Note: Descriptions are shown in the official language in which they were submitted.
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PROCESS FOR THE MANUFACTURE OF DIESEL RANGE
HYDROCARBONS
Field of the invention
The invention relates to an improved process for the manufacture of
hydrocarbons, particularly diesel range hydrocarbons from bio oils and fats,
wherein the formation of higher molecular weight compounds is reduced. The
invention also relates to processing of feedstock containing free fatty acids,
using
a high product recycle/fresh oil-ratio at reduced reaction temperatures.
Background of the invention
Environmental interests and an increasing demand for diesel fuel, especially
in
Europe, encourage fuel producers to employ more intensively available
renewable
sources. In the manufacture of diesel fuels based on biological raw materials,
the
main interest has concentrated on vegetable oils and animal fats comprising
triglycerides of fatty acids. Long, straight and mostly saturated hydrocarbon
chains of fatty acids correspond chemically to the hydrocarbons present in
diesel
fuels. However, neat vegetable oils display inferior properties, particularly
extreme viscosity and poor stability and therefore their use in transportation
fuels
is limited.
Conventional approaches for converting vegetable oils or other fatty acid
derivatives into liquid fuels comprise transesterification, catalytic
hydrotreatment,
hydrocracking, catalytic cracking without hydrogen and thermal cracking among
others. Typically triglycerides, forming the main component in vegetable oils,
are
converted into the corresponding esters by the transesterification reaction
with an
alcohol in the presence of catalysts. The obtained product is fatty acid alkyl
ester,
most commonly fatty acid methyl ester (FAME). Poor low-temperature properties
of FAME however limit its wider use in regions with colder climatic
conditions.
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Said properties are the result of the straight chain nature of the FAME
molecule
and thus double bonds are needed in order to create even bearable cold flow
properties. Carbon-carbon double bonds and ester groups however decrease the
stability of fatty acid esters, which is a major disadvantage of
transesterification
technology. Further, Schmidt, K., Gerpen J.V.: SAE paper 961086 teaches that
the
presence of oxygen in esters results in undesirable higher emissions of NO, in
comparison to conventional diesel fuels.
Undesired oxygen may be removed from fatty acids or their esters by
deoxygenation reactions. The deoxygenation of bio oils and fats, which are
oils
and fats based on biological material, to produce hydrocarbons suitable as
diesel
fuel products, may be carried out by catalytic hydroprocessing, such as
hydrocracking, but also more controlled hydrotreating conditions may be
utilized.
During hydrotreating, particularly hydrodeoxygenation oxygen containing groups
are reacted with hydrogen and removed through formation of water and therefore
this reaction requires rather high amounts of hydrogen. Due to the highly
exothermic nature of these reactions, the control of reaction heat is
extremely
important. Impure plant oil/fat or animal fat/oil, high reaction temperatures,
insufficient control of reaction temperature or low hydrogen availability in
the
feed stream may cause unwanted side reactions, such as cracking,
polymerisation,
ketonisation, cyclisation and aromatisation, and coking of the catalyst. These
side
reactions also decrease the yield and the properties of diesel fraction
obtained.
Unsaturated feeds and free fatty acids in bio oils and fats may also promote
the
formation of heavy molecular weight compounds, which may cause plugging of
the preheating section and decrease catalyst activity and life.
The fatty acid composition, size and saturation degree of the fatty acid may
vary
considerably in feedstock of different origin. The melting point of bio oil or
fat is
mainly a consequence of saturation degree. Fats are more saturated than liquid
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oils and in this respect need less hydrogen for hydrogenation of double bonds.
Double bonds in a fatty acid chain also promote different kinds of side
reactions,
such as oligomerisation/polymerization, cyclisation/aromatisation and cracking
reactions, which deactivate catalyst, increase hydrogen consumption and reduce
diesel yield.
Plant oils/fats and animal oils/fat may contain typically 0-30% of free fatty
acids,
which are formed during enzymatic hydrolysis of triglycerides especially when
oil
seeds are kept in humid atmosphere. Free fatty acids can be also formed during
purification of bio oils and fats, especially during caustic wash i.e. alkali
catalyzed
hydrolysis. The amount of free fatty acids present in plant/vegetable oils is
typically 1-5 wt % and in animal fat 10-25 wt-%. Free fatty acids are
corrosive in
their nature, they can attack against materials of unit or catalyst and can
promote
some side reactions. Free fatty acids react very efficiently with metal
impurities
producing metal carboxylates, which promote side reaction chemistry.
Fatty acids may also promote the formation of heavy compounds. The boiling
range of these heavy compounds is different from the range of diesel fuel and
may
shorten the life of isomerisation catalyst. Due to the free fatty acids
contained in
bio oils and fats, the formation of heavy molecular weight compounds are
significantly increased compared to triglyceridic bio feeds, which have only
low
amount of free fatty acids (<1%).
Biological raw materials often contain metal compounds, organic nitrogen,
sulphur and phosphorus compounds, which are known catalyst inhibitors and
poisons inevitably reducing the service life of the catalyst and necessitating
more
frequent catalyst regeneration or change. Metals in bio oils/fats inevitably
build up
on catalyst surface and change the activity and selectivity of the catalyst.
Metals
can promote some side reactions, but blocking of catalyst active sites
typically
decreases the activity and thus metal impurities such as Na, Ca, and Mg
compounds should be removed as efficiently as possible.
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Hydrolysis of triglycerides produces also diglycerides and monoglycerides,
which
are partially hydrolyzed products. Diglycerides and monoglycerides are surface-
active compounds, which can form emulsions and make liquid/liquid separations
of water and oil more difficult. Bio oils and fats can also contain other
glyceride-
like surface-active impurities like phospholipids (for example lecithin),
which
have phosphorus in their structures. Phospholipids are gum like materials,
which
can be harmful for catalysts. Natural oils and fats also contain other types
of
components, such as waxes, sterols, tocopherols and carotenoids, some metals
and
organic sulphur compounds as well as organic nitrogen compounds. These
compounds can be harmful for catalysts or pose other problems in processing.
