U.S. patent application number 15/293712 was filed with the patent office on 2017-04-20 for continuous process for pretreating a ligno-cellulosic feedstock.
This patent application is currently assigned to Beta Renewables S.p.A.. The applicant listed for this patent is Beta Renewables S.p.A.. Invention is credited to Francesco Cherchi, Dario GIORDANO.
Application Number | 20170107664 15/293712 |
Document ID | / |
Family ID | 55027676 |
Filed Date | 2017-04-20 |
United States Patent
Application |
20170107664 |
Kind Code |
A1 |
GIORDANO; Dario ; et
al. |
April 20, 2017 |
CONTINUOUS PROCESS FOR PRETREATING A LIGNO-CELLULOSIC FEEDSTOCK
Abstract
It is disclosed a continuous process for pre-treating a
ligno-cellulosic feedstock. The ligno-cellulosic feedstock is
introduced in a pressurized reactor vessel and subjected to a
hydrothermal treatment with steam by inserting steam from at least
a two steam streams having different temperatures. The
ligno-cellulosic feedstock is then subjected to steam explosion.
Preferably, at least a portion of the steam in the reactor is
superheated steam and the superheated steam is located in a
superheated zone which is in proximity of the outlet of the
pressurized reactor vessel.
Inventors: |
GIORDANO; Dario; (Tortona,
IT) ; Cherchi; Francesco; (Novi Ligure, IT) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Beta Renewables S.p.A. |
Tortona |
|
IT |
|
|
Assignee: |
Beta Renewables S.p.A.
Tortona
IT
|
Family ID: |
55027676 |
Appl. No.: |
15/293712 |
Filed: |
October 14, 2016 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
D21B 1/36 20130101 |
International
Class: |
D21B 1/36 20060101
D21B001/36 |
Foreign Application Data
Date |
Code |
Application Number |
Oct 16, 2015 |
EP |
15425084 |
Claims
1. A continuous process for the pre-treatment of a ligno-cellulosic
feedstock, comprising the steps of: a) introducing the
ligno-cellulosic feedstock in a pressurized reactor vessel; b)
hydrothermally treating the ligno-cellulosic feedstock with steam
at a reactor pressure, by inserting at least a first steam stream
comprising a first steam and a second steam stream comprising a
second steam in the pressurized reactor vessel, the first steam
having a first steam temperature and the second steam having a
second steam temperature, wherein the second steam temperature is
greater than the first steam temperature; and c) steam exploding
the ligno-cellulosic feedstock.
2. The process of claim 1, wherein the first steam stream is
inserted into the pressurized reactor vessel through a first steam
inlet or inlets and the second steam stream is inserted into the
reactor vessel through a second steam inlet or inlets, wherein at
least one second steam inlet has a distance from a feedstock outlet
of the pressurized reactor vessel which is less than the distance
from a feedstock inlet of the pressurized reactor vessel.
3. The process of claim 1, wherein at least a portion of the steam
in the pressurized reactor vessel is superheated steam.
4. The process of claim 3, wherein the temperature of at least a
portion of the superheated steam in the pressurized reactor vessel
is at least 10.degree. C. greater than the steam saturation
temperature at the reactor pressure.
5. The process of claim 3, wherein the superheated steam is
included in a superheated zone of the pressurized reactor vessel,
wherein the superheated zone has a volume which is less than a
percent value selected from the group consisting of 50%, 30%, and
10% of the total volume of the pressurized reactor vessel.
6. The process of claim 5, wherein the superheated zone is located
in proximity of the feedstock outlet of the pressurized reactor
vessel.
7. The process of claim 1, wherein hydrothermally treating the
ligno-cellulosic feedstock is conducted for a residence time which
is a value in a range selected from the group consisting of from 1
minute to 30 minutes, from 2 minutes to 20 minutes, and from 3
minutes to 10 minutes.
8. The process of claim 1, wherein the first steam temperature is
in a range selected from the group consisting of from 170.degree.
C. to 230.degree. C., from 175.degree. C. to 210.degree. C., and
from 180.degree. C. to 195.degree. C.
9. The process of claim 8, wherein the second steam temperature is
greater than the first steam temperature by at least a value
selected from the group consisting of 10.degree. C., 30.degree. C.,
50.degree. C., and 100.degree. C.
10. The process of claim 8, wherein the first steam and the second
steam are saturated steam.
11. The process of claim 8, wherein the second steam is superheated
steam.
12. The process of claim 8, wherein the reactor pressure is greater
than a value selected from the group consisting of 13 bar, 15 bar,
and 18 bar.
13. The process of claim 12, wherein the first steam stream is
inserted at a first steam pressure and the second steam stream is
inserted at a second steam pressure, and the reactor pressure is at
least a percent value selected from the group consisting of 60%,
80%, 90%, and 95% and less than 100% of the lower of the first
steam pressure and the second steam pressure.
14. The process of claim 12, wherein the reactor pressure is
homogeneous.
15. The process of claim 8, wherein the second steam stream has a
mean flow which is greater than 0 and less than a percent value
selected from the group consisting of 70%, 50%, 30%, 10% of a mean
flow of the first steam stream and the second steam stream.
16. The process of claim 8, wherein the total amount of steam per
Kg of ligno-cellulosic feedstock on a dry basis introduced in the
pressurized reactor vessel is in a range selected from the group
consisting of from 0.2 Kg/Kg to 2 Kg/Kg, from 0.4 Kg/Kg to 1.5
Kg/Kg, and from 0.6 Kg/Kg to 1 Kg/Kg.
17. The process of claim 1, wherein steam exploding the
ligno-cellulosic feedstock comprises releasing a pressure applied
to the feedstock through a blow line operatively connected to the
feedstock outlet of the pressurized reactor vessel.
18. The process of claim 1, wherein the ligno-cellulosic feedstock
is introduced in the pressurized reactor vessel at a temperature
which is in a range selected from the group consisting of from
20.degree. C. to 100.degree. C., from 40.degree. C. to 95.degree.
C., and from 60.degree. C. to 90.degree. C.
19. The process of claim 18, wherein the ligno-cellulosic feedstock
introduced in the pressurized reactor vessel has a moisture content
in a range of from 40% to 70% by weight of the ligno-cellulosic
feedstock on a wet basis.
Description
PRIORITIES AND CROSS REFERENCES
[0001] This patent application claims the priority from European
Patent Application No. 15425084 filed on 16 Oct. 2016, the teaching
of which are incorporated herein by reference in their entirety
BACKGROUND
[0002] For converting a ligno-cellulosic feedstock to useful
compounds, such as biofuels and bio-chemicals, a pretreatment is
usually required to break down the ligno-cellulosic structure in
order to increase the accessibility to the carbohydrates contained
therein. The pre-treated ligno-cellulosic feedstock may then be
further processed, for example, by enzymatic hydrolysis, to obtain
a hydrolyzed mixture comprising monomeric sugars.
