GB2353732A - Apparatus and method for separating mixed liquids - Google Patents

Abstract

In a biomass extraction circuit there is a need for an improved evaporator. An evaporator (12) includes an inlet for a liquid solvent/biomass mixture, the inlet including a spray nozzle (27). The evaporator (12) includes a heated vessel wall (20a, 20b). When the solvent/biomass mixture flows through the nozzle (27), the pressure drop vaporises some of the solvent. The heated walls (20a, 20b) vaporise the remainder of the solvent. The solvent vapour vents from the evaporator 12 for recycling. Liquid biomass extract collects in the base of the vessel for draining and subsequent use.

Description

2353732 APPARATUS AND METHOD FOR SEPARATING MIXED LIQUIDS This invention

concerns apparatus and a method for separating mixed liquids, especially liquids of differing volatilities. Such mixtures arise e.g. in processes (described in exemplary fashion below) for extracting "biomass". A typical intermediate product in such processes is a mixture of a solvent such as a hydrofluorocarbon ("HFW) (e.g. 1,1,1,2 tetrafluoroethane); a chlorofluorocarbon ("CFC'); or a hydrocholorofluorocarbon ("HCFW), together with a waxy or oily liquid extracted from naturally occurring matter known as biomass.

By the term "hydrofluorocarbon" we are referring to materials which contain carbon, hydrogen and fluorine atoms only and which are thus 1.5 chlorine-free.

Preferred hydrofluorocarbons are the hydrofluoroalkanes and particularly theCl-4hydrofluoroalkanes. Suitable examplesOf Cl-4hydrofluoroalkanes which may be used as solvents include, inter alia, trifluoromethane (R 23), fluoromethane (R41), 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 -dilluoroethane (R-152a), heptafluoropropanes 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-hexafiuoropropane (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 365mfe). Mixtures of two or more hydrofluorocarbons may be used if desired.

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

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

Biomass extraction 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).

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, curnin, 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, such as those mentioned above, or e.g. liquified carbon dioxide, in such extraction processes.

It is possible to carry out biomass extraction using mixtures of solvents.

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 Figure 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 2 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 5 liquefied solvent is then optionally collected in intermediate storage vessel (receiver) 15 or returned directly to the extraction vessel 11 to complete the circuit.

A feature of this process is that the principal driving force for circulation 10 of solvent through the biomass and around the system is the difference in pressure between the condenser/storage vessel and the evaporator. This difference in pressure is generated by the compressor. Thus to increase the solvent circulation rate through the biomass it is necessary to increase this pressure difference, requiring a larger and more powerful is compressor.

The large difference in solvent liquid and vapour densities means that a modest increase in liquid circulation rate can require significant additional capital and operating cost because of this increase in compressor size.

This means that the system designer has to compromise between the rate at which liquid can be made to flow through the biomass and the rate at which vapour can be compressed.

In such extraction systems the material being extracted is typically an involatile liquid or waxy solid and so the solvent evaporator serves the dual purpose of both separating the solvent from the product by distillation and of acting as a storage location for the product. The conventional art design is typically a jacketed pressure vessel with solvent and product forming a pool of liquid or slurry. Heat is supplied through the vessel walls to vaporise the solvent and the heat transfer regime is that of pool 3 boiling. Liquid feed is introduced through a dip-pipe.

This arrangement is straightforward to fabricate but can give rise to a number of problems.

Firstly, because the solvent and product form an intimate mixture the system can be considered close to thermodynamic equilibrium. This means that as the amount of product in the vessel increases through the progress of the extraction, the concentration of product in the evaporator's liquid pool steadily rises. Most extract materials are of very low volatility compared to the solvent and this means that, if the evaporation temperature is fixed e.g. by the temperature of a heat source, then the vaporisation pressure must fall with time.

A decrease in vapour pressure will reduce the vapour density and therefore mass throughput of the compressor. If this reduction in density is not counteracted by increasing the evaporation temperature then the overall solvent circulation rate around the system (which is determined by compressor mass throughput) must then drop.

The evaporation temperature attained in the evaporator will be constrained by the temperature at which heat is supplied. If, as is normally the case, some form of heat integration (heat recovery) is employed to recycle heat from the condenser to the evaporator, then there will be a definite upper limit to the temperature of heat supply. This when combined with the thermal resistances in the evaporator will limit the ability to raise the evaporator temperature to offset changes in evaporation pressure.

In addition there will be a maximum temperature difference which can be 30 applied to the evaporator vessel 12, beyond which the boiling regime will 4 change from pool boiling to so-called film boiling, the characteristic of which is greatly reduced heat flux when compared to the pool boiling regime. Thus any attempts to compensate for changes in boiling pressure are greatly restricted by the properties of the equipment and the 5 thermodynamic properties of the solvent/extract fluid system.

Secondly, it is known that the addition of a second, involatile component to a boiling fluid can have a detrimental effect on the rate of heat transfer (normally expressed as a reduction in heat transfer coefficient). This arises from the introduction of so-called mass transfer resistance in addition to the thermal resistances encountered in boiling of a pure fluid. Therefore as the quantity of extract retained in the evaporator rises during the extraction, there is an increased likelihood of impaired heat transfer. This can cause reduced solvent circulation rates, as described hereinabove.

