Apparatus and method for reducing residual solvent levels

Abstract

A method of reducing residual solvent levels in extracted biomass that is contaminated with HFC solvent includes sparging the biomass extract with a solvent stripping gas such as air or Nitrogen.

Description

[0001] This invention concerns apparatuses and a method for reducing residual solvent levels, especially following “extraction” of biomass. This is the extraction of flavours, fragrances or pharmaceutically active ingredients from materials of natural origin (these materials being referred to as “biomass” in the body of this text).

[0002] Examples of biomass materials include but are not limited to flavoursome or aromatic substances such as coriander, cloves, star anise, coffee, orange juice, fennel seeds, cumin, ginger and other kinds of bark, leaves, flowers, fruit, roots, rhizomes and seeds. Biomass may also be extracted in the form of biologically active substances such as pesticides and pharmaceutically active substances or precursors thereto, obtainable e.g. from plant material, a cell culture or a fermentation broth.

[0003] There is growing technical and commercial interest in using near-critical solvents in such extraction processes. Examples of such solvents include liquefied carbon dioxide or, of particular interest, a family of chlorine-free solvents based on organic hydrofluorocarbon (HFC) species.

[0004] By the term “hydrofluorocarbon” we are referring to materials which contain carbon, hydrogen and fluorine atoms only and which are thus chlorine-free.

[0005] Preferred hydrofluorocarbons are the hydrofluoroalkanes and particularly the C1-4 hydrofluoroalkanes. Suitable examples of C1-4 hydrofluoroalkanes which may be used as solvents include, inter alia, trifluoromethane (R-23), fluoromethane (R-41), difluoromethane (R-32), pentafluoroethane (R-125), 1,1,1-trifluoroethane (R-143a), 1,1,2,2-tetrafluoroethane (R-134), 1,1,1,2-tetrafluoroethane (R-134a), 1,1-difluoroethane (R-152a), heptrafluoropropanes and particularly 1,1,1,2,3,3,-heptafluoropropane (R-227ea), 1,1,1,2,3,3-hexafluoropropane (R-236ea), 1,1,1,2,2,3-hexafluoropropane (R-236cb), 1,1,1,3,3,3-hexafluoropropane (R-236fa), 1,1,1,3,3-pentafluoropropane (R-245fa), 1,1,2,2,3-pentafluoropropane (R-245ca), 1,1,1,2,3-pentafluoropropane (R-245eb), 1,1,2,3,3-pentafluoropropane (R-245ea) and 1,1,1,3,3-pentafluorobutane (R-365mfc). Mixtures of two or more hydrofluorocarbons may be used if desired.

[0006] R-134a, R-227ea, R-32, R-125, R-245ca and R-245fa are preferred.

[0007] An especially preferred hydrofluorocarbon for use in the present invention is 1,1,1,2-tetrafluoroethane (R-134a).

[0008] It is possible to carry out biomass extraction using other solvents, such as chlorofluorocarbons (“CFC's”) or hydrochlorofluorcarbons (“HCFC's”) and/or mixtures of solvents.

[0009] Known extraction processes using these solvents are normally carried out in closed-loop extraction equipment. A typical example 10 of such a system is shown schematically in FIG. 1.

[0010] In this typical system, liquefied solvent is allowed to percolate by gravity in downflow through a bed of biomass held in vessel 11. Thence it flows to evaporator 12 where the volatile solvent vapour is vaporised by heat exchange with a hot fluid. The vapour from evaporator 12 is then compressed by compressor 13. The compressed vapour is next fed to a condenser 14 where it is liquefied by heat exchange with a cold fluid. The liquefied solvent is then optionally collected in intermediate storage vessel (receiver) 15 or returned directly to the extraction vessel 1 to complete the circuit.

