Note: Descriptions are shown in the official language in which they were submitted.
CA 02219244 1997-10-27
W 096~34226 PCTAN096/00098
Method and appara~us for the manu~acture of a
hydrocarbon product as well as the product
itself
The present invention relates to a method and a plant
for producing a hydrocarbon product in the form of a 5Us--
pension comprising hydrate particles of at least one gas
hydrate suspended in a hydrocarbon-based liquid. The hydro-
carbon product itself also repr2sents a paF~ or this lnven-
tion.
Suspensions including particles of gas hydrate sus-
pended in a hydrocarbon-based liquid, are previously known
per se, especially as a temporary intermediate product used
in connection with treatment or transport of gas hydrate.
In this connection reference is made to US patent No.
2.363.529 which in particular relates to a suspension used
in connection with a controlled fractioning o~ different
hydrate-forming hydrocarbons from a fluid, and also to US
patent No. 2.356.407 which in particular relates to the use
of a similar suspension for transporting gas hydrate from
one place to another, for instance for storage. Finally US
patent No. 3.514.274 should be mentioned. This patent
describes how natural gas may be transported as a hydrate in
a slurry based on liquid propane. However, such a slurry
will be unstable at atmospheric pressure unless the tempera-
ture is below -42~C. In contrast to this the present inven-
tion provides a hydrocarbon product being stable at atmos-
pheric pressure even when the temperature is only a few
degrees below zero.
According to the present invention a simplified and
improved method for the generation of a hydrocarbon product
is obtained, in which the gas hydrate particles are sur-
rounded by or suspended in a hydrocarbon medium; and the
invention also relates to a plant for producing a hydro-
carbon product. Finally the present invention relates to a
hydrocarbon product as mentioned above, with enhanced
product qualities.
The gas being included in the gas hydrate when
3S liberated may be used in many different manners. The gas
may be used in power production, for instance in power
plants, or in central heating, or may be distributed to
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consumers through pipelines. The hydrocarbon component in
the product may also be used as a raw material for the
production of chemicals, and products such as synthetic gas,
methanol, acetic acid etc. The heavier components in the
product may be used as components of fuel or propellants,
and as raw material within the petrochemical industry.
In this connection it can also be mentioned that it is
previously known to produce gas hydrate for transporting
and/or storing gas at advantageous pressure and temperature
conditions, cf. Norwegian patent No. 175.656.
However, it has been found that great problems arise
when gas hydrates are used in connection with transporting
and storing gas using conventional technigues, for instance
because the gas hydrate is cintered into a compact material
during storage, which material is difficult to handle as it
sticks to walls in the storage tanks and to the inner sur-
faces of pipelines.
In addition, hydrate which is treated in a conventional
manner has to be stored at high pressure or at very low
temperatures. It would have been advantageous to reduce the
storing pressure of the hydrocarbon product to 1 atmosphere.
This may be done when the present invention is used, as the
gas hydrate included in the hydrocarbon product then obtains
a stable and even temperature through the entire volume of
the product, and this temperature may easily be maintained
at a level ensuring a stable storage also at normal
pressure.
The main object of the present invention is to provide
a method and a plant for the efficient generation of large
quantities of a new product including gas hydrate in large
guantities, which requires an efficient heat transfer during
the hydrate generation.
An additional object of the invention is to provide a
new, preferably pumpable hydrocarbon product which is easy
to handle, which in practical operation includes a hydro-
carbon slurry or paste having a highest possible hydrate
percentage, and in particular a product being stable at the
prevailing pressure and temperature conditions in the
-
CA 02219244 1997-10-27
W 096/34226 PCTA~O~ C9
transport and storing areas, and accordingly a product which
does not release gas which can lead to an undesirable
increase of the pressure. Still a further object is to
provide a hydrocarbon product which does not comprise any
free water or ice or only contains insignificant amounts of
~ free water, i.e. water which is not converted into hydrate,
as the occurrance of such free, non-converted water is
deemed to represent one major reason why gas hydrate
previously has been difficult to handle. In addition such
free, non-converted water, will represent additional losses,
as the water both represents an additional weight which
again requires additional power during transport, and does
not contribute to the transport of gas at all.
The expressions Uinsignificant'' or "unimportant" amounts
of water or frozen water, indicate that the content of free,
non-converted water should not be at such a level that the
overall content of hydrate-forming, gaseous components in
the product will be below an acceptable level. Economical
considerations have shown that the conditions are acceptable
when the relative volume of hydrate-forming gaseous com-
ponents before the generation of hydrate as compared to the
volume of solid gas hydrate and frozen water, after such
generation, is 130 or higher. Accordingly the hydrocarbon
product should include at least 130 Sm3 gas/m3 solid matter.
In particular it should be mentioned that the process condi-
tions are set to obtain a final product where the solid,
hydrate-containing material has a gas content corresponding
to a density of at least 130 Sm3/m3, preferably above 150
Sm3/m3 solid material, when methane has been used as the
hydrate-forming material.
Yet another object is to provide a method for gene-
rating large amounts of a hydrocarbon product, continuously
or in batches, by means of known and well-proven chemical
engineering equipment.
Yet another object is to provide a new method for
generating a new hydrocarbon product by a process including
two-step direct cooling of the original materials and
intermediate products with the aid of two identical or two
CA 02219244 1997-10-27
W 096/34226 PCTA~03610~~9
different cooling agents.
Yet another object is to provide a new plant which
either uses a common container for both generating and
cooling, or which instead uses separate containers for
carrying out each process step.
It is also an object to achieve a method which reduces
the risk of ice and hydrate formation at undesired locations
in the plant, for example in places where there is danger of
clogging.
A further object of the present invention is to provide
a suspension in which large amounts of gas hydrate occur in
the form of particles being surrounded by, or suspended in,
a carrying liquid, which makes possible an effective heat
transfer between the gas hydrate included in the mass and
the outside environment, and which thereby ensures an
effective regulation and control of the overall temperature
in the product.
Thus, both an energy carrying medium which can be
stored and handled simply, substantially by means of
traditional storage and transport equipment for liquids,
pastes, dispersions and slurries, and at the same time a
suspension with a very high energy content lying between the
energy content of liquified gas (LNG) and that of compressed
(CNG) gas, have been achieved without encountering corre-
sponding problems of very high pressure and/or very lowtemperatures.
In order to fulfil the object of effective generation
of large quantities of gas hydrate, direct contact is made,
according to the present invention, between a first cooling
medium which is added and the hydrate-forming hydrocarbons,
where the latter usually exist in the form of gas. Large,
direct contact surface between gas and cooling agent is the
deciding factor. Such direct cooling has been shown
empirically to be the cooling method which gives the highest
rates of hydrate production and which therefore best lends
itself to industrial applications.
Another advantage of the invention is that the process
cannot only be carried out in a stationary plant on land,but
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W O 96134226 PCT~N096~'~0C9
can also be adapted for use on floating installations and
ships at sea where there is a need for storing gas produced,
either alone or associated with other petrochemical pro-
~ ducts. Such compact plants can be built because the plant
according to the present invention is relatively simple, andto a large extent is comprised of components which already
are well tested and commercially available in the form of
pumps, valves, cooling systems, tanks and so on.
The listed advantages and objectives can be achieved by
using a method according to one, or several of the patent
claims below, by implementing the method by means of equip-
ment in accordance with the apparatus claims set out below,
and by a product according to the product claims set out
below.
