Alkylation and Polymerization Flashcards
What is Alkylation. What is the catalyst used. What is the product
Chemically combining isoparaffin with a low molecular weight olefin to form higher molecular weight isoparaffin by controlling T & P in the presence of an acid catalyst.
Sulfuric acid or hydrofluoric acid
Product is alkylate (an isoparaffin), w/ high-octane value. This is blended into motor and aviation gasoline to improve antiknock property.
Typical Product Rating
MON: 88 to 94
RON: 94 to 99.
Is Alkylation endo or exo
highly exothermic
What is the feedstock mixture in alkylation
A mixture of olefins (mainly propylene and butylene). These come from catalytic cracking and coking operations
Alkylation process variables
- Reaction temperature
- Acid strength
- Isobutane concentration
- Reaction time
Effect of reaction temperature. Typical Temperature range for each type of catalyst
Low = higher quality
Sulfuric acid: typically 40-50F (min of 30 and max of 70)
Hydrofluoric acid: 70-100F
Effect of Acid strength. Typical numbers for each catalyst type
High water conc = lower catalytic activity
Increasing S.A strength from 89 - 93 wt% increases quality by 1-2 octane numbers
In HF.A, the highest octane number is obtained in the range of 86-90 wt% acidity
Isobutane Concentration Expression and Effect. Do we want high or low? Why ?
Typical numbers
Expressed in terms of isobutane/olefin ratio.
High ratios increase octane number and yield, and reduce side reactions and acid consumption.
In industrial practice the isobutane/olefin ratio in the feed varies from 5:1 to 15: 1.
Reaction Time Expression and Definition
Expressed in two ways: olefin space velocity & contact time.
Olefin space velocity is defined as the volume of olefin charged per hour divided by the volume of acid in the reactor.
Contact time is defined as the residence time of the fresh feed and externally recycled isobutane in the reactor
Reaction Time effect of Olefin space. Typical numbers for contact time.
Lowering the olefin space velocity increases the product octane and lowers acid consumption.
Contact time for HF.A alkylation ranges from 5 to 25 mins and for S.A alkylation from 5 to 40 mins.
what is used to promote contact between acid and hydrocarbon phases
Agitation
1st step in Hydrofluoric acid alkylation unit.
How is it done. Why. What happens after.
Feed (olefin and isobutane) is dehydrated by passing them through a solid bed desiccant unit. This is done to minimize potential corrosion of process equipment. After dehydration the feed is mixed with hydrofluoric acid at sufficient pressure to maintain all components in liquid phase.
2nd step in Hydrofluoric acid alkylation unit.
The reaction mixture is allowed to settle into two liquid layers. The acid has a higher density than the hydrocarbon mixture and is withdrawn from the bottom of the settler.
Then it is passed through a cooler to remove the heat gained from the exothermic reaction. This is then recycled and mixed with more fresh feed, thus completing the acid circuit.
A small stream of acid is withdrawn from the settler and fed to an acid rerun column (5 trays 150psig) to remove dissolved water and undesired (polymerized) hydrocarbons. This rerun operation is necessary to maintain the activity of the hydrofluoric acid catalyst.
The overhead product from the rerun column is clear hydrofluoric acid which is condensed and returned to the system.
What happens to the top layer of the acid settler?
The hydrocarbon layer (top) is a mixture of propane, isobutane, normal butane, and alkylate along with small amounts of hydrofluoric acid.
These components are separated by fractionation and the isobutane is
recycled to the feed.
Propane and normal butane products are passed through caustic treaters to remove trace quantities and hydrofluoric acid.
Many of the reactor systems are similar to a horizontal shell and tube heat exchanger with cooling water flowing inside the tubes
What does the Autorefrigeration process use
The autorefrigeration process uses a multistage cascade reactor with
mixers in each stage to emulsify the hydrocarbon–acid mixture.
Describe the Autorefrigeration process (S.A alkylation unit)
Olefin feed (or mix of olefin and isobutane feed) is introduced into the mixing compartments. The reaction is held at a pressure of approximately 10 psig and a temperature of about 40 °F.
The gases (vaporized to remove the heats of reaction) are compressed and liquefied.
A portion of this liquid is vaporized in an economizer to cool the olefin hydrocarbon feed before it is sent to the reactor.
The remainder of the liquefied hydrocarbon is sent to a depropanizer column for removal of the excess propane.
The liquid isobutane from the bottom of the depropanizer is pumped to the first stage of the reactor.
What happens at the last stage of the reaction
The acid–hydrocarbon emulsion from the last reactor stage is separated into acid and hydrocarbon phases in a settler.
The hydrocarbon phase is pumped through a caustic wash to eliminate trace amounts of acid and then sent to a deisobutanizer.
The deisobutanizer separates the hydrocarbon feed stream into isobutane (returned to the reactor), n-butane, and alkylate product.
What does polymerization do, what is the feed.
What are the stats
Capital cost lower or higher?
Propylene and butylene (olefins) can be polymerized to form a high octane product (an olefin) in the gasoline boiling range.
For a similar olefin feed, polymerization produces almost 50% of the gasoline produced by alkylation, and the product has a high octane sensitivity (unleaded 97 RON & 83 MON).
But capital and operating costs are much lower than for alkylation.
Common catalyst for polymerization
Phosphoric Acid on an inert support
What happens to the feed in polymerization (2)
What happens after reaction (1)
It is pretreated to remove sulfur and reduce water concentration to about 400 ppm.
The olefin feed stream is heated to about 400 °F (204 °C) and passed over the catalyst bed. Reactor pressures are about 500 psig (3450 kPa).
After leaving the reaction, the product is fractionated to separate butane and propane form polymer gasoline.
Is polymerization endothermic or exothermic.
How is temperature controlled
exothermic
Temp is controlled by injecting a cold propane quench or by generating steam.
Additionally the propane and butane in the feed act as heat sinks to help control the rate of reaction.
