PETROLEUM REFINING TECHNOLOGYCHE441

PRESENTATION ON: ALKYLATION UNIT

BYGROUP 9

DEPARTMENT OF CHEMICAL ENGINEERING400 LEVEL

6TH FEBRUARY, 2013

ALKYLATION

The addition of an alkyl group to any compound is an alkylation reaction but in petroleum refining terminology the term alkylation is used for the reaction of low molecular weight olefins with an isoparaffin to form higher molecular weight isoparaffins.

ALKYLATION UNIT

It is simply the unit in the petroleum refining process where alkylation take place.

LEARNING OUTCOMES1. ALKYLATION/ ALKYLATION UNIT 2. ALKYLATION REACTIONS 3. PROCESS VARIABLES 4. ALKYLATION FEEDSTOCKS 5. ALKYLATION PRODUCTS 6. CATALYSTS 7. HYDROLUORIC ACID ALKYLATION PROCESSES -Philips Process -UOP 8. SULPHURIC ACID ALKYLATION PROCESSES -Auto Refrigeration Process -Effluent Refrigeration Process 9. COMPARISON OF PROCESSES10. ALKYLATION YIELDS AND COST11. HAZARDS AND SAFETY MEASURES

ALKYLATION REACTIONS

• The principal reactions which occur in alkylation are the combinations of olefins with isoparaffins as follows:

• The first step is the addition of a proton to the olefin to form a t-butyl cation.

This reaction with sulfuric acid results in the production of alkyl sulfates. Occasionally alkyl sulfates are called esters. Propylene tends to form much more stable alkyl sulfates than either C4 or C5 olefins.With either 1-butene or 2-butene, the sec-butyl cation formed may isomerize via methyl shift togive a more stable t-butyl cation.

These initiation reactions are required to generate a high level of ions but become less important at steady state. Typically, this can be observed as a higher rate of acid consumption initially when using fresh acid.

The t-butyl cation is then added to an olefin to give the corresponding C8 carbocation:

These C8 carbocations may isomerize via hydride transfer and methyl shifts to formmore stable cations…

Then the C8 cations undergo rapid hydride transfer as isobutane, or other species, regenerates the t-butyl cation to perpetuate the chain sequence…

Unfortunately, these are not the only reactions occurring during alkylation. There are a number of secondary reactions that, in general, tend to reduce the quality of the alkylate.

PROCESS VARIABLE OF ALKYLATIONALKYLATION PROCESS

Sulfuric acid process

Hydrofluoric acid process

ESSENTIAL PROCESS VARIABLE OF ALKYLATION

Reaction TemperatureAcid Strength

Isobutane Concentration

Reactor Pressure

Olefin Space Velocity

PROCESS VARIABLES The most important process variables are reaction temperature, acid strength,

isobutane concentration, and olefin space velocity. Changes in these variables

affect both product quality and yield.

Reaction temperature has a greater effect in sulfuric acid processes than in those using hydrofluoric acid. Low temperatures mean higher quality and the effect of changing sulfuric acid reactor temperature from 25 to 55°F (4 to 13°C) is to decrease product octane from one to three numbers depending upon the efficiency of mixing in the reactor. In hydrofluoric acid alkylation, increasing the reactor temperature from 60 to 125°F (16 to 52°C) degrades the alkylate quality about three octane numbers [16]. In sulfuric acid alkylation, low temperatures cause the acid viscosity to become so great that good mixing of the reactants and subsequent separation of the emulsion is difficult. At temperatures above 70°F (21°C), polymerization of the olefins becomes significant and yields are decreased. For these reasons the normal sulfuric acid reactor temperature is from 40 to 50°F (5 to 10°C) with a maximum of 70°F (21°C) and a minimum of 30°F (1°C).

For hydrofluoric acid alkylation, temperature is less significant and reactor

temperatures are usually in the range of 70 to 100°F (21 to 38°C).

At temperatures above 70°F (21°C), polymerization of the olefins becomes significant and yields are decreased. For these reasons the normal sulfuric acid reactor temperature is from 40 to 50°F (5 to 10°C) with a maximum of 70°F (21°C) and a minimum of 30°F (1°C). For hydrofluoric acid alkylation, temperature is less significant and reactor temperatures are usually in the range of 70 to 100°F (21 to 38°C).

