cong nghe butamer
TRANSCRIPT
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Department of Chemical Engineering
Title: The Hydroisomerization of Butane to Isobutane
Class: CHE 594: Refining of Oil and Synthetic Liquids
Report Written by: Ryan Lee Robles, 1100884
Submitted to: Dr. Arno de Klerk
Date Submitted: December 8, 2010
Signature of Report Writer: _______________________________________________
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Summary
The renewed interest in the hydroisomerization of butane has become an emergent process in
the wake of increased environmental concerns. The phase out and reduction of octane boosters,
such as MTBE and aromatic compounds has left a niche void in the fuel industry. Isomerization
serves as a viable precursor for the production of high performance gasoline, possessing the
ability to meet more stringent environmental regulations and standards. Specifically, iso-butane
is used as feed to an alkylation unit, responsible for the production of high octane fuel additives.
The hydroisomerization of butane to isobutane was overviewed.
The dominant hydroisomerization technology in industry is the UOP Butamer process,
capable of adaptive flow schemes dependent on process requirements.
The process is a fixed-bed, catalytic process occurring in the vapour phase. Optimal feed
will contain relatively high amounts of normal butane. However, the process is equilibrium
limited, typically peaking at 60% yield. Operation using the classic bifunctional catalyst-
platinum impregnated aluminum- occurs under moderate conditions due to the high selectivity.
Acidity is maintained through continuous injection of chlorinated compound, potentially
introducing corrosion to both process and downstream equipment.
The reaction proceeds through an alkene intermediate formed by dehydrogenation on the
metal sites and completes with hydrogenation of an iso-alkene to isobutane.
Extra precaution must be taken during feed pre-treatment as the catalyst is highly sensitive to
feed poisons, especially water and sulphur. A suggested alternative catalyst uses a zeolitic
structure more resistant to contaminants, but requires harsher operating conditions.
Process economics will always have a significant role in many factors of the overall
process. Thus, it is crucial to pay close attention to the significant operating parameters
including temperature, pressure and the hydrogen-to-hydrocarbon ratio of the feed. Alternative
schemes and consideration of advanced process control techniques can lead to optimal operating
conditions, making hydroisomerization a practical process in meeting continually changing fuel
specifications.
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Table of Contents
Summary i
Table of Contents ii
List of Figures iii
List of Tables iii
Introduction 1
1 The Process
Feed
Products
Process Performance
Process Unit: DriersProcess Unit: Reactors
Process Unit: Stabilizer
Utilities
2
5
5
5
56
6
7
2 Reaction Chemistry
Temperature
Pressure
Hydrogen-to-Hydrocarbon Ratio (H2/HC)
7
8
8
9
3 Catalysis
New Generation Catalysts
9
10
4 Process Engineering
Mass and Energy Balance
Concerns
11
11
13
5 Supplement
Process Control
Process Economics
13
13
14
Conclusion 15
References 16
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List of Figures
Figure 1.1 Hydroisomerization Process with Complete Recycle 3
Figure 1.2 Basic Process Flow Diagram for the UOP HOT Butamer Process 4
List of Tables
Table 4.1 Estimated Operating Requirements 11
Table 4.2 Composition of Field Butane Feedstock 12
Table 4.3 Estimated Product Yield Based on the Field Butane Feedstock 12
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Introduction
Based on biogenic theory, oil is composed of compressed hydrocarbons formed ages ago
in a process that began when ancient biomass and other fossilized organic material, buried under
millions of years of sediment, were subjected to extreme pressures and temperatures. The oil
industry began to develop over five thousand years ago19. Oil seeping from the ground was
utilized in the Middle East for waterproofing boats and baskets. It was also proposed the
Neolithic period pioneered the oil movement wherein high commodity whale oil was used as
lamp oil. It was not until the advent of kerosene production through simple atmospheric
distillation that the modern era began. From then, two major developments changed the face of
the oil industry: Edisons economical light bulb virtually eliminated the use of kerosene as a light
source and the emergence of the internal combustion engine created demand for both diesel fuel
and motor gasoline fractions. New techniques were required in order to meet the growing
demand for these specific petroleum fractions. For instance, alkylation, catalytic cracking (the
hammer) and catalytic reforming (including hydroisomerization) are all processes used to
maximize the volume and overall quality of fuels.
