EP0302923A1 - A method of separating a volatile component from a mixture utilizing a carrier gas for vapour transport from an evaporator to a condensor - Google Patents

Abstract

A volatile component (1) is separated from a mixture (2) by vaporization in an evaporator (4) and by condensation in a condenser (5), the vapor being transported by a carrier gas (3), circulated in and between the evaporator and the condenser. A flow (1 ') of liquid (11, 1) is put into circulation, by heat transfer stages carried out one after the other, from a point of contact with the mixture (2), via a cooler (62 ), via a point of contact with said carrier gas (3) in the condenser (5) and via a heater (61), until returning to said point of contact with the mixture (2).

Description

A method of separating a volatile component from a mixture utilizing a carrier gas for vapour transport from an evaporator to a condensor.

The present invention refers to a method of separating a volatile component from a mixture, in which said volatile component is incorporated, said method comprising circulating a flow of a carrier gas from an evaporator to a condensor and back to said evaporator, said carrier gas being supplied to the evaporator to contact said mixture and subsequently being transferred to the condensor together uith volatile component of the mixture, a part of said volatile component being precipitated by condensation in said condensor.

As exemples of processes, where a volatile component of a mixture is a desired product, may be mentioned ethanol distilled out of an ethanol/water solution, and water, distilled out of salt water. As other examples of processes, where a lowered fraction of the volatile component is desired, may be mentioned concentrations of solutions and drying of moist material. The invention is thus applicable to a wide span of distillation and drying processes.

The use of a carrier gas as a transport medium for vapour from an evaporator to a condensor has been demonstrated in the German patent application 2 459 935 in referance to distillation of water. Thus a spray of heated salt water is in an evaporator evaporatvely cooled by a flow of a carrier gas, said gas being subsequently propelled in a condensor in heat exchange with the flow, of salt water, preheating the same under precipitation of condensate. The amount of available heat of vaporization is in the process very limited, namely to the sensible heat in the spray drops and consequently is the obtainable evaporation rate of tbe spray drops small. A remarkably improved process effiency is according to the invention obtained by the introduction of a separate heating agent in the form of a circulated liquid flow;, transferring to the mixture any desired amount of heat of vaporization and being a total enthalpy sink for condensate.

A separation process may thus be carried out with high energy effiency at any desired pressure level or at any desired temperature range below the boiling point of the volatile component, which constitutes a prime object of the invention.

Especially pressure level may be chosen equal to athmosferic pressure, which constitutes a second object.

Temperature range for a separation process may further be chosen to prevent deterioration in quality of heat sensitive mixtures or to fit a use of heat transfer surface out of plastics, which is a third object of the invention.

These objects and others, listed below, are obtained by a method further characterized in that a flow, of a liquid is circulated from heat transfer contact to said mixture, via heat transfer contact to an external heat sink in a cooler, via heat transfer contact to said carrier gas in said condensor, via heat transfer contact to an external heat source in a heater and back to said heat transfer contact to said mixture.

A further object is simplified and highly efficient condensation surface by the use of the volatile component as the heat transporting liquid.

Another further object is amode to conduct a separation process over a wide temperature range with utterly improved energy efficiency.

Another object is to provide a separation process, well adapted to be driven by a liquid/liquid heat pump for further improved energy efficiency.

The invention, will be described in more detail below in reference to the drawings. The basic principle of the method is in fig. 1 illustrated in a temperature and enthalpy diagram and in fig. 2 in an apparatus section. The method extended over a widened temperature range is illustrated in fig. 3 in a temperature/enthalpy diagram and in fig. 4 in an appartus section.

Figures 5-13 illustrate the method, utilizing differant modes of heat transfer in the evaporator to a mixture. Thus fig. 5 shows in a plan section direct heat transfer through membranes to a fluent mixture. Fig. 6 shows in a vertical cross section and fig. 7 in a longitudinal section another mode of direct heat transfer through tube walls to a fluent mixture. This mode is further illustrated in figure 8 by a section of an apparatus for distillation of a fluent mixture.

