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Kurt Detrick (UOP)

One metallurgy issue that has been a hot topic in recent years is the specification for low Residual Elements (RE) in carbon steel for HF service. Based on recommendations made in NACE paper 03651, ASTM developed Supplemental Specifications for carbon steel that can be called out in purchase orders for steel that is to be used in HF Alky service. However, steel that meets those Supplemental Specifications has sometimes been difficult and/or expensive to procure. Recently, there have been one or two suppliers that have made a commitment to maintaining a supply of steel that meets the low RE spec, but price and availability are still a bit of a concern.

Normalization of the steel is one of the requirements listed in the ASTM Supplemental Specifications mentioned above. Many prominent metallurgists agree that Normalization of the steel is not beneficial toward resistance to corrosion from HF and it has been suggested that the requirement for Normalization be dropped from the ASTM Supplemental Specs for carbon steel in HF service.

The use of Hastelloy C-276 in HF service has increased significantly in recent years. In most cases, it does not appear to be significantly better or worse than Monel from a corrosion standpoint, but it is a much harder material than Monel and it is often easier to cast, so it is an attractive alternative to Monel in some specific cases even though the material cost is typically somewhat higher than Monel. Hastelloy C22 and C-2000 has also been successfully used in HF service on a very limited scale.

Kurt Detrick (UOP)

The n-butane stream from an alkylation unit contains some organic fluorides. The fluoride must be removed in alumina treaters before being fed to an isomerization unit because the fluoride is a catalyst poison. When the fluoride is removed from the organic fluoride molecule, an olefin is formed. There should not be any olefins in the n-butane going into the alumina treaters, so the only olefins in the product are those created from the organic fluorides. So, the key to preventing excessive olefins in the n-butane product from the HF Alkylation unit is to prevent formation of an excessive number of organic fluorides in the n-butane going to the alumina treaters. This can be done by:

- Avoiding excessively low acid purity. Organic Fluoride production can increase significantly bat acid purity below about 85% HF.

- Avoiding excessively low reactor temperature. Organic Fluoride production can increase significantly bat acid purity below about 80 °F (27 °C).

- Avoiding excessively high concentration of iso-butane in the n-butane product. The iso-butane fraction from the main fractionator in the HF Alkylation unit typically has a higher concentration of organic fluorides than the n-butane fraction. (The organic fluorides in the iso-butane stream should be kept in the recycle isobutane stream to the reactor where the organic fluorides can be converted back to HF).

Kurt Detrick (UOP)

Some advantages to alkylating amylenes are:

- It helps reduce the RVP of the FCC gasoline

- It helps reduce the RVP of the entire gasoline pool (although not as much as it might appear on the surface due to the Hydrogen Transfer Reaction)

- It reduces the total olefin content of the gasoline pool.

- It increases the volume of gasoline produced from the amylenes (or another way to look at it is that it allows the refiner to convert some iso-butane into gasoline)

Some challenges are:

- Dealing with the increased amount and type of feed contaminants (sulfur and diolefins)

- Increased ASO production – difficulty maintaining acid purity

- More olefin feed means lower iso-butane/olefin ratio for most units

- Increased iC5 production in the Alky (due to the Hydrogen Transfer Reaction). This can cause higherRVP of the alkylate or more iC5 in the n-butane produc

Jason Noe (UOP)

Normally clay downstream of extraction lasts a long time. Units utilizing glycol can damage the clay easily if there is any glycol carry over. SulfolaneTM generally does not damage clay. For the glycol processes, it is possible to use a scrubber to reduce glycol carry over to the clay. This is a standard requirement of UOP’s CaromTM units but the older UdexTM and Union Carbide TetraTM units could benefit if this is a problem for them.

Kurt Detrick (UOP)

The feed to an HF Alkylation unit typically contains about 10 – 20 wppm of Sulfur. Nearly all of the sulfur in the HF Alky feed makes ASO, which mostly stays in the acid phase in the reactor. This ASO is removed from the acid in the Acid Regeneration (or Rerun) column and so most of the sulfur typically is rejected from the unit in the ASO product stream. However, during normal operation of the unit, as much as 20% of the sulfur in the feed to an HF Alkylation unit can wind up in the alkylate, so if the feed to the HF Alkylation unit increases above the typical 10 – 20 wppm, the amount of sulfur in the alkylate willincrease. Also, the following things can cause more than 20% of the sulfur in the HF Alky feed to wind up in the alkylate product:

- Acid carryover from the Settler. Incomplete settling of acid in the settler (due to high velocities or emulsion) can cause acid to be carried over with the hydrocarbon feed to the main fractionator. This will allow the ASO to go to the bottom of the main fractionator with the alkylate and will cause higher sulfur concentration in the alkylate.

- ASO leaving the top of the Acid Regenerator or Rerun column. In some unit configurations, if the Acid Regenerator or Rerun column is over-stripping, flooding, or otherwise operating in a way that causes ASO to go out the top of the column, this ASO (with the associated sulfur) can get into the main fractionator and then it will wind up in the alkylate.

- Internal Regeneration. When internal regeneration (regeneration of the acid in the Iso stripper or main fractionator) is done, all of the sulfur in the feed will wind up in the alkylate. Some units have stopped the use of internal regeneration for this reason.