Alkylation and Polymerization

Alkylation & PolymerizationAlkylationAlkylation is the process of producing gasoline range material (alkylates) from olefins such as propylene, butylenes, amylene and isobutene.The process combines an unsaturated light hydrocarbon with isobutane to produce alkylate.

(Source: U.S. Energy Information Administration)

In practice only isobutane is used because isopentane has a sufficiently;High octane number Low vapor pressure to allow it to be effectively blended directly into finished gasolines.Either sulfuric or hydrofluoric acid is used as the catalyst for the alkylation reaction. Alkylate is high in octane but has low volatility and can be added to motor gasoline.High octane hydrocarbons are needed to help prevent auto ignition of gasoline (knocking) in an engine.Each catalyst also has different operating conditions (like temperature and pressure) along with different safety considerations.Sulfuric acid is a liquid at unit operation conditions, while hydrofluoric acid is a gas at unit operating conditions.The current trend towards elimination of methyl tertiary butyl ether (MTBE) has resulted in in increased attention to alkylation technology.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 the reaction of low molecular weight olefins with an isoparaffin to form higher molecular weight isoparaffins.

PolymerizationPolymerization in petroleum refining is the process of converting light olefin gases including ethylene, propylene, and butylene into hydrocarbons of higher molecular weight and higher octane number that can be used as gasoline blending stocks. Polymerization combines two or more identical olefin molecules to form a single molecule with the same elements in the same proportions as the original molecules. Polymerization may be accomplished thermally or in the presence of a catalyst at lower temperatures.Both processes used to make gasoline components from materials that are too light to be otherwise used in gasoline.

Catalyst (H2SO4 & HF)In alkylation processes, it using hydrofluoric or sulfuric acids as catalystsFrom a safety and environmental standpoint, H2SO4 has a clear advantage over HF. In some areas of the world, HF is no longer considered an acceptable option for a new unit due to concerns over safety. HF is an acute poison that may immediately and permanently damage lungs and the corneas of the eyes.

Source:www.dupont.com alkylation process

Table 1:Light Olefin Alkylate Octane 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 from pure olefins are quite different for each catalyst as shown in Table 1.Catalyst and chemical costs

Catalyst and chemical costs favor HF units, with the main difference being acid cost. Although HF is more expensive, much less is used and can be regenerated on site. The operating cost of H2SO4 alkylation depends heavily on reactor design, feed pretreatment, residual contaminants, and the cost and availability of H2SO4 regeneration. Presently, refiners can either regenerate the catalyst on site or send it to an outside regenerator. H2SO4 vs. HF SummaryFrom a safety and environmental standpoint, H2SO4 has a clear advantage over HF.The actual choice for a particular refinery is governed by a number of site-specific factors, which require a detailed analysis. UNIT INVESTMENT UTILITY COSTS CATALYST & CHEMICALS SAFETY/ENVIRONMENTAL CONSIDERATIONS PRODUCT QUALITY FEED TYPE/ISOBUTANE AVAILABILITY production

Typical modern refinery processes for producing gasoline blending components are given below:Catalytic Naphtha Reforming- converts saturated, low octane hydrocarbons into higher-octane products containing about 60% aromatics.Fluidised Catalytic cracking FCC- breaks larger, higher-boiling hydrocarbons into gasoline range product containing 30% aromatics and 20-30% olefins.Isomerisation- raises gasoline fraction octane by converting straight chain hydrocarbons into branched isomers.

Alkylation- reacts gaseous olefin streams with isobutane to produce liquid high octane iso-alkanes.

Among all the options for lead phase out,Catalytic Naphtha ReformingandFluidized Catalytic Crackinghave been the most commonly employed processes in refineries to provide gasoline blending high-octane components.

Octane Enhancement

Olefin polymerizationOlefin polymerization to - obtain polymer gasoline with good octane numbers.RON of the polymer gasoline product < reforming and alkylation. Comparatively poor quality but for the sake of enhancing octane number, polymerization is carried out.Polymer gasoline product quality : Polymerization < alkylation unit. Typical feedstock for polymerization process are C3 and C4 olefins that are obtained from catalytic cracking.The end product from polymerization reactor is a dimer or a trimer of the olefins.Polymerization combines two or more identical olefin molecules to form a single molecule with the same elements in the same proportions as the original molecules. Polymerization may be accomplished thermally or in the presence of a catalyst at lower temperatures.

Polymerization in petroleum refining.

Caustic wash: C3-C4 olefin feed subjected to caustic wash to remove H2S and other sulphur compounds (such as mercaptans). These tend to poison the catalyst.Water scrubbing: Eventually water scrubbing is carried out to remove dissolved impurities and generate waste water.