Patents US 4,992,605 and US 5,705,722 describe processes for the production of
diesel fuel additives by conversion of bio oils into saturated hydrocarbons
under
hydroprocessing conditions with CoMo and NiMo catalysts. The process operates
at high temperatures of 350-450 C and produces n-paraffins and other
hydrocarbons. The product has a high cetane number but poor cold properties
(melting point >20 C), which limits the amount of product that can be blended
in
conventional diesel fuels in summer time and prevent its use during winter
time.
The formation of heavy compounds with a boiling point above 343 C was
observed, especially when a fatty acid fraction was used as a feed. A reaction
temperature with a lower limit of 350 C was concluded as a requirement for
trouble-free operation.
A two-step process is disclosed in FI 100248, for producing middle distillates
from vegetable oil by hydrogenating fatty acids or triglycerides of vegetable
oil
origin using commercial sulphur removal catalysts, such as NiMo and CoMo, to
give n-paraffins, followed by isomerising said n-paraffins using metal
containing
molecular sieves or zeolites to obtain branched-chain paraffins. The
hydrotreating
was carried out at rather high reaction temperatures of 330-450 C, preferably
390 C. Hydrogenating fatty acids at those high temperatures leads to shortened
catalyst life resulting from coking and formation of side products.
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one being
a hydrodeoxygenation step and the second one being a hydroisomerisation step
utilizing counter-current flow principle, and using biological raw material
containing fatty acids and/or fatty acid esters as the feedstock. The process
comprises an optional stripping step.
Deoxygenation of plant oils/fats and animal fats with hydrogen use a large
amount
of hydrogen and at the same time releases significant amount of heat. Heat is
produced from deoxygenation reactions and from double bond hydrogenation.
Different feedstocks produce significantly different amounts of reaction heat.
The
variation of reaction heat produced is mainly dependent on double bond
hydrogenation. The average amount of double bonds per triglyceride molecule
can vary from about 1.5 to more than 5 depending on the source of bio oil or
fat.
FR 2,607,803 describes a process for hydrocracking of vegetable oils or their
fatty
acid derivatives under pressure to give hydrocarbons and to some extent acid.
The
catalyst contains a metal dispersed on a support. A high temperature of 370 C
did
not result complete oxygen removal or high selectivity of n-paraffins. The
product
mixture formed, contained also some intermediate fatty acid compounds.
Formation of water during hydrotreatment results from the deoxygenation of
triglyceride oxygen by the means of hydrogen (hydrodeoxygenation).
Deoxygenation under hydrodeoxygenation conditions is to some extent
accompanied by a decarboxylation reaction pathway and a decarbonylation
reaction pathway. Deoxygenation of fatty acid derivatives by decarboxylation
and/or decarbonylation reactions forms carbon oxides (CO2 and CO) and
aliphatic
hydrocarbon chains with one carbon atom less than in the original fatty acid
molecule. Decarb-reactions mean here decarboxylation and/or decarbonylation
reactions.
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The feasibility of decarboxylation varies greatly with the type of carboxylic
acid
or derivative thereof used as the starting material. Alpha-hydroxy, alpha-
carbonyl
and dicarboxylic acids are activated forms and thus they are more easily
deoxygenated by decarb-reactions. Saturated aliphatic acids are not activated
this
way and generally are difficult to deoxygenate through decarb-reactions.
Decarboxylation of carboxylic acids to hydrocarbons by contacting carboxylic
acids with heterogeneous catalysts was suggested by Maier, W. F. et al:
Chemische Berichte (1982), 115(2), 808-12. Maier et al tested Ni/A1203 and
Pd/Si02 catalysts for decarboxylation of several carboxylic acids. During the
reaction the vapors of the reactant were passed through a catalytic bed
together
with hydrogen. Hexane represented the main product of the decarboxylation of
the
tested compound heptanoic acid. When nitrogen was used instead of hydrogen no
decarboxylation was observed.
Patent US 4,554,397 discloses a process for the manufacture of linear olefins
from
saturated fatty acids or esters, suggesting a catalytic system consisting of
nickel
and at least one metal selected from the group consisting of lead, tin and
germanium. With other catalysts, such as Pd/C, low catalytic activity and
cracking
to saturated hydrocarbons, or formation of ketones when Raney-Ni was used,
were observed.
Object of the invention
An object of the invention is an improved process for the manufacture of
diesel
range hydrocarbons from bio oils and fats, with high selectivity, essentially
without side reactions and with high diesel yield.
A further object of the invention is an improved process for the manufacture
of
diesel range hydrocarbons from bio oils and fats, wherein the extent of high
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molecular weight compounds formed during hydrotreating is decreased and the
stability of the catalyst is increased.
A still further object of the invention is an improved process for the
manufacture
of diesel range hydrocarbons from bio oils and fats, wherein the
hydrotreatment of
triglyceride feedstock containing free fatty acids is carried out using
dilution of
fresh feed and reduced reaction temperature.
A still further object of the invention is an improved process for the
manufacture
of diesel range hydrocarbons from bio oils and fats, which process produces
high
quality diesel component with high yield.
The present invention provides a process for the manufacture of diesel range
hydrocarbons wherein a feed is hydrotreated in a hydrotreating step and
isomerised in
an isomerisation step, characterized in that a feed comprising fresh feed
which may
contain more than 5 wt% of free fatty acids and at least one diluting agent is
hydrotreated at a reaction temperature of 200-400 C, in a hydrotreating
reactor in the
presence of catalyst, and the ratio of the dilution agent/fresh feed is 5-
30:1.
The feed can contain more than 10 wt% of the free fatty acids. In one
embodiment, the
hydrotreating step the reaction temperature is 250-350 C, preferably 280-340
C. The
dilution agent can be selected from hydrocarbons and recycled products of the
process
or mixtures thereof. The feed can contain less than lOw-ppm alkaline and
alkaline
earth metals, calculated as elemental alkaline and alkaline earth metals, less
than 1Ow-
ppm other metals, calculated as elemental metals, and less than 30w-ppm
phosphorus,
calculated as elemental phosphorus.