A broad outline of the production of a hydrocarbon
product according to the present invention is given in the
following four steps.
- In step a) large quantities of hydrate are generated.
- In step b) redundent water is removed from the hydrate.
- In step c) the hydrate is cooled by the addition of a
cold liquid containing hydrocarbon while it is ensured
that the hydrate does not dissociate, and,
- In step d) the end product is taken out of the process.
Any residual non-converted water will form a film
around the individual hydrate particles, and hydrate
products which contain large quantities of non-converted
water will be difficult to handle if they are subjected to
temperatures below the freezing point of water. Any re-
dundant water can be removed from the hydrate by many means
in order to form a "dry" hydrate, i.e. a hydrate where large
quantities of non-converted water are no longer present, at
least not in an amount sufficient to cause transport prob-
lems. The three most important methods of removing non-
converted water are:
- The hydrate can be treated mechanically, e.g. drained,
compressed or compacted so that superfluous water is pressed
out. Known treating units such as filters, centrifuges, or
hydrocycolones can be of use. In any case this method will
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not remove all the water.
- Further amounts of hydrate-forming hydrocarbons can be
added, as liquids or gases, and brought into contact with
the non-converted water so that also the non-converted water
is converted into hydrate. By ensuring the addition of an
extra amount of hydrate-forming components at suitable
pressure and temperature conditions, all the residual free
water can be converted to hydrate(s) so that the finished
hydrate will be completely dry.
- Excess amounts of water can also be removed by the
addition of a water-absorbant medium, e.g. an alcohol or a
ketone, e.g. acetone. Such media will however have a
certain tendency also to dissolve hydrates and for that
reason alone should only be used in special circumstances.
The expression "remove" therefore includes all these
methods and combinations of them.
According to the present invention direct cooling is
used, as already mentioned, where the product to be cooled
and the cooling medium being used, come into direct contact
with each other. This direct cooling can be conducted
substantially in at least two steps, by the use of a first
and a second cooling liquid, also denoted cooling agents.
The first cooling liquid is used during hydrate generation
in step a) and has as its most important object to remove
the amount of heat generated during the formation of
hydrates, so that the temperature in the hydrate-generating
zone is kept within the hydrate-generating limits at a given
process pressure. The cooling liquid thus shall not only
cool down "the gas" or the hydrocarbons which can form
hydrates, but also the hydrate produced and the water
present, to the extent being necessary. However, the cool-
ing in the first step only goes down to a temperature which
ensures that hydrates are formed in the desired amounts.
The first cooling liquid can be water, and in that case must
be removed or converted to hydrate in step b), before the
second cooling liquid, under process step c), brinqs the
average temperature of the hydrocarbon product below the
freezing point of water
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The second cooling liquid will perform several tasks,
but first and most importantly it will cool the hydrates
generated so that they are stable at the pressure of the
~ surroundings, e.g. at atmospheric pressure. The second
cooling liquid cools the product down to temperatures of
below 0~C but first after virtually all water is removed.
The non-converted water which is removed from the
intermediate product in step b) can be, wholly or partially,
cooled again and recirculated into the process, e.g. to step
a) for producing further amounts of gas hydrate.
Any recirculation of the first and the second cooling
liquids for maintaining the desired temperature of the
product, takes place in separating the cooling agent from
the hydrate to a greater or lesser extent, being cooled anew
and recirculated alone. It is preferred that the recircu-
lation flow which is cooled, should not contain hydrate
particles, ice or water, as such components will have a
tendency to precipitate as ice or hydrate on the cooling
surfaces in the heat exchangers. The recirculated, newly
cooled cooling liquid again cools the product by direct
contact.
A crucial point concerning the present invention is
that all the gas hydrate particles are in close contact with
a liquid hydrocarbon. This fact ensures a stable temperature
through the complete mass of hydrate and makes fast tempera-
ture changes within the mass of hydrate possible when
desired, as the mass nowhere will be thermally insulated
from the temperature controlling medium, that is the liquid
based on hydrocarbons, also referred to as the second cool-
ing medium.
The suspension of hydrate particles in the firstcooling medium, possibly including a minor amount of free,
non-converted water as it is found at the end of stage a, is
referred to as the first intermediate product and has an
average temperature just above the freezing point of water
and a pressure equal to the hydrate-generating pressure.
The suspension of hydrate particles in the second
cooling medium, with a ~;n; of free, non-converted water,
CA 02219244 1997-10-27
W 096/34226 PCTANO9~ 9
as is found at the end of the process step b, is referred to
as the second intermediate product. This has a temperature
T4. The end product itself, however, shall be cooled down to
a temperature being so low that the hydrate is stable at the
prevailing storage pressure. The temperature in the end pro-
duct may e.g. be reduced to -40~C and the pressure be reduced
to approximately 1 atmosphere. A more detailed explanation
is given below.
When the product has obtained stable temperature and
pressure conditions, and superfluous cooling liquid has been
removed, so that the product preferably has obtained a
pumpable/transportable consistence, the desired end product
has been obtained.
The end product may be handled by means of conventional
transporting and storage equipment, developed for other
products such as paste and slurries.
The conditions which have to be met to ensure that
hydrate is generated are on the one hand, that pressure and
temperature are within the hydrate-generating limits. In
addition it is rather important that the hydrate-forming
hydrocarbons and the water, possibly in frozen condition as
snow or ice, are in close contact for long enough that the
conversion into hydrate becomes as complete as possible.
When the hydrate generation takes place by spraying atomized
water into the upper part of the hydrate-generating zone in
the container 2 it is important that the container is high
and that generated hydrate does not build up in too high
piles in the container. This will ensure that the contact
time between water and gas is sufficiently long-lasting so
that large amounts of hydrate may be generated. In fig. 1, 2
and 3 it i5 assumed that the container 2 can have a very
great height.
If instead a design solution is selected in which the
gas is bubbled from the bottom of the container through
water, it is important that the gas is well distributed
through nozzles and the height up to the surface of the
water is sufficient.
Another important condition is that the flow of
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W 096t34226 PCT~0~61C~~9~
material out from and into the hydrate-generating zone is
sufficiently large.
The inter-relations of the temperatures referred to in
this application are as follows:
T1 is the temperature of the first cooling medium when it
enters the hydrate-generating zone. T, has to be so much
below the equilibrium temperature for generation/dis-
sociation of gas hydrate at the prevailing working
pressure, that hydrate will be generated.
T2 is the temperature of the first cooling medium when it
leaves the hydrate-generating zone. This temperature is
rather close to the equilibrium temperature of hydrate.
T3 is the temperature of the second cooling medium when it
is supplied to the cooling zone.
T4 is the temperature of the end product.
T5 is the temperature of the second intermediate product.
T6 is the temperature within the hydrate when the hydrate
product in step c is cooled down to a temperature below
the freezing point of water, at which the cooling
medium comprising destabilizing amounts of volatile
components may be replaced by a medium having a
substantially lower content of such components,
and the most important relative conditions can be expressed
as follows:
T, << T og oCc ~ T~
T3 < T4 << oCC og
T4 < T~ < oCC s TL.
Below some further details are mentioned, which can be
of great importance.
It should be noted that the first cooling liquid may be
supplied at so low temperature that small amounts of ice are
generated locally for a short time. However, it is important
that the first cooling medium is not supplied in so large
~ amounts or at so low temperatures that large quantities of
ice are generated. Accordingly this invention also covers
methods according to which some ice is generated in the
hydrate-generating zone, but is later melted by mutual heat
transfer between the remaining quantities of gas and liquids
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W O 96/34226 PCTA~0~6~C~9
in the hydrate-generating zone.
