IL29261A - Vapour compression distillation unit - Google Patents

Description

PATENT ATTORNEYS · D'DID D ' □ 111) DR. REINHOLD COHN |Π3 TJinj"! Ί.7 DR. MICHAEL COHN Ι Π 3 IK -I ' D ·'π ISRAEL SHACHTER B.Sc. .D.-l I D -I 111 Ί ΝΤ ΠΙ' Filet/ PATENTS AND DESIGNS ORDINANCE SPECIFICATION A vapour compressioa dist nction I/We ISRAEL DESALIHATIOI EMIH1ERIHG (ZARCHIH PROCESS) LTD., an Israel company, if Pilot Plant Area, Tel-Baruch, Tel Aviv; Ϊ-IOSHE PACHTER, an Israel citizen, of 8 Liberman Street, Tel Aviv and AMITZUR ZEE? BARAK, an Israel citizen, of 7 U.N. Boulevard, Tel Aviv do hereby declare the nature of this invention and in what manner the same is to be performed, to be particularly described and ascertained in and by the following statemen This invention relates to a vapour compression ^ distillation unit.

Known vapour compression distillation units are designed to have a relatively small capacity, for example, less than 50,000 gallons per day and are generally arranged to operate at relatively high temperatures of about 100°C and with a high concentration factor which is usually not less than 2. Under such temperature and concentration conditions, scale deposition and corrosion become increasingly likely and steps must therefore be taken both in the chemical treatment of the feed stream to be distilled and in the actual design of the distillation unit so as to reduce such hazards to a manageable level. It is found in practice, however, that such steps considerably increase the cost of the equipment involved as well as the cost of carrying out the process, fhus, in order to reduce the danger of corrosion at the relatively high temperature of operation, it is necessary to construct all those portions of the unit (particularly the heat exchange components thereof) of suitably corrosion-resistant material, for example of cupro-nickel alloys, and these as well as other suitable materials which are resistant to corrosion at these relatively high temperatures are relatively expensive.

In an attempt to reduce the overall amount of expensive corrosion-resistant material, it has been proposed so to design the equipment as to render it capable of operation with a high temperature differential across the heat transfer surface* Such a design, however, leads to an increased power consumption.

Furthermore, with operation at an elevated temperature (e.g. 1G0°C) it is necessary to subject the incoming sea water to preheating from an ambient temperature to the elevated operating temperature. For this purpose a relatively expensive feed preheater must be provided and the equipment requires a relatively long start-up time.

Despite the above referred to disadvantages, most of the known vapour compression distillation units are nevertheless designed to operate at an elevated temperature and this in view of the fact that the density of saturated vapour increases rapidly with temperature. In consequence, for the same distilled water capacity a relatively small and inexpensive conventional compressor may be employed to operate at say 100°C as compared with the very much larger and more expensive conventional compressor which would be required if operation was to take place at 40°C.

It is an object of the present invention to pro- * vide a relatively inexpensive vapour compression distillation unit in which the above referred to hazards or disadvantages are substantially avoided or reduced.

According to the present invention there is provided a vapour compression distillation unit comprising an evaporator chamber, a plurality of heat transfer tubes formed of aluminium extending transversely through the evaporator chamber, a thin bladed compressor having an inlet communicating with the evaporator chamber and an outlet communicating with one set of ends of the plurality of tubes, a distillate outlet communicating with the other set of ends of the plurality of tubes, means for supplying under gravity and under substantially laminar or near laminar conditions over said tubes, the arrangement being such that distillation of the feed stream is arranged to take place on the tubes and condensation of the vapour thus produced after compression thereof by the flexible blade compressor is arranged to take place in the tubes at ambient temperatures not exceeding 50oG and with a condensation-evaporation temperature difference not exceeding 3°C.

