Apparatus and methods for removing solvent residues

Details Apparatus and methods for removing solvent residues Apparatus and methods for removing solvent residues

2002-02-27

2004-01-13

Drodge, Joseph (Department: 1723)

Liquid purification or separation

Processes

Liquid/liquid solvent or colloidal extraction or diffusing...

C210S143000, C210S175000, C210S511000, C210S634000, C210S739000, C210S774000, C210S805000, C426S425000, C554S008000

Reexamination Certificate

active

06676838

ABSTRACT:

This invention concerns an apparatus and methods for removing solvent residues in particular after “extraction” of biomass. Biomass extraction is the extraction of flavours, fragrances or pharmaceutically active ingredients from materials of natural origin (these materials being referred to as “biomass”).
BACKGROUND OF THE INVENTION
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.
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.
By the term “hydrofluorocarbon” we are referring to materials which contain carbon, hydrogen and fluorine atoms only and which are thus chlorine-free.
Preferred hydrofluorocarbons are the hydrofluoroalkanes and particularly the C
1-4
hydrofluoroalkanes. Suitable examples of C
1-4
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), 1,1,1,2,3,3-hexafluoropropane (R-236ea), heptafluoropropanes and particularly 1,1,1,2,3,3,3-heptafluoropropane (R-227ea), 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.
R-134a, R-227ea, R-32, R-125, R-245ca and R-245fa are preferred.
An especially preferred hydrofluorocarbons for use in the present invention is 1,1,1,2-tetrafluoroethane (R-134a).
It is possible to carry out biomass extraction using other solvents such as chlorofluorocarbons (“CFC's”) and hydrochlorofluorocarbons (“HCFC's”), and/or mixtures of solvents. CFC's and HCFC's are not approved for food use and consequently are rarely employed in extraction processes in which the depleted biomass residue is intended as e.g. an animal feed.
Known extraction processes using solvents are normally carried out in closed-loop extraction equipment. A typical example 10 of such a system is shown schematically in FIG.
1
.
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 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
15
or returned (line
16
) directly to the extraction vessel
11
to complete the circuit.
One of the key areas of concern relating to the use of solvents such as are used in biomass extraction processes is the level of residual solvent on the biomass material after extraction is complete. High levels of residual HFC (or other) solvent may be regarded as undesirable from a number of aspects:
loss of HFC to atmosphere;
loss of HFC from the recycle process potentially increasing top-up costs;
landfill, incineration, composting and other biomass disposal regulatory issues; and
suitability of depleted biomass for use as an animal feed supplement.
When an extraction is completed it is customary to allow as much as possible of the solvent to drain from the extraction vessel
11
to the solvent evaporator, whether by gravity or by imposed pressure difference between the extractor
11
and evaporator
12
. This does not however guarantee the complete removal of liquid solvent: some may be trapped in the voids of the extractor bed and some will be chemically adsorbed onto the surface of the biomass.
Any liquid solvent which is not removed from the biomass prior to the vessel being opened for cleaning will obviously be lost from the system. This is undesirable from a financial viewpoint. It also means that the solvent is emitted to the environment, either by direct vaporisation on opening, or by slow desorption and vaporisation from the biomass which is also undesirable. If the biomass is disposed of by incineration, and the solvent contains fluorine, this may attract an additional cost penalty for incineration because most commercial incineration plant can only handle fluorine-containing wastes by blending with other material.
In order to improve the rate of solvent extraction, the biomass is usually chopped or ground in some manner in order to increase the surface area in contact with the extraction solvent. Whilst beneficially increasing the rate of extraction of the desired components during biomass extraction, this increased surface area acts to increase the quantity of solvent that can remain adsorbed onto and in the biomass after extraction. Clearly some cost-effective method of achieving acceptable residual HFC solvent levels in the exhausted biomass would be of significant value in the development of the technology.
One way to remove residual solvent from a bed of biomass is to supply heat. This forces evaporation/desorption and therefore allows substantial removal of the residue. This can be accomplished conventionally by admission of heated air or steam, or by supply of external heat to the walls of the packed bed extractor vessel
11
.
Methods involving heat supply are however not very efficient for the following reasons.
In the case that hot air is used to remove the solvent, the solvent is driven off as a low concentration of condensable vapour in an inert air vent. The recovery of solvent from such vents is a well known problem in the chemical process industry: owing to the boiling points of most volatile solvents the refrigeration equipment required to condense and recover solvent from such vents is rarely economically justified at small scale, yet most extraction equipment for the flavour and fragrance industry is at relatively small scale. Thus although the biomass is cleaned of solvent, this solvent is still lost to the user and still represents an emission to the environment.
When steam is used as a heat source, recovery of solvent is possible: in this case the steam would first be condensed, leaving a vent of mostly solvent, then a second refrigerated condenser could be used to condense the solvent vent. However this too is potentially costly in terms of additional equipment. Furthermore the condensed steam will be contaminated with solvent, the condensed solvent will be contaminated with water, and there will probably still be losses of solvent to the atmosphere.
If heat is supplied to the external wall of the extractor as the final stage of the extraction process, then some vaporisation of solvent can be expected. In this case any solvent vaporised will be recovered in the normal way and thus is not lost from the system. The problem with this approach is that the thermal conductivity of a packed bed of material (biomass and solvent) which is substantially free of liquid will be very low—approaching the thermal conductivity of the solvent vapour. This means that the rate at which heat can be transmitted radially into the vessel will be low and consequently the time taken to raise the whole bed temperature to a level which is effective for desorption will be large. This time represents a loss of potential proc

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