[0003] Typically, mechanical, thermal and hydrothermal, physical,
biological and chemical pretreatments are used.
[0004] Steam explosion is a well-known pre-treatment technique in
which the ligno-cellulosic feedstock is first subjected to a
hydrothermal treatment in the presence of steam at high temperature
and pressure, followed by rapid release of the steam pressure to
produce an explosive disruption of the ligno-cellulosic structure.
Thereby, the feedstock is inserted in a pressurized reactor,
wherein the pressure is usually obtained by inserting saturated
steam in the reactor. Steam is used to heat the feedstock to the
process temperature, ideally reaching a temperature which is close
to the steam temperature. A relevant portion of the steam will
therefore condense to liquid water causing a significant reduction
of the reactor pressure. Sufficient steam must be added to the
reactor vessel to achieve the desired pressure for steam explosion
pulping, such as 15 bar. A difficulty with this approach is that it
requires a large amount of steam to heat the material and to
efficiently steam explode it out of the reactor vessel. The
required large volume of steam is expensive in terms of energy
consumption, especially in the case that the feedstock is
introduced in the reactor vessel at a low temperature.
[0005] Therefore, a first problem to be solved is to reduce the
amount of steam needed in a steam explosion process, while
achieving at the same time an effective steam explosion of the
feedstock.
[0006] A second problem to be solved is to reduce the amount of
condensed liquid water present together with the feedstock during
steam explosion. Namely, condensed water, having a higher
gravimetric density than the feedstock, reduces the effectiveness
of steam explosion and may cause the plugging of the blow lines
typically used for implementing steam explosion.
[0007] A first solution to reduce the amount of steam while
preserving steam explosion effectiveness is presented in
US20080277082, which discloses a method and device for steam
explosion pulping including: impregnating a cellulosic biomass feed
material in a pressurized reactor vessel; discharging the
impregnated feed material from the vessel to a high pressure
compressor; elevating a pressure of the feed material in the
compressor; discharging the pressurized feed material from the
compressor to a conduit coupled to a blow valve; rapidly reducing
the pressure of the pressurized feed material as the feed material
passes through the blow valve, and pulping the feed material by
expansion of fluid in the feed material during the rapid pressure
reduction. The high pressure discharge compressor applies
centrifugal force to increase the pressure of a feed material
stream from a pressurized reactor vessel. The centrifugal force
applied to the stream increases the pressure to, for example, at
least 0.5-1 bar above the pressure inside the cooking reactor. A
first drawback of the offered solution is that a relevant amount of
mechanical energy is required in providing centrifugal force to the
feed material stream to reach sufficiently high pressure to ensure
an effective subsequent steam explosion. A second drawback is that
the incoming material may plug the rotating components of the
compressor. As the rotation speed is high, disruptive damages may
occur.
[0008] A further solution to reduce the amount of steam while
preserving steam explosion effectiveness is originally disclosed in
Boehm, R. M. "The Masonite process", Industrial and Engineering
Chemistry, 22(5), pag. 493-497, 1930, and described in Fiberboard
Manufacturing Practices in the United States, by Otto Suchsland and
George E. Woodson, United States Department of Agriculture Forest
Service Agriculture Handbook No. 640, 1984, p. 62. The disclosed
sequence of operation is as follows: 1) Gun is loaded with green
chips through the port on the top; 2) Chip inlet valve is tightly
closed; 3) Low-pressure steam (350 lb/in.sup.2--just over
430.degree. F.) is admitted immediately. This brings the chips to a
temperature of about 375.degree. F.; 4) The chips remain at
375.degree. F. for 30 to 40 s; 5) High-pressure steam is admitted
and the gun pressure is elevated within about 2 to 3 s to 1,000
lb/in.sup.2 equivalent to a temperature of about 540.degree. F.; 6)
The chips remain at this pressure for about 5 s; 7) The hydraulic
discharge valve is opened; 8) The chips explode due to the pressure
differential and at the same time are forced by the expanding steam
through the slotted bottom port plate where they are shredded into
a mass of fiber bundles; 9) Steam and fibers are separated in a
cyclone. Thereby, the disclosed process is a batch process using
two steam sources, namely the low-pressure steam and the high
pressure steam, sequentially. These types of batch processes are
known to be difficult to be implemented on an industrial scale.
[0009] The use of superheated steam for treating a ligno-cellulosic
biomass is known in the art.
[0010] As an example, Dave Barchyn, Stefan Cenkowski, "Process
analysis of superheated steam pre-treatment of wheat straw and its
relative effect on ethanol selling price", Biofuel Research Journal
4 (2014) 123-128, examined the use of superheated steam as a
process medium by which wheat straw ligno-cellulosic material is
pre-treated as an alternative to steam explosion. In the paper, it
is said that superheated steam has been successfully implemented
into industrial processes such as food processing and drying and
biomass decontamination and has led to substantial increases in
energy efficiency due to high penetration and energy delivery. In
the disclosed pre-treatment, the samples were subjected to 15 min
of hot water treatment in pressurized hot water (193 kPa,
119.degree. C.) followed by 2, 5, or 10 min of superheated steam
treatment in a batch process.
[0011] In WO2011044282A2 discloses a process for the
thermal-mechanical pretreatment of biomass. The process includes
subjecting a biomass feedstock to thermal reaction under conditions
exceeding atmospheric pressure, at a temperature exceeding ambient
temperature, at a predetermined moisture content and for a
predetermined amount of time. Subsequently, the pressure of said
thermal reaction is reduced under conditions resulting in explosive
decompression of said biomass. The decompressed biomass is then
subjected to axial shear forces to mechanically reduce the size of
the fibers of the biomass to obtain treated biomass. The resultant
treated biomass has a high level of enzymatic digestibility and a
low concentration of degradation products. The thermal reaction
conditions are provided by a live steam injection. In one
embodiment, the steam injection is provided at a minimum pressure
of 290 psig and reduced adiabatically to the thermal reactor
operating pressure, therefore allowing the steam to enter the
reactor slightly superheated in order to compensate for any ambient
heat loss in the reactor. Generally, the higher the steam pressure,
the more superheat can be transferred to the reactor.
[0012] The present invention is believed to solve in an effective
manner the above mentioned problems occurring in steam explosion
processes disclosed in the art.
BRIEF DESCRIPTION OF THE INVENTION
[0013] It is disclosed a continuous process for the pre-treatment
of a ligno-cellulosic feedstock, comprising the following steps of:
introducing the ligno-cellulosic feedstock in a pressurized reactor
vessel; hydrothermally treating the ligno-cellulosic feedstock with
steam at a reactor pressure, by inserting at least a first steam
stream comprising a first steam and a second steam stream
comprising a second steam in the pressurized reactor vessel, the
first steam having a first steam temperature and the second steam
having a second steam temperature, wherein the second steam
temperature is greater than the first steam temperature; and steam
exploding the ligno-cellulosic feedstock.
