Alkylation | Chemical Technology - Chemical Engineering PDF Download

9.1 Introduction 

• In this lecture we present a brief overview of the alkylation process.
• 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.

9.2 Reaction Mechanism

• There are three basic reaction steps to achieve alkylation
• Step 1 involving 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.

• Step 2 involving carbonium ion intermediate formation: In this reaction, the carbonium ion formed in step 1 reacts with the olefin to produce an intermediate carbonium ion.

• Step 3 involving 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.

9.3 Reaction conditions

• To avoid olefin polymerization, high isobutane to olefin ratios are used.
• Typical isobutene to olefin ratios are 5:1 to 15:1
• Acid catalysts are used. Primarily sulphuric acid (H₂SO₄) or 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 - 20 oC using H₂SO₄ and 25 – 40 ^oC using HF
• Reaction temperature: 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.

9.4 Sulfuric acid based alkylation process technology (Figure 9.1) 

Figure 9.1 Sulphuric Acid Alkylation Unit

• 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 

9.5 HF process technology (Figure 9.2) 

Figure 9.2 HF Alkylation Processes

• 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

9.6 Technical questions

1. Why isobutane is fed to the deisobutanizer unit in the sulphuric acid flow sheet?

Ans: The isobutane feed consists of n-butane and it should be separated so that the alkylation reactor is fed with only isobutane. Now, the issue is to use a separate distillation column for separating isobutane feed into isobutane and butane. This will incur additional costs. Therefore, instead of two separate columns, both separations are carried out in the same distillation column. Ofcourse, costs will be saved by this concept in processing.

2. Explain how compression and cooling will enable the separation of propane and olefin+isoparaffins mixture? 

Ans: The separation of propane from isoparaffins and olefins is very difficult due to close boiling points of these streams. Now, compression is also required so as to avoid refrigeration system in the condenser of propane. This is also due to the fact that higher pressures increasing boiling point of the propane and enable the usage of cooling water in the condenser. On the other hand, removing heat from the system will enable adiabatic flash conditions which can be exploited to separate the propane and olefin+isoparaffin mixture.This entire operation has significant research in the field of thermodynamics and mass transfer subjects. Probably, good number of simulation studies have finally yielded the flowsheet to obtain such configurations.

3. What are the advantages of H₂SO₄ process when compared to the HF process? 

• Additonal equipment is not need in this process. This is not the case in HF process where HF recovery or neutralization requires additional processes.
• HF can form a toxic vapour cloud therefore, safety parameters need to be more effectively considered.
• Higher capital costs in HF process - Good quality product is produced in sulphuric acid process but not the HF process.

4. Why is deisobutanizer placed before the depropanizer?

Ans: Isobutane is used in excess. If propane is removed first then both the columns have high flow rates. Since column costs are proportional to the flow rates, deisobutanizer is placed before depropoanizer so that the depropanizer can process less feed and hence have lower diameters and cost. 

5. What additional units are there in the HF process flow sheet when compared to the sulphuric acid flow sheet? 

a. Drier

b. Deflourinator

c. Safety instrumentation and accessories