An indirect method of heat transfer to fluent or solid mixture by means of a vapour saturated carrier gas is illustrated in fig. 9 in an apparatus section and in figures 10 and 11 in detail sections thereof.

Another mode of indirect heat transfer to a granulated, solid mixture by means of intermittantly circulated heating flows of gas is illustrated in apparatus cross sections in figures 12 and 13.

In reference to fig. 1 and 2 a flow 3' of an inert carrier gas 3 is propelled along an elongated evaporator 4 in evaporative transfer contact to a mixture 2, outspred in the evaporator 4. From its inlet, cool end to its oulet, hot end of the evaporator the gas flou 3' undergoes a temperature rise +∆ t₂ and an enthalpy gain

+ ∆i₂, the latter being mainly composed of the latent heat of a flow of a volatile component 1 of the mixture 2, dissipated in vapour form 1" from the mixture. Heat of vaporisation may to a small extent be supplied by sensible heat in the mixture, but is essentially transferred to the mixture 2 from a flow 1' of a liquid 11. Heat may in direct or indirect manners, as described below, be transferred to the mixture 2 in a countercurrent mode tα the flow direction of carrier gas 3, the liquid undergoing a corresponding enthalpy loss ~Δi₂ and an essentially equal temperature drop - ∆ t₂ .

In a temperature/enthalpy diagram, according to fig. 1 the state of the carrier gas 3 is described by a curve C_k' , representing a portion "k" of a curve C, that denotes enthalpy of carrier gas, saturated with vapour of the volatile component 1. The corresponding state of the liquid 11 is in the diagram described by a straight line "A", enthalpy and temperature being proportional.

A reversed process takes place in a condensor 5, where the liquid flow 1' and the carrier gas flout 3' both flows being transferred from the evaporator 4, are propelled in countercurrent enthalpy transfer to each other. In the diagram, fig. 1, the condensor state of the carrier gas 3 is described by a curve C_k", the gas changing in temperature -∆ t₄ and in enthalpy -∆ i₄ under precipitation of said flow 1" of condensed volatile component 1. Conversely the liquid 11 undergoes essentially a temperature rise + ∆ t₄ and an enthalpy gain +∆ i ₄, and the condensor state of the liquid is in the diagram described by a straight line "B". For working condituons with the carrier gas 3 fully saturated with vapour of the volatile component 1, the curves C_k' and C_k" are indentical.

Preferably volatile component 1 is chosen for said liquid 11. Thus condensation may in known manner be processed as direct uet enthalpy transfer and condensation between the carrier gas 3 and the liquid 11, the latter being spred over a contact body 55, distributed in a vertically elongated condensor 5. As follows, the liquid flow 1' is enriched by precipitated, said flow. 1" in the condensor. The latter flow is continously bled off, preferably from the cool end of the condensor 5 or evaporator 4. The straight lines "A" and "B" must in the diagram, fig. 1, obviously enclose the curved lines C_k' and C_k", together with the necessary allowance of heat gradient needed for the described heat and vapour transfers. A closed process circuit and a driving force for the process is thus obtained by a cooling step in a cooler 62, where the liquid 11, being transferred from "A" to "B", is cooled against an external heat sink 8, changing in temperature

-∆t₃ and in enthalpy -∆i₃. In a heating step, the liquid 11 being transferred from "B" to "A", the liquid 11 is heated in a heater 61 from an external heat source 7, the liquid changing in temperature + Δ t₁ and in enthalpy

+ Δi₁.

A driving force for the separation process is a heat amount Δi₁, approx. equal to Δi₃, being degradated in temperature level from a heat source 7 to a heat sink 8. An essense of the invention is, that the driving force may be essentially smaller than the enthalpy amounts

Δ i₂≅ Δi₄ engaged in the actual process of separation of a volatile component 1 from a mixture 2 in the evaporator 4 and condensing the same in the condensor 5. Thus a first energy efficiency factor e₁ = Δi₂/ Δi₁ is defined. Obviously said factor is improved by extending a temperature range "k" of the process and. by narrowing of the temperature gap between the straight lines "A" and "B". The latter is accomplished by using large and. efficient transfer surface for heait and vapour. Since the temperature re range "k" may be chosen at will below the boiling point of the volatile component 1, temperature conditions may be chosen to suit a heat sensitive mixture 2 or to fit the use of plastics for cheap, efficient and noncαrrosive evaporator and. condensor transfer surfaces. Further described condensation process over a wetted contact body 55 may be highly effective. Using a temperature range k=15º, a first efficiency factor e₁ = 2 may be obtained.