[0011] The extraction of flavours, fragrances or pharmaceutically active components from materials of natural origin using chlorine-free solvents based on HFC's and other solvents as noted is of growing technical and commercial interest. One reason for use of the closed loop extraction circuit of FIG. 1 is to avoid the undesirable release of HFC or other solvents to atmosphere.

[0012] Residual solvent levels in extracts to be used in foodstuffs are governed by legislation in many countries. The levels of residual solvents in extracts for other applications are generally less strictly defined. Even where legislation does not dictate a specific level of solvent residue, it is generally desirable to minimise the residue levels where economic and practicable to do so. In the prior art, residual solvent residues have been reduced through a combination of heating and evacuation, or in the case of CO2 solvents by relatively mild evacuation alone. Unfortunately, the process of heating and evacuation often has a detrimental effect on the aroma of the finished extract, many of the volatile “top note” components having been removed along with the solvent.

[0013] One of the key benefits of using a solvent such as 1,1,1,2-tetrafluoroethane (R-134a) for extraction of materials of biological origin is its ability to capture the relatively volatile flavour and fragrance components. In order to maintain the high quality of the extracts obtained with R-134a, a method of reducing the residual solvent levels that does not impact significantly on the organoleptic behaviour of the extract is required.

[0014] According to a first aspect of the invention there is provided apparatus as defined in claim 1.

[0015] According to a second aspect of the invention there is provided a method as defined in claim 5.

[0016] Preferred features of the apparatus and method are set out in the dependent claims.

[0017] There now follows a description of preferred embodiments of the invention, by way of non-limiting example, with reference being made to the accompanying drawings in which:

[0018] FIG. 1 is a schematic representation of a prior art biomass extraction plant;

[0019] FIG. 2 shows part of the FIG. 1 plant modified to include features in accordance with the invention;

[0020] FIG. 3 shows an alternative vessel, according to the invention, in which the invention may be practised; and

[0021] FIGS. 4 and 5 are art graphs illustrating the results of comparative experiments including the method of the invention.

[0022] FIG. 2 shows part of the FIG. 1 circuit, modified in accordance with the invention. In FIG. 2 components having the same function as their FIG. 1 counterparts have like reference numerals and are not described again in detail.

[0023] The solvent vapour delivery line 12a exiting the upper part of the evaporator 12 is switchably connectable, by means of flow control valve 29, to compressor 13 (when the apparatus is in use to extract biomass); to a vacuum (for purging evaporator 12 at the end of the extraction process) or to a sparging gas disposal line 31.

[0024] Flow control valve 29 is controllable e.g. manually or by means of a computer.

[0025] FIG. 2 shows a sparging wand 27 that is immersible into the biomass extract liquor 26 in evaporator 12 that, during and immediately after extraction, is contaminated with HFC solvent.

[0026] Wand 27 is a hollow elongate member having one or more apertures therein. Wand 27 is open at one end that is connected to a supply line 27a of pressurised sparging gas such as air or nitrogen.

[0027] Wand 27 may be inserted into evaporator 27 e.g. via an access plate or hatch and submerged under the surface of the extract 26. When sparging gas flows through line 27a and into the hollow interior 27 it exits via the apertures in wand 27 and perfuses through liquor 26.

[0028] An alternative arrangement is shown in FIG. 3, in which the sparging occurs in a hollow vessel 25 remote from evaporator 12. In this case a wand 27 could be used in the same way as in FIG. 2. Alternatively, as shown, a sparging gas (air or N2) supply 30 may be permanently connected via a port beneath the surface of liquor 26 in the vessel 30 or evaporator 12.

[0029] The proposed method of residual HFC solvent reduction is as follows. After the extraction process is complete, the collected extract is subject to mild evacuation, e.g. by means of the vacuum connection in FIG. 2, to remove the bulk of the solvent for recovery. At this stage the residual solvent level is likely to be of the order of 10000 ppm. The sparging gas is then introduced into the extract e.g. using the wand 27 and sparging is continued at a rate and period sufficient to achieve the desired level of solvent residue, typically of the order of 1-100 ppm.