[0014] It is further disclosed that the first steam stream may be
inserted into the pressurized reactor vessel through a first steam
inlet or inlets and the second steam stream is inserted into the
reactor vessel through a second steam inlet or inlets, wherein at
least one second steam inlet has a distance from a feedstock outlet
of the pressurized reactor vessel which is less than the distance
from a feedstock inlet of the pressurized reactor vessel.
[0015] It is also disclosed that at least a portion of the steam in
the pressurized reactor vessel may be superheated steam.
[0016] It is further disclosed that the temperature of at least a
portion of the superheated steam in the pressurized reactor vessel
may be at least 10.degree. C. greater than the steam saturation
temperature at the reactor pressure.
[0017] It is also disclosed that the superheated steam may be
included in a superheated zone of the pressurized reactor vessel,
wherein the superheated zone has a volume which is less than a
percent value selected from the group consisting of 50%, 30%, and
10% of the total volume of the pressurized reactor vessel.
[0018] It is also disclosed that the superheated zone may be
located in proximity of the feedstock outlet of the pressurized
reactor vessel.
[0019] It is further disclosed that hydrothermally treating the
ligno-cellulosic feedstock may be conducted for a residence time
which is a value in a range selected from the group consisting of
from 1 minute to 30 minutes, from 2 minutes to 20 minutes, and from
3 minutes to 10 minutes.
[0020] It is also disclosed that the first steam temperature may be
in a range selected from the group consisting of from 170.degree.
C. to 230.degree. C., from 175.degree. C. to 210.degree. C., and
from 180.degree. C. to 195.degree. C.
[0021] It is further disclosed that the second steam temperature
may be greater than the first steam temperature by at least a value
selected from the group consisting of 10.degree. C., 30.degree. C.,
50.degree. C., and 100.degree. C.
[0022] It is also disclosed that the first steam and the second
steam may be saturated steam.
[0023] It is further disclosed that the second steam may be
superheated steam.
[0024] It is also disclosed that the reactor pressure may be
greater than a value selected from the group consisting 13 bar, 15
bar, and 18 bar.
[0025] It is further disclosed that the first steam stream may be
inserted at a first steam pressure and the second steam stream is
inserted at a second steam pressure, and the reactor pressure is at
least a percent value selected from the group consisting of 60%,
80%, 90%, and 95% and less than 100% of the lower of the first
steam pressure and the second steam pressure.
[0026] It is also disclosed that the reactor pressure may be
homogeneous.
[0027] It is further disclosed that the second steam stream has a
mean flow which may be greater than 0 and less than a percent value
selected from the group consisting of 70%, 50%, 30%, 10% of a mean
flow of the first steam stream and the second steam stream.
[0028] It is also disclosed that the total amount of steams per Kg
of ligno-cellulosic feedstock on a dry basis introduced in the
pressurized reactor vessel may be in a range of from 0.2 Kg/Kg to 2
Kg/Kg, from 0.4 Kg/Kg to 1.5 Kg/Kg, and from 0.6 Kg/Kg to 1
Kg/Kg.
[0029] It is further disclosed that steam exploding the
ligno-cellulosic feedstock may comprise releasing a pressure
applied to the feedstock through a blow line operatively connected
to the feedstock outlet of the pressurized reactor vessel.
[0030] It is also disclosed that the ligno-cellulosic feedstock may
be introduced in the pressurized reactor vessel at a temperature
which is in a range selected from the group consisting of from
20.degree. to 100.degree. C., from 40.degree. to 95.degree. C., and
from 60.degree. to 90.degree. C.
[0031] It is further disclosed that the ligno-cellulosic feedstock
introduced in the reactor may have a moisture content in a range of
from 40% to 70% by weight of the ligno-cellulosic feedstock on a
wet basis.
BRIEF DESCRIPTION OF FIGURES
[0032] FIG. 1 is an exemplary embodiment of the disclosed
process
[0033] FIG. 2 is another exemplary embodiment of the disclosed
process
DETAILED DESCRIPTION
[0034] It is disclosed a pre-treatment process of a
ligno-cellulosic feedstock comprising carbohydrates and lignin. The
pre-treatment process increases the accessibility of the
carbohydrates to a biological agent such as an enzyme or enzyme
mixture. Therefore, the pre-treated ligno-cellulosic feedstock may
be subjected to a subsequent hydrolysis step, to produce a
hydrolyzed mixture comprising water soluble monomeric sugars. A
detailed description of a ligno-cellulosic feedstock may be found
in WO2015028156A1, pag. 11-14, which is herein incorporated by
reference. A preferred ligno-cellulosic feedstock is selected from
the group of agricultural residues, in particular straws such as
wheat straw, rice straw, or bagasse, such as sugar cane bagasse.
The hardwoods and softwoods also benefit from this process.
[0035] The disclosed process is a continuous process, which
comprises introducing the ligno-cellulosic feedstock in a
pressurized reactor vessel through a feedstock inlet, and
subjecting the ligno-cellulosic feedstock to a hydrothermal
treatment while the feedstock moves, or is conveyed, to a feedstock
outlet of the pressurized reactor. The hydrothermal treatment is
realized by inserting at least two steams having different
temperature in the pressurized reactor vessel, which is thereby
pressurized at a reactor pressure by the at least two steams
inserted therein. The ligno-cellulosic feedstock is then subjected
to steam explosion by rapidly releasing the pressure applied to the
feedstock.
[0036] According to one aspect, the disclosed process significantly
reduces the total amount of steam which is needed to pre-treat the
ligno-cellulosic feedstock, with respect to the amount of steam
needed to pre-treat the ligno-cellulosic feedstock with steam at a
unique temperature.
[0037] According to another aspect, the disclosed process greatly
reduces the amount of liquid water which is formed in the
pressurized reactor vessel due to steam condensation. Because of
the high pressure, it is difficult to separately withdraw liquid
water from the pressurized reactor vessel and its presence during
steam explosion may strongly reduce the disruptive effects of the
steam explosion on the ligno-cellulosic feedstock.
[0038] According to a further aspect of the invention, the
disclosed process prevents or strongly reduces the steam pressure
drop in the pressurized reactor vessel due to steam condensation.
The pressure drop may occur in the pressurized reactor vessel
typically in the case that the ligno-cellulosic feedstock is
inserted at low temperature while inserting the steam at a low flow
rate to limit steam consumption.
[0039] In order for the process to be continuous, it is not
necessary that the ligno-cellulosic feedstock is continuously
introduced into the pressurized reactor vessel, but it can be
introduced at steady aliquots or pulses. Thus there are moments
when there is no ligno-cellulosic feedstock entering the
pressurized reactor vessel. But, over time, the total mass
introduced into the pressurized reactor vessel equals the total
mass subjected to steam explosion. In the case that a portion of
the ligno-cellulosic feedstock is withdrawn in liquid and/or solid
form from the pressurized reactor vessel from auxiliary outlets
without being steam exploded, the mass balance applies to the total
amount of withdrawn and steam exploded ligno-cellulosic feedstock.
