AU2012201386B2 - A method for heating a liquid comprising a solvent and solute, and separating the solvent and solution - Google Patents

Abstract

A method of treating a liquid comprising a solvent and a solute, and separating the solvent and solute, comprising (i) introducing a heating fluid into a treating zone, the heating fluid including a fluid which contains the same molecules as the solvent; (ii) introducing the liquid to be treated into the treatment zone, (iii) causing the heating fluid to move relative to and to heat the liquid by convective heat transfer; and (iv) separating the solute from the liquid.

Description

1 A METHOD FOR HEATING A LIQUID COMPRISING A SOLVENT AND SOLUTE, AND SEPARATING THE SOLVENT AND SOLUTION Field of the invention [0001] The invention relates to a method for heating a liquid comprising a solvent and solute, and separating the solvent and solute. In one form of the invention, the method is adapted for liquid purification and waste treatment system. [0002] It is the intention of the invention to provide a thermally efficient and innovative means of separating a solvent and solute. As such, in one form of the invention, it is possible to use evaporation to purify polluted industrial waste water or sea water, or the like. The application of this technology, when used for desalination purposes, also produces sodium chloride concentrate as a useable by-product. Background of the invention [0003] In this specification unless the contrary is expressly stated, where a document, act or item of knowledge is referred to or discussed, this reference or discussion is not an admission that the document, act or item of knowledge or any combination thereof was at the priority date, publicly available, known to the public, part of common general knowledge; or known to be relevant to an attempt to solve any problem with which this specification is concerned. [0004] Whilst the invention is described with reference to water as the liquid, it will be understood that the term "liquid" is not so limited and other liquids are useable with the invention. Description of the invention [0005] According to a first aspect of the invention, a method of treating a liquid comprising a solvent and a solute, and separating the solvent and solute, comprising: (a) introducing a heating fluid into a treating zone, the heating fluid including a fluid which contains the same molecules as the solvent; (b) introducing the liquid to be treated into the treatment zone , (c) causing the heating fluid to move relative to the liquid so as to heat the liquid by convective heat transfer; and 2 (d) separating the solute from the liquid; (e) taking off a sub-stream of heating fluid following separation step (d) and compressing the sub-stream; and (f) passing the compressed sub-stream in heat exchange relationship with the heating fluid to be introduced into the treating zone. [0006] Preferably, the relative movement between the heating fluid and the liquid is a turbulent flow. [0007] Preferably, the heating fluid is steam (advantageously superheated steam) and the solvent is water. [0008] The process of the present invention permits high energy efficiencies by using vapour re-compression with the benefits of direct-contact evaporation to significantly reduce or eliminate the scale formation on heat transfer surfaces. The high energy efficiency is achieved by extracting a sub-stream of the heating fluid following the separation step, compressing it in eg. a mechanical compressor, to raise its temperature and then condensing it in a heat exchanger to recover the latent heat by heating the circulating heating fluid. [0009] The methods of the invention also facilitate management of relatively large phase change energy from liquid to gas and gas to liquid by keeping it within the one operating environment. The method may operate as a closed system from an energy perspective but may be an open system from a mass flow perspective. Therefore only a small amount of energy input to the control components is required to manage this considerably larger energy. [0010] It is well known that there are impurities in water that affect its hardness. Hard water may cause scaling in conventional water treatment processes and equipment. Hardness refers to the presence of dissolved ions, mainly of calcium Ca2+ and magnesium Mg2+ ions which are acquired through water contact with rocks and sediments in the environment. The positive electrical charges of these ions are balanced by the presence of anions (negative ions), of which bicarbonate HC0 3 2 -, carbonate CO 3 2 - and sulphate S0 4 2 - are the most important. The solubility of sulphates and bi-carbonates decrease with increasing temperature, which means that in a traditional thermal process the sulphates and carbonates preferentially form on the hottest surfaces, primarily the heat exchangers. This causes fouling (scaling) and loss of performance of the heat exchangers and results in rapid decline in system thermal performance as well as increased maintenance costs and downtime.

