U.S. patent application number 13/551136 was filed with the patent office on 2013-04-25 for efficient process for producing alcohol.
This patent application is currently assigned to ZEACHEM, INC.. The applicant listed for this patent is Timothy J. Eggeman, Dan Verser. Invention is credited to Timothy J. Eggeman, Dan Verser.
Application Number | 20130102043 13/551136 |
Document ID | / |
Family ID | 22413872 |
Filed Date | 2013-04-25 |
United States Patent
Application |
20130102043 |
Kind Code |
A1 |
Verser; Dan ; et
al. |
April 25, 2013 |
Efficient process for producing alcohol
Abstract
A process for producing ethanol including a combination of
biochemical and synthetic conversions results in high yield ethanol
production with concurrent production of high value coproducts. An
acetic acid intermediate is produced from carbohydrates, such as
corn, using enzymatic milling and fermentation steps, followed by
conversion of the acetic acid into ethanol using esterification and
hydrogenation reactions. Coproducts can include corn oil, and high
protein animal feed containing the biomass produced in the
fermentation.
Inventors: |
Verser; Dan; (Menlo Park,
CA) ; Eggeman; Timothy J.; (Englewood, CO) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Verser; Dan
Eggeman; Timothy J. |
Menlo Park
Englewood |
CA
CO |
US
US |
|
|
Assignee: |
ZEACHEM, INC.
Menlo Park
CA
|
Family ID: |
22413872 |
Appl. No.: |
13/551136 |
Filed: |
July 17, 2012 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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13101900 |
May 5, 2011 |
8236534 |
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13551136 |
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12693533 |
Jan 26, 2010 |
7964379 |
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13101900 |
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12051668 |
Mar 19, 2008 |
7682812 |
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12693533 |
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11172376 |
Jun 29, 2005 |
7351559 |
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12051668 |
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10310552 |
Dec 4, 2002 |
6927048 |
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11172376 |
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09720930 |
Dec 29, 2000 |
6509180 |
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PCT/US00/06498 |
Mar 10, 2000 |
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10310552 |
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60124276 |
Mar 11, 1999 |
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Current U.S.
Class: |
435/161 ;
435/155; 435/157 |
Current CPC
Class: |
C07C 67/08 20130101;
C12P 7/56 20130101; C12P 7/54 20130101; Y02P 20/10 20151101; Y02E
50/10 20130101; Y02E 50/17 20130101; C12P 7/14 20130101; Y02E 50/16
20130101; C12P 7/06 20130101; C07C 29/149 20130101; Y02P 20/127
20151101; C07C 29/149 20130101; C07C 31/08 20130101; C07C 67/08
20130101; C07C 69/14 20130101 |
Class at
Publication: |
435/161 ;
435/155; 435/157 |
International
Class: |
C12P 7/06 20060101
C12P007/06 |
Claims
1-75. (canceled)
76. A method comprising: a) culturing a microorganism to produce a
dilute solution of a carboxylic acid salt, the cation of which is
capable of producing an insoluble salt when combined with a weak
acid; and, b) chemically converting the carboxylic acid to an
alcohol.
77. The method of claim 76, wherein the microorganism is cultured
in medium comprising a carbohydrate source.
78. The method of claim 77, wherein the carbohydrate source is
obtained from biomass.
79. The method of claim 78, wherein the biomass is selected from
the group consisting of grass, trees, wood, paper, corn, rice,
manure, crop residue, agricultural waste, and municipal waste.
80. The method of claim 78, wherein prior to fermentation, the
biomass is treated using at least one treatment selected from the
group consisting of heating and enzymatic hydrolysis.
81. The method of claim 77, wherein at least 60% of the carbon in
the carbohydrate source is converted into the alcohol.
82. The method of claim 77, wherein essentially none of the carbon
in the carbohydrate is evolved as carbon dioxide.
83. The method of claim 76, wherein the carboxylic acid is selected
from the group consisting of lactic acid and acetic acid.
84. The method of claim 76, wherein the cation is calcium.
85. The method of claim 76 wherein the weak acid is CO.sub.2.
86. The method of claim 76, wherein the insoluble salt is a
carbonate salt.
87. The method of claim 76, wherein the insoluble salt is calcium
carbonate.
88. The method of claim 76, wherein the step of culturing
comprises: (i) producing lactic acid, lactate, or mixtures thereof,
by fermentation; and (ii) converting the lactic acid, lactate, or
mixtures thereof, into acetic acid, acetate or mixtures
thereof.
89. The method of claim 88, wherein the lactic acid is converted
into acetic acid by fermentation.
90. The method of claim 76, wherein the step of culturing comprises
using at least one homofermentative organism.
91. The method of claim 76, wherein the step of culturing comprises
using at least one homoacetogenic organism.
92. The method of claim 76, wherein the step of culturing comprises
using at least one organism from a genus selected from the group
consisting of Lactobacillus, Clostridium, Acetobacterium, and
Peptostreptococcus.
93. The method of claim 76, wherein the step of culturing comprises
using at least one organism selected from the group consisting of
Lactobacillus casei, Clostridium formicoaceticum, and Clostridium
thermoaceticum.
94. The method of claim 76, wherein the alcohol is selected from
the group consisting of methanol, ethanol and mixtures thereof.
95. The method of claim 76, wherein the step of chemically
converting is not fermentation.
96. The method of claim 76, wherein the step of chemically
converting comprises: (i) acidifying the dilute carboxylic acid
salt solution with a weak acid to produce a carboxylic acid and an
insoluble salt of the cation and the weak acid; (ii) reacting the
carboxylic acid with an amine to produce an acid/amine complex;
(iii) recovering and decomposing the acid/amine complex to
regenerate the amine and form the carboxylic acid; and (iv)
converting the carboxylic acid to an alcohol.
97. The method of claim 96, wherein the step of converting the
carboxylic acid to an alcohol comprises forming an ester of the
carboxylic acid and a second alcohol and hydrogenating the ester to
produce the alcohol and recover the second alcohol.
98. The method of claim 96, wherein the hydrogen for the step of
hydrogenating is produced by a method selected from the group
consisting of steam reforming, water electrolysis, gasification
from biomass, partial oxidation of hydrocarbons, and partial
oxidation of coal.
99. The method of claim 96, wherein the step of converting the
carboxylic acid to an alcohol comprises directly hydrogenating the
carboxylic acid.
Description
FIELD OF THE INVENTION
[0001] This invention relates to a process for the conversion of
carbohydrates from any of a number of sources into ethanol for fuel
or chemical use. The invention uses a combination of fermentation
and chemical conversion to greatly increase the yield of ethanol
from carbohydrates compared to the current art. In addition a high
value coproduct may be produced for use in animal feed.
BACKGROUND OF THE INVENTION
[0002] Ethanol is a major chemical used in human beverages and
food, as an industrial chemical, and as a fuel or a component in
fuels, such as reformulated gasoline to reduce emissions from
automobiles. This invention relates mainly to the production of
ethanol for use as a chemical or fuel.
[0003] There are several traditional ethanol processes based on
fermentation of carbohydrates. In the most typical process, a
carbohydrate derived from grain is hydrolyzed to its component
sugars and fermented by yeast to produce ethanol. Carbon dioxide is
generated in the process from a fraction of the carbohydrate by the
metabolism of the yeast. The generation of carbon dioxide is
inherent in the metabolism of the yeast. This production of
CO.sub.2 by yeast limits the yield of ethanol from yeast to about
52% maximum on a weight basis. This is a major limitation on the
economic production of ethanol as the CO.sub.2 is of low value and
is typically wasted into the atmosphere and may become a burden on
the environment.
