Seawater Desalination with Multiple Effect Distillation (MED)Waterman Engineers Australia
The earliest desalination method is multiple-effect distillation (MED), which is very efficient thermodynamically. The MED process takes place in a sequence of evaporators known as effects, and it works on the premise of lowering ambient pressure. This method allows the seawater feed to boil numerous times without the need for extra heat after the first effect.
After being preheated in tubes, seawater enters the first effect and is elevated to the boiling point. To encourage quick evaporation, seawater is sprayed onto the surface of evaporator tubes. Externally supplied steam from a normally dual-purpose power plant heats the tubes. The steam condenses on the opposite side of the tubes, and the steam condensate is recycled as boiler feedwater at the power plant. The steam economy of the MED plant is related to the number of impacts. The overall number of effects is constrained by the total temperature range allowed and the minimum temperature difference between each effect.
In the first effect, only a portion of the seawater poured to the tubes evaporates. The remaining feed water is fed to the second effect and applied to a tube bundle once more. The vapours produced in the first effect then heat these tubes. This vapour is condensed into a freshwater product, while a portion of the remaining seawater feed is evaporated in the next effect. The evaporation and condensation processes are repeated from effect to effect at decreasing pressures and temperatures. This continues for numerous effects, with a typical large plant having 4 to 21 effects and a performance ratio of 10 to 18 in a large plant.
Some plants have been designed to run with a top brine temperature (TBT) of around 70°C in the first phase, which decreases the risk for seawater scaling but increases the need for additional heat transfer space in the form of tubes. The power consumption of a MED plant is substantially lower than that of an multi stage flash (MSF) plant, and the MED plant’s performance ratio is significantly greater. As a result, MED is more efficient than MSF in terms of thermodynamics and heat transport.
Horizontal MED plants have been successfully running for nearly three decades. The tubes in MED plants might be horizontal, vertical, or submerged. Low-temperature MED units have gradually grown. Two MED units in Sharjah, United Arab Emirates, each have a capacity of 22,700 m3 per day, A 45,400 m3 /day unit has a design and demonstration module for the MED process. The Middle East has seen the most recent applications for major MED plants. Although MED plants are currently in the minority compared to MSF plants, their numbers have been growing.
We will study the following types one by one
- Multiple Effect Distillation (MED)
- Multiple Effect Distillation with Thermal Vapour Compression (MED-TVC)
- Multiple Effect Distillation with Mechanical Vapour Compression (MED-MVC)
In a sequence of effects, this procedure is repeated (Multiple Effect Distillation). The produced steam condenses on a traditional shell and tubes heat exchanger in the final cell. The “distillate condenser,” as it is known, is cooled by seawater. Part of the warmed saltwater is used as make-up for the unit at the condenser’s outlet, while the rest is rejected to the sea. Brine and distillate are collected from cell to cell until they reach the last one, where centrifugal pumps extract them.
The number of kilos of distillate generated per kilo of steam supplied into the system is a measure of the evaporator’s thermal efficiency. The Gain Output Ratio (GOR) is a term for this figure. The installation of a thermocompression between one of the cells and the hot one can improve the GOR of the evaporation. This static compressor will recycle portion of the vapour created in one of the cells into higher pressure vapour to be utilised as heating media for the first one, using LP or MP steam.
An MED evaporator with thermocompression in a typical configuration (MED-TVC). Whereas the GOR of evaporation would be around 3 (using LLP steam), the GOR of condensation would be around 6. (Using LP or MP steam).
When steam is not available, the MED procedure can still be used with a Mechanical Vapour Compressor (MED-MVC). A centrifugal compressor powered by an electrical engine recycles the vapour from the cold cell to the hot one in this case. The system’s electricity usage is in the range of 8 to 15 kWh/m3. The maximum capacity of MED-MVC units is 5000 m3/day due to existing compressor technology limitations.
The multiple effect evaporation system where it’s not really a schematic, but it looks like a photograph of an existing system that has been done by a leading specialist in water treatment. It was famous, much more famous working with heat exchangers and cooling towers, this company, and it is also producing units like that. There are some companies that produce these units not much, by the way, but maybe three, four or five companies throughout the world for producing these desalination systems in general, talking about the large-scale server desalination units.
Introduction of MED
It’s a replication of a single effective operation unit. The reason of the replication is that not to waste high temperature streams that are leaving the unit we have high temperature brine leaving the unit and also have vapour which is leaving a high temperature and we had to condense it to preheat the feed.
However, there are other ways that we can make use of this energy by having the vapour which is leaving what effect is going as a source of heat as that for the next effect and so on. You’re better making use of the vapour to generate additional amount of desalinated water vapour and then as the vapour condenses, you have desalinated water and you’re going to need one down condenser at the end after the last effect.