In the modern era, environmental consciousness has become equally important as
meeting demands. The push to become more green has led to more stringent environmental
conditions. Concern over the usage of fuel and oil has resulted in drastic changes in regulations,
having an impact on gasoline, jet fuels and lubricating oils. Regulations are calling for more
efficient and cleaner burning fuels. To overcome these new challenges, there is a continual need
for both the progressive improvement of current technologies and the development of practical,
innovative ideas.
For instance, the hydroisomerization (isomerization) process was greatly affected by the
advent of World War II. With the increasing demand for high octane aviation fuels, came the
need for improved refinery technologies. Aliphatic alkylation processes based on the Friedel-
Crafts reaction were developed in the 1930s as a collaborative effort of several American
companies to fuel Allied warplanes17. During alkylation, an alkene is essentially bonded with
isobutane (iC4) to form a high octane gasoline additive. Despite being adequate for the times,
there were still several issues with the process. Excessive corrosion, sludge formation and high
catalyst consumption meant alkylation units were plagued with costly maintenance and operating
expenses. As well, the supply of iC4 from straight-run sources was limited. Renewed interest in
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the hydroisomerization process began to emerge with the initial phase out of tetra ethyl lead in
the 1970s. Following amendments to the Clean Air Act25 which led to the phase out of leaded
gasoline in Europe and the US, oxygenates (such as MTBE, ETBE) became primary gasoline
additives as octane boosters. However, oxygenates have recently garnered environmental
attention due to their discovery as a surface and groundwater contaminant2, suspected of emitting
toxic formaldehyde or peroxyacetyl nitrate. Further, the European Program on Emission, Fuel
and Engine Technologies has suggested the environmental sustainability of gasoline may be
improved through the reduction of alkene, aromatic, oxygen and sulphur contents12. Aromatics
and alkenes react with nitrogen oxides to form ozone, contributing to smog formation. Despite,
the reduction of aromatic content in motor gasoline will have negative effects on the fuel quality.
An alternative process must be found to replace the void left by these components.
Light branched alkanes are a viable replacement for the production of environmentally
sustainable fuels. However, as mentioned before, the straight-run fraction of i-alkanes in crude
oil is minimal. This shortcoming stimulated the original development of the bifunctional catalyst
and its use in the hydroisomerization process of converting normal butane (nC4) to isobutane.
1 THE PROCESS
The hydroisomerization of alkanes is of substantial importance in petroleum refining.
The transformation of a normal straight-chain alkane into an isomerised branched alkane results
in a high octane additive, meaning a more efficient and environmentally cleaner burning fuel. In
general, the degree of branching is directly related to the octane number. There are two main
isomerization processes: the isomerization of lower n-alkanes (nC5-nC7) for the production of
high octane blending components; and the nC4 conversion as feed for the production of alkylate.
Of interest is the hydroisomerization of butane to isobutane, which is a result of skeletal
rearrangement in the presence of hydrogen. The end products are simply isomerized
hydrocarbons containing the same carbon number distribution as the feed.
The iC4 is used primarily as feed to an alkylation unit. Alkylate blending has seen a
significant increase in demand due to its high octane, low vapour pressure blending
characteristics and the gradual shift away from certain oxygenates. For instance, the reduction of
MTBE as a gasoline additive must be compensated with other environmentally sound fuel
quality boosters. Motor fuel alkylate can effectively fill this void, resulting in reduced carbon
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monoxide (CO) and hydrocarbon (HC) emissions, crucial to meeting current environmental
regulations.
Figure 1.1 outlines the main hydroisomerization process with a complete recycle stream
for unconverted hydrogen and normal alkanes.