[0004] In addition, yeast have a limited ability to utilize sugars
other than glucose. While glucose is the major sugar produced from
the hydrolysis of the starch from grains, it is not the only sugar
produced in carbohydrates generally. A large research effort has
gone into the potential conversion of biomass into ethanol. Biomass
in the form of wastes from agriculture such as corn stover, rice
straw, manure, etc., and biomass crops such as switch grass or
poplar trees, and even municipal wastes such as newspaper can all
be converted into ethanol. However a major limitation of these
processes is the complexity of the hydrolyzate that results from
treatment of the biomass to produce the fermentation medium. The
hydrolyzate typically contains glucose, but also large amounts of
other sugars such as xylose, which yeast cannot metabolize. This is
another potential yield limitation on yeast based ethanol
processes.
[0005] Research has been directed to the use of organisms other
than yeast which in contrast to yeast, do consume many if not most
of the sugars derived from the hydrolysis of biomass. Examples
include Zymomonas sp. bacteria and E. coli bacteria which have been
genetically engineered to utilize xylose. Thereby the potential
range of substrate sugars which can be converted to ethanol has
been increased. There is a class of organism that has been proposed
for the production of ethanol, typically of the Clostridium sp.
These thermophiles usually produce both acetic acid and ethanol.
However, it is believed that these organisms produce a limited
yield of ethanol. It is generally assumed in the literature on
ethanol fermentation that this yield limitation is fixed by the
biochemical pathway called the Embden-Myerhof pathway by which
ethanol is produced in all of the organisms so far proposed for
production of ethanol, including the thermophiles.
[0006] Thus none of this development has addressed the inherent
problem of the yield of ethanol from sugar based on the
coproduction by the organisms of CO.sub.2.
[0007] An important part of the commercial processes for producing
ethanol is the production of valuable coproducts mainly for use in
animal feed or food. In the corn dry milling process the coproducts
include distillers dried grains and solubles (DDG, DDGS). In the
corn wet milling process the coproducts include germ, gluten meal
and fiber. These coproducts find large markets in the animal feed
business. However in both processes to a very large extent, the
ingredients in the original grain, that is the oil, protein and
fiber fractions, are passed through the processes unchanged in
composition, while the carbohydrate fraction is converted largely
to ethanol. Therefore the value of these coproducts is based on the
inherent composition of the plant components.
[0008] There are other chemicals that can be produced by industrial
fermentation from carbohydrates besides ethanol. Major examples are
acetic acid and lactic acid. Acetic acid is a major food ingredient
in the form of vinegar and a major industrial chemical. Vinegar for
food use is typically produced from potable ethanol by the action
of Acetobacter sp. which oxidize ethanol to acetic acid using
oxygen from the air.
[0009] Major industrial uses for acetic acid are as a solvent, as
an intermediate in the synthesis of other chemicals such as vinyl
acetate and in the production of cellulose acetate. Major new uses
for acetic acid have been proposed such as the production of
calcium magnesium acetate (CMA) for use as a road deicer in place
of sodium chloride (NaCl). CMA has a much reduced environmental
impact compared to NaCl since it is much less corrosive and is
biodegradable.
[0010] Researchers have proposed the production of industrial grade
acetic acid by fermentation from carbohydrates. However no
production by fermentation currently exists due to economic factors
related mainly to recovering acetic acid from dilute fermentation
broths. Acetic acid is typically produced at low concentrations of
around 5% or less in water as a fermentation broth. Since acetic
acid has a higher boiling point than water, all of the water, about
95% of the broth, must be distilled away from the acetic acid to
recover the acid or other more complex processes must be used to
recover the acetic acid.
[0011] Related to this field of acetic acid production is the use
of so called acetogens, a class of bacteria which utilize a unique
biochemical pathway to produce acetic acid from sugars with 100%
carbon yield. For example, one mole of glucose can be converted to
three moles of acetic acid by Clostridium thermoaceticum. These
bacteria internally convert CO.sub.2 into acetate. These bacteria
are called homofermentative microorganisms or homoacetogens. They
do not convert any of the carbohydrate to CO.sub.2 and only produce
acetic acid. Examples of homoactogens are disclosed in Drake, H. L.
(editor), Acetogenesis, Chapman & Hall, 1994, which is
incorporated herein by reference in its entirety. In addition these
homofermentative organisms typically convert a wide range of sugars
into acetic acid, including glucose, xylose, fructose, lactose, and
others. Thus they are particularly suited to the fermentation of
complex hydrolyzates from biomass. However this line of research
has not overcome the economic limitations of the acetic acid
fermentation process to make it competitive with the natural gas
based route.
[0012] Therefore, industrial acetic acid is today made from coal,
petroleum or natural gas. The major process is the conversion of
natural gas to methanol and the subsequent carbonylation of the
methanol using carbon monoxide directly to acetic acid. U.S. Pat.
No. 3,769,329 describes this process.
[0013] Related to the natural gas route, it has been proposed to
produce ethanol from acetic acid by way of synthesis of esters of
acetic acid produced in this process, or a related modification,
and subsequent hydrogenation of the esters. U.S. Pat. Nos.
4,454,358 and 4,497,967 disclose processes to produce acetic acid
from synthesis gas, which is then esterified and hydrogenated to
produce ethanol, and are incorporated herein by reference in their
entirety. The hydrogenation of esters to produce alcohols is well
known. None of these processes are based on the conversion of
carbohydrates to ethanol.
[0014] There is another class of well known fermentations that have
the property of converting carbohydrates at 100% carbon yield,
using homofermentative lactic bacteria. These bacteria convert one
mole of glucose for example into two moles of lactic acid. The
relevance of this is that lactic acid may also be used as the
substrate for fermentation to acetic acid by homofermentative
acetogens again with 100% carbon yield. Two moles of lactic acid
are converted into three moles of acetic acid by Clostridium
formicoaceticum for example. Prior to the present invention, no one
has been known to have devise a process to produce ethanol in high
yield from carbohydrates, which is the main objective of this
invention.
SUMMARY OF THE INVENTION
[0015] In accordance with one embodiment the present invention,
carbohydrates are converted to ethanol with very high carbon yield
by a combination of fermentation and chemical conversion, thus
overcoming the major limitation of known processes for the
conversion of carbohydrates to ethanol. The present invention
combines several chemical and biochemical steps into a new process
with many advantages. The basic process of this invention comprises
three steps:
[0016] 1. Converting a wide range of carbohydrates, with very high
carbon yield (>90% potentially) using a homoacetic fermentation
(or a combination of homolactic and subsequent homoacetic
fermentations) into acetic acid,
[0017] 2. Recovering, acidifying (if necessary), and converting the
acetic acid to an ester (preferably, the ethyl ester using recycled
ethanol product), and
[0018] 3. Hydrogenating the ester, producing ethanol, and
regenerating the alcohol moiety of the ester.
[0019] The net effect of this process is to convert carbohydrates
in very high carbon yield to ethanol. No CO.sub.2 is produced from
carbohydrates as a byproduct of this process.
[0020] Another benefit of the current invention is the production
of a higher value byproduct due to the conversion of the plant
proteins into bacterial single cell protein. The conversion of the
plant protein into single cell bacterial protein increases the
concentration of the protein, restructures the protein to have a
more valuable composition for animal feed in terms of essential
amino acids, for example, and potentially provides other benefits,
for example, in milk production.
[0021] The conversion of the fiber fraction, and the cellulose and
xylan fractions of the grain contributes to the overall yield of
ethanol.
[0022] While the production of single cell protein and the
utilization of fiber are important additional benefits of the
invention, the yield factor alone is a major improvement and can be
practiced on its own in conjunction with the corn wet milling
process, without the production of single cell protein or the
utilization of cellulose fiber.
[0023] Advantages of the invention over the current state of the
art can include one or more of the following:
[0024] 1. Very high yield of product from raw material with obvious
economic benefits compared to known ethanol processes,
[0025] 2. No production of CO.sub.2 from carbohydrate by the
process with benefits to the environment, i.e. the much more
efficient conversion of renewable resources to ethanol,
[0026] 3. Inherently wide substrate range for ethanol production,
i.e. a wide range of potential biomass sources and their component
sugars, and
[0027] 4. High value byproducts, e.g., single cell protein;
restructuring of plant protein, produced with high efficiency.
