Note: Descriptions are shown in the official language in which they were submitted.
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IMPROVED THERMODYNAMIC SEPARATION OF IiEAVIER COMPONENTS
FROM NATURAL GAS
The present invention relates to the separation of hydrocarbon gases
into components of differing boiling points. The invention relates more
specifically to
a method and an apparatus especially suited for separating propane, methane or
ethane from natural gas.
BACKGROUND OF THE INVENTION
The applicant's prior United States Patent 4,770,683, issued
September 13, 1988, describes a process and an apparatus for distillation of
two
materials of differing boiling points. A process for distillation of two
materials of
differing boiling points particularly propane, ethane or carbon dioxide from
natural
gas is described in which the conventional distillation tower is divided into
a first
tower at higher pressure than a conventional tower and a second tower at lower
pressure. Liquid drawn from the first is expanded to the lower pressure
through two
or more stages with cool extracted at each stage and used to cool gas
withdrawn
from the top of the first tower to keep the top tray at a required
temperature. Gas
withdrawn from the second tower is compressed and cooled for return to the
first
tower as a reflux. The use of the cool from the expanded liquid and the use of
the
two towers provides an improved thermodynamic efficiency and avoids the use of
costly turbo-expanders.
In addition, a further arrangement by the present applicant in US
Patent No: 6,098,425 issued Aug 8t" 2000 discloses an improvement to the above
patent in which efficiency is enhanced by the provision of a third tower and
an
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arrangement by which additional cool is supplied to the top of the high
pressure
tower as a reflex.
Ethane recovery is similar to lpg recovery in concept except that more
energy is required for refrigeration and reflex compression. This process also
applies to situations where the low pressure gas is sold at higher pressures
but the
benefits compared to other processes are much less than that described in the
first
paragraph where, essentially there are na other processes that are ever
considered
unless the desired residue gas pressure for the sales pipeline is above 200
psig.
The use of this technology for the recovery of ethylene in ethylene plants,
will reduce
the power requirements and capital cost of the de-methanizer portion of these
plants. The above US patent of the applicant was described as being very
applicable to the separation of ethane and ethylene. That patent could also be
used
for the de-methanizer in an Ethylene Plant but it is believed that this patent
will be an
improvement when combined with that patent.
In US Patent No: 6,182,468 issued Feb 6t" 2001 of the present
Applicant is disclosed distillation processes for the separation of close
boiling point
materials. Such a process is used in the extraction of various materials
generally
using a distillation tower. Examples of such separations are:
1. Recovering ethane from natural gas
2. Recovering propane from natural gas
3. Recovering carbon dioxide from natural gas
4. Recovering helium from natural gas
5. Rejecting nitrogen from natural gas
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6. Recovering ethylene in ethylene plants.
The above patent has optimal advantage when utilised in conjunction
with a two tower or multi-tower process described in the above United States
patent.
It may also be used to advantage with other distillation patents for example
the
various arrangements described in patents held by the Ortloff Corporation.
SUMMARY OF THE INVENTION
According to the invention there is provided a method for separating a
supply material into two materials of different boiling points comprising:
providing two separate vessels connected to provide transfer of
materials from one vessel to the other;
feeding supply material to one of the vessels arranged to form a mixed
gas and liquid phase such that one material in the vessels separates into said
liquid
phase and the other material separates into said gas phase;
extracting the separated gas from the top of one vessel and the
separated liquid from the bottom of the other vessel;
arranging the supply and the vessels such that the pressure in one of
said vessels is higher than the other;
and injecting reflux into the top of one of the towers;
wherein the 'reflex' flow rate enters the top of one of the towers under
flow control so that there is a constant flow rate of flashed liquid into the
top of the
tower.
Preferably the flow is measured when the overhead fluid is a gas prior
to cooling.
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Preferably said one of the towers is the high pressure tower.
Preferably the high pressure tower is controlled by maintaining a
constant ratio between this 'reflex' flow and the flow rate of dehydrated gas
feed to
the process.
Preferably, in order to smooth the variation of low pressure tower
overhead gas, a small amount of dehydrated gas feed is added to the 'reflex'
gas in
order to make the desired ratio of 'reflex' flow.
Preferably, in situations, where there is an excess of overhead gas
from the low pressure tower, the surplus is processed with the main dehydrated
feed
gas which is cooled, chilled, partially condensed arid fed to the bottom of
the high
pressure tower.
