Removal of chloride compounds
A review of the use of chloride guard beds to treat liquid phase and gas phase streams to prevent operational problems from hard-to-detect organic chlorides, while at the same time avoiding side reactions and high partial pressures
Peter V Broadhurst
Johnson Matthey Catalysts
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Catalytic reforming catalysts are dosed with organic chloride to condition the catalyst, which leads to chloride compounds in the product streams at low ppm levels. If untreated, these chlorides, containing both hydrogen chloride (HCl) and various organic chlorides, cause operational related problems. These problems include formation and deposition of ammonium chloride, chloride related corrosion, poisoning of downstream catalysts and product specification issues. As a result, many operators install chloride guard beds to remove the chloride.
Until a few years ago, the focus on preventing operational problems from the chloride compounds in the catalytic reformer product stream was to remove HCl. Absorbents were developed to effectively remove this HCl. Even so, performance of the ad- or ab-sorbent products has proven to be variable between one catalytic reformer duty and another. This variation in performance can be observed in terms of chloride pick up, lifetime-to-breakthrough, pressure drop build-up and byproduct formation.
More recently, a growing concern for many refinery operators has become the removal of organic chloride species. These compounds are less easy to detect and measure and also are less readily ad- or ab-sorbed. The effectiveness of the available chloride guard products is limited although improvements are being made to the formulations.
At the operating conditions in the reformer, the organic chloride used for dosing the catalyst breaks down to HCl that is believed to be the active chloriding species. This catalyst treatment promotes surface acidity and enhances the isomerisation activity, thus improving the properties of the reformate product. The chloride, however, is not irreversibly bonded to the catalyst surface. Hence, the need for continuous dosing, and chloride is inevitably found in the product streams leaving the catalytic reforming reactors at low ppm levels. The chloride species was known to cause a number of problems in the separation section of the catalytic reformer and in downstream equipment and units. Examples of the problems include ammonium chloride formation and deposition, corrosion, poisoning of downstream catalysts and product speci- fication issues. Depending on the nature and severity of the problems experienced by the refinery operator, the problems may be resolved through routine maintenance, particularly if fouling and/or corrosion are the dominant issues.
Many refineries, however, have installed dedicated chloride guard beds to remove the chloride at various locations in the downstream sections of the catalytic reformer. The nature and severity of the problems experienced also influence the number and location of these guard beds.
Until a few years ago, a prevalent view was that the chloride was predomi-nantly HCl or that the problems experienced were dominated by the HCl component of the chlorides. One factor in this may have been the relative difficulty in accurately detecting organic chloride compounds at the low levels that are present, particularly as there will be a range of organo-chloride species. Comparatively, HCl is easier to measure and may have been detected more reliably. As a result, chloride guard product developments for these duties were focused on improving the capacity for HCl removal.
In the last five years, however, there has been a marked increase in concern for detection and removal of the organic chloride species and this is the subject of product development work.
Chloride related problems
Some of the locations that may suffer due to chloride attack are illustrated in Figure 1.
Both ammonium chloride formation and induced corrosion are a consequence of the HCl that is present. Ammonium chloride is formed by reaction of HCl with traces of ammonia formed from traces of basic nitrogen compounds in the catalytic reformer feed. At high temperatures, it is not an issue because the ammonium chloride readily dissociates into HCl and NH3 but once temperatures in sections of the plant fall below 100°C, the compound is stable as NH4Cl and deposits on to equipment.
In addition to the problems associated with HCl, any organo-chlorides tend to cause issues for downstream catalyst poisoning and product specification. For example, catalysts based on nickel, copper and palladium are very susceptible to rapid deactivation by chloride ions.
Locating guard beds
Chloride guard beds can be installed in a variety of locations around the cataly-tic reformer. These can be divided into gas phase duties (make gas, off-gas, recycle gas) and liquid phase duties (LPG, reformate, unstabilised reformate) and are illustrated in Figure 2.
In many respects, it seems preferable to treat the unstabilised reformate, as the chloride guard bed can be located to treat the stream at elevated temperature since the process stream is heated upstream of the stabiliser column. This ensures that the HCl and ammonia are dissociated (allowing HCl removal), eliminating the possibility of ammonium chloride issues in the stabiliser. A guard bed at this location also eliminates the need for separate beds on the offgas, LPG and reformate streams. There are some points to note if this is attempted.
Chloride guards relying on adsorption work better at low temperature as the compound to be adsorbed increasingly favours remaining desorbed from the active surface as temperature increases. Thus, a chloride guard relying on absorption (chemical reaction) must be chosen. Two-phase flow should be avoided as this will affect the performance of the chloride guard and may lead to physical breakage and resultant pressure drop problems.
The guard bed will tend to be relatively large as it will treat the total unstabilised reformate flow. This increases capital expenses (capex) and operating expenses (opex) of the bed.
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