Note: Descriptions are shown in the official language in which they were submitted.
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TITLE: ETHANOL PROCESSING WITH VAPOUR SEPARATION
MEMBRANES
FIELD
[0001] This specification relates to alcohol processing or gas or vapour
separation.
BACKGROUND
[0002] The following is not an admission that anything discussed below
is citable as prior art or part of the common general knowledge.
[0003] Plant matter, for example carbohydrates or cellulose, may be
fermented to produce a liquid, sometimes called beer, that is primarily water
but includes ethanol. Dewatering this beer may produce ethanol that is
substantially free of water, for example having less than about 1% water by
volume, which may be used as a fuel or a fuel additive suitable for use in,
for
example, internal combustion automobile engines. Distillation can be used to
partially dewater the beer, but the energy required in the distillation column
reflux loop per volume percent of water removed increases as the ethanol
content increases for a given number of trays in the column. At about 97%
ethanol by volume, the ethanol/water azeotrope has been reached and simple
distillation is no longer effective. Other techniques, such as azeotropic
distillation or molecular sieves may then be used. The energy requirement of
these processes is a significant problem as is the amount of water required
for
fermentation. Solids produced in the fermentation process, sometimes called
stillage, may be useful for animal feed but must be dewatered. Dewatering
involves a first step in which the stillage is dewatered typically to about a
70%
moisture content measured on a dry basis and a second step in which the
stillage is dried further to about a 15% moisture content measured on a dry
basis as required for sale as distillers dried grains and solubles, which
requires a significant amount of energy.
[0004] U.S. Patent No. 4,978,430 describes a process in which an
evaporation vessel produces a mixture of an organic compound vapour and
water vapour. The mixture permeates through an aromatic polyimide gas
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separation membrane. The permeated vapour has an increased concentration
of water vapour and a product vapour has a reduced concentration of water
vapour. The permeated vapour passes through a condenser and is then
returned to the evaporation vessel.
[0005] International Patent Application No. PCT/CA2004/001047 filed
on July 16, 2004 describes an asymmetric integrally skinned membrane. The
membrane can have a vapour permeance to water at least 1 x 10"'
mol/m2sPa at a temperature of about 30 C to about 200 C. The membrane
may have a vapour permeance selectivity of at least 50, preferably at least
250 for water/ethanol at a temperature of about 140 C. Application No.
PCT/CA2004/001047 is incorporated herein in its entirety by this reference to
it.
INTRODUCTION
[0006] The following introduction is not intended to limit or define any
claim. One or more inventions may reside in any combination of one or more
process steps or apparatus elements drawn from a set of all process steps
and apparatus elements described below or in other parts of this document,
for example the detailed description, claims or figures.
[0007] This specification describes a process for removing water from
an aqueous alcohol mixture, for example ethanol, using a heat driven
process, for example distillation, and a gas separation membrane unit. A
permeate is produced from the membrane unit that is substantially water
vapour or ethanol free. This water vapour is compressed and used to transfer
heat to one or more other parts of the process, for example distillation,
drying
stillage, heating beer or pre-heating vapours before membrane separation.
The water vapour may be condensed and used in the process, for example as
make up water for fermentation. Heat energy in a product vapour may also be
used, for example to dry stillage or pre-heat the beer before distillation.
[0008] This specification also describes processes for dewatering an
ethanol feed using a plurality of gas separation membrane stages, for
example two, three or more. The stages may be arranged in series in relation
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to a feed/retentate/product flow. Permeate from an upstream stage may be
compressed and used to heat another process step. Permeate from a
downstream stage may be condensed for use as make up water, or
compressed and added to an upstream permeate stream. A permeate stream
may be fed through another membrane stage for further dewatering before
being used to transfer heat. One or more of the possibilities described above
may be combined.
[0009] A membrane unit may also be used to process a partially
dehydrated stream within a plant. For example, a combined process having a
gas separation membrane unit and a molecular sieve unit is described.
BRIEF DESCRIPTION OF THE FIGURES
[0010] Figure 1 is a simplified schematic flow sheet of an ethanol
processing plant.
[0011] Figure 2 is a simplified schematic flow sheet of a scrubber of the
plant of Figure 1.
