Alkylation is a process for chemically combining isobutane with light olefinic hydrocarbons, typically C3 and C4 olefins, (e.g. propylene, butylene) in the presence of an acid catalyst, usually sulphuric acid or hydrofluoric acid. The product, alkylate (an isoparaffin) has a high-octane value and is blended into motor and aviation gasoline to improve the antiknock value of the fuel. The light olefins are most commonly available from the catalytic crackers.
Alkylate is one of the best gasoline blending components because it is a clean burning, very low sulphur component, with no olefinic or aromatic compounds and with high octane and low vapour pressure characteristics.
1. Introduction
1.1 Alkylation
Alkylation is a process for chemically combining isobutane with light olefinic hydrocarbons, typically C3 and C4 olefins, (e.g.propylene, butylene) in the presence of an acid catalyst, usually sulphuric acid (H2SO4) or hydrofluoric acid (HF). The product, alkylate (an isoparaffin) has a high-octane value and is blended into motor and aviation gasoline to improve the antiknock value of the fuel. The light olefins are most commonly available from the catalytic crackers. Alkylate is one of the best gasoline blending components because it is a clean burning, very low sulphur component, with no olefinic or aromatic compounds and with high octane and low vapour pressure characteristics [1].
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1.2 Advances in alkylation technologies
The alkylation process will continue to be a favoured technology for producing clean fuels.MTBE(methyl-tert-butyl ethanol) phase out in the USA, implementation of the latest european specifications, enlargement of the EU and adoption of cleaner fuels specifications worldwide are major drivers for refiners requiring more, high octane, gasoline blending components that do not contain aromatics, benzene, olefins and sulphur. Also as the types of gasoline engine in use worldwide become more uniform, there will be a general decline in the markets for low octane gasoline requiring more components to be upgraded to high quality fuel.
Table 1 shows the major technical and mechanical advances. Reactor design improvements are one of the most important developments. The early plants used a pump and time-tank reactor system which was designed to mix the reactants intimately with the catalyst and to remove the exothermic heat of reaction for temperature control [2] .It is required that for the desired reactions to continue with the removal of the unwanted reactions, good mixing of higher concentrations of dissolved isobutane in the acid phase is necessary. Since the early reactors were inadequate in this respect, new reactor designs evolved which improved the degree of acid-hydrocarbon contacting. The importance of good temperature control was also realized in the course of time as commercial experience was gained. Regulating the temperature of the reaction mixture in the suitable range was essential for good alkylation. Inadequate temperature control resulted in decreased alkylate yields and octanes and increased acid consumption. Therefore, to avoid these penalties the new reactor designs included improved temperature control techniques as well as improved mixing. The two most commonly used reactor systems which grew out of the reactor development work for H2SO4 alkylation are the Stratford Engineering Company’s Stratco contactor and the M. W. Kellogg Corporation Cascade reactor were bubbled up through liquid HF.
There have been improvements in the preparation of feed and this has given rise to growth in alkylation technology [4, 5]. The ability to design better fractionators has made higher quality feedstocks available, and feed pretreatment facilities have been developed to remove water, mercaptans, sulfides, and diolefins effectively. Bauxite treating, hot water washing, and electrostatic precipitation are some of the significant developments which have improved product quality and reduced fouling and corrosion in downstream equipment. The sulfuric acid recovery process (SARP), developed to reduce the acid consumption in H2SO4 alkylation units was another contribution to alkylation technology. In this process the spent acid from an alkylation unit reacts with a portion of the olefin feed to form dialkylsulfates. The dialkylsulfates are extracted from the reaction mixture with isobutane, and the extract is charged to the alkylation unit.
Table I: Advances in alkylation technology [3]
1) Improved reactors
A) better mixing
B) better temperature control
2) Recognition and control of operating variables
3) Improved feed preparation
4) Improved product treatment
5) Sulfuric acid recovery process
6) Catalyst promoters
7) Mechanical and construction improvement
2. Types of alkylation processes
The alkylation process can be divided into the sulfuric alkylation process and the hydrofluoric acid alkylation process, indirect alkylation by acidic resin, indirect alkylation by solid phosphoric acid and olefin hydrogenation.