Acid strength has varying effects on alkylate quality depending on the effectiveness of reactor mixing and the water content of the acid. In sulfuric acid alkylation, the best quality and highest yields are obtained with acid strengths of 93 to 95% by weight of acid, 1 to 2% water, and the remainder hydrocarbon diluents. The water concentration in the acid lowers its catalytic activity about 3 to 5 times as much as hydrocarbon diluents, thus an 88% acid containing 5% water is a much less effective catalyst than the same strength acid containing 2% water. The poorer the mixing in a reactor, the higher the acid strength necessary to keep acid dilution down [16]. Increasing acid strength from 89 to 93% by weight increases alkylate quality by one to two octane numbers. In hydrofluoric acid alkylation the highest octane number alkylate is attained in the 86 to 90% by weight acidity range. Commercial operations usually have acid concentrations between 83 and 92% hydrofluoric acid and contain lessthan 1% water. increasing the reactor temperature from 60 to 125°F (16 to 52°C) degrades the alkylate quality about three octane numbers [16].In sulfuric acid alkylation, low temperatures cause the acid viscosity to become so great that good mixing of the reactants and subsequent separation of the emulsion is difficult.

Isobutane concentration is generally expressed in terms of isobutane/olefin ratio. High isobutane/olefin ratios increase octane number and yield, and reduce side reactions and acid consumption. In industrial practice the isobutane/olefin ratio on reactor charge varies from 5:1 to 15: 1. In reactors employing internal circulation to augment the reactor feed ratio, internal ratios from 100:1 to 1000:1 are realized.

Olefin space velocity is defined as the volume of olefin charged per hour divided by the volume of acid in the reactor. Lowering the olefin space velocity reduces the amount of high-boiling hydrocarbons produced, increases the product octane, and lowers acid consumption. Olefin space velocity is one way of expressing reaction time; another is by using contact time. Contact time is defined as the residence time of the fresh feed and externally recycled isobutane in the reactor. Contact time for hydrofluoric acid alkylation ranges from 5 to 25 minutes and for sulfuric acid alkylation from 5 to 40 minutes . Although the

relationship is only approximate, Mrstik, Smith, and Pinkerton developed a correlating factor, F, which is useful in predicting trends in alkylate quality where operating variables are changed.

F= IE(I/O)F

100(SV)O

where

IE = isobutane in reactor effluent, liquid volume %

(I/O)F = volumetric isobutane/olefin ratio in feed

(SV)O = olefin space velocity, v/hr/v

The higher the value of F, the better the alkylate quality. Normal values of F

range from 10 to 40.

SUMMARY (Alkylation process variable)

Reaction temperature: If too low, increases acid viscosity & hinder good mixing of reactant. If too high, side reaction set in.

Acid strength: Its effect depends on the effectiveness of reactor mixing & H2O content of the acid.Isobutane concentration: It’s express interms of isobutane/olefin ratio. The higher the ratio the better the octane number.

Olefin space velocity: It’s the ratio of volume of olefin per hour to volume of acid in the reactor. The lower the ratio the higher the octane number.

Reactor Pressure: The reactor pressure at which the reactant are maintain in the liquid phase is sufficient.

ALKYLATION FEEDSTOCK

A feedstock is the basic material from which a good is manufactured or made.

Olefins (ranging from C3-C5) and isobutane are used as alkylation unit feedstock

Olefin feed originates from the FCC Unit.The olefin feed is also likely to contain diluents (such as

propane, n-butane and n-pentane), non-condensable (such as ethane and hydrogen) and contaminants.

Olefins can also be produced by dehydrogenation of paraffins.

Isobutane feed obtained from the hydrocracker.

The Isobutane feed to an alkylation unit can be either low or high purity.

The isobutane feed does not normally contain any significant level of contaminants.