[0028] In one embodiment of the present invention a method to
produce ethanol with a very high yield is provided. The method
includes the steps of fermenting a medium which contains a
carbohydrate source into acetate, acetic acid or mixtures thereof.
The acetate, acetic acid, or mixtures thereof are chemically
converted to ethanol. Preferably, at least about 60%, more
preferably at least about 80% and more preferably at least about
90% of the carbon in the carbohydrate source is converted to
ethanol. Essentially none of the carbon in the carbohydrate source
is converted into carbon dioxide. However, if hydrogen is produced
later in the process by steam reforming, carbon dioxide will be
produced at that stage. Preferably, the fermentation medium
comprises less than about 20% nitrogen and yields a biomass
byproduct which is useful as an animal feed, with preferably at
least about 10% by weight biomass product. The carbohydrate source
can include any appropriate source such as corn, wheat, biomass,
wood, waste paper, manure, cheese whey, molasses, sugar beets or
sugar cane. If an agricultural product such as corn is employed,
the corn can be ground to produce corn and corn oil for recovery.
The carbohydrate source, e.g., corn, can be enzymatically
hydrolyzed prior to fermentation. Preferably, the fermentation is
conducted using a homofermentative microorganism. The fermentation
can be a homoacetic fermentation using an acetogen such as a
microorganism of the genus Clostridium, e.g., microorganisms of the
species Clostridium thermoaceticum or Clostridium
formicoaceticum.
[0029] In an embodiment of the present invention, the fermentation
includes converting the carbohydrate source into lactic acid,
lactate or mixtures thereof by fermentation and subsequently
converting the lactic acid, lactate or mixtures thereof into acetic
acid, acetate or mixtures thereof by fermentation. The lactic acid
fermentation can be a homolactic fermentation accomplished using a
microorganism of the genus Lactobacillus. Alternatively, the
carbohydrate source can be converted into lactic acid, lactate,
acetic acid, acetate or mixtures thereof in an initial fermentation
using a bifido bacterium. Typically, one mole of glucose from the
carbohydrate source is initially converted to about two moles
lactate and the lactate is converted to about three moles
acetate.
[0030] Acetic acid which is formed in connection with the
fermentation can be in the form of acetate depending on the pH of
the fermentation medium. The acetate can be acidified to form
acetic acid. For example, the acetate can be reacted with carbonic
acid in and an amine to form calcium carbonate and an amine complex
of the acetate. The amine complex can be recovered and thermally
decomposed to regenerate the amine and form acetic acid. The
calcium carbonate can be recovered for reuse. The acetic acid can
be esterified and hydrogenated to form an alcohol. Alternatively,
the acetic acid may be directly hydrogenated to form ethanol. The
esterification is preferably accomplished by reactive
distillation.
[0031] In another embodiment of the present invention, the acetate
can be acidified with carbon dioxide to produce acetic acid and
calcium carbonate and esterified to acetate ester for recovery.
Preferably, the process takes place at low or nearly atmospheric
pressure. Preferably, the calcium carbonate is recycled to a
fermentation broth in order to maintain a desired pH. Preferably,
the ester is a volatile ester. As used herein, the term "volatile
ester" means that the ester is capable of recovery by distillation,
and therefore the ester should be more volatile than the water from
which it is recovered. The alcohol employed in the esterification
is preferably methanol, ethanol or mixtures thereof. The ester is
preferably recovered by distillation, such as by reactive
distillation, and subsequently converted to ethanol.
[0032] The reactive distillation can be accomplished by acidifying,
esterifying and recovering the ester in a reaction column. A dilute
solution of acetate salt in water mixed with ethanol is introduced
near the top of a reaction section of the column. Carbon dioxide
gas is introduced near the bottom of the reaction section of the
column. The carbon dioxide reacts with acetate salt and ethanol in
the reaction zone to form calcium carbonate and ethyl acetate.
Ethyl acetate can be concentrated, e.g., by vaporizing a mixture
containing excess ethanol in water and an azeotrope comprising
ethyl acetate, water and ethanol. The azeotrope can be separated
from the excess ethanol and water, e.g., by the addition of water,
thereby causing a phase separation between an ethyl acetate-rich
portion and a water and ethanol-rich portion. The ethanol and water
can be returned to the reaction zone and the calcium carbonate can
be recycled to a fermentation broth to control pH.
[0033] In one embodiment of the present invention, ethanol is
produced from a carbohydrate source, with essentially none of the
carbon and the carbohydrate source converting to carbon
dioxide.
[0034] In another embodiment of the present invention, ethanol is
produced from a carbohydrate source wherein at least 60%,
preferably 70%, more preferably 80% and more preferably 90% and
more preferably 95% of the carbon in the carbohydrate source is
converted to ethanol.
[0035] In accordance with another embodiment of the present
invention, an ester is recovered from a dilute solution of a
carboxylic acid salt. The carboxylic acid salt is acidified with
carbon dioxide to produce the corresponding carboxylic acid and
calcium carbonate, and simultaneously esterified with an alcohol to
form an ester. The ester is recovered. Preferably, the ester is a
volatile ester and the alcohol is methanol, ethanol or mixtures
thereof. The ester can be recovered by distillation, such as by
reactive distillation. The ester can be converted to ethanol. The
acidification, esterification and recovery can take place in a
reaction column. Initially, a dilute solution of the carboxylic
acid salt in water mixed with alcohol is introduced near the top of
a reaction section of the column. Carbon dioxide gas is introduced
near the bottom of the reaction section of the column. The carbon
dioxide and carboxylic acid salt and alcohol react to form calcium
carbonate and a volatile ester of the carboxylic acid salt. The
ester can be concentrated by vaporizing a mixture containing excess
alcohol and water and an azeotrope made up of the ester, water and
alcohol. The azeotrope can be separated from the excess alcohol and
water, e.g., by the addition of water, thereby causing a phase
separation between an ester-rich portion and a water and
alcohol-rich portion. The excess alcohol and water can be returned
to the reaction zone.
[0036] In accordance with an embodiment of the present invention, a
carbohydrate source and natural gas are converted to an easily
transportable liquid product. The carbohydrate source is converted
to acetic acid, acetate or mixtures thereof by fermentation. The
acetic acid, acetate or mixtures thereof is converted to ethyl
acetate. At least part of the ethyl acetate is converted to ethanol
using hydrogen obtained from the natural gas source. The ethanol
and/or ethyl acetate which is produced is then transported to a
location remote from where it is produced. Preferably, the
carbohydrate source and natural gas source are located within a
distance that makes it economically feasible to produce the
transportable liquid product, and the remote location is a
sufficient distance away that it is not economically feasible to
transport the carbohydrate and natural gas to the remote location
for processing. Preferably, the economically feasible distance is
less than about five hundred miles and the uneconomical remote
distance is greater than about a thousand miles. For example, the
natural gas source and carbohydrate source can be located on a
Caribbean island such as Trinidad and the remote location can be on
the Gulf Coast, such as the Texas Gulf Coast. Alternatively, the
carbohydrate source and the natural gas source can be located in
Australia and/or New Zealand and the remote location can be Asia,
e.g., Japan, Taiwan, Korea or China.
[0037] In another embodiment of the present invention at least 80%
of the carbon in a carbohydrate source is converted into ethanol.
The method includes enzymatically hydrolyzing the carbohydrate
source to sugars and amino acids. A carbohydrate, sugars and amino
acids (from the original source or another source) are converted
into lactic acid, lactate or mixtures thereof by homolactic
fermentation. The lactic acid, lactate or mixtures thereof are
converted into acetic acid, acetate or mixtures thereof by
homoacetic fermentation. The pH of the fermentation broths are
maintained in a range from about pH 6 to about pH 8, using a base.
