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Energy & Fuels 2000, 14, 2-5
Mechanism of Reboiler Fouling during Reforming Glen Brons* and Irwin A. Wiehe† Exxon Research & Engineering Co., Corporate Research, P.O. Box 997, Annandale New Jersey 08801-0997 Received May 27, 1999
Friedel-Crafts chemistry pathways catalyzed by iron chloride and/or aluminum chloride were determined to be responsible for fouling observed on the shell side of a reformer reboiler unit. Characterization of the foulant material and reformate led to elucidating the mechanism and proposed mitigation routes. The reboiler solid foulant material was found to contain primarily coke (86 wt %) and inorganics (14 wt %). The inorganics are mostly iron-based materials as iron sulfide, chloride, and oxide. These corrosion products can also increase the lay-down and subsequent coking of heavy-end materials. In addition to the iron compounds, the balance of metal types are mostly ppm levels of alumina and silica. Analyses of the reformate revealed the presence of multi-ring compounds in the product. Additional evidence that aluminum chloride is present in the reformate lead to the conclusion that the heavy ends are more than likely forming via Friedel-Crafts reactions. With the significant liquid hold-up in the reboiler and skin temperatures near 500 °C, these heavy-end materials have the opportunity to lay down on the shell-side surface of the heat bundle and degrade to coke over time. The addition of a chloride trap to this reformer should eliminate or reduce this fouling while possibly also reducing system corrosion and observed ammonium chloride fouling in the stabilizer.
Introduction The primary purpose of catalytic reforming is to convert lower octane normal paraffins and naphthenes into higher octane iso-paraffins and aromatics. The catalysts commonly used today contain platinum supported on a silica or silica-alumina base. In most cases, rhenium is combined with platinum to form a more stable catalyst which permits operation at lower pressures. Platinum is thought to serve as a catalytic site for hydrogenation and dehydrogenation reactions, and chlorinated alumina provides an acid site for isomerization, cyclization, and hydrocracking reactions. The reforming catalyst activity is a function of surface area, pore volume, and active platinum and chlorine contents. This activity is reduced during operation by coke deposition and chloride loss. The activity can be restored, or the catalyst regenerated, by high-temperature oxidation of the carbon followed by chlorination.1 There are three basic types of reforming processes: continuous, cyclic, and semi-regenerative. During continuous reforming, the catalyst is regenerated continuously and can be maintained at a higher activity. The semi-regenerative reformer regenerates the catalyst depending on need and can vary between 3 and 24 months. This type requires that the reformer be taken off-stream. The third type of reformer is between these extremes. The cyclic reformer uses what is termed swing * Corresponding author. Fax: (908) 730-3198. E-mail: gbbrons@ erenj.com. † Current address: Soluble Solutions, 3 Louise Lane, Gladstone, NJ 07934. Fax: (908) 470-0939. E-mail:
[email protected]. (1) Gary, J. H.; Handwerk, G. E. Petroleum Refining, Technology and Economics, 3d ed., Marcel Dekker: New York, 1994; pp 201-230.
reactors which can be taken off-line to regenerate the catalyst without having to shut the unit down. Catalytic reforming tail reactor effluent is typically cooled through heat exchange with feed plus recycle gas, through heat exchange with other streams, and finally through airrfin/cooling water exchangers to about 100 °F. The cooled effluent is sent to a separator drum where the vapor and liquid are separated. The condensed liquid is typically sent to a stabilizer (or debutanizer) tower, where light ends are recovered. The stabilized bottoms product can be sent to tankage for subsequent motor gasoline blending, aromatics recovery, etc. Such stabilizers can experience ammonium chloride salt fouling due to the presence of both ammonia and hydrogen chloride in the separator drum liquid. The source of heat for the stabilizer reboiler can be tail reactor effluent at 900-920 °F (ca. 500 °C) as in this case study. The primary purpose of the reboiler is to heat and partially vaporize the bottoms from the stabilizer. The 900-920 °F heat supplied by the tail reactor effluent comes into the tubes (tube side) and results in very high film or skin temperatures on the shell side of the reboiler. No coking is observed on the tube side due to the presence of hydrogen still in the tail reactor effluent. The reboiler does not vaporize all of the stabilizer bottoms and, as a result, has an immediate concentrating effect on any larger molecules that might be present in the reformate. This liquid holdup in the reboiler results in exposures of the reformate to the high skin temperatures for longer times and creates an opportunity for fouling to occur.
10.1021/ef990106j CCC: $19.00 © 2000 American Chemical Society Published on Web 12/29/1999
Mechanism of Reboiler Fouling during Reforming Table 1. Foulant Composition Summary summary
wt %
C, H, N, S(organic) Fe2O3 FeCl2 FeS other total metals
86.44 11.11 0.72 1.32 0.31
The resulting fouling experienced in stabilizer reboilers can be quite severe. In this particular case, it was observed during a turnaround that the reboiler had contained substantial solids which had built up on the heat bundle of the reboiler’s shell side. The extensive fouling observed was enough that standard cleaning approaches (e.g., solvent washing) were ineffective. The bundle inevitably had to be re-tubed due to mechanical damage experienced during attempts to free the bundle from the exchanger shell. In an effort to mitigate this type for fouling, efforts were made to determine the mechanism by which it occurs. This was carried out by fully analyzing the foulant material and the reformate products. The results are presented below and discussed.
Results and Discussion Reboiler Foulant Investigation. A sample of the solids recovered from the reboiler had been retrieved and subjected to several analyses. During recovery, the sample was exposed to the atmosphere and/or not preserved. As a result, some oxidation products were expected. The sample was heterogeneous in that the pieces/chunks varied in appearance (color, texture, etc.) Pieces of each type were selected in order to obtain a representative sample. These were combined, ground using a grinding mill to -60 mesh (e250 µm) in particle size, and shaken well to homogenize the sample prior to analysis. The sample was analyzed for its elemental composition and metals contents. Thermal gravimetric analysis (TGA), microconcarbon residue (MCR), 13C NMR, scanning electron microscopy (SEM), temperature-programmed oxidation (TPO), X-ray diffraction (XRD), and neutron activation analysis (NAA) were also used to study the makeup of the sample in more detail. The results from these tests are discussed below. The resulting composition for the homogenized sample is summarized in Table 1. The types of metals present were determined and quantified using a needed combination of different techniques (i.e., inductively coupled plasma optical emission spectroscopy (ICP-OES), flame atomic absorption (FLAA), graphite furnace atomic absorption (GFAA), neutron activation analyses (NAA), and combinations thereof). These analyses are necessary because all of the metals cannot be found or quantified using only one of these tests. For example, ICP alone cannot detect the presence of Si, Re, Pt, and others. The sum of the elements and metals account for 99+ wt % of the sample. An indication that the analytical data are accurate is the oxygen level. The oxygen level measured directly by NAA is 2.99 wt %, which is in very good agreement with the oxygen level calculated by difference at 3.44 wt %.
Energy & Fuels, Vol. 14, No. 1, 2000 3 Table 2. Composition of Reformates before and after Catalyst Regeneration analyses
before
after
carbon (wt %) hydrogen (wt %) nitrogen (ppm) total sulfur (ppm) chlorine (ppm) aluminum (ppm) iron (ppm) atomic H/C ratio
88.73 11.49 90 < 10 < 20 3.5
