WO2008156612A1

WO2008156612A1 - Process for the purification of crude glycerol compositions

Info

Publication number: WO2008156612A1
Application number: PCT/US2008/007293
Authority: WO
Inventors: Harapanahalli S. Muralidhara; Myong K. Ko
Original assignee: Cargill, Incorporated
Priority date: 2007-06-15
Filing date: 2008-06-11
Publication date: 2008-12-24

Abstract

Disclosed is a process for purifying a crude glycerol composition. The process comprises the steps of: (a) providing a crude glycerol composition; (b) filtering the crude glycerol composition though two or more nanofiltration or reverse osmosis filters that are connected in series to form a purified glycerol composition. At least one of the filters comprises filter media having a contact angle between about 44 and about 56 degrees.

Description

PROCESS FOR THE PURIFICATION OF CRUDE GLYCEROL COMPOSITIONS

CROSS-REFERENCE

This application is an international application claiming priority to U.S. Provisional Application No. 60/934,847 file 15 June 2007 incorporated herein by reference. BACKGROUND

With rising fossil fuel prices, the production of biodiesel is expected to increase. Typically, biodiesel is produced by transesterifying a glyceride-containing vegetable oil (e.g., soybean oil) and animal fats (e.g., tallow) with a monohydric alkyl alcohol (e.g., methanol) to form fatty acid alkyl esters (i.e., biodiesel) along with the co-product glycerol (propane-l,2,3-triol). An idealized reaction scheme for the production of biodiesel is shown below.

1 Triglyceride + 3 Alkyl Alcohol → 1 Glycerol + 3 Fatty Acid Alkyl Ester

Typically, excess alkyl alcohol is used in the reaction. Upon completion of the reaction, the products exist in two phases - a biodiesel phase (fatty acid alkyl ester phase) and a glycerol phase. Since the crude glycerol phase is heavier, it may gravity separated from the biodiesel phase. The separated crude glycerol phase typically comprises a major portion of glycerol, along with excess alkyl alcohol, inorganic salts, fats, low MW organic compounds (matter organic not glycerol, i.e., MONG), and traces of monoglycerides, diglycerides, and triglycerides. The crude glycerol phase is typically combined with biodiesel wash water and neutralized prior to processing through methanol evaporation. The methanol evaporated crude glycerol (crude glycerol -water) typically contains glycerol, water, salt, small amount of methanol, MONG, color bodies, and traces of glycerides. If the glycerol has been heated significantly, oligomers/polymers of glycerol may also be present. When an acidic catalyst is used in the transestification reaction, traces of various ethers formed between the alkyl-alcohol and glycerol may also be present.

As a high volume byproduct of biodiesel production, it is desirable to purify the crude glycerol so that the glycerol can be used in applications such as food manufacturing, pharmaceuticals, plastics, and chemical synthesis. The purification of crude glycerol by distillation is energy intensive and costly. Glycerol tends to degrade at temperatures above about 150°C to form undesirable products such as acrolein and allyl alcohol. In order to successfully distill glycerol a high level of vacuum (e.g., down to about 1 to 10 mm Hg) is required. This type of process, however, requires expensive processing equipment in order to achieve and maintain this high level of purity.

What is desired, therefore, is a process to purify a crude glycerol-containing composition to provide glycerol at a high purity level.

SUMMARY The present invention provides a process for purifying crude glycerol compositions. The process comprises the steps of (a) providing a crude glycerol composition; (b) filtering the crude glycerol composition though two or more nanofiltration or reverse osmosis filters that are connected in series to form a purified glycerol composition, wherein at least one of the filters comprises filter media having a contact angle between about 44° and about 56°.

In some embodiments of the invention, the process comprises three or more filters (e.g., 3, 4, or 5, etc.) that are positioned for series flow of the crude glycerol composition. The filters may be nanofiltration filters or reverse osmosis filters. For example, the process may comprise 3 or 4 nanofiltration filters. In an exemplary embodiment, the process comprses four nanofiltration filters positioned in series, wherein the second, third, and fourth filters have filter media with a contact angle of between about 44° and about 56°. In other embodiments, the process comprises a first nanofiltration filter followed by three reverse osmosis filters, wherein the second, third, and fourth filters comprise filter media having a contact angle between about 44° and about 56°. In many embodiments, the crude glycerol that is purified using the process of the invention is obtained from biodiesel production. Typically, the crude glycerol is an aqueous solution comprising between about 30 %wt. and 60 %wt. glycerol. In some embodiments, the crude glycerol may comprise, for example one or more salts selected from NaCl, Na₂SO₄, or NaHCCβ. In some embodiments, the crude glycerol comprises about 30 %wt. to about 88 %wt. glycerol, about 2 %wt. to 6.5 %wt. ash, about 0 to about 0.3% MONG, about 0.3 %wt. mono- and diglycerides, about 0 to about 0.15 %wt. methanol, and water.