In one embodiment, the diluting agent:fresh feed ratio is 10-30:1 preferably
12-25:1.
The feed can contain 50-20000 w-ppm of sulphur, calculated as elemental
sulphur. The
fresh feed can be of biological origin, selected from plant oils/fats, animal
fats/oils, fish
fats/oils, fats contained in plants bred by means of gene manipulation,
recycled fats of
the food industry and mixtures thereof The fresh feed can be selected from
rapeseed
oil, colza oil, canola oil, tall oil, sunflower oil, soybean oil, hempseed
oil, olive oil,
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linseed oil, mustard oil, palm oil, peanut oil, castor oil, coconut oil, lard,
tallow, train
oil or fats contained in milk.
In one embodiment, a mixture of feed of biological origin and a
hydrocarbon/hydrocarbons is used as the fresh feed. The hydrotreating catalyst
bed
system can comprise one or more catalyst beds. The hydrotreating step pressure
can
vary in the range of 20-150 bar, preferably in the range of 30-100 bar. The
isomerisation step pressure can vary in the range of 2-15 MPa, preferably in
the range
of 3-10 MPa, the temperature varying between 200 and 500 C, preferably between
280
and 400 C. The hydrotreating can be carried out in the presence of a
hydrogenation
catalyst, said hydrogenation catalyst containing a metal from the Group VIII
and/or
VIB of the Periodic System. The hydrotreating catalyst can be a supported Pd,
Pt, Ni,
NiMo or a CoMo catalyst, the support being alumina and/or silica.
In one embodiment, an isomerization catalyst containing molecular sieve is
used in the
isomerization step. The isomerisation catalyst can contain a metal from the
Element
Group VIII. The isomerization catalyst can contain A1203 or Si02. The
isomerization
catalyst can contain SAPO-11 or SAPO-41 or ZSM-22 or ZSM-23 or ferrierite and
Pt
or Pd or Ni and A1203 or Si02.
Definitions -
Here hydroprocessing is understood as catalytic processing of organic material
by
all means of molecular hydrogen.
Here hydrotreatrnent is understood as a catalytic process, which removes
oxygen
from organic oxygen compounds as water (hydrodeoxygenation, HDO), sulphur
from organic sulphur compounds as dihydrogen sulphide (H2S)
(hydrodesulphurisation, HDS), nitrogen from organic nitrogen compounds as
arnin0nia (NH3) (hydrodenitrogenation, HD1=1) and halogens, for example
chlorine
from organic chloride compounds as hydrochloric acid (1-1C1)
(bydrodechlorination, ITDCI), typically under the influence of sulphided NiMo
or
sulphided CoMo catalysts.
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Here deoxygenation is understood to mean removal of oxygen from organic
molecules, such as fatty acid derivatives, alcohols, ketones, aldehydes or
ethers by
any means previously described.
Here hydrodeoxygenation (IIDO) of triglycerides or other fatty acid
derivatives or
fatty acids is understood to mean the removal of carboxyl oxygen as water by
the
means of molecular hydrogen under the influence of catalyst.
Here decarboxylation and/or decarbonylation of triglycerides or other fatty
acid
derivatives or fatty acids is understood to mean removal of carboxyl oxygen as
CO2 (decarboxylation) or as CO (decarbonylation) with or without the influence
of molecular hydrogen. Decarboxylation and decarbonylation reactions either
together or alone are referred to as decarb-reactions.
Here hydrocracking is understood as catalytic decomposition of organic
hydrocarbon materials using molecular hydrogen at high pressures.
Here hydrogenation means saturation of carbon-carbon double bonds by means of
molecular hydrogen under the influence of a catalyst.
Here n-paraffins mean normal alkanes or linear alkanes that do not contain
side
chains.
Here isoparaffins mean alkanes having one or more Ci ¨ C9, typically Ci - C2
alkyl side chains, typically mono-, di-, tri- or tetramethylalkanes. .
The feed (total feed) to the hydrotreating unit is here understood to comprise
fresh
feed and at least one dilution agent.
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Summary of the invention
The present invention relates to an improved process for the manufacture of
hydrocarbons from renewable sources, such as plant oils/fats and animal
oils/fats,
comprising a hydrotreating step and an isomerisation step. Particularly the
invention relates to the transformation of the starting materials comprising
triglycerides, fatty acids and derivatives of fatty acids or combinations of
thereof,
into n-paraffins with reduced formation of high molecular weight hydrocarbons
using dilution of fresh feed and reduced reaction temperature in the
hydrotreating
step and converting the obtained n-paraffins into diesel range branched
alkanes
using isomerisation, with high diesel yield. The hydrotreating step is carried
out
contacting the feed comprising fresh feed and at least one diluting agent with
a
hydrotreatment catalyst under hydrotreatment conditions. Then the obtained
product is isomerised with an isomerisation catalyst under isomerisation
conditions. The hydrocarbon oil formed via this process is a high quality
diesel
component.
Detailed description of the invention
It was surprisingly found that dilution of fresh feed in the hydrotreatment
step, in
combination with decreased reaction temperature reduces the undesired side
reactions and improves reaction selectivity, particularly when a starting
material
containing free fatty acids is used. The diluting agent can be a hydrocarbon
of
biological origin and/or non-biological origin. The dilution agent can also be
recycled product from the process (product recycle). The diluting agent /
fresh
feed-ratio is 5-30:1, preferably 10-30:1 and most preferably 12-25:1.
A preferable embodiment of the invention and of the hydrotreatment step is
illustrated in Figure 1, wherein a hydrotreatment process configuration is
provided, comprising one or more catalyst beds in series, hydrotreated product
recycle introduction on the top of the first catalyst bed and fresh feed,
quench
liquid and hydrogen introduction on top of each catalyst beds. This results in
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control of the reaction temperature in the catalyst beds and hence
diminishes undesired side reactions.