2.1. The sulphuric acid process
This process uses sulphuric acid as the catalyst and its feedstock are propylene, butylene, amylene, and fresh isobutane.
Feedstocks are fed into the reactor which is divided into zones, each containing sulfuric acid, isobutane and olefins feed. The reactor product contains hydrocarbon and acid phases which are split in the settler; the hydrocarbon phase is washed with caustic and hot water for pH control and then depropanized, deisobutanized, and debutanized. The alkylate product so formed can then be used for motor fuel blending or for producing aviation grade blends. The isobutane goes back to the feed.
Figure 1: Acid catalyzed isobutene dimerization to 2, 4, 4-trimethyl-1-pentene and 2, 4, 4trimethyl2-pentene by the standard Whitmore-type carbocation mechanism [3].
2.2 The hydrofluoric acid process
This process employs hydrofluoric acid as the catalyst. The two types of hydrofluoric acid alkylation process commonly used are the Philips and UOP (a Honeywell company) processes. While Philips uses a reactor/settler combination system, UOP uses two reactors with separate settlers [2].
The major differences between sulfuric and hydrofluoric alkylations (HF) are temperature and acid consumption. Sulfuric alkylation requires refrigeration to maintain a low reactor temperature. The acid consumption rate for sulfuric alkylation is over a hundred times that of HF [8].
Figure 2: Aliphatic alkylation mechanism with hydrofluoric acid as catalyst: (a-b) initiation by addition of HF to the olefin and in the case of a sec. butylcation, hydride transfer from isobutane to produce a tert. butyl cation, (c) olefin addition to the tert-butyl cation, and (d) hydride transfer form isobutane to yield alkylate and regenerate the tert-butylcation [3].
Table II: Research Octane Number (RON) and Motor Octane Number (MON) of alkylates typically produced by HF alkylation of isobutane with various olefins [3].
Olefin feed
RON
RON + MON / 2
MON
Propene
91 – 92
–
89.5 – 90.0
1-butene
94.4
–
91.6
2-butene
97.8
–
94.6
Isobutene
95.9
–
93.4
Pentenes
90 – 91
–
93.4
n-pentenes
–
82.5
–
Table III: Research Octane Number (RON) and Motor Octane Number (MON) of alkylates produced by H2SO4 alkylation of isobutane with various olefins at 9-10 °C,
94-95 % H2SO4 concentration, and isobutane:olefin ratio of 7-9:1 [3]
Olefin feed
RON
MON
Propene
89.0
87.1
n-butene
97.8
93.9
Isobutene
93.2
90.3
n-pentene
91.0
88.0
Isopentene
91.2
88.8
2.3 Indirect alkylation by acidic resin
This process employs the use of a polar solvent to limit the activity of the acid resin in order to improve the dimerization selectivity. High conversion of isobutene can be obtained at low temperature usually less than 100 °C [8, 9 – 12]. On an industrial scale, the recovery of the polar solvent (tertiary butyl alcohol) could serve to regulate the product distribution and also to reduce the amount of oligomer formed during production to less than 10 % [8].
The alkylate produced from this technology has a research octane number (RON) of 99 101 and motor octane number (MON) of 96 – 99.
2.4 Indirect alkylation by solid phosphoric acid
The principle of indirect alkylation by solid phosphoric acid (SPA) is the same as by acidic resin catalysis; the difference being that dimerization over SPA follows an ester-based mechanism [13]. Heavy oligomer formation is mechanistically limited, [10] because the strength of the phosphoric acid ester bond decreases with increasing carbon number of the olefin.
Indirect alkylation by SPA is carried out in two steps: selective dimerization of isobutene (from C4 streams) to form diisobutene; followed by hydrogenation to form the saturated product isooctane. Selectivity problems and catalyst deactivation hinder the isobutene dimerization reaction. Because this reaction decides the quality and properties of the alkylate formed, it is a crucial step in this process.
