IL24856A - Evaporators for distillation of sea water and other liquids - Google Patents

Description

C O H E N Z E D E K & S P I S B A C H E GD. PATEN T ATT OR N E YS 24, LEVONTIN STR., P. O. B. 1169 P A T E N T S & D E S I G N S O R D I N A N C E 14006/65 SPECIFICATION IMPROVEMENTS IN EVAPORATORS FOB DISTILLATION OF SEA WATER AND OTHER LIQUIDS BURNETT & BOLFE LIMITED, a British Company, of Commissioners Road, St ood, Rochester, Kent, England, 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 statement: A major part of the cost of fresh water produced by the distillation of sea water lies in the cost of the energy required to bring about the distillation. Energy obtained at a low temperature may be much cheaper then energy obtained at a higher temperature and waste heat from some previous use may be employed. For example, if heat energy is obtained from the exhaust system of a steam turbine, the cost may be negligibly small if all the heat is taken at a temperature below that at which the turbine is capable of operating. Modern steam turbines are capable of rejecting heat at temperatures as low as 90°F and it is therefore desirable to be able to use heat at this temperature for distillation purposes..

If multiple effect distillation is used, the energy cost decreases with an increa.se in the number of effects, but this increase increases the capital cost of the plant. Generally speaking it has been found too expensive to increase the number of effects beyond six and this makes the cost of the energy requirements so high that it has been hitherto thought not possible for multiple effect distillation to comp-Erete with flash distillation in the production of fresh ater from sea water. have The inventors fesce examined the use of multiple effect distillation for the production of fresh water and has appreciated that the following requirements are' essential in order to make the use of multiple effect distillation economic.

Firstly, the. drop in pressure in the vapour be kept as small as possible with only a low level of of carry-over & liquor with the vapour* Secondly, the temperature drop necessary for the transfer of heat from the condensing vapour of one effect into the evaporated vapour of the next effect must be as small as possible.

Low values for these two factors enable a large number of effects to be used for a given temperature range of operation over the whole equipment.

Thirdly, the equipment must be simple to operate, durable, and cheap to construct in large units.

Similar considerations apply in cases other than the distillation of sea water where large quantities of water or predominantly water containing liquor have to be evaporated, for example, in the recovery of products in solution from large volumes of industrial wastes such as the extraction of material suitable for animal foodstuffs from the waste liquor .from hisly distillation.

According to this invention, a multiple effect evaporator for the distillation of sea water to provide fresh water, or of other-water containing liquors, comprises a series of metal plates arranged face to face with spaces between them, each space being closed to form an evaporation cell having an unobstructed path for the flow of vapour from the plate on one side to that on the other side, a liquor supply header extending along the top of each cell arranged to supply a plate on one side of the cell, a trough extending along the bottom of each face of the plates in each cell, an outlet connected to each trough, a vacuum device having a connection for producing a vacuum in each cell and means for heating one end plate in the series so that, in use, a film of liquor flowing down the face of this plate in the end cell in the series is partially evaporated and the vapour from it flows along the unobstructed path and condenses on the face of the next plate forming the other side of .the cell and heats this plate to evaporate the film of liquor flowing down its other side, the vapour from this being condensed on the next plate and so on through the series of cells, the unevaporated concentrated liquor being withdrawn through one outlet of each cell and condensed distilled water being withdrawn through the other outlet of each cell.

This arrangement is based on the inventor*s ' discovery that a thin film of sea water or other predominantly water containing liquor flowing under gravity down a heated plate will not become sufficiently superheated to cause bubble boiling. Vapour evaporates from the surface without vapour bubbles bursting on the surface of the film of liquid.

Satisfactory bubble free evaporation may be obtained with liquor films having a thickness corresponding to flow rates of up to 15 lb. of liquor per linear horizontal inch per hour when flowing down a vertical plate under gravity. This is the definition of the term "thin film" as used herein► Such films have been found to have a four crested waves which may be as much as 0.02 inch in height.

This surface evaporation without bursting bubbles produces a vapour which has a sufficient purity to make any further separation unnecessary. Thus in the case of sea water distillation the vapour is generated without contamination by salt water spray. It is for this reason that the separator which has previously been necessary in multiple effect evaporators is dispensed with so that the vapour has only to flow across the thickness of the cell to the plate on the other side where it is condensed and gives up its heat to the next cell forming the next effect. The pressure drop in the vapour in its. transfer from one effect to the next is thus reduced almost to vanishing point. Films of water flowing down a plate under these conditions are such as to produce high heat transfer coefficients. Therefore a largo number of cells forming a large number of effects can be provided with a comparatively small temperature difference between the heat supply to the first effect and the temperature of the condensed water produced from the last effect.

