Chloride management in reformer product streams
A review of chloride activity in reformer operations and best practices to counter it .
HPCL- Mittal Energy Limited
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Chlorides have been a long- standing issue in catalytic reformer operation. In view of greater insights into the types of chlorides in reformer streams and their impact on units, there is increasing interest in removing all chloride species. Chlorides can be controlled through the use of appropriate adsorber/absorbent guard beds along with operational adjustments to avoid problems like formation and deposition of ammonium chloride, chloride related corrosion, poisoning of downstream catalysts, and product specification issues. Total chloride (HCl and organic chlorides) removal or control poses a different challenge and requires altered guard bed formulations.
The tracking of chlorides and use of proper analysis techniques for the measurement of HCl and organic chlorides in reformer product streams is critical to the management of chlorides in a running reformer unit to prevent associated operational issues.
The location, number, and philosophy of chloride guard beds during the design or revamp phase is a key factor to be evaluated to optimise both capital and operational expenses without compromising downstream chloride specifications. The nature and severity of the problems experienced influence the number and location of guard beds. Many refiners favour treatment of individual product streams where problems have been encountered or, in the case of a new design, where the licensor’s experience suggests that a problem is expected.
The selection of adsorbent/absorbent formulation is also an important decision with regard to effective removal of different chloride species encountered or expected in a particular unit, depending upon reformer catalyst age and how the unit is being operated. The run length between changeouts (chloride capacity) as well as cost of removal of chloride per kilogram of adsorbent are other factors which influence the selection of adsorbent.
The expected life of an adsorbent, especially in a gas phase stream, may be greatly impacted by process upsets and chloride slip may happen earlier than estimated. Units where the available design margins in the hydraulics have been exploited to operate at higher rates may experience issues like liquid slip to the adsorbent bed, higher gas hourly space velocity, and so on. This inhibits the mass transfer rate in the bed, leading to chloride slip from the adsorbent sooner than expected.
The factors leading to process upsets need to be considered and addressed to achieve maximum efficiency of chloride removal and to avoid frequent adsorbent changeouts and chloride slippage to downstream operations.
It is equally important to follow the chloride saturation of beds and to establish criteria for bed replacement to avoid undetectable slippage of chlorides as well as the side reaction chemistry of polymerisation/ green oil formation due to over- saturation of the guard bed.
Until recently, the predominant focus on preventing operational problems from chloride compounds in catalytic reformer product streams was to remove HCl. More recently, an emerging concern for many refiners has become the removal of organic chlorides. One of the contributing factors to this may be the relative difficulty of measuring organic chloride compounds accurately at low levels, especially considering the various different types of organic chloride species. HCl is comparatively easy to measure and detect more reliably. As a result, chloride guard product developments have focused on improving the capacity of HCl removal.
In the last few years, there has been increased focus on and concern for the removal of organic chlorides as well as HCl. Total chloride removal or control has also been an area for the development of new chloride guard formulations.
Organic chlorides are formed due to the presence of olefins in reformer product streams. These olefins react with HCl in the presence of an acid catalysed bed (for example, AlCl3) to form organic chlorides (see Figure 1).
The lower catalyst surface area hinders maintenance of critically important platinum dispersion. Loss of platinum dispersion increases olefin production which, along with higher HCl content, can increase organic chloride formation as the catalyst ages. Higher organic chloride formation further accelerates reduction of the chloride adsorbent’s lifetime. Catalysts with higher surface area stability extend the lifetime of downstream adsorbents, units, and equipment.
The leakage rate of chlorides from the reformer increases as the catalyst incrementally loses surface area with each regeneration. A lower surface area catalyst requires a lower H2O/HCl ratio in order to maintain constant catalyst chloride content. Chloride addition to the catalyst must be increased in the regenerator to obtain the required lower H2O/HCl ratio since the H2O concentration remains relatively constant. This increased addition of chloride to the catalyst results in an equivalent increased loss from the catalyst once back on oil that will shorten the lifetime of the chloride adsorbent.
The impact of organic chlorides becomes more severe with aging of the reformer catalyst. With aging, the olefin content of the reformer product streams increases and more HCl dosing is required to maintain the same level of chloride on the catalyst.
Partitioning of olefins happens in the gas phase as well as the liquid phase. Typically, 800-1000 ppmv olefins in net gas and 0.7-0.9 wt% olefins have been observed in reformate. However, this strongly depends upon weighted average inlet temperature (WAIT), loss in catalyst surface area, and platinum dispersion in the catalyst.
Organic chlorides are less polar than HCl and less readily adsorbed in the guard bed. They may break through months before HCl breakthrough and remain invisible in the absence of testing, leading to operational problems in downstream sections (see Figure 2).
The effectiveness of current bed formulations for organic chloride control is limited but a lot of research has been done and is being done by manufacturers for the improvement of formulations. An ideal adsorbent formulation will co-adsorb HCl and organic chlorides, offer competitive chloride capacity, minimise polymerisation or minimisation of organic chloride formation, and offer optimum cost of chloride removal.
The perfect situation is to replace the bed at organic chloride breakthrough to avoid operational and product specification problems in the downstream sections.
It is worth mentioning that if testing has indicated that HCl is not detectable in the product streams but downstream corrosion is still occurring it may be that chlorides are present but in the form of organic chlorides.
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