The C4 stream, consisting mainly of isobutene, n-butane, isobutene, and n-butenes, is fed to the dimerization reactor, where isobutene is dimerized selectively in the presence of SPA catalyst. The reaction is exothermic, and heat must be removed to avoid temperature rises that can lead to the formation of undesired oligomers. These oligomers have relatively high molecular weights and boiling points and are not suitable as gasoline blends; they also rapidly deactivate the catalyst. Depending on the catalyst, an appropriate solvent may be needed to increase the selectivity toward the dimers. At higher operating temperatures the isobutene derived alkylate quality quickly deteriorates due to trimerization and cracking [11].
Propene forms a stronger ester bond with the phosphoric acid than the butenes, and it will become the dominant carbocation source [12]. The product stream from the reactor is fed to a distillation column, where dimerized and heavy products are separated from the unreacted C4 components and solvent. The dimer is then saturated in a separate reactor to form alkylates in the presence of a hydrogenation catalyst. In order to obtain alkylate quality hydrogenated products from an n-butene rich, isobutene lean feed, the reaction temperature should be less than 160 °C and the feed should not contain more than 5 % propene or 10 % pentenes.
3. Flow diagrams of direct and indirect alkylation process
Figure 3: Block flow diagrams of the direct alkylation (HF and H2SO4 catalysed alkylation) configurations evaluated [3].
Flow diagram 1: This is the base case for direct alkylation, using a straight run Iron-Based High Temperature Fischer-Tropsch (Fe-HTFT) C4 feed. There is little isobutane in the straight run feed, which constrains the alkylate yield.
Flow diagram 2: In order to overcome the constraint imposed by the low straight run isobutane content of C4 feed, a hydroisomerization unit is included in this two-step flow diagram to convert the straight run n-butane to isobutane. The hydroisomerization unit has an internal recycle, with an overall high isobutane yield. Although the alkylate yield may have been considerably improved compared to the base case, most of the C4 olefins have not been converted.
Flow diagram 3: The ratio of paraffins to olefins necessary for direct alkylation can be balanced by hydrogenating some of the C4 olefins to C4 paraffins in order to increase the alkylate yield.
Flow diagram 4: The alkylate yield may be further increased by using propene as the alkylating olefin. Propene is more abundant than the C4 hydrocarbons in straight run HTFT feed, which implies that all the hydrocarbons can be hydrogenated and hydroisomerized to isobutane for alkylation with propene. In this case an alkylate yield above 100 % based on the C4 feed can be obtained, but at lower octane number than with C4 material only.
Figure 4: Block flow diagrams of the indirect alkylation (acidic resin and solid phosphoric acid dimerization) configurations evaluated [3].
Flow diagram 5: It consists of acid catalyzed dimerization followed by hydrogenation. The direct conversion of isobutene in straight run HTFT syncrude with an acidic catalyst has a low alkylate yield (8 %), since only 8 % of the C4 olefins are isobutene. However, this alkylate has an octane number of almost 100.
Flow diagram 6: By use of skeletal isomerization, the alkylate quality and yield of n-butenes to isobutene can be improved. The n-butene conversion in the case of acidic resin dimerization is very low, and it is best to isomerize all n-butenes to isobutene. This results in an alkylate yield of 81 %.
4 Product yield and quality
In a fuels refinery there is an incentive to convert normally gaseous products into liquid transportation fuels. The quantity and the quality of the liquid fuel being produced are both important, and in terms of alkylate production, the quality is related to the octane number (ON) (1/2) RON + (1/2) MON) of the motor-gasoline. The investment economics is refinery dependent, with octane constrained refineries putting a premium on quality, while refineries with an unsaturated market putting a premium on volume.
Table IV: Alkylate yield and alkylate octane number calculated for the indirect alkylation flowschemes shown in figure 4 [3]
s/n
Dir.alkyl.fowscheme
Alkyl.tech
Alkyl.yld(m%C4)
Oct.no.(1/2)RON+(1/2)MON
1
Base case straight run
HTFT
HF
H2SO4
2
2
94
96
2
Case 1 + C4
hydroisomerisation
HF
H2SO4
21
20
94
96
3
Case 2 + butane
hydrogenation
HF
H2SO4
102
101
94
96
4
Case 3 + propene
alkylation
HF
H2SO4
197
189
91
88
The alkylate yield is based on the mass of alkylate produced per mass of total straight run high temperature Fisher – Tropsch C4 cut material.