A second energy efficiency factor e₂ may further be obtained by a coupling of the external heat sink 8 and heat source 7 with a heat pump process. This may be done, very favorably, using the fairly clean, distilled volatile component 1 as. a heating/cooling agent. The temperature range for a heat pump is in the mentioned range k=15º also favorable and a heat factor e₂ ≅ 5 may be obtained. A total energy efficiency = e₁xe₂ ≅ 10 will result.

Gas pressure may be chosen at will, but athmospheric pressure may be preferred for low apparatus cost. Enclosed carrier gas 3 may also be chosen at will or specifically to suit quality demand's of a mixture 2, for example the absence of oxygen. Alow molecule weight gas such as helium may be chosen for high diffusion rate for vapour. In case of high vaporisation rate in the evaporator 4 or vapour pressure reducing forces in the mixture 2, the carrier gas 3 may not attain full vapour saturation in the evaporator 4. Process efficiency may in this case be improved by precooling the flow 3' to deu point before injection of the flow into the condensor by cooling it against the cooled flou 3', ejected from the condesαr, in a heat exchanger 63.

In fig. 2 is illustrated the method in an apparatus section showing carrier gas 3 propelled in a circuit in and between an evaporator 4, a condensor 5 and a heat exchanger 63 and a liquid flou 1' propelled in subsequent heat transfer steps in contact to a mixture 2 in an evaporator, to a cooler 62, to carrier gas 3 in a condensor, to a heater 61 and back to the evaporator 4.

Energy efficiency may be increased significantly by a further inventive step, described below, in reference to fig. 3 and 4. Thus a number of part flows 31', 32'.. are branched off from the mixed flow of carrier gas 3 and volatile component vapour at succesive steps along the flow path in the evaporator 4 to be remerged with the mixed flow of carrier gas and vapour at succesive steps along the gas flow path in the condensor 5. Further the temperature range "k" is extended significantly. Utilizing tw.o part flows 31' and 32', fig. 3 illustrates evaporator state of the carrier gas 3 as described by three separate curves C₁' , C₂' and C₃', representing subsequently diminished mass flows of carrier gas, increased temperature levels and essentially equal transport capacity of vapour. Reversely the state of the carrier gas 3 along its flou in the condensor is described by corresponding curves C₃", C₂" and C₁" , representing subsequently increased mass flows of carrier gas 3, decreased temperature levels and essentially equal transport capacity of vapour. The state of the liquid flow 1' is described by the straight lines "A" and "B". The described differentiation of mass flow of the carrier gas 3, using an adequate number of part flows, will permit a small tempera ture gap between the lines "A" and "B" in combination with a wide temperature range "k", resulting in significantly increased first efficiency factor e₁ in comparison to previously described method, using a uniform mass flow 3'. A distillation process is exemplified in reference to fig. 3, uith the volatile component 1 being water, the carrier gas 3 being air and the mixture 2 being a water solution to be concentrated. Corresponding mass flous, temperature changes Δt and enthalpy changes Δi appear from the figure. The driving force is a heat amount Δ i ₁ deteriorated from a heat source 7 of approx. +85ºC to a heat sink 8 of approx. +15°C. First energy efficiency factor clculated e₁ = Δi₂/ Δ i₁ = 4,3.

Heat demand for distillation of 1 kg water amounts to 0,19 kwh, which is another expression of energy efficiency.

The heat transfer between the liquid flow 1' and the mixture 2 may according to the nature of the mixture be arranged in differant modes described below in referance to figures 5-13.