A biomass byproduct which is useful as an animal feed can be
recovered from the fermentation. The acetate is acidified with
carbon dioxide to produce acetic acid and calcium carbonate and the
acetic acid is simultaneously esterified with an alcohol to form a
volatile ester. The volatile ester can be recovered using reactive
distillation. Hydrogen can be produced by any number of methods,
e.g., steam reforming of natural gas. The acetate ester is
hydrogenated to form ethanol.
BRIEF DESCRIPTION OF THE DRAWINGS
[0038] FIG. 1 is a block diagram of one embodiment of the process
of the present invention.
[0039] FIG. 2 illustrates the metabolic pathway for the conversion
of glucose to lactate.
[0040] FIG. 3 illustrates the metabolic pathway for the conversion
of glucose to acetate.
[0041] FIG. 4 illustrates one embodiment of reactive
distillation.
DETAILED DESCRIPTION OF THE INVENTION
[0042] One embodiment of the ethanol process of the present
invention uses a unique combination of enzymatic milling, indirect
fermentation, and chemical synthesis to produce a slate of high
valued products. FIG. 1 is a simplified block flow diagram for the
process. Corn 10 is ground 20 the oil 30 is extracted, and the
remaining material is then enzymatically converted 40 using enzymes
44 into fermentable sugars and amino acids. Acetic acid and
bacterial biomass 60 are produced by a two step fermentation 50.
The first step uses a lactic acid bacteria such as Lactobacillus
casei to convert the fermentable sugars into lactic acid. The
second fermentation step uses an anaerobic bacteria such as
Clostridium formicoaceticum to convert the lactic acid and residual
sugars from the first fermentation into acetic acid without any
CO.sub.2 byproduct. This combination of fermentation steps results
in a high yield of acetic acid with the coproduction of bacterial
biomass 60 that can meet the US FDA requirements on direct-fed
microbials. The resulting acetic acid is converted into ethanol 70
using chemical synthesis steps (esterification+hydrogenation) 80.
The bacterial biomass 60 from the fermentations is directly usable
or can be processed into a high protein animal feed concentrate
called Single Cell Protein (SCP) 60.
[0043] The overall chemistry for the major steps are as
follows:
TABLE-US-00001 Enzymatic (1/n) Starch + H.sub.2O -> Dextrose
Treatment: Fermentation 1: Dextrose -> 2 Lactic Acid
Fermentation 2: 2 Lactic Acid -> 3 Acetic Acid Esterification: 3
Acetic Acid + -> 3 Ethyl Acetate + 3 Ethanol 3 H.sub.2O Steam
Reforming: 1.5 Methane + 3 H.sub.2O -> 1.5 CO.sub.2 + 6 H.sub.2
Hydrogenation: 3 Ethyl Acetate + 6 H.sub.2 -> 6 Ethanol Overall:
(1/n) Starch + 1.5 Methane + -> 3 Ethanol + H.sub.2O 1.5
CO.sub.2
where n is the number of dextrose units in a starch molecule. The
above equations show starch as the only source of fermentable
sugars in corn. However, the ethanol process of the present
invention uses an enzymatic milling process which also makes the
cellulose and hemicellulose fractions of corn available for
fermentation. The enzymatic milling process increases the amount of
fermentable sugars derived from a given amount of corn by about 20%
over traditional wet milling.
[0044] Another reason for the high yield of the ethanol process of
the present invention is the amount and source of the CO.sub.2
produced by the present process. In the ethanol process of the
present invention, only 0.5 moles of CO.sub.2 are produced for
every mole of ethanol. In contrast, traditional fermentation routes
produce 1 mole of CO.sub.2 for every mole of ethanol. Furthermore,
the ethanol process of the present invention uses less expensive
methane rather than dextrose as the carbon source for carbon
dioxide. Lower CO.sub.2 production from less expensive feedstocks
leads to better process economics.
Preparation of Suitable Fermentation Substrate
[0045] There are many processes which are well known in the state
of the art to provide suitable fermentation media for lactic acid
or acetic acid fermentations. These can be media which minimize the
amount of nitrogen in the media and thus minimize the amount of
single cell protein. On the other hand there are processes which
attempt to increase the utilization of nitrogen in the feed.
[0046] Any suitable media preparation process may be used for the
purposes of this invention.
[0047] As an illustrative example only, one can consider using corn
as the raw material. Several pretreatment steps are typically used
in corn milling such as cleaning, germ removal for oil production,
etc as is well know to those in the milling art.
[0048] Typically enzymatic treatment is used to convert the corn
into a media that is suitable for metabolism by the bacteria in the
downstream fermentations, although acid hydrolysis has also been
used. The ground corn is mixed with water to form a slurry which is
then heat sterilized. A continuous sterilizer that heats the corn
slurry to 120-140 C and provides 1 to 5 minutes of residence time
can be used. Preferably, the retention tubes are designed to
provide turbulent flow (Re>4000) with minimal dead spots, so
that good sterilization without excessive carmelization occurs.
[0049] Heat sterilization also begins the liquefaction process.
During liquefaction the starch granules swell and the viscosity of
the slurry rises dramatically. Heat stable a-amylase is used to
limit the rise in viscosity by depolymerizing the starch
molecules--a process called saccharification. a-amylase is an
enzyme which hydrolyzes the 1,4 linkages in the starch molecule. It
is classified as an endoenzyme since it randomly attacks bonds in
the interior of the starch molecule. Sufficient reduction in
viscosity is achieved with 10-15% hydrolysis of the starch in less
than 10 minutes residence time at pH 5.5-7.0.
[0050] Glucoamylase is preferably used to complete the hydrolysis
of the starch molecule. Glucoamylase is an exoenzyme since it only
attacks the ends of the starch molecule. The enzyme hydrolyzes both
1,4 and 1,6 linkages, so nearly complete hydrolysis of the starch
can be achieved. Optimal conditions for glucoamylase are typically
58-62 C, pH 4.4-5.0, and 24-48 hours of residence time. Longer
residence times are typically not beneficial since the enzyme also
catalyzes the formation of non-fermentable disaccharides--processes
called reversion and retrogradation.
[0051] In addition to the utilization of the starch fraction of
corn it is desirable to utilize the other major fractions,
including the hemicellulose and cellulose, as well as the protein
in this invention. This is not typically done in current ethanol
processes. The higher yield of this invention and the wide
substrate utilization capability of the fermentation used enhance
the value of these added steps as opposed to the current processes
as will be shown.
[0052] Hydrolysis of hemicellulose can be carried out in several
ways. Much research is known on acid hydrolysis, but enzymatic
hydrolysis is also well known. Complete enzymatic hydrolysis of
hemicellulose requires a mixture of enzymes. The pendant arabinose
and glucuronic acids are removed from the xylose backbone using
a-L-arabinofuranosidase and a-glucuronidase. The xylose backbone is
hydrolyzed using endo-b-1,4-xylanase and b-xylosidase.
[0053] Cellulose utilization is also of value. Several methods are
know for the hydrolysis of cellulose to fermentable sugars. For
example, cellulose is hydrolyzed by the synergistic action of three
cellulase enzymes: endo-b-glucanase, exo-b-glucanase, and
b-glucosidase. The endo-b-glucanase is an endoenzyme which randomly
hydrolyzes the 1,4 linkages in the interior of the cellulose
molecule. Exo-b-glucanase removes cellobiose units (a disaccharide
of b linked glucose) from the non-reducing end of the cellulose
chain. b-glucosidase hydrolyzes a cellobiose unit into two glucose
molecules. Working together, the three enzymes can convert
cellulose into glucose monomer. It is also a feature of this
invention that lactic acid bacteria as used in this invention
utilize cellobiose directly which reduces feedback inhibition of
the hydrolysis.
[0054] The hemicellulose and cellulose enzymes have been the focus
of much research work over the past 10-20 years. These enzymes are
required for efficient conversion of woody biomass materials into
fermentable sugars, which can then be used as fermentation
feedstocks for ethanol and other fermentation products by
traditional processes. Biomass materials such as grass, wood,
paper, and crop residues are much less expensive than starch based
materials such as corn starch.
