Alkylation -2002-1

Isobutane/Olefin Alkylation
•
•
•
•
•
•
Boundary conditions and options to high-octane gasoline
Alkylation catalysts
Mechanistic issues in alkylation
Impact of important process parameters
General features of alkylation technology
Liquid acid based processes
– Sulfuric acid based
– Hydrofluoric acid based
• Solid acid based process developments
• Conclusions and outlook
Process units in a refinery
Naphta
C
r
u
d
e
D
e
s
t.
Jet /
Light
diesel
Heavy
diesel
VGO
Naphta
hydro
treater
H2 unit
Isomer
ization
Aromatics
saturation
Reformer
Hydro
cracker
Resid
Acid
H2
Dehexa
nizer
Gasoline
desulfur.
Alkylation
RFCCU
Sulfur
LPG
HDS
FCCU
Tail gas
unit
Gasoline
Sweet
fuel
Sulfur
recovery
Distillate
Amine
treating
Acid
regen.
Fuel Oil,
Asphalt
Gas
RFG specifications in Europe
1999
2000
2005
500
150
50 (10)*
Aromatics, max vol.% no spec.
42
35
Benzene, max vol.% 5
1
1
Octane RON min
95/98
95/98
95/98
Olefins, max vol.%
no spec.
18
18
RVP max kPa
80
60
60
Sulfur, max ppm wt.
* Potentially in 2005, possibly mandatory in 2007/2008
Sulfur in gasoline
Sulfur,
ppm
Typical % of
Gasoline
% Contribution
to sulfur
FCC Gasoline
800
30-50
90
LSR Gasoline
150
3
5
Alkylate
16
10
2
MTBE
20
5
1
Butanes
10
5
<1
Reformate
0
20-40
0
Isomerate
3
5
<1
Blending component
Pathways to high octane alkanes
Isomerization
• Converts n-pentane, n-hexane from LSR
• C5/C6 iso-alkanes (RON ca. 80)
Dimerization plus hydrogenation
• Converts isobutene (possible
substitute for etherification)
• 2,2,4-trimethylpentane (RON
100) with pure isobutene.
• Mixture of dimethylhexanes
and trimethylpentanes (RON
ca. 80) with mixed butene
feed.
Alkylation
• Converts C 3-C5 alkenes with
isobutane
• Mixture of C 5-C12 iso-alkanes,
mainly trimethyl pentanes
(RON 92-96)
Defining alkylation
Alkylation
iso-C4 +
iso-butane
n-C4=
n-butene
Multiple alkylation
iso-C9+
iso-C8
iso-octanes
Cracking
iso-C5-7
Alkylation in refining is the chemical reaction of a low-molecularweight olefin with an isoparaffin to form a liquid product, alkylate, that
has a high octane number and is used to improve the antiknock
properties of gasoline. The reaction takes place in the presence of a
strong acid catalyst, and at controlled temperature and pressure.
Ullmann, Encyclopedia of Chemical Technology
Alkylation – the boundary conditions
C4H10 + C4H8 ?
C8H18
? H° ~ - 90 kJ/mol C4H10
• Feed
isobutane (+ isopentane) and butenes (+ propene and pentenes)
• Typical alkylate composition
70% C 8-alkanes (mainly trimethylpentanes), 15% C5-C7, 15% C9-12
• Product quality
Free of aromatics and alkenes, low in sulfur, low RVP, RON 92-96,
only slightly lower MON
• Worldwide annual alkylate production
74 Mio tons (2001)
• Commercial catalysts used
Sulfuric acid, hydrofluoric acid
• Catalyst market
Total catalyst value 340 million US $ (increasing 5% per year)
Alkylation catalysts
Sulfuric acid
• Corrosive
• Highly viscous liquid
• Very low hydrocarbon solubility
– Vigorous stirring mandatory for achieving satisfactory
emulsion quality with sufficiently large phase boundary
areas.
• Optimum reaction temperature 4-10°C
– Refrigeration necessary to remove process heat
– T > 18°C lead to oxidation of HC (forming SO2 and water)
• High acid consumption of 70-100 kg/ton alkylate
– On site regenerated acid 3 times more expensive than fresh
acid
Alkylation catalysts
Hydrofluoric acid
•
•
•
•
Corrosive
Approximately 30 times higher isobutane solubility compared to H2SO4
Injection through nozzles sufficient for intensive mixing
Optimum reaction temperature 16-40°C
– Cooling water sufficient for process heat removal
• Low catalyst consumption
• Highly toxic liquid with a boiling point of 20°C
• Forms stable aerosols drifting downwind on ground level for several
kilometers when released into the atmosphere
– Legislative pressure against HF alkylation
Alkylation catalysts
Solid acids
• A large variety of solid acids have been proposed
– Large pore zeolites (FAU, BEA, etc.)