It is possible for each cell to operate with a temperature difference of only -¾-°F between adjacent cells, but with such a low temperature difference, the cost of the evaporator is relatively high in relation to the quantity of distillate produced. A practical working level for the , temperature difference across a single cell or effect is 3°F to 15°E but this may be increased to 25° if the energy cost is low. For example, if reject heat from a steam turbine is taken at 24-0°F and temperature drop of 10°F.

Experiments have been carried out to verify the operation in the range between -g-0!1 and 2^>°F' temporatur difference between adjacent cells but this is not thought to be the ultimate, limit.

The temperature difference between two cells for a given heat transfer rate varies with absolute pressure and tends to increase at lower pressures. This temperature difference also depends on the flow rate of 'water down the plate and increases as the flow rate increases.

A continuous film of water has been established with bubble free surface evaporation at a flow rate as low as lb. per inch run measured horizontally per hour.

As the flow rate of water down the plate is increased, the temperature difference to maintain a given heat transfer rate is increased but bubble free surface evaporation is maintained up to the maximum flow rate of 15 lb. per inch per hour.

The apparatus may be operated over a wide range of pressures and tests have been made with a vapour pressure of 0.1 lbs./sq. inch absolute. The upper range of operation is thought to be not less than 1,000 lbs/sq. inch absolute.

The supply header at the top of each cell may be in the form of a box which may be formed by welding further plates to the edge of the cell and the distribution of the sea water to the face of the plate from which it is evaporated takes place extends froia the "box downwards parallel to the face of the plate. The width of this slit is a little greater than the thickness of the sea water film required. Of course other forms of distibutor can be used to produce satisfactory" starting films.

The length of the plates, which governs the height of the individual cells, nay vary between wide limits, but it is preferably not less than 3 feet and not more than 15 feet.

The distance by which the plates in the series are spaced apart from each other, that is the thickness of each cell from plate to plate nay also vary, but the smallest convenient space between the plates from a manufacturing consideration may be of the order of ¾· inch, but is preferably ·¾· inch. Any increase of this dimension is allowable but increases the size of cell and thus the overall size of the evaporator without increasing the performance of the evaporator in any way.

It will be appreciated that the vapour in each effect traverses the thickness of the cell from the evaporating surface to the condensing surface with very small pressure drop.

Tho width of the plates of the cells does not affect their operation in any way and may be determined by the size of the metal plates available or by other structural considerations. The cross sectional area of the sea water supply header is also not critical, but it must be sufficient to ensure that there is only a small pressure drop of the sea water along it.

The two troughs at the bottom of each cell may most two side plates. This baffle plate needs to be only of sufficient height to contain the sea water and the fresh water deposited in the troughs on each side of it and separation of these two flows of water must be maintained even under the effect of temporary flooding in either of the outlet headers. The baffle plate should not be too high because it will then obstruct the passage of vapour across the cell. A height of a few inches is preferred.

Permanent gases must be recovered from the cells and it is for this reason that the vacuum producing device and its connections to the cells are provided .

The connections are preferably placed near the bottoms of the cells because the most common permanent gases are heavier than water vapour. Each cell is enclosed but some dissolved permanent gases are introduced in the feed water, The connection to each cell may be provided with a small orifice, which must be of sufficient size to ensure that all the permanent gases are efficiently removed3 but the orifice should not be large enough to receive an appreciable quantity of water vapour. Preferably, each orifice is provided with a throttle adjustment and these may be set initially when the evaporator is put into operation and this setting: nay be maintained. The fresh and salt water outlet headers nay also be provided with throttling devices which are also initially set in the same way.

In operation, heat is applied to the plate at one end of the series by a condensing vapour, a fluid, or some other means . concentrate from the cells may be conducted separately to a heat exchanger in which they give up their heat to the inconing feed. The points of inlet for the hot v/ater and take off points for feed water to the cells nay be positioned so that feed water is at the optimum tenperature .