[0055] . Redaction in enzyme cost can be obtained by overlapping
the saccharification activity with the fermentation process in a
design called Simultaneous Saccharification and Fermentation (SSF).
Product inhibition of the cellulases is avoided by conversion of
the glucose into ethanol or other desired fermentation product. The
SSF philosophy has been used for decades by the ethanol industry
with starch enzymes. Research also shows that this concept works
for the hemicellulase and cellulase enzyme systems. This process
may also be used in the current invention. It is a preferred
process because the fermentation used in this invention utilizes
more of the types of sugars produced in the hydrolysis and further
accelerates the hydrolysis compared to a yeast fermentation which
consumes the glucose fraction largely.
[0056] In addition to the utilization of the fiber fraction
comprising hemicellulose and cellulose, it may be desirable in this
invention to utilize the protein fraction.
[0057] Protease enzymes are used to hydrolyze the corn proteins
into smaller peptides and amino acids. These amino acids and
peptides are a major nitrogen source for the fermentation bacteria.
Hydrolysis of the proteins is required to speed nitrogen
assimilation in the fermentation. U.S. Pat. No. 4,771,001 shows the
use of protease enzymes to increase the utilization of proteins by
a lactic acid fermentation. This patent also illustrates the use of
a different raw material, in this case cheese whey. For the
purposes of the current invention the protein used to supplement
the fermentation can come from the corn as illustrated, or from
other protein sources and can be mixed into the media. Any protein
source that produces a suitable fermentation media for lactic acid
or acetic acid fermentation and does not inhibit the fermentation
may be used.
[0058] In its most general embodiment the current invention does
not depend upon a specific carbohydrate or protein source, but any
suitable source may be used.
Fermentation
[0059] The overall purpose of the fermentation part of the current
invention is to convert the fermentable carbohydrates and amino
acids into acetic acid and single cell bacterial protein. In a
preferred embodiment a two step fermentation process is used. The
first step uses a homofermentative lactic acid bacteria to convert
the bulk of the fermentable sugars into lactic acid and single cell
protein. The second step uses a homofermentative acetogenic
bacteria to convert lactic acid and residual carbohydrates into
acetic acid.
[0060] The tactic acid fermentation step uses a homofermentative
lactic acid bacteria such as Lactobacillus casei to convert the
fermentable sugars into lactic acid. Lactic acid bacteria are
gram-positive, non-spore forming, aerotolerant anaerobes. These
bacterial are found in the mouths and intestinal tracts of most
warm blooded animals including humans. None are pathogenic and many
are approved by the US FDA as viable organisms for use in
direct-fed microbials for animal feeds. Viable cultures are also
present in many yogurts consumed by humans.
[0061] As shown in FIG. 2, lactic acid is the sole metabolic
product for homofermentative strains. Glucose is metabolized to
pyruvate using the regular Embden-Meyerhof glycolytic pathway.
Pyruvate is convert to lactic acid in a single NAD coupled step.
Most lactic acid bacteria are mesophilic with optimal temperatures
for growth between 35 to 45 C. Cell growth is pH sensitive with
optimal pH around 6.0. Product inhibition begins to affect the
kinetics of cell growth and acid production at lactic acid levels
above 4 wt %. Complete inhibition of growth occurs around 7 wt %
while complete inhibition of acid production occurs around 10-12 wt
%.
[0062] The feed to the fermentation is very dilute in carbohydrates
with only about 5 wt % fermentable sugars. A single stage
continuous stirred tank reactor (CSTR) type fermentor is
appropriate for this step. However, any suitable bioreactor can be
used, including batch, fed-batch, cell recycle and multi-step CSTR.
The low carbohydrate concentration in the feed will limit the
effects of product inhibition on the cell growth and acid
production kinetics, thus 90+% conversion of the dextrose with
about 18-24 hour residence times is possible. Most homofermentative
strains will readily metabolize a range of substrate sugars. It is
advantageous to combine the lactic acid fermentation with the
subsequent acetic acid fermentation in such a manner so as to
utilize all of the sugars.
[0063] In contrast to many industrial lactic acid fermentations,
the current invention may be operated in a mode in which the
fermentation is carbohydrate limited rather than nitrogen limited.
Thus biomass production is maximized by keeping most of the
fermentation in the growth associated state and ensuring that
sufficient nitrogen is available for growth. For any growth
associated fermentation the biomass yields are typically about 10.5
g per mole of ATP produced. Since lactic acid fermentations produce
a net of 2 Moles of ATP per mole of glucose, the biomass yield will
be around 2 (10.5/180)=0.12 g per g of glucose. By stoichiometry,
the remaining 0.88 g of glucose are converted into 0.88 grams of
lactic acid.
[0064] The efficient production of biomass as single cell protein
is an important part of this invention. In contrast to the
production of single cell protein historically, the use of an
anaerobic homofermentative fermentation is very advantageous. This
is because all of the energy production of the organism comes from
the production of the desired metabolite whether lactic acid or
acetic acid. This means that there is no wasted byproduct CO.sub.2
as is the case in aerobic fermentations. In addition, because of
the lack of production of CO.sub.2, the heat produced by the
fermentation is also minimized. Therefore the utilization of energy
contained in the raw material carbohydrates is maximized toward the
production of valuable single cell protein or lactic and acetic
acid. The traditional yeast fermentation, in addition to wasting
mass as CO.sub.2, also requires the removal of heat.
[0065] The fermented broth from the first fermentation step is
clarified using a centrifuge. The concentrate contains the lactic
acid bacteria and is sent to single cell protein recovery. The
amount of single cell protein produced is related to the amount of
nitrogen in the form of hydrolyzed proteins as amino acid and
peptides that is supplied to the fermentation in the medium. This
can range from a very small amount, but not zero, as lactic acid
bacteria require some complex nitrogen sources, such as 1% up to
about 15% overall yield of single cell protein based on the total
nitrogen plus carbohydrate in the medium. It is a feature of the
invention that the production of single cell protein can be
controlled over a wide range. The single cell protein can be
processed by any suitable means, such as spray drying, to produce a
salable product.
[0066] Another important feature of the current invention is the
production of a single cell protein which is enhanced in value as
an animal feed ingredient. The single cell protein from the lactic
acid fermentation has these features. It has a high protein
concentration of about 70%, depending on the strain of organism and
the specific conditions of the fermentation. It has a good amino
acid profile. That is, it contains a high percentage of so called
essential amino acids comprising, for example, lysine, methionine,
isoleucine, tryptophan, and threonine. The combined percentage of
these amino acids in lactic acid bacteria is about 10.5%, compared
to corn protein which has about 1% of the total corn kernel. The
protein composition of corn depends on the fraction of the corn
considered. Corn gluten meal, for example, has about 7.5%, but corn
gluten feed has about 2.5% of essential amino acids. This enhanced
amino acid composition is directly related to the value of the
protein as an animal feed ingredient.
[0067] In a preferred embodiment, the current invention can produce
single cell protein at high efficiency and with high value.
[0068] The centrate, from the separation of the lactic acid
bacteria from the fermentation broth of the first fermentation, is
fed to a second fermentor where the lactate is converted into
acetate using an acetogenic bacteria. Lactate can be a preferred
substrate for acetogenic bacteria in many of their natural
environments. The rate of fermentation and yield on lactate
substrate can be very high, e.g., over 98% yield of acetate from
lactate.
[0069] Incomplete removal of the lactic acid bacteria is typically
acceptable since the acetic acid fermentation typically uses a
thermophilic strain and the second fermentation is done at a higher
temperature. Contamination of the acetic acid fermentation with a
mesophilic lactic acid bacteria is typically not an issue since the
lactic acid bacteria typically cannot grow at these higher
temperatures. Also, near complete conversion of the glucose is
expected in the first fermentor, so the lactic acid bacteria which
do happen to bleed through the centrifuge into the second fermentor
will not have a carbohydrate source.
