APPARATUS TO SEPARATE A LIQUID FROM DISSOLVED SOLIDS BY EVAPORATION

CLOSE TO ITS CRITICAL POINT

Technical Field

The invention relates to an apparatus for distillation of a purifyable liquid near or above its critical point, wherein the purifyable liquid contains a dissolved solid, said apparatus comprising:

a separation vessel for separating the liquid into a vapour phase in a super critical state and a liquid residue separated by a liquid surface within said separation vessel, heat exchange means for transmitting heat from the vapour phase and the liquid residue and from the vapour phase and the liquid residue to the purifyable liquid,

means for pumping the purifyable liquid into said heat exchanger and for feeding the purifyable liquid into said separation vessel for separating said purifyable liquid into the vapour phase and the liquid residue separated by the liquid surface, said separation vessel having a liquid inlet, a vapour outlet and a residue outlet, said vapour outlet being positioned at the top of the separation vessel, said liquid inlet being positioned at the bottom of the separation vessel and said liquid residue outlet being positioned in said separation vessel between the liquid inlet and the vapour outlet and immediately below the surface of the liquid to maximise the concentration of the dissolved solid discharged through the liquid residue outlet, and

means for establishing a pressure and a temperature in the separation vessel sufficient to bring the liquid to a state near or above the critical point so as to form the vapour phase and the liquid residue and for establishing a temperature profile in the purifyable liquid, which increases from the liquid inlet to the vapour phase at the vapour outlet, first means for discharge of the liquid residue and second means for discharge of a distillate.

Background Art

US patent No 5,591,310 describes an apparatus for distillation of a liquid near its critical point, said liquid containing a dissolved solid. The apparatus includes a separation vessel in which the liquid may be separated into a vapour and a liquid, residue sepa- rated by a liquid surface. The apparatus also includes a pump for pumping liquid into the vessel so as to establish and maintain the liquid and vapour in a separation section

at a desired pressure. The pump is a reciprocating pump with substantially horizontal displacement means such as pistons for liquid, residue and distillate respectively which are characterised in that each of the pumping out cylinders associated with the residue displacement means and the distillate displacement means communicates with an out- let valve and a slide valve which are controlled by the piston movement such that the slide valve shuts off discharge of liquid to a discharge outlet when the piston has reached in a position before its top position, and opens the output valve when the pressures above are comparable. However, it may be difficult to control the operational parameters and it may be difficult to start the apparatus.

Disclosure of Invention

The apparatus according to the invention characterised by the means for pumping the purifyable liquid comprises at least three reciprocating pump with substantially vertical displacement and means for supplying liquid into the heat exchanger, withdrawing of liquid residue from the heat exchanger and withdrawal of the distillate from the heat exchanger.

As a result of the vertical positioning of the displacement the volume over the dis- placement means could be used for detection of different operational parameters such as the pressure.

According to the invention the pumps with substantially vertical displacement means may be provided with valves controlled by means of a super adjacent cam shaft.

According to the invention the heat exchange means may be composed of adjacent tubes of heat conducting material formed like coils.

According to the invention the means for establishing a sufficient temperature in the separation vessel and for providing said temperature profile in the purifyable liquid may comprise inner heating means.

Brief Description of the Drawinq(s)

The invention is explained in detail below with reference to the drawing(s), in which

Fig. 1 illustrates a distillation apparatus according to the invention comprising a separation vessel and associated pumping means,

Fig. 2 a sectional view of the separation vessel,

Fig. 3 another view of the separation vessel,

Fig. 4a sectional view of the pumping means,

Fig. 4b a perspective view of the pumping means.

Fig. 5a sectional view of the pumping means in an angle position of 0°,

Fig. 5b a perspective view of the pumping means in an angle position of 0°,

Fig. 6a a sectional view of the pumping means in an angle position of 90°,

Fig. 6b a perspective view of the pumping means in an angle position 90°,

Fig. 7a a sectional view of the pumping means in an angle position of 180°,

Fig. 7b a perspective view of the pumping means in an angle position of 180°,

Fig. 8a a sectional view of the pumping means in an angle position of 270°,

Fig. 8b a perspective view of the pumping means in an angle position of 270°,

Fig. 9 a time diagram of the first piston and associated valves, and

Fig. 10 a time diagram of the second and the third pistons and associated valves.

Best Mode(s) for Carrying out the Invention

Fig. 1 illustrates a distillation apparatus comprising a separation vessel 2 for separating liquid into a vapour phase in a supercritical state and a liquid residue separated by a liquid surface within the separation vessel 2.

Heat exchange means 4 are placed in the said separation vessel 2 for transmission of heat from the vapour phase and the liquid residue into the purifyable liquid.

The apparatus also comprises pumping means 6 for pumping the purifyable liquid such as salt water through the heat exchange means 4 and into the separation vessel 2 for separating the purifyable liquid into vapour phase and liquid residue separated by the liquid surface. The pumping means 6 are able to provide a pressure of at least about 260 bars.