Preferably all of the low pressure tower overhead is fed as 'reflex' and
there is added some dehydrated feed to achieve a constant 'reflex' flow rate.
Preferably a constant ratio is maintained of condensed reflex liquid to
the dehydrated inlet gas or a constant mass ratio (rather than volume ar molar
ratio).
Preferably there is provided a near constant flow of liquid to the top
tray of the high pressure tower.
The new invention is a process control scheme to use with all of the
above distillation patents. It also applies to the ~rtloff Two Tower Process
and a
patent by some Southern U.S. inventors who sold their idea to a company in
Houston. These processes are described below.
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The present invention therefare includes all of the features disclosed in
the above patents to which reference should be made for further details and
the
subject matter of which is incorporated herein by reference.
All of my patents involve dividing a distillation tower into two or more
5 towers having different operating pressures. When the concept is utilized
for
recovering propane from natural gas, all of the sweetened (usually, if
required),
dehydrated natural gas is cooled, chilled and contacted in a high pressure
tower and
liquid from this tower is processed in a lower pressure tower(s). Recycle from
the
low pressure tower is compressed, cooled, chilled, condensed, and sub-cooled
at a
high pressure, typically between 700 and 1300 psia and flashed into the top
tray
(usually) in the high pressure tower.
The operation of the low pressure tower (known as the de-ethanizer in
propane recovery situations) may vary resulting ire a variation in flow rate
of its
overhead vapour. The overhead vapour is compressed, cooled, condensed, sub-
cooled and used as 'reflex' for the high pressure tower.
The object of this invention is to dampen variations in flow of this reflex
to result in a more stable operation and a higher average propane recovery.
The 'reflex' flow rate enters the top of the high pressure tower, known
as the gas fractionator (or de-methanizer) under flow control so that there is
a
constant flow rate of flashed liquid into the top of the de-methanizer. The
flow can
be measured when the overhead fluid is a gas prior to cooling. The de-
methanizer
is controlled by maintaining a constant ratio between this 'reflex' flow and
the flow
rate of dehydrated gas feed to the process.
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In order to smooth the variation of de-ethanizer overhead gas, a small
amount of dehydrated gas feed is added to the 'reflex' gas in order to make
the
desired ratio of 'reflex' flow. In situations, where there is an excess of de-
ethanizer
overhead gas, the surplus could be processed with the main dehydrated feed gas
which is cooled, chilled, partially condensed and fed to the bottom of the de-
methanizer. (Normally, it would be preferred to design to utilize all of the
de-
ethanizer overhead as 'reflex' and add some dehydrated teed to achieve a
constant
'reflex' flow rate.)
Variations of this concept could be to maintain a constant ratio of
condensed reflex liquid to the dehydrated inlet gas ~or a constant mass ratio
(rather
than volume or molar ratio).
This concept works by providing a near constant flow of liquid to the
top tray. When more make-up to the reflex stream is required from the main
dehydrated gas stream that enters the bottom of the de-methanizer, there is a
higher
methane content in the reflex, so more flashing on the top tray and slightly
less liquid
leaving the top tray. There is also slightly less vapour entering the bottom
tray so
the ratio of liquid enter the top to vapour entering the bottom remains closer
to
constant.
This concept will also work for the Ortloff Two Tower process which is
a variation of my first patent except that their second tower operates at a
higher
pressure than their first tower. To achieve this higher pressure, they must
pump
liquid with a cryogenic pump from the first tower to the second tower. This
process
is not as thermo-dynamically efficient and it requires a cryogenic pump, but
does not
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require a compressor to recycle de-ethanizer overhead vapour for condensing
and
feeding to the top of the de-methanizer. In a similar manner to that described
in
relation to my own prior patents, the flow rate of 'reflex gas' in the Ortloff
type
process in accordance with my invention to the top of the de-methanizer can be
smoothed with a slip-stream of dehydrated feed gas.
Also this concept will work with the mast recent of the above patents
that is another variation of my first patent. This patent recycles de-
ethanizer
overhead product (ethane) which is cooled, condensed, and sub-cooled then fed
to
the top of the de-methanizer. In their process, the de-ethanizer is usually a
refluxed
tower, whereas I usually have a quench-fed de-ethanizer. In this design, the
reflex
flow would be maintained at a constant ratio to the inlet feed by using make
up from
the inlet dehydrated gas.