A direct heat transfer through a membrane is illustrated in fig. 5, showing a plan section of inside the evaporator 4 vertically mounted pairs of flexible, thin plastic membranes 64. The liquid 11 is propelled in between inner surfaces of the membranes by gravity as thin liquid films, spred over the surfaces by capillary forces. A fluent mixture 2 is propelled as open, falling liquid films 21 along outer surfaces of the membranes in evaporative contact to a carrier gas flow 3', propelled upwards in gas spaces 41. In referance to fig. 4 a continous mixture flow 2' of a process fluid may be injected at the top of a vertically extended evaporator, a continous flow 2" of processed fluid being ejected from the bottom of the evaporator. The transfer material 64 is cheap, efficient and noncorrosive and may easily be extended in surface area and height for excellant transfer conditions.

Above described mode has some disadvantage in that the flow speed in the falling liquid films, changing with viscosity, and the retainment time for the liquid in the evaporator are difficult to regulate.

This difficulty is overcome by another mode of direct heat transfer from the liquid 11 to the mixture 2, described below in referance to figures 6-8. According to fig. 6 and 7 an horizontally elongated evaporator 4 encloses contact bodies 44, horizontal heat transfer tubes 67 for tbe heating flou 1' and a bottom vessel for a fluent mixture 2. Said flow 3' of carrier gas is propelled in horizontal flow direction through the contact bodies 44 and along the tubes 67. Substantial part flows 22 of the mixture 2 are circulated vertically through the contact bodies 44, efficiently uetting the same, and over the tubes 67 under reheating. Circulation devices are pumps 46, conduits 47 and spray nozzles 48. The evaporative contact surface betueen the mixture 2 and the carrier gas 3 may thus be extended at will, independantly of the heat transfer surface of the tubes 67 and further be uetted at uill by the part flows 22, independently of the actual mass, flow of the mixture 2 along the vessel 41 and along the evaporator 4.

Described mode of heat transfer is further exemplified in an apparatus according to fig. 8 for distillation of a volatile component 1 out of a fluent mixture 2, in conformance essetially to the apparatus previously described in referance to fig. 4. The evaporator 4 encloses three separate agglomerates of said contact bodies 44, heat transfer tubes 67 and bottom vessel 41, positioned vertically above each other. Said flow 3' of carrier gas is propelled subsequently through the agglomerates from the cool, top end of the evaporator to the hot, bottom end. Said heater liquid flow 1' is propelled in opposite direction in sequence through the heating tubes 67. Said part flous 31', 32' are branched off from the flow) 3' in. the evaporator 4 and remerged with the flow 3' in the condensor 5. Said part flows 22 are arranged for uetting contact bodies 44 and tubes 67.

A fluent mixture 2, to be distilled., is. propel led by gravity through the vessels 41 in sequence from the cool top end to the hot bottom end of the evaporator. Said precipitated flow 1" of the volatile component is bled off the liquid flow 1', preferably at the cool end of the evaporator.

The mass flow of carrier gas decreases stepwise under its path from cool to hot end in the evaporator. In order to maintain a fair. gas velocity for high evaporation the flow area of the contact bodies 44 are correspondingly decreased.

One further object with shown design of the apparatus according to fig. 8 is to achieve direct and short gas conduits for the part flows 31' and 32'. The apparatus is exemplified with two part flows and three agglomerates. A further differentiation of the mass flow of carrier gas by means of increased number of part flows and agglomerates may easily be achieved within the design pattern, in order to improve thermodyπamic efficiency.

An indirect mode of heat transfer to the mixture 2, being spred in the evaporator 4 as fluent or solid layers 21, is illustrated in referance to fig. 9-11.

A further flow 9' of the carrier gas 3 is propelled in a closed circuit in evaporative, countercurrent enthalpy transfer along a wetted, further contact body 99 to the heated, liquid, flou 1' of the volatile component 1 and subsequently in enthalpy transfer to the mixture layers 21 through the walls of tubes-65. A further flow 1" of volatile component, condesed along the tube walls, is merged with the flou 1'. The tubes.65 are positioned inclined for drainage of condensate. The tubes may be made of flexible plastic material, for example extruded polyeten hose, and may further be inflated by means of a small pressure differance between the. flows 9' and 3'.