In Figure 1 the hydrotreatment reactor 100 comprises two catalyst beds 10 and
20.
Fresh feed 11 is introduced as streams 12 and 13 on the catalyst beds 10 and
20,
10 respectively, and hydrogen as stream 22 and 23 on the catalyst beds
10 and 20,
respectively. The fresh feed stream 12 is first mixed with the hydrotreated
product
recycle stream 41 and quench liquid stream 43 and the resulting mixture 31,
diluted in the fresh feed concentration, is then introduced on the catalyst
bed 10.
In order to obtain a required sulphur concentration in the feed stream 31,
required
amount of sulphur make up is added to the fresh feed stream 11 via stream 15.
As
mixture 31 passes through the catalyst bed 10 with the hydrogen stream 22,
fatty
acids and fatty acid derivatives of the fresh feed stream 12 are converted to
the
corresponding reaction products. A two-phase stream 32 is withdrawn from the
bottom of the catalyst bed 10 and is mixed with the fresh feed stream 13,
quench
liquid stream 44 and the hydrogen stream 23. The formed vapor-liquid mixture
33, diluted in the fresh feed concentration, is then introduced on the
catalyst bed
20 at reduced temperature due to cooling effect of the hydrogen, quench liquid
and fresh feed, passed through the catalyst bed 20 and finally withdrawn from
the
catalyst bed as a product stream 34. The stream 34 is separated in to a vapor
stream 35 and liquid stream 36 in the high temperature separator 101. Vapor
stream 35 is rich in hydrogen and is directed to further treatment. Part of
the liquid
stream 36 is returned to the reactor 100 as recycle stream 40, which is
further
divided to dilution stream 41 and total quench liquid stream 42. The quench
liquid
stream 42 is cooled in the heat exchanger 102 to provide adequate cooling
effect
on the top of the catalyst beds 10 and 20. Hydrotreated product stream 51 is
directed from the hydrotreatment step to further processing.
The catalyst beds 10 and 20 may be located in the same pressure vessel or in
separate pressure vessels. In the embodiment where the catalyst beds are in
the
same pressure vessels the hydrogen streams 22 and 23 may alternatively be
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introduced on the catalyst bed 10 and then be passed through the catalyst beds
10
and 20. In the embodiment where the catalyst beds are in separate pressure
vessels, the catalyst beds may operate in parallel mode with separate dilution
streams, hydrogen streams and quench liquid streams. The number of catalyst
beds may be one or two or more than two.
The sulphur make up to the hydrotreatment step may be introduced with the
fresh
feed stream 11. Alternatively, required amount of sulphur may be fed with the
hydrogen streams 22 and 23 as gaseous sulphur compound such as hydrogen
sulphide.
Hydrogen is fed to the hydrotreating reactor in excess of the theoretical
hydrogen
consumption. During the hydrotreating step, triglyceride oils, fatty acids and
derivatives thereof are almost theoretically converted to n-paraffins without
or
almost without side reactions. Additionally, propane is formed from the
glycerol
part of the triglycerides, water and CO and/or CO2 from carboxylic oxygen,
II2S
from organic sulphur compounds and NH3 from organic nitrogen compounds.
Using the above described procedures in the hydrotreating step, the
temperature
needed for reactions to start up is achieved in the beginning of each catalyst
bed,
the temperature increase in the catalyst beds is limited, harmful and
partially
converted product intermediates can be avoided and the catalyst life is
extended
considerably. The temperature at the end of the catalyst bed is controlled by
net
heat of reactions and to the extent of the dilution agent used. The dilution
agent
may be any hydrocarbon available, of biological origin or non-biological
origin. It
can also be recycled product from the process. Fresh feed content from feed
(total
feed) is be less than 20 wt-%. If the product recycle is used, product
recycle/fresh
feed ratio is 5-30:1, preferably 10-30:1, most preferably 12-25:1. After the
hydrotreatment step, the product is subjected to an isomerization step.
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Feedstock
The bio oil and/or fat used as the fresh feed in the process of the present
invention
originates from renewable sources, such as fats and oils from plants and/or
animals and/or fish and compounds derived from them. The basic structural unit
of a typical plant or vegetable or animal oil/fat useful as the feedstock is a
triglyceride, which is a triester of glycerol with three fatty acid molecules,
having
the structure presented in the following formula I:
0
if=-= 401`- R2
Rrs'''01_, (kr R3
Formula 1. Structure of triglyceride
In formula I R1, R2 and R3 are alkyl chains. Fatty acids found in natural
triglycerides are almost solely fatty acids of even carbon number. Therefore
R1,
R2, and R3 typically are C5 - C23 alkyl groups, mainly C11-C19 alkyl groups
and
most typically C15 or C17 alkyl groups. R1, R2, and R3 may contain carbon-
carbon
double bonds. These alkyl chains can be saturated, unsaturated or
polyunsaturated.
Suitable bio oils are plant and vegetable oils and fats, animal fats, fish
oils, and
mixtures thereof containing fatty acids and/or fatty acid esters. Examples of
suitable materials are wood-based and other plant-based and vegetable-based
fats
and oils such as rapeseed oil, colza oil, canola oil, tall oil, sunflower oil,
soybean
oil, hempseed oil, olive oil, linseed oil, mustard oil, palm oil, peanut oil,
castor oil,
coconut oil, as well as fats contained in plants bred by means of gene
manipulation, animal-based fats such as lard, tallow, train oil, and fats
contained
in milk, as well as recycled fats of the food industry and mixtures of the
above.
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Bio oil and fat suitable as fresh feed may comprise C12 ¨ C24 fatty acids,
derivatives thereof such as anhydrides or esters of fatty acids as well as
triglycerides of fatty acids or combinations of thereof. Fatty acids or fatty
acid
derivatives, such as esters may be produced via hydrolysis of bio oils or by
their
fractionalization or transesterification reactions of triglycerides.
