ARTICLE pubs.acs.org/EF
Fouling Characteristics of Hydrocarbon Streams Containing Olefins and Conjugated Olefins Zhiming Fan,† Parviz Rahimi,*,† Teclemariam Alem,† Andrew Eisenhawer,‡ and Patricia Arboleda† † ‡
CanmetENERGY, Natural Resources Canada, Devon, Alberta T9G 1A8, Canada Nalco Canada Company, Fort McMurray, Alberta T9H 4C4, Canada ABSTRACT: The fouling characteristics of a coker gas oil (CGO) containing measurable amounts of olefins and conjugated olefins were investigated using a bench-scale hot liquid process simulator and a batch autoclave reactor at different temperatures. At low surface temperatures (∼200 °C), the fouling propensity of CGO is very low, and at surface temperatures between ∼250 and 325 °C, the fouling propensity is slightly higher because of polymerization of unsaturated olefins. When the surface temperature is further increased to above 350 °C, fouling is observed to increase significantly because of familiar coking reactions. Slow polymerization of olefins and conjugated olefins was observed at temperatures of 270 and 300 °C. However, at 350 °C, thermal cracking reactions that produce olefins and conjugated olefins take place and significant amounts of fouling deposits can be formed at this temperature. The fouling deposits of CGO exhibit typical polyaromatic coke structures. A better understanding of how the various mechanisms interact with temperature will improve management of fouling in process streams containing olefins and conjugated olefins.
’ INTRODUCTION Olefins and conjugated olefins (diolefins and/or olefins conjugated to aromatics) can be formed during thermal cracking of petroleum or petroleum products. Carbonaceous fouling deposits have been observed in downstream processing units, such as heat exchangers, reboilers, fractionators, and hydrotreator reactors associated with thermal cracking. In industrial practice, condensed carbonaceous deposits are found at locations that experience high temperatures (e.g., spray nozzles in the distillation tower), gums are found in the lower temperature areas (e.g., draw trays), and mixtures of condensed and gum deposits are observed in the preheat exchangers. In addition, the fouling deposits that occur in the hydrotreater bed are suspected to result from the exposure of the hydrocarbon streams to oxygen in the intermediate tankages. The fouling mechanisms of hydrocarbon streams containing olefins and diolefins are very complex and can change with the operating environment. In addition, there is lack of suitable methods for analyzing the detailed composition of olefins and conjugated olefins in petroleum products,1-4 especially in heavy oil fractions. In recent years, there has been a lot of work5-11 investigating the fouling mechanisms of crude oil and gas oil and fouling mitigation methods; however, the literature on coker gas oils (CGOs) is very limited.12 The main reasons for fouling in oil streams are believed to be mass transfer of suspended particulates, precipitation of asphaltenes, and chemical reactions.6 Suspended particulates can originate from crude oil production (e.g., clays) or from corrosion products (e.g., FeS) from upstream transportation or processing units. Asphaltene precipitation occurs when changes in composition, temperature, or pressure destabilize the colloidal system, which is made up of saturates, aromatics, resins, and asphaltenes (SARA). Chemical reaction fouling13 can be caused by auto-oxidation associated with small amounts of oxygen, thermal decomposition at high temperature, and polymerization of unsaturated hydrocarbons. r 2011 American Chemical Society
In the absence of oxygen at high temperatures (>300 °C), thermal decomposition may lead to the accumulation of highmolecular-weight species on the wall surfaces and eventually lead to the formation of carbonaceous fouling deposits. At high temperatures, conversion of organic compounds to carbonaceous deposits can be described as14 cracking cyclization
hydrocarbon mixture sf aromatic, dehydrogenation
dealkylation
cyclic residues sf polycyclic aromatic, polycondensation