One distinguishing feature between a continuous and a batch process
is that, in a continuous process, a fresh portion of the
ligno-cellulosic feedstock is introduced in the pressurized reactor
vessel at the same time that a hydrothermally treated portion of
the ligno-cellulosic feedstock is subjected to steam explosion.
Such steam explosion is done in a continuous manner which includes
an aliquot or pulse removal.
[0040] The ligno-cellulosic feedstock is preferably subjected to a
previous soaking process or step to remove a portion of
non-ligno-cellulosic compounds contained in the raw
ligno-cellulosic feedstock such as inorganic salts, waxes, and
organic acids prior to being introduced into the pressurized
reactor. In the soaking step or process, external contaminants,
such as ground, stones, and harvesting residues may also be
separated. The soaking process preferably comprises introducing the
ligno-cellulosic feedstock in a soaking liquid comprising water at
a temperature from 20.degree. C. to 100.degree. C., more preferably
from 40.degree. C. and 70.degree. C. and for a soaking time which
is from 30 seconds to 30 minutes, more preferably from 3 minutes to
15 minutes. The soaking step or process is preferably conducted at
atmospheric pressure. After soaking, some liquid is removed from
the ligno-cellulosic feedstock by draining and/or by applying
mechanical shearing/compression forces. Preferably, all the free
liquid and at least a portion of the soaked liquid in the biomass
are withdrawn before inserting the feedstock in the pressurized
reactor. Thereby, in the disclosed process, the ligno-cellulosic
feedstock may be introduced in the pressurized reactor vessel at a
temperature in a range from 20.degree. C. to 100.degree. C., more
preferably from 40.degree. C. to 95.degree. C., and most preferably
from 60.degree. C. to 90.degree. C. The moisture content of the
ligno-cellulosic feedstock may be from 40% to 70%, more preferably
from 45% to 65%, and most preferably from 50% to 60% on a wet
basis.
[0041] In another embodiment, the ligno-cellulosic feedstock is
subjected to a preliminary hydrothermal treatment in water or a
liquid comprising water to solubilize a portion of the water
insoluble carbohydrates contained in the ligno-cellulosic feedstock
prior to being introduced in the pressurized reactor vessel. The
preliminary hydrothermal treatment is conducted in pressurized
conditions in the presence of water in a steam or liquid phase, or
mixture thereof, at a temperature from 100.degree. C. to
190.degree. C., preferably from 130.degree. C. to 180.degree. C.,
and most preferably from 140.degree. C. to 170.degree. C. The
preliminary hydrothermal treatment is conducted for a time in a
range from 10 minutes to 3 hours, preferably from 15 minutes to 3
hours, and most preferably from 20 minutes to 60 minutes. The
preliminary hydrothermal treatment solubilizes mainly the
hemicellulosic component of the ligno-cellulosic feedstock, which
may subjected to thermal degradation at high temperature, and a
liquid comprising water and water soluble xylose polymers and
oligomers and optionally other hemicellulose-derived sugars is
thereby separated from the solid ligno-cellulosic feedstock before
treating the solid ligno-cellulosic feedstock according to the
disclosed process.
[0042] To describe the disclosed process, reference is made to FIG.
1 and FIG. 2, which represent two exemplary reactor assemblies
which may be used to implement the process. Each reactor assembly
comprises a pressurized reactor vessel, wherein the hydrothermal
treatment occurs, and a steam explosion device for rapidly reducing
the pressure applied to the feedstock.
[0043] The ligno-cellulosic feedstock is introduced in the
pressurized reactor vessel from a zone which is at a lower pressure
than the reactor pressure. The feedstock may be introduced from a
zone at atmospheric pressure, i.e. 1 bar, or from an already
pressurized environment at a pressure greater than 1 bar. A
pressure sealing device is therefore used for introducing the
feedstock in the pressurized reactor vessel. The pressure sealing
device is preferably a continuous plug forming device such as a
biomass compressor, also known as a plug screw feeder, or worm
screw feeder. In this case, the feedstock is conveyed from an inlet
of the pressure sealing device connected to the low pressure zone
to an outlet of the pressure sealing device by means of an internal
screw, which compresses the feedstock to form a plug capable of
dynamically sustaining a difference of pressure between the two
zones at different pressure, while continuously introducing the
feedstock in the pressurized reactor vessel. The feedstock plug may
be mechanically fragmented at the outlet of the pressure sealing
device and the feedstock preferably enters the pressurized reactor
vessel under the action of gravity. A rotary cell, which works in a
discontinuous or semi-continuous mode, may also be used to
introduce sequential aliquots of feedstock in the pressurized
reactor vessel. The pressure sealing device is connected to the
pressurized reactor vessel, that is the outlet of the pressure
sealing device may be directly or indirectly connected to the
feedstock inlet of the pressurized reactor vessel. In the case of
direct connection, typically the outlet of the pressure sealing
device and the inlet of the pressurized reactor vessel are joined
by means of one or more flanges tightened to avoid steam leaks. In
the case of indirect connection, a connection system is interposed
between the outlet of the pressure sealing device and the inlet of
the pressurized reactor vessel. The connection system may comprise
one or more pipes, vessels or apparatuses.
[0044] The pressurized reactor vessel comprises a feedstock inlet,
a feedstock outlet and two or more steam inlets for inserting steam
in the pressurized reactor vessel, and it is designed to operate at
a maximum internal pressure of at least 20 bar, preferably at least
40 bar, and more preferably at least 50 bar, according to
well-known reactor design rules. Depending on the specific type of
pressurized reactor vessel, the feedstock inlet and feedstock
outlet on the pressurized reactor vessel may be located in
different positions. The pressurized reactor vessel used to
implement the disclosed process may be any kind of pressurized
reactor.
[0045] In FIG. 1, it is represented a first exemplary reactor
assembly comprising a vertical pressurized reactor vessel, wherein
the feedstock inlet is positioned at a higher height than the
feedstock outlet with respect to gravity, preferably at the top of
the reactor, in such a way that the flow of the ligno-cellulosic
feedstock from the feedstock inlet to the feedstock outlet is
promoted by the action of gravity force. The pressurized reactor
vessel may include a means to convey, or move, the ligno-cellulosic
feedstock from the feedstock inlet to the feedstock outlet. The
inclination angle of the pressurized reactor vessel may be
different from vertical, without limiting the scope of the
invention.