Hydrocrackers produce a great deal of the isobutane used in alkylation but it is also obtained from catalytic reformers, crude distillation, and natural gas processing

CATALYST Concentrated sulfuric and hydrofluoric acids are the only catalysts used

commercially today for the production of high octane alkylate gasoline . The desirable reactions are the formation of C8 carbonium ions and the

subsequent formation of alkylate. The main undesirable reaction is polymerization of olefins. Only strong acids can catalyze the alkylation reaction but weaker acids can cause polymerization to take place. Therefore, the acid strengths must be kept above 88% by weight H2SO4 or HF in order to prevent excessive polymerization. Sulfuric acid containing free SO3 also causes undesired side reactions and concentrations greater than 99.3% H2SO4 are not generally used.

Isobutane is soluble in the acid phase only to the extent of about 0.1% by weight

in sulfuric acid and about 3% in hydrofluoric acid. Olefins are more soluble in the acid phase and a slight amount of polymerization of the olefins is desirable as the polymerization products dissolve in the acid and increase the solubility of isobutane in the acid phase. If the concentration of the acid becomes less than 88%, some of the acid must be removed and replaced with stronger acid.

In hydrofluoric acid units, the acid removed is redistilled and the polymerization products removed as a thick, dark ‘‘acid soluble oil’’ (ASO). The concentrated HF is recycled in the unit and the net consumption is about 0.3 lb per barrel of alkylate produced. Unit inventory of hydrofluoric acid is about 25–40 lb acid per BPD of feed.

The sulfuric acid removed usually is regenerated in a

sulfuric acid plant which is generally not a part of the alkylation unit. The acid consumption typically ranges from 13 to 30 lb per barrel of alkylate produced. Makeup acid is usually 98.5 to 99.3 wt% H2SO4.

ALKYLATION PRODUCTSIn addition to the alkylate stream, the products leaving the alkylation unit includethe propane and normal butane that enter with the saturated and unsaturated feedstreams as well as a small quantity of tar produced by polymerization reactions.The product streams leaving an alkylation unit are:1. LPG grade propane liquid2. Normal butane liquid3. C5 alkylate4. Tar Theoretical Yields and IsobutaneRequirements Based on Olefin Reacting Alkylate Isobutane vol% vol%

Ethylene 188 139Propene 181 128Butenes (mixed) 172 112Pentenes (mixed) 165 96

Only about 0.1% by volume of olefin feed is converted into tar. This is not truly a tar but a thick dark brown oil containing complex mixtures of conjugated cyclopentadienes with side chains

HYDROFLUORIC ACID ALKYLATION PROCESSES

Phillips and UOP are the two common types of hydrofluoric acid alkylation processes in use.

THE PHILLIPS PROCESS In the Phillips process, olefin and isobutane feedstock are dried and fed to a combination reactor/settler system. Upon leaving the reaction zone, the reactor effluent flows to a settler (separating vessel) where the acid separates from the hydrocarbons. The acid layer at the bottom of the separating vessel is recycled. The top layer of hydrocarbons (hydrocarbon phase), consisting of propane, normal butane, alkylate, and excess (recycle) isobutane, is charged to the main fractionator, the bottom product of which is motor alkylate. The main fractionator overhead, consisting mainly of propane, isobutane, and hydrogen fluoride (HF), goes to a depropanizer. Propane with trace amount of HF goes to an HF stripper for HF removal and is then catalytically defluorinated, treated, and sent to storage. Isobutane is withdrawn from the main fractionator and recycled to the reactor/settler, and alkylate from the bottom of the main fractionator is sent to product blending.

REACTOR

SETTLER

ACID RERUN COLUMNSETTLER

MAIN FRACTIONATOR

REACTOR

DEPROPANIZERALKYLATE

`

O LEFIN S A N D ISO B U TA N E

PROPANE N-BUTANEISOBUTANE

ISOBU

TANE

RECIRCULATIO

NPU

MP

HYD

ROCARBO

N LAYER

HF

UOP HYDROFLUORIC ACID ALKYLATION PROCESS

O LEFIN S A N D ISO B U TA N E

HF

HF

THE UOPThe UOP process uses two reactors with separate settlers. Half of the dried feedstock is charged to the first reactor, along with recycle and makeup isobutane. The reactor effluent then goes to its settler, where the acid is recycled and the hydrocarbon charged to the second reactor. The other half of the feedstock also goes to the second reactor, with the settler acid being recycled and the hydrocarbons charged to the main fractionator. Subsequent processing is similar to the Phillips process. Overhead from the main fractionator goes to a depropanizer. Isobutane is recycled to the reaction zone and alkylate is sent to product blending.