Polymerization reactor: The reaction mixture is heated, compressed and fed to a polymerization reactor. The reactor design is a shell and tube type design where catalyst is placed in the tube for the reaction to take place and cooling water is circulated in the shell side to control the temperature increase due to the exothermic reaction.

Fractionation: Subsequently, the reactor product is fed to a depropanizer and debutanizer to produce propanes, butanes and polymer gasoline. The polymeric product is further stabilization using hydrogenation stabilizer which converts any freely available double bonds to single bonds. The end product is polymer gasoline.

The propane produced is partially recycled to the reactor and the other part taken out as a product.

Reaction mechanism Comprises of four basic steps

Carbonium ion formation : Here, olefin reacts with acid catalyst to yield carbonium ion.

Additon reaction : Carbonium ion reacts with olefin to generate intermediate carbonium ion.

Regeneration : The intermediate carbonium ion converts to the dimer and generates back the proton on the catalyst surface.

Isomerization : Straight chain proton substituted olefins convert to isomeric carbonium ions.

Operating conditionsCatalysts used: Acid catalysts (H2SO4 ) are used.

Temperature: 150 220oC are used. Too high temperatures give tar deposits.

Pressure: 25 100 atms.

Polymerization ties two or more olefins together to make polymer gasoline. The double bond in only one olefin is changed to a single bond during each link between two olefins. This means the product will still have a double bond. For gasoline, these polymer stocks are good for blending because olefins tend to have higher octane numbers than their paraffin homologs.

http://nptel.ac.in/courses/103103029/11ALKYLATIONThe addition of an alkyl group to any compound is an alkylation reaction but in petroleum refining terminology the term alkylation is used for upgrade light olefins (from FCC and cokers viz breakers) and isobutene into a highly branched paraffins. In an alkylation process, olefins are reacted with isoparaffins to yield alkylate product.The basic purpose of alkylation is to enhance the octane number of the feed stock.For instance, octane number of butane alkylate is about 92 97. This is due to the formation of a hydrocarbon with side chain arrangement of carbon and hydrogen atoms.Although alkylation can take place at high temperatures and pressures without catalysts, the only processes of commercial importance involve low temperature alkylation conducted in the presence of either sulphuric or hydrofluoric acid.

Reaction Mechanism

Three basic reaction steps to achieve alkylation

Carbonium ion formation: In this reaction, alkene reacts with a proton (acid catalyst) to produce a proton substituted olefin. The proton substituted olefin reacts with isoparaffin to generate a reactive carbonium ion and alkane.

Carbonium ion intermediate formation: In this reaction, the carbonium ion formed in step 1 reacts with the olefin to produce an intermediate carbonium ion.

Regeneration of carbonium ion: In this reaction, the intermediate carbonium ion reacts with the isoparaffin to produce alkylate product and carbonium ion. Thus carbonium ion is again regenerated to take part in step 2 reactions along with other additional unreacted olefin molecules.

Reaction conditions

To avoid olefin polymerization, high isobutane to olefin ratios are used.Typical isobutene to olefin ratios are 5:1 to 15:1Acid catalysts are used. Primarily sulphuric acid (H2SO4) or Hydrofluoric acid (HF)are used.Depending on the acid catalysts choosed the process complexity varies. We present both process technologies to indicate the pertinent process complexity.Reaction operating temperature: 10 - 20C using H2SO4 and 25 40C using HFReaction pressure: 4.4 bar for H2SO4 and 7.8 bar for HF.When H2SO4 is used refrigeration is used.When HF is used, refrigeration is not used.Sulfuric acid based alkylation process technology

Caustic wash: The feed mixture (olefin + C4 compounds) are first subjected to caustic wash. During caustic wash, sulphur compounds are removed and spent caustic is recycled back to the caustic wash. Fresh caustic solution is added to take care of the loss.Refrigeration: The olefin feed enters a refrigeration unit to reduce the feedstock temperature.Alkylation reactor: The reactor is arranged as a series of CSTRs with acid fed in the first CSTR and feed supplied to different CSTRs. This arrangement is for maximizing the conversion.In the alkylation reactor it is important to note that the olefin is the limiting reactant and isoparaffin is the excess reactant.

The alkylator unit therefore will have two phases in due course of reaction namely the olefin + isoparaffin mixture which will be lighter and the alkylate stream which will be heavier and will be appearing as a bottom fraction if allowed to settle.

Since excess isoparaffin is used, the isoparaffin can be easily allowed as a bypass stream.

Eventually, the alkylate product from the last reactor will be taken out as a heavy stream.

Thus, the alkylation reactor produces two streams. These are (a) isoparaffin rich organic phase and (b) alkylate rich phase along with acid and isobutane phases.

These streams should be subjected to further purification.

