Choosing a selective hydrogenation system
Data has shown the effectiveness of palladium catalyst for the removal of diolefins and acetylenes in steam cracking as well as catalytic cracking
Ronald G McClung and Steve Novalany, BASF Catalysts
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There are many applications for selective hydrogenation catalysts and processes within the petrochemical and petroleum refining industries. The common source for many of these applications is a cracking process producing olefinic streams containing diolefins and/or acetylenes that require removal by selective hydrogenation before further processing. For the petrochemical industry the steam cracker is the source of these olefinic streams and for the petroleum refining industry the source is the catalytic cracker.
The most common steam cracker selective hydrogenation applications include acetylenes/diolefins removal form C2s and C3s, vinyl acetylene and butadiene removal from C4s and diolefin removal from C5+ pyrolysis liquids.
In the petroleum refining industry where gross hydroprocessing is practised to make cleaner fuels, the most common selective hydrogenation applications are for the removal of diolefins from either a C4 or C5 olefin stream. The catalyst technology applied to the treating of the C4 and heavier streams is similar if not the same for both industry segments. Even though base metal catalysts are used at times for selective hydrogenation, the majority of applications are based around the use of a palladium catalyst.
There are some principles around catalyst and reactor system selection that are common to both the petrochemical and petroleum refining applications for selective hydrogenation. In this article, guidelines are provided for both catalyst and reactor system selection for selective hydrogenation of C4 and heavier streams. The effect of contaminants, reactor flow direction, hydrogen /reactant distribution, inlet distributor design, palladium loading as well as palladium distribution are discussed.
The most frequently used reactor system for selective hydrogenation applications is a fixed-bed system with hydrogen and liquid always co-current but with flow direction (down-flow vs up-flow) being the major question. Even for the process in which a substantial exotherm is experienced, a fixed-bed reactor with attendant product recycle is preferred over a tubular design for its simplicity in loading, in normal operation and in lower initial capital cost.
The primary criterion for choosing flow direction in a selective hydrogenation reactor system with liquid and vapour mixed at the reactor inlet conditions is gas/liquid molar ratio entering the reactor system. Intimate mixing of hydrogen and reactants at the point of reaction for each level in the reactor bed is essential to making the best use of the palladium catalyst as well as providing good two-phase flow distribution.
Down flow reactors
A state-of-art distribution tray is needed for the down-flow reactor system at possibly more than one elevation within the bed. These reactor systems are commonly called trickle bed reactor systems because the liquid must move at a slow rate over the catalyst particles while the hydrogen is co-currently moving through the bed void space. These reactors work well in a narrow range of flows around the original design rates. However, as a practical matter, most operating units in a refinery or petrochemical plant are subject to “capacity creep” resulting in process units being pushed well beyond their nameplate capacities.
Of course, as the liquid flow increases above the original design rate, the voids are gradually filled with liquid making it increasingly difficult to maintain good gas/liquid contact. This maldistribution of reactants leads to poor catalyst utilisation and potentially much shorter process run lengths. Nonetheless, the down-flow reactor system does have significant advantages, including lower reactor pressure drop with un-fouled catalyst bed and easier dumping for maintenance purposes.
However, the down-flow design has several disadvantages. For example, allowable liquid rates well above design rates are limited. Operating at elevated liquid rates requires a much more complex hydrogen distribution system, including multiple injection points along the length of the catalyst bed. For the most common, single-point hydrogen injection systems, achieving good hydrogen distribution is practically impossible when hydrogen rates are required in excess of that dissolved at reactor conditions.
Poor hydrogen distribution leads to inadequate diolefin hydrogenation. Consequently, at certain points in the reactor bed, polymer forms causing reduced catalyst accessibility and eventually excessive pressure drop. Polymer formation leads to shortened process cycles because of premature catalyst ageing or high reactor pressure drop. Additional equipment for high temperature hydrogen stripping is required for removal of polymer in order to extend the useful life of the catalyst.
In many cases, if a down-flow reactor operation is experiencing poor performance due to the previously noted mal-distribution, the reactor system can be converted to an up-flow reactor system to resolve the problem. Designs are offered for conversion to an up-flow operation requiring only a change of the external piping and no vessel welding. Having licensed over 30 selective hydrogenation process units since the early 1980s, Engelhard has a strong preference for an up-flow reactor design for reactor inlet gas/liquid molar ratios of 0.2 or less.
A typical flow diagram for an up-flow licensed unit is shown in Figure 1. The preference for up-flow design at low gas/liquid ratios (<0.2) is supported by the long first cycles that have been achieved in operations for the removal of trace diolefins from a butene or pentene stream. Good contacting of hydrogen and reactant and uniform gas distribution help minimise oligomers and polymers that otherwise would form, thereby shortening the cycle.
Eight-year first cycles are not uncommon in C4 and C5 operations. This rationale readily extends to higher carbon number selective hydrogenation applications processes as long as the gas/liquid ratio is in the recommended range of 0.2 or less. A summary of the merits and demerits of the up-flow reactor system design versus the down flow design are:
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