Reactor(s)
Stabilizer
nC4 Feed
Makeup Hydrogen
Gas
Isomerate
Desorption
Recycle of Uncoverted H2 and nC4
Adsorption
LPG
Figure 1.1 Hydroisomerization Process with Complete Recycle5
Hydroisomerization is a fixed bed process occurring in the vapour phase which employs
the use of a platinum impregnated chloride-alumina catalyst. Due to the high activity of the
catalyst, the process is able to operate under moderate conditions (180-220C, 1.5-3MPa and
space velocity of 2h-1)18. However, a continuous injection of a chlorinated organic compound
(carbon tetrachloride) is necessary to maintain the acidity of the support which will inherently
produce a corrosive by-product. A make-up stream of hydrogen is also introduced and serves
two main purposes. First, in conjunction with platinum, there is prevention of coke deposits.
Second, the hydrogen acts as a suppressant to the polymerization of alkene intermediates formed
during the reaction.
The normal butane is separated from the isobutane through fractionation. Since the
reaction is equilibrium limited, any unconverted feed material and hydrogen is recovered for
recycling to the isomerization reactors.
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The first hydroisomerization unit was commissioned in 1953 by UOP17, followed by
British Petroleums C4 Isomerization unit and Shells HYSOMER unit in 1970. The Butamer
process currently dominates industry as the pioneer technology for the hydroisomerization of
butane. Depending on the specific application, the overall process will vary slightly for each
hydroisomerization unit. A simplified process flow diagram of the hydrogen-once-through
(HOT) Butamer process is shown in Figure 1.2.
Dryer
Dryer
Reactor(s)
Stabilizer
Separator
nC4 Feed
Makeup Hydrogen
Gas to Scrubb ing
and Fuel
Isomer ised
Product
Figure 1.2 Basic Process Flow Diagram for the UOP HOT Butamer Process23
Many C4 isomerization units (and specifically the Butamer Process) have been
commissioned. Facilities currently employing this technology include Amoco (Texas City,
Texas), Chevron (Salt Lake City, Utah), Marathon (Garyville, LA) and the Paso Refinery (El
Paso, Texas)24.
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Feed
The catalyst used in the process is highly susceptible to heteroatom and water poisoning.
Before reaching the isomerization unit, the feed is commonly pretreated through hydrotreating
processes such as hydrodesulphurization (HDS) and hydrodenitrogenation (HDN).
According to UOP (2007), the Butamer unit is able to process natural gas liquids (NGL)
from one of their other proprietary processes, the UOP NGL recovery unit. Otherwise, the
optimal feedstock into the hydroisomerization unit will contain high amounts of normal butane
with lower concentrations of isobutane, pentane (nC5) and other heavier components. For feed
streams with considerable amounts of iC4 or nC5 (approximately 30% or more17), an isostripper
column or deisobutanizer (DIB) can be used to enrich the stream with nC4. Feeds already rich in
nC4 do not require any pre-treatment and are fed directly to the reactor section. The initial
feedstock is combined with make-up hydrogen, preheated and fed to the reactor. The hydrogen
to carbon ratio does not significantly influence the reaction, but more importantly prevents
deactivation of the catalyst through decreasing the coke deposits. The presence of hydrogen also
suppresses the polymerization of alkene intermediates in the reaction. A recycle gas stream and
product separator may be necessary for excess hydrogen and unconverted feed material recovery.
Products
The process produces iC4and hydrogen streams. Heavier by-products and light gas may
also be created. Typical yields, regardless of feed content are approximately 60 % by volume17.
Typical compositions will be outlined in section 4.
Process Performance10
Naturally, the isobutane ratio is the main indicator of performance for a butane
isomerization unit, indicating the amount of desired product.
(1)
Process Unit: Driers13
A molecular sieve is commonly used to remove water from both the hydrogen and
hydrocarbon feeds. As an extra precaution to prevent catalyst deactivation, certain molecular
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sieves are also capable of removing sulphur compounds. Driers may be operated in parallel to
allow for continuous operation during off-line water desorption in the saturated molecular sieve.
