HF ALKYLLATION
Hanoi University Of Mining and Geology
CTTT K4
Group 10
+Lê Thanh Tuấn
+ Nguyễn Văn Khải
+Đỗ Bá Đạo
+Lê Hòa Thuận
+Nguyễn Trọng Hùng
Alkylate was found to have excellent aviation gasoline properties. It was the highest octane fuel component then known, with high motor octane and excellent lead response
Alkylation unit is one of the conversion processes used in the petroleum refineries. It is used to convert isobutane and low-molecular-weight alkenes (primarily a mixture of propene and butene) into an alkylate, a high octane gasoline component. The process occurs in presence of a strong acting acid as catalyst. The acid can be either sulfuric acid or hydrofluoric acid (HF). Depending on the acid used as catalyst the unit takes the name of SAAU (Sulphuric Acid Alkylation Unit) or HFAU (Hydrofluoric Acid Alkylation Unit). Since crude oil generally contains only 10 to 40 percent of hydrocarbon constituents in the gasoline range, refineries use a fluid catalytic cracking unit (FCCU) process to convert high molecular weight hydrocarbons into smaller and more volatile compounds, which are then converted into liquid gasoline-size hydrocarbons. Alkylation processes transform low molecular-weight alkenes and iso-paraffin molecules into larger iso-paraffins with a high octane number. FCCU is a very common unit in a modern oil refinery, but it is not uncommon for a refinery not to have an alkylation unit. Indeed, by 2010 there are some countries in the world without any installed alkylation unit.
Process chemistry
The reactions taking place in the alkylation reactor are many and relatively complex. First the olefin reacts with the acid to form an ester; then the ester is alkylated by a t-butyl carbenium ion chain mechanism.
Propylene, butylene, & pentenes are olefins used —butylene preferred.
High octane isooctane alkylate produced.
Lower reactant consumption.
Alkylation reactions have complex mechanisms & it may produce many different varieties.
Primary alkylation reactions
Most of the alkylate product is made by primary alkylation reactions. In these reactions one mole of olefin reacts with one mole of isobutane to form an isoparaffin exactly 4 carbon numbers heavier.
Feed pretreatment
In the UOP HF alkylation process olefin-rich feeds from the FCC gas plant are typically deethanized, Merox treated to remove H2S and mercaptans and dried. Some refiners have also added MTBE or Selective Hydrogenation (SHP) plants upstream the alkyl.
Reaction
After pretreatment, the olefin feeds are combined with a large excess of recycle isobutane to provide an 6–14 isobutane:olefin molar ratio and injected into the circulating HF acid catalyst at the shell side inlet to the water-cooled reactor. Cooling water flows through the reactor tubes to remove the highly exothermic heat of reaction and to maintain reaction conditions at 80–100◦F. The alkylation reaction is very fast with 100% olefin conversion. The excess isobutane, alkylate product, non-reactive hydrocarbons (propane, n-butane) in the feeds and the acid catalyst pass on to the settler vessel. The dense acid phase separates from the hydrocarbons rapidly by gravity and is then pumped back to the reactor. The hydrocarbons containing dissolved HF flow off the top of the settler to the isostripper.
Fractionation

It consists of an isostripper, a depropanizer, and an HF Stripper. The isostripper is a large tower with two sidedraws with the primary function recycling isobutane to support the high isobutane:olefin molar ratio of the reactor. The tower typically has two reboilers; the upper reboiler typically maximizes the use of relatively lowcost low or medium pressure steam and the lower reboiler uses a heating medium that can give 400–450◦F process temperatures. Alkylate is drawn off the bottom of the tower, cooled in exchangers, and sent to product storage. The next product draw up the tower is the n-butane sidedrawand above that is the isobutane recycle draw.On mostUOPunits the isobutane draw is located below the feed tray tominimize HF in the isobutane recycle.
The isostripper overhead vapor is a propane-enriched isobutane stream and HF which is condensed and separated into settling drum. The HF phase is pumped back to the reactor section. The HF saturated hydrocarbon phase is charged to the depropanizer.
The depropanizer and its associated HF stripper remove propane from the isobutane recycle. The depropanizer bottoms is returned to the reactors as part of the recycle isobutane. The depropanizer overhead containing the propane product and HF are condensed and separated in the overhead receiver. The acid phase is returned to the reactor section and the acid-saturated propane is stripped free of acid in the HF Stripper column. The HF stripper bottoms is an acid-free propane product which is treated with hot alumina to remove organic fluorides, cooled and treated with KOH pellets to remove traces of HF and water.
Acid regeneration