The process of the invention can be used to purify crude glycerol to a defined grade, for example, fuel grade, technical grade, or pharmaceutical grade. In many embodiments, the purified glycerol has a purity of about 99% or greater.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 is a schematic representation of a process of the present invention. FIG. 2 is a schematic representation of a process of the present invention. FIG. 3 is a schematic representation of a process of the present invention.

DETAILED DESCRIPTION

The invention provides processes for the purification of crude glycerol compositions. In many embodiments, the processes of the invention provide purified glycerol compositions that achieve fuel grade, technical-grade, or pharmaceutical-grade purity. In some embodiments of the invention, the process comprises treating a crude glycerol composition with two or more nanofiltration steps that are conducted in series. In other embodiments, the process comprises treating a crude glycerol composition with one or more nanofiltration steps followed by one or more reverse osmosis filtration steps that are conducted in series. In many embodiments, the process comprises three to four nanofiltration steps, or the process comprises one nanofiltration step followed by two or more reverse osmosis filtration steps.

In many embodiments, the crude glycerol compositions are formed as a byproduct of the production of biodiesel. Biodiesel production is a well-known process wherein glyceride-containing starting materials are chemically converted from their glyceride form into fatty acid methyl esters. Biodiesel production typically comprises one of three types of reaction schemes, for example, (1) base catalyzed transesterification of tri glyceride- containing starting material with an alcohol (2) direct acid-catalyzed esterification of a triglyceride-containing starting material with methanol or other primary alcohol; (3) conversion of a triglyceride-containing starting material to its fatty acids with subsequent conversion of the fatty acids to alkyl esters using acid catalysis; and (4) conversion of a triglyceride-containing starting material with alcohol to a fatty acid alkyl ester using mild base, neutral, mild acid conditions along with a homogeneous or heterogeneous catalyst. The majority of biodiesel is produced using base catalyzed transesterifi cation of a triglyceride-containing starting material with an alcohol, hi this process a triglyceride- containing starting material is reacted with a stoichiometric amount of a primary alcohol (e.g., methanol) in the presence of a catalyst to yield glycerol and fatty acid methyl esters (biodiesel). Typical catalysts include sodium methoxide (NaOCH₃), sodium hydroxide (NaOH), or potassium hydroxide (KOH).

Crude glycerol-water compositions resulting from base catalyzed bodies production typically contains a major portion of glycerol, along with water, inorganic salts, fats, MONG, and traces of monoglycerides, diglycerides, and triglycerides. In some embodiments, the crude glycerol compositions from biodiesel production comprise about 30 %wt. to about 88 %wt. glycerol, about 2 %wt. to 6.5 %wt. ash, about O to about 0.3% MONG, about 5 %wt. to about 10 %wt. water, about 0.3 %wt. mono- and diglycerides, about 0 to about 0.15 %wt. methanol, and the balance water. Trace amounts of other impurities such as color bodies are also typically present. Examples of salts include Na₂SO₄, NaHCO₃, NaCl, and mixtures thereof.

In many embodiments, purification according to the processes of the invention results in purified glycerol compositions having a desired degree of purity, for example, fuel grade purity, technical grade purity, or pharmaceutical grade purity. A comparison of glycerol purified by distillation with that prepared by an embodiment of the present invention is provided in TABLE A.

TABLE A

Embodiments of the invention include one or more nano filtration steps and/or one or more reverse osmosis steps in order to purity the crude glycerol.