[0046] In FIG. 2, it is represented a second exemplary reactor
assembly comprising a tubular pressurized reactor vessel, having
preferably a cylindrical shape. The tubular pressurized reactor
vessel is preferably disposed in a horizontal or approximately
horizontal position, thereby the main axis of the pressurized
reactor vessel may be at an angle which is less than 15.degree.,
preferably less than 10.degree., and more preferably less than
5.degree. with respect to a horizontal plane. If the inclination
angle is different from 0.degree., the tubular pressurized reactor
vessel is preferably oriented in such a way to promote the flow of
the condensed water or liquids to the feedstock outlet under the
action of gravity, which corresponds to a clockwise inclination
angle in FIG. 2. The feedstock inlet and the feedstock outlet are
preferably located at or close to the opposite ends of the tubular
pressurized reactor vessel. The feedstock inlet and feedstock
outlet may be located on the circular bases or on the lateral
surface of the tubular pressurized reactor vessel. Preferably, the
feedstock inlet is located at a higher height than the feedstock
outlet with respect to gravity so that the ligno-cellulosic
feedstock is inserted and it is removed under the action of
gravity, even if a mechanical or pneumatic extractor may be used to
force the removal of the ligno-cellulosic feedstock from the
tubular pressurized reactor vessel. The tubular pressurized reactor
vessel may comprise an internal screw conveyor having a rotation
axis which is coincident with the main axis of the pressurized
reactor vessel. The shaft of the screw conveyor is connected with
rotation means, which typically comprise a motor and a transmission
coupling stage external to the pressurized reactor vessel. The
screw conveyor conveys the ligno-cellulosic feedstock from the
feedstock inlet to the feedstock outlet. Preferably, the conveyor
flights are designed to promote the mixing of the ligno-cellulosic
feedstock with steam while it is conveyed from the feedstock inlet
to the feedstock outlet.
[0047] The steam inlets are located on the pressurized reactor
vessel, each steam inlet being connected to a steam source through
a steam line. With reference to the exemplary reactor vessels of
FIGS. 1 and 2, the steam inlets, or the majority of the steam
inlets, are preferably located on the lateral surface of the
tubular pressurized reactor vessel, with some steam inlets
distributed along the longitudinal section of the pressurized
reactor vessel so that they progressively infuse steam on the
ligno-cellulosic feedstock advancing in the pressurized reactor
vessel. A steam inlet is typically connected to a steam controlling
device, which is preferably located on the steam line in proximity
of the steam inlet. A suitable steam controlling device which can
be used is a steam valve, indicated as V1 and V2 in FIGS. 1 and 2.
Steam pipes connect the steam inlets with at least a steam source.
Preferably, each steam inlet is connected to one steam source,
while a steam source is connected to one or more steam inlets. The
steam source may be any suitable apparatus to generate steam at
high pressure and temperature, such as a steam boiler. The steam
line is pressurized at a steam pressure higher than the reactor
pressure and the steam controlling device typically regulates the
steam flow entering the pressurized reactor vessel through the
specific steam inlet. As the steam entering the pressurized reactor
vessel may be subjected to turbulent flow immediately downstream
the steam controlling device, the steam pressure of each steam
entering the pressurized reactor vessel is defined as the steam
pressure upstream of the steam controlling device and it may be
measured by means of a pressure gauge positioned preferably
immediately upstream of the steam controlling device. In the same
way, the steam temperature of each steam entering the pressurized
reactor vessel is defined as the steam temperature upstream of the
steam controlling device and it may be measured by means of a
temperature gauge positioned preferably immediately upstream of the
steam controlling device.
[0048] The steam inlets are distributed on the surface of the
pressurized reactor vessel, in order to progressively infuse the
ligno-cellulosic feedstock with steam while the ligno-cellulosic
feedstock progressively advances from the feedstock inlet to the
feedstock outlet of the pressurized reactor vessel. Thereby, each
steam inlet is positioned at a certain distance from the feedstock
inlet and from the feedstock outlet. Because the feedstock inlet
and a steam inlet are two geometrical surfaces, from a geometrical
point of view the distance of a steam inlet from the feedstock
inlet may defined as the minimum linear distance of any point of
the steam inlet to any point of the feedstock inlet. From an
alternative physical point of view, the distance of a steam inlet
from the feedstock inlet may be defined as the minimum length a
steam molecule travels in the pressurized reactor vessel to reach
the feedstock inlet. Equivalently, the distance of a steam inlet
from the feedstock outlet may be geometrically defined as the
minimum linear distance of any point of the steam inlet to any
point of the feedstock outlet. From an alternative physical point
of view, the distance of a steam inlet from the feedstock outlet
may be defined as the minimum length a steam molecule travels in
the pressurized reactor vessel to reach the feedstock outlet. With
reference to FIG. 1 and FIG. 2, the distance of the steam inlet 52
from the feedstock outlet is less than the distance of the steam
inlet 52 from the feedstock inlet, while the distance of the steam
inlet S1 from the feedstock inlet is less than the distance of the
steam inlet S1 from the feedstock outlet.
[0049] The pressurized reactor vessel may further comprise
auxiliary inlets for introducing liquids or gas in the reactor
vessel, and discharge outlets for removing liquids from the
reactor, such as for instance condensed water.
[0050] In the disclosed process, the ligno-cellulosic feedstock is
introduced in the pressurized reactor and subjected to a continuous
hydrothermal treatment while it is moved through the pressurized
reactor vessel from the feedstock inlet to the feedstock outlet.
The hydrothermal treatment is preferably conducted for a short
residence time, which may be from 1 minute to 30 minutes,
preferably from 2 minutes to 20 minutes, and most preferably from 3
minutes to 10 minutes. In order to obtain an effective hydrothermal
treatment of the lingo-cellulosic feedstock in a short time, at
least two steams in the pressurized reactor vessel are used, the
steams having different temperatures. Even if more than two steams
may be used, the process may be described for clarity in the
preferred embodiment of two steams. Thereby, a first steam stream
comprising the first stream having a first steam temperature is
inserted in the pressurized reactor vessel through at least a first
steam inlet, while a second steam stream comprising the second
steam having a second steam temperature is inserted the pressurized
reactor vessel through at least a second steam inlet. Preferably
each steam is inserted through a set of steam inlets, in order to
achieve a homogeneous mixing of steam with the ligno-cellulosic
feedstock. Preferably, the first steam inlets are upstream
connected to a unique first steam generator, or steam source,
providing the first steam, and the second steam inlets are
connected to a unique second steam generator providing the second
steam. It is noted that the redundancy of steam generators
introduced by the disclosed process is apparent, as a real
ligno-cellulosic feedstock conversion plant, or biorefinery,
usually includes a main steam generator and a supplementary boiler
to produce heat and electric energy by burning ligno-cellulosic
residues and by-products. In the case that only one steam generator
producing steam at a unique temperature is available, the first
steam and the second steam may be derived from the unique steam
generator, optionally reusing recycled steam streams from the
conversion plant.
[0051] The first steam is characterized by having a first steam
temperature and a first steam pressure which are preferably
measured immediately upstream of the steam controlling device.