An exaraple of an evaporator constructed in accordance with the invention is illustrated in the accompanying drawings in which Figure 1 is a vertical section through a part of the evaporator taken nornal to the plates which are flat; Figure 2 is a section through one cell taken on the line II - II in Figure 1 parallel to the planes of the plates; and, Figure 3 is a flow diagram showing the connections between the various parts of the evaporator.

The evaporator illustrated by way of example in the drawings is one which has been built for experimental purposes and it has only three effects. A larger evaporator may, however, be built in a similar manner with nony more effects and its capacity nay be increased by having a number of series of plates arranged in parallel with each other and supplying all the series of plates separately with the liquid to be evaporated. The evaporator illustrated will be described with reference to the evaporation of sea water to obtain fresh water, but it can also be used for evaporating other liquors.

The evaporator O'omprises a series of metal plates 1 closed down the side edges of the plates 1 by further plates 3 and 4 which are in each case welded to the left hand one of the two plates 1 "between which they are held. The bottoms of the cells are closed by further plates 5 which are similarly fixed and at the tops oft a cells are sea water supply headers 6.

Each of the headers 6 has its botton formed by an angle-shaped strip 7 fixed in position by three rivets 8- passing through its vertical leg. A narrow slot 9 is formed between the face of the vertical leg of the angle-shaped strip and the adjacent face of the plate 1. This slot is 0.005 inch wide. The top of each supply header is formed by an inclined plate 10 which is welded along one edge to the plate 1 and along the other edge to the horizontal arm of the L-shaped strip 7· A sea water supply pipe 11 is welded to each of the plates 10 and the three pipes 11 lead through flow control valves 12 to a common supply pipe 13.

The bottoms of the spaces 2 forming the individual cells are divided into two troughs side by side by vertical plates 14 welded to the middles of the plates 5· pipe 15 is welded to the plate and communicates with a trough 16 on the left-har.d side of each of the plates 14 and a pipe 1 is also welded to the plate 5 and communicates with a trough 18 on the right-hand side of each of the plates 14. The pipes 15 lead through throttle valves 19 to a common pipe 20 and ~he pipes 17 similarly lead through throttle valves 21 to a common pipe 22.

A steam chest 25 having a steaii supply pipe 24 and a condensate return pipe 2 i attached to the left-hand The stean supply chan er 23 is not permanently welded to its plate 1 and neither are the plates 3 , 4-, 5 and 7 welded to the plates 1 at their right-hand sides.

Instead, these parts are clamped together with, sealing gaskets 2? interposed in the .joints. The clanping is effected by four beans 28 with four tie bolts and nuts 29 and 30 respectively. The unit is held together in this way so that it can easily be dismantled for cleaning purposes .

In addition to the pipes already mentioned, the unit has a cooling water supply pipe 31 leading to the condenser 26 and a cooling water outlet pipe 32 leading away fron it. There is also a pipe 33 welded to each of the plates 4 and thus connunicating with each of the spaces 2 forning a cell and these pipes 33 lead through throttle control valves 34 to a common vacuun pipe 35 · The stean supply pipe 24 and the condensate return pipe 25 lead fron and to a stean generator 36 . In this experimental example, this is a boiler fitted with an electric immersion heater, but of course, usually waste stean from the exhaust of a steam 'turbine or any other waste heat nay be used.

As is shown diagrammatic lly in Figure 3 of the drawings, the sea water feed to the pipe 13 is brought about by a feed pump 37 and the water first of all passes through a pre-heater in the form of a heat exchanger 33.

The vacuun pipe 35 leads to a vacuun punp 39 and this discharges to a drain 40. The £>ipes 20 and 22 lead to extraction pumps 41 and 42 respectively for the concentrate and distillate. These punps discharge through pipes 43 and 44 to the heat exchanger 38 where the residual heat of the Ill operation, sea water is fed by the feed punp 37 through the pipes 13 and 11 to the sea water supply headers 6. The supply to these headers is controlled by the valves 12 and a thin filn of sea water fron each header trickles through the slot 9 down the right-hand face of one of the plates 1. In this exanple it has been found that the initial thickness of the filn of sea water as soon as any disturbances caused by the flow fron the slot 9 has a snoothed out, is only about 0.001 inch thick when the liquor flow rate is 2 lb. per horizontal inch per houS. The filn of water fron the left-hand one of the plates 1 is evaporated as it flows downwards and the vapour fron it is condensed on the next one of the plates 1 in the nanner already described. The unevaporated concentrated sea water flows into the troughs 16 whence it is punped away by the extraction punp 2 and the fresh distilled water flowB into the troughs 8 and is punped away by the punp 41. The varying degree of vacuun necessary in the spaces 2 is provided by the vacuun punp 39 and is adjusted by neans of the valves 3 . These valves are opened sufficiently to naintain the vacuun by t e extraction of any air or other gases dissolved in the inconing sea water, but excessive opening which would ause the vacuun punp to suck out too nuch of the vapour evaporated in the cells is avoided.