– Supported Brønsted and Lewis acids
– Supported Friedel-Crafts metal halides
– Organic resins
– Heteropolyacids
• All materials deactivate quickly through pore/site blocking
– Effective regeneration is a key issue
• No process based on solid acids is operated on industrial scale.
• Several developed process are offered for licensing.
Initiation steps
• Initiation: Activation of the alkene by proton addition
– Formation of acid esters (butyl sulfates, butyl fluorides, with solid
acids - alkoxides)
+ HX
¦
¦
X
+
+ X-
Sec-butyl carbenium ions experience hydride transfer from isobutane
- sometimes after the addition of further butene molecules
The tert-butyl carbenium ion is the central species in the catalytic
cycle
+
Hydride transfer
+
+
+
Simplified alkylation/dimerization mechanism
CnH2n + H+ ?
alkene
Olefin addition
[CnH2n+1]+
CnH2n
alkoxy species
alkene
Initiation
[i-C4H9
]+
tert-butoxy species
i-C2nH4n
Desorption
i-C2nH4n+2
iso-alkane
[C2nH4n+1]+
alkylated alkoxy
i-C4H10
iso-butane
Hydride transfer
Reaction pathways to different TMPs
60
40
30
20
224TMP
25 DMH/ 223 TMP
234 TMP
24 DMH
233 TMP
23 DMH
Thermodynamic dist. (wt%)
C8 isomer selectivity (wt%)
50
10
0
3
5
8
10
13
40
30
20
0
H3C
2,2,3-TMP
2,3,3-TMP
2,2,4-TMP
2,3,4-TMP
C
+
CH3
H CH
C
CH3 CH
3
18.4
18.6
14.5
10
Time on stream (hours)
CH3
48.5
50
223TMP224TMP233TMP234TMP
CH3
H2
C
+ C
H3C CH C
CH3
3
CH3
The primary product, 2,2,3 TMP
is formed in lower amounts than
thermodynamically expected.
è No readsorption of products!
Simplified alkylation cycle
Single alkylation
Multiple alkylation
+
Hydride transfer
+
BAS of at
least
moderate
strength
Olefin addition
+
Primary products
Equilibrium products
Isomerization
Hydride transfer vs. olefin addition
C4=
kC1
C4+
kC2
C8+
kA1
kB1
C12=
kC3
C12+
kA2
C16+
kA3
kB3
kB2
C4
High kC þ
C8=
C8
C12
dimerization, oligomerization
High kA
þ
polymerization, catalyst deactivation
High kB
þ
alkylation
How to achieve high rB / rA(+rC) ratios?
• Minimize the olefin concentration (e.g., CSTR-type reactor)
• Maximize hydride transfer rate
Reactant concentrations in ideal reactors
Conc. vs. time
Conc. vs. location
CSTR
Tub. reactor
Reactor type
¦
Only CSTR allows nominally low paraffin/olefin ratio at high
conversion without large recycle.
Side reactions
Cracking
• Cracking via β-scission is responsible for the production of
hydrocarbons with “odd” carbon numbers (C5-C7, C9-C11).
Cracking products are also highly branched and, thus, give a
relatively high RON.
Self alkylation
• Hydrogen redistribution may occur between n-butene and isobutane leading to n-butane and iso-butene. This side reaction is
responsible for a considerable production of n-butane,
especially when using HF as catalyst. With propene as feed
alkene propane will be produced.
Side reactions
Dimerization/Oligomerization
• Weak acid sites catalyze only oligomerization reactions. Hydride
transfer requires a higher acid strength. Oligomerization proceeds on
intrinsically weak acid sites in solid acids and in too diluted liquid
acids.
Catalyst deactivation
• Multiple hydride transfer of alkenes and subsequent cyclization lead to
highly unsaturated cyclic species, which strongly bind to the acid and
lead to deactivation with liquid and solid acids (acid soluble oil,
conjunct polymers).
