WO1991011239A1

WO1991011239A1 - High temperature vapor phase dehydration of aqueous alcohol mixtures

Info

Publication number: WO1991011239A1
Application number: PCT/US1991/000457
Authority: WO
Inventors: Michael R. Ladisch
Original assignee: Purdue Research Foundation
Priority date: 1990-01-24
Filing date: 1991-01-23
Publication date: 1991-08-08
Also published as: AU7328091A; BR9104191A

Abstract

Process for the enrichment of aqueous alcohol mixtures comprising contacting the aqueous alcohol mixture while in the vapor state with dried starch having a temperature in excess of about 105 °C under conditions effective for water in the vapor to be adsorbed by the starch and thereby produce an alcohol enriched vapor, and recovering the alcohol enriched vapor.

Description

HIGH TEMPERATURE VAPOR PHASE DEHYDRATION OF AQUEOUS ALCOHOL MIXTURES

BACKGROUND OF THE INVENTION

The present invention is directed to a process for the dehydration of aqueous alcohol mixtures, and in particular, to a process for the enrichment of aqueous alcohol mixtures using starch having a temperature in excess of about 105°C.

Fermentation ethanol has been proposed as a liquid fuel alternative to those which have been historically supplied by petroleum, and as an octane enhancer for gasoline. Such applications typically require the ethanol to contain less than about 1% water.

Alcohols produced from either grains or biomass yield a broth containing 6 to 12 percent ethanol using conventional fermentation technology. Traditionally, ethanol has been recovered from the broth by distillation to its azeotrope (95.57 percent ethanol by weight) followed by distillation using a third component which breaks the azeotrope. However, the energy-efficiency of this process is substantially less than what is desired.

In an effort to provide an energy efficient process for the dehydration of alcohol, it has been proposed to contact the aqueous alcohol mixture in the vapor state with a dehydration agent selected from the 2 group consisting of cellulose, carboxymethylcellulose, cornmeal, cracked corn, corn cobs, wheat straw, bagasse, starch, hemicellulose, wood chips, and other agricultural products at a temperature of approximately 90°C. See e.g., Ladisch et al., U.S. Patent 4,345,973 at col. 2, lines 25-36 and lines 63-66.

Among the dehydration agents proposed, corn grits (particles of corn derived from dry milling) has received considerable interest because it is an inexpensive, renewable resource having favorable sorption and pressure drop characteristics. However, the operational capacity pf corn grits on a volumetric basis is not as great as would be desired. Accordingly, the volume of corn grits necessary to dehydrate alcohol streams requires a substantial capital investment in equipment in which the grits are used. In addition, reconditioning (dehumidification) of the gas used to regenerate the corn grits upon completion of an adsorption cycle typically involves refrigeration and constitutes a substantial operating expense.

As noted above, dehydration of aqueous alcohol mixtures by adsorption onto starch at 90°C has also been proposed. In fact, starch has been demonstrated to possess greater capacity than corn grits at 90°C on a volumetric basis (see, e.g., Hong et al., "Adsorption of Ethanol- ater Mixtures by Biomass Materials," Biotechnology and Bioenαineerinα. Vol. XXIV, pg. 725-730 (1982), and Ladisch et al., "Dehydration of Ethanol: New Approach Gives Positive Energy Balance," Science. Vol. 205, pages 898-900 (August 31, 1979)). Nevertheless, corn grits have been preferred commercially because of the significant pressure drop associated with starch, and because it is perceived that corn grits have greater tolerance than does starch to conditions which cause the adsorbent to gel and thereby lose separation capability.

SUMMARY OF THE INVENTION

Among the objects of the present invention, therefore, is the provision of an energy efficient process for the dehydration of aqueous alcohol mixtures, the provision of such a process in which the aqueous alcohol mixture is contacted with an adsorbent having favorable mass transfer properties, the provision of such a process in which the adsorbent has a greater capacity than corn grits on a volumetric basis, and the provision of such a process in which the adsorbent is resistant to gelatinization.