SULPHURIC ACID ALKYLATION PROCESSThis is an alkylation process which uses sulphuric acid(H₂SO₄) as a catalyst to yield paraffinic products with high octane blending components which make them suitable for automobile fuel and aviationfuel.Basically the major processes involving the use of sulphuric acid asa catalyst in the unit are

•AUTO-REFRIDGERATION PROCESS•EFFULENT REFRIDGERATION PROCESS

AUTO REFRIGERATION This process uses a multistage cascade reactor with mixers ineach stage to emulsify the hydrocarbon–acid mixture. An olefin feed or a mixture of olefin feed and isobutane feed is introduced into the mixing compartments with enough mixing energy is introduced to obtain sufficient contact of the hydrocarbon reactants and the sulphuric acid catalyst to obtain good reaction selectivity.The reaction is held at a pressure of approximately 10 psig (69 kPag) in order to maintain the temperature at about 40˚F (5˚C). To prevent vaporization of the hydrocarbons, the gases are vaporized,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 vapors are returned for recompression. The remaining liquefied hydrocarbon is sent to a depropanizer column for removal of the excess propane whichaccumulates in the system. The liquid isobutane from the bottom of the depropanizer is pumped to the first stage of the reactor. The acid–hydrocarbon emulsion from the last reactor stage is separated into acid and hydrocarbon phases in a settler.

The acid is removed from the system for reclamation, and the hydrocarbon phase is pumped through a caustic wash followed by a water wash (or a fresh acid wash followed by either caustic or alkaline water washes) to eliminate trace amounts of acid and then sent to a deisobutanizer. The deisobutanizer separates the hydrocarbon feed stream into isobutane (which is returned to the reactor), n-butane, and alkylate product

EFFULENT REFRIGERATION PROCESS: The effluent refrigeration process uses a single-stage reactor in which the temperature is maintained by cooling coils.This process is the divided into four processes •REACTION SECTION•REFRIGERATION SECTION•EFFLUENT TREATING SECTION•FRACTIONATION SECTION REACTION SECTION:In the reaction section, olefins and isobutane are alkylated in the presence ofsulfuric acid catalyst.The olefin feed is initially combined with the recycle isobutane. The olefin and recycle isobutane mixed stream is then cooled to approximately 60°F (15.6°C) by exchanging heat with the net effluent stream in the feed/effluent exchangers.REFRIGERATION SECTION:This a sectionwhere the heatof reaction is removedandlight hydrocarbon are purged from the unit NET EFFLUENT TREATING SECTION:The net effluent stream from the reaction section contains traces of free acid, alkylsulfates and di-alkyl sulfates formed by the reaction of sulfuric acid with olefins. Thesealkyl sulfates are commonly referred to as “esters.” Alkyl sulfates are reactionintermediates found in all sulfuric acid alkylation units, regardless of the technology utilized. If the alkyl sulfates are not removed they can cause corrosion in downstream equipment. FRACTIONATION SECTION:The fractionation section configuration of grassroots alkylation units, either autorefrigeratedor effluent refrigerated, is determined by feed composition to the unit andproduct specifications. As mentioned previously, the alkylation reactions are enhancedby an excess amount of isobutane. In order to produce the required I/O volumetric ratioof 7:1 to 10:1 in the feed to the Contactor reactors, a very large internal recycle stream is required. Therefore, the fractionation section stream.BLOW DOWN SECTION: The spent acid is degassed,waste water ph is adjusted and acid vent stream are neutralised before been sent off-site.

Block Flow Diagram of a STRATCO® Effluent Refrigerated Sulfuric acid alkylation.