Nanofiltration is a pressure-driven membrane separation process that is positioned between reverse osmosis and ultrafiltration. The driving force of the separation process is the pressure difference across the semi permeable nanofiltration membrane. In nanofiltration, monovalent ions typically pass partially or completely through the membrane, whereas more highly charged divalent and multivalent ions and low molecular weight organics are rejected by the nanofiltration membrane to a greater degree. That is, nanofiltration membranes are more effective at rejecting ions that have two or more negative charges (e.g., SO₄ ^2") than they are in rejecting ions that have a single negative charge (e.g., Cl^"). For this reason, purification of crude glycerol compositions that contain salt impurities comprising divalent and polyvalent ions (e.g., Na₂SO₄ and/or NaHCO₃) can be effectively achieved using nanofiltration. Similar to nanofiltration, reverse osmosis is a pressure-driven membrane separation process. In contrast to nanofiltration, reverse osmosis is capable of removing monovalent ions as well as removing divalent and multivalent ions from a solution.

Nanofiltration/reverse osmosis filtration is typically operated at pressures ranging from about 100 psi to about 800 psi. In the process of the present invention it is generally advantageous to purify the crude glycerol composition using nanofiltration/reverse osmosis membranes that have a filter surface having an average contact angle ranging from about 44 to about 56 degrees. The contact angle of a nanofiltration or reverse osmosis membrane can be measured, for example, suing a Kruss drop shape analysis system DSA 10. Examples of some useful filters having a contact angle ranging from about 44 to about 56 degrees include type DL (NF) (50.5 degrees, from GE Osmonics); type SG (RO) (49.4 degrees, from GE Osmonics); type SR2 (NF) (55.5 degrees, from Koch); type SR3 (NF) (45.9 degrees, from Koch); and type TRS83 (NF) (48.0 degrees from TRISPE).

In many embodiments, it is desirable to initially filter the crude glycerol composition using a guard filter. The guard filter functions to reject nearly all foulants causing irreversible membrane fouling and to withstand under severe chemical environment during membrane cleaning-in-place. Typical foulants are oily matters, particulate matters, MONG, molecular weight of organic matters larger than the pore size of guard filter, color bodies, and some level of salts. Therefore, the guard filter functions to protect subsequent NF/RO membranes from fouling, to extend membrane service life, and to reduce overall membrane cleaning-in-place. Useful guard filters include nanofiltration membranes that have a contact angle of 0 degrees and chemical compatibility of pH 1-14 and methanol (e.g., MPS34 from Koch).

A process flow diagram of an embodiment of the invention is shown in FIG. 1. The process of FIG. 1 includes three nanofiltration filters that are configured to run in series. In the process of FIG. 1 a crude glycerol stream 20 is fed to a first nanofiltration device 30. First nanofiltration device 30 filters crude glycerol stream 20 resulting in the formation of retentate stream 32 and permeate stream 34. Retentate stream 32 is fed to a two-stage water/glycerol evaporator 70. Permeate stream 34 is fed to a second nanofiltration device 40 which filters permeate stream 34 resulting in the formation of retentate stream 42 and permeate stream 44. Retentate stream 42 is fed to evaporator 70 along with retentate stream 32. Permeate stream 44 from nanofiltration device 40 is fed to a third nanofiltration device 50 which filters permeate stream 44 resulting in the formation of retentate stream 52 and permeate stream 54. Retentate stream 52 is fed back into permeate stream 34 so that it can be refiltered though second nanofiltration device 40. During the process, the combined retentate stream 32 and retentate stream 42 become stream 32' and are fed into evaporator 70. A two-stage water/glycerol evaporator 70 functions to remove water at the 1^st stage and to recover glycerol at the 2^nd stage from retentate streams 32 and 42 resulting in the formation of stream 72 and stream 74. Stream 74 is fed into permeate stream 34 so that it can be further processed by nanofiltration device 40. Stream 72 is an evaporator bottom containing salt, glycerol, and impurities. After being processed by nanofiltration device 50, permeate stream 54 is then fed to water evaporator 80. Water evaporator 80 functions to remove water from permeate stream 54 resulting in the formation of product stream 84 and water stream 82.

TABLE B summarizes typical compositions for the streams of the process of FIG. 1. TABLE B

Basis: lOOkg/min feed MPS34: 30% rejection Na₂SO₄ at 2:1 glycerol to water ratio at 50°C & 500 psi SR2: 88% Na₂SO₄ rejection at 2:1 glycerol to water ratio at 50°C & 500 psi SR3: 95% Na₂SO₄ rejection } at 2:1 glycerol to water ratio at 50°C & 500 psi The process of FIG. 1 is particularly suitable for purifying crude glycerol compositions containing multivalent salt species (e.g., Na₂SO₄). In many embodiments, the process has a yield of glycerol of about 99% or greater. The purity of the resulting glycerol may reach technical or pharmaceutical grade.