Variations of this concept could be to maintain a constant ratio of
'reflex flow' to residue gas flow leaving the top to the de-methanizer.
Another
variation of this concept is to take the make-up "reflex" flow from the vapour
space in
the bottom of the de-methanizer. This vapour could be letdown through an
expansion valve to the inlet to the top tray of the de-ethanizer. Sometimes,
retrograde effects will result in this letdown vapour having a small liquid
phase which
would add to the reflex of the de-ethanizer. This stream also tends to load
level the
recycle compressor so that it has a constant inlet flow gate thus a constant
inlet
pressure.
Another variation of this concept utilizes low temperature separator
overhead vapour as make-up. In this concept, there is a low temperature
separator
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in the stream of partially condensed dehydrated feed which sometimes is
letdown
through a turbo-expander into the inlet of the de-methanizer.
Another variation would be to inflate the flow of make-up gas by a
certain fraction so that it would take less make-up gas to make up for a
deficiency of
de-ethanizer overhead gas. Of course, in some situations, it may be better to
deflate
the flow of make-up gas, rather than inflate it.
In addition to recovering propane from natural gas, this concept also
works for recovering ethane and heavier liquids from natural gas or recovering
carbon dioxide and heavier liquids from natural gas. It also works for
separating
closed boiling point liquids and other separations discussed in my prior
patents.
BRIEF CDESCRIPTION OF TFiE DRAV1PINGS
Figure 1 is a schematic illustration of the elements of a first process
according to the present invention using the two tower system of the above
prior patent
which is particularly but not exclusively designed for supplying the residue
gas at a low
pressure.
Figure 2 is a schematic illustration of the elements of a second process
according to the present invention using the single tower.
Both figures are substantially as shown in DS patent no: 6,182,468 above but
have been modified to show the present invention. The following description
has also been
primarily taken from the above patent and includes the elements with which the
present
invention is concerned.
DETAILED DESCRIPTION
Turning firstly to Figure 1 there is shown an arrangement for separating Ipg+
products from a feed of natural gas leaving a residue sales gas for sale at
low pressure that
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is less than 100 psig.
The arrangement provides a feed supply line 10 which feeds to an inlet
separator 11 which acts to separate gas from any incoming liquid. The liquid
can be
handled in a number of different ways including supply to the free water knock
out system of
the crude oil processing plant in arrangements where such is available.
Alternatively, the
liquid can be passed through a dehydrator and fed to the de-ethanizer.
The inlet gas from the inlet separator 11 is supplied to an inlet compressor
12
having an after-cooler 13. The gas is compressed to a sufficient pressure in
the compressor
12 so that after the compressor the gas can be dehydrated in a molecular sieve
14 and
processed in the Ipg recovery plant and then has sufficient pressure for the
gas entering the
sales pipeline 15.
Prior to entering the dehydrator 14 in the form of a molecular sieve, a
further
liquid separator 16 is provided for recycling a liquid through a return fine
17 having a let
down valve 18.
As stated above, the arrangement described herein is particularly designed
for low pressure residue gas. However if the desired pipeline pressure in the
residue gas is
intended to be above 600 psig, it is preferred that compression be added to
the residue gas
downstream of the recovery plant so that the tower assembly described
hereinafter can
operate at approximately 400 psig.
After the inlet gas is compressed, after-cooled and the liquids extracted in
the
separator 16, the gas is dehydrated in the dehydrator 14 which is preferably a
molecular
sieve as described above or can possibly be a "Dryso" T"" process which is a
tri-ethylene
glycol process. In such an arrangement a sophisticated regeneration system as
shown can
be provided using extractive distillation to reduce the water content of
ethylene glycol. The
extracted material from the regeneration system is returned to the feed as
indicated in the
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supply line 20.
Downstream of the dehydrator 14, there is provided a supply line 21 which is
divided into two supply lines 22 and 23 acting to effect a proportional
division of the feed in
the supply line 21. Each line includes a flow control valve 22A and 23A which
are controlled
5 using conventional flow control systems well known to ore skilled in the art
to maintain the
required proportions depending upon the measurement of various parameters of
the
process.