3 [0011] In the method of the present invention when applied to such water, the sulphates and carbonates are substantially trapped within the liquid droplets as they move along with the heating fluid and are dried. As such the droplets coalesce on the solute collector (mist eliminator) and drain away with the waste stream. By positioning the heat exchangers downstream of the solute collector, any adverse effect of the sulphates and carbonates is significantly reduced or eliminated as they have no material contact upon those heat exchangers. This dramatically reduces fouling of heat exchangers and significantly increases their long term thermal performance. Additionally, since the solute collectors (mist eliminators) operate at a lower temperature than the droplets, they will only cause minimal scale build-up in any event. [0012] It will be appreciated that the methods of the invention may be implemented in combination with conventional pressure and heat balance control systems. Computer software incorporating control loops for internal chamber pressure and heat balance control will be incorporated. The design and function of such software is within the ordinary competencies of computer programmers. [0013] The methods of the invention are also highly suited for treating hyper-saline solutions, and such like solutions. They are capable of operating at much higher solute concentrations than competitive membrane processes. Brine solutions and other high concentration liquids are beyond the technical or economic capabilities of reverse osmosis and other similar approaches. Those approaches require high pumping pressures which together with the cost of membranes limit the concentrations of solutions they may treat. In contrast, the methods of the invention may be operated below or above atmospheric pressure at low to medium temperatures. Description of the drawings [0014] The methods of the invention are now further illustrated in certain devices with reference to the figures in which: [0015] Figure 1 is a sectioned view of the continuous flow treatment chamber. [0016] Figure 2 is a process flow diagram of the chamber illustrated in Fig. 1. [0017] Figure 3 is a drawing of a drying zone of the invention. [0018] Figure 4 is a drawing of a forward cascade injection drying zone of the invention. [0019] Figure 5 is a drawing of a backward cascade injection drying zone of the invention.