Another embodiment of the process of the invention is shown in FIG. 2. The process of FIG. 2 includes four nanofiltration filters that are configured to run in series. In the process of FIG. 2 a crude glycerol stream 120 is fed to a first nanofiltration device 130. Nanofiltration device 130 filters crude glycerol stream 120 resulting in the formation of retentate stream 132 and permeate stream 134. Retentate stream 132 is fed to evaporator 170. Permeate stream 134 is fed to a second nanofiltration device 140 which filters the stream resulting in the formation of retentate stream 142 and permeate stream 144. Retentate stream 142 is combined with retentate stream 132 and the combined streams 132' are fed to evaporator 170. Permeate stream 144 is fed to a third nanofiltration device 150 which filters permeate stream 144 resulting in the formation of retentate stream 152 and permeate stream 154. Retentate stream 152 is fed back into permeate stream 134 so that it can be refiltered though second nanofiltration device 140. Permeate stream 154 is fed into a fourth nanofiltration device 160 which filters the stream resulting in the formation of retentate stream 162 and permeate stream 164. Retentate stream 162 is then fed back into permeate stream 134 so that it can be refiltered though third nanofiltration device 140. During the process, retentate streams 132 and 142 are both fed into a two-stage water/glycerol evaporator 170. The two-stage water/glycerol evaporator 170 functions to removed water at the 1^st stage and to recover glycerol at the 2^nd stage resulting in the formation of streams 172 and 174. Stream 174 is fed into filtrate stream 134 so that it can be processed by nanofiltration device 140. Filtrate stream 164 from nanofiltration device 160 is fed to water evaporator 180. Water evaporator 180 functions to remove water from filtrate stream 164 resulting in the formation of water stream 182 and product stream 184.

TABLE C summarizes typical compositions for the streams of the process of FIG. 2. TABLE C

Basis: 1 OOkg/min feed

MPS34: 30% NaHCO₃ rejection at 2:1 glycerol to water ratio and at 50° C & 500 psi

TS83: 95% NaHCO₃ rejection at 2:1 glycerol to water ratio and at 50° C & 500 psi

The process of FIG. 2 is particularly suitable for purifying crude glycerol compositions containing multivalent salt species (e.g., NaHCO₃). In many embodiments, the process has a yield of glycerol of about 99% or greater. The purity of the resulting glycerol may reach technical or pharmaceutical grade. Another embodiment of the process of the invention is shown in FIG. 3. The process of FIG. 3 includes one nanofiltration filter and three reverse osmosis filters that are configured to run in series. In the process of FIG. 3, a crude glycerol stream 220 is fed to a first nanofiltration device 230. Nanofiltration device 230 filters crude glycerol stream 220 resulting in the formation of retentate stream 232 and permeate stream 234. Retentate stream 232 is fed to a two-stage water/glycerol evaporator 270. Permeate stream 234 is fed to a first reverse osmosis device 240 which filters permeate stream 234 resulting in the formation of retentate stream 242 and permeate stream 244. Retentate stream 242 is combined with retentate stream 232 and the combined stream 232' is fed to two-stage water/glycerol evaporator 270. Permeate stream 244 is fed to a second reverse osmosis device 250 which filters permeate stream 244 resulting in the formation of retentate stream 252 and permeate stream 254. Retentate stream 252 is fed back into permeate stream 234 so that it can be refiltered though first reverse osmosis device 240. Permeate stream 254 is fed into a third reverse osmosis device 260 which filters the permeate stream resulting in the formation of retentate stream 262 and permeate stream 264. Retentate stream 262 is fed back into permeate stream 234 so that it can be refiltered though second reverse osmosis device 240. During the process, retentate streams 232 and 242 are both fed to two-stage water/glycerol evaporator 270. Two-stage water/glycerol evaporator 270 functions to remove water at the 1^st stage and to recover glycerol at the 2^nd stage resulting in the formation of streams 272 and 274. Stream 274 is fed from two-stage water/glycerol evaporator 270 to permeate stream 234 so that it can be processed by first reverse osmosis device 240. Stream 272 is two-stage water/glycerol evaporation bottom containing salt, glycerol, and impurities. After being processed by the nanofiltration and reverse osmosis devices, permeate stream 264 is then fed to ion exchange device 290 and then to water evaporator 280. Ion exchange 290 functions to remove residual salt (NaCl) from stream 264 to produce salt-free stream 264. Water evaporator 280 functions to remove water from permeate stream 264 resulting in the formation of water stream 282 and product stream 284.