Fig. 9, showing a plan section of the evaporator 4, illustrates, heat transfer from said flow 9' to a fluent mixture 2, being propelled as open, falling films 21 along the perimeter of vertically extended and inflated tubes 65. Fig. 10, showing another plan section of the evaporator, illustrates heat transfer from said flow 9' to a solid mixture 2, being distributed in the evaporator as parallel! layers 21. Flat tubes 65 are inflated against the layers 21 during processing, for example a batch type drying of a moist sheet material 2. The tubes may be deflated under loading or unloading to facilitate positioning of the solid mixture layers 21.

A further method of indirect heat transfer to the mixture 2 is described below in referance to fig. 12 and 13. The method refers to a granular and gas penetrable type of mixture. The method may for exemple be applied for drying moist grain, chopped organic material or lumber. Such a mixture may in known manner be charged intermittantly with sensible heat and intermittantly be discharged of vapour to a carrier gas contacting the mixture, the heat of vaporisation being taken from sensible heat in the mixture. The. intermittant heat charging of the mixture is accomplished with gas circulated in a closed circuit through the mixture and a heater.

This procedure may houever not allow any high temperature gradient along the path of the heating gas through the moist mixture, due to vapour diffusion from thus a heated, warmer part to a colder part of the mixture bulk. The method of the invention, directly applied to the above mentioned heat transfer procedure, would, have a very poor first energy efficiency factor e ₁ < 1 and thus be useless. However, by splitting a bulk of moist, granular mixture 2 into a number of part lots, operated at separate stepwise increased temperature levels, the method described in referance to fig. 3 and 4 may be applied.

In referance to fig. 12, showing an apparatus plan section, an evaporator 4 encloses two separate chambers 4a and 4b, each containing a bulk of granular mixture 2 arranged for gas penetration.

Refesring to fig. 13, showing a corresponding vertical apparatus section, the mixture 2 is in each chamber split into a number of part lots 2_r, as exemplified by four vertically arranged part lots 2_r, index r running upwards 1 to 4. As shown in fig. 12, any pair of part lots

2_r is coupled, with a cyclically operated valve device 75_r, a circulation fan 76_r and a part heater 66_r, by means of which a part flow 10 of gas may be circulated intermittantly through either of the part lots 2_r, transferring heat to the same from the part heaters 66_r. Uith the part flows 10_r thus engaged in heating the part lots 2_r of one of the chambers 4a or 4b, the carrier gas flow 3' is directed through the mixture 2 of the other chamber.

The flow pattern of the carrier gas 3 appears from fig. 13. Thus the part lots 2_r are penetrated in succesion, index r running from 4 to 1. Three of said part flows of carrier gas 31', 32' and 33' are branched off from the evaporator 4 and remerged with the flow of carrier gas in the condensor 5. Said, liquid flow 1' is propelled through the heaters 66_r in succesion, index r running from 1 to 4.

The apparatus shown in fig. 13 is essentially conforming to the apparatus described in referance to fig. 4, the thermodynamic properties and efficiency being of the same order as described in referance to fig. 3.

The part lots 2_r may be stationary in shoun pos ition s during the dry in g process. They may also be moved continously or discontinously through the apparatus, thus passing different temperature zones under drying.

The apparatus is exemplified in fig. 13 uith vertically arranged part lots 2_r. The method of the invention may uell also be used within. an arrangement with the part lots 2_r lined up horizontally along a horizontally extended tunnel formed evaporator 4.

The purpose of mentioned contact bodies 44, 55 and 99 is to promote efficient contact surface between falling liquid films and said carrier gas 3, propelled through the contact bodies.. A suitable contact body may in known manner comprise crosswise positioned layers of corrugated sheet material.