[0070] The acetogenic bacteria have been known and studied since
the 1930's. Drake, H. L. (editor), Acetogenesis, Chapman &
Hall, 1994, gives a good overview of the field. The acetogenic
bacteria include members in the Clostridium, Acetobacterium,
Peptostreptococcus and other lesser known species. The habitats of
these bacteria are: sewers, anaerobic digesters at municipal waste
treatment plants, natural sediments, termite guts, rumens, and
intestinal tracts of non-ruminants including humans. Pathogenicity
is rare. All of these organism are strict anaerobes, which means
that contact with oxygen is often fatal to the microorganism.
Clostridium are spore formers. Spores are resistant to many
sterilization techniques and special procedures have been
established for handling spore-forming bacteria. The Acetobacterium
and Peptostreptococcus species are not spore formers.
[0071] FIG. 3 is a simplified sketch of the metabolic pathways used
by most acetogenic bacteria. The organism metabolizes glucose to
pyruvate using the normal Embden-Meyerhof glycolytic pathway.
Lactic acid is also metabolized by first converting it back to
pyruvate. From pyruvate, the organism makes acetic acid and carbon
dioxide using the regular oxidation pathways. The main
distinguishing feature of acetogenic bacteria is that the CO.sub.2
produced in this oxidation step is not released to the environment.
Instead, the acetogenic bacteria have a metabolic pathway which
will fix the CO.sub.2 and make an additional mole of acetic
acid.
[0072] The novel acetogenic pathway provides three functions for
the organism:
[0073] 1. Like all anaerobes, a terminal electron acceptor other
than oxygen is required to balance the redox reactions of
metabolism. In this case, the reduction of carbon dioxide acts as
the electron sink.
[0074] 2. Cellular energy (i.e. ATP) is produced from this pathway.
The metabolic pathways for conversion of one mole of glucose into
two moles of acetic acid and two moles of carbon dioxide produce
four ATP per mole of glucose consumed. Addition of the acetogenic
pathways creates another acetic acid molecule from the carbon
dioxide and increases the ATP yield to 4-6 ATP per mole of glucose.
The additional ATP are not made directly from the substrate-level
phosphorylation but are made in other processes such as the
electron transport chain and from ion pumps located in the cell
membranes. The exact amount of ATP produced from the secondary
sources varies from strain to strain and is also dependent upon the
cell environment.
[0075] 3. Carbon dioxide can be converted into cellular carbon
needed for growth using the cell's anabolic pathways, even when
common carbon sources such as glucose are not available.
[0076] Some acetogens will produce other organic acids such as
formic, propionic, succinic, etc. in addition to acetic acid. These
organisms are described as heterofermentative as opposed to the
homofermentative organisms which only produce acetic acid. The
heterofermentative pathways represent a potential yield loss in the
current invention, and proper strain selection and elucidation of
the factors which cause the formation of these other organic acids
will minimize the impact.
[0077] By far, most work to date has been with the Clostridium
strains. Many of these strains are thermophilic with optimal
temperatures for growth around 60 C. Several kinetic studies (Yang,
S. T., Tang, I. C., Okos, M. R., "Kinetics and Mathematical
Modeling of Homoacetic Fermentation of Lactate By Clostridium
formicoaceticum", Biotechnology and Bioengineering, vol. 32, p.
797-802, 1988, Wang, D. I., Fleishchaker, R. J.; Wang, G. Y., "A
Novel Route to the Production of Acetic Acid By Fermentation",
AIChE Symposium Series-Biochemical Engineering: Renewable Sources,
No. 181, vol. 74, p. 105-110, 1978; and Tang, I. C., Yang, S. T.,
Okos, M. R., "Acetic Acid Production from Whey Lactose by the
Co-culture of Streptococcus lactis and Clostridium
formicoaceticum", Applied Microbiology and Biotechnology, vol. 28,
p. 138-143, 1988, which are incorporated herein by reference in
their entirety) have been conducted to examine the effects of pH
and acetate levels on both cell growth and acid production. These
organism are sensitive to low pH and product inhibition occurs at
much lower concentrations than in lactic acid bacteria. Optimal pH
is around 7 and maximum acetate tolerance is only about 30 g/l in
batch fermentation.
[0078] A one or two stage CSTR fermentor design is typically
appropriate for the second fermentation step. However, any suitable
bioreactor can be used, including batch, fed-batch, cell recycle,
and multi-step CSTR. In contrast to the first fermentation step,
the acetic acid fermentation is nitrogen limited rather than
carbohydrate limited. Yield of acetic acid from lactic acid can be
greater than 85% of theoretical.
[0079] In one embodiment, the broth from the second fermentation
step is prepared for the second part of the current invention which
is the chemical conversion. As an example, the broth is clarified
with a combination of a centrifuge and a microfilter. The
centrifuge removes the bulk of the biomass and reduces the size of
the downstream microfilter by reducing its load. The microfilter
permeate is sent to a nanofiltration unit. The microfilter acts as
a prefilter for the nanofiltration unit. The nanofiltration unit
removes proteins, unconverted sugars, etc. which have molecular
weights above about 300. The nanofiltration unit removes the bulk
of the impurities in the acetate broth and produces a water white
permeate that can be sent to downstream processes.
[0080] The concentrates from the centrifuge, microfilter and
nanofilter may be processed to recover values useful in the single
cell protein or recycled to one of the fermentation steps.
Alternatively, they may be disposed of in any acceptable manner
such as composting or incineration.
[0081] Although a preferred embodiment of the current invention
utilizes two fermentation steps and the production of single cell
protein, this is not required in the most general case. A suitable
medium for the acetic acid fermentation alone may be provided.
Although single cell protein may not be produced, the increased
yield form the carbohydrate source will still provide an important
advantage for the current invention.
[0082] In addition, it is not necessary to utilize the
hemicellulose or cellulose fraction of the raw material in order to
get the advantages of the current invention. An example is the
utilization of the invention in conjunction with a corn wet mill
where the medium would be almost pure starch and corn steep water
produced by the mill. The current invention would still increase
the ethanol yield compared to current technology by 75% providing a
huge economic advantage.
[0083] The key feature of the fermentation step is therefore the
conversion of carbohydrate from any source into acetic acid.
Acidification and Esterification
[0084] In the next step of the invention, the acetic acid or
acetate produced in the fermentation is converted to an ester of
acetic acid, preferably methyl or ethyl ester and more preferably
ethyl ester. Any suitable process that will convert the acetic acid
or acetate salt to the ester is acceptable as part of this
invention.
[0085] Acetic acid is a weak organic acid with pKa=4.76. If the
fermentation is conducted at near neutral pH (i.e. pH=7.0), the
product of the fermentation will actually largely be an acetate
salt rather than the acid. In the fermentation, any suitable base
can be used to neutralize the fermentation. The preferred
neutralizing agent is Ca(OH).sub.2, which can be supplied by CaO
(lime) or calcium carbonate (CaCO.sub.3) which can be recycled from
later in the process. Other neutralizing agents can be used, such
as NaOH or NH.sub.2OH, as determined by the conditions required by
the fermentation organism. However, even the acetate salt is
inhibitory and the maximum concentration of acetate is usually
limited to about 5% in the fermentation broth.
[0086] Thus, there are two problems in the recovery of acetic acid
salts from a solution such as a fermentation broth. The acetate
salt must usually be converted to the acid, and the acid must be
removed from the dilute solution in water. In addition it is
desirable to recycle the base used to neutralize the fermentation
to reduce costs and avoid potential environmental impact.
[0087] The most typical route is the sequential acidification of
the salt to produce acetic acid and then the subsequent recovery of
the acid. Even after the salt is converted to a dilute acid
solution, there is still the need to recover the product from the
water. Many different process approaches have been proposed to
recover such dilute solutions. Since acetic acid has a higher
boiling point than water, the bulk of the water, about 95% of the
broth, must be distilled away from the acetic acid to recover the
acid if simple distillation is used. Alternatively, some more
complex process may be used to recover the acetic acid, usually in
conjunction with solvent extraction. However this line of research,
that is, acidification with subsequent recovery from the dilute
solution, has not overcome the economic limitations of the acetic
acid fermentation process to make it competitive with the synthesis
gas based route. Therefore, all industrial acetic acid is currently
made from synthesis gas derived from coal, petroleum or natural
gas.