Briefly, therefore, the present invention is directed to a process for the enrichment of aqueous alcoho mixtures. The process comprises contacting the aqueous alcohol mixture while in the vapor state with starch havin a temperature in excess of about 105°C under conditions effective for water in the vapor to be adsorbed by the starch. The alcohol enriched vapor is then recovered. The present invention is further directed to a process for dehydrating water ethanol mixtures having an ethanol content of between about 6% and 12% by weight. Th process comprises subjecting the mixture to distillation t produce an overhead product having an ethanol concentratio of less than about 95.6% by weight. The overhead product while in the vapor state is then dried by contacting- with starch having a temperature in excess of about 105°C unde conditions effective for water in the vapor to be adsorbe by the starch and thereby produce an ethanol enriched vapor. The ethanol enriched vapor is thereafter recovered. Other objects and features will be in part apparent and in part pointed out hereinafter.

BRIEF DESCRIPTION OF THE DRAWINGS

Fig. 1 is a chromatogram depicting the results o a prior art process outlined in Example 1.

Figs. 2 and 3 are chromatograms depicting the results of the process outlined in Example 2.

Fig. 4 is a chromatogram depicting the results o the process outlined in Example 3 which was carried out in accordance with the present invention. 5

DESCRIPTION OF THE PREFERRED EMBODIMENTS

In accordance with the present invention, it has been discovered that aqueous alcohol mixtures in the vapor state can be efficiently dehydrated using starch at a temperature in excess of about 105°C. Starch at a temperature of in excess of about 105°C has a significant selective operational capacity for water vapor over ethano and can be resistant to gelatinization. In addition, operation of the adsorbent bed at a higher temperature allows a hotter gas with a higher moisture content to be used to dry the starch after each adsorption cycle. • Consequently, the use of chiller water and the costs associated therewith to obtain a regenerating gas with the desired relative humidity may be minimized. The process of the present invention may be used to dehydrate alcohols, and ethanol in particular. Water/ethanol mixtures having an ethanol concentration les than about 95.6% by weight may thus be conveniently and efficiently enriched to produce ethanol having less than about 4% water, and most preferably less than 1% water.

Various types of starch can be used in accordanc with the present invention, including corn, potato, rice, wheat, sorghum, tapioca, and others known to those of ordinary skill in the art. Corn starch is preferred. The starch may be isolated in granular form fro its plant source by either wet or dry milling, preferably wet milling. Once isolated, the starch is a dry, soft. usually white powder. Starch is insoluble in cold water, alcohol and most organic solvents and is stable in storage for indefinite periods of time, if kept dry. Though the starch granules are physically strong, they begin to imbibe water if held in a water suspension and heated. Under such conditions, the granules increase in size and the suspension increases in viscosity and eventually forms a paste; this process is referred to as gelatinization. The gelatinization temperature of a starch is dependent upon such factors as starch concentration, pH of the suspension, rate of heating, and the presence of certain salts.

In accordance with the present invention, the starch may be in various forms. For instance, it may be in the form of granules isolated by the milling process. Corn starch granules vary in diameter from 5-25 microns, rice starch granules vary from 3-8 microns in diameter, potato starch granules vary from 15-100 microns, while tapioca starch granules vary in diameter from 5-35 microns. Alternatively, the starch granules may be agglomerated into spherical, cylindrical or other larger size particles.

The starch may be provided in its native state, or it may be provided in a pregelatinized state. Pregelatinized starch is starch that has been gelatinized and dried so that it forms a gel readily without heat on contact with a sufficient quantity of water. In addition, the starch may be chemically modified in any of a number of ways known in the art. 7

The size of the starch adsorbent effects both th mass transfer and pressure characteristics of the system. In general, a reduction in size of the starch absorbent results in an increase in the mass transfer characteristics and pressure drop of the system whereas an increase in siz of the starch absorbent results in a decrease in mass transfer characteristics and pressure drop of the system. Nevertheless, starch granules agglomerated into larger particles retain the sorption character of starch and have the pressure drop advantages of corn grits.

In accordance with the present invention, the• starch has a temperature of at least about 105°C for adsorption. The upper limit is the temperature at which the starch begins to undergo thermally induced chemical degradation and the optimal range in any one adsorption process is achieved through a trade-off between the cost o reconditioning the regeneration gas which decreases as the adsorption temperature increases and the loading capacity of the starch which decreases as the adsorption temperature increases. Presently, it is preferred that the starch be at a temperature between about 105°C and 170°C for adsorption.