The process further includes a processing tower arrangement generally
indicated at 30 including a high pressure tower 31 and a low pressure tower
32. These two
10 towers are generally as described in the above United States Patent and the
disclosure of
that document is incorporated herein by reference. The two towers each
comprise a
distillation tower section for effecting separation of the components in the
feed so that the
high pressure tower section 31 discharges lighter gas components at an upper
discharge 33
and heavier components at a lower discharge 34. The low pressure tower 32 has
an upper
discharge 35 and a lower discharge 36. The upper gas discharge 33 provides the
residue
sales gas 15 while the bottom discharge 36 of the low pressure tower provides
the heavier
Ipg+ product 37.
The first portion of the feed gas divided into the supply line 23 is supplied
as
a feed to the lower part of the high pressure tower 31. Prior to supply to the
tower
arrangement, the gas in the supply line 23 is passed through a heat exchanger
R which
includes a component 38A on the fine 23 and second component 38B forming a
reboiler for
material at the bottom of the low pressure tower component 32. Thus the heat
exchanger R
extracts cool into the component 38A to cool the supply on the line 23 and
applies heat to
the component 38B acting as a reboiler to return the material as a side feed
to the lower part
of the lower pressure component 32.
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The supply on the line 23 is further passed through a second heat exchanger
S having a first component 39A and a second component 39B which again acts to
extract
cool for the material in the line 23 and acts as a heat supply for a side
reboiler on the lower
pressure tower component 32.
Gas from the top discharge 35 of the low pressure tower 32 is returned to the
feed through a supply duct 40. Prior to return to the feed, cool is extracted
from the return
gas in a further heat exchanger 41 and that cool is applied to the feed on the
line 23.
Finally a refrigerator unit 42 is used to apply external cooling to the feed
prior
to injection into the high pressure tower component 31 at a feed position 43.
The second proportion on the line 22 is passed to a compressor system 44
including a compressor 45 and a heat extractor 46. The second proportion of
the gas is
compressed to a pressure in the range 500 to 1400 psig so that it can be
cooled and
condensed and used for injecting into the tower arrangement as a cooling top
supply.
The prior patent and the prior published application of the present inventor
disclose the use of liquid injection at the top of the high pressure tower for
maintaining a
cool temperature in the high pressure tower'. In the prior application this is
termed as
"reflux". However in the present invention the compressed material includes a
component of
the original supply from the feed 10 and in addition includes a component from
the
discharge gas from the discharge outlet 35 of the low pressure tower.
The second proportion is thus compressed in the compressor system 44 and
cooled by the cooling arrangement 46. It is then passed through a heat
exchanger 47 which
extracts cool from the residue gas and supply line 48. Further cooling is
effected in a further
heat exchanger L which includes first component 49~ on the line 22 and a
second
component 49B extracting cool from the product 37. Further refrigeration
cooler 50 is
provided using external refrigerant. ~ownstream of the refrigerator 50 is
provided a further
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heat exchanger 51 extracting cool from the residue gas on the supply line 48.
After the passage through the heat exchangers, the second proportion of the
feed is usually totally condensed and sub-cool is provided by the heat
exchanger 51. The
second proportion of the feed is then passed through a let down valve 52
before injection
into the high pressure tower 31 at a feed entry 53.
The compression of the second proportion only provides significant
advantages in economical recoveries. In the past, all processes considered
compressing all
of the inlet gas to the high pressure before processing. In the present
invention only the
proportion in the line 22 is compressed thus avoiding the necessary power
requirements for
compression and also reducing capital cost.
In some situations a phase envelope of the second proportion gas has to be
considered so that an optimum pressure is chosen which provides optimum cool
recovery by
the gaslliquid and thus the most economical system. The above optimum cool
recovery is
usually at a pressure that is close to the maximum cool recovery.
Turning now to Figure 2 there is shown substantially the same arrangement
having the same first and second proportions divided into the first and second
feed systems.
In this arrangement, however, the two tower process is replaced by a more
conventional
single tower process as indicated in the single tower 55 as is well known from
the processes
of Ortloff.
In Figure 1, the speed of compressor 45 is adjusted to obtain the flow
through the Bine following Aerial cooler 46. Valve 52 is a back pressure
control
valve. The difference between FR 54 8~ FR55 indicates flow of make-up gas in
line
22. Desired "reflux ratio" is FR 54 I Flow in line 21.
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1~
Figure 2 is a modification of the old single tower process. In a similar
manner to Figure 1, desired flow in FR 54 can be achieved by adjusting speed
of
compressor 45. ~esired "flow ratio" is FR54 / FR55.