In order to avoid catalyst deactivation and undesired side reactions the feed
shall
comply with the following requirements: The amount of alkaline and alkaline
earth metals, calculated as elemental alkaline and alkaline earth metals, in
the feed
is below 10, preferably below 5 and most preferably below 1 w-ppm. The amount
of other metals, calculated as elemental metals, in the feed is below 10,
preferably
below 5 and most preferably below 1 w-ppm. The amount of phosphorus,
calculated as elemental phosphorus is below 30, preferably below 15 and most
preferably below 5 w-ppm.
In many cases the feedstock, such as crude plant oil or animal fat, is not
suitable
as such in processing because of high impurity content and thus the feedstock
is
preferably purified using suitably one or more conventional purification
procedures before introducing it to the hydrotreating step of the process.
Examples of some conventional procedures are provided below:
Degumming of plant oils/fats and animal oils/fats means the removal of
phosphorus compounds, such as phospholipids. Solvent extracted vegetable oils
often contain significant amounts of gums, typically 0.5-3% by weight, which
are
mostly phosphatides (phospholipids) and therefore a degumming stage is needed
for crude plant oils and animal fats in order to remove phospholipids and
metals
present in crude oils and fats. Iron and also other metals may be present in
the
form of metal-phosphatide complexes. Even a trace amount of iron is capable of
catalysing oxidation of the oil or fat.
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Degumming is performed by washing the feed at 90-105 C, 300-500 kPa(a), with
II3PO4, NaOH and soft water and separating the formed gums. A major amount of
metal components, which are harmful for the hydrotreatment catalyst, are also
removed from the feedstock during the degumming stage. The moisture content
of the degummed oil is reduced in dryer at 90-105 C, 5-50 kPa(a).
A feedstock, which is optionally degummed or refined in another conventional
way, may be bleached. In the bleaching the degummed or refined feedstock is
heated and mixed with natural or acid-activated bleaching clay. Bleaching
removes various impurity traces left from other pretreatment steps like
degumming, such as chlorophyll, carotenoids, phosphoipids, metals, soaps and
oxidation products. Bleaching is typically carried out under vacuum to
minimize
possible oxidation. Generally the goal of bleaching is to reduce the color
pigments
in order to produce an oil of acceptable color and to reduce the oxidation
tendency
of oil.
Optionally the triglyceride structures of the feedstock may be decomposed by
prehydrogenating the double bonds using reduced reaction temperature with
NiMo or other catalyst, prior to the of by hydrodeoxygenations in order to
prevent
double bond polymerisation of unsaturated triglycerides.
The process according to the invention is particularly advantageous when the
fresh feed contains more than 5 % of free fatty acids and even more than 10 %
of
free fatty acids. Thus also naturally occurring fats and oils containing
significant
amounts of free fatty acids can be processed without the removal of free fatty
acids.
In the following the process according to the invention comprising a
hydrotreating
step and an isomerisation step is described in more detail.
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In the first step of the process, i.e. in the hydrotreating step, fatty acids,
triglycerides and other fatty acid derivatives comprised in the feed are
deoxygenated, denitrogenated and desulphurisated.
The feed comprises fresh feed and at least one dilution agent and the ratio of
the
dilution agent/fresh feed is 5 ¨ 30:1, preferably 10-30:1, most preferably 12-
25:1.
The dilution agent is selected from hydrocarbons and recycled product of the
process i.e. product recycle or mixtures thereof.
In the hydrotreating step, the pressure range may be varied between 20 and 150
bar, preferably between 50 and 100 bar, and the temperature between 200 and
400
C, preferably between 250 and 350 C and most preferably between 280 and
340 C.
It was found that the selectivity of decarb-reactions and the deoxygenation
through decarb-reactions can be promoted during hydrotreating over the
hydroteatment catalyst, by using sulphur content of 50 ¨ 20000 w-ppm,
preferably
1000-8000 w-ppm, most preferably 2000-5000 w-ppm of sulphur in the total feed,
calculated as elemental sulphur. The specific sulphur content in the feed is
able to
double the extent of n-paraffins formed by removal of COx. Complete
deoxygenation of triglycerides by decarb-reactions can theoretically lower the
consumption of hydrogen about 60% (max) compared with pure deoxygenation
by hydrogen.
At least one organic or inorganic sulphur compound may optionally be fed along
with hydrogen or with the feed to achieve the desired sulphur content. The
inorganic sulphur compound can be for example II2S or elemental sulphur or the
sulphur compound may be an easily decomposable organic sulphur compound
such as dimethyl disulphide, carbon disulfide and butyl thiol or a mixture of
easily
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decomposable organic sulphur compounds. It is also possible to use refinery
gas
or liquid streams containing decomposable sulphur compounds.
In the hydrotreatment/hydrodeoxygenation step, known hydrogenation catalysts
containing metals from Group VIII and/or VIB of the Periodic System may be
used. Preferably, the hydrogenation catalysts are supported Pd, Pt, Ni, NiMo
or a
CoMo catalyst, the support being alumina and/or silica, as described for
instance
in FI 100248. Typically, NiMo/A1203 and CoMo/A1203 catalysts are used.
In order to control the increase of temperature resulting from the
aforementioned
reactions over catalyst beds and side reaction formation, an improved reactor
configuration is presented in Figure 1. The hydrotreatment section comprises
one
or more catalyst beds in series, dilution agent introduction on the top of the
first
catalyst bed and fresh feed, recycle liquid and hydrogen introduction on top
of
each catalyst beds. If the dilution agent is product recycle, the product
recycle/fresh oil-ratio is from 5-30:1, preferably 10-30:1 and most preferably
12-
25:1. The catalyst beds can be located in same pressure vessel or each bed in
a
separate pressure vessel. Hydrogen is fed in excess to the theoretical
chemical
hydrogen consumption and the feedstock is converted totally or almost totally
within each catalyst bed. Using these procedures, harmful, partially converted
product intermediates are avoided, the temperature needed for reaction
initiation is
achieved in the beginning of each catalyst bed, the rise of reaction heating
is
controlled in the catalyst beds and the catalyst life is improved
considerably.