The temperature of the first cooling medium may be
below 0~C, in particular when the cooling medium is a
hydrocarbon, when it is supplied to the hydrate-generating
zone. This fact is also understood from the conditions
below:
T4 < T6 ~ 0~C ~ Ts
T~ is the temperature in the hydrate after it is
generated and after removal of non-converted water in step
b, i.e. before the cooling process in step c. Accordingly T~
has to be above or equal to 0~C. T6 is the temperature which
the hydrate at least must have to ensure that the hydro-
carbon medium which incorporates the stabilizing amounts of
volatile components, can be replaced by a medium having a
low content of such components so that the generated hydrate
shall not dissociate due to the absence of stabilizing
consentrations of hydrate-forming components. When the
temperature in the gas hydrate has come below the tempera-
ture T6, the gas within the gas hydrate for all practial
purposes will be irreversibly included in the gas hydrate
structure. This is also according to the conditions stated
above.
The water which is to be converted into hydrate may
already at the start of the process be introduced as snow or
ice. Then the requirement for cooling the first cooling
medium is reduced. It should therefore be pointed out that
the first cooling medium can be supplied at such a tempera-
ture and in such an amount that a small amount of ice is
generated or is maintained, but not in such amounts that ice
is carried over to the next step in the process.
The second cooling liquid which is used in step c,
consists of a hydrocarbon-based liquid, and the temperature
in this liquid has to be sufficiently low that at the output
from the cooling zone there is obtained a mixture of gas
hydrate and hydrocarbon liquid having a temperature which
leads to a stable mixture at the surrounding pressure which
normally will be approximately 1 bar. However, it is impor-
tant that the first cooling liquid, in particular when this
CA 02219244 1997-10-27
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11
liquid includes water, must not have a temperature below its
own freezing point. The first cooling medium can, however,
also include hydrate-forming hydrocarbons which may be other
hydrocarbons than those which exist as gas in the hydrate-
generating zone.
When the composition of the second cooling medium is
considered, the following should also be noted:
On the one hand the total partial pressure of the
hydrate-forming hydrocarbons should not be reduced substan-
tially below the limit for hydrate generation at the presenttemperature. This is to ensure that the second intermediate
product which still has a temperature above O'C is maintained
stable or does not dissociate.
If the second cooling medium does not include any
hydrate-forming components, the supply of this cooling
medium will reduce the partial pressure from those hydro-
carbons and accordingly the hydrate will become unstable and
will dissociate. Therefore the cooling zone, at least at the
beginning of step c, should be supplied with sufficient
amounts of hydrate-forming hydrocarbons to ensure that the
hydrate is kept stable until the temperature has reached
T=T~, i.e. the temperature of the end product.
If insufficient amounts of hydrate-generating
components are present during the cooling in step c, there
is a risk that a portion of hydrate will dissociate before
the temperature T=T4has been reached.
On the other hand it is required that the content of
destabilizing components should be removed or reduced. These
components may be methane, ethane, propane or other volatile
components. These components should be removed from the
hydrocarbon medium to obtain a stable end product. Accord-
~ ingly a hydrocarbon medium included in the end product will
not at the temperature T=T include the stabilizing compo-
- nents in amounts leading to a collective partial pressure
from these components over the hydrocarbon medium, exceeding
the pressure in the environments (normally approximately 1
bar) at temperature T=T .
The hydrocarbon medium in the end product should not
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include the stabilizing amounts of light hydrocarbons such
as methane and ethane. The partial pressure for each of the
destabilizing components can, at least as a first approxi-
mation, be calculated from Henrys law: Pi = H . C , in
which:
P~ x;mum acceptable partial pressure of component i,
Hi = Henrys constant found experimentally, and
ci = the consentration (measured in mol/volume unit) of
said component.
The sum of the partial pressure of such volatile components,
~Pi must be below the prevailing pressure in the surround-
ings. If the sum of the partial pressures exceeds this
limit, the end product becomes unstable as it releases
volatile gases such as methane, ethane and to a certain
degree also propane when the end product is exposed to the
prevailing pressure (- 1 bar) of the environments, even when
the final temperature T=T4 (<<O~C) has been reached.
The method and the apparatus according to the invention
can comprise a mechanical treatment, e.g. implemented by
means of at least one mixer. The purpose of this solution is
to prevent generation of agglomerates and large quantities
of hydrate and thus contribute to an increased transport of
hydrate-forming components to the interfaces between hydrate
and non-converted water in the compound, and also to an
equalization of the temperature in the hydrocarbon product.
Even if the hydrate product may be exposed to mechani-
cal treatment to produce a suspension of hydrate particles
in a cooling liquid, such a mechanical treatment will not
always be required. Depending on the composition of said
liquids, and the pressure and temperature conditions, the
hydrate can in many circumstances disintegrate naturally and
thereby generate small separated particles so that a sus-
pension is generated as soon as a hydrate is brought
together with the hydrocarbon containing liquid.
In other words the object is to obtain a product which
represents a mixture of
- a dry hydrate produced from hydrate-forming hydro-
carbons, and
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13
- a hydrocarbon-containing liquid,
as none o~ these components includes essential amounts of a
free, i.e. non-converted water, so that the mixture may be
- exposed to temperatures below the freezing point of water
without any risk of ice generation. Accordingly, the
temperature of said mixture can be controlled within wide
limits, and the mixture will be stable at a pressure of down
to 1 atmosphere. Especially this last condition makes the
product unique and well adapted to transport and storing.
Besides, it is very advantageous that the hydrocarbon liquid
has good thermal contact with all the particles within the
suspension and accordingly acts as an efficient temperature
stabilizing and controlling medium for these particles.
Again it is assumed that irreversible generation of large
amounts of ice takes place.
As soon as free water no longer exists, at least not in
considerable quantities, after the generation of hydrate,
e.g. as it is removed according to one of the methods
explained above, the second cooling medium, e.g. the liquid
hydrocarbon, preferably consisting of the so-called
condensate fraction of crude oil anyhow can be supplied
relatively soon after the generation of hydrate, and then at
a temperature which may be substantially below the freezing
point of water, as the risk of ice generation resulting in
clogging, is then strongly reduced.
When the cooling in step c is considered, the following
essential features should be mentioned:
- The cooling is undertaken in the presence of necessary
amounts of hydrate-generating components until the tempera-
ture T has reached a value T.; below O C.- The cooling is preferably undertaken in the absence of
destabilizing hydrocarbon components when T is below T~.
- The destabilizing components may be removed in several
different manners:
i) A medium comprising destabilizing components may be
replaced by a cold hydrocarbon medium which does not
include destabilizing components exceeding a limit
value determined from the stability requirements for
CA 02219244 1997-10-27
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14
the end product, either when the temperature T has
reached the value T6, or when the compound of hydrate
has reached the temperature T=T4, or
ii) step c is undertaken using a different cooling medium
and in the presence of necessary amounts of hydrate-
forming components until the temperature T=T~, whereupon
the content of destabilizing components is removed from
the second intermediate product as the product, (gas
hydrate included in a hydrocarbon medium) is exposed to
a sufficiently low pressure so that the destabilizing
components are released as gas from the hydrocarbon
medium, until an end product satisfying the stability
requirements has been generated. Gas being released may
be compressed anew and recirculated to the hydrate-
generating step a. Pressure relief and removal of
residual portions of volatile components in the end
product can be undertaken while the product is still in
the cooling zone. Remaining portions o~ volatile
components released as gas upon the pressure relief
process, can also be removed after the end product has
been transferred to a storage tank. This storage tank
then has to be equipped with a gas outlet at its upper
point and also must be connected to equipment necessary
for the further handling of released gas, e.g. pipe-
lines and compressors adapted for recirculation of the
gas to step a.
