooled to 291 K and the feedstock injection was started. As soon as the injection was finished, the nitrogen purge was applied until atmospheric pressure was reached. Then the collection system was heated
to ambient temperature and samples of liquid and gaseous products were collected. The spent catalyst was separated from glass beads by sieving. The following experimental conditions used in this study were kept constant: reaction temperature 803 K, feed mass 1.75 g, and contact time 12 s. To vary cracking severity and thus conversion, the amount of feed and the volume of catalyst bed (8.5 mL) were kept constant while the amount of catalyst was changed. The catalyst-to-feed ratio was varied in the range 2.0, 2.4, 2.8, 3.2, and 3.6 m/m. 2.2. Feedstocks and Catalyst. The feedstock from an in eska rafinerska, a.s. (Czech dustrial FCC unit operated by C Republic) was used as a base material for preparation of feedstocks containing 15 and 30% (m/m) of FTS wax and also for comparison purposes. A sample of a commercial equilibrium FCC catalyst was used. FTS wax was prepared at an in-house experimental unit for FTS process. The unit consists of four fixed-bed reactors placed in an oil bath that makes it possible to operate the unit at nearly isothermal conditions. The catalyst volume of each catalyst bed is up to 250 mL. The FTS wax used in the MAT experiments, i.e., the fraction of FTS products that condensed in a hot separator (∼393 K), was collected from the FTS unit working under following reaction conditions: H2/CO = 2 (mol/mol), 1.9 MPa, GHSV = 750 h1, and temperature 473513 K. The catalyst, Co/Al2O3, was prepared by incipient wetness impregnation using Co(NO3)2 3 6H2O as the source of Co. The amount of the used precursor corresponded to the desired Co concentration, i.e., 15% (m/m), and the Co concentration determined by XRF analysis of the catalyst was 14.98% (m/m). After impregnation with an aqueous solution, the catalyst was dried for 2 h at 378 K and calcined at 673 K for 8 h. A commercial γ-alumina support purchased from Sasol Germany (spheres 1.8 mm) was used. The specific surface area and pore volume of the support were 186 m2/g and 0.489 cm3/g, respectively. After impregnation with Co these values dropped to 138 m2/g and 0.317 cm3/g, correspondingly. The catalyst was activated by reduction ex situ at 673 K for 24 h followed by catalyst passivation using N2/air mixture (0.521% O2) and not allowing the temperature to exceed 333 K. After loading the catalyst into the reactor, the catalyst was re-reduced in situ at 423 K for 24 h. Hence, active catalyst phase Co0 was obtained. The extent of cobalt reduction was estimated to be ca. 70% based on temperature-programmed reduction measurements. The basic physicochemical properties of FCC feedstock, FTS wax, and their mixtures are given in Table 1 together with the codes of individual feedstocks. The feedstocks were denoted 8850
dx.doi.org/10.1021/ie201969s |Ind. Eng. Chem. Res. 2012, 51, 8849–8857
Industrial & Engineering Chemistry Research
ARTICLE
Figure 1. Dependence of conversion to products with boiling point