Phase separator: It so happens that the acid enters the organic rich stream and will be subjected to phase separation by settling. Similarly, the olefin/isoparaffin mixture will be also separated by gravity settling. Thus the phase separator produces three streams namely (a) olefin + isoparaffin rich phase (b) acid rich stream (c) alkylate rich stream.

Olefin + Paraffin processing: The olefin + paraffin stream is first subjected to compression followed by cooling. When this stream is subjected to throttling and phase separation, then the olefin + paraffin rich stream will be generated. The propane rich stream from this stream is generated as another stream in the phase separator.

Propane defractionator: The propane rich stream after cooling is fed to a fractionator where propane is separated from the olefin+isoparaffin mixture. The olefin+isoparaffin mixture is sent back to mix with the olefin feed.

Caustic wash for alkylate rich stream: The caustic wash operation ensures to completely eliminate acid concentration from the alkylate.

Alkylate fractionation: The alkylate is fed to a distillation column that is supplied with isobutane feed and alkylate feeds to produce isobutane as a top product and alkylate + butane mixture as a bottom product.

Debutanizer: The debutanizer separates butane and alkylate using the concept of distillation.

Hydrofluoric ACID ALKYLATION PROCESS

The process is similar to the sulphuric acid plant. However, additional safety issues make the process complex.The feed is first subjected to drying followed by pre-cooling.

After pre-cooling the reaction mixture the reaction mixture is fed to a reactor.

Unlike CSTRs in series here impeller reactors are used. The reactor consists of cooling tubes to absorb the heat generated.

The reaction products enters a settler where oil and the HF are separated.

Since there can be traces of HF in the oil rich phase and vice-versa additional processing is followed.

The HF rerun column removes traces of oils from the bulk of the HF. Thus HF purified will be recycled back to the reactor. The bottom product thus generated in this unit is acid oils.

A HF stripper is used to remove the HF in lower quantities from the alkylate product. Eventually, the HF stripper produces HF that is sent back to the reactor and the alkylate product.

The alkylate product is sent to a deisobutanizer and depropanizer units. The final alkylate product is produced by using a deflourinator which is basically a caustic wash or adsorption unit. Finally n-butane + alkylate is produced as the bottom product.

http://nptel.ac.in/courses/103103029/9Safety and Health

Alkylation and Polymerization in RefineriesExtremely Hazardous Chemical ProcessesU.S. refineries use very large quantities of Hydrofluoric Acid (HF) in alkylation and Sulfuric Acid (SA) for both alkylation and polymerization Some have >1/2 million pounds on site of chemical catalystHF readily vaporizes in the atmosphere. A large release can form a vapor cloud that can travel great distancesSA is a highly corrosive acid and exist in liquid form at atmospheric pressure. Can cause chemical and thermal burn if in contact33Alkylation and Polymerization CatalystHF and SA Extremely Toxic OSHA & EPA regulate as highly toxic a Toxic Inhalation Hazard (TIH) Damages eyes, skin, nose, throat, respiratory system and bonesFast acting, can cause deep, severe burns and can cause permanent damageHigh concentrations are immediately dangerous to life and health (IDLH 30 ppm) Serious exposures require a knowledgeable health practitioner to administer antidote calcium gluconate as soon as possible after exposure34Recent ReleasesHF Related Oil Industry Incidents Marathon Canton, OH (February 23, 2011) release of 145 pounds of HF CITGO Corpus Christi, TX (July 19, 2009) explosion, fire, HF release critically injured oneSunoco Philadelphia, PA (March 2009) HF release sends 13 workers to hospitalGiant Industries, Ciniza, NM (April 4, 2004) 4 workers seriously in fire on HF unit35CITGO Incident

Citgo, Corpus Christi, TX. July 2009 explosion and fire in the HF alkylation unit severely injured one worker and burned for two days. According to the CSB investigators, about 10% of the estimated 42,000 pound release traveled beyond the refinery fenceline.36Most Recent37

HF release in South Korea, Gumi National Industrial Complex in the southern city of Gumi. Five workers killed, 18 injured, 3,000 treated for exposure. Numerous cattle and crops were affected.

Aftermath

Health and Safety Considerations for Alkylation Fire PotentialAlkylation units are closed processesHowever, the potential exists for fire should a leak or release occur that allows product or vapor to reach a source of ignition.

SafetyFor chemical: Sulfuric acid and hydrofluoric acid are potentially hazardous chemicals Loss of coolant water, which is needed to maintain process temperatures, could result in disasterPrecautions are necessary to ensure that equipment and materials that have been in contact with acid are handled carefully and are thoroughly cleaned before they leave the process area or refineryImmersion wash vats are often provided for neutralization of equipment that has come into contact with hydrofluoric acid.Hydrofluoric acid units should be thoroughly drained and chemically cleaned to remove all traces of iron fluoride and hydrofluoric acid. Following shutdown, where water has been used the unit should be thoroughly dried before hydrofluoric acid is introduced.