The method in its most simple embodiment, can be
performed in one single production line in which each
operation is undertaken consecutively and continuously. Then
the generation of hydrate takes place first, whereupon any
water present is removed from the hydrate and the dry
hydrate is cooled by means of a suitable cooling liquid.
However, such a simple production line with only one course,
requires a batchwise treatment of gas, both at the input and
at the output. A preferred method therefore is to use a
process including at least two parallel production lines,
each adapted for performing at least some of the above-
mentioned production steps. By an arrangement according to
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W O 96/34226 PCTANO~6/n~9
which the various production lines all the time are at
different steps of the process, so that the production lines
start the production of hydrate at different instants of
- time, the complete plant which accordingly comprises two or
several parallel production lines running in different
"phases", will when considered as a unit obtain a relatively
steady or uniform flow of gas at the input and will also
provide a relatively steady flow of the end product from the
output, and this will in many cases be an essential require-
ment for commercial plants intended for gas management.
Even if a hydrocarbon based liquid is preferred as the
cooling medium, and even if it is deemed to be preferable
that the same cooling liquid is used in all parts of the
plant where cooling is required, the invention also covers
methods and plants using different cooling agents at differ-
ent places in the process. Accordingly a cooling medium may
for example be the same in all the process steps mentioned
above, or a different cooling medium may be used of differ-
ent places, and of these some may comprise or be consisted
of water.
It should also be emphasized that a stable end product
requires conditions which ensure that the hydrocarbon pro-
duct is stable for all practical purposes. The term stable
here also covers the conditions which in the literature
often is referred to as "meta-stable".
The method described above will also more or less
include a description of the plant itself, and this plant
may, in its simplest embodiment comprise only one single
container or reactor provided with inlets for gas, water and
at least one cooling medium, and also provided with outlets
for generated hydrate and excess water and/or cooling
liquid. Such a container may, if required, also be provided
with inlets and outlets adapted for the circulation of at
least portions of the cooling liquid which then may be
cooled down in an external heat exchanger arrangement, and
then lead back to the container for repeated cooling of the
product. In a similar manner the container may, if required,
be provided with a recirculating loop which leads at least
CA 022l9244 l997-l0-27
W 096/34226 PCT~NOg~'03~9 16
some of the excess water back for repeated cooling in a
second, external heat exchanger before the water is returned
to the container.
The different components described above must of course
be interconnected by means of the necessary communication
connections, and must also be provided with the necessary
valves, detectors and control accesories. In a different
embo~;~e,Pt the plant can finally comprise two or several
containers, as the product is transferred to new containers
as the production steps are completed. The plant therefore
preferably may include a separate storage container for
hydrate. This storage container can preferably be heat
insulated and can also be connected to an external heat
exchanger via a circulation loop through which at least a
portion of a liquid fraction of the hydrocarbon product can
circulate. Details of such a plant is described below in
several different embodiments and with reference to the
drawings.
To give a still clearer understanding of the present
invention reference is made to a detailed description of the
method and the plant according to the present invention, and
to the accompanying drawings in which:
Figure 1 illustrates one simple embodiment of the plant
according to the present invention, in which water
which is to be converted into hydrate, may
circulate several times through the generator,
while being cooled in between. The hydrate-
generating zone and the cooling zone is in this
embodiment built in one common container. This
will give a clear presentation of the main
principle.
Figure 2 illustrates a somewhat different embodiment of the
plant, also according to the present invention,
but here the water which is to be converted passes
through the process only once (the once through
principle).
Figure 3 illustrates a different embodiment in which ~he
cooling zone is represented by a separate unit,
-
CA 022l9244 l997-l0-27
W 096134226 PCT~NO~ 9
17
Figure 4 illustrates a flow chart for an industrial plant
and here some calculated values and capacities for
the plant moduls are given, and also some parallel
process paths are assumed in some of the process
stages.
It should be noted that some of the details of the
implementation has been omittet in the drawings, which
mainly comprise the principles of the present invention. It
should also be mentioned that same reference numbers are
used in all of the drawings when considered appropriate,
while the different Figures and parts of Figures are not
necessarely shown in the same scale.
A first explanation of the principle is given with
reference to Figure 1 which illustrates one of the simplest
possible implementations of the present invention. The
Figure shows the main features of a plant adapted to perform
the method according to the invention.
The first embodiment of the method according to the
invention is undertaken in a plant comprising a pressure
tank or container 2 which in step a acts as the hydrate-
generating zone 1 and which in step c acts as the cooling
zone 80, and its associated cooling circuits for water
and/or for the first and the second cooling medium. These
components represent the main components of the system. As
shown in Figure 1 the container or reactor 2 is connected to
a storage unit 50 for storing the end product.
In the following a first embodiment will be explained,
in which water, possibly seawater, is used as the first
cooling medium. A different modification of this first
embodiment, in which a hydrocarbon liquid is used as the
first cooling medium, will be explained later on.
The container or the reactor 2 is made of a suitable
material, e.g. stainless steel, and the construction is such
that the container will endure a selected internal working
pressure, with sufficient margins.
Hydrate-forming hydrocarbons, e.g. a natural gas
comprising 90% methane, 4% ethane, 2% propane and a minor
residual portion comprising heavier hydrocarbons and other
CA 02219244 1997-10-27
W 096/34226 PCTANO~GJ~OO9~
gaseous components (N2, C02, etc.), are supplied through a
pipeline 7 to the upper part 11 of the container 2 filled
with gas. Apart from the requirement that the gas supplied
through the pipeline 7 must have a pressure according to the
selected working pressure, no specific conditions have to be
met when the qualities of the gas are considered, and
accordingly no specific retreatment is required.
Water is supplied and enters the volume of gas 11 in
the upper part of the reactor 2 through a pipeline 5 and is
flooded into the gas volume through at least one nozzle 6.
The water comes from any available source, e.g. a cold
source of fresh water (not shown on the Figure), and must,
when it enters the reactor 2 through the nozzle 6, have a
temperature T=T1 below the equilibrium temperature for
generation/dissociation of gas hydrate at the prevailing
pressure. The relation between the equilibrium temperature
of hydrate and the required gas pressure is known for a
person skilled in the art from literature such as Slaan,
E.D. Jr., "Clathrate hydrates of natural gases", Marcel
Dekker, Inc., New York 1990. Ref. is also made to the
conditions mentioned in the first part of this specifi-
cation.
If the working pressure is selected to 60 bar, a
temperature T=Tl of +10 - +12'C will be sufficiently low for
generation of hydrate in the reactor container 2. However,
it is obvious that the temeprature T; can preferably be much
lower, e.g. down towards 0CC. If the first cooling liguid is
water, this temperature should preferably not be below the
freezing point of water.