Preferably said ambient temperatures do not exceed 40°0. The temperature differential across the heat transfer surfaces formed by the tubes should preferably be less than 1.5°C. The feed concentration factor should be less than 2. fhe tubes which extend transversely through the evaporator chamber, being formed of aluminium, combine good heat transfer characteristics with relative cheapness. The use of aluminium is only made possible by virtue of the fact that distillation takes place at ambient temperatures. Thus, if higher temperatures were employed such a material could not be used in view of its high corrosion factor at elevated temperatures. Furthermore, the fact that the flow of the feed stream over the aluminium tubes takes place under substantially laminar or near laminar conditions also permits the use of the cheap aluminium. Thus, under turbulent flow conditions the protective layer formed on the surface of the tubes may become damaged thus leading to an accelerated corrosion of the tubes.

By virtue of the fact that it is possible to between 3·5 to 4.5 times cheaper than corresponding cupro-nickel tubes) for the heat exchange surfaces, the capital cost of this element of the equipment is substantially reduced. It is therefore possible considerably to increase, within economic limits, the overall area of the aluminium heat transfer surfaces and in consequence it is possible to obtain a smaller temperature differential across the heat transfer suraces and a lower power consumption. .

Furthermore, the very fact of operating at the relatively low, ambient temperatures considerably reduces the rate of scale deposition and minimizes, if not entirely removes, the necessity for chemical treatment of the feed thereby reducing the cost of operation.

A further distinct advantage of operating at ambient temperatures arises out of the fact that the feed does not require any preheating and therefore the unit in accordance with the invention is capable of immediate start-up.

It is furthermore an essential feature of the unit in accordance with the present invention that a thin-bladed compressor is employed rather than a conventional compressor. A suitable thin-bladed compressor is a flexible strip steel blade compressor* Alternatively, the compressor can be provided with thin flat aluminium strip blades of thickness of about 1.5 mm. In all cases the thin-bladed compressor is considerabl cheaper than the conventional compressor. The thin-bladed compressor, e.g. a flexible blade comressor can readil be desined to co e with the relatively large specific volume of water vapour involved at low temperature operation and which is no more expensive than a conventional compressor dimensioned to cope with the relatively small specific volume of water vapour which arises at high temperature operation.

It is in fact by virtue of the use of the thin-bladed compressor that the economic design of the unit as a whole becomes practicable.

Furthermore, with a unit in accordance with the present invention only a very low degree of compression is required and as a consequence very small pressure differences exist between the inlet end of the compressor and the outlet end thereof. Advantage can be taken of this fact by making the partitions in the unit, which separate the region communicating with the input end of the compressor (the evaporation region) and the region in cosistunication with the outlet end of the compressor (the condensing region) of relatively thin materials this again leading to a cheapening of the cost of the equipment. The low pressure difference facto also simplifies considerably the problem of providing an effective seal between the two regions thereby again leading to a cheapening of the cost of equipment* In particular, this low pressure difference factor facilitates the mounting of the tubes with respect to end plates of the evaporator chamber in such a way that they pass freely through these end plates, the tube clearances being substantially sealed, for example by rubber sheets arranged to back the end plates. In this way there is avoided the necessity of forming expensive rolled joints between the tubes and the end plates and the ready removal of damaged tubes for repair and their return is rendered possible.

The avoidance of the requirement of effecting a roll joint seal between the tubes and the end plates renders it possible to use tubes of non~circular cross-sectional shape and in particular tubes having vertically elongated cross-sectional shapes, such tubes having better hea transfer coefficients than corresponding circular tubes.

In the event that some perforation of the aluminium tubes does occur as a result of pitting the evaporating brine will not contaminate the condensate seeing that the latter is at a slightly higher pressure than the former. On the other hand as a result of the small pressure dif erence which exists between the two regions and the resistance proffered to through flow of condensing vapour as a result of the gas-liquid surface tension at the apertures, only a fractional amount of product need be wasted, before the damage to the tubes is detected and repaired. As indicated above, repair of a damaged tube is rendered very simple in view of the facility of withdrawing the tube from the unit and its reassembly in the unit. Repair itself can take the form of simply smearing the perforations with an appropriate adhesive.

One embodiment of a vapour compression distillation unit in accordance with the present invention will now be described by way of example and with reference to the accompanying drawings in whic i Fig. 1 is a schematic longitudinal sectional Fig. 2 is a cross-sectional view of the unit shown in Fig. 1 taken along the line II-II, and Fig* 3 is a view on an enlarged scale of a detail of the unit shown in Fig. 1.

