CA2581761C - Ethanol processing with vapour separation membranes - Google Patents

Abstract

In a process for removing water from ethanol, a distillate is fed to a gas separation membrane unit. Permeate from the membrane is compressed and used as heating steam for further distillation. The membrane unit may have two or more stages. Permeate from a stage may be condensed and used as fermentation make up water, compressed and fed to the permeate from an upstream stage or heating steam, or fed to another membrane stage for further dewatering.

Description

TITLE: ETHANOL PROCESSING WITH VAPOUR SEPARATION MEMBRANES
FIELD
[0001] This specification relates to alcohol processing or gas separation.
BACKGROUND

[0002] The following is not an admission that anything discussed below is citable as prior art or part of the common general knowledge.

[0003] Plant matter, for example carbohydrates or cellulose, may be fermented to produce a liquid, sometimes called beer, that is primarily water but includes ethanol. Dewatering this beer may produce ethanol that is substantially free of water, for example having less than about 1% water by volume, which may be used as a fuel or a fuel additive suitable for use in, for example, internal combustion automobile engines. Distillation can be used to partially dewater the beer, but the energy required per volume percent of water removed increases as the ethanol content increases. At about 97% ethanol by volume, the ethanol/water azeotrope has been reached and simple distillation is no longer effective. Other techniques, such as azeotropic distillation or molecular sieves may then be used. The energy requirement of these processes is a significant problem as is the amount of water required for fermentation. Solids produced in the fermentation process, sometimes called stillage, may be useful for animal feed but must first be dewatered which requires a significant amount of energy.

[0004] U.S. Patent No. 4,978,430 describes a process in which an evaporation vessel produces a mixture of an organic compound vapour and water vapour. The mixture permeates through an aromatic polyimide gas separation membrane. The permeated vapour has an increased concentration of water vapour and a product vapour has a reduced concentration of water vapour.
The permeated vapour passes through a condenser and is then returned to the evaporation vessel.

[0005] International Patent Application No. PCT/CA2004/001047 filed on July 16, 2004 describes an asymmetric integrally skinned membrane. The membrane can have a vapour permeance to water at least 1 x i0 mol/m2sPa at a temperature of about 30 C to about 200 C. The membrane may have a vapour permeance selectivity of at least 50, preferably at least 250 for water/ethanol at a temperature of about 140 C.
INTRODUCTION

[0006] The following introduction is not intended to limit or define any claim.
One or more inventions may reside in any combination of one or more process steps or apparatus elements drawn from a set of all process steps and apparatus elements described below or in other parts of this document, for example the Detailed Description, Claims or Figures.

[0007] This specification describes a process for removing water from an aqueous alcohol mixture, for example ethanol, using a heat driven process, for example distillation, and a gas separation membrane unit. A permeate is produced from the membrane unit that is substantially water vapour or ethanol free.
This water vapour is mechanically compressed and used to transfer heat to one or more other parts of the process, for example distillation, drying stillage, heating beer or pre-heating vapours before membrane separation. The water vapour may be condensed and used in the process, for example as make up water for fermentation. Heat energy in a product vapour may also be used, for example to dry stillage or pre-heat the beer before distillation.

[0008] This specification also describes processes for dewatering an ethanol feed using a plurality of gas separation membrane stages, for example two, three or more. The stages may be arranged in series in relation to a feed/retentate/product flow. Permeate from an upstream stage may be compressed and used to heat another process step. Permeate from a downstream stage may be condensed for use as make up water, or compressed and added to an upstream permeate stream. A permeate stream may be fed through another membrane stage for further dewatering before being used to transfer heat. One or more of the possibilities described above may be combined.
BRIEF DESCRIPTION OF THE FIGURES

[0009] Figure 1 is a simplified schematic flow sheet of an ethanol processing plant.

[0010] Figure 2 is a simplified schematic flow sheet of a scrubber of the plant of Figure 1.

[0011] Figures 3 to 6 are simplified schematic flow sheets of alternate membrane units of the plant of Figure 1.
DETAILED DESCRIPTION

[0012] Various apparatuses or processes will be described below to provide an example of an embodiment of each claimed invention. No embodiment described below limits any claimed invention and any claimed invention may cover processes or apparatuses that are not described below. The claimed inventions are not limited to apparatuses or processes having all of the features of any one apparatus or process described below or to features common to multiple or all of the apparatuses described below. It is possible that an apparatus or process described below is not an embodiment of any claimed invention. The applicants, inventors and owners reserve all rights in any invention disclosed in an apparatus or process described below that is not claimed in this document and do not abandon, disclaim or dedicate to the public any such invention by its disclosure in this document.