A key advantage of HF alkylation over sulfuric is the ability to recover the acid from the byproduct polymer, water, and other contaminants. In the UOP HF Alkylation Process, a small stream of circulating acid is stripped with superheated isobutane in a small monel tower called the Acid Regenerator. The regenerator overhead is HF and isobutane that are recycled to the reactor; the regenerator bottoms is polymer and the HF–water azeotrope which are neutralized with aqueous KOH. The neutralized polymer has good fuel value. The amount of polymer produced is generally only 1 to 2 barrels per 1,000 barrels of alkylate product.
KOH regeneration

The UOP process minimizes chemical costs by regenerating the KOH used to treat products and allwaste and stormwater. ThisKOHregeneration is accomplished using lime. As the lime is mixed into the KOH, regeneration of the KOH takes place by the following reaction:
2 KF + Ca(OH)2 → 2KOH + CaF 2
The calcium fluoride forms a precipitate and can be easily separated from the regenerated KOH.
Process variables in HF alkylation

Key variables are reactor temperature, isobutane to olefin molar ratio, acid strength, and acid to hydrocarbon volume ratio.
Reaction temperature is one of the more important process variables as it has a significant influence on the octane number of the product. Almost all HF alkylation reactors are operated below 100◦F. At higher temperatures a decrease in alkylate octane number will occur. Above 120◦F polymerization and cracking side reactions become excessive reducing alkylate quality. In many cases, acid regeneration capacity of the HF alkylation unit would not be able to maintain proper control of the acid strength at temperatures above 110◦F. Extremely lowreaction temperatures may cause incomplete alkylation. Thus reaction temperatures below 80◦F are typically not used.
Acid strength is usually kept between 85 and 95 mass% HF. Maintenance of this strength level results from a balance between the performance of the unit feed treating systems for sulfur and water removal and acid regeneration operation. In some cases oxygenate removal systems or diene removal systems are also used on the feed where there are known to be high oxygenates (such as downstream of an MTBE unit, or high diolefins (from severe FCC conditions). The action of the acid on reactions is a complex phenomenon and is dependent on the type as well as the amount of diluents.
The fresh acid is supplied by acid manufacturers at 99.0+ wt% HF. This purity is too high for optimum performance of the HF alkylation process. As the water content of the circulating acid increases, carbon steel that is not attacked by anhydrous HF, becomes less resistant to acid attack
HF feed contaminants

As with many refining processes, the control of contaminants coming into the unit with the normal feedstocks is critical to the long and dependable operation of the HF alkylation unit. Above the recommended maximum levels of feed stock contaminants, acid consumption, acid regeneration requirements, and in some cases unit corrosion and product quality are all measurably affected. Generally all contaminants are kept as low as possible within the capabilities of the feed treating systems. The major feed contaminants normally found in alkylation feeds are water or oxygenates, sulfur compounds, nitrogen compounds, non-condensibles, and diolefin.
HF alkylation maintenance

Because HF is highly corrosive to most materials, careful control and maintenance of equipment metallurgy and condition is required. Carbon steel is the primary material used for vessels and piping and it can be used only because of a corrosion barrier layer of iron fluoride that forms on any carbon steel surface exposed to HF. The iron fluoride layer is tenacious and serves as a barrier against further carbon steel corrosion as long as the deposit remains undisturbed. Under certain conditions, such as when wet acid is in the unit, this iron fluoride scale can soften and break off leading to fouling and corrosion issues. In severe cases this can lead to considerable unscheduled feed outages as well as clear safety issues.
Most refiners aggressively monitor their equipment’s remaining corrosion allowance and use regularly scheduled valve and flange replacement to head off any problems. For maintenance during an FCC turnaround, the normal time most alkylation turnarounds are maintained, many refiners choose to dissolve all the iron fluoride scale by using a chemical cleaning company
Safe handling

MEA is transported by road or rail tanker in its concentrated form. It is transferred in the normal way to an onsite storage bullet, which is blanketed by an inert gas. MEA is degraded on exposure to air. The use of this, and other enthanolamines, in the refinery processes is in a dilute form. This dilution and its onsite storage is very often in a suitably constructed pit usually in the proximity of the user plant. In some cases, a cone roof tank may be used for onsite storage. In all cases, though, the product must be kept free from exposure to air by inert gas blanketing. The dilution of MEA for use in the refinery process is between 15 and 20 wt%. The water for this dilution is usually treated boiler feed water, which is essentially free of impurities.