4 [0020] A sectioned view of the main chamber is shown in Fig. 1. In this figure the arrows show the flow paths followed by the gas filling the chamber. In operation this gas 100 consists of a mixture of super-heated steam at close to atmospheric pressure, and contaminated water droplets which travel along the left- and right-hand drying zones 101, 102. However, it will be understood that that there can be significant advantages in some cases in operating the process at pressures well above or below atmospheric pressure. For example if we operate at well below atmospheric pressure then we can operate the chamber temperature at say 50 deg. C, which is below the saturation concentration inversion temperatures for Mg and Ca salts. [0021] These zones will be referred to in the description of Figs 1 & 2 as zones A, 101, and B, 102, respectively. The drying zones are joined at the front and rear ends by the fan end box 103 (zone) and return end box 104 (zone) respectively. They thus form a closed circuit with the heating gas being driven around it by the circulating fans 105, in a manner akin to an aerodynamic wind-tunnel. [0022] In each drying zone 101 and 102 shown there are a number of elements required for the operation of the device. At the gas entry end of each zone, in the direction of the gas flow, there is first a set of flow straightening screens 106, followed by a heat exchanger (HX3) 107 and then a spray nozzle assembly 108. At the gas exit end of each zone there is a mist eliminator (ME) 109.. The purpose of the flow straightening screens 106 is to ensure uniformly distributed gas flow over the heat exchangers 107 and smooth gas flow in the drying zones 101 and 102. [0023] In addition to the main chamber (zones A and B) 101, 102, the system includes a compressor 110 and motor 111, which is used as part of the open Rankine cycle heat pump. This component is only shown in Fig. 2 and typically draws steam from the return end box 104, compresses it, and then feeds it to the heat exchangers HX3 107 where it condenses and yields the purified water product stream of the machine. A consequence of this scheme is that the heat energy given up by the condensing steam is used to heat the gas in the main chamber 101, 102. As a result, a significant proportion of the thermal energy contained in the condensing steam is re-cycled in the process thus increasing the overall energy efficiency by a significant factor. [0024] The contaminated feedwater 112 which is to be purified is pre-filtered and chemically treated prior to being fed to the device. This feedwater is pumped by a high pressure pump 113 through a heat exchange 114 to the misting spray nozzles 108 and mixed with the circulating gas 100 in the drying zones (A and B) 101, 102. As the fine mist travels down the drying zones 101, 102, heat energy is exchanged between the gas 100 and the droplets thus causing water in the 5 droplets to evaporate and form steam. As a result the mass of the droplets decreases and the salt concentration within each droplet increases. After the droplets have travelled a distance down the drying zone and the bulk of the water within them has been removed, the remaining droplets are collected by the mist eliminators 107 and collected at the base of the machine as the concentrate stream 115. [0025] As purely illustrative, in the operation of the process the droplet diameter may be, for example, in the range 15 to 40pm, and the gas velocity in the range 1 to 3.5 m/s. For an NaCl/H 2 0 solution of 3.5 percent by mass (typical of seawater) it has been experimentally demonstrated that the concentrate volumetric flow rate is significantly below 30 percent of the contaminated feedwater flow rate. In addition it has been demonstrated that the condensate stream meets potable water standards. [0026] As can be seen from Fig.2, the main chamber is composed of two droplet drying zones 101, 102, and the fan-end and return-end boxes 104, 105. As shown the chamber gas 100 is driven around the closed circuit by circulating fans 105 whose drive motor speeds are controlled by an electronic variable speed drive (VSD) 116. As the drying process for both zones is essentially the same, the following explanation concentrates on Zone B 101. [0027] Following the gas 100 along the Zone B, it is essentially dry super heated steam as it exits the fan end box 103 and enters the screens 106 whose purpose is to straighten up the gas flow and to ensure the flow velocity is uniform across the face of the heat exchanger HX3 107. Typically the steam will be at or near atmospheric pressure with slight superheat as it enters the heat exchanger and exits with a temperature rise of around 10 C to 20'C. [0028] Immediately following the heat exchanger 107 is the spray nozzle array 108 which emits a spray of finely atomized contaminated feedwater at a temperature of approximately 99'C which travels down the zone 102 mixed with the steam exiting from HX3 107. Because of the droplet's close proximity to the surrounding steam there is significant heat transfer from the superheated steam to the droplets. By the time the droplets have reached the mist eliminator ME 109 the bulk of its water content has evaporated into steam, and the contaminants are concentrated in the remaining droplets. Experience has shown that over 70 percent of the water can be evaporated during this process. A feature of this evaporation process is that it occurs in a volume devoid of hot heat transfer surfaces so that problems of scale formation on the hot surfaces, particularly by Mg2+ and Ca2+ ionic compounds, is considerably reduced.

