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Nov-2019

Vent gas treatment in platforming

Unlike a conventional scrubbing system, the latest regenerator vent gas chloride treatment for platforming units does not employ caustic

CANDICE CARRINGTON
Honeywell UOP
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Article Summary
In the CCR Platforming process, low octane C6 to C11 paraffins and naphthenes are converted to higher octane aromatics and hydrogen. The process typically consists of a stack of three or four reactors, where the catalyst flows downwards from one reactor to the next by gravity. The spent catalyst is transported to the regenerator section, where it is continuously regenerated. From the regenerator, the catalyst is returned to the top of the reactor stack in ideal condition to facilitate the platforming reactions. As a result of advances in the technology, modern CCR Platforming catalysts can function for more than 10 years.

UOP is an industry leader for semi-regenerative and continuous catalytic regeneration (CCR) reforming solutions, first commercialising catalytic reforming in 1949 and first commercialising CCR in 1971. The company has developed catalytic reforming technology improvements to increase octane, yield, and hydrogen production. Currently, UOP has more than 75% segment participation in CCR reforming technology1 for the production of Euro IV/V gasoline and BTX aromatics.

CCR Platforming regenerator vent gas treatment
During the regeneration process, an organic chloride is added to the process to adjust the catalyst chloride levels and to enable the redistribution of platinum on the catalyst surface. Most of this chloride is retained by the catalyst. However, some of it exits with the vent gas from the regenerator section. It is desirable to treat this vent gas before it is released to the atmosphere to remove the chloride products for the following reasons:
• Environmental: the treatment minimises the amount of chloride compounds, such as HCl and dioxins, that are emitted to the atmosphere. This is especially beneficial in regions where environmental regulations limit emission levels. For example, in the US, the Environmental Protection Agency requires at least a 97% control efficiency on emissions, or a maximum limit of 10 vol ppm of HCl. Europe is also working continuously on improving legislation and reducing emissions, and specific environmental regulations exist in some local regions.

• Corrosion control: some of the chloride compounds in the vent gas are acidic when they contact moisture in the atmosphere. This could lead to corrosion of nearby ladders, platforms and other structures posing potential safety concerns. Corrosion of nearby equipment could reduce the reliability of that equipment.

In the earliest designs, the vent gas was treated by neutralising with a diluted solution of caustic in a multi-stage scrubbing system (see Figure 1). Acidic hydrogen chloride reacts with the sodium hydroxide to form sodium chloride and water, whilst the other chloride compounds dissolve into the aqueous solution. Then the salts and other chloride compounds are purged from the circulating caustic and this spent caustic is disposed of, requiring the addition of fresh caustic to maintain the neutralisation capability.

Regenerator vent gas chloride treatment
UOP’s Regeneration Vent Gas (RVG) Chlorsorb system is the next step in regenerator vent gas chloride treatment (see Figure 2). Like the scrubbing system, it removes chlorides from the regeneration vent gases. However, it does not require the use of caustic, so there is no need for specialist disposal arrangements for spent caustic and the continuous purchasing of fresh caustic has been eliminated. The RVG Chlorsorb process uses the platforming unit’s own catalyst as the adsorbent for the chlorine compounds so they are removed from the vent gas. The HCl is adsorbed onto the catalyst, and the dioxins are destroyed by combustion in the regenerator.

An added benefit is that consumption of fresh organic chloride is reduced because the chlorides that would have exited with the vent gases or been disposed of in the spent caustic are recycled back into the process. Table 1 compares RVG Chlorsorb and the scrubbing system and illustrates the benefits of the former.

RVG Chlorsorb now forms part of the standard offering for new CCR Platforming units and has been added during revamp projects to many units worldwide. In total, there are 85 operating units and another 41 currently in design and construction.

The first Chlorsorb unit
The Swedish Refiner, Preem, is the owner of the CCR Platforming unit that was the first to include and operate a Chlorsorb unit on a regenerator vent gas stream. In the late 1990s, there was increased focus in some parts of Europe on pollutants in water and air, and the refiner (then known as Scanraff) increased its focus on environmental emissions from the CCR Platforming unit at Lysekil. The refiner decided to revamp the unit to reduce chlorine emissions and worked with UOP to implement the RVG Chlorsorb design.

The unit requires some additional equipment, which was installed around the existing equipment whilst the unit continued in normal operation (see Figures 3 and 4). After construction, UOP’s field service and inspection team checked out the unit on-site to ensure quality and accurate construction. This is a typical UOP project deliverable to ensure successful unit performance.

When construction was approved and all punch list items were addressed, there was a short shutdown of the regenerator whilst the CCR Platforming side was kept online and the final tie-ins were performed. In March 2001, the regenerator was restarted and the first RVG Chlorsorb unit was commissioned at Lysekil.

As Figure 5 shows, regenerator emission control performed well from the outset and more than met emission requirements.

The data showed that the installation was effective at removing emissions and the results more than met the refiner’s dioxin emission requirements.

Preem continues to be satisfied with the performance of the Chlorsorb unit, and continues to pursue advancements to improve the capability, reliability, and profitability of the CCR Platforming unit.

UOP offers the technology in two ways, depending on the type of CCR unit (see Figures 6 and 7). For pressurised or CycleMax units, RVG Chlorsorb is offered as a licensed technology and engineering solution. The existing CCR unit is revamped in place, with new equipment and piping added to the existing unit. The revamp of the existing unit is performed by the customer. For atmospheric CCR Platforming units, RVG Chlorsorb is offered as a licensed technology and equipment solution. UOP provides a modular standalone Chlorsorb unit, reducing modifications to the existing unit to tie-ins.

The equipment solution provides a complete unit that is installed next to the existing structure. It provides a drop-in solution that capitalises on UOP Modular’s experience with CCR construction, offering several benefits:
• Typically, revamps of this nature are scheduled during planned turnarounds. A modular solution provides on-time certainty, which can be critical to meet turnaround objectives.
• The modifications within the existing unit are minimised, providing cost assurance for the overall implementation.
• The installation activities are minimised at the site, providing a shorter shutdown and a reduced risk of interference with the operations of the existing atmospheric CCR Platforming unit.
• Moving the majority of the fabrication works to an external shop provides improved safety and smoother site works.
Technology provided as an equipment package provides a more complete solution.
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