The exanple described with reference to the drawings has plates which are essentially flat and it is one advantage of this invention that the use of conparatively snail tenperature differences and therefore of correspondingly low pressure differences between one cell and the next on Eiotal plates which of itself represents an inportant saving in the cost of construction of the evaporator. Even with the snail pressure difference corresponding to a tenperature difference of 3°F there is set up an appreciable curvature in the plates due to the difference in pressure fron one side to the other and in order to keep thii curvature within practical linits so as. to naintain a reasonably constant plate to plate distance, two arraiigenents nay be enployed. The first is a forn of grid-like structure which is placed between the walls of each cell and is constructed of thin netal sheet affixed edgewise across the thickness of the cell fron plate to plate. With a plate to plate distance of approxinately this grid does not sensibly interfere with the vapour flow fron one plate to the other, that is to say the path f.or the vapour renains unobstructed neither does it pernit liquid to travel fron one pla^e to the other when the cells are naintained in a sensibly vertical position.

The second arrangenen- deliberately inposes a curvature on the plates of each -iell. The curvature has a vertical axis and the general shape and dinensional stability of the collection of cells thus forned is kept stable by firnly cLcnping together the franes surrounding the cells, bu~ either of these two arrange-nents pernits the use of cells of indefinite width in a horizontal direction.

This evaporator has bee in use in the distillation of sea water.

It is suitable for use or, a wide rangc^ of other We have also used it for the distillation of river water and other contaninated water and also for a range of organic and inorganic liquids such as inpure ethyl alcohol, acetone, and toluene nixed with water.

We have also used it for the recovery of snail quantities of such organic liquids as N-butyl acetate fron water.

We have also used it for the concentration of solutions of various materials dissolved in water.

Claims (1)

1. particularly described and ascertained the nature of our said and the same to be we declare that what we claim A multiple effect evaporator for the evaporation of sea water or other water containing the evaporator comprising a series of metal plates face to face with spaces between each space being closed to form an evaporation cell having an unobstructed path for the flow of vapour from the plate on one side to that on the other a liquor supply header extending along the top of each cell arranged to supply a thin film of liquor which flows down the face of the plate on one side of the a trough extending along the bottom of each face of the plates in each an outlet connected to each a vacuum device having a connection for producing a vacuum in each cell and means for heating one end plate in the series so in a film of liquor flowing down the face of this plate in the end cell in the is evaporated and the vapour from it flows along the unobstructed path and condenses on the face of the next plate forming the other side of the cell and heats this plate to evaporate the film of liquor flowing down vapour its other this being condensed on the next plate and so on through the series of the unevaporated concentrated liquor being withdrawn through one outlet of each cell and condensed distilled water being withdrawn through the other outlet of each An evaporator according to claim 1 in which the supply header at the top of each cell is in the form of a box and a narrow slit for the distribution of liquor to the plate on one side of the cell is formed between the plate and a short distributor plate which extends downwards from the box parallel to the the height of the plates forming the cells is between feet and An evaporator according to any one of the preceding claims in which the distance adjacent plates in the series is from inch to An evaporator according to any one the preceding in which the two troughs at the of each cellare formed by an upright baffle plate projecting upwards from the bottom of the cell between the two plates in the series forming the sides of the An evaporator according to any one of the preceding claims in which the means for heating the end plate in the series comprises a chamber extending over the face of the plate which is remote from the next plate in the an inlet for steam to the chamber and an outlet for condensate from the An evaporator according to any one of the preceding in which the outlets are connected separately to a heat exchanger through which a supply pipe leading to the supply headers extends so that the distillate concentrate warm the incoming An evaporator according to any one of the preceding claims in which the vacuum producing device is connected to each cell through an opening near the bottom of the An evaporator according to claim in which the connection of the vacuum device to each cell has an orifice with a throttle An evaporator to any one of the preceding in which each outlet is provided with an throttling An evaporator according to claim constructed substantially as described with reference to the accompanying day of Attorneys insufficientOCRQuality