6 [0029] Because of the heat transfer from the steam to the water droplets as they travel together along the drying zone 102 there is a corresponding drop in the steam temperature from its exit temperature to a few degrees of superheat. Since this heat transfer process cannot occur instantaneously the length of the drying zone 102 needs to be carefully selected to ensure adequate time for the drying process to occur. The relationship between the drying zone length, the gas velocity, the droplet size, the initial dissolved salt concentration, the heat exchanger exit temperature, the temperature gradient along the zone and the required concentrate yield is exceedingly complex; but is subject to mathematical modelling and computer simulation. [0030] As mentioned above when the droplets arrive at the mist eliminator 109 under the above operating conditions they contain less than 30 percent of their initial mass of water. By varying the operating conditions, and especially the operating temperatures and drying length, the magnitude of the concentrate yield can be controlled within a wide range so that if necessary it can be reduced to less than 5 percent of the initial mass of water. [0031] Once the gas loaded with the dried droplets arrives at the mist eliminator 109 it functions to separate the droplets from the gas 100. The gas 100 passes through the eliminator 109 as dry superheated steam, while the droplets collect on the eliminator and drain under gravity into the concentrate drainage pipe-work 115. Typically, the mist eliminator 109 can separate the droplets from the gas 100 with almost 100 percent efficiency so long as the droplet diameters do not fall below a critical size of about 5pm. For the present experimental designs it is important to control the spray droplet diameters so that this critical size limit is not breached. After passing through the mist eliminator 109 the gas 100 flows around the return end box 104 and enters Zone A 101, where the process described for Zone B is repeated. OTHER IMPROVEMENTS [0032] To improve the operating efficiency of the device a number of steps of energy recovery have been implemented as discussed below. Main Compressor Circuit [0033] As discussed above, each HX3 107 is used to raise the gas temperature flowing around the main chamber by around 1 0 0 C to 20'C. To supply the heat energy to achieve this, superheated steam is drawn into the compressor suction line 117, as shown in Fig. 2, from the return end box 104 and fed to the compressor inlet port 110. This steam is typically adiabatically compressed in the compressor 110 to a pressure ratio in the range of 1.7-2.0. In addition 7 compressor cooling water is injected into the inlet cooling port so as to ensure that the steam temperature at the compressor output is only slightly above the steam saturation temperature Tsat for the selected pressure ratio. Under some circumstances additional de-superheat water needs to be injected closer to the heat exchanger inlets as shown at 118 in Fig. 2. The saturated steam is then fed to the two HX3 heat exchangers 107 and since its temperature exceeds the gas inlet temperature to each HX3 107, heat energy will be transferred from the inlet steam to the gas. As a result of this heat transfer the steam will condense to water, and HX3 107 may thus be termed a condensing heat exchanger. [0034] As a result of the steam condensation in each HX3 107, the latent heat of vaporization of the steam is effectively recovered and used to heat the gas in the main chamber 101, 102 as it passes through each heat exchanger 107. The condensate, which is purified water, is then collected from the bottom of each HX3 107 and fed out through the condensate line 119 to heat exchanger HX2 114. [0035] It will be noted that this energy recovery is achieved by the compressor doing mechanical work and thus the process is not in contravention of the second Law of Thermodynamics. Feedwater Circuit [0036] The condensate leaves the bottom of each HX3 107 at the saturation temperature corresponding to the selected compressor pressure ratio and is fed to heat exchanger HX2 114 which is used to recover the sensible heat in the condensate when it is discharged at near ambient temperature. This sensible heat is used to pre-heat the contaminated feedwater 112 prior to being fed to the spray nozzles 108. Main Chamber Mass Balance Circuit [0037] To maintain heat and mass balance in the main chamber an additional heat exchanger HX4 120 is used. Steam is drawn off the return end box 104 and fed to HX4 120 where it is condensed and then fed to the condensate line 119. The mass of steam drawn off is controlled by a control valve (not shown) so as to preferably maintain the main chamber pressure at ±500Pa around atmospheric pressure. This heat exchanger 120 also has a secondary effect of varying the heat balance of the main chamber and so can be used for fine-tuning the chamber internal temperatures.

8 Cascade Multi-Staging [0038] In cascade multi-staging the treated liquid is injected into the drying zone multiple times rather than using a single, long, drying zone. The original drying zone concept as illustrated in Fig 3 consists of one heat exchanger 107 placed at the drying zone 102 inlet in front of a nozzle system 108. At the end of the drying zone 102, a solute collector (mist eliminator) 109 is placed to collect the solute but let the solvent gas pass through. [0039] Cascade multi-staging breaks up each drying zone 102 into a number of sub-zones along the path of the liquid being treated to form mini drying cells 121. As shown in Figs. 4 and 5, each drying cell 121 consists of one heat exchanger 107 and nozzle assembly 108 at the inlet, and solute collector (mist eliminator) 109 at the downstream, outlet end of the drying cell 121. [0040] The general mode of operation is as follows: e the liquid to be treated is initially sprayed through nozzles 108 into the first drying cell 121 where a percentage of each droplet is evaporated by interaction with the heating fluid and then collected by the downstream solute collector (mist eliminator) 109. e the solute so collected is circulated to another drying cell 121 pumped through the nozzles 198 of that drying cell 121 where those droplets are evaporated further and collected at the end of that drying cell 121 in the downstream solute collector (mist eliminator) 109. e the same sequence is followed in relation to other drying cells 121. [0041] By way of example only, cascade multi-staging may have two configurations as discussed below. However, there are innumerable possible arrangements of multi-staging which will be readily apparent to those skilled in the art. [0042] First is forward-cascading along the length of the drying zone where the drying cells are located sequentially along the solvent steam path. The second is a backward-cascading where the initial injection takes place in a drying cell and the collected solute is circulated to an upstream drying cell. [0043] It will be also understood that an approach which uses a combination of the two is possible.

