Note: Descriptions are shown in the official language in which they were submitted.
1
Removal of Dissolved Carbohydrates from a Sulfuric Acid Solution
Field of the Invention
This invention pertains to processes for removing dissolved carbohydrates from
sulfuric acid
solutions.
Background of the Invention
Sulfuric acid is used in many industrial applications, including the
production of cellulose
nano crystallites (CNC). In this process the sulfuric acid becomes diluted but
is not
consumed to any significant degree. It is desirable to re-concentrate the acid
to the original
inlet concentration such that the acid can be re-used in the CNC reactor. This
is
accomplished by feeding the spent acid to a sulfuric acid re-concentrator
(SARC) unit.
However, the spent sulfuric acid contains dissolved carbohydrates formed from
the
hydrolysis of the cellulose. These carbohydrates are mainly sugar monomers and
oligomers (primarily glucose (C6H1206), but also others from the residual
hemicellulose
fraction) and some degradation products such as furfural (C5H402). These
carbohydrates
must be substantially removed from the acid for it to be re-used in CNC
production and
many other applications.
Additionally, in the presence of a strong acid, such as sulfuric acid,
residual carbohydrates
may also undergo a number of complex dehydration and polymerization reactions
to form
larger species, which may precipitate and are often coloured.
The dehydration of the sugar molecules is known as carbonization and is
dependent on the
temperature for a given sulfuric acid concentration. In general, as the acid
concentration
increases, the temperature required for the reaction to occur decreases.
Carbonization
leads to darkening of the product acid, causing product colour and quality
issues for the
CNC. If this happens within the sulfuric acid re-concentrator unit it can also
cause
undesirable fouling of heat exchanger surface area and foaming.
There are several conventional and commercially available techniques for the
separation of
neutral sugars from acids. These include nano-filtration, ion exchange and
CA 3002039 2018-04-17
2
chromatography. However, these involve disadvantages such as high capital cost
and
production of a concentrated sugar stream, still containing significant levels
of acid, which
must be dealt with at considerable additional cost. These conventional
techniques also
often involve dilution of the acid and/or sugar stream, requiring later
removal of the water
through additional evaporation.
Summary of the Invention
The invention provides an improved method of removing dissolved carbohydrates
from a
sulfuric acid solution, in particular a sulfuric acid solution with dissolved
sugars from a
process for making CNC, so that the acid solution can be recycled for use in
the CNC
process or other applications.
According to one aspect of the invention, there is provided a method of
removing dissolved
carbohydrates from a sulfuric acid solution, comprising the steps of holding
the sulfuric acid
solution at an elevated temperature to carbonize the carbohydrates and form a
carbonized
solid, and removing the carbonized solid from the sulfuric acid solution.
According to a further aspect of the invention, there is provided a method of
removing
dissolved carbohydrates from a sulfuric acid solution, comprising the steps of
increasing the
concentration of the sulfuric acid solution, holding the sulfuric acid
solution at an elevated
temperature to carbonize the carbohydrates and form a carbonized solid,
removing the
carbonized solid from the sulfuric acid solution to produce a purified
sulfuric acid solution,
and exposing the purified sulfuric acid solution to a colour removal treatment
to remove
colour from the sulfuric acid solution.
Further aspects of the invention and features of specific embodiments are
described below.
Brief Description of the Drawings
Figure 1 is a schematic block diagram of the purification process according to
one
embodiment of the invention.
Figure 2 is a graph showing the absorbance spectra of sulfuric acid solutions
treated with
various colour removal materials.
CA 3002039 2018-04-17
3
Figure 3 is a graph showing the absorbance spectra of a sulfuric acid solution
treated with
colour removal material in accordance with Example 3.
Detailed Description
In one embodiment, the process of the invention starts with a sulfuric acid
solution with
dissolved sugars, resulting from a process for the production of CNC. The acid
solution has
become diluted during the CNC process and accordingly has a concentration
lower than
that of fresh acid solution for use in the CNC process. For example, the acid
solution may
have become diluted from about 60-65 wt.% down to about 15-25 wt.%.
Acid Concentration
As an optional step in the process, the sulfuric acid solution with dissolved
sugars can be
concentrated prior to further treatment steps, to increase the concentration
of the solution to
a concentration suitable for use in the process for producing CNC. This is
done by
evaporation, using a single effect or multi-effect evaporator.
To avoid undesirable carbonization in the concentrator itself we have found
that it is
preferable to minimize the coincident temperature/concentration as well as the
exposure
time. This can be done in a single effect operating at deep vacuum. However,
at higher
capacities it is often desirable to improve steam economy, so a multiple-
effect evaporator
can be configured which avoids the problematic higher concentration and
temperature
combinations by using either a fully, or partially, feed and steam forward
arrangement. The
sulfuric acid solution and steam are fed into the first vaporization unit, and
the sulfuric acid
solution is processed through the series of vaporization units to concentrate
the sulfuric acid
solution by evaporation of water. Undesirable carbonization is avoided by
reducing the
pressure and thus evaporation temperature of the more concentrated acid.