Process Unit: Reactors17
There is immediate catalyst deactivation at the inlet of the reactor which progressively
deteriorates moving down the catalytic bed. A two reactor swing system can be employed to
counteract this loss of on-stream efficiency introduced by this deactivation profile. Once the
catalyst in one reactor is killed, the swing operation to the second reactor will allow
simultaneously, the replacement of the spent catalyst in reactor one and the continuous
isomerization of feed in the current reactor. In addition, the first reactor may be operated at high
temperatures in order to achieve the faster reaction rates, while the second reactor can be
operated at moderate temperatures to increase the selectivity through a more favourable
thermodynamic equilibrium. The equilibrium concentration of iC4 is 60% at 180C, but only
40% at 300C6.
There are two main advantages with the dual reactor operation. First, the cycle length
will only be contingent on regular shutdown and maintenance allowing for continued operation
during catalyst replacement. The spent catalyst is usually vacuum- or gravity dumped from the
reactor. Prior to replacement, residual hydrocarbons may be removed from the catalyst through a
nitrogen and oxygen sweep
24
. The removal and replacement of spent catalyst may becomparatively lengthy and will require, at minimum, shut down of the reactor.
The second advantage is a reduction in catalyst consumption. When operating with a
single reactor, the effectiveness of the catalyst will be insufficient far before the catalyst is
completely spent. This will result in considerable amounts of capable catalyst being discarded.
A double reactor system will allow for the complete deactivation of catalyst, virtually resulting
in complete catalyst utilization. However, process economics will always be employed in
refinery design. Regardless of the apparent advantages, the expense of installing two reactors
will factor in whether or not a refinery will choose to run a single or dual reactor system.
Process Unit: Stabilizer
The reactor effluent is cooled and then flows to the stabilizer unit where any light gas
coproduct is removed. The isomerate is sent downstream to a deisobutanizer for separation.
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Utilities
Aside from the regular utilities such as power, the process will include various grades of
steam for heating. As well, the reactor effluent is cooled prior to entering the stabilizer
suggesting a cooling medium, such as cooling water, is also necessary. Section 4 will outline the
operating requirements of the Butamer process.
2 REACTION CHEMISTRY
Bifunctional catalysts comprised of a noble metal and acidic function are favourable for
alkane isomerization. The Butamer process uses a fixed-bed reactor containing a highly-
selective, chloride promoted catalyst to perform the desired conversion of isomerizing the nC4 to
iC4. The reaction as specified for butane is17
(2)
The primary reaction pathway is proposed to initiate through an alkene intermediate
formed through dehydrogenation on the platinum site18
(3)
Hydrogenation (equilibrium to the left) is favoured as the reaction is operated under high
hydrogen pressure. Despite the unfavourable conditions, alkene formation is promoted as a
sufficient amount of alkenes created are converted to carbonium (carbenium7) ions (driving the
equilibrium to the right). Carbonium ions are formed from the reaction of neutral hydrocarbon
molecules with the acid sites. Protonation of the alkene on the acid site to form ions is given by
(4)
Skeletal isomerization reaction is proposed to follow the rearrangement of the carbonium
ion through a cyclo-alkyl intermediate4
(5)
The isomerized carbonium ion loses a proton to the catalyst site and is converted to an
isomerized alkene in a process analogous to reaction (3)
(6)
The iso-alkene intermediate is hydrogenated on the metal site to isobutane
(7)
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Without termination, chain propagation would continue indefinitely given a sufficient
feed supply. However, acid consumption and catalyst deactivation can be attributed to chain
termination reactions. As proposed by Sie (2008), chain termination occurs when highly
unsaturated molecules with strong proton binds are created. For instance, the hydride transfer
between an alkene and carbonium ion will form an alkane and unsaturated carbonium ion. These
molecules are no longer capable of participating in the reaction recycle and are responsible for
both a loss in catalyst activity and effectiveness of the acid sites.
As shown in the reaction steps, the hydroisomerization process is a reversible reaction
and it is equilibrium conversion limited8. Since there are several transformations taking place -
namely isomerization, hydrocracking and coking- the influence of equilibrium on each individual
reaction step must be considered. The following operating parameters affect the degree of
isomerization.
Temperature
The operating temperature is the dominant influence on the equilibrium of the process.
The dehydrogenation on the metal sites is favoured by increased temperatures; while only the
reaction rate is affected on the acid sites. In spite of the initiation step being favoured, higher
temperature will also favour hydrocracking of the feed to propane and other lighter components.