The separation vessel 2 has an inlet 2a for purifyable liquid, a vapour outlet 2b (through the heat exchange means 4) and a residue outlet 2c (also through the heat exchange means 4). The vapour outlet 2b is positioned at the top of the separation vessel 2. The liquid inlet 2a is positioned at the bottom of the separation vessel 2. The liquid residue outlet 2c is positioned in the separation vessel 2 between the liquid inlet 2a and the va- pour outlet 2b and immediately below the surface of the liquid to maximise the concentration of dissolved solid discharged through the liquid residue outlet 2c.

The apparatus also comprises means for establishing a pressure and a temperature in the separation vessel 2, which are sufficient to bring the liquid into a state near or above the critical point so as to form the vapour phase and the liquid residue and establish a temperature profile in the purifyable liquid, which increases from the liquid inlet 2a to the vapour phase at the vapour outlet 2b. The pumping means 6 comprises three pumps, 6a, 6b, 6c, having reciprocating pistons with substantially vertical displacement, a first piston pump 6a for supplying purifyable liquid into the bottom (at 2a) of the separation vessel 2 through the valve 6a', a second piston 6b for withdrawal of liquid residue from the separation vessel 2 through the heat exchanger 4, and a third pump 6c for withdrawal of distillate from the separation vessel 2 through the heat exchanger 4.

The pumps with the substantially vertical displacement means such as pistons are moved up and down by means of a crankshaft 11.

Each of the pumps 6a, 6b, 6c is provided with valves controlled by means of a super- adjacent cam shaft 9. The cam shaft 9 is adapted to open and close the valves 6a", 6a'", 6b", 6b"', 6c", 6c'" as indicated in the time diagram illustrated in Figs. 9 and 10. The cam shaft 9 is through a belt 17 in rotational connection with the crank shaft 11. A

fly wheel 12 is positioned at the end of the crank shaft 11 so as to secure a smooth rotation.

A fourth pump may be provided for supplying a gas, such as oxygen into the separation vessel 2 so as to burn out impurities in the liquid to be distillated. The liquid and the gas are preferably supplied by means of the same pump 6a.

The means for regulating the pressure in a separation vessel 2, preferably at a constant pressure of about 240-260 bars, may be composed of a valve 19 having a prede- termined opening pressure, e.g. a ball valve, said valve being located at the top of the pump 6a for the liquid to be distillated.

Alternatively, the means for regulating the pressure in the separation vessel 2 may be composed of a pressure sensitive mechanism regulating at least one of the valves, preferably the relief valve 19, preferably the close angle of the said relief valve 19 in response to the pressure in the separation vessel 2, cf. Fig. 4b, illustrating the pumps and the valves associated with each of the pumps 6a, 6b, 6c. The regulation means comprises a chamber 13, which is connected to the separation vessel 2, so as to detect the pressure in the separation vessel 2. The pressure sensitive chamber 13 has a movable spring-biased wall 14, which is connected to a rotatable shaft 15. In the surface of the rotatable shaft 15, two sets of helicon-shaped grooves 15a, 15b are provided.

Each set of grooves 15a, 15b is rotationally connected to a gear wheel 15a', 15b', con- trolling the rotation of the underlying cam shaft 9 with two gears 9a, 9b, the first gear 9a being fixed to the cam shaft 9, and the other gear 9b being rotatable in relation to the cam shaft 9. The two sets of helicon-shaped grooves 15a, 15b are oppositely turned, i.e. turned anti-clockwise and clockwise, respectively. By shifting the rotatable shaft 15 in axial direction, the gear wheels 15a', 15b' connected thereto will be turned in relation to each other, such that the opening and the opening phase of the valve 6b'"controlled by the cam shaft 9 (indirectly) is changed accordingly, i.e. in response to the pressure in the pressure sensitive chamber 13. This setup can also be used in connection with other valves for controlling the opening and the opening phase, e.g. for regulating the opening interval of a valve supplying or removing purifyable liquid to or from the sepa- ration vessel.

A negative feedback is established, if the opening interval of the valve 6b'" changes accordingly when the pressure in the pressure sensitive chamber 13 increases. As a result the pressure regulating means automatically seeks towards equilibrium. The equilibrium pressure is set by means of an adjustable screw 16 for compressing a spring for biasing the movable wall 14 of the pressure sensitive chamber 13. The adjustable screw 16 is calibrated to indicate the pressure in response to the angle position of the screw. The inclination of the helicon-shaped grooves 15a, 15b should merely be suitable.

Figs. 9 and 10 illustrate the time diagrams of the valves controlled by means of the cam shaft 9.

Fig. 9a illustrates the stroke of the piston of the pump 6 vs. the angle position of the crank shaft 11.

Fig. 9b illustrates the opening and closing of the water inlet valve 6a'".

Fig. 9c illustrates the opening and closing of the gas inlet valve 6a".

Fig. 9d illustrates the opening and closing of the outlet valve 6a', preferably in response to the pressure in the reactor (the separation vessel).

Fig. 9e illustrates the opening and closing of a special relief valve 19, which is a very short period at the beginning of the process cycle.