[0013] Figure 1 shows a plant 10 used to produce a product 12. The product 12 may be used as fuel grade ethanol or may have about 99% or greater ethanol by volume. The raw feed 14 to the plant 10 is a plant material that may be fermented to produce ethanol, for example carbohydrates or cellulose, for example from corn kernels, sugarcane or switchgrass. The raw feed 14 passes to a fermenter 16 which is also fed with water 18 as well as yeast and other fermentation inputs. The fermenter 16 outputs a beer which may be temporarily stored in a beer tank 18. The beer contains ethanol but is mostly water. The beer may contain about 10 to 12 percent ethanol by volume, although up to 20 percent by volume or more may be possible. The beer then flows, optionally passing through a beer pre-heater 20, to a distillation column 22. The distillation column 22 may be a single, multi-stage or multi-effect column or columns for producing a distilled ethanol 24 with an increased ethanol content. For example, the distillation column 22 may be or comprise a stripping column or zone or a beer column or zone and increase the ethanol content to at least 45% by volume but typically less than 75% by volume. Alternately, the distillation column 22 may further comprise a rectification column or zone and increase the ethanol content to 75% by volume or more, more typically 90% or more up to a value approaching 97% by volume. Although aspects of the invention may be useful when the beer is distilled to an ethanol content of 75% by volume or more, energy consumption for the plant 10 as a whole is likely to be less when the beer is distilled to 75% ethanol by volume or less which may be done in a single column. The distillation column 22 may have a reflux loop 32 and a reboiler loop 34.

[0014] The distilled ethanol 24, as a vapour, passes through a scrubber 26. Scrubber 26 will be described further below but removes particles from the distilled ethanol 24. The particles are contained in a first liquid 28 which may be returned to the fermenter 16 as make up cook water and a second liquid 30 which may be returned to the beer tank 18.

[0015] Scrubbed ethanol 36 leaves the scrubber 26 and flows to the membrane unit 38. The membrane unit 38 will be described in further detail below. In general, the membrane unit 38 produces a product vapour 40 that is nearly water free, for example having 99% or more ethanol by volume. The membrane unit 38 also produces compressed vapour permeate 44 and, optionally, condensed permeate 46. Both permeates 44, 46 have only trace ethanol contents, for example 2% ethanol by volume or less. Condensed permeate 46, if any, may be returned to the fermenter 16 as make up cook water.
For reasons that will be discussed further below, compressed vapour permeate 44 carries heat energy and may be used to heat another part of the process. In Figure 1, for example, the compressed vapour permeate 44 is used in a second reboiler loop 46 to heat the liquids in the bottom of distillation column 22.
Optionally, condensed vapour permeate 44 may be used to replace or further supply heat to reboiler loop 34, beer preheater 20, a stillage dehydrator, a scrubbed ethanol heater 48 or other apparatuses or processes. After transferring its heat energy, compressed vapour permeate 44 may become a liquid, primarily water, and be re-used, for example as make up cook water for fermenter 16.

[0016]
Stillage 50 may be withdrawn from distillation column 22 or optionally from the beer feed to distillation column 22. Stillage 50 may be partially dewatered by mechanical means and then sent through a drying circuit 52. In drying circuit 52, stillage passes through one or more heat exchangers 54.
Heat exchangers 54 use heat from product vapour 40 and suction from pumps 56 to encourage water to evaporate from stillage 50. Pumps 56 also transport the evaporated water until it condenses into reclaimed stillage water 58 which may be used, for example, in fermenter 16. Dried stillage 60 may be, for example, about 30 percent solids by weight or more.

[0017]
Figure 2 shows scrubber 26 in greater detail. Scrubber 26 has a spray tank 80, tank 84, pumps 86, and forward cleaners 82 configured and connected as shown. Scrubber 26 removes particles from the vapours by entraining the particles in water.

[0018]
Various alternate membrane units 38 will be described below with reference to Figures 3 to 6. Each of Figures 3 to 6 show a different example of a membrane unit 38. Other examples of membrane units 38 may be created by combining all or parts of one or more of the examples of Figures 2 to 6. The membrane units 38 have multiple membrane stages 80. Each membrane stage 80 may be a membrane module, a stage in an internally staged module, or a set of modules or internal stages in parallel. Membrane modules may use polymeric membranes, for example of polyimide hollow fibers. A hollow fibre module may be fed to the insides of the hollow fibres. The membranes may be asymmetric integrally skinned polyimide membranes as described, for example, in International Patent Application No. PCT/CA2004/001047. Such membranes can have a vapour permeance for water of 4 x 10-7 mol/m2sPa or more at about 80 C.
The membranes can have a vapour permeance selectivity of 250 or more for water/ethanol at about 140 C. The membrane unit 38 may also have a vapour compressor 82. The vapour compressor 82 compresses permeate gases adiabatically which causes them to rise in temperature. The increased temperature allows the heat energy in the permeate gases to flow to, and heat, lower temperature gases or liquids. Heat energy in the permeate gases, as sensible heat or latent heat of condensation, can then be used as heating steam in other parts of the process. The vapour compressor 82 may be a radial type fan or compressor which provides a compression ratio of, for example, between 1:20 and 1:40. Although the vapour compressor 82 requires energy to turn the compressor, a relatively low compressor output is sufficient to activate a vapour flow carrying a relatively large energy flow.