If the temperature in the gaseous phase 11 in the upper
part of the reactor container Z is maintained at at least 2-
3'C below the equilibrium temperature at the prevailing work-
ing pressure by supplying a sufficient amount of cold water
as the cooling medium, gas hydrate will be generated and
take the form of a slurry comprising particles of gas
hydrate in water. Just after generation this material will
have a consistancy and a look similar to wet snow and it
will contain a larqe percentage of non-converted water
CA 02219244 1997-10-27
W 096/34226 PCT~N096/00098
19
Generated gas hydrate and non-converted water will
gather in the lower part of the reactor container 2. Gas
hydrate is, just as ice, lighter than water, and the slurry
comprising gas hydrate and water will to a certain degree
separate into one upper fraction containing substantially
all of the gas hydrate as the water containing gas hydrate
slurry, and a lower fraction comprising non-converted water
and residual amounts of gas hydrate particles. However, the
distinction between the two fractions may be unclear or non-
existant if the floating phase comprises relatively largeamounts of gas hydrate particles and the material is moving
or turbulent.
During the generation of hydrate non-converted water
having a temperature T=T2 (which is a little above the
generating temperature T=T) is discharged from the lower
part of the reactor container 2 through the pipeline 13.
When required, water can also be discharged from the system
via a pipeline 19 connected to the pipeline 13. Water to be
returned to the hydrate-generating zone is applied through a
pump 14 and a heat exchanger 17 returning to the water inlet
5 between the pipelines 16 and 18.
The heat exchanger 17 can be cooled by a suitable,
external cooling medium. If large amounts of seawater having
a low temperature, e.g. 5 C or below, are available, it is
viable as a cooling medium. In many cases it would, however,
be more proper to use a cooling medium such as propane,
ammonia or similar media for cooling the recirculated water,
since such media having a normal boiling point substantially
below O'C give higher temperature differences and accordingly
also more compact heat exchangers 17.
The water used for production of gas hydrate has to be
replaced by supplying more water.
When the desired amount of gas hydrate has been gene-
rated in the reactor container 2, the process step a is
finished and accordingly the water supply is stopped, e.g.
by closing a valve (not shown), and non-converted water is
separated from the hydrate in step b, e.g. by draining. If
required a filter (not shown) can be installed above the
CA 02219244 1997-10-27
W 096t34226 PCTA~O~G
discharge pipe at the bottom of the reactor to avoid loss of
gas hydrate.
Considerable amounts of water will still be bound to
the hydrate a~ter such a simple draining process, mainly as
a film of water on the outer surface of the hydrate
particles and in between the particles due to capillary
attraction. These remaining amounts of water may be removed,
as mentioned in the general part of the specification, in
different ways previously known per se. Additional amounts
of hydrate-forming gases and a cooling hydrocarbon agent may
for example be supplied to flow through the hydrate com-
pound, and thus cause a conversion of remaining water into
gas hydrate. This takes place in process step b, but in this
embodiment in the same container.
When the substantial amount of free, non-converted
water has been removed, a second cooling medium containing
hydrocarbon is supplied to the hydrate compound, which is
still kept in the reactor container 2 during process c. This
second cooling medium containing hydrocarbon is supplied
through an inlet 25 for the cooling medium. Accordingly the
product may now be considered as being in the cooling zone
80, even if the product itself has not been taken out of the
container 2. As described in other places in the descrip-
tion, the cooling zone 80 may possibly be arranged within a
different container. The second cooling medium which is
supplied to the reactor container 2 during the process step
c, is supplied in such an amount and at such a temperature
that the mixture of gas hydrate and hydrocarbon obtains the
assumed final temperature T=T , at which the gas hydrate is
stable or meta-stable at atmospheric pressure, i.e. general-
ly at temperature T=T~=- 10 C or below. In the process stage d
the stable end product is transferred to a storage tank 51.
A quite simple estimate based on the specific heat
capacities of hydrocarbon and gas hydrate, will lead to
indications about required amounts of hydrocarbon cooling
medium with a certain amount of gas hydrate, a selected
output temperature T=T of the gas hydrate, and the
temperature T~ in the second cooling medium supplied as well
CA 02219244 1997-10-27
W 096/34226 PCT~No~ Dn3S
as the final ~r~ature T=T4.
The second hydrocarbon cooling medium is preferably a
mixture of light, liquid hydrocarbons, and in particular a
so-called condensate fraction. This medium should preferably
not contain components which may participate as wax or
solids or possibly higly viscous materials on cool surfaces
within the plant. At the same time the hydrocarbon liquid
used as the second cooling medium as explained above, con-
tains a minimum of hydrate-forming components.
Heated cooling medium, i.e. the second cooling medium
after passing through the gas hydrate in order to to cool
it, is discharged from the container at the temperatures
T=T5, and is recirculated through a second cooling circuit
which can comprise for example a pump 21, a heat exchanger
24 and the required circulation pipelines 20,23 and 25. The
heat exchanger 24 is fed by a suitable cooling medium such
as amonia, propane, mixtures of light hydrocarbons or freon.
Supplemental amounts of the second cooling medium containing
hydrocarbon adapted to replace the amount of hydrocarbon
liquid included in the end product, may be supplied via a
pipeline 22 connected to the cooling circuit.
When the desired final temperature T~ has been reached
in the gas hydrate within the container 2, the end product
which can comprise gas hydrate particles within a hydro-
carbon liquid, is discharged through the pipe 8 and thevalve 9, and transferred to a storing tank 51. Theoretically
the end product may be stored in the same container 2, but a
specific storage container 51 is preferred so that the
generator 2 again is free for further production. To reduce
the flow of heat into the storage tank 51, this tank may be
insulated thermally by a suitable material 57. The tempera-
ture in the stored gas hydrate may be controlled by draining
and circulation of the hydrocarbon liquid through a separate
- cooling circuit (not shown) connected to the container 51
via pipes 52 and 53. The storage tank 51 is provided with an
outlet 64 adapted for the transfer of the hydrocarbon
product or the end product (gas hydrate in a hydrocarbon
liquid) to further transport, storage or processing units.
_ _ _ _ ,
CA 02219244 1997-10-27
W 096/34226 PCT~NO9~'~~D3~
Prior to the transfer of the product ~rom the reactor
container 2, superfluous amounts of hydrocarbon liquid may
be drained from the gas hydrate.
The end product will, as previously explained, contain
particles of gas hydrate suspended in or surrounded by a
liquid containing hydrocarbon at the temperature T4. The
dimensions and the shape of the particles will vary, and
will be a result of process conditions and possible post
treatment of the gas hydrate compound. The size of the
particles may vary from fractions of 1 mm up to several
centimeters, all within the scope of the present invention.
A stirring apparatus 31,32, respectively 55,56 may be
installed in the hydrate-generating zone 1/the cooling zone
80 and/or in the storage zone 50. Such stirring units may be
required to obtain a sufficiently fine grain in the materi-
ale and effective heat exchange between the components at
different stages of the process. The stirring of the product
in the storage container may also reduce the tendency for
sintering in the end product.
As an alternative to supplying the gas through the
pipeline 7 to the upper part of the reactor 2, the gas may
be supplied to the lower part of the container through a
pipeline 61. Using such a method for supplying the gas, the
gas may be bubbled through a mixture of solids and liquids
in the lower part of the reactor 2. Such a solution willcontribute to a high concentration of hydrate-generating
components from the gas in the liquid phase, and therefore
also contribute to a powerful generation in the liquid phase
during step a, and possibly step b of the process. Non-
converted gas or gas which to a large extend has been poorwhere hydrate-forming components are considered, may, when
such an embodiment is used, be discharged from the plant as
a flow of gas through an outlet 62 close to the top of the
reactor container 2. Supplies of gas from both upper and
lower parts of the container 2 may also be combined.