RECYCLE ISOBUTANE

PROPANE PRODUCT

n-butane PRODUCT ACID

OLEFIN FEED MAKEUP ISOBUTANE ACID

SPENT ACID FRESH ACID PROCESS ALKYLATE PRODUCT WATER

REFINERY SPENT ACID WASTEWATER TREATMENT

REFIGERATION

ALKYLATION REACTION EFFLUENT

TREATING

FRACT IONAT ION

BLOWDOWN

DEISOBUTANIZER OPERATION (DIB)

The DIB is a part of the fractionation column it is basically used for purification and separation of liquids into various components. The feed into the DIB contains excess isobutene, n- butane, C8

+ alkylate, and residual olefinic C4’s. Figure 1 below illustrates a typical DIB.

When the feed enters the DIB it is being heated and evaporation takes place. The isobutane in the mixture having the lowest boiling point (-11.7OC 0r 10.9oF) is evaporated first and it comes out as the overhead product. Some of the isobutane is refluxed back to the DIB with a reflux ratio of 11.588 for further purification and it is being recycled back to the feed stream of the alkylation unit. The n-butane having a boiling point of -0.5oC (31.1oF) comes out at the side stream of the column. The final and most important product which is the alkylate comes out at the base of the column.

COMPARISION OF THE ALKYLATION PROCESSES

Commercial alkylation catalyst options for refiners today consist of hydrofluoric (HF) and sulfuric (H2SO4) acids. In some areas of the world, HF is no longer considered an acceptable option for a new unit due to concerns over safety; however, this is not the case everywhere. Due to site-specific differences in utility economics, feed and product values, proximity to acid regeneration facilities, etc., both H2SO4 and HF alkylation technologies should be evaluated. The evaluation criteria can be divided into the following categories: Feed Availability and Product Requirements, Safety and Environmental Considerations, Maintenance, Operating Costs (Utilities and Catalyst/Chemical Costs) and Capital Investment.

Feed Availability and Product Requirements Historically, butylenes from the FCC were the traditional olefins fed to the alkylation unit. Today, alkylation units are using a broader range of light olefins

including propylene, butylenes and amylenes. Alkylate composition and octane number from pure olefins are quite different for each catalyst as shown in table 1

Table 1. Light Olefin Alkylate Octanes

RON MON HF H2SO4 HF H2SO4

Propene 91-93 91-92 89-91 90-92

Butene-1 90-91 97-98 88-89 93-94

Butene-2 96-97 97-98 92-93 93-94

Isobutene 94-95 94-95 91-92 92-93

Amylene 90-92 89-92 88-89 88-90

• Safety & Environmental Considerations• Maintenance• Operating Costs• Utility Costs• Catalyst and Chemical Costs• Capital Investment In summary, a process comparison of the alkylation processes

shows that neither has an absolute advantage over the other. From a safety and environmental standpoint, H2SO4 has a clear advantage over HF. Economics of the processes are sensitive to base conditions for feedstocks and operating conditions, as well as refined product pricing. Thus, the actual choice for a particular refinery is governed by a number of site-specific factors, which require a detailed analysis.

HAZARDS AND SAFETY MEASURES IN ALKYLATION UNIT

There are two main hazards inherent in the alkylation unit, which are:1. Large volumes of light hydrocarbon which are highly flammable and are

potential explosives if released.2. The acid catalyst (H2SO4 and HF) which is corrosive and toxic are used.

Both sulfuric acid and HF contains similar volume and similar risk but the risk associated with HF is more severe than sulfuric acid and as such HF demands stricter precautions. For this, the American petroleum institute (API) issued a recommended practice specifically for HF alkylation unit. The publication recommends that access to an HF alkylation unit be restricted due to the potential hazards of HF. No similar specification was recommended for sulfuric acid alkylation.

The major risk-related differences of sulfuric acid and HF as a catalyst in the alkylation unit lies in each compound’s VOLATILITY. HF has a boiling point of 19.4oC at atmospheric pressure and vaporizes in the event of any leak to the atmosphere.