To dehydrate aqueous alcohol mixtures, a vapor o the mixture is contacted with the starch under conditions effective for the starch to selectively adsorb the water 8 vapor. The vapor may be the overhead product from the top of a distillation column as discussed in Ladisch, U.S. Patent 4,345,973 col. 3, lines 35-62. Alternatively, it may be produced by passing an inert carrier gas stream through an aqueous alcohol mixture whereby a portion of the mixture is carried by the gas. Care must be taken to avoid the introduction of liquid water to the starch which will cause the starch to gel into a solid mass and thereby lose its sorption capacity. In any event, the aqueous alcohol vapor preferably has a temperature approximately equal to that of the starch at the start of the adsorption cycle, i.e., at least about 105°C.

The aqueous alcohol vapors are contacted with the starch until shortly before breakthrough of the water vapor. Thereafter, the starch is regenerated using a gas having a relative humidity less than 1 to desorb the water from the starch. The regeneration gas preferably has a temperature equal to the desired temperature of the starch at the start of the adsorption cycle and may be introduced to the starch countercurrent to that of the aqueous alcohol vapor.

Because of its relatively high operational capacity for water on a volumetric basis, adsorption columns designed for starch at temperatures in excess of about 105°C may be of size smaller than adsorption columns conventionally designed for corn grits at 80-90°C. The following examples illustrate the invention.

EXAMPLE 1 (Prior Art) A column (standard 1/4 inch o.d. x 10 cm long stainless steel tubing having a nominal inside diameter of 4 mm and a volume of 1.57 mL) was packed with corn grits, i.e., particles of corn derived from dry milling which wer screened to an average particle size of about 2.2 mm). Th grits had a composition of about 12% to 14% moisture, 8% protein, 0.5% to 1% fat, and a balance of starch. The grits were added while the column was gently tapped to settle the material. The materials were packed as shipped, without any further screening.

After packing, the column was connected to a standard Carle GC, heated and then dried at a helium flow rate of 20 to 23 mL/min. This corresponds to a gas superficial velocity of 128 to 147 cm/min (calculated base on empty column cross-sectional area, r=0.223 cm). The helium flow was started as soon as the column was placed in the oven; hence, drying and heating to 85°C were initiated simultaneously. The column was dried for a minimum of 2 hours to periods lasting overnight. Heating the column from one temperature to the next required about two hours. The separation characteristics of the corn grits was determined at a temperature of 85°C. The carrier gas, helium, containing essentially no moisture was passed 10 through the GC column at a flow rate of 20 to 23 mL/min. Gas flow rate was measured at the column outlet using a standard GC bubble meter at room temperature and atmospheric pressure. Separation characteristics were, based on injection of 0.5 microliter of 190 proof ethanol. The ethanol peak emerged almost immediately (0.09 to 0.12 minutes) while the water peak^"followed. Although the water concentration was only 7.6%, by weight (5% by volume), of the sample, the water peak was broader and larger than the ethanol peak. This was partly due to the water sorption characteristics, as well as the difference of the thermal conductivity detector's ("TCD") response of water relative to ethanol. The TCD detector was set at an attenuation of lx for these runs. The resulting chromatograms were recorded on an HP integrator and a representative chromatogram is presented in Figure 1..

EXAMPLE 2 The procedures of Example 1 were repeated except that the separation characteristics of the corn grits were determined at temperatures of 105, 140-145 and 170°C. Representative chromatograms are presented in Figures 2 and 3.

The corn grits separated water from ethanol at 85 and 105°C (see Figures 1 and 2), but there was significant overlap between the ethanol and water peaks. Duplicate chromatograms were run at each condition, with a typical result shown in Figures 2(a) and 2(b). Run to run variability in water peak retention was found to range fro 7% to 16%, within a duplicate set of runs. The variabilit appeared to be less at the lower temperatures and then increased with increasing temperature. Separation betwee ethanol and water is lost at 145°C and 170°C (see Figure 3), for the corn grits. The broad peaks in Figures 3(a) and (b) are due to the ethanol and water peaks overlapping. When ethanol and water separates, the first peak, ethanol, is much more narrow than the combined peak (see Figures 1 and 2).