Fig. 10a illustrates the stroke of the pistons of the pumps 6b and 6c vs. the angle position of the crank shaft 11. The angle position of the pistons of the

pumps 6b and 6c is 180° shifted in relation to the angle position of the piston of the pump 6a.

Fig. 10b illustrates the opening and closing of inlet of either distillate and gas (pump 6b) or concentrate and gas (pump 6c) from the reactor (the separation vessel).

Fig. 10c illustrates the opening and closing of the outlet valve of either distillate and gas (pump 6b) or concentrate and gas (pump 6c).

Fig. 1Od illustrates almost the same as Fig. 9d.

The heat exchange means for the separation vessel 2 are composed of tubes of heat- conducting material shaped like coils. The liquid to be distillated is moved along said coils, while the liquid residue and the vapour are transported in the opposite direction in the tubes, so as to provide a heat exchange.

The means for providing a predetermined temperature profile and a sufficient tempera- ture in the separation vessel 2 is composed of inner heating means 10 in the form of electrical resistors. Temperature sensing means for sensing the temperature profile in the separation vessel 2 and an electronic regulating circuit 19 for regulating the electrical current to one or several of the heating means 10 in response to the actual temperature profile has been provided, so as to obtain the desired temperature profile. The temperature at the top of the reactor should be about 600° C, in the middle of the reactor about 450° C. A number of coils for the circulation of a refrigerant is provided outside the reactor.

The separation vessel 2 is provided with an outer tube 21 of stainless steel. This tube is exposed to a rather heavy thermal load, especially in the upper part. As a result thereof, it will be necessary to exchange said tube after a predetermined number of operational hours, preferably 2000 hours. Such a tube 21 is rather expensive. The applicant has therefore constructed the separation vessel in such a manner that the said tube is symmetric and capable of being reversed so that the upper end is turned down, and the lower end is turned up. As a result, the tube 21 can have a much longer lifetime, preferably two times the lifetime without such a reversion. The reason is that the tube is exposed to the highest thermal loading of the upper part. The tube should naturally be symmetric, and it should be possible to dismantle the separation vessel.

Examples

Example 1

Energy Consumption

The energy consumption for 1000 kg of distillate for the desalination apparatus was calculated under the following conditions:

raw water, 3% NaCI: 1125 kg/h, temperature 8° C distillate: 1000 kg/h, temperature 15 ⁰ C residue, 24% NaCI: 125 kg/h, temperature 15° C

pressure in heat exchanger and separator: 250 bars efficiency of water pump: 85% radiation loss from the apparatus: 10% of added heat

a) Energy consumption of pump: 0.15x250x10 ⁵ x1125/ (3600x10 ⁶ )=1.172 kWh

b) Heat consumption 4x184x(15-8)x125/3600=9.150 kWh

c) Radiation loss: 9x150x0.1 =0.915 kWh

The total energy consumption depends upon the construction of the pump, heat ex- changer and insulation.

Example 2

Energy Consideration for the Apparatus

To evaluate added theoretical heat in the separation zone the following enthalpies were calculated for pure water: Raw water into the heating zone: 1.5 kg, temperature 10° C, pressure 250 bars, 99 kJ;

Distillate at inlet in the top of the separation zone (vapour pipe): 1.0 kg, temperature 420° C, pressure 250 bars, 2774 kJ;

Distillate out of the heating zone: 0.5 kg, temperature 390° C, pressure 250 bars, 107 kJ;

Residue at inlet in the bottom of the separation zone (residue pipe outlet): 0.5 kg, temperature 390° C, pressure 250 bars, 1195 kJ; and

Residue out of the heating zone: 0.5 kg, Temperature 20° C, pressure 250 bars, 55 kJ. 162 kJ-99 kJ = 63 kJ per 1.0 kg of distillate are added.

Example 3

Comparison of Vapour and Water Compression

To evaluate the size of the mechanical work for a distillation apparatus according to the invention a calculation was made of the compression work for a distillation of raw water at a water production (distillate) of 1 kg/h under the following conditions:

raw water in: 1.5 kg/h, temperature 10° C. distillate out: 1.0 kg/h, temperature 20° C. residue out: 0.5 kg/h, temperature 20° C.

a) Heat loss of outgoing water (common to vapour and water compression):

Q=dt x C _p xm/3600=17.43 Wh

C _p =4184 J/kg at 250 bars.

b) Vapour Compression Work:

pressure in evapourator: 1 bar, temperature 100° C. pressure in condensator: 1.5 bars, temperature 110° C.

Theoretical work:

L=k/(k-1) x P ₁ xV ₁ ((P ₂ /P ₁ )((k-1)/k)-1)/3600=19.86 Wh

Expected Work: 19.86 / 0.5=39.7 Wh.

c) Water Compression Work

Pressure in heat exchanger and separator: 250 bars

Theoretical work:

L=P x V/3600=10.42 Wh,

which gives L=5.21 Wh with a recovery degree of 50%.

For a continuous water production of 1 kg/h there is a saving in work of about 5 times by water compression with respect to vapour compression, which enables distillation of a liquid with a lower energy consumption than in the heretofore known methods, and the distillate is always sterile.