A scientific HF tests conducted in 1986 in the Nevada desert surprised the researchers when 100% of the HF liquid released formed a dense white rolling cloud of toxic gas. The cloud expanded rapidly and toxic concentrations were measured at a distance of 5-10km from the released point. Thus, unless mitigated, an HF released in a refinery will place workers and the surrounding community in severe danger. Sulfuric acid on the other hand, is a non-volatile liquid with a boiling point of over 290oC at atmospheric pressure. A test of its behaviour when combined with isobutane in an alkylation condition were carried out in 1990, the test shows that sulfuric acid consistently remained in its liquid state and no aerosols or vapours were observed during any portion of the test. Other Oil Company like Exxon Mobil, Chevron, Amaca, Texaco etc. also carried out test on sulfuric acid which was concluded that sulfuric acid does not vapourize.

MITIGTION PROCESS OF HF

1. RAPID DEINVENTORY SYSTEM: The main aim here is to reduce the amount of HF that would be released during an accident. In the cause of an accident the acid is transferred to an empty vessel by an available system pressures or pumps. The total time to transfer the acid will depend on the size of the unit, transfer mechanism and the pipe diameters.

2. WATER SPRAY SYSTEM: There are two main benefits to a water spray system for HF, the first is derived from the ability of water to absorb HF, thus removing the HF from the released cloud. The second benefit comes from the enhanced turbulence generated by the water mixing with the released cloud. The increased turbulence adds air to the system, thereby reducing the concentration of HF in the cloud. There different types of water spray that can be used for this function they are; water curtains, fixed water monitors, portable water cannons etc.

3. ADDITION OF ADDITIVE: Over the years several efforts have been made to identify a material(s) that when added to HF would reduce the potential size of the hazard zone following a release. In general, a desirable additive would have the following characteristics :

The additive increases the ratio of HF that falls to the ground versus the HF that remains airborne during a release.

The additive dilutes the acid thereby reducing its concentration.

The additive works to reduce the vapour pressure of the HF phase, thus making the HF/ additive mixture less volatile.

The additive does not affect the alkylation efficiencies.

NOTE: The additive reduces the amount of HF that remains airborne following a release when compared to the non-additive release. This will reduce the risk of inhalation of the compound.

PERSONAL PROTECTIVE EQUIPMENT (PPE)

These are pieces of equipment or apparel used or worn to protect an individual working in an alkylation unit. Within both HF and sulfuric acid alkylation units, readily available safety showers, water hydrant and hose, and eye wash fountains should be provided and regularly inspected to assure good working order.

Other PPE to be provided includes: Face shield, goggles or acid resistance gauntlets- To protect the eyes from HF

and sulfuric acid. Acid resistance gloves- to protect the hands from HF and sulfuric acid in the

alkylation unit. Acid resistance boots and coverall- To protect the feet and body from acid

burns. Air-supplied respirator (self-contained breathing apparatus)- It supplies

oxygen or compressed air to personnel exposed to alkylation unit environment.

CONCLUSION. Definition of Alkylation unit. Alkylation reaction. Effects of the process variables: temperature, acid strength, isobutane concentrations, olefins space velocity. Feed stocks in alkylation unit Products of Alkylation units Catalysts used in Alkylation units

Comparison of processes. Hydrofluoric Acid (HF):I. Smaller and simpler reactor designs are feasibleII. Cooling water can be used instead of refrigeratorIII. Smaller settlling devices are needed for emulsion.IV. Essentially complete regeneration of hydrofluoric

acid catalyst occurs.V. There is increased flexibility of operations relative

to temperature, external ratio of isobutane to olefins

VI. There is decreased need for turbulence or agitation when acid and hydrocarbon streams are combined

Sulphuric acid

I. The entire effluents hydrocarbon stream is neutralized, no additional equipment needed unlike in hydrofluoric acid process.

II. Drying is not required, whereas in hydrofluoric acid process, drying equipment is needed.

III. Maintenance costs and the amount of safety equipment in hydrofluoric acid process are greater.

IV. Capital cost is lesser because there is no mitigation equipment.

V. Isobutane is fully used for production of Alkylate.VI. There are greater limitations on obtaining Alkylates with high

octane numbers with HF acid processVII. Safety and environmental restrictions limit the use of

hydrofluoric system in highly populated Areas.

HAZARDS AND SAFETY

• Large volumes of light hydrocarbon which are highly flammable and are potential explosives if released.

• The acid catalyst (H2SO4 and HF) which is corrosive and toxic are used.

THANK YOU FOR LISTENING!!!