Hydrodeoxygenation of triglycerides facilitates controlled decomposition of
the
triglyceride molecule contrary to uncontrolled cracking. Double bonds are also
hydrogenated during the controlled hydrotreatment. Light hydrocarbons and
gases
formed, mainly propane, water, CO2, CO, II2S and NH3 are removed from the
hydrotreated product.
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It was surprisingly observed in examples that product recycle dilution can
prevent
or remarkably decrease the reactions between free fatty acids and the
formation of
high molecular weight compounds during hydrotreating, when at least 5:1
(product recycle):(fresh oil)-ratio was used. The effect of product recycle is
based
on two phenomena: dilution effect of recycle and more controllable and reduced
reaction temperatures used over catalyst bed during hydrodeoxygenation. Higher
temperatures and especially hot spots of catalyst bed promote ketonisation
reactions. Due to this invention, it is possible to use various sources of bio
oils
and fats without the need to remove fatty acids. After the hydrotreatment
step, the
product is subjected to an isomerization step.
Isomerisation of n-paraffins formed during hydrotreatment
In the second step of the process, i.e. in the isomerization step,
isomerization is
carried out which causes branching of the hydrocarbon chain and results in
improved performance of the product oil at low temperatures. The isomerisation
produces predominantly methyl branches. The severity of isomerisation
conditions and choice of catalyst controls the amount of methyl branches
formed
and their distance from each other and therefore cold properties of bio diesel
fraction produced. The product obtained from the hydrotreatment step is
isomerised under isomerisation conditions with an isomerisation catalyst.
In the process according to the invention, the feed into the isomerisation
reactor is
a mixture of pure n-paraffins and the composition of it can be predicted from
the
fatty acid distribution of individual bio oils. During the hydrotreating step
of the
process, triglyceride oils and other fatty acid derivatives and fatty acids
are almost
theoretically converted to n-paraffins. Additionally propane is formed from
the
glycerol part of triglycerides, water and COx from carboxylic oxygen, I-12S
from
organic sulphur compounds and NH3 from organic nitrogen compounds. It is
substantial for the process that these gas phase impurities are removed as
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completely as possible before the hydrocarbons are contacted with the
isomerization catalyst.
The isomerization step may comprise an optional stripping step, wherein the
reaction product from the hydrotreatment step may be purified by stripping
with
water vapour or a suitable gas such as light hydrocarbon, nitrogen or
hydrogen.
The optional stripping step is carried out in counter-current manner in a unit
upstream of the isomerization catalyst, wherein the gas and liquid are
contacted
with each other, or before the actual isomerization reactor in a separate
stripping
unit utilizing the counter-current principle.
In the isomerisation step, the pressure varies in the range of 20-150 bar,
preferably
in the range of 30-100 bar and the temperature varies between 200 and 500 C,
preferably between 280 and 400 C.
In the isomerisation step, isomerisation catalysts known in the art may be
used.
Suitable isomerisation catalysts contain a molecular sieve and/or a metal
selected
from Group VIII of the Periodic Table and/or a carrier. Preferably, the
isomerisation catalyst contains SAPO-11 or SAPO-41 or ZSM-22 or ZSM-23 or
ferrierite and Pt, Pd or Ni and A1203 or 5i02. Typical isomerization catalysts
are,
for example, Pt/SAP0-11/A1203, Pt/ZSM-22/A1203, Pt/ZSM-23/A1203 and
Pt/SAP0-11/5i02. Most of these catalysts require the presence of hydrogen to
reduce the catalyst deactivation.
An isomerised product, which is a mixture of branched hydrocarbons and
preferably branched paraffins boiling in the range of 180 ¨ 350 C, the diesel
fuel
range, and having one carbon atom less than the original fatty acid chain, is
obtained. Additionally some gasoline and gas may be obtained.
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Advantages of the invention
The invention provides a method for reducing the formation of higher molecular
weight compounds during the hydrotreatment of a feed obtained from plant oils
and animal fats and which may contain free fatty acids.
It was surprisingly found that the problems of prior art processes may be
avoided
or at least significantly reduced by the improved process according to the
invention, comprising a hydrotreatment step and an isomerisation step wherein
product recycle or another dilution agent in the hydrotreatment step in
combination with reduced operation temperature result in important
improvements, particularly when the fresh feed contains more than 5 wt% of
free
fatty acids. A special reactor configuration and high dilution of fresh feed
introduced into hydrotreatment are used in the method. The extent of side
reactions is decreased and the stability of catalyst during hydrotreating is
increased during the hydrotreatment step.
In the examples it was be seen that the ratio of at least 5:1 (recycle:fresh)
significantly decreased the formation of high molecular weight products, when
the feedstock contains 10 wt-% of free fatty acids (calculated from fresh oil)
is
used. Using at least 5:1 recycle ratio and reduced reaction temperature, free
fatty
acids can be processed without the need for deacidification. High quality
hydrocarbons are obtained, suitable for the diesel fuel pool with high yield.
The invention is illustrated in the following with examples presenting some
preferable embodiments of the invention. However, it is evident to a man
skilled
in the art that the scope of the invention is not meant to be limited to these
examples.
Examples
All hydrotreatment tests were performed in the presence of hydrogen.
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Tall oil feed (100 % free fatty acids) without product recycle
Hydrotreating of tall oil (100 % free fatty acids) with NiMo catalyst was
carried
out at 50 bars pressure, LHSV 1.5 and reaction temperatures from 340-360 C
10 without product recycle. Hydrogen oil ratio was 900 normal liters 112
per liter oil
fed. The hydrotreating of tall oil 100 % free fatty acid feed caused rapid
deactivation of NiMo catalyst, and formation of heavy weight compounds and
aromatics was observed. Bromine indexes increased during the run even if
temperature compensation of catalyst was used (Figure 8). Product oil
contained
15 about 7 wt-% aromatics and about 7 wt-% heavies (>375 C boiling).