heavy residues ðsemi-cokeÞ f coke Small amounts of dissolved oxygen are believed to be the key cause of fouling in naphthas,15 gas oils,11 and crude oils.16 Watkinson et al.11 indentified soluble and/or insoluble gums, which are formed by auto-oxidation and deposited on heated surfaces, as the main fouling mechanisms for a heavy vacuum gas oil in the presence of air. Wiehe reported12 that popcorn coke, a puffed carbonaceous fouling solid, was often found in equipment downstream from processing units used to crack petroleum residues, such as cokers and visbreakers. Popcorn coke collected from the fractionators downstream of fluid coker and flexicoker was found to be isotropic, with a H/C atomic ratio of 0.65-0.75, and contained less than 1 ppmw nickel and vanadium. Oil samples collected from fractionator trays were found to contain measurable amounts of conjugated olefins (diene value around 4 g of I2/100 g). Wiehe12 believed that the popcorn fouling deposits were caused by the polymerization of olefins conjugated to aromatics. In addition, on the basis of the hypothetic fouling mechanisms of coked samples, namely, the polymerization of Received: December 7, 2010 Revised: January 11, 2011 Published: February 10, 2011 1182
dx.doi.org/10.1021/ef101661n | Energy Fuels 2011, 25, 1182–1190
Energy & Fuels olefins and conjugated olefins, a fouling mitigation strategy was proposed and proven to be effective in industrial practice. Crittenden et al.17 and Watkinson et al.18 used styrene polymerization as a model chemical system to study the fouling mechanisms associated with chemical reactions. However, the bulk and surface temperatures in their experiments were relatively low (maximum surface temperature of 230 °C), and their feedstocks were not as complex as oil systems. In this work, the fouling propensity of CGO, which contains significant olefins and conjugated olefins, was tested using a bench-scale Alcor hot liquid process simulator (HLPS) at different surface temperatures (around 200-360 °C). In addition, the CGO was treated in a 5 L batch reactor at 270, 300, and 350 °C under inert atmosphere up to 20 days. Optical micrographs of samples collected at different reaction times were examined, along with the bromine number, diene value, density, viscosity, and molecular weight of the CGO feed. As well, fouling deposits collected from the autoclave wall were characterized by elemental analysis, scanning electron microscopy (SEM), thermogravimetric analysis (TGA), and mid-infrared photoacoustic spectroscopy (PAS-IR). The possible CGO fouling mechanisms under inert conditions are discussed.
ARTICLE
Table 1. Main Physical and Chemical Properties of CGO quantity analyzed
Characterization of the CGO Sample. The main physical and chemical properties of the CGO sample were analyzed in the CanmetENERGY analytical lab following applicable American Society for Testing and Materials (ASTM) methods, in which viscosity and density are measured by ASTM D445 and D4052, respectively; molecular weights of different CGO samples were examined by vapor pressure osmometry (VPO) according to ASTM D2503. SARA analysis of CGO feed was followed by ASTM D2007M: first asphatenes were precipitated by n-pentane; then saturates and resins (polars) were separated; and whatever that is left is regarded as aromatics. Indirect methods (bromine number and diene value measurements) were used to semi-quantitatively measure the contents of olefins and conjugated olefins in the oil samples, respectively. The bromine number is obtained by ASTM D1159, which gives an indication of aliphatic unsaturation (olefins) in petroleum samples. Diene values were measured by following the UOP method 326-82. The procedure is to reflux the oil with a known excess of maleic anhydride in toluene solution. The maleic anhydride forms a water-insoluble adduct with conjugated olefins in the oil. Thus, when the product is washed with water, the unreacted maleic anhydride (watersoluble) reacts to form maleic acid. The quantity of resulting maleic acid is determined by titration with sodium hydroxide. This quantity can be used to calculate the number of moles of reacted maleic anhydride, which is equivalent to the number moles of conjugated olefins. Metals in the oil sample were detected by inductively coupled plasma-mass spectrometry (ICP-MS) analysis. Materials. CGO samples were collected from a bitumen-delayed coking unit, with a bromine number of 21.24 g of Br2/100 g and diene value of 4.48 g of I2/100 g. Similar diene values were reported by Wiehe12 for the oils from coker fractionator locations where fouling deposits formed. The boiling points of the CGO are in the range of 228-630 °C, with 15 wt % above 450 °C. It has a density of 0.983 g/cm3. The other physical and chemical properties of the CGO are given in Table 1: sulfur content of 4.76 wt % and saturates, aromatics, and polars of 26.15, 66.58, and 7.27 wt %, respectively. No C5-asphaltenes were detected in the CGO sample. The metal contents in the CGO were low (