As seen in the drawings, the unit consists of an outer cylindrical shell 1 which is divided by a pair of end plates 2 and 3 into a central evaporator chamber 4, a» outer compression chamber 5 and, opposite thereto, an outer distillate chamber 6.

Extending transversely through the evaporator chamber 4 is a plurality of heat transfer tubes 7 which extend through and are supported at either end by the end plates 2 and 3. The mode of supporting and sealing the ends of these heat transfer tubes 7 with respect to the end plates 2 and 3 wiH be described below w h reference to Fig. 3 of the drawings.

A flexible blade compressor 8 is disposed coaxially in the compression chamber 5» the intake of the compressor 8 communicating via a centrally located suction tube 9, with the interior of the evaporator chamber 4. The compressor 8 is arranged to be driven by a motor 10 which is coupled to the compressor hub U by means of a drive shaft 12, which extends sealingly through the end wall of the shell 1. Mounted on the hub 11 are a plurality of flexible blades 13· She compressor stator consists of shrouds 14 and a multivaned diffuser 15. Both the rotor blades 13 and the vanes of the diffuser 15 are made of thin steel strips. She flexible blade compressor 8 will not be described in any further detail in the present specification seeing that it is of can be made. A saline water feed inlet 17 is provided at the base of the evaporator chamber 4 whilst a saline water outlet 18 is also provided in the base of the evaporator chamber 4 and communicates on the one hand via a pump 19 with a distillant inlet 20 provided in the upper region of the evaporator chamber 4 and on the other hand with a brine blow down 21. The distillant inlet 20 is coupled to a multi-nozzled manifold 22 located in the upper region in the evaporator chamber 4 above the heat transfer tubes 7.

A non-condensible gas outlet 23 is provided in the upper region of the distillate chamber 6 whilst a distillate outlet 24 is provided in the lower region thereof and is provided with a pump 25.

A droplet separator device 26 extends horizontally through the evaporator chamber 4 and serves to separate the saline, carry over droplets from the suction tube 9· Referring to Pig. 3 of the drawings, there can be seen the mode of supporting and sealing the end of a heat transfer tube 7 to the end plate 2. As can be seen the end plate 2 is formed with an aperture 31 which is Of slightly larger diameter than that of the heat transfer tube 7.

Secured to the end plate' 2 is a slightly flexible sealing sheet 32 (made preferably of rubber or of a plastic material) having apertures formed therein which are aligned with the apertures formed in the end plate but being of slightly lesser diameter than the diameter of the heat transfer tubes. As can be readily seen, when the heat transfer tube 7 is passed through the end plate 2 and the sealing sheet 32, the rim 33 of the hole in the sealing sheet 32 becomes slightly splayed and presses against the wall of the heat transfer tube 7 providing an effective seal and securing the heat transfer tube 7 in position. Such a mode of securing the heat transfer tube 7 in the evaporator chamber 4 allows for the ready removal of the tube 7 when this may be required for cleaning and replacement.

In operation, non-condensible gases are removed through the non-condensible outlet 23 whic is coupled to a suitable aspirating means. Saline water is introduced into the evaporator chamber via the inlet 17. This saline water mixes with the relatively concentrated brine which has already accumulated in the bottom of the evaporator chamber 4 as a result of continuing partial distillation and the mixed saline solution is removed through the outlet 18 by means of the pump 19, a portion thereof being discarded as blowdown 21 whilst the remainder is introduced into the evaporator chamber through the inlet 20 and manifold 22. The incoming saline streams descend under gravity flowing over the heat transfer tubes 7 in the form of relatively thin films and partially evaporating. The vapours thus formed pass through the suction tube 9 into the inlet of the flexible blade compressor 8 and after compression pass into the heat transfer tubes 7 where condensation takes place. The heat of condensation is transferred through the walls of the heat transfer tubes 7 to the thin film of saline liquid flowing ove the external surfaces of the tubes 7 causing the evaporation of this liquid. The distillate thus formed passes out of the heat transfer tubes into the distillate chamber 6 from whence it is removed via the outlet 24 and the pump 25. relatively concentrated brine, it is of course possible to feed the inlet saline solution directl to the inlet 20.