Density
(50 C) of product oil was high 777.1 kg/m3 compared to typical values with
rapeseed oil hydrotreated product oil (761-762 kg/m3) using lower reaction
temperature and optimized reaction conditions.
20 Example 2. Comparative example
Tall oil fatty acid feed (100% FFA) at high reaction temperatures without
product recycle
Hydrotreating of tall oil fatty acid feed (100% FFA) at high reaction
temperatures
370-385 C was carried out without product recycle. Rapid deactivation of NiMo
catalyst and formation of heavy weight compounds and aromatics was observed.
Density of hydrotreated oil (table 1) was significantly higher than in
rapeseed oil
runs (typically 761-762 kg/m3). Both oils contained mainly C18 fatty acids (-
90-
wt-%) and rather steady formation of water was observed during run. During the
tall oil hydrotreating about 7-8 wt-% heavier molecular weight compounds and
8.1 wt-% aromatics were formed. These side reactions are caused by
concentrated
fatty acid feed and too high reaction temperatures. Deactivation of catalyst
is
clearly seen from increasing bromine indexes. During the satisfactory
operation
bromine index should be below 50. Table 1 describes densities, bromine
indexes,
reaction temperatures and water formed during test runs for 2 to 14 days using
tall
oil fatty acid feed (100% FFA) without recycling.
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Table 1.
Duration of 2nd 4th 6th 9th llth 12th 13th 14th
test run day day day day day day day day
Temperature, 370 375 378 381 385 385 385 385
C
Density, 771.8
773.1 773.7 776.5 779.1 779.8 780.5 781.2
50 C, kg/m3
Bromine 101 150 188 198 247 269 300 330
index
Product 9.37 9.5 9.81 10.3 10.2 10.0 10.1 10.2
water, %
Example 3. (Comparative example)
Effect of metal impurities of bio oils on the catalyst performance
Tube reactor hydrotreatment test runs were carried out using crude rapeseed
oil,
crude animal fat and purified rapeseed oil. Analysis of these feeds are shown
in
Table 2. Crude feeds contained significant amount of metals, organic
phosphorus,
sulphur and nitrogen compounds. Purified feeds contained only trace levels of
these impurities
Table 2. Impurity levels of crude and purified plant oils and animal fats
Impurity Unit r¨Crude Purified Crude
Rapeseed Rapeseed Animal fat
oil oil
I PPm 90 µn-70
162
Metals (total) ............
õ ............
riPm
33 7.2 1125
rOrg.nitrogen
0.8 0.7 10.8
Free Fatty acid, GPC
KOH/g 1.0 0.1
21.5
Total Acid Number
[13T3In 110<1 86
[¨Phosphorous
ppm 3 1 85
rSulphur (original)
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Test runs using crude, unpurified oils/fats showed that catalyst needed higher
temperatures to work properly, but gradually lost its activity (Figure 5).
Triglycerides and increased bromine number of product oil was found. High
amount of metals were also detected on to the catalyst. Temperature profile of
the
catalyst bed showed that top of the catalyst bed was deactivated and reaction
section moved forward (Figure 4), when reactor heating was maintained steady.
Metals adsorbed on to the catalyst also promote side reactions like decarb-
reactions.
First hydrotreatment test run was carried out using crude rapeseed oil.
Purified
rapeseed oil was used as a reference feed. Purified rapeseed oil achieved
complete
IMO conversion at 305 C using WHSV=2. Crude rapeseed oil gave total IMO
conversion not until reaction temperature 330 C was used with space velocity
WHSV=1. It was however seen from temperature profiles over the catalyst bed
that first part of catalyst was deactivated very quickly. In Figure 4,
reaction
temperature profile over catalyst bed and performance of crude rapeseed oil
are
presented.
Second hydrotreatment test run was carried out using purified rapeseed oil and
crude animal fat. Purified rapeseed oil was used as a reference feed. Purified
rapeseed oil with product recycle achieved complete IMO conversion at 305 C
using WHSV=1. Crude animal fat with product recycle did not give complete
IMO conversion at 305 C using WHSV=1. It was seen from GPC analyses that
product oil contained triglycerides and catalyst also significantly
deactivated
during crude animal fat feed. Pumping problems was also observed during crude
animal fat feeding. Performance of crude animal fat is presented in Figure 5.
Example 4. (Comparative example)
Effect of free fatty acids (10 wt-% in fresh feed) on the formation of high
molecular weight hydrocarbons
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Hydrotreatment was carried out using purified rapeseed oil as reference feed
without product recycle. A test run was carried out at 305 C and 50 bars
pressure
using WHSV= 1 and H2/oil-ratio=1000. Sulphur content of feed was 570 ppm.
During a second hydrotreatment test period stearic acid was fed (10 wt-% from
rapeseed oil) along with purified rapeseed oil using same reaction conditions
without product recycle. It was right away observed that the extent of high
molecular weight compounds increased gradually from initial level ¨3 wt-% to
¨8
wt-%. These higher molecular weight compounds (molecular weight double or
more of the feed) are not in the boiling range of diesel fuel and thus
decrease
diesel yield and potentially shorten the catalyst life. Thus free fatty acids
in bio
oils make their processing more difficult. In Figure 2 the increase of
formation of
high molecular weight hydrocarbons is observed, when 10 wt-% free fatty acids
was fed along with purified rapeseed oil triglycerides without product
recycle.
Example 5.
Effect of product recycle on preventing formation of unwanted heavy side
reaction compounds when the feed contained 10 wt-% free fatty acids
A hydrotreatment test run was carried out using 10 wt-% stearic acid
containing
purified rapeseed oil as reference feed without product recycle under
following
reaction conditions: WHSV= 1, 50 bars, 305 C, 112/oil-ratio=1000 and sulphur
content of feed =570 ppm. During the second hydrotreatment test run period
same
feed was diluted with product hydrocarbons so that (fresh oil )/ (product
recycle) -
ratio was 1:5. WHSV of fresh oil was maintained at 1, therefore WHSV of total
oil feed increased to 6. The reaction temperature was kept at 305 C and
reaction
pressure at 50 bars. H2/(fresh oil)-ratio was maintained at 1000. IMO product
(n-
paraffins) simulated product recycle, which was mixed in advance with fresh
oil.