Correspondingly, the second steam is characterized by having a
second steam temperature and a second steam pressure which are
preferably measured immediately upstream of the steam controlling
device. Preferably, the first steam temperature is in a range from
170.degree. C. to 230.degree. C., more preferably from 175.degree.
C. to 210.degree. C., and, and most preferably from 180.degree. C.
to 195.degree. C. The second steam temperature is greater than the
first steam temperature, that is the second steam temperature is
greater than the first steam temperature by at least 10.degree. C.,
more preferably 30.degree. C., even more preferably 50.degree. C.,
and most preferably 100.degree. C. Even if the maximum value
allowed for the second steam temperature will vary according to the
specific configuration and process conditions, preferably the
second steam temperature is less than 300.degree. C. In one
embodiment, the first steam and the second steam are saturated
steam, that is they are in equilibrium with heated water at the
same pressure, i.e., it has not been heated past the boiling point
for that pressure. Again, it is reminded that this property refers
to the steam before being inserted in the pressurized reactor
vessel. Thereby, the first steam pressure and the second steam
pressure are fixed by the thermodynamic equilibrium condition, and
are easily defined by the temperature-pressure conversion tables of
saturated steam. In another embodiment, at least the second steam
is superheated steam, thereby the second steam pressure is less
than the saturation pressure at the second steam temperature. The
superheated steam may be obtained from the saturated steam drawn
from a boiler by passing it through a separate heating device (a
super-heater) which transfers additional heat to the steam by
contact or by radiation.
[0052] The first steam and the second steam are introduced in the
pressurized reactor vessel in the form of a stream. While a
continuous steam stream is preferred, being more easily operatively
controllable, the stream may also be pulsed. In a preferred
embodiment, the first steam is the main steam used for
hydrothermally treating the ligno-cellulosic feedstock, while the
second steam is used as a refining steam. Thereby, in this
preferred embodiment, the amount of first steam exceeds, or greatly
exceeds, the amount of second steam. The amount of the second steam
introduced in the pressurized reactor vessel may be defined in
terms of the mean flow of the second steam stream relative to the
mean flow of the first steam stream and the second steam stream
introduced in the pressurized reactor vessel. The instantaneous
steam flow may vary to a great extent, as it happens in the case of
a pulsed steam stream. Thereby, a mean flow is measured over a time
which is equal or comparable to the residence time of the
ligno-cellulosic feedstock in the pressurized reactor vessel. Even
if the mean flow of the second steam stream may be less than 70% of
the mean flow of the total steam streams, it is preferred that it
is less than 50%, more preferred less than 30%, and most preferred
less than 10% of the mean flow of the first steam stream and the
second steam stream. One of the improvements of the disclosed
process over the prior art processes is the reduction of total
amount of steam needed in the hydrothermal treatment, thereby also
the reduction of condensed water or liquid in the pressurized
reactor vessel. The amount of steam used in the process may be
quantified as the total amount of steam used for pretreating a Kg
of ligno-cellulosic feedstock on a dry basis, which in the case of
two steams is the sum of the amount of the first steam and the
second steam. The total amount of steams per Kg of ligno-cellulosic
feedstock on a dry basis introduced in the reactor is preferably in
a range of from 0.2 Kg/Kg to 2 Kg/Kg, more preferably from 0.4
Kg/Kg to 1.5 Kg/Kg, and most preferably from 0.6 Kg/Kg to 1
Kg/Kg.
[0053] Preferably, the ligno-cellulosic feedstock does not
completely fill the pressurized reactor vessel in such a way that
the hydrothermal treatment is conducted in the presence of steam.
Thereby, preferably in a portion of the pressurized reactor vessel
only steam is present. The filling factor of the pressurized
reactor vessel, which is the percent volume of the pressurized
reactor vessel occupied by the ligno-cellulosic feedstock, may be
less than 80%, preferably less than 60%, and most preferably less
than 50%. Thereby, the first steam and the second steam, entering
the pressurized reactor vessel, will mix with the steam already
present in the pressurized reactor vessel to reach the reactor
pressure, which is the pressure of steam in the pressurized reactor
vessel. The reactor pressure in the pressurized reactor vessel is
preferably spatially homogeneous. At a certain instant, spatial
variations of the reactor pressure may occur in a limited portion
of the pressurized reactor vessel, especially in proximity of the
steam inlets due to fluid dynamic turbulences generated by steam
insertion. The reactor pressure may be measured by means of a set
of pressure gauges sampling the internal pressure. The pressure
gauges are preferably homogeneously positioned in the pressurized
reactor vessel, taking care to avoid the reactor regions in
proximity of the steam inlets, which are not statistically
representative of the reactor pressure. The reactor pressure is
represented by the arithmetic mean of the sampled pressure, and the
reaction pressure is considered homogeneous if the standard
deviation of the sampled pressure is less than 10%, preferably less
than 5%, and most preferably less than 2% of the reaction
pressure.
[0054] Preferably, the reactor pressure is greater than 13 bar,
more preferably greater than 15 bar, and most preferably greater
than 18 bar.
[0055] The first steam pressure and the second steam pressure are
greater than the reactor pressure, to permit both the steam streams
to enter the pressurized reactor vessel, but preferably the reactor
pressure is close to the lower of the first steam pressure and the
second steam pressure. In the preferred embodiment that the first
steam and the second steam are saturated steam, the reactor
pressure is close to the first steam pressure, which is the steam
at lower temperature. The reactor pressure is at least 60%, more
preferably at least 80%, even more preferably at least 90%, and
most preferably at least 95% of the lower value of the first steam
pressure and the second steam pressure.
[0056] The first and the second steam streams are inserted through
separated inlets which are located in different positions on the
surface of the pressurized reactor vessel, thereby that the steam
temperature in the pressurized reactor vessel is preferably not
homogeneous. Namely, in a zone of the pressurized reactor vessel in
proximity of the second steam inlet the steam temperature will be
close to the second steam temperature. Thereby, in that zone the
steam temperature will be greater than the steam temperature in a
zone located in proximity of the first steam inlet. As the second
steam diffuses from the second steam inlet in the pressurized
reactor vessel, the temperature of the internal steam will
progressively decrease. Thereby, in the disclosed process, at least
a portion of the steam in the pressurized reactor vessel may be in
a superheated state. The presence of steam in superheated state, or
superheated steam, can be verified by locally measuring the steam
temperature in different positions in the pressurized reactor
vessel. A steam temperature greater than the steam saturation
temperature at the reactor pressure indicates that the steam in the
measurement position is in a superheated state. The superheated
steam may reach a temperature far exceeding the steam saturation
temperature, which is a further important advantage offered by the
disclosed process to reduce the amount of condensed liquids in the
pressurized reactor vessel. Namely, the extra energy of superheated
steam, with respect to saturated steam at the same pressure, may
heat the ligno-cellulosic feedstock without generating condensed
water or liquid. Thereby, at least a portion of the superheated
steam preferably has a temperature which is at least 10.degree. C.