As indicated above, distillation takes place at ambient temperatures whilst the difference in pressure between the evaporator and condenser chambers is very low. As a consequence the evaporation is not accompanied by boiling and scale formation is minimal. At these temperatures scale consists mostly of calcium carbonate which can readily be removed and thus obviating the necessity for chemical treatment of the feed. Furthermore, as a direct consequence of operating at ambient temperatures the dangers of corrosion or erosion are minimal thus allowing for the use of the relatively cheap aluminium for the heat transfer surfaces. Similarly, the low pressure differential referred to above allows for the use of thin materials for the heat transfer tubes and for the end plates 2 and 3 which can also advantageously be formed of aluminium. Furthermore, there is no longer any need for providing for a tight seal between the heat transfer tubes and the supporting partitions the seal described above with reference to Fig. 3 of the drawings being perfectly adequate and being of course much simpler and cheaper in manufacture, assembly and disassembly than known connections and seals of the heat transfer tubes.

Finally, the fact that a tight seal does not require to be provided between the heat transfer tubes and the end plates allows for the easier assembly and consequent use of heat transfer tubes whose cross-sections are elongated in the vertical plane, such a tube shape facilitating the production of thin liquid films and thereby increasing the degree of heat transfer. It will be realised that in conventional equipment the provision of an effective seal between such non-circular heat transfer tubes and the supporting partitions is complicated and expensive.

In a particular example using the distillation unit described above the temperature in the evaporator chamber is 30°C whilst the temperature in the condensation chamber is 31.3°C. 'The pressure prevailing in the evaporator chamber is 30 mm Hg whilst the pressure prevailing in the compression chamber is 34·5 mm Hg.

It has been found in practice that with a distillation unit in accordance with the present invention, a specific example of which has been given above, whilst the thermodynamic efficiency may be slightly less than that of a conventional vapour compression distillation unit operating with a standard compressor at elevated temperatures, the economies achieved in using relatively cheap materials for the heat transfer surfaces and, in consequence, in carrying out the distillation at ambient temperatures are extremely considerable and render the unit as a whole very much cheaper than corresponding conventional units.

Claims (6)

HAVING MOW particularly described and ascertained the nature of our said invention and in what manner the same is to be performed, ¾?e declare that what we claim is;-

1. A vapour compression distillation unit comprising an evaporator chamber, a plurality of heat transfer tubes formed of aluminium extending transversely through the evaporator chamber, a thin-bladed compressor having an inlet communicating with the evaporator chamber and an outlet communicating with one set of ends of the plurality of tubes, a distillate outlet communicating with the other set of ends of the plurality of tubes, means for supplying a feed stream through the evaporator chamber so as to flow under gravity over said tubes, the arrangement being such that distillation of the feed stream is arranged to take place on the tubes and condensation of the vapour thus produced after compression thereof by the flexible blade compressor is arranged to take place in the tubes at ambient temperatures not exceeding 50°G and with a condensation evaporation temperature difference not exceeding 3°C«

2. A vapour compression distillation unit according to Claim 1, wherein said thin-bladed compressor is a flexible blade compressor.

3. A vapour compression distillation unit according to Claim 1 or 2, wherein said tubes are of circular cross-section*

4. A vapour compression distillation unit according to Claim 1 or 2, wherein said tubes are of substantially elliptical cross-section having their major axes located in substantially vertical planes.

5. A vapour compression distillation unit according I to any one of the preceding Claims, wherein s¾id compressor is located in a compression chamber and said distillate outlet is provided in a distillate chamber, the evaporator chamber being separated from the compressor and distillate chambers by respective relatively thin end plates, the ends of the heat transfer tubes passing through corresponding apertures in the end plates, each end plate being provided with flexible sealing sheets which are apertured and through which pass said tubes causing the sealing sheets to flare and press seal on to the walls of the tubes.

6. A vapour compression distillation unit substantially as hereinbefore described by way of example and with reference to the accompanying drawings. Dated this 1st day of January, 1968. Por the Applicants RTNERS