[0012] Figures 3 to 6 are simplified schematic flow sheets of alternate
membrane units of the plant of Figure 1.
[0013] Figure 7 is a flow sheet of the distillation and dehydration
section of an ethanol plant using molecular sieves provided as the
background for a comparative example.
[0014] Figure 8 is a flow sheet of the distillation and dehydration
section of an ethanol plant using vapour separation membranes rather than
the molecular sieves of Figure 7.
[0015] Figure 9 is a flow sheet of the distillation and dehydration
section of an ethanol plant using vapour separation membranes in
combination with molecular sieves.
DETAILED DESCRIPTION
[0016] Various apparatuses or processes will be described below to
provide an example of an embodiment of each claimed invention. No
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embodiment described below limits any claimed invention and any claimed
invention may cover processes or apparatuses that are not described below.
The claimed inventions are not limited to apparatuses or processes having all
of the features of any one apparatus or process described below or to
features common to multiple or all of the apparatuses described below. It is
possible that an apparatus or process described below is not an embodiment
of any claimed invention. The applicants, inventors and owners reserve all
rights in any invention disclosed in an apparatus or process described below
that is not claimed in this document and do not abandon, disclaim or dedicate
to the public any such invention by its disclosure in this document.
[0017] Figure 1 shows a plant 10 used to produce a product 12. The
product 12 may be used as fuel grade ethanol or may have about 99% or
greater ethanol by volume. The raw feed 14 to the plant 10 is a plant material
that may be fermented to produce ethanol, for example carbohydrates or
celiulose, for example from corn kernels, sugarcane or switchgrass. The raw
feed 14 passes to a fermenter 16 which is also fed with water 17 as well as
yeast and other fermentation inputs. The fermenter 16 outputs a beer which
may be temporarily stored in a beer tank 18. The beer contains ethanol but is
mostly water. The beer may contain about 3 to 15 percent, or about 10 to 12
percent, ethanol by volume, although up to 20 percent by volume or more
may be possible. The beer then flows, optionally passing through a beer pre-
heater 20, to a distillation column 22. The distillation column 22 may be a
single, multi-stage or multi-effect column or columns for producing a
distilled
ethanol 24 with an increased ethanol content. For example, the distillation
column 22 may be or comprise a stripping column or zone or a beer column
or zone and increase the ethanol content to at least 45% by volume but
typically less than 75% by volume. Alternately, the distillation column 22 may
further comprise a rectification column or zone and increase the ethanol
content to 75% by volume or more, more typically 90% or more up to a value
approaching 97% by volume. Although aspects of the invention may be useful
when the beer is distilled to an ethanol content of 75% by volume or more,
energy consumption for the plant 10 as a whole is likely to be less when the
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beer is distilled to 75% ethanol by volume or less which may be done in a
single column. The distillation column 22 may have a reflux loop 32 and a
reboiler loop 34.
[0018] The distilled ethanol 24, as a vapour, passes through a scrubber
26. Scrubber 26 will be described further below but removes particles and any
liquid droplets from the distilled ethanol 24. The particles are contained in
a
first liquid 28 which may be returned to the fermenter 16 as make up cook
water and a second liquid 30 which may be returned to the beer tank 18.
[0019] Scrubbed ethanol 36 leaves the scrubber 26 and flows to the
membrane unit 38. The membrane unit 38 will be described in further detail
below. In general, the membrane unit 38 produces a product vapour 40 that is
nearly water free, for example having 99% or more ethanol by volume. The
membrane unit 38 also produces compressed vapour permeate 44 and,
optionally, condensed permeate 46. Both permeates 44, 46 have only trace
ethanol contents, for example 2% ethanol by volume or less. Condensed
permeate 46, if any, may be returned to the fermenter 16 as make up cook
water, or optionally sent to the distillation column 22. For reasons that will
be
discussed further below, compressed vapour permeate 44 carries heat energy
and may be used to heat another part of the process. In Figure 1, for
example, the compressed vapour permeate 44 is used in a second reboiler
loop 46 to heat the liquids in the bottom of distillation column 22.
Optionally,
condensed vapour permeate 44 may be used to replace or further supply heat
to reboiler loop 34, beer preheater 20, a stillage dehydrator, a heater 48 or
other apparatuses or processes. After transferring its heat energy,
compressed vapour permeate 44 may become a liquid, primarily water, and
be re-used, for example as make up cook water for fermenter 16.