Concerted mechanism of alkylation
+
H+
Alkylation
+
“Selfalkylation”
+
+
+
+
+
Dimerization
Cracking
Multiple alkylation or
addition of C5+ alkene
+
+
+
Important process parameters
RON
C5-C7
RON
Acid
Consumption
Isobutane/alkene ratio
RON
Reactor
Volume
Alkene space velocity
C9+
Reaction temperature
Isobutane/Alkene ratio
¦ Determines the concentration of
isobutane in the reactor, which
controls the hydride transfer rate
Alkene space velocity
¦ Influences alkene addition rates
Reaction temperature
¦ Optimum temperature different for
each individual catalyst
General process scheme
Make up isobutane
Recycle isobutane
Propane
Reaction heat removal
Make up acid
Reactor
system
Spent acid
Pretreatment
Reg. acid
Alkene feed
Acid
regeneration
Product
treatment
n-Butane
Fractionation
Alkylate
Oil
Feed pretreatment
Feed impurities
• Water, di-olefins, sulfur and nitrogen compounds and – when olefins
from an etherification unit are used – traces of oxygenates.
• These compounds lead to increased acid consumption due to dilution
(water, ethers) and irreversible reactions.
þFeed pretreatment necessary for impurity removal
– Selective hydrogenation of di-olefins
– Caustic wash or Merox™ treatment for removal of sulfur compounds
– Alumina for adsorbing water, etc.
Removal of process heat
• Approximately 90 kJ per mole of reacted isobutane have to be
removed.
• HF catalyzed processes operate at 16-40°C and can be cooled
with cooling water.
• Sulfuric acid based processes operate at subambient
temperatures (oxidation of hydrocarbons) and typically are
cooled utilizing the hydrocarbon process stream itself.
– Partial evaporation of isobutane (plus added propane to
increase the efficiency).
Product separation
• The hydrocarbon stream has to be separated from the acid.
– With liquid acids settlers are used to separate the emulsion
phases by gravity.
• Fractionation of hydrocarbon stream by distillation into propane, nbutane, recycle isobutane and alkylate. Sometimes also a separate
pentane-stream is separated.
n-Butane
Alkylate
OR
n-Butane
Pentane
Depentanizer
Net effluent
Debutanizer
Deisobutanizer
Net effluent
Deisobutanizer
Recycle
isobutane
Recycle
isobutane
Alkylate
Product treatment
• With both liquid acids, traces of esters are solved in the hydrocarbon
stream.
– Corrosion problems in downstream equipment.
þ Hydrocarbon stream has to be treated.
• Sulfuric acid
– Bauxite (alumina) sorbent (adsorbs esters, has to be regenerated)
– Alkaline water wash (decomposes esters and neutralizes acid)
– Acid wash (decomposes esters, acid is sent to reactor)
• Hydrofluoric acid
– Thermal decomposition of alkyl fluorides
– Caustic and/or alumina treatment of all hydrocarbon streams
Liquid acid regeneration
• Spent hydrofluoric acid can be distilled and recycled back into
the reactor.
• Spent sulfuric acid has to be burned at about 1000°C.
– SO2 is catalytically oxidized to SO3.
– SO3 is reacted with dilute H2SO4 to give concentrated acid.
• The price of regenerated acid is about two to three times the
market price of sulfuric acid.
• Regeneration either on site or shipment to chemical plant.
Solid acid regeneration
• Regeneration by burning off the deposits
– Requires a catalyst with high temperature stability.
– Significant amounts of process heat have to be removed.
• Regeneration by (supercritical) extraction in situ or ex situ
– Most likely not sufficiently effective.
• Regeneration by hydrogenation/hydrocracking
– Treatment at moderate temperature for hydrogenation of
highly unsaturated compounds (frequent).
– Treatment at high temperature for hydrocracking of bulky
species blocking the pores (occasional).
Liquid acid catalyzed processes
Sulfuric acid based processes
• Stratco effluent refrigerated sulfuric acid alkylation
• ExxonMobil autorefrigerated sulfuric acid alkylation
• Time tank process (no longer in use)
Hydrofluoric based processes
• UOP HF alkylation
• Phillips HF alkylation
Stratco effluent refrigerated sulfuric
acid alkylation
Detailed view of Stratco contactor reactor
Coolant
out
Emulsion
to Settler
Circulation
Tube
Acid
Hydrocarbon
Feed
Impeller
Coolant
in
Tube
bundle
Stratco effluent refrigerated sulfuric
acid alkylation
• Contactor™ reactor
– Heat exchanger tube bundle to remove the heat of reaction
– Mixing impeller on one end
– Hydrocarbon feed and recycle acid enter on the suction side
of the impeller inside the circulation tube
• Hydrocarbon stream is routed through heat exchanger and
partially evaporated
• Reaction is typically staged with respect to the acid flow (up to
four reactors in series)
• Typically operating conditions: isobutane/alkene feed ratio 7-10,
alkene space velocities (WHSV) 0.06 to 0.19 hr –1, reaction
temperatures 6 – 10°C, sometimes up to 18°C.