9 Forward Cascade Injection [0044] This configuration is depicted in Fig. 4. The first drying cell 121 is placed at the start of the drying zone and subsequent drying cells 121 are placed serially along the drying zone until the final drying cell 121 as shown. Each drying cell 121 takes advantage of the drying zone phenomenon that droplet evaporation is not a linear function of the drying length of the entire drying zone. It is believed that the evaporation function is closer to a logarithm response, so a higher proportion of mass is lost from each droplet at the beginning of the drying zone where the temperature difference between the solvent gas and solution droplets is highest. [0045] The advantages of forward cascade injection over a normal drying zone are that drying cells are simple to arrange, and pumping with collected solute are easily designed. A penalty capital cost is incurred with more heat exchangers, solute collectors (mist eliminators), nozzles and pumps required than a simple extended drying zone. However, significantly higher processing volumetric throughput is achieved using drying cells while there is only a marginal increase in cost. In addition, closer control can be achieved in the droplet drying process with this approach. Backward Cascade Injection [0046] A further variation of the cascade multi-staging is to use backward cascade where the drying cells 121 are placed in reverse order relative to the fluid flow. As illustrated in Fig. 5, instead of the first drying cell 121 being placed at the beginning of the drying zone, it is instead placed at the end of the drying zone. Each subsequent drying cell 121 is placed in front of the previous until the start of the drying zone is reached. Essentially the higher concentrate collected solute is circulated backwards through the drying zone process. [0047] This configuration has the advantage of using greater differences in temperature within the overall drying zone compared with forward cascade injection, and hence allows further shrinkage of the drying zone. However, there are additional costs with this configuration as the pumping mass flows need to closely managed and a higher order control system is required to balance the mass and energy flows within the system.Whilst three drying cells are shown in the Figs. 4 and 5, the number of drying cells in a drying zone is not so limited. [0048] The word 'comprising' and forms of the word 'comprising' as used in this description and in the claims does not limit the invention claimed to exclude any variants or additions.

10 [0049] Those skilled in the art will appreciate that the invention described herein is susceptible to variations and modifications other than those specifically described. It is to be understood that the invention includes all such variations and modifications. The invention also includes all of the steps, features, compositions and compounds referred to or indicated in this specification, individually or collectively, and any and all combinations of any two or more of said steps or features.

Claims (4)

1. A method of treating a liquid comprising a solvent and a solute, and separating the solvent and solute, comprising: (a) introducing a heating fluid into a treating zone, the heating fluid including a fluid which contains the same molecules as the solvent; (b) introducing the liquid to be treated into the treating zone , (c) causing the heating fluid to move relative to the liquid so as to heat the liquid by convective heat transfer; (d) separating the solute from the liquid; (e) taking off a sub-stream of heating fluid following separation step (d) and compressing the sub-stream; and (f) passing the compressed sub-stream in heat exchange relationship with the heating fluid to be introduced into the treating zone.

2. The method of claim 1 wherein the relative movement between the heating fluid and the liquid is preferably a turbulent flow.

3. The method of claim 1 wherein the heating fluid is steam and the solvent is water.

4. The method of claim 1 wherein the heating fluid is superheated steam and the solvent is water.