[0020] Stillage 50 may be withdrawn from distillation column 22 or
optionally from the beer feed to distillation column 22. Stillage 50 may be
partially dewatered by mechanical means and then sent through a drying
circuit 52. In drying circuit 52, stillage passes through one or more heat
exchangers 54. Heat exchangers 54 use heat from product vapour 40 and
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suction from pumps 56 to encourage water to evaporate from stillage 50.
Pumps 56 also transport the evaporated water until it condenses into
reclaimed stillage water 58 which may be used, for example, in fermenter 16.
Dewatered stillage 60 may be, for example, about 30 percent solids by weight
or more.
[0021] Figure 2 shows scrubber 26 in greater detail. Scrubber 26
has a spray tank 80, tank 84, pumps 87, and forward cleaners 82 configured
and connected as shown. Scrubber 26 removes particles and liquid droplets,
if any, from the vapours by entraining the particles and droplets in water.
[0022] Various alternate membrane units 38 will be described below
with reference to Figures 3 to 6. Each of Figures 3 to 6 show a different
example of a membrane unit 38. Other examples of membrane units 38 may
be created by combining all or parts of one or more of the examples of
Figures 3 to 6. The membrane units 38 have multiple membrane stages 80.
Each membrane stage 80 may be a membrane module, a stage in an
internally staged module, or a set of modules or internal stages in parallel.
Membrane modules may use polymeric membranes, for example of polyimide
hollow fibers. A hollow fibre module may be fed to the insides of the hollow
fibres. The membranes may be asymmetric integrally skinned polyimide
membranes as described, for example, in International Patent Application No.
PCT/CA2004/001047. Such membranes can have a vapour permeance for
water of 4 x 10-' mol/m2sPa or more at about 80 C. The membranes can have
a vapour permeance selectivity of 250 or more for water/ethanol at about 140
C. The membrane unit 38 may also be equipped with a vapour compressor
82. The vapour compressor 82 compresses permeate vapours adiabatically
which causes them to rise in temperature. The increased temperature allows
the heat energy in the permeate vapours to flow to, and heat, lower
temperature vapours, gases or liquids. The heat energy in the permeate
vapours (sensible heat plus latent heat of condensation) can then be used for
heating purposes in other parts of the process. The vapour compressor 82
may be, for example, a radial type fan or compressor that provides a
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compression ratio of less than 1:40, for example, between 1:2 and 1:10.
Although the vapour compressor 82 requires energy to turn the compressor,
at relatively low compression ratio the temperature rise of the vapours
permits
their use as a heat source, for example in a re-boiler.
[0023] Figure 3 shows a two stage membrane unit 38a. Permeate from
a first stage 80a is sent to a vapour compressor 82 and used as heating
steam for distillation column 22 as described above. Retentate from the first
stage 80a becomes feed for a second stage 80b. Permeate from second
stage 80b passes through a condenser 84 before being reused as cook water
for fermentation as described above. The use of a compressor 82 to increase
the retentate pressure from first stage 80a is an option, or to compress
permeate from the first stage 80a before it reaches a cooler 86.
[0024] Figure 4 shows a second membrane unit 38b having three
stages 80a, 80b and 80c. Permeate from these stages 80a, 80b, 80c may
have a temperature of about 100 C, but declining downstream, and
pressures of about 30-60 kPa (absolute), 5-15 kPa (absolute) and 1.5 to 4
kPa (absolute) respectively. Optionally, the third stage 80c and its permeate
flow may be omitted to create a two stage membrane unit. For each
downstream unit 80b, 80c, the permeate is collected and passed through a
cooler 86 and a vapour compressor 82 before joining the permeate from an
adjacent upstream stage 80b upstream of its vapour compressor 82. Cooler
86 may assist in creating a permeate side vacuum to withdraw permeate and
also allows the permeate vapour to be compressed to a higher pressure while
contributing to reducing the outlet temperature of the compressed permeate.
By recompressing permeate, and recycling it as heating steam, the second
membrane unit 38b maximizes energy recovery. Compressed vapour
permeate may have a temperature of 150 C or more and a pressure of 200
kPa (absolute) or more.