ExxonMobil autorefrigerated
sulfuric acid alkylation
T= 5°C, WHSV = 0.03 hr
–1
The heat of reaction is removed by evaporating isobutane plus added
propane from the reaction zones making additional cooling unnecessary.
Summary sulfuric acid
• Both processes give a comparable alkylate quality at
comparable costs.
• Both processes are constantly improved with respect to their
mixing characteristics.
• Both licensors explore the increased utilization of propene and
pentenes.
Phillips HF alkylation
• Non-cooled riser-type reactor.
• The hydrocarbon mixture is
introduced through nozzles at
the bottom and along the length
of the riser.
• The acid flow is maintained by
gravity and cooled in a heat
exchanger with cooling water to
remove the reaction heat.
• Typical process parameters are
temperatures in the order of
24°C, isobutane/alkene ratios of
about 14-15.
UOP HF alkylation
• Vertical reactor / heat
exchanger.
• The reaction heat is removed
by cooling water, which is
flowing through cooling coils
inside the reactor.
• During normal operation, the
acid is distilled with the
product, so that no external
regeneration is necessary.
• Additional acid regeneration
column necessary for startup
or in case of feed
contamination.
UOP HF alkylation process
Safety in HF alkylation
• Options for refiners to respond to regulatory pressure intended to
assure the safety of HF alkylation units
• Mitigation
– Leak detection, water spray towers, rapid acid deinventory
systems
• HF modifiers
– Reduce the volatility of HF
• Conversion to another catalyst, e.g., sulfuric acid
Stratco Alkysafe™
• Conversion of an HF alkylation plant into a sulfuric acid plant.
• Reuses as much as possible of the existing HF plant.
– Reaction and distillation sections can be reused.
– Acid blowdown section may be reconstructed from existing
equipment.
• New equipment
– packaged refrigeration unit (propane as coolant)
– effluent treating system
– acid blowdown
– tankage sections
• Due to lower Isobutane/Alkene ratio (lower throughput in the
deisobutanizer) either a higher product quality or a higher production
capacity can be achieved.
• Costs are claimed to be comparable to installment of HF mitigation
systems.
HF alkylation modifiers
• Phillips/ExxonMobil ReVap™
– Additive on sulfone (possibly sulfolane) basis
– No chemical reaction with HF
• UOP/ChevronTexaco Alkad™
– Additive based on amine salts of HF, forming liquid “onium” poly
hydrogen fluoride complexes with HF
• Both additives reduce HF
aerosol formation in case of
leaks.
• Additional separation
equipment has to be installed.
• Both additives may increase
alkylate quality with propene,
isobutene and pentenes as
feedstock.
Summary HF alkylation
• Due to legislative pressure many refiners installed
expensive mitigation systems and/or are using
additives to reduce the volatility of HF.
• Some refineries are interested in converting their HF
alkylation unit into a H2SO4 unit or possibly also into a
solid acid based unit.
Installed process capacities
Alkylate capacity, BPD
700.000
600.000
500.000
400.000
300.000
200.000
100.000
0
ExxonMobil Kellogg
Autorefrigerated
process
UOP
Phillips
Stratco
Solid acid catalyzed alkylation
• UOP Alkylene™ process
• AkzoNobel/ABB Lummus AlkyClean™ process
• Lurgi Eurofuel® process
– The three processes are based on solid acids with
regeneration section utilizing purge and hydrogenation/
hydrocracking strategies
• Haldor Topsøe FBA™ process
– Liquid acid supported on solid carrier
– Acid handling similar to liquid acid processes
UOP Alkylene™ process
• Solid catalyst most likely alumina supported AlCl3 catalyst modified with
alkaline metal cations and a Ni, Pd or Pt hydrogenation function No catalyst
or sludge disposal problems.
• The process operates at 10-40°C and at an isobutane/alkene ratio of 6-15.
UOP Alkylene™ process
• Vertical riser reactor.