Table V: Alkylate yield and alkylate octane number calculated for the indirect alkylation flowschemes shown in figure 3 [3]
s/n
Indir. Alkyl. flowscheme
Dim. tech
Alkyl. yld (m%C4)
Oct.no(RON+MON)/2
5
Base case straight run
HTFT
Acidic resin
SPA
8
72(90)b
99
87
6
Base case + skeletal
isomerisation
Acidic resin
SPA
81
85
99
99
The alkylate yield is based on the mass of alkylate produced per mass of total straight run high temperature Fischer-Tropsch C4 – cut material.b yield including coproduced kerosen
5 Environmental aspects
The environmental burdens due to the treatment of free hydrofluoric acid (HF) losses from an alkylation unit cannot be overlooked. The reality is that hydrofluoric acid losses from the unit do occur through side-reactions, forming organic fluorides, which become entrained in product streams, and through direct entrainment of free HF in a heavy hydrocarbon waste stream [6, 7].
The environmental aspects associated with the liquid phase direct alkylation processes led to the development of solid acid direct alkylation.
From an environmental stand point, indirect alkylation is preferred to direct alkylation and that flowscheme 5 (figure 4) is the most environmentally friendly [3].
6 Conclusion
It was found that the choice of technology depended on the different refining priorities, namely, the following: (a) Least complexity, (b) Highest alkylate yield
7 Literature
[1] Encyclopedia of Earth Home page. http://www.eoearth.org/
article/alkylation_in_petroleum_refining (accessed Aug.30, 2010)
[2] Albright, L.F.; ‘Comparison of Alkylation Processes’: Chem.Eng.,
209, Oct. 10, 1996.
[3] Wang, Y.; Subramaniam, B., 6874 ,Ind.Eng.Chem.Res., Vol.47,number 10, 2008.
[4] Albright, L.F.; ´Alkylation Processes Using Sulfuric Acid As Catalyst´, Ibid,
143, Aug. 15, 1997.
[5] De Klerk, A.; ´Isomerisation of 1-butene to isobutene at low temperature,
Ind.Eng.Chem.Res., 43, 6325, 2004.
[6] Occupational Safety and Health Administration Homepage. http://www.osha.gov/
dts/osta/otm/otm_iv/otm_iv_2.html (accessed Aug.31, 2010).
[7] Warren, R.T.; ´Alkylation and Isomerisation´, oil and gas journal, vol 97,
Issue 4, Jan.26, 1999.
[8] UOP Home page. http://www.uop.com/objects/NPRASpr2003HFAlkyd.pdf /
Article/advances in hydrofluoric (HF) acid catalyzed alkylation
(accessed Sept. 14, 2010).
[9] Kamath, R. S.; Qi, Z.; Sundmacher, K.; Aghalayam, P.; Mahajani,S. M.,
´Process analysis for dimerization of isobutene by reactive distillation´,
Ind.Eng.Chem.Res. 45, 1575, 2006.
[10] De Klerk, A. ´Reactivity differences of octenes over solid phosphoric acid´,
Ind. Eng. Chem. Res. 45, 578, 2006.
[11] De Klerk, A.; Engelbrecht, D.J.; Boikanyo, H. ´Oligomerization of
Fischer-Tropsch olefins: effect of feed and operating conditions on
hydrogenated motor-gasoline quality´, Ind. Eng.Chem. Res. 43, 7449, 2004.
[12] De Klerk, A. ´Distillate production by oligomerization of Fischer-Tropsch olefins over solid phosphoric acid´, Energy Fuels, 20, 439, 2006.
De Klerk, A.; ´Isomerisation of 1-butene to isobutene at low temperature´,
Ind.Eng.Chem.Res., 43, 6325, 2004.
[13] Nelson, W.L., McGraw-Hill,New, petroleum refinery engineering third edition, p 660,
2003.
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