This doubt condenser is going to be used for condensing the vapour which is coming from the last effect only because the vapour which is leaving any other effect is condensing the next one and by the condensation. It transfers its latent heat to the sprayed seawater accordingly, you are getting additional vapour on every effect and that’s why the performance of multiple effects desalination system is dependent mainly on the number of the effects.
Numbers of effects
You can have dependence on temperature, related dependence on the quality of the water which is coming in some dependence on the arrangement be it forward, parallel, or backward. But it is mainly dependent on the number of effects.
If you can increase the number of effects, then you are going to get higher performance ratio. Increase in the number of effects depends on what is the top brine temperature and what is the bottom brine temperature. Bottom brine temperature normally is greater than the seawater by 10. So, we’re talking about something in the order of 30 degrees centigrade in summer.
Whereas the top brine temperature is limited by the fact that we are afraid from scaling to occur to the external surface of tubes. This is unlike the MSF in which the saltwater is passing inside tubes and accordingly, you can just simply do clean it by ball cleaning.
They have ball and the size almost of a squash ball. So, they are sent inside that use the water is driving them as so, they are floating on the sea water moving inside the tubes and then they wrap the surfaces internal surface of the tubes and do cleaning while in operation.
We cannot do that for MED we have to limit our temperature to the scale limit definitely we’re using anti scaling’s anti fouling found as pre-treatment process for the unit, however, still are limited to some temperature which is relatively low or took about 65 degrees or so and there are some trials to increase this temperature by doing either nano filtration or maybe ultra-filtration with reduced graphene oxide coating on the membranes accordingly. You will require less power to because definitely talking about nanofiltration or ultra-filtration you need power in a pump that’s going to drive your seawater through these membranes, but it’s much less than that. You can increase the top brine temperature maybe to 80, 85 degrees.
Some companies did that by increasing the kind of those they have like acid cleaning and the increase the acid in the seawater, which is coming in, which is fine, you can increase the temperature, but it eats up the material so that the material wear rate is a little bit on the higher side.
The classification is
- Forward Feed
- Backward Feed
- Parallel Feed
The parallel feed when we come to look at it, we’ll see that it can be also sub classified into
- Parallel across
The differences between them are in the direction of the sea water flow, if the sea water is all directed to the first effect, and then whatever is rejected from there will be sprayed and the second effect, whatever rejected from the second effect will be sprayed and the third effect and so on this is what we call follow up feed.
The whole feed has gone from stage effect number 1, the remainder goes to 2, the remainder goes to 3 to 4 in a forward manner. On the other hand, when we’re talking about backward, it’s the opposite. So, you’re going from the last effect to the effect before the last which is basically the same principle as the forward effect but in the opposite direction, which is also in the opposite direction of the vapour flow.
Is there a benefit from the backward feed? It’s not frequently used in seawater desalination, because of the simple reason that when you are moving from effect number n to n-1 to n-2 ending up at effect number 1. You are going in the direction of increased temperature and increased salinity, which means that you can reach for the saturation condition for the soul earlier and then precipitation occurs, and you can find substantial scaling occurring on the outer sides outer surface of the tubes that’s exactly what you don’t want to see.
The benefit is that you are going to heat less amount of water every effect. So, it may have advantages of being optimised in terms of heating, you don’t heat the whole fluid, but you only heat the remainder to a higher temperature. So, in terms of energy utilisation, it looks better but it might be good for brackish water or maybe for some sort of that has been used for juice extraction or pharmaceutical kind of applications. This is where you don’t really have high salinity or corrosive sort of seawater where you need to protect your material.
The third one is the parallel feed is basically the forward feed all that feed is going through this feed pipe it reaches to effect number 1, it is a sprayed part will evaporate that will go to condense in the second effect and provide additional amount of heat and the brine which is rejected from this effect is going to be sprayed and the second effect. There’s another benefit of what we see here is that normally effect number 1 is the effect at the highest temperature and pressure.
Effect number 2 is at a lower pressure temperature, effect 3 at a lower pressure and temperature to the extent that some of these effects can operate in vacuum and that’s not surprising. Why because lower pressure you have lower evaporation temperature and then you can have saturation condition at low temperature, and you are getting more and more vapour.
The other benefit is the brine, which is leaving effect number 1, when it goes to effect number 2, it’s already saturated maybe it will cool down as it’s moving down the sub cooled for this but remember effect number 2 is at a temperature less than effect number 1.
The assumption that we always do is that the brine which has been sprayed here is sprayed as saturated liquid. So, any additional amount of heat that will be given due to the condensation of the vapour is immediately going to be spent and evaporating the water.