Claims

1. A method of separating a volatile component (1) from a mixture (2) in which said volatile component (1) is incorporated, said method comprising circulating a flow (3') of a carrier gas (3) from an evaporator (4) to a condensor (5) and back to said evaporator (4), said carrier gas being supplied to the evaporator (4) to contact said mixture (2) and subsequently being transferred to the condensor (5) together with volatile component (1) of the mixture (2), a part (1") of said volatile component (1) being precipitated by condensation in said condensor (5) c h a r a ct e r i s e d i n that a flou (1') of a liquid (11) is circulated from heat transfer contact to said mixture (2), via heat transfer contact to an external heat sink (8) in a cooler (62), via heat transfer contact to said carrier gas (3) in said condensor (5), via heat transfer contact to an external heat source (7) in a heater (61) and back to said heat transfer contact to said mixture (2).

2. A method according to claim 1 c h a r a c t e r i z e d i n that said liquid (11) is said volatile component (1) and that it is supplied to said condensor (5) by being sprayed over a contact body (55) in said condensor (5) to provide direct contact with said carrier gas (3).

3. A method according to claim 1 or claim 2 c h a r act e r i z e d i n that a number of part flows ( 31', 32'...) are branched off from said carrier gas (3) in succesive steps along the path of said flou(3') in said evaporator (4) and are remerged with said carrier gas (3) at succesive steps along the path of said flou (3') in said condensor (5),

4. A method according to claim 3 c h a r a c t e r iz e d i n that said mixture (2) is distributed in sais evaporator (4) as layers (21) on membranes (64), said liquid flow (1') being propelled in heat exchange with one surface of said layers (21) through said membranes (64), another surface of said layers being in said evaporative contact to said carrier gas (3).

5. A method according to claim 3 c h a r a c t e r iz e d i n that said mixture (2), being a fluid, is propelled along bottom vessels (41 ) incorporated into an essentially horizontally elongated evaporator (4), that a number of part flows (22) of said mixture (2) are vertically circulated from said vessels (41) over and through contact bodies (44), incorporated into said evaporator (4), to contact said carrier gas (3), being propelled through said contact bodies (44), that said part flows (22) further are brought into said heat transfer contact to said flow. (1') through walls of heater tubes (6-7) and further back to said vessels (41).

6. A method according to claim 2 or claim 3 c h a r a ct e r i z e d i n that said mixture (2) is distributed in said evaporator (4) as layers (21 ), said, volatile component (1) being propelled in direct wet contact, counter current enthalpy exchange to a circulated further flow (9') of said carrier gas (3) along a contact body (99), said further flow (9') being subsequently propelled through tubes (65) in said, heat transfer contact to said mixture layers (2,21) and subsequently being transferred back to contact said contact body (99).

7. A method according to claim 6. c h a r a c t e r iz e d i n that said tubes (65), being made of flexible, plastic membrane material, are kept inflated against said layers (21) by means of a pressure difference between said flows (3', 9') of said carrier gas (3).

8. A method according to claim 3 in which said mixture (2) is treated as tuo gaspenetrable lots in two separate chambers ( 4a and 4b ) in said evaporator (4), said lots being arranged in intermittant heat transfer contact to a circulated heating gas c h a r a c t er i z e d i n that said lots are split into a number of part lots (2_r) operated at differant, succesive temperature levels, that said part lots (2_r) of one of the chambers (4a or 4b) are iπtermittantly heated by part flows, of. gas (10_r) in separate circuits propelled through said part lots (2_r) and part heaters (66_r), that simultanously said flow of carrier gas (3') is propelled through the part lots (2_r) of the other, said chamber (4b or 4a) in succesion and further that said liquid flow (1') is propelled through said part heaters (10_r) in succesion.

9. A method according to claim 1 c h a r a c t e r i z e d i n that said external heat source (7) and heat sink (8) are coupled into a heat pump process.

10. A method according to claim 1 c h a r a c t e r i z e d i n that said flou (3') of carrier gas (3) is precooled before injection into said condensor (5) in a heat exchanger (63) against said flou (3'), cooled in and ejected out of said condensor (5).