[0019] Figure 3 shows a two stage membrane unit 38a. Permeate from a first stage 80a is sent to a vapour compressor 82 and used as heating steam for distillation column 22 as described above. Retentate from the first stage 80a becomes feed for a second stage 80b. Permeate from second stage 80b passes through a condenser 84 before being reused as cook water for fermentation as described above.

[0020] Figure 4 shows a second membrane unit 38b having three stages 80a, 80b and 80c. Permeate from these stages 80a, 80b, 80c may have a temperature of about 100 C, but declining downstream, and pressures of about 30-60 kPa (absolute), 5-15 kPa (absolute) and 1.5 to 4 kPa (absolute) respectively. Optionally, the third stage 80c and its permeate flow may be omitted to create a two stage membrane unit. For each downstream unit 80b, 80c, the permeate is collected and passed through a cooler 86 and a vapour compressor 82 before joining the permeate from an adjacent upstream stage 80b upstream of its vapour compressor 82. Cooler 86 may assist in creating a permeate side vacuum to withdraw permeate and also allows the permeate vapour to be compressed to a higher pressure. By recompressing permeate, and recycling it as heating steam, the second membrane unit 38b maximizes energy recovery.
Compressed vapour permeate may have a temperature of 150 C or more and a pressure of 200 kPa (absolute) or more.

[0021] Figure 5 shows a third membrane unit 38c. The third membrane unit 38c combines aspects of the first membrane unit 38a and second membrane unit 38b. Two permeate streams 44, 46 are produced, but the condensed permeate 46 is produced from two downstream stages 80b, 80c connected with recycle and compression of the further downstream permeate to the adjacent upstream permeate as in the first membrane unit 38a. The combined permeate of downstream stages 80b, 80c passes through a condenser 88, and a holding tank 90 and is then recycled to the fermenter 16. The configuration of membrane unit 38c optimizes cost/energy.

[0022] Figure 6 shows a fourth membrane unit 38d. Permeate from first and second stages 80a, 80b is compressed and fed to third stage 80c individually as shown in the solid line or by joining the further downstream permeate to the adjacent upstream permeate before its compressor 82 as shown with the dashed line. Permeate from the third stage 80c is recycled upstream of the heater 48 upstream of the first stage 80a. Permeate vapour from the third stage is compressed and recycled as has been discussed above. In the third membrane unit 38d, the permeate is re-separated which increases ethanol recovery over the previous membrane units 38a, b, c. Compressed vapour permeate 44 may be 0.1% ethanol by volume or less, or essentially steam.
Similarly, the permeate from any one or more stages described in Figures 3 to may be further separated as shown in Figure 6 to improve ethanol recovery.

Claims (13)

1. A process for removing water from a mixture comprising ethanol and water, the process comprising steps of a) distilling the mixture to produce distillate;
b) feeding at least a portion of the distillate to a gas separation membrane;
c) collecting permeate from the gas separation membrane;
d) compressing at least some of the permeate and using heat carried by the permeate to assist in distilling the mixture; and e) removing water from at least some of the distillate before feeding at least some of the distillate to the gas separation membrane, the step of removing water from at least some of the distillate including a step of passing the distillate through a molecular sieve.

2. The process of claim 1 further comprising compressing permeate and recycling it to a fermenter.

3. The process of claim 1 further comprising collecting a product vapour from the membrane and passing it through a heat exchanger.

4. The process of claim 1 wherein the permeate is 2% ethanol by volume or less.

5. The process of claim 1 wherein the mixture is partially de-watered after distilling the mixture and before feeding the distillate to the gas separation membrane.

6. The process of claim 1 wherein distillate for feeding to the gas separation membranes is collected from a purge stream of the molecular sieve.

7. The process of claim 1 wherein the step of distilling the mixture is performed in a single distillation column to an ethanol content of at least about 45%
by weight.

8. A process of removing water from a mixture comprising ethanol vapour and water vapour, the process comprising steps of feeding the mixture through two or more membrane vapour separation stages wherein permeate from one stage is compressed for use as a supply of heat to an upstream distillation unit, and passing the distillate through a molecular sieve to remove water from at least some of the distillate.

9. The process of claim 8 wherein permeate from another stage is condensed and used as a source of water.

10. The process from claim 8 wherein permeate from a downstream stage is compressed and added to permeate from an upstream stage.

11. The process of claim 8 wherein permeate from a first stage is fed to a second stage and permeate from the second stage is compressed for use as a supply of heat.

12. The process of claim 1, wherein step d) comprises compressing the permeate substantially adiabatically to increase the temperature of the permeate.

13. The process of claim 1, wherein step d) comprises compressing the permeate using a compressor having a compression ratio of less than about 1:40.