greater than the steam saturation temperature at the reactor
pressure, more preferably at least 30.degree. C. greater than the
steam saturation temperature at the reactor pressure, and most
preferably at least 50.degree. C. greater than the steam saturation
temperature at the reactor pressure. The extra temperature of the
superheated steam in the pressurized reactor vessel with respect to
the saturation temperature will depend on many factors, and can
therefore be controlled to a certain extent. A first factor is the
difference of the second steam pressure and the reactor pressure. A
second factor is the mode in which the second steam stream is
inserted in the pressurized reactor vessel: namely, the faster the
insertion, the less the temperature of the second steam entering
the pressurized reactor vessel will drop. Preferably, the second
steam stream is inserted by means of an adiabatic or nearly
adiabatic expansion, thereby without a significant heat exchange
with the environment. In one embodiment, the second steam is in a
superheated state before entering the pressurized reactor vessel,
in which case at least a portion of the second steam will remain in
a superheated state in the pressurized reactor vessel. It is noted
that if the second steam is in a superheated state, the second
steam pressure may be close to the reactor pressure, provided that
it is greater to permit the insertion of the steam. In this case,
the steam in the pressurized vessel is in a superheated state
because the second steam is already in a superheated state before
being inserted in the pressurized vessel.
[0057] In one embodiment, the steam in the pressurized reactor
vessel may be in a superheated state also in a zone located in
proximity of the first steam inlet, and the first steam may be
suitably selected as in the case of the second steam.
[0058] In a preferred embodiment, the first steam is saturated
steam at a first steam pressure which is slightly greater that the
reactor pressure to permit the insertion in the pressurized reactor
vessel, thereby the steam in the reactor vessel is at most in a
superheated state in a very limited zone close to the first steam
inlet. Thereby, in a zone in proximity of the first steam inlet the
steam in the reactor vessel is saturated steam. In this case, the
reactor pressure is at least 60%, more preferably at least 80%,
even more preferably at least 90%, and most preferably at least 95%
of the of the first steam pressure.
[0059] The inventors have found that by suitably inserting two or
more steams having different steam temperatures, the local
temperature of the steam in the unique pressurized reactor vessel
may be controlled to a great extent, and that a portion of the
internal steam may be maintained in a superheated state. Thereby,
in the unique pressurized reactor vessel, there may be the presence
of steam in a saturated state and a superheated state in different
zones of the pressurized reactor vessel. The extent and position of
the different zones may be controlled by suitably locating the
first steam inlets and the second steam inlets on the pressurized
reactor vessel. This may be realized for instance by concentrating
the second steam inlets, or the majority of the second steam
inlets, on a specific region of the pressurized reactor vessel
surface, in such a way to have in the pressurized reactor vessel an
extended hot steam zone, wherein steam is preferably in a
superheated state. On the basis of the present disclosure, the
position and extent of the different temperature zones may be
easily defined or changed by a person skilled in the art.
[0060] In a preferred embodiment, the superheated steam is included
in a small zone of the reactor vessel, which is a superheated zone,
in such a way that the ligno-cellulosic feedstock is treated with
superheated steam for a short superheating time, which can be of
the order of a few minutes, or less than 2 minutes, or less than 1
minute, or less 30 seconds. The superheating time may be less than
50% of the total residence time of the hydrothermal treatment,
preferably less than 30%, and most preferably less than 10%. In
this embodiment, the second steam temperature may be extremely
high, i.e. greater than 250.degree. C., without causing a
significant sugars degradation as the exposure of the
ligno-cellulosic feedstock to high temperature steam occurs for a
short time. Moreover, the small superheated zone may be sustained
by using a limited amount of second steam, thereby in this
embodiment the percent flow of the second steam stream is less than
20% of the flow of total steam streams. The superheated zone may
have a volume which is less than a 20%, more preferably less than
10% of the total volume of the reactor vessel.
[0061] In a preferred embodiment, the second steam is inserted
through a second steam inlet on the pressurized reactor vessel
which has a distance from the feedstock outlet which is less than
the distance from the feedstock inlet. In the case that the second
steam is inserted through multiple steam inlets, preferably at
least one of the second steam inlets has a distance from the
feedstock outlet which is less than the distance from the feedstock
inlet, even more preferably the majority of the second steam inlets
are positioned to have a distance from the feedstock outlet which
is less than the distance from the feedstock inlet, and most
preferably all the second steam inlets have a distance from the
feedstock outlet which is less than the distance from the feedstock
inlet. Even if the first steam inlets may be positioned without
specific requirements, at least one of the first steam inlets has a
distance from the feedstock inlet which is less than the distance
from the feedstock outlet, even more preferably the majority of the
first steam inlets are positioned to have a distance from the
feedstock inlet which is less than the distance from the feedstock
outlet, and most preferably all the first steam inlets have a
distance from the feedstock inlet which is less than the distance
from the feedstock outlet. In this embodiment, the steam
temperature in the zone of the pressurized reactor vessel in
proximity of the feedstock outlet is greater than the steam
temperature in the zone of the pressurized reactor vessel in
proximity of the feedstock inlet, more preferably reaching a
superheated state at or in proximity of the feedstock outlet.
Thereby, the superheated zone is preferably located in proximity of
the feedstock outlet, meaning that at least 50% of the superheated
zone, i.e. the points of the superheated zone, has a distance from
the feedstock outlet which is less than the distance from the
feedstock inlet. Preferably at least 80%, more preferably at least
90%, and most preferably at least 99% of the superheated zone has a
distance from the feedstock outlet which is less than the distance
from the feedstock inlet.
[0062] Therefore, according to a preferred embodiment, the
ligno-cellulosic feedstock is inserted in the pressurized reactor
vessel through a feedstock inlet and it is subjected to a
hydrothermal treatment in the presence of steam, wherein the steam
temperature increases while the feedstock advances to the steam
outlet. Preferably, first the ligno-cellulosic feedstock is steam
treated by saturated steam in the majority of the reactor vessel,
or for the majority of the residence time, and subsequently it is
steam treated by superheated steam for the remaining part of the
residence time. The inventors have found that by progressively
treating the ligno-cellulosic feedstock with steam at increasing
temperature, and particularly creating a superheated zone in
proximity of the feedstock outlet, the following step of the
process, which is the steam explosion of the ligno-cellulosic
feedstock, greatly improves.