Further, as reaction on the sites is accelerated, increased alkene concentrations on the catalyst
surface will favour coking. As a result, the conversion equilibrium to the iso structure is highly
reliant on temperature and can be increased with lower conditions. However, the inherent caveat
of lowering the operating temperature is a decreased reaction rate- requiring the selection of a
highly active catalyst.
Pressure
As the process is a rearrangement, there is no change in the number of moles meaning the
equilibrium is not significantly affected by pressure. Ultimately, an increase in pressure will
only cause a decrease in the rate of conversion to branched isomers. This minor influence on
conversion occurs as there is an inhibition of the initiation step due to the increase in the partial
pressure of hydrogen. However, one main advantage involves the reduced rate of the
hydrocracking and coking reactions.
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Hydrogen-to-Hydrocarbon Ratio (H2/HC)
The conversion can be increased by operating at lower hydrogen to feed ratios. However,
the hydrogen used in the process serves a more valuable application in preventing the
deactivation of the catalyst by reducing coke formation. It is also a dominant variable in
determining the process economics. Operating at certain H2/HC ratios will affect the size and
ultimately the needs for the hydrogen recycle. Typically the H2/HC ratio is about 0.5-2.06.
Selection of optimal operating conditions is contingent on obtaining a practical hydrogen
partial pressure through balancing the operating pressure and the H2/HC ratio which will allow
for an extended catalyst life. The ability to operate at lower temperatures has become obtainable
through the advancement of initial catalysts developments.
3 CATALYSIS
The aim of catalyst development throughout history was to obtain a high activity catalyst
which could be practically implemented. In the 1960s, early aluminum chloride catalysts were
plagued with high corrosivity, high consumption and high processing costs7. The main
advantage, however, was the low operating temperatures (< 200C) due to the high acidity.
Catalysts which incorporated platinum on an acid support of Al2O3were later developed, but
were less active and required harsh operating temperatures (~320-450C).
The third generation of catalysts consists of chlorinated alumina combined with platinumand is predominantly used in industry today. Similar catalysts are commonly used in the
isomerization of other light alkanes (C5/C6), suggesting the catalysts is not highly sensitive to
the carbon number in the feed; provided adequate nC4 amounts are present in the feed.
Examples of this type of catalysts are the commercially available I-12 and I-20 catalysts readily
used in UOPs Butamer process.
The balance between the acidic and metallic sites plays a role significant role in the
performance of the process. It implicitly determines the probability of acid catalyzed side-
reactions taking place by determining the alkene partial pressure6. Production of the catalyst is
performed by treating the platinum alumina matrix with carbon tetrachloride at high
temperatures without affecting the pore structures. It is crucial to control acidity so as not to
favour undesired hydrocracking reactions. As well, the amount of platinum will depend on the
molecular mass of the feed- heavier feeds are more likely to have coke formation. The alumina
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matrix will consist of about 8-15 wt%Cl2and 0.3-0.5 wt%Pt8. Continuous injection of a
chlorination agent maintains the high acidity and allows for lower operating temperatures.
The advantage of a bifunctional catalyst is the opportunity for stable isomerization under
adequately high hydrogen pressure. From the chain termination reactions, formation of the
unwanted, unsaturated molecules can be reduced by ensuring alkene intermediates are kept
saturated with hydrogen. In addition, the presence of platinum prevents coke deposition.
The catalyst is highly sensitive to poisons and pre-purification of the feed is a necessity.
The catalyst is especially susceptible to water. Molecular sieve drying systems are effectively
used to remove any water present in the hydrocarbon or hydrogen feeds. Other heteroatoms,
such as sulphur, must also be removed before being fed into the hydroisomerization unit.
Typical processes would involve various hydrotreating reactions (HDN, HDS, HDO) and caustic
extraction to remove sulphur contents. Fluoride which may be introduced from coupling with an
alkylation unit will degrade the catalyst and can be removed by passing the feed over a hot bed
of alumina. The molecular sieves may also have the capacity to remove heteroatoms.