The above two problems are both of concern because they will cause a need for continual adjustment of overall solvent circulation rates (to balance output of the evaporator with demand of the compressor) and will tend to increase the time required to extract a material.

There is a third problem which arises from the known design, again related to the thermodynamic properties of the system. Once the extraction is substantially complete it is necessary to vaporise the residual solvent in the extractor vessel, evaporator and connecting pipework so as to allow recovery of as much of the solvent as possible. If this vaporisation takes place purely from a pool of boiling liquid mixed with product then as the concentration of product extract rises some of the product must vaporise. This may admittedly be at quite a low concentration in the vapour but it will still contaminate the solvent. 5 This loss of valuable product into the solvent system means that additional recovery and cleaning of the solvent will be required before it can be reused. This will be costly both in equipment required and in process 5 downtime.

An additional problem may arise with some extract materials when vaporised in this way. Many flavour and fragrance extracts are mixtures of several molecules: thus it is possible to lose some of the characteristic components of the essence and thereby change its worth if a portion is allowed to vaporise into the solvent system as outlined above. This is also undesirable.

To summarise:

0 The use of pool boiling vaporisers for solvent/extract separation is the standard art technology used in closed-loop solvent extraction processes as outlined above.

Such vaporisers do not separate solvent from product material and so give risks of: poor heat transfer leading to extended cycle time; thermal degradation of product; possible partial alteration in extract characteristic by fractionation, and loss of productive time by contamination of the solvent system with extract.

According to a first aspect of the invention there is provided apparatus as

Claims (21)

1. defined in Claim 1.

   Advantages of this apparatus include:

   The vaporisation of solvent occurs partly by direct contact between the 6 droplet spray and the vapour in the vessel, and partly by evaporation from a thin film. Both regimes are well known as offering high heat transfer coefficients and hence good thermal performance.

   There is little reduction in evaporation capability as the apparatus operates because the level of extract in the vaporising solvent is always low (at the concentration supplied via the inlet from e.g. an extractor vessel such as vessel 11 of Figure 1) and thus there is little change in boiling temperature or solvent vapour density with time.

   Some biomass extracts are known to solidify when concentrated at temperatures close to or below ambient. The invention allows extract to stay liquid in the vaporising film (by virtue of the choice of the temperature of the heated part) and thus it is possible to hold the extract in the liquid phase and allow it to run to the base of the vessel, even if the bulk solvent vaporisation temperature is below the extract melting point.

   Preferred arrangements of the components of the apparatus that give rise to efficient separation are defined in Claims 2 and 3.

   Preferably the nozzle is removable from the inlet. The use of a removable feed spray nozzle means that the optimum spray angle and droplet size can be easily altered from extraction to extraction depending on the properties and concentrations of the extracts being processed.

   Optionally the inlet includes a preheater for the solvent/liquid mixture. Preferably the heating by the preheater is controllable, e.g. by means of a microprocessor control, in dependence on the precise feed being processed. This advantageously controls the amount of flash vaporisation of the solvent in the apparatus.

   7 Desirable features of the vapour outlet are defined in Claims 7 and 8. These features advantageously ensure complete withdrawal of the solvent vapour from the vessel, while avoiding entrainment of the biomass 5 extract.

   The demisting barrier of Claim 9 also beneficially helps to prevent entrainment of the biomass extract.

   Claims 10 and 11 define features of the dimensions of the vessel that assist in providing good flow and separation effects.

   The invention is considered to reside in apparatus as defined herein when operatively connected in a biomass extraction plant, especially such a plant including a closed loop solvent circulation path of which the pressure vessel, in particular the interior thereof, forms part.

   According to a further aspect of the invention there is provided a method as defined in Claim 14. Preferred aspects of the method are defined in Claims 15 to 17.

   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 Figure 1 shows a prior art, typical, closed loop biomass extraction circuit; and Figure 2 shows in schematic, cross-sectional view apparatus according to the invention in the form of an evaporator.

   8 Referring to Figure 2 there is shown an evaporator 12 that may readily replace the conventional evaporator 12 in the Figure 1 circuit.

   Evaporator 12 of Figure 2 is a jacketed pressure vessel 20 with a heat 5 source 21 flowing through the jacket 22. An alternative embodiment would use an electric heating element wound or embedded around the side and bottom walls 20a, 20b of the vessel 20 in a manner similar to that of jacket 22 in Figure 2.

   Liquid solvent bearing extract material from the extraction stage 11 of the process is fed via a supply line 23 optionally through a preheater 24 and thence is introduced to the interior of the vessel through a short vertical pipe 26, at the end of which is mounted a removable, angled spray nozzle 27.

   When the solvent/biomass extract liquid mixture flows through line 23 and pipe 24 to and through nozzle 27 the feed pressure drops over the nozzle 27 and a portion (about 5%-7% typically) of the feed is therefore vaporised as it enters the vessel 11. The amount of this so-called flash vapour can be altered according to experience by preheating of the liquid solvent feed.