The initial content of heavy hydrocarbons in the recycle was ¨0,4 wt-%.
It was unexpectedly observed that the formation of heavy hydrocarbons was
almost totally prevented or at least very significantly decreased when product
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recycle was used (Figure 3). This is most probably caused by significantly
diminished side reactions of free fatty acids wherein a carboxylic acid
molecule
can react with another carboxylic acid molecule to form a higher molecular
weight compounds. In Figure 3 the effect of product recycle on preventing the
formation of unwanted higher molecular weight by-product is presented. Table 3
presents analysis results of the feed and products.
Table 3. Analysis results of the feed and products
Feed analyses Product
analyses
AR AR + 10 wt- AR + 10 wt- AR + 10 wl
(10% % stearic % SA + % SA +
AR SA) + acid Recycle
Recycle
Recycl (10% FtEC without after after
Property Method Units AR e feed SA) 1: 5 recycle
196 hours 552 hours
Density, 15 C
calc. D4052 kg / m3 920.4 788.1 915.8 807.2 790.8
788.3 788.3
Density, 50 C D4053 kg / m3 897.6 761.4 893.2 781.2
764.2 761.7 761.7
Br-index D2710 mg/100 g 53.7 21.5 26
Br number D 1159 g/100 g 56 49.1 6.3
Iodinenumber D5554 g/100 g 112 103 18
HC GPC area-% 99.6 83.0 94.3 99.6
99.6
Fatty acids GPC area-% 0.7 0 10.6 1.8 0 0 0
Heavy HC GPC area-% 0 0.4 0.5 5.7 0.4
0.4
Diglycerides GPC area-% 2.3 0 2.4 0 0 0
Triglycerides GPC area-% 97 0 87 14.7 0 0 0
SA = Stearic acid, AR = purified rapeseed oil, REC = product recycle, ITC =
hydrocarbons, heavy ITC = high molecular weight hydrocarbons
Example 6. (Comparative example)
The effect of lower reaction temperature on the selectivity of n-paraffins and
oil yield
Studies were carried out with NiMo catalyst using rapeseed oil as feed and
reaction temperatures 280-330 C and 340-360 C, WHSV=1 and reactor pressure
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5 of 50
bars. Alkali raffinated rapeseed oil triglycerides contained mainly C18 fatty
acids. C18 fatty acids contributed about 89 wt-% of all fatty acids in
rapeseed oil.
Theoretical amount of n-paraffins formed from rapeseed oil fed is about 86.4
wt-
% (calculated from rapeseed oil fed).
10 Complete IMO conversion with almost theoretical n-paraffin yield was
accomplished, when well controlled reaction temperatures <330 C were used.
Almost theoretical n-paraffin yields tell us from complete IMO conversion and
very controllable operation without significant side reactions. High amount of
side
reactions (cyclisation, aromatisation and cracking) and low n-paraffin yield
were
15 observed when unnecessary high reaction temperatures 340-360 C was
used. In
Figure 6 the conversion of rapeseed oil triglycerides to n-paraffins is
presented.
Example 7.
Stability of catalyst
The stability of NiMo-catalyst using palm oil model feed (impurities added)
along
with product recycle (catalyst life test) was carried out using following
reaction
conditions: Reaction temperature = 300-305 C, Reactor pressure = 40 bars,
WHSV (fresh) = 0,5, WHSV (total) = 3, 112/0i1 (fresh) = 900, Sulphur in feed =
100 w-ppm. Palm oil was used as a main component of feed, but it was modified
with animal fat, fractions of free fatty acids, crude rapeseed oil, and
lecithin in
order to get suitable specification of impurities of test feed. Fresh feed
analysis is
presented below in table 4. Fresh oil was then diluted in advance with 1:5
ratio of
IMO product (simulates product recycle). The duration of test run was over 9
months. Stabile operation was maintained (table 4 and Figure 7) and the
formation
of heavies was steady over the whole test run Figure 7.
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Table 4. Stability of catalyst
Fresh Feed
analysis Product oil analysis
1898 3408 5601
Run duration 383 hours hours hours
hours
Analysis Method Unit
Density, 15 C D4052 kg / m3 804.9 787.4 785.6 785.3
784.9
Density, 50 C D4052 kg / m3 778.8 760.7 758.9 758.6
758.1
Br-index D2710 mg/100 g 29200 33 48 33 11
HC GPC area-% o 99.3 99.4 99.3 99.4
Fatty acids GPC area-% 1.2 o o o o
Monoglyc/high
molec. weight HC GPC area-% 0.3 0.7 0.6 0.7
0.6
Diglycerides GPC area-% 6.3 o o o o
Triglycerides GPC area-% 92.1 o o o o
TAN D664 mg KOH/g 2.1 -0 -0 -0 -0
Sulphur D 5453 ppm 3 1.2 2.0 2.7 2
Nitrogen D4629 mg/kg 6 <1 <1 1.2 <1
Sodium, oil AAS mg/kg 3 0.4 <0.1 <0.1 <0.1
Calcium, oil AAS mg/kg 2 0.3 <0.1 <0.1 <0.1
Magnesium, oil AAS mg/kg 0.3 <0.1 <0.1 <0.1 <0.1
Molybdenum, oil AAS mg/kg - <0.5 <0.5 <0.5 <0.5
Aluminum, oil ICP metals mg/kg <2 <2 <2 <2 <2
Iron, oil ICP metals mg/kg <1 <1 <1 <1 <1
Nickel, oil ICP metals mg/kg <1 <1 <1 <1 <1
Phosphorus, oil ICP metals mg/kg 4 <1 <1 <1
<1