A further modification of the embodiment described
above is that water is replaced, completely or partly by a
hydrocarbon medium already as a first cooling medium. This
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W O 96/34226 PCTAN09~J'~C9
may take place as the cooling circuit for hydrocarbon
liquids is connected to the reactor container 2 which,
according to the Figure 1, comprises the circulation pump 21
and the heat exchanger 24, which is constructed in such a
5 manner that the cooling requirements of step a is covered by
circulation of a hydrocarbon medium instead of water. If a
substantial part of the hydrate generation is to take place
within the gas-filled volume 11 in the reactor container 2,
it is necessary that the cooling medium containing hydro-
10 carbon is supplied at least partly to this volume of gas,
preferably as droplets (sprayed or flooded in) through an
alternative supply line 25' (indicated by means of a dotted
line in Figure 1).
In Figure 2 a different embodiment is shown, substan-
15 tially distinguished from the embodiment in Figure 1 as non-
converted water is not recirculated during the generating
step a, but only flows through the plant one single time (in
principle once). Gas is supplied as previously via the
pipeline 7. Cooled water, preferably cold seawater, is
20 supplied to the reactor container 2 through the pipeline 5
and the nozzles 6, both as a raw material for generating
hydrate and as the first cooling medium. During steps a and
b non-converted water will gather at the bottom of the
reactor container 2. This amount of non-converted water is
25 just discharged through the pipeline 19. Diluted gas which
may be found in the discharged water, may if required be
removed by means of a hydrocyclone 41 or a similar liquid/-
gas separator. In many embodiments it will however, be
possible to reduce the pressure so much that the remaining
30 amounts of diluted gas may be removed from the water and
handled in a suitable manner without any use of other
G equipment than a simple gas/liquid separator.
The supplied amount of cooled water through the pipe 5
- and the nozzles 6 may be controlled, e.g. by means of
35 valves, so that all the heat which is generated in the
process is removed from the reactor container 2 as heated,
non-converted water through the outlet 19. In this manner
the need for further cooling is reduced or avoided. A better
CA 02219244 1997-10-27
W O 96/34226 PCTAN096/00098
24
cooling is in other words obtained simply by increasing the
input of cooling water from the pipe 5.
The reactor or the hydrate generator 2 will be exposed
during the working process to a medium high pressure (50-80
bar a). Even if substantially larger amounts of water have
to be pumped through the reactor against this pressure, a
corresponding increase of the pumping power is not re~uired.
In a rather simple way a pressure sluice assembly may be
arranged in which the discharged flow of liquid at a high
pressure is sluiced against an inwardly directed flow of
liquid at low pressures. Theoretically, only the water which
has been used for generating gas hydrate within the reactor
will require external pumping power.
The central outlet 43 from the hydrocyclone 41 will
contain hydrocarbons as gas or liquid, and these hydrocar-
bons may again be put under high pressures whereupon they
may flow back into the process loop, or they may be used as
power source for motors in pumps, compressors and similar
e~uipment in a plant, e.g. by using suitable engines.
The hydrate compound may preferably be cooled down to a
temperature at least 15CC, typically 20-30CC, below the
prevailing temperature in steps a and b of the process.
Accordingly the pressure requirements for the storage
container 51 is much lower than the pressure requirements
for the generator 2 which has to endure a pressure of at
least 60 bar. Accordingly it may be preferred to let step c
and step d be effected in a different container than the
reactor which has been used in step a and step b of the
process.
A process plant using separate cooling in a specific
container 81 is shown in Figure 3, in which the reference
number 80 still refers to the cooling zone where the step c
is undertaken. The cooling container 81 is itself preferably
insulated by a layer of heat insulating material 82. In
Figure 3 it is also assumed that, during draining of liquids
in process step b it may be the case that the liquid
discharged from the bottom of the reactor 2 through the pipe
75 will contain a mixture of a liquid medium containing
CA 02219244 1997-10-27
W 096/34226 PCT~NO~ c~9s
hydrocarbon and water. This mixture may be separated in a
specific separator 78. When step b is finished in the
reactor container 2, the hydrate compound is transferred to
the container 81. The fluid communication through a tube 8 5
which connects the gas volumes 11 and 86, found in upper
parts of the containers 2 and 81 respectively, will give the
required pressure compensation which allows easy passage of
compound into and out of the container 81, as soon as the
valve 9 is opened. The hydrate compound in the container 81
is cooled down, in a similar manner as described in step a,
by direct cooling using recirculation of a second cooling
medium containing hydrocarbon through a cooling loop this
time comprising a heat exchanger 87.
When the hydrate compound is cooled to the desired
temperature which is preferably below -10~C, the compound is
transferred to the storage tank Sl, of which a small part is
seen in the lower part of the Figure.
In connection with the embodiments shown above as a
plant adapted for carrying out the method according to the
present invention, the following possibilities for modifi-
cations should be mentioned:
When water has been drained off in step b, the hydrate
compound which still may contain small amounts of free
water, can be exposed to an additional, hydrate-generating
step in which the free water is brought into contact with
hydrate-forming gas components such as-methane, ethane and
propane. This may take place for example as such components
can be supplied through a pipe 61 (Figure 3) near the bottom
of the reactor 82. In this manner an additional drying of
the hydrate compound can be obtained, and the result is a
hydrate compound comprising only gas hydrate without free
waters or with ~uite insignificant amounts of free water.
Larger amounts of free water in the hydrate compound, would
- as already mentioned, lead to problems when the hydrate is
cooled down in step c in the process, as the free water
would then freeze to ice and form bridges of ice on and
between the surface of each gas hydrate particle. However,
as mentioned above, very small amounts of free water may be
CA 02219244 1997-10-27
W 096/34226 PCT~NO
tolerated.
In connection with certain embodiments it is deemed
preferable that the second cooling medium should not include
hydrate-forming components or that such components are not
present in this step of the process, as such components
could lead to reduced stability in the end product. In such
cases it is recommended that the content of volatile
components in the hydrocarbon medium be kept at a level
which gives a partial pressure from the hydrocarbon, at the
storage temperature below the pressure in the environment.
This again may be obtained if the second cooling medium, at
least quite at the end of step c, is a hydrocarbon medium
which is comprised substantially of hydrocarbons with at
least five carbon atoms.
The hydrate compound obtained after the steps b, c or
d, may be exposed to a draining or compressing stage in
which superfluous humidity is reduced, removed or squeezed
out. A proper composition of the end product is a suspension
having approximately 80 volume % hydrate and approximately
20 volume % liquid hydrocarbon, mainly identical with the
second cooling liquid but possibly including small fractions
of free water in a frozen condition, and residual amounts of
the first cooling liquid if this liquid had a different
composition from the second cooling liquid.
A somewhat more detailed description of a realistic
plant for carrying out the hydrate-generating process which
is explained in connection with the principle drawings
above, is also given in the following example referring to
Figure 4. Here too, the capacity of the plant is vaguely
assumed and the values of some of the parameters are given
as calculated examples.