   The angled nozzle 27 directs a fine spray of the unvaporised liquid towards the wall 20a of the vessel 20 and the liquid then runs down the inside wall 20a of the vessel in a thin film. As this film flows downward it vaporises because the wall temperature is maintained above the vaporisation temperature for the solvent at evaporation pressure. The extract material (i.e. the liquid biomass extract) (whose dewpoint is lower than that of the solvent) flows down to the base 20b of the vessel 20, where it collects through the duration of the cycle and can be harvested 9 through a drain valve 28 in the bottom wall 20b as required.

   The evaporated vapour rises through the vessel, undergoing heat and mass transfer with the droplet spray, and is removed from a duct 29 at the top 5 of the vessel by the action of the solvent compressor 13.

   The section 20c of vessel above the point of solvent introduction can be used (by suitable extension of the heating jacket 22 or other heating elements) to provide some additional superheat to the vapour (to help dry out the vapour) and also acts as a disengagement space to minimise entrainment of liquid droplets.

   To aid this disengagement of vapour the internal diameter D of the vessel is selected to give vapour velocities low enough to substantially avoid entraining droplets of the particle size formed by the spray nozzle. An additional demister pad or baffle arrangement 30 may be fitted as desired at the top of the internal vapour space to further hinder liquid entrainment.

   The length L of the vessel is selected to ensure that in the normal 20 operating cycle there is sufficient heat transfer surface to ensure complete evaporation of solvent before the anticipated level of product in the vessel base 20b is reached.

   A CLAIMS Apparatus for separating a volatile solvent and a less volatile liquid, mixed together, in liquid form, from one another, the apparatus comprising:

   a hollow pressure vessel having an inlet connected to a pressurised source of solvent/liquid mix, a vapour outlet and an outlet for liquid; a heater for heating part of the interior of the pressure vessel; and a spray nozzle, connected to the inlet, for introducing the solvent/liquid mix into the pressure vessel as a fine spray, whereby the pressure drop across the nozzle causes vaporisation of at least some of the solvent, the nozzle and the heated part of the interior being so positioned relative to one another that the spray contacts the heated part to vaporise the remainder of the solvent.
2. 2. Apparatus according to Claim 1 wherein the heated part is so positioned and heated that the liquid flows in liquid form along the heated part to a liquid reservoir in the vicinity of the outlet for liquid.
3. 3. Apparatus according to Claims 1 and 2 wherein the spray nozzle and the vapour outlet are located at or near the top of the pressure vessel; the outlet for liquid is at or near the base of the pressure vessel; and the heater heats an interior wall of the pressure vessel.
4. 4. Apparatus according to any preceding claim, wherein the nozzle is removable from the inlet.
5. 5. Apparatus according to any preceding claim wherein the inlet includes a preheater for the solvent/liquid mixture.
6. 6. Apparatus according to Claim 5 wherein the heating by the preheater is controllable.
7. 7. Apparatus according to any preceding claim wherein the vapour 5 outlet is connected to suction.
8. 8. Apparatus according to any preceding claim wherein the vapour outlet is in a disengagement zone, the apparatus including a superheater for heating vapour in the said zone of the vessel.
9. 9. Apparatus according to Claim 8 including a demister pad or baffle plate separating the disengagement zone from the remainder of the interior of the vessel.
10. 10. Apparatus according to any preceding claim wherein the transverse dimensions of the interior of the pressure vessel are such as to ensure vapour velocities low enough to avoid entraining droplets of the fine spray towards the vapour outlet.
11. 11. Apparatus according to Claim 2 or any claim dependent therefrom, wherein the heated part is of sufficient length as to ensure complete vaporisation of solvent mixed with the liquid, before the liquid reaches the reservoir.
12. 12. Apparatus according to any preceding claim when included in a biomass extraction plant.
13. 13. Apparatus according to Claim 12 wherein the biomass extraction plant includes a closed loop solvent circulation path, of which the pressure 30 vessel forms part.

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14. 14. A method of separating a volatile liquid solvent and a further, less volatile liquid mixed together, the method comprising the steps of supplying the mixed liquids under pressure to the interior of a pressure vessel, via a nozzle that reduces the pressure of the liquid mix and forms a fine spray thereof, whereby some of the solvent evaporates; heating part of the interior of the pressure vessel; contacting the heated part with the fine spray, causing evaporation of the remainder of the solvent; allowing the liquid to flow along the heated part to a reservoir; draining the liquid from the reservoir; and venting the vapour from the vessel.
15. 15. A method according to Claim 14 including the step of preheating the liquid mixture before passing it via the nozzle.
16. 16. A method according to Claim 14 or Claiin 15 including the step of superheating the solvent vapour in the pressure vessel.
17. 17. A method according to any of Claims 14 to 16 when practised as part of a biomass extraction process.
18. 18. Apparatus generally as herein described, with reference to and/or as illustrated in Figure 2 of the accompanying drawings.
19. 19. A method generally as herein described, with reference to and/or as illustrated in Figure 2 of the accompanying drawings.
20. 20. Biomass extract obtained by the method of any of Claims 14 to 17 or 19.

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21. 21. Solvent obtained by the method of any of Claims 14 to 17 or 19.

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