TABLE D summarizes typical compositions for the streams of the process of FIG. 3. TABLE D

Basis: 100kg/min feed

MPS34: 15% NaCl rejection at 2: 1 glycerol to water ratio and at 50⁰C & 500 psi CG: 55% NaCl rejection at 2:1 glycerol to water ratio and at 50⁰C & 500 psi The process of FIG. 3 is particularly suitable for purifying crude glycerol compositions containing both monovalent and mixtures of monovalent and multivalent salt species, for example, Na₂SO₄ , NaHCO₃, and NaCl. In many embodiments, a yield of about 99% or greater can be achieved using nanofiltration followed by three stages of reverse osmosis. The resulting purified glycerol may be technical or pharmaceutical grade.

TABLE E provides NF/RO membrane performance characteristics for salt/color body removal from crude glycerol -water stream (2:1 water to glycerol ratio with 2% solt and color bodies). TABLE E

All publications and patents mentioned herein are hereby incorporated by reference. The publications and patents disclosed herein are provided solely for their disclosure. Nothing herein is to be construed as an admission that the inventors are not entitled to antedate any publication and/or patent, including any publication and/or patent cited herein.

Other embodiments of this invention will be apparent to those skilled in the art upon consideration of this specification or from practice of the invention disclosed herein. Various omissions, modifications, and changes to the principles and embodiments described herein may be made by one skilled in the art without departing from the true scope and spirit of the invention which is indicated by the following claims.

Claims

WHAT IS CLAIMED IS:

1. A process for purifying a crude glycerol composition, the process comprising the steps of: (a) providing a crude glycerol composition; (b) filtering the crude glycerol composition though two or more nanofiltration or reverse osmosis filters that are connected in series to form a purified glycerol composition, wherein at least one of the filters comprises filter media having a contact angle between about 44 and about 56 degrees.

2. The process of claim 1, wherein the process comprises three or more filters that are connected in series, wherein the filters comprise nanofiltration or reverse osmosis filters.

3. The process of claim 1, wherein the process comprises four or more filters that are connected in series where the filters comprise nanofiltration or reverse osmosis filters.

4. The process of claim 2, wherein the process comprises a first nanofiltration filter; a second nanofiltration filter; and a third nanofiltration filter.

5. The process of claim 4, wherein the second and the third nanofiltration filters comprise filter media having a contact angle between about 44 degrees and about 56 degrees.

6. The process of claim 3, wherein the process comprises a first nanofiltration filter; a second nanofiltration filter; a third nanofiltration filter; and a fourth nanofiltration filter.

7. The process of claim 6, wherein the second, the third, and the fourth filters comprise filter media having a contact angle between about 44 degrees and about 56 degrees.

8. The process of claim 3, wherein the process comprises a first nanofiltration filter; a second reverse osmosis filter; a third reverse osmosis filter; and a fourth reverse osmosis filter.

9. The process of claim 8, wherein the second and the third and the fourth filters comprise filter media having a contact angle between about 44 degrees and about 56 degrees.

10. The process of claim 1, wherein the crude glycerol is an aqueous solution comprising between about 30 %wt. and 60 %wt. glycerol.

11. The process of claim 1, wherein the crude glycerol is obtained from biodiesel production.

12. The process of claim 1, wherein the crude glycerol comprises one or more salts selected from NaCl, Na₂SO₄, or NaHCC-3.

13. The process of claim 1, wherein the crude glycerol comprises about 30 %wt. to about 88 %wt. glycerol, about 2 %wt. to 6.5 %wt. ash, about 0 to about 0.3% MONG, about 0.3 %wt. mono- and diglycerides, about 0 to about 0.15 %wt. methanol, and water.

14. The process of claim 1 , wherein the purified glycerol is technical grade.

15. The process of claim 1 , wherein the purified glycerol is pharmaceutical grade.

16. The process of claim 1, further including the step of treating the crude glycerol with an ion exchange resin.

17. The process of claim 1 , wherein the process has a yield of purified glycerol of about 99% or greater.