[0088] A number of methods have been proposed to acidify the acetic
acid salt solution. One method is the reaction of the acetate salt
with a strong acid such as sulfuric acid to form acetic acid (HAc)
and calcium sulfate (CaSO.sub.4). The CaSO.sub.4 precipitates and
is easily separated from the acetic acid solution. However, this
method requires the consumption of acid and base and produces a
byproduct waste salt that may become an environmental burden.
Another method is bipolar electrodialysis that splits the salt into
an acid and base (this does not work well with Ca salts, but one
could substitute Na in this case). Other routes to produce dilute
acetic acid from the salt are well known.
[0089] Reaction of a carboxylic acid salt with an amine and
CO.sub.2 with the precipitation of CaCO.sub.3 and the formation of
an acid amine complex that can be extracted and thermally
regenerated has also been proposed, as shown by U.S. Pat. No.
4,405,717, which is incorporated herein by reference in its
entirety.
[0090] U.S. Pat. No. 4,282,323, which is incorporated herein by
reference in its entirety, discloses a process to acidify acetate
salts using CO.sub.2 in a number of ways. In the referenced patent
the acetic acid formed is removed by a solvent to a separate
phase.
[0091] Esterification of acetic acid to form ethyl acetate is a
well understood reaction:
##STR00001##
[0092] Esterification is typically performed in the liquid phase.
The equilibrium constant for this reaction is 4.0 and is nearly
independent of temperature. Acid catalysts for the reaction
include: strong Bronsted acids such as sulfuric acid and methane
sulfonic acid, acidic ion exchange resins, zeolites, and a number
of other materials, including carbonic acid formed by the
dissolution of CO.sub.2 in water. The reaction rate is influenced
by the type and concentration of catalyst, the reaction
temperature, and the degree of departure from equilibrium.
[0093] Alternative routes exist that attempt to avoid the separate
acidification and esterification steps. A carboxylic acid salt may
be reacted directly with an alcohol such as ethanol to produce the
ester directly. An intermediate step may be inserted to convert the
Ca salt to an ammonia salt. In this step the dilute Ca(Ac).sub.2 is
reacted with NH.sub.3 and CO.sub.2 to form NH.sub.4Ac and
CaCO.sub.3 which precipitates. The ammonia salt of acetic acid may
then be esterified directly as shown by U.S. Pat. No. 2,565,487,
which is incorporated herein by reference in its entirety.
Preferred Approach
[0094] The preferred approach is to combine chemical and phase
change operations into a new efficient process to directly produce
a volatile ester of acetic acid and distill the ester away from the
broth.
The three parts are:
[0095] 1) Acidification of the fermentation broth with CO.sub.2 at
low or nearly atmospheric pressure to produce acetic acid and
precipitate CaCO.sub.3 which can be recycled directly to the
fermentation as the base;
[0096] 2) Simultaneous esterification of the formed acetic acid
with an alcohol, such as methyl or ethyl alcohol, to form a
volatile ester, and
[0097] 3) Reactive distillation to push the acidification and
esterification equilibria to high conversion.
[0098] Since esterification is an equilibrium reaction, high
conversion can be obtained by driving the reaction to the right
with continuous removal of one or more products. Reactive
distillation similar to that developed by Chronopol for lactide
synthesis (See U.S. Pat. No. 5,750,732, which is incorporated
herein by reference in its entirety) and by Eastman Chemical for
methyl-acetate production (see U.S. Pat. Nos. 4,939,294 and
4,435,595 and Agreda, V. H., Partin, L. R., Heise, W. H.,
"High-Purity Methyl Acetate Via Reactive Distillation", Chemical
Engineering Progress, p. 40-46, February 1990, which are
incorporated herein by reference in their entirety) is an
economically attractive method. U.S. Pat. No. 5,599,976, which is
incorporated herein by reference in its entirety, discloses the
conversion of very dilute acetic acid to the ester in a continuous
reactive distillation process. Xu and Chaung (Xu, Z. P, Chuang, K.
T., "Kinetics of Acetic Acid Esterification over Ion Exchange
Catalysts", Can. J. Chem. Eng., pp. 493-500, Vol. 74, 1996) show
that reactive distillation to produce the ester of acetic acid from
dilute solution is the preferred method to remove acetic acid from
very dilute solutions, as are produced in the current invention. In
this concept, the acetic acid flows in a counter current fashion to
the esterifying ethanol in a distillation column. In the current
invention, ethyl acetate is more volatile than acetic acid so the
ethyl acetate is distilled away from the liquid mixture and the
esterification reaction is pushed to the right, thus enabling high
conversions in a single vessel. The process proposed here goes
beyond these examples in that its combines simultaneous
acidification with the reactive distillation esterification. All of
the cited processes start with acetic acid (or lactic acid in the
Chronopol case) and not a salt.
[0099] The net effect of the reactive distillation process, the
preferred route, is to remove the acetic acid from the dilute
solution without vaporizing the water which forms the bulk of the
stream.
[0100] In addition, the use of CO.sub.2 as the preferred acidifying
agent with the precipitation of CaCO.sub.3 allows the recycle of
the neutralizing agent to the fermentation without the consumption
of chemicals. The CaCO.sub.3 can be used directly in the
fermentation or can be converted first to CaO by calcination.
The reactive distillation process 80a is shown in FIG. 4.
[0101] Reaction Section:
[0102] The raw material, a dilute (5%) solution of calcium acetate
410 (Ca(Ac).sub.2) in water 414 is mixed with ethanol 418 and fed
to the column 422 at the top of the reaction section 424. CO.sub.2
420 is fed to the column 422 at the bottom of the reaction section
424. The simultaneous reaction of CO.sub.2 420 with Ca(Ac).sub.2
410 and ethanol 418 takes place in the reaction zone 424 in the
center section of the column 422 with the formation of CaCO.sub.3
428 and ethyl acetate (EtAc) 432.
CO.sub.2(g)+H.sub.2O->H.sub.2CO.sub.3
Ca(Ac).sub.2+H.sub.2CO.sub.3->CaCO.sub.3(s)+2HAc
2HAc+2EtOH->2EtAc
[0103] The most volatile component in the reaction mixture is the
ethyl acetate/water/ethanol azeotrope 436. The azeotrope
composition is 82.6% ethyl acetate, 9% water and 8.4% ethanol and
has a normal boiling point of 70.2.degree. C. The azeotrope 436 is
removed from the reaction mixture by vaporization along with some
EtOH and water. The bottom product from the reaction zone is a
water and ethanol solution containing the suspended CaCO.sub.3
flowing to the stripping section.
[0104] Separation Section:
[0105] In the upper separation zone 450 the azeotrope is separated
from the ethanol and water also vaporized from the reaction
mixture. The ethanol water mixture 454 is recycled to the reaction
zone 424 and the overhead product is the azeotrope 436. The
CO.sub.2 is separated from the overhead condensate and recycled to
the column with makeup CO.sub.2. The azeotrope can be broken by the
addition of water, which causes a phase separation, with the water
and ethanol rich phase returned to the appropriate point in the
reactive distillation column (not shown).
[0106] Stripping Section:
[0107] Since excess ethanol is used to favor the forward
esterification reaction in the reaction section, the stripping
section 458 returns the excess ethanol to the reaction zone. In the
stripping section 458 the ethanol is removed from the
CaCO.sub.3-containing water stream which is discharged from the
column 422 and separated by a simple liquid/solid separation 462
such as centrifugation or filtration, into the solid base 466 for
recycle and water 470.