[0025] Figure 5 shows a third membrane unit 38c. The third membrane
unit 38c combines aspects of the first membrane unit 38a and second
membrane unit 38b. Two permeate streams 44, 46 are produced, but the
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condensed permeate 46 is produced from two downstream stages 80b, 80c
connected with recycle and compression of the further downstream permeate
to the adjacent upstream permeate as in the first membrane unit 38a. The
combined permeate of downstream stages 80b, 80c passes through a
condenser 88, and a holding tank 90 and is then recycled to the fermenter 16.
The configuration of membrane unit 38c provides balanced cost and energy
improvements. A compressor 82 connects the holding tank 90 to an outlet 91
to atmosphere.
[0026] Figure 6 shows a fourth membrane unit 38d. Permeate from first
and second stages 80a, 80b is compressed and fed to third stage 80c
individually as shown in the solid line or by joining the further downstream
permeate to the adjacent upstream permeate before its compressor 82 as
shown with the dashed line. Permeate from the third stage 80c is recycled
upstream of the heater 48 upstream of the first stage 80a. Permeate vapour
from the third stage is compressed and recycled as has been discussed
above. In the third membrane unit 38d, the permeate is re-separated which
increases ethanol recovery over the previous membrane units 38a, b, c.
Compressed vapour permeate 44 may be 0.1% ethanol by volume or less, or
essentially steam. Similarly, the permeate from any one or more stages
described in Figures 3 to 5 may be further separated as shown in Figure 6 to
improve ethanol recovery.
[0027] Two examples of design applications, shown in Figures 8 and 9,
using a membrane unit 38 in the ethanol industry and exhibiting benefits in
reducing the process energy demand or increasing production compared to a
process using molecular sieves without a membrane separation unit, shown in
Figure 7, are described hereinafter.
[0028] Figure 7 shows a conventional process flow diagram of the
distillation and dehydration sections 100 of a fuel ethanol plant, for example
a
fuel ethanol plant with an upstream fermentor fed for example with sugarcane,
producing 71 429 Uday of anhydrous ethanol at 99.5 % EtOH w/w (by
weight). The primary pieces of equipment in the distillation and dehydration
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sections 100 of the plant are a stripping column A, a rectification column B,
an
evaporator C, and a pressure swing molecular sieve semi-continuous
dehydration system D comprising three molecular sieve units, MSU1, MS2U,
MSU3. Stripping column A and rectification column B are parts of a two-
column distillation unit.
[0029] From a fermenter, beer at 9 % EtOH w/w is fed at a rate of 26
329 kg/h into the stripping column A, from which a stream at 42 % EtOH w/w
is extracted and directed into the rectification column B. Stillage from the
stripping column A comprises 0.02 % EtOH w/w typically. A condensed
stream from the rectification column B at 93 % EtOH w/w is evaporated and
pre-heated in the evaporator C prior to being fed into the molecular sieve
system D, from which dehydrated ethanol vapour is recovered and condensed
afterwards as a 99.5 % EtOH w/w product.
[0030] The typical energy loads for the various components, as shown
in Figure 7, is 12.9 GJ/h required for steam for the stripping column A, 5.34
GJ/h required for steam for the rectification column B, and 5.26 GJ/h required
for steam for the evaporator.
[0031] An alternate distillation and dehydration section 110 using a
membrane unit 38e to replace the rectification column B, the evaporator C
and the molecular sieve dehydration system D of Figure 7 is shown in Figure
8. The alternate distillation and dehydration section 110 of Figure 8 reduces
the energy demand of the distillation and dehydration operations compared to
the distillation and dehydration section 100 of Figure 7.
[0032] The membrane unit 38e replaces the rectification column B, the
evaporator C and the molecular sieve dehydration system D of Figure 7, such
that the distillation and dehydration section 110 of the ethanol plant now
comprised two main processes and equipment units, the stripping column A
and the membrane unit 38e.