For piping and tank:Leaks, spills, or releases involving hydrofluoric acid or hydrocarbons containing hydrofluoric acid can be extremely hazardous Care during delivery and unloading of acid is essentialProcess unit containment by curbs, drainage, and isolation so that effluent can be neutralized before release to the sewer system is consideredVents can be routed to soda-ash scrubbers to neutralize hydrogen fluoride gas or hydrofluoric acid vapors before releasePressure on the cooling water and steam side of exchangers should be kept below the minimum pressure on the acid service side to prevent water contaminationFor process units:Some corrosion and fouling in sulfuric acid units may occur from the breakdown of sulfuric acid esters or where caustic is added for neutralizationThese esters can be removed by fresh acid treating and hot-water washingTo prevent corrosion from hydrofluoric acid, the acid concentration inside the process unit should be maintained above 65% and moisture below 4%HealthThis is a closed process, exposures are expected to be minimal during normal operations. There is a potential for exposure should leaks, spills, or releases occur. Sulfuric acid and hydrofluoric acid are potentially hazardous chemicals so special precautionary emergency preparedness measures and protection appropriate to the potential hazard and areas possibly affected need to be provided. Safe work practices and appropriate skin and respiratory personal protective equipment are needed for potential exposures to hydrofluoric and sulfuric acids during normal operations such as reading gauges, inspecting, and process sampling, as well as during emergency response, and maintenance.Procedures should be in place to ensure that protective equipment and clothing worn in hydrofluoric acid activities are decontaminated and inspected before reissue. Appropriate personal protection for exposure to heat and noise also may be required.Health and Safety Considerations for PolymerizationFire potentialPolymerization is a closed process where the potential for a fire exists due to leaks or releases reaching a source of ignitionSafetyThe potential for an uncontrolled exothermic reaction exists should loss of cooling water occurSevere corrosion leading to equipment failure will occur should water make contact with the sulfuric acid, such as during water washing at shutdownsCorrosion may also occur in piping manifolds, reboilers, exchangers, and other locations where acid may settle outHealthThis is a closed system, exposures are expected to be minimal under normal operating conditions. There is a potential for exposure to caustic wash (sodium hydroxide), to sulfuric acids used in the process or washed out during shutdown Safe work practices and/or appropriate personal protective equipment may be needed for exposures to chemicals and other hazards such as noise and heat, and during process sampling, inspection, maintenance, and turnaround activities

The collection, transportation, and disposal of garbage, sewage, and other waste products.

Also encompasses management of all processes and resources for proper handling of waste materials, from maintenance of waste transport trucks and dumping facilities to compliance with health codes and environmental regulations

Objectives

Ensure environmental protectionEncourage waste into energy optionEnsure an efficient and effective waste managementTo design manufacturing process that can avoid and minimize waste Increase in reuse and recycling rates and product

Waste Water Any kind of water that has been adversely affected in quality by anthropogenic influenceSource of wastewater in refinery Process WaterBoiler feed waterCooling waterFire water Utility water

Why we need to treatContaminantConcentration (mg/l)BOD150-205 COD300-600Phenol20-200Benzene1-100Heavy Metal0.1-100Pyrene1-100Wastewater TreatmentPhysical Treatment method consists of:ScreeningSedimentationFlotationFiltrationChemical treatment:PrecipitationIon ExchangeChemisorptionBiological Treatment Method:Aerobic degradationAnaerobic degradationmicroorganism

Environmental Issues

Environmental IssuesAir IssuesUnder the Clean Air legislation 1978 states have the primary responsibility to address air-related impacts from energy development. States are required under the Act to maintain - or come into attainment with -National Ambient Air Quality Standards (NAAQS) Criteria air pollutants (ozone, CO, SO2, PM, and their precursors, including Nox and VOCs)Hazardous Air pollutants (HAPs, primarily fugitive VOC emissions from oil and gas production)Haze precursors (which include ozone, Nox SO2, and particulates)greenhouse gases (GHGs, which include CO2 and CH4)

Environmental IssuesWater IssuesWater Enactment 1920 (Act 418)produced waterdrilling fluids, cuttings and well treatment chemicalsprocess, wash and drainage watersewerage, sanitary and domestic wastesspills and leakage

Act Environmental Quality Act 1974OSHA 1994Petroleum (Safety Measure) Act 1984Petroleum Development Act 1974Gas Supply Act 1993International Act1963 Clean Air Act:Establishes levels for chemicals and particulates. Can regulate hazardous chemicals.1972 Noise Control Act1974 Safe Drinking Water ActGives EPA authority to formulate and enforce drinking water standards. Covers flushing chemicals down the drain.