Carbonization
Although carbonization is undesirable during evaporation we have found that it
can be used
positively at the desired point in the carbohydrate-removal process. The
sulfuric acid
solution with dissolved sugars (whether re-concentrated or not), is thermally
treated, i.e.,
heated and held at the elevated temperature for a residence time sufficient to
carbonize the
CA 3002039 2018-04-17
4 =
bulk of the sugars to create a carbonized solid material. Direct steam
injection, concentrated
product recycle and/or careful design of the heating system is required to
avoid fouling of
exchanger surfaces due to carbonization.
Greater removal of sugars during the carbonization step occurs at higher acid
concentrations and at higher temperatures. The acid concentration may be
greater than
50 wt.%, and preferably in the range of 55-65 wt.%, for best integration back
into the CNC
process. The temperature to which the acid solution is heated may be at least
80 C,
alternatively at least 100 C, or at least 120 C, or at least 150 C. The
residence time during
which the acid solution is held at an elevated temperature is at least 1 hour,
alternatively at
least 4 hours, alternatively at least 5 hours, or at least 6 hours.
The carbonization step can be done before, during or after the optional
concentration step
depending on the starting concentration. The carbonized solid material is then
removed by
conventional solid/liquid separation techniques such as filtration,
centrifugation or settling,
leaving a purified acid solution.
The carbonized solids may be washed and safely disposed of or burned to
recover energy
and thus offset the operating costs of the process.
Colour Removal
Residual coloured compounds may have an effect on the whiteness of the CNC
product
when the purified acid solution is recycled for use in the CNC process, and
accordingly their
removal from the acid solution is often desirable. As an optional step
following thermal
treatment and filtration, the purified acid solution is treated with a colour
removal method,
such as granular activated carbon, resin and/or nano-filtration, etc., to
remove colour
compounds formed in the carbonization process.
In some embodiments of the invention, the process also includes the
application of one or
more conventional techniques for the separation of neutral sugars from acids.
These
include nano-filtration, ion exchange and chromatography. Nano-filtration
would typically be
employed at the incoming, dilute, acid concentration while ion exchange and
chromatography would typically be employed after some acid concentration;
however, all
CA 3002039 2018-04-17
= 5
= three would normally require further acid concentration afterwards due to
low incoming
concentration and/or additional dilution that occurs during purification. The
carbonization
technique could also be used as a pre-treatment method upstream of any of
these
techniques to provide bulk sugar removal.
The Embodiment of Figure 1
An embodiment of the process for the removal of dissolved sugars is
schematically
illustrated in Figure 1, in which the process includes the steps of acid
concentration,
carbonization and colour removal. In the process 10, a sulfuric acid re-
concentrator 12 has
a sulfuric acid feed 14, received from a CNC process. The output from the
sulfuric acid re-
concentrator 12 is a concentrated acid stream 16 and a condensate (water)
stream 18. The
concentrated acid stream 16 is heated and fed to carbonization unit 20, in
which the acid is
held for a suitable residence time at elevated temperature. The carbonized
acid stream 22
is then optionally cooled 24 (as indicated by broken lines in Figure 1), and
subsequently
clarified by removal of the solids filtered in the solid/liquid separation
unit 26. The isolated
carbonized solids 28 from the solid/liquid separation unit 26 are sent for
disposal or
incineration 30. The suspended solids-free acid product stream 32 from the
solid/liquid
separation unit 26 is, optionally, pumped to a colour removal unit 34. The
colour removal
unit 34 comprises, for example, activated carbon beds, fed by an activated
carbon stream
36. The colour removal unit 34 produces a purified acid stream 38 which is
recycled to the
CNC reactor or other application. Spent activated carbon 40 from the colour
removal unit
34 is sent for disposal or incineration 42, or alternatively, is regenerated.
In an exemplary embodiment of the process 10, the acid feed 14 is centrifuge
centrate
comprising 25.6 wt.% sulfuric acid and 2 wt.% sugars. The concentrated acid
stream 16
from the sulfuric acid reconcentrator 12 comprises 54.9 wt.% sulfuric acid and
4.3 wt.%
sugars. The carbonization unit 20 holds the heated acid at about 150 C for 5
hours. The
filtrate stream 32 from the filtration unit 26 comprises 56.2 wt.% sulfuric
acid and 2.0 wt.%
sugars. The carbonized solids 28 comprise 20 wt.% acid and have a dry LHV of
8360
BTU/lb. The colour removal unit 34 operates at 45 C. The purified acid stream
38
comprises 56.6 wt.% sulfuric acid and 1.3 wt.% sugars. The spent activated
carbon stream
comprises 20 wt.% acid.
CA 3002039 2018-04-17
6
Examples
Example 1
Samples of sulfuric acid solution with dissolved sugars from a CNC process
were subjected
to thermal treatment to carbonize the sugars. The tests were conducted at a
range of acid
concentrations from 10.6 to 60.4 wt.%, temperatures from 80 to 160 C, and
durations from
0.5 to 18 hours.