• Hydrocarbon and catalyst flow co-currently upwards in the riser.
• At the top of the riser, the catalyst particles sink downwards into the
reactivation zone, where they are washed with isobutane and
dissolved H2 under reaction conditions.
• A small slipstream of catalyst is withdrawn and directed to a
reactivation vessel, in which the catalyst is regenerated semi-batch or
batch wise at elevated temperature in a circulating H2 stream.
• Catalyst: most likely alumina supported AlCl3 catalyst modified with
alkaline metal cations and a Ni, Pd or Pt hydrogenation function.
• The process operates at 10-40°C and at an isobutane/alkene ratio of
6-15.
• The process is available for license.
AkzoNobel/ABB Lummus AlkyClean™
process
Isobutane
Olefin feed Pretreatment
(optional)
Reactor
system
Catalyst
Regeneration
Product
Distillation
Light Ends
Isobutane
Feed
Alkylate
Product
• Unknown reactor type, with „high degree of mixing“.
• Serial reaction stages with distributed alkene feed injection for high
internal isobutane/alkene ratios.
• Catalyst most likely zeolite USY based.
AkzoNobel/ABB Lummus AlkyClean™
process
Reactor
Effluent
Continuously
i-C4 feed
Olefin
Olefin
Mild regeneration
Occasionally
H2
Regeneration at
250°C (1 reactor)
• Multiple reactors are used,
which swing between
reaction and regeneration.
• Mild regeneration at
reaction temperature and
pressure with hydrogen
dissolved in isobutane is
performed frequently (far
before the end of the
theoretical catalyst lifetime).
• When necessary, the
catalyst is fully regenerated
at 250°C in a stream of gasphase hydrogen.
Reaction column
Separation column
Lurgi Eurofuel® process
Catalyst regeneration
H2/i-C4
optional
Lurgi Eurofuel® process
• Tray distillation tower reactor.
• The evolving reaction heat is dissipated by the evaporation of
the reaction mixture.
• Intermittently the catalyst is exposed to hydrogen rich operating
conditions for low temperature regeneration. Infrequent
regeneration occurs in a proprietary section under elevated
temperatures.
• Catalyst presumably based on faujasitic zeolite.
• The process operates at temperatures around 50-100°C with an
isobutane/alkene ratio between 6 to 12 and a higher alkene
space velocity than in the liquid acid based processes.
Haldor Topsøe FBA™ process
• Fixed bed reactor.
• Reaction temperature is in the range of 0-20°C. The reactor is
operated adiabatically and the reaction heat is removed by a cooled
reactor effluent recycle.
• Liquid triflic (trifluoromethanesulfonic) acid is supported on a porous
support material.
Haldor Topsøe FBA™ process
• At the upstream end of the catalyst zone, ester intermediates
are formed, which are soluble in the hydrocarbons and are
transported into the acid zone.
• They react to form the products and free acid.
• The active zone slowly migrates through the bed in the
direction of the hydrocarbon flow.
Comparison of processes
Feed
Cooling
treatment
Product
treatment
Alkene
space
velocity
ASO
formation
Equipment
corrosion
Alkylene
o
-
-
n.a.
+
+
AlkyClean
+
+
+
n.a.
+
+
Eurofuel
+
n.a.
+
+
+
+
FBA
-
+
-
n.a.
-
-
n.a.
o
+
no information available
no improvement
possible improvement
clear improvement
Comparison of process parameters
HF
H2SO4
Zeolites
16-40
4-18
50-100
11-14
7-10
6-15
0.1-0.6
0.03-0.2
0.2-1.0
Exit acid strength (wt.-%)
83-92
89-93
-
Acid per reaction volume (vol.-%)
25-80
40-60
20-30
1000-2500
6-18
4-10
Reaction temperature (°C)
Feed
isobutane/alkene
(mol/mol)
Alkene space velocity (kg
hr)
ratio
Alkene /kg Acid
Catalyst productivity (kg Alkylate /kg Acid)
Comparison of alkylate quality
Mixed C4 olefin feed
Conclusions and outlook
• The process modifications and the new development
show the high dynamic in this area.
• The modifications in H2SO4 and HF based processes
target improved product quality, reduced feed
pretreatment and a significantly better safety situation.
• The laboratory and pilot plant data demonstrate that solid
acid catalysts are a viable alternative to the liquid acid
catalysts.
• Innovative reactor technology and timely catalyst
regeneration are the key components in the new
processes based on solid acids.