[0033] The membrane unit 38e comprises two membrane stages 80a,
80b in series, with a compressor 82 between, which raises the pressure of the
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retentate issued from the first stage 80a from about 110 kPa to about 225
kPa. The permeate from both stages 80a, 80b is condensed. Permeate from
the first stage 80a, at 0.1 % EtOH w/w, is directed to a fermention section of
the plant, and permeate from the second stage 80b at 1.7 % EtOH w/w is
directed back for re-distillation to the stripping column A.
[0034] Using the membrane system 38e to replace the units B, C and D
of the distillation and dehydration section 100 of Figure 7 corresponds to a
total savings of 5.34 + 5.26 = 10.60 GJ/h in steam energy. In addition, there
is
the potential to recover energy from the superheated compressed retentate
from the first stage 80a, 171 MJ/h, and the latent heat of condensation of the
retentate, dehydrated ethanol, issued from the second stage 80b, 2.47 GJ/h.
However, there is 64 MJ/h demand for steam to pre-heat the feed stream from
the stripping column A to the membrane unit 38e which is subtracted from the
energy savings of the distillation and dehydration section 110 of Figure 8. A
total net energy savings, in the form of reduced energy required to generate
steam, of 13.2 GJ/h results from using one distillation column (stripping
column A) with further dehydration by the membrane unit 38e over a
conventional coupling of a two column distillation unit (stripping column A
and
rectification column B) with further dehydration using an evaporator C and
molecular sieve dehydrators D.
[0035] Figure 9 illustrates another example of a design application of a
membrane vapour separation process in a distillation and dehydration section
120 of an ethanol plant. The design results in energy savings and an
increase in the dehydrated ethanol product yield compared to Figure 7.
[0036] In this example, a rich-ethanol purge at 69.1 % EtOH w/w issued
from the molecular sieve system D is directed to a membrane unit 38f
comprising two membrane stages 80a, 80b arranged in series. After
evaporation, in a second evaporator Cb, and pre-heating, the rich-ethanol
purge at 105 C and a pressure of 162 kPa, is fed into the membrane unit 38f.
The pressure of the retentate stream from the first stage 80a is reduced to
157.6 kPa and so there is no compression required on this stream ahead of
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the second membrane stage 80b. In the permeate line from the first stage
80a, the vacuum is set at 10 kPa absolute. In the permeate line from the
second stage 80b, the vacuum is set at 2.5 kPa absolute. After condensation,
the permeate from the first stage, at 1.2 % EtOH w/w, and the permeate from
the second stage, at 17.5 % EtOH w/w, are routed back for re-distillation to
the stripping column A. The total mass flow from both of these permeate
streams corresponds to about 0.1 % w/w of the flow of beer into the stripping
column A. The flow rate of anhydrous ethanol product from the membrane
unit 38f is 17 714 L/day, an increase in production by 24.8 % compared to the
conventional distillation and dehydration section 100 of Figure 7.
[0037] Dehydrating the ethanol-rich purge, at 845 kg/h and 69.1 %
EtOH w/w, into the membrane 38f results in a significant reduction of the
total
mass flow that is distilled in the rectification column B and afterwards
evaporated in the evaporator C. Consequently, the steam heat loads for units
B and C are now reduced to the following values: 3.33 GJ/h steam for the
rectification column B and 3.28 GJ/h steam for the evaporator C. Vvlth respect
to the data presented for the conventional distillation and dehydration
section
100 of Figure 7, the total steam heat load for units B and C in Figure 9 is
reduced by 4.0 GJ/h.
[0038] With the membrane unit 38f, the electrical energy required for
the powered equipment, which is chiller load for condensing the permeate
from the membrane unit 38f and condensing the additional anhydrous ethanol
produced by the membrane unit 38f and circulation and vacuum pumps, totals
45 MJ/h. The steam energy loads associated with the membrane unit 38f are
respectively 1.34 GJ/h and 87 MJ/h for evaporation and pre-heating the
ethanol-rich purge from the molecular sieve system D, for a total of 1.43 GJ/h
of energy required.
[0039] Using the membrane unit 38f to treat the molecular sieve purge
reduces the energy required to heat steam by 2.56 GJ/h, and the overall
energy required is reduced by 2.51 GJ/h, with the additional benefit of a
24.8% increase in anhydrous ethanol production.
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[0040] While various examples of devices or processes have been
described above, various other specific devices or processes may also be
within the scope of the invention defined by the following claims.