Three parallel reactors are now used, in the Figure 4
referred to as 2A,B,C; and of which only 2A is shown in
detail, while the containers 2B and 2C are referred to only
as their connections are shown. The reactor containers 2A,
2B and 2C will, when working, be at different stages in the
production process, so that the reactors transfer the
manufactured hydrate in sequence to the cooling tank 81,
CA 02219244 1997-10-27
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27
which may be common for all the reactors. How many reactors
2 which may be connected to a common cooling tank, depends
among other things on how long the different steps of the
- process need. The figure shows the situation at the end of
5 the process stage a, and the text is referred to on the
r figure ~or understanding the positions of di~ferent valves,
and which fluid flows are affected at this stage.
Here follows a short description of the reactor
container 2A and its associated units in the plant, to the
10 point where the finished end product is transferred to the
storage tank 51 (lower right on Figure 4).
The hydrate-generating process is based on the use of
seawater both as hydrate-generating water and as cooling
water in the reactor arranged according to the "once through"
15 principle which, as the name assumes, only uses one single
through-flow of the water which is to be included in the
hydrate. According to this the seawater which is supplied,
flows through the pump 100 and the water inlet 5, through
the divided hydrating reactor 2A and is then lead directly
out into the sea (after simple treatment in a hydrocyclone
plant 41). The incomming seawater is at a temperature of 8''C
pumped into the reactor system by means of the seawater pump
100. The reactor 2A operates at 60 bar a. Within the reactor
the seawater is evenly distributed within the total volume
by means of nozzles 6 installed in the ceiling and/or in the
cylinder wall. The generation of hydrate takes place as the
seawater contacts the natural gas which has entered the
system via the pipe 7. In the bottom of the reactor 2 the
temperature is about 13'C (the equilibrium temperature~. The
natural gas which is entering the reactor system may e.g.
amount to 700 OOO Smj/d (standard cubic meter each day). The
reactor 2A itself is a so-called "semi-batch' unit in which
the generation of the hydrate takes place continueously,
while the output of the product takes place in batches as
the hydrate product at intervals is discharged into a
gathering tank 2A, arranged below the reactor 2A.
As mentioned above the reactor system consists of three
parallel reactors, 2A/BtC, which may have separate or (as
CA 02219244 1997-10-27
W 096/34226 PCTA~O~GI'~
28
shown on Figure 4) one common collector tank 81. The units
are sequencially controlled, so that they operate in cycles
where each cycle consists of three sequences or intervals.
In the first interval the reactor 2A is discharged of the
hydrate product and seawater as the valve is open between
the reactor and the collector tank Z'. At the same time the
discharge line for seawater close to the bottom of the
reactor is closed. When the reactor 2A is emptied, a valve
between the reactor 2A and the collector tank 2A is closed.
Then as much as possible of the seawater which has been
included in the hydrate compound, is squeezed out from the
collector tank 2A, e.g. by means of gas supplied under
pressure. The hydrate compound which in this manner has been
"dried" is supposed to have a compacting density of approxi-
mately 130 Sm3 gas/m3 hydrate.
When the seawater has been squeezed out of the product,the second interval starts, in which the hydrate product in
the reactor 2A is flooded with condensate delivered from a
cooling tank 81 by means of condensate pump 101. In this
manner a slurry product is obtained, containing hydrate and
condensate, and this slurry product is more easy to handle.
In the third and last interval this hydrate slurry is
transferred from the collector tank 2A to the cooling tank
81, in which the hydrate slurry is cooled down to -20'C. The
power behind this operation is the great differensial
pressure between the collector tank 2A (60 bar) and the
cooling tank 81 (15 bar).
Each interval is calculated to 4 minutes and
accordingly one cycle of the process is 12 minutes. The
sequential operation of the three reactor units A, B, C is
controlled by a control system not shown, in such a manner
that each unit at a given time works in different intervals.
In that manner the adjacent common process equipment such as
the cooling tank 81, the condensate pumpe 101 etc. work
3S continuously against that of the reactors 2A, B or C which
at each time is connected. During the different intervals it
is necessary to compensate the pressure between the reactor
2A and the collecting tank 51. This is achieved by means of
CA 02219244 1997-10-27
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29
an open pressure balancing connection (not shown) between
the two tanks). The cooling down of the hydrate product
takes place secondarily within the collector tank 81 in
which the hydrate slurry is cooled down during flooding by
S means of cooled condensate (-20CC). As said hydrate slurry
from the collector tank 2A is partly cooled down, the
cooling tank 51 may operate at 15 bar without any problems
in connection with dissociation of the hydrate product. The
complete cooling down process is obtained by means of a cold
condensate circuit 87 connected to the cooling tank, and in
this the filtrated condensate is delivered from the cooling
tank at -20~C, is then cooled further down to -30~C in a
circulation cooling loop 87 for condensate, and then
returned to the cooling tank 81. In the circulating cooler
87 the condensate is cooled down by evaporation of propane
by means of a cooling circuit compressor and a propane
condensator 79 (based on seawater).
The cooled slurry product from the cooling tank 81 is
supplied to a hydrate/condensate separator 111, in which the
product is separated as a hydrate paste (20 volume % conden-
sate and 80 volume % hydrate) and stored at atmospheric
pressure. Separated condensate is returned to the cooling
tank. Make-up condensate is added to the cooling tank 81 to
cover the requirement for condensate included in the hydrate
product (the paste).
Excess seawater from the reactor units 2A, 2B, 2C is
treated first in a plant comprising flash tanks and a
battery 41 of hydrocyclones, which respectively degases and
separates oil/condensate droplets from the seawater which
afterwards is let out into the ocean.
Below follows a list showing capacity, power
~ requirements, pressure and temperature at some important
places in the plant:
- seawater inlet (at 5) 3495 m /h
35 seawater pump (100) 9015 kW differensial pressure 65
bar,
gas inlet tat 7) 700.000 Sm3/d
hydrate reactor (2A) 60 bar, 13 c
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W 096/34226 PCTA~09~'C0~9
outlet of seawater (from 2A) 1098 m3/h
slurry valve (from 2A) 673 m~/h, oGC, 15 bar
cooling tank ( 81) 15 bar
circulating pump (for 87) 585 kW
condensate cooler (87) 11465 kW, -20GC to -30CC
Besides, the text on Figure 4 states how the different
sequences are mainly controlled.
The method and the apparatus can be modified in many
ways within the scope of the claims below.
10 Below some specific conditions which may be of some
importance when the present invention is to be implemented,
are mentioned. To obtain a stable end product such as this
term is defined above, the hydrocarbon carrier in the end
product should have a low content of volatile hydrocarbon
components. This may be obtained in two different ways:
1) By replacing one hydrocarbon medium (used as the second
cooling medium) which comprises many volatile components,
with a cold hydrocarbon medium having a low content of such
components.
2) After reduction of the pressure, i.e. when the pressure
is released and has come down to the pressure in the
environment, the volatile components which are released from
the hydrocarbon medium (the second cooling medium) are
released as gas, if the hydrocarbon containing medium at the
end of step c still incorporates significant amounts of
volatile components.
The stabilizing may of course also include a combi-
nation of these techniques.
When step c is finished, the product will still be
under high pressure (approximately the same pressure as in
step a) within the cooling zone 80. Normally the end product
will therefore be exposed to pressure in the environment
when removed from the cooling zone.
Releasing the pressure may take place while the hydrate
product is still in the cooling zone 80, or at the same time
as the hydrate product is taken out of the cooling zone.