EXAMPLE 3 The procedures of Examples 1 and 2 were repeated except that pregelatinized corn starch granules (-20 mesh particle size range between about 0.05 mm and 0.8 mm; average particle size of about 0.2 mm) were used instead o corn grits. Unlike the corn grits, the pregelatinized starch granules separated ethanol/water at 145 and 170°C (compare Figures 3 and 4). In addition, the separation achieved at 105°C with the pregelatinized starch is much superior to that of grits at 85°C (compare Figure 4(c) to Figures 1 and 2). At 85°C, the pregelatinized starch has such strong adsorption, that a water peak is difficult to detect. EXAMPLE 4 The procedures of Examples 1 and 2 were repeated, except that agglomerated particles made from the pregelatinized corn starch granules of Example 3 were substituted for the corn grits. The particles were made by compressing the starch powder of Example 3 to ultimately give particles of an average size of about 1.25 mm. For comparison purposes, the results for the agglomerated particles, the pregelatinized granules (Example 3) and the corn grits (Example 1) are presented in Table I. The agglomerated particles gave similar results to starch granules and both have significantly greater capacity factors than corn grits at the 3 temperatures.

The capacity factor is for water, k^' _wat_er, ^^s:

_φ t_r - t₀' ^k water^5* T^~" (-¹-)

•-o where t_r is the retention time (peak maximum) of the water while t_Q ^' is usually the retention time of an excluded component. The excluded component in this case is assumed to be ethanol. At a retention time of 0.09 minutes, the elution volume is 2 mL. The total column volume is 1.56 mL. Hence, some ethanol retention occurred, although at the conditions used the retention appears to be small. As a first approximation, we assumed that the ethanol retention is equivalent to the retention of an excluded and non-adsorbed component which is an approximation. We have chosen to refer to the ethanol retention time as t_n ' . Comparison of the data show that the capacity factor for water decreases with increasing temperature by about 10-fold or more between 105 and 170°C (see Table I). In addition, the capacity factor is 5-fold higher for pregelatinized starch granules and agglomerated starch particles at 105°C (Table I) than it is for corn grits. The data indicate that dramatic improvements in operational sorption capacity could result from properly formulated starch sorbents. This is further indicated by plate height

10 estimates (based on water) summarized in Table II. The plate count, N, and plate height, h, is:

where

h - — (3)

N

20 where t_r is peak retention of water, t_w is peak width at the base, and L is column length.

The higher the value of the plate count (or the lower the plate height), the less is the dispersion and the sharper the break through curve. In this respect the

25 pregelatinized starch granules have the lowest plate height with the agglomerated particles being comparable (Table II) TABLE I Comparison of Capacity Factors** water'^'

Packing Wgt^Λ Density^x Packed^x ^UP (Capacity factor) at

Sorbent (q/ml) (JU (mm) 85°C 105°C 145°C 170°C

Agglomerated Starch 0.71 1.12 (1.26) ND⁺ (339?)⁺ 69 22 Starch Granules 0.68 1.06 (0.14) ND⁺ 366 95 33 Corn Grits (8/10) 0.63 0.99 2.2 ND⁺ 65 >0* >0*

10 ++ Based on sample volume of 0.5 μl of 190 proof ethanol at a He flow of 23 mL/min (v=147 cm/min, superficial velocity) . t k'_M.ι._pr = (t_r - t_n') where t_r is retention time for water and t,-.' is retention time for ethanol.

X Measured at room temperature and moisture content.

15 * Ethanol and water peaks overlap almost completely. Water sorption is small.

Water peak difficult or impossible to quantify.

TABLE II Estimate of Plate Heights for Water

Plate Height (cm) at

145°C 170°C

1.9 2.0 1.2 3.6

NA^ NA^"1

⁺ NA = not available—width of water peak could not be measured.