[0030] During sparging the evaporator or vessel outlet may be connected to sparging gas disposal line 31.

[0031] This process may be conducted within the evaporator/collector 13 of the extraction equipment (FIG. 2 method); in a separate vessel (FIG. 3 method) or storage container 25 for the extract; or in a countercurrent stripping column where the (liquid) extract is passed over a packing material in contact with the sparging gas. While air can be used as the sparging gas to good effect, the use of nitrogen is beneficial in that it provides an inert atmosphere over the extract thus improving storage stability, especially when sparging is conducted in the extract storage vessel itself. When the extract is in the form of a low melting solid or paste, the extract may beneficially be warmed before sparging, to generate a liquid.

[0032] The efficiency of the method of the invention is illustrated by the following non-limiting example:

EXAMPLE

[0033] The samples were subjected to a combination of heat (up to 4° C.) and vacuum for known periods of time as well as nitrogen sparging. Graphs were plotted which indicated the rate of removal of 1,1,1,2-tetrafluoroethane (“R-134a”).

[0034] Removal methods used;

Conditions
a Before
b 10 mins @ 40° C.
c 10 mins vacuum @ RT
d 10 mins vacuum @ 40° C.
e 20 mins vacuum @ 40° C.
f N2 sparge 10 mins @ RT
g N2 sparge 20 mins @ RT

[0035] In order to determine the amount of residual R-134a the extract (0.5 g) was weighted into a 30 mil Hypo-seal vial and crimp sealed with a PTFE coated silicone rubber septum. The vial was heated to 80° C. for 30 mins to liberate the R-134a from the extract. The vial was removed from the oven and pressurised with 20 mls of air from a syringe. The syringe was then allowed to refill and immediately injected onto the GC via a gas sample valve. The GC had been previously calibrated.

[0036] The results were then calculated in % (or ppm) w/w R-134a/weight of biomass or extract. The limit of detection was around 50 ppm w/w R-134a in the extract.

[0037] FIGS. 4 and 5 show the results of the treatments.

DISCUSSION

[0038] For the star anise extract used in this experiment the use of vacuum and heat (40° C.) will remove the relatively large quantities of residual R-134a in a relatively short space of time (i.e. 2700 to 100 ppm w/w in less than 20 mins) but with prolonged time required for further reduction. Similar results are expected from other extracts.

[0039] It can be seen from FIG. 5 that the most effective means of removal of R-134a is the nitrogen sparge method (10,000 to <1 ppm w/w in 10 mins). This method also has the added advantage of not requiring any heat treatment of the extract. In addition nitrogen sparging provides an inert atmosphere for further storage.

Claims

1. Apparatus for reducing residual solvent levels in extracted biomass, comprising a vessel for containing liquid biomass extract contaminated with solvent; and a gas supply for sparging biomass extract in the vessel with a solvent stripping vapour.

2. Apparatus according to claim 1 wherein the gas supply includes a hollow wand having a gas outlet and being connected to a source of pressurised solvent stripping vapour, whereby the solvent stripping vapour flows from the source through the hollow wand to exit via the gas outlet, the gas outlet of the wand being moveably immersed in the liquid biomass extract to permit sparging thereof by the solvent stripping vapour.

3. Apparatus according to claim 1 wherein the gas supply includes a gas inlet rigidly secured to supply solvent stripping vapour, through a wall of the vessel, to sparge any biomass extract in the vessel.

4. Apparatus according to claim 1 wherein the gas supply supplies air and/or Nitrogen for sparging of biomass extract.

5. A method of reducing solvent levels in extracted biomass, comprising sparging liquid biomass extract, contaminated with solvent, with a solvent stripping vapour.

6. A method according to claim 5 wherein the solvent stripping vapour is air or Nitrogen.

7. Biomass extract obtained by the method of claim 5 or claim 6.