The main disadvantage of the catalysis is hydrogen chloride (HCl) formation directly
caused by a combination of the hydrogen pressure and continued injection of chloride. Elution
of chlorine from the catalysts will affect all downstream components due to the highly corrosive
nature of HCl. A caustic scrubber is necessary to neutralize any HCl present in the off-gas.
New Generation Catalysts
A fourth generation of catalysts has recently been developed. It is composed of a
predominantly platinum impregnated zeolite. The high resistance to contaminants avoids the
need for feed pre-treatment. In addition, the zeolitic process does not require expensive drying
facilities or chloride promotion, avoiding the need for off-gas treatment. The catalysts are also
regenerable. However, since the activity is lower, operation temperatures are generally higher
resulting in lower concentration of branched isomers. The higher temperatures suggest the
necessity of a fired heater which may be financially cumbersome. Industrially used zeolites are
platinum-containing, modified synthetic mordernite17. Examples include, UOPs HS10 and
HYSOPAR from Sd-Chemie. Platinum loaded sulphated zirconia was recently commercialized
for the n-butane isomerization, but possesses shorter catalyst lifetime. Ultimately, the
development of new catalysts should naturally seek to incorporate the advantages of the zeolitic
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structure, but improve on activity to become more economically feasible and competitive with
the dominant process.
4 PROCESS ENGINEERINGMass and Energy Balance
Table 4.1 outlines an estimated utility requirement based on figures for the UOP Butamer
process. The basis feed rate is 115 000MTA(3800BPSD). A simple estimated yield based on a
given feed was also extracted from Meyers (2003) and is summarized in Table 4.2 and Table 4.3.
The make-up hydrogen added in this case is about 65.6m3/h.
While many stochastic optimization procedures seek to predict and optimize utility costs,
it should be noted utility costs are inherently volatile in nature and expenses can never truly be
predicted or known (unless a contract is signed for fixed rates)20, 26. For instance, the average
electricity rate for the general service large" customer class varied in increments from 2005 to
201015. The cost basis in the tables are as follows: electric power, $0.05/kWh; MPS, $3.50/klb;
LPS, $2.50/klb.
Table 4.1 Estimated Operating Requirements
Utility Requirement Deisobutanizer Butamer Unit $US/SD
Power, kW 200 300 600
Medium Pressure Steam
14.1 kg/cm3, 1000 kg/h
200 lb/in2gage, 1000 lb/h
5.0
11.1 936
Low Pressure Steam
3.5 kg/cm3, 1000 kg/h
50 lb/in2gage, 1000 lb/h
16.3
35.9 2153
Cooling Water, m /h 35 89 77
Catalyst and chemical consumption
$US/SD 523 523
Total 4289
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Table 4.2 Composition of Field Butane Feedstock
Feed MTA wt %
Propane 978 0.85
Isobutane 29 325 25.50
n-butane 82 282 71.55
Isopentane 1 805 1.57
n-pentane 610 0.53
Total 115 000 100
MTA = metric tons per annum
Table 4.3 Estimated Product Yield Based on the Field Butane Feedstock
Product MTA wt %
Isobutane
Propane
Isobutane
n-butane
978
104 190
3 922
0.85
90.60
3.41
Heavy-end by-product
Isobutane
n-butane
Isopentane
n-pentane
69
2 702
1 058
978
0.06
2.35
0.92
0.85
Light gas
Methane
Ethane
Propane
252
357
541
0.22
0.31
0.47
Total 115 000 100
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Concerns
The environmental footprint of this process is increased with the continual need for
chlorination. There is concern downstream regarding the hydrochloric acid (HCl) inherently
produced from this process. Regardless, carbon steel equipment is still successfully used due to
the dry conditions. However, caustic scrubbers are still needed to neutralize the acid in the off-
gas formed to prevent corrosion of downstream equipment.
A significant cost which should be addressed along with utilities is that of the catalyst
and chemical consumption. Since the process is highly reliant on a continuous injection of a
chlorinated compound, readily available amounts of compound must be maintained.