Remaining amounts of volatile (destabilizing) components in
the hydrocarbon medium will in both cases be released as
CA 022l9244 l997-l0-27
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31
gas. Such gas may be taken away, eventually to be recom-
pressed and returned to the earlier stages of the process.
If pressure release takes place at the same time as the
hydrate product is removed from the cooling zone 80, the
final stabilizing (removal of remaining amounts of volatile
components) of the product may take place in the storage
tank 51, so that the end product will not be ready until
after such stabilizing in the storage tank.
The end product itself may have a paste or slurry
consistence, and the size of the hydrate particles may vary
within wide limits so that the hydrate may comprise rather
big lumps with a dimension going up to several centimeters
within a liquid hydrocarbon medium. Of course it may also be
preferable with hydrate particles of varying size in one end
lS product, as the small hydrate particles will fill up spaces
between larger hydrate particles without any substantial
reduction of the gas content.
Of course the storage tanks have to be dimensioned to
endure a certain excess pressure. If the pressure in the
environments is 1 bar, this does not mean that the final
pressure also has to be 1 bar. With an excess pressure of
0,5 bar the final pressure in the end product may for
example be approximately 1,5 bar.
When it is stated that the second cooling medium shall
have a partial pressure which at the final temperature is
below the final pressure, it is accepted that the cooling
medium may contain a certain amount of volatile hydrocarbons
such as isobuthane and propane, and this does not jeopardize
the stability requirements. However, it is an assumption
that the sum of the partial pressures of the single com-
ponents in the cooling medium mixture is below the final
pressure as stated in connection with Henrys formula in the
specification.
If the method used leads to such a condition that the
water added to the hydrate-generating zone is so strongly
cooled that it comprises or consists of ice or snow, the
hydrate conversion and the temperature control taken place
in process step a has to go on until all ice or snow has
CA 02219244 1997-10-27
W 096/34226 PCTA~096/00098 32
been converted into hydrate and meltet water.
It is also preferred that the process conditions during
step a) are set so that an end product is obtained in which
the solid hydrate-containing material has a gas content
corresponding to a density of at least 130 Sm~/m3, preferably
above 150 Sm3/m3 solids, when methane is used as the hydrate-
generating hydrocarbon.
It should also be emphasized that the hydrate-
generating pressure and temperature conditions in process
step c has to be maintained until the hydrate compound has
reached a temperature at which the tendency to dissociate in
the generated hydrate will be negligible for practical
purposes. If cooling takes place at a high speed, this
temperature will be reached just after the moment at which
the water freezes.
Finally it should be emphasized that the final pressure
or the storage pressure is normally preselected according to
design requirements for containers and piping. The final
pressure is a nominal pressure which mainly is dependent on
the construction of the plant.
The method, the plant and the product according to the
present invention may be used in different industrial
connections. Thus the invention may be used for conversion
of natural gas into a hydrocarbon product which may be
stored and transported at technically speaking simple
conditions. The method may therefore be used in connection
with production and transport of natural gas from primary
gas fields, especially from remote gas fields to a terminal
plant in connection with the market or the location of the
users. Gas from so-called associated gas fields, that is oil
fields which in addition to the hydrocarbon liquid contain
larger or smaller amounts of gas components, may also be
converted into hydrocarbon products according to the present
invention, and the conversion of such gas may accordingly
lead to a profitable oil and gas production from such oil
and gas fields.
Further, the invention may be used where there is a
need for ta~ing care of and storing volatile hydrocarbon
CA 02219244 1997-10-27
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33
compounds during a shorter or longer period. Such needs may
also rise where superfluous gas is developed in connection
with oil production or oil raffing, in connection with
loading and unloading as well as transporting crude oil,
where gathering of volatile connections (VOC) from the crude
oil, and in connection with loading refined products as
petrol, diesel etc.
The product according to the invention may be used for
several purposes, e.g. as a medium for storage and transport
of natural gas, as a fuel for motors or in heating plants,
or again as a source of natural gas components and light
hydrocarbon liquids which may be further treated in petro-
chemical plants. In particular it may be of interest to use
a product as a propellant for vessels, e.g. as an environ-
mental friendly propellant for ferries.
The plant according to the invention may be installedon vessels or on offshore platforms, or may be built as
stationary plants on land.
There follows below a short explanation of Figure 4, in
particular so as to indicate capicities in a practical
embodiment of such a hydrate plant.
Figure 4 shows the actual generating and cooling part
of a hydrate-generating plant according to the present
invention, the plant being based on the use of seawater both
for the hydrate generation and for cooling. Presumably it
is possible to obtain a packing degree of about 130 Sm3/m'.
The elements of the figure will be described broadly from
the inlet at the upper, lefthand part of Figure 4, to the
outlet at the lower, righthand part of Figure 4.
At the lefthand side of the figure there is shown a
hydrate reactor 2A and a collecting/flushing tank 2A
connected thereto. It is to be understood that the plant
can comprise several hydrate reactors connected in parallel,
as indicated in the figure by arrows leading respectively to
and from such parallel reactors, as denoted 2B and 2C.
The pump P-100 takes in seawater to all the generators
2A, 2B, 2C through the supply pipes 5 in an amount of
approximately 3.500 m /h. The seawater is considered to have
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W 096/34226 PCTAN09
34
a temperature o~ about 8~C and the pump P-100 will increase
the pressure by about 65 bar. The power consumption of this
pump will then be about 9 MW.
Gas is introduced into the reactors through supply
conduits 7 ~rom the oil and gas processing in an amount of
approximately 700.000 Sm3/day. Superfluous seawater is
removed from the reactor via the treatment plant 41 for
discharged water, and it is estimated that the total output
of seawater at 42 is about 3.300 m3/h at a temperature of
about 13~ C. Moreover it is seen from the le~thand side of
figure 4 how the reactor 2A, 2A can be subdivided into an
upper reactor operating under a pressure of about 60 bar and
where the temperature at the bottom of the reactor is about
13~ C, and a lower collecting/flushing tank where the
pressure can also be about 60 bar. At the bottom of flush-
ing tank 2A there is indicated a sieve located for retain-
ing hydrates being thereby collected above this sieve.
Through pipe connections and control valves hydrate
slurry is conveyed from all the reactors in the plant
according to a controlled se~uence, to the cooling tank 81
which operates at a pressure of about 15 bar. The cooling
in the cooling tank is effected by means of the circulation
cooler 87 which can for example be cooled by means of a
propane cooling circuit 79, which in turn is cooled with
seawater being supplied by means of the pump P-104, which
has an estimated power consumption of somewhat less than 400
KW. The actual propane circuit 79 will have a power con-
sumption of somewhat less than S MW and the propane cooling
circuit 79 is considered to cool the condensates in the
circulation cooler 87 down to about -30' C. The condensate
is circulated by means of the pump 102 which has an esti-
mated power consumption somewhat less than 600 KW when
handling about 3.000 m condensate per hour. The slurry
product containing hydrate is delivered at the bottom of the
cooling tank 81 and is conveyed to the hydrate/condensate
separator 111 which returns condensate to the cooling tank
and discharges the finished hydrate product, which may well
be in paste form, further for storage in tank 51 being
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W 096/34226 PCT~NO95'dG098
located outside this figure of the drawings.
Moreover the figure contains the most important valves
and pumps and connecting pipes being necessary for imple-
- menting the generating and cooling part of the plant. The
figure illustrates in particular how several reactor tanks
can cooperate with a single cooling tank.
-