[0108] The net effect of the reactive distillation process is to
recover the acetic, acid from the dilute salt solution thereby
producing a relatively concentrated product stream at the top and
without vaporizing the water that forms the bulk of the stream. The
integration of the three sections reduces the energy requirement.
The simultaneous removal of the product ester shifts the
esterification equilibrium and leads to higher conversion in a
short time.
[0109] It is unusual to handle precipitates in a distillation
system. However, in this case the precipitation reaction occurs in
the bulk phase and is not due to the concentration of the solution
at a heat transfer surface, a common type of fouling. Ethanol beer
stills in the corn dry milling hanol industry typically handle
solids loading in the stripping section through the use of trays
with simple construction and large openings. Alternatively, it
would be possible to operate the reaction section in other
configurations, such as a series of stirred tanks with a common
vapor manifold, to simulate the column reaction section.
[0110] The successful development of a low cost, low energy,
integrated acidification, esterification and purification process
for ethyl acetate would potentially allow the economic production
on an industrial scale of major chemicals from renewable resources,
which are now produced from non-renewable resources.
[0111] One major benefit of using renewable resources is the
reduction of CO.sub.2 production with the replacement of fossil raw
materials. There would be a benefit to the U.S. economy from the
replacement of imported petroleum with domestic renewable
resources. The use of agricultural commodities to produce chemicals
and liquid fuels without subsidy has important benefits to the farm
community in terms of product demand and stable markets and reduces
the cost of U.S. government subsidies.
Hydrogenation
[0112] The third major step in the invention is the conversion of
the ester of acetic acid into two alcohols by hydrogenation. The
hydrogenation of esters to produce alcohols is a well-known
reaction.
##STR00002##
[0113] U.S. Pat. Nos. 2,782,243, 4,113,662, 4,454,358, and
4,497,967, which are incorporated herein by reference in their
entirety, disclose processes for the hydrogenation of esters of
acetic acid to ethanol.
[0114] For the particular case at hand, hydrogenation can be
performed in either the liquid phase or the gas phase. Any suitable
hydrogenation process can be used. This reaction is also an
equilibrium reaction. The reaction can be driven to the right by
using high partial pressures of hydrogen. Typical reaction
conditions are 150-250 C and 500-3000 psi depending upon the
desired conversion and selectivity. The reaction can be catalyzed
by any suitable hydrogenation catalysts, such as copper chromite,
nickel, Raney nickel, ruthenium, and platinum. A copper chromite,
nickel, or Raney nickel catalyst is preferred for the hydrogenation
since these catalysts are not poisoned by water. In the liquid
phase process, an alcohol such as ethanol is a good solvent.
[0115] In the gas phase process, the ethyl acetate feed is
vaporized and fed to the hydrogenation reactor with an excess of
hydrogen. After passing through the bed, the vapors are cooled and
flashed into a low pressure knockout drum. The hydrogen rich vapor
phase is recycled back to the reactor. The liquid phase is
distilled to remove residual water and unreacted ethyl acetate. The
water is not made by the hydrogenation chemistry; it's source is
the liquid-liquid equilibrium level present in the upstream reflux
drum of the reactive distillation column.
[0116] Another distillation column may be needed as a final
polishing step, depending upon the nature and quantities of side
products from the esterification and hydrogenation units.
[0117] The preferred ester is ethyl acetate, as it avoids the
introduction of a second compound into the process which must be
purified away from the product stream.
[0118] The water stripper collects water streams from the
acidification, esterification, and hydrogenation units. The water
is steam stripped to recover solvent values, then the water is sent
to final treatment and discharge or recycled to the fermentation
section.
[0119] Many potential sources of hydrogen for use in the present
invention exist. Any suitable hydrogen source can be used that
produces hydrogen of sufficient purity for the hydrogenation
reaction and that will not poison the catalyst. Raw materials for
hydrogen production include water from which hydrogen can be
produced by electrolysis. Many fossil and renewable organic
feedstocks can also be used. If a fossil feedstock is used, such as
methane from natural gas, some CO.sub.2 will be produced along with
the hydrogen. However, if a renewable feedstock is used then the
CO.sub.2 production will be neutral to the environment. For
example, feedstocks which contain carbon and hydrogen at the
molecular level can be used to produce hydrogen. Wood chips,
sawdust, municipal wastes, recycled paper, wastes from the pulp and
paper industry, solid agricultural wastes from animal and/or crop
production are all examples of renewable feedstocks that can be
used for hydrogen production, e.g., using gasification
technology.
[0120] Steam reforming of methane to produce hydrogen is a well
know process. As shown in FIG. 1, natural gas 90 and water 92 are
reacted in a steam reformer 94 to form hydrogen 96 and carbon
dioxide 98. Other methods to produce hydrogen (partial oxidation of
hydrocarbons, partial oxidation of coal, water electrolysis, etc.)
could also be used. Where pure oxygen is available, such as in a
fenceline operation with an air separations plant, the partial
oxidation processes can be economically viable. Where inexpensive
sources of electricity are available, electrolysis can be
viable.
[0121] Another advantage of the current invention, compared to
prior art technology for ethanol production, is the heat balance in
the process. In the current invention, if hydrogen is made by steam
reforming on site, excess heat is available at high temperature and
in an integrated plant due to the hydrogenation reaction of the
ester being a highly exothermic process. Therefore, the overall
process is highly energy efficient. In addition, none of the
carbohydrate raw material is wasted as CO.sub.2 with the attendant
generation of heat, which must be wasted to cooling water.
[0122] Another advantage of the current invention is the ability to
convert natural gas via hydrogen to a liquid product, e.g.,
ethanol, at very high yield. This feature can be utilized in
situations where any carbohydrate source is located close to a
source of natural gas production or easy transportation by
pipeline. This allows the utilization of gas in remote geographies,
such as islands that produce both gas and sugar cane or other
carbohydrate crop, to produce an easily transported liquid chemical
or fuel, again at high efficiency. For example, plant using the
process of the present invention could be located on the island of
Trinidad, where natural gas and carbohydrate sources are available
at economically attractive prices. The plant can produce
substantially pure ethanol for transport in liquid form to a remote
location where it can be economically utilized, such as the Texas
Gulf Coast. The ethanol can be used as a fuel or a feedstock for
further processing. For example, the ethanol can be converted to
ethylene and sold through existing ethylene pipeline systems.
Alternatively, the ethanol can be recycled within the plant for
production of ethyl acetate. In other words, the plant can be used
to produce a liquid product comprising substantially all ethyl
acetate, substantially all ethanol or any combination of the two.
Because the natural gas and carbohydrate source are located
relatively close to each other, these feedstocks can be converted
to a higher value liquid product which can be easily transported to
a remote location in an economic manner.
[0123] Preferably, the carbohydrate source and the natural gas
source are located within five hundred miles of each other, more
preferably within three hundred miles of each other and more
preferably within two hundred miles of each other. Preferably, the
remote location to which the transportable liquid product is
located where economic transportation of the carbohydrate and
natural gas is not viable, e.g., more than eight hundred miles,
more preferably more than a thousand miles and more preferably more
than fifteen hundred miles from the point of production. It will be
appreciated that it is generally easier to transport the
carbohydrate source to the source of the natural gas. It will also
be appreciated that the specific distances that favor an economic
advantage will vary depending on the price of the feedstocks, the
price of the transportable liquid product and transportation costs.
As will be appreciated by one skilled in the art, transportation
costs are influenced by a number of factors, including geographic
barriers, such a mountain ranges, bodies of water, etc. and are not
solely dependent on distances. As a further example, natural gas
and carbohydrate sources can be found in Australia and/or New
Zealand. A plant located on these island nations could produce
ethanol and/or ethyl acetate for transport to Asia, and in
particular, Japan, Taiwan, Korea and China.
[0124] While various embodiments of the present invention have been
described in detail, it is apparent that modifications and
adaptations of those embodiments will occur to those skilled in the
art. However, it is to be expressly understood that such
modifications and adaptations are within the spirit and scope of
the present invention.
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