According to a study published in 199624, most refineries place spent catalyst directly
into closed containers (e.g. 55 gallon drums, flow-bins, 1 cubic yard supersacks). The
frequency of generation occurs between 2 and 10 years. Waste reduction methods may include
sending the exhausted catalyst to reclaim platinum which is a precious metal. Despite no oxygen
is present during operation, the presence of chlorine in the process may cause the spent catalyst
to contain toxic dioxins formed during the unit turnaround and catalyst replacement.
Commercial solutions to dioxin removal are readily available (e.g. ADIOX, MercOx).
5 SUPPLEMENT
Process Control
A study by Lukec et al (2007) proposes the implementation of advanced process control
techniques, namely model predictive control (MPC), on the pentane hydroisomerization process
at the Rijeka Refinery in Croatia. MPC theories have been applied in industry to allow for
optimal control for instance, in terms of minimizing energy consumption, maximizing plant
capacity and maintaining quality product. Proposals outlined in the paper can be feasibly applied
to butane isomerization. The basics of soft sensor design include statistical determination of
significant inputs and outputs. A mathematical model can then be developed to enable on-line
estimation of the desired properties, such as iC4 yields, and any highly correlated inputs can be
adjusted accordingly using the MPC control scheme. Through the initial investigation, it is
known the temperature and the H2/HC ratio are highly influential on the yield. Soft-sensor
design can both build on and reinforce this supposition. Although, the proposal is not without its
caveats. The practical implementation of this type of control holds various barriers. If historian
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data of the parameters is not readily available, sensors need to be installed in order to collect
data. In addition, it is unlikely online estimation of the product composition is performed. This
may mean disparity between the lab analysis and online data could exist, introducing
unnecessary data processing work. Internal obstacles to overcome are red tape measures upon
unproven/new techniques and the requirements of other processes in the refinery- optimizing the
hydroisomerization unit will affect both up and downstream processes. Further, excitation of the
system may be necessary for proper process identification. It is highly unlikely a company will
allow costly experimental changes to an operating process. Regardless, proper application of
control techniques can lead to reduction in utility costs, improvement in catalystslifetime and
ultimately, more efficient operation.
Process Economics
It should be recognized additional costs need to be factored in to the overall economics of
the hydroisomerization process. For instance, labour, repair and in this case, royalties and
patents must be included when considering this process.
Alternative means of saving capital costs can be found in a slightly altered process. It
suffices to mention, UOPs hydrogen-once-through flow scheme (Figure 1.2) eliminates the need
for a high pressure separator and recycling compressor for hydrogen.
A common practice in industry is the integration of a hydroisomerization unit with an
alkylation unit23. Unconverted nC4 can be removed from the alkylation process isostripper and
fed back into the isomerization unit. With the increased iC4 concentrations, reduction in size of
the isostripper will ultimately lead to reduction in utilities. As refinery economics is paramount
in industry, synergy of the hydroisomerization and alkylation processes can lead to favourable
decreases in both capital and operating costs.
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Conclusion
The hydroisomerization of butane to isobutane has become a commonplace technology in
meeting the demand for high quality gasoline. Light branched alkanes, in this case, iso-butane is
used as feedstock to an alkylation unit, responsible for the production of high octane additives.
The technological overview of the process can be summarized as follows:
Hydroisomerization is a fixed-bed, catalytic process occurring in the vapour phase UOPsButamer process is the dominant technology in industry Operating conditions are typically: 180-220C, 1.5-3MPa, space velocity of 2h-1and
H2/HC ratio of 0.5-2
Equilibrium limitations yield about 60% iso-butane per pass; recycling ofunconverted material can result in virtually 100% conversion
Acidity of the chlorinated-Pt/Al2O3catalyst is maintained by continuous injection ofchlorinated compound
The catalysts is highly sensitive to feed poisons, including water and sulphur and pre-treatment of the feed is a necessity
The main issue is corrosion of process and downstream equipment. Thus, a causticscrubber is needed to remove any traces of HCl in the off-gas
Process economics may be improved through direct combination with an alkylationunit; improved catalysts and process flow schemes; and the implementation of
advanced process control techniques
Ultimately, the hydroisomerization process of butane to isobutane can be a very practical
process to implement in meeting the perpetually changing demands in both environmental and
fuel regulations and standards.
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