This is not like the 1st effect, the first effect normally is having some sort of a sub cooled or compressed liquid, even if you have preheaters. The preheaters increase the temperature of the field, but it might not increase it to the level that it reaches for saturation condition. So, in the 1st effect, you may have some sensible heating and then evaporation, but from the 2nd effect onward always the sprayed vapour is assumed to be saturated conditions.
So, any energy given to it, it’s energy that will result in evaporation right away. The preheaters can be using a couple of heat sources one of them is a portion of the vapour which is formed in this effect.
The second could be a quantity of the flesh of vapour in the flesh boxes. So, it goes up here to condense and this one plus portion of the form vapour both they will condense into liquid water, they are going to release the latent heat of condensation to preheat the feet and when they are formed when they are converted into water, liquid water, they are again joining the production line.
Backward Feed MED
Seawater introduced to the last stage (lowest T, P) and proceeds toward the first step. The increase of T and P leads to using brine pumping units between the effects. This increases pumping power, maintenance, and air leaks (disadvantages) In this system, too, brine of high concentrations is admitted to effects of high T crossing solubility limit of calcium sulphate (disadvantage).
Parallel Feed MED
The advantage is the steam which is coming from the boiler from the power plant does not have to heat all the water that we have, it only needs to hit only a fraction which has come to this effect.
This is a substantial saving in the quantity of steam which is coming here nothing is free, if this is that we are losing in every effect, we should start by sensible heating until this the field comes to the saturation temperature of the effect and then evaporation occurs.
This is not going to be the same in all effects. This effect is at a temperature of 65. So, you have to heat the field that the field is at 40. You have to heated from 40 to 65, you need to heat it from 40 to 60, you needed to heat it from 40 to 55, you need to heat it from 40 to 50.
The Delta decent support for each one of them is not the same, but there is a sensible component everywhere.
The signs that we see by looking at the existing MED desalination plants that are in the industry, and noticing that they are basically parallel, which means that parallel stream is something that is superior in performance.
Forward Feed MED (MED-FF)
This is a forward feed again, which is the one that we’re going to start in analysing. Remember, only the 1st effect is the one that receives steam from the power plant and the steam which would condense, and we’ll leave it was going to go back again to the power plant. The rest of the effects are receiving the vapour that has been generated in the previous effects.
So, that’s something that you should always keep in mind keep remembering that there are different layouts, if the effect is going to go to the preheater and the rest is going to go as a source of heat for that next effect where it will condense inside the tubes.
If you have flash boxes, then you have additional vapour coming to the preheaters. So here only a portion of the vapour leaving the effect is going to abbreviate.
All the vapour which is leaving the effect is going to pass by the feet preheater first and then it will go there, which means that whatever is going to go to the next effect, it will not all be saturated vapour but it will be a mixture of saturated vapour and saturated liquid, because part of the vapour has been converted into liquid by condensation and the rest is moving as vapour.
You have a mixture of liquid and vapour coming it means that when you’re talking about the design of multiple effects, desalination, there’s no limit, you can come up with any different arrangement of effects of flow rates, or even of the flow direction itself, you can change the way that you like provided that you are going to provide to give or to result in an acceptable performance.
It’s basically talking about how much better is the performance ratio? How much smaller is the specific mass flow rate of the cooling water? How much smaller is the surface area because performance issue when it’s high you’re guaranteeing that you have an efficiency system which is running efficiently to care about the smaller surface area it means that you’re talking about a smaller capital cost.
Smaller or less quantity of mastery of cooling water would definitely is going to mean that you have smaller capital costs by downsizing the pumps smaller running costs by operating smaller pumps rather than using bigger pumps.
So that is basically the kind of measure that we test against. It’s not only that, but you will see that in our assumption, we assume that the surface area in every effect is constant. So, area 1 is equal to area 2 is equal to area 3 is equal to area 4 is equal to area n and reality. This is not a limit, no one said that they should be the same. It’s all up to you to make similar same area or different areas.
Typically, we have certain number of effects, which means that if evaporator is an effect will feed preheaters. Normally, if you have 8 effects, the preheaters are going to be 6, if you have 8 effects, the flash boxes are going to be 7. So that’s something that has been again, it’s not sacred, it’s not a Bible, I mean, that’s something that a designer can select to change this, it’s all up to him, except for something like to do to draw your attention to.
The first effects are not associating any flash boxes, and we cannot put a flash box there. The reason is, if you put a flash box there, this is basically coming from the power plant, and it should go back again to the power plant. So, there’s no point to do that does it means that you cannot do that ever.
This is basically steam coming from the power plant and it’s coming at high temperature and pressure it passes inside the tubes, and the temperature and pressure of the steam is greater than the temperature and pressure of the effect itself. Accordingly, if you have a flashing box, at the same temperature and pