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Energy & Fuels 2007, 21, 2964-2968
Improved Designs of FCC Gasoline Hydrodesulfurization Units by Properly Measuring the Olefin Content of the Gasoline Feed Gary G. Podrebarac* and Arvids Judzis, Jr. CDTECH, 10100 Bay Area BouleVard, Pasadena, Texas 77507 ReceiVed NoVember 21, 2006. ReVised Manuscript ReceiVed July 9, 2007
Hydrodesulfurization of cracked gasoline is now a vital step for the production of clean fuels. Along with sulfur removal, however, olefin saturation occurs. Knowing the olefin content of the gasoline is key to achieving a proper design, as olefin saturation largely sets the heat release and hydrogen consumption that will be experienced. More specifically, the molar concentration of the olefins must be known, and determining this in cracked gasoline is not as straightforward as it seems. There are a number of seemingly appropriate analytical methods for olefin measurement. This study examines several of the more common methods used in the refining industry and compares their performance on a sample of full-range FCC gasoline. A case is made that the bromine number is the most appropriate measurement to use as the basis for a reactor design.
Introduction The removal of sulfur from gasoline has become a vital refining step as governments have enacted laws requiring cleaner burning fuels. Of particular importance is the desulfurization of cracked gasoline, which typically contributes the bulk of the sulfur in the gasoline pool. In the capacity of a process licensor, CDTECH has gained considerable experience in the area of gasoline hydrotreating. CDTECH’s first commercial CDHDS unit to desulfurize a heavy FCC gasoline fraction came onstream in May 2000, and many more designs have followed. Olefin saturation is a side reaction that occurs as cracked gasoline undergoes hydrodesulfurization (HDS). It is an undesirable exothermic reaction that consumes hydrogen and also causes reduction of the octane rating of the gasoline product. In cracked gasoline, the sulfur content may be 1000 ppmw or 0.017 mol of S/L, and the concentration of olefins may be 3 mol of Cd/L. These are just typical figures, but they illustrate how the olefin concentration can be 2 orders of magnitude higher than the sulfur concentration. To properly design a gasoline HDS reactor, the heat balance and the hydrogen consumption rates need to be known with confidence. Given the relative concentrations of olefins and sulfur, it is clear that the olefin content of the gasoline has a greater impact on these engineering considerations than the sulfur content. Thus, it is very important to know the molar olefin concentration of the gasoline to be treated. A variety of techniques is available to determine the olefin content of gasoline, with most reporting results in vol % (or wt %) olefins in an attempt to be historically consistent with the FIA method (ASTM-D1319). During the development and commercialization of CDTECH’s FCC gasoline HDS processes, the following techniques were commonly encountered: (1) Br #: bromine number, ASTM-D1159;1 (2) PONA: 50 m high* Corresponding author. Fax: (281) 474-0660; e-mail: gary.podrebarac@ cdtech.com. (1) ASTM-D1159-93. Standard Test Method for Bromine Numbers of Petroleum Distillates and Commercial Aliphatic Olefins by Electrometric Titration; ASTM International: West Conshohocken, PA.
resolution GC, ASTM-D6733;2 (3) PIANO: detailed hydrocarbon analysis, ASTM-D5134;3 and (4) MD-GC Reformulizer method, ASTM-D6839.4 Each of these methods is a reproducible technique. However, we observed that comparisons of these various methods tend to provide inconsistent results. This becomes especially apparent when one tries to convert to molar units of concentration. Studies in the literature also compare some of the measurement techniques and also confirm that differences may occur. In 1990, Kosal et al.5 compared FIA with a capillary column technique (similar to the PONA and PIANO methods) and a multidimensional (MD) column technique (similar to the MDGC/Reformulizer) to identify paraffins, naphthenes, and aromatics. The authors were somewhat critical of the FIA method, citing long analysis times and difficulty reading the exact boundary between hydrocarbon types. In general, it was reported that the two GC methods agreed with FIA but that problems with coelution of components were observed. The coelution problem grew worse as the boiling point of the sample increased, and the authors subsequently confined the testing to those samples with an endpoint of less than 215 °C (419 °F). The multicolumn method worked quite well, even with some heavy samples beyond the 215 °C endpoint. However, the authors cautioned that this technique is quite a bit more complex than the capillary method, requiring significantly more maintenance work. No information was gathered on olefin measurements. In a study by Barman,6 a comparison was made between FIA, Br #, and an MD-GC method ASTM-D5443, commonly called PIONA. The PIONA technique showed good agreement with (2) ASTM-D6733-01. Standard Test Method for Determination of IndiVidual Components in Spark Ignition Engine Fuels by 50 Meter Capillary High Resolution Gas Chromatography; ASTM International: West Conshohocken, PA. (3) ASTM-D5134-98. Standard Test Method for Detailed Analysis of Petroleum Naphthas through n-Nonane by Capillary Gas Chromatography; ASTM International: West Conshohocken, PA. (4) ASTM-D6839-02. Standard Test Method for Hydrocarbon Types, Oxygenated Compounds, and Benzene in Spark Ignition Engine Fuels by Gas Chromatography; ASTM International: West Conshohocken, PA. (5) Kosal, N.; Bhairi, A.; Ali, M. A. Determination of hydrocarbon types in naphthas, gasolines, and kerosenes: A review and comparative study of different analytical procedures. Fuel 1990, 69, 1012-1019.
10.1021/ef060593k CCC: $37.00 © 2007 American Chemical Society Published on Web 08/29/2007
Designs of FCC Gasoline Hydrodesulfurization Units
Energy & Fuels, Vol. 21, No. 5, 2007 2965
Table 1. Summary of Analytical Techniques To Be Compared method
description
limitations
Br # D1159
known weight of sample is dissolved in a solvent, then titrated with a bromide/bromate solution electrometric detection of titration endpoint
samples need to be free of material lighter than isobutane samples should have D-86 90% distillation temperature under 327 °C (626 °F) this technique does not identify individual components some minor variation in the response of different olefins, which is quantified in Table A1.1 of the method some sulfur compounds interfere with the titration results generally, only one of the double bonds in a diene molecule is titrated
PONA D6733
determines individual hydrocarbon components of spark ignition engine fuels with boiling ranges up to 225 °C (437 °F) technique can be used for iindividual blend-stocks, but statistical data for the method were only gathered with blended fuels
benzene coelutes with 1-methyl-cyclopentene toluene coelutes with 2,3,3-tri-methylpentane when technique is used for PONA determination, user must be cautioned about errors from coelution of components procedure is applicable to samples with less than 20 mass % olefins significant coelution is possible with olefins >C7 and particularly when samples are derived from FCC gasoline; total olefin content with these samples may not be accurate
PIANO D5134
determines individual hydrocarbon components of petroleum samples sample should have an endpoint by D-3710 less than 250 °C (482 °F)
method is applicable to olefin-free liquid hydrocarbons olefin content should be