EXAMPLE 5 This set of runs were carried out in a 6-foot long x 4 mm i.d. (0.25-inch o.d., volume = 23 ml) column packed with 15 gm of wet-milled, native corn starch sold under the trade designations 16F-0089, and S-4126 by Sigm Chemical Company (St. Louis, MO). The starch granules ha a size on the order of 10 microns. The column was place in a Carle GC, heated to 105°C and conditioned (i.e, drie overnight with a helium carrier gas (105°C) . Before packing, the starch was washed with deionized water and then dried at 20°C to give a flowing powder.

A sample of ethanol (equivalent to 0.2 ml liqui was injected directly onto the column, and a peak eluted 2.6 min at a gas flow rate of 14.4 ml/min. Gas flow was measured in a bubble meter at room temperature. Several injections were made and in each case, the peak exhibite tailing. This indicates that some adsorption of ethanol may have occurred. When water was injected, an eluting peak was not detected even after 24 hours.

EXAMPLE 6 The procedures of Example 5 were repeated, except that the column of Example 5 was shortened by a factor of 18 to 10 cm. The bed was retained with 5 micron end-fittings. The column volume was 1.26 ml. A free volume of 1.8 ml starch (3.5 g dry weight) was packed into the column. The column was conditioned at 105°C, as before. This time the water peak was detected. Injection of a 10 μl sample of water gave a broad, flat peak which started to elute after 2 hours. The peak was hardly detectable. When ethanol was injected, the peak eluted almost immediately. As a consequence of these results, the column was shortened even further.

EXAMPLE 7 The 10 cm column in Example 6 was shortened to 4 cm. This time, injection of 5 μl of a 50/50 (volume) mixture of ethanol/water resulted in an ethanol peak which eluted in seconds and water peak which eluted at about 41.4 minutes (measured at peak max.). The 10cm. column was then used to measure retention times of the additional components.

1. nitrogen;

2. methanol; 3. t-butanol; and 4. isopropanol.

In all cases, elution occurs almost immediately after injection. These results confirm that water can be separated from other alcohols in addition to ethanol. In view of the above, it will be seen that the several objects of the invention are achieved and other- advantageous results attained.

As various changes could be made in the above processes and products without departing from the scope of the invention, it is intended that all matter contained i the above description shall be interpreted as illustrativ and not in a limiting sense.

Claims

WHAT IS CLAIMED IS:

1. A process for the enrichment of aqueous alcohol mixtures comprising contacting the aqueous alcohol mixture while in the vapor state with dried starch having a temperature in excess of about 105°C under conditions effective for water in the vapor to be adsorbed by the starch and thereby produce an alcohol enriched vapor, and recovering the alcohol enriched vapor.

2. The process of claim 1 wherein the alcohol is ethanol.

3. The process of claim 1 wherein the aqueous alcohol mixture contains less than 95.6% by weight ethanol.

4. The process of claim 1 wherein the aqueous alcohol mixture is a fermentation product.

5. The process of claim 1 wherein the starch temperature is less than about 170°C.

6. The process of claim 1 wherein the starch is selected from the group consisting of corn, wheat, potato, rice, sorghum and tapioca starches.

7. The process of claim 1 wherein the starch i corn starch.

8. The process of claim 1 wherein the vapor is produced by heating the alcohol water mixture.

9. The process of claim 1 wherein the vapor is produced by passing an inert carrier gas through an aqueou mixture.

10. The process of claim 1 wherein the starch has a particle size of between about 5 and 25 microns.

11. A process for dehydrating a water ethanol mixture having an initial ethanol concentration of betwee about 6 and about 12% by weight comprising subjecting the mixture to distillation to produc an overhead product having an ethanol concentration of le than about 95.6% by weight, contacting the overhead product while in the vapor state with dried starch having a temperature in excess of about 105°C under conditions effective for water in the vapor to be absorbed by the starch and thereby produce an ethanol enriched vapor, and recovering the ethanol enriched vapor.

12. The process of claim 11 wherein said ethanol enriched vapor has an ethanol concentration of less than about 1% by weight.

13. The process of claim 11 wherein the starch temperature is less than about 170°C.

14. The process of claim 11 wherein the starch is selected from the group consisting of corn, wheat, potato, rice, sorghum and tapioca starches.

15. The process of claim 11 wherein the starch is corn starch.

16. The process of claim 11 wherein the starch has a particle size of between about 5 and 25 microns.