[0063] During steam explosion, the pressure applied to the
ligno-cellulosic feedstock in the pressurized reactor vessel is
suddenly reduced while the ligno-cellulosic feedstock is removed
from the reactor vessel or reactor vessel assembly to a downstream
low pressure zone. It is generally recognized in the art that the
physical effects produced by the steam explosion on the
ligno-cellulosic feedstock, thereby producing a pre-treated
ligno-cellulosic feedstock, may vary and may be controlled to a
certain extent by suitable choice of operating parameters, which
include the absolute pressure drop applied to the feedstock and the
steam explosion time, that is the time used to release the pressure
applied to the feedstock. Although the steam explosion time is
difficult to be quantitatively measured and it may depend on the
setup used, a person skilled in the art may easily define how to
operate a steam explosion in order to obtain a steam explosion of
the ligno-cellulosic feedstock. For example, one way to do so is to
compare the enzymatic accessibility of the pre-treated
ligno-cellulosic feedstock with the enzymatic accessibility of a
reference case, wherein following the hydrothermal treatment the
pressure applied to the feedstock is released to 1 bar in a very
long time of 1 minute or more. The increase of the enzymatic
accessibility by more than 10% with respect to the reference case
may be considered as indicative of a steam explosion of the
ligno-cellulosic feedstock. The enzymatic accessibility is the
percent ratio of the total monomeric sugars obtained in a reference
hydrolysis test to the total amount of sugars present in the
pre-treated feedstock. The total amount of sugars include water
insoluble sugars, mainly glucans and xylans, and water soluble
oligomers and monomers already present in the pre-treated
ligno-cellulosic feedstock. The enzymatic accessibility may be
conducted according to many protocols known in the art, which
typically require to hydrolyze the pre-treated ligno-cellulosic
feedstock in the presence of a great amount of enzyme or enzyme
cocktail. For instance, the accessibility may be conducted using a
reference amount of 10 ml of Cellic Ctec3 by Novozymes A/S,
Bagsvaerd, Denmark, per gram of cellulose in the pre-treated
feedstock, for an hydrolysis time of 48 hours, at a temperature of
45.degree. C. and at a pH of 5 under stirring agitation.
[0064] In a preferred embodiment, the steam explosion of the
ligno-cellulosic feedstock following the hydrothermal treatment
produces a steam explosion of the cell, thereby substantially
disrupting the cell walls. This result typically corresponds to
short steam explosion time and high pressure drop, and the
enzymatic accessibility is enhanced. In certain embodiments, the
pressure applied to the ligno-cellulosic feedstock may be released
in a time to produce a cell expansion, thereby causing what is
known in the art as a steam flash, provided that the enzymatic
accessibility of the pre-treated ligno-cellulosic feedstock is
increased with respect to the reference case. In further
embodiments, the steam explosion device may be operated to obtain a
refining of the feedstock, meaning that a pretreated feedstock with
a smaller mean particle size is obtained, again provided that the
enzymatic accessibility of the pre-treated feedstock is increased
with respect to the reference case.
[0065] The steam explosion is conducted by means of a steam
explosion device which preferably comprises a blow valve, which is
a valve interposed between the pressurized reactor vessel or
reactor assembly and a downstream low pressure expansion zone. The
blow valve can be automatically operated in closed and open
positions, wherein the open position may range from a full open
position to a certain level of partial open position. In certain
blow valves, the switch time, that is the time needed to pass from
the closed to the open position, may also be regulated. The steam
explosion device may be operated in continuous or semi-continuous
mode. In the case of continuous operation, the ligno-cellulosic
feedstock is continuously admitted to the steam explosion device to
flow to the low pressure zone. If a blow valve is used, the blow
valve is kept in the open position during stationary operation. In
semi-continuous mode, the blow valve is operated with a duty cycle
corresponding to the ratio between the open time and the total
cycle time.
[0066] The steam explosion device is upstream operatively connected
to the feedstock outlet of the pressurized reactor vessel. By the
expression "operatively connected", it is meant that the steam
explosion device may be directly or indirectly connected to the
feedstock outlet of the pressurized reactor vessel. In the case
that the steam explosion device is directly connected to the
feedstock outlet of the reactor vessel, the feedstock is steam
exploded while it is removed from the feedstock outlet. In the case
that the steam explosion device is indirectly connected to the
feedstock outlet of the pressurized reactor vessel, a connection
system is interposed between the feedstock outlet of the
pressurized reactor vessel and the inlet of the steam explosion
device. The connection system may comprise one or more pipes,
vessels or apparatuses, provided that the pressure at in the
connection system does not differ significantly from the reactor
pressure in the pressurized reactor vessel. Thereby, the pressure
in the connection system may be at least 80%, preferably at least
90%, and more preferably at least 95% of the reactor pressure in
the pressurized reactor vessel. As an example, in the reactor
assembly of FIG. 1 it is represented a steam explosion device which
is directly connected to the feedstock outlet.
[0067] In FIG. 2 it is represented a steam explosion device which
is operatively connected to the feedstock outlet by means of a
vertical connection pipe or vessel, so that the ligno-cellulosic
feedstock is conveyed to the feedstock outlet of the pressurized
reactor vessel and then fall by gravity at the bottom of the
connection pipe or vessel to be steam exploded through the steam
explosion device BV. One or more optional steam inlets S3 may be
located on the surface of the connection pipe or vessel. As these
optional steam inlets are characterized by having a shorter
distance from the feedstock outlet than from the feedstock inlet of
the pressurized reactor, in an alternative embodiment at least a
portion of the second steam is inserted through the optional steam
inlets S3. The second steam may therefore not directly enter the
pressurized reactor vessel, but may enter through the connection
pipe or vessel.
[0068] In FIG. 2, the steam explosion device may further comprise a
feedstock distribution device for partitioning the ligno-cellulosic
feedstock in feedstock portions and sequentially exposing the
ligno-cellulosic feedstock portion to a blow valve, thereby
sequentially releasing the pressure applied to each portion of the
feedstock. Thereby, the feedstock distribution device is located
upstream of the blow valve in the high pressure zone. A preferred
feedstock distribution device is disclosed as a high pressure
compressor in US2008277082A1, which is herein incorporated by
reference. It comprises a rotating disk provided of radially
disposed walls defining circular sector vanes. The feedstock
distribution device received the feedstock from the feedstock
outlet of the pressurized reactor vessel while around a vertical
axis, thereby partitioning the feedstock in the vanes.
[0069] The steam explosion device is downstream connected to a low
pressure expansion zone which is at a pressure lower than the
reactor pressure, preferably by means of one or more blow lines.
Preferably the low pressure expansion zone comprises a separation
cyclone, wherein the pre-treated ligno-cellulosic feedstock is
collected and steam is recovered. The low pressure expansion zone
is preferably at a pressure in a range from 0.2 bar to 4 bar, more
preferably from 0.9 bar to 2 bar. Thereby, in certain embodiments,
the low pressure expansion zone may be at a sub atmospheric
pressure, and the pressure expansion zone may be provided of
extraction systems to dynamically maintain a pressure of less than
1 bar. Preferably, the low pressure is atmospheric pressure, that
is 1 bar, or slightly super atmospheric, and the pressure reduction
to the expansion pressure preferably occurs in one step.
* * * * *