Tests 1-7
A glass pressure tube was charged with the feed solution and immersed in a
temperature-
controlled oil bath. The solutions were heated to the target temperatures and
incubated until
blackening occurred, or for a desired length of time. For the initial tests
the temperature was
maintained for 1 hour incubations at increasingly higher target temperatures
until darkening
of the feed was observed. Later tests were held for longer incubation periods.
The material
was then cooled to ambient temperature and filtered through a 1.5 urn pore-
size glass
microfiber filter to remove dark solids. The solids were rinsed with deionized
(Dl) water and
dried in a 105 C oven.
Test 8
The feed solution was heated in three segments of 6 hours. After each heating
period the
sample was cooled, filtered, and a sample of the filtrate was extracted to be
tested for
carbon oxygen demand (COD). A clean pressure tube was then charged with the
remaining
filtrate and heating was continued.
Test 9
The concentrated acid was heated in a 1L round bottom flask using a
temperature
controlled oil bath. The flask was equipped with a condenser and the system
was open to
atmosphere. The acid was heated to 150 C for 6 hours one day and 7 hours the
following
day. Black solids were removed by filtration using a 1.5 um pore-size glass-
fibre filter.
The conditions and results of the nine tests are summarized in Table 1.
CA 3002039 2018-04-17
7
= Table 1
Carbonization Conditions
H2SO4 COD removed
Test # (wt %) Temperature Duration (0/0)
.
( C) (hrs)
100 1
1 10.6 120 1 4.9%
150 1
80 1
2 28.4 11.9%
120 1
80 0.5
3 20.8 120 1 14.6%
150 1
80 1
4 52.0 26.6%
100 2
55.3 120 4 51.1%
6 55.3 120 5 60.6%
7 60.4 150 12 72.7%
8.1 160 6 35.2%
8.2 20.8 160 6 -16.4%
8.3 160 6 2.2%
8 total 20.8 160 18 26.3%
54.6%
9 57.23 150 13 (69.4% after
activated carbon
treatment)
The tests show that greater removal of sugars (as COD) occurred at higher acid
concentrations and temperatures. COD removal was greatly increased at acid
5 concentrations above 50 wt.%, as seen in Tests 4 & 5.
COD was still being removed after 4 hours of heating, as shown by Tests 5 and
6. The bulk
COD was removed in the first six hours of heating, as shown in Test 8, whereas
further
heating (Tests 8.2 and 8.3) showed no appreciable additional removal.
CA 3002039 2018-04-17
8
Example 2
Tests were conducted using filtrate from Test 9 of Example 1 to determine the
effectiveness
of colour removal of several commercially available absorption media,
including granular
activated carbon (GAC), ion exchange resin and adsorbent resins. The results
are set out
in Table 2.
Table 2
Dosage Dosage Colour of
Sample Material (g/mL) (wt%) Treated acid
1 HYDRODARCO 3000 40.2 2.73 Yellow/brown
2 SEPABEADS SP850 40.8 2.79 Dark brown
3 SEPABEADS SP825L 40.1 2.75 Light brown
4 DIAION HP20 39.7 2.70 Light brown
5 DIAION HP2MGL 39.9 2.75 Dark brown
6 DIAION HP21 40.3 2.75 Brown
HYDRODARCO is a trademark of Cabot Corporation. SEPABEADS and DIAION are
trademarks of Sigma-Aldrich Co.
The absorbance spectra of samples 1 to 6 are shown in Figure 2. It was
concluded that the
Cabot HYDRODARCO 3000 granulated activated carbon was most effective at
removing
the colour from the concentrated acid.
Activated carbon loading of about 40 g/L is higher than what would be expected
in an
commercial process. This is due to the fact that the lab testing was performed
as a single
stirred batch, whereas a continuous commercial process would be carried out in
a multi-
stage packed column. The staging benefit of using a column would provide at
least an order
of magnitude reduction in required loading.
Example 3
A 498.5g batch of hydrolysate containing 57 wt.% H2504 was prepared, heated to
CA 3002039 2018-04-17
9
160 C and held at temperature for a total of 13 hours. COD was reduced by
54.6% after
filtration. This material was subjected to two stages of colour removal using
Hydrodarco
3000 as indicated in Table 3 below, which reduced the colour of the final
material to a pale
yellow with an absorbance spectrum as shown in Figure 3.
Table 3
Filtrate Mass 1103000 Dosage Dosage
(g) Mass (g/L) (wt%)
added
(g)
Step 1 498.52 17.25 50.62 3.46
Step 2 From 1 above 15.41 45.22 3.09
Final / Total 428.43 32.66 95.84 6.55
The colour removal step further reduced the COD (from 54.6% to 69.4% COD
removed).
Example 4
Samples of the dried carbonized solids from Example 1 were subjected to
calorific testing to
determine their heating value. The dry Higher and Lower heating values were
determined to
be 8680 and 8360 BTU/lb respectively.
As will be apparent to those skilled in the art in the light of the foregoing
disclosure, many
alterations and modifications are possible in the practice of this invention
without departing
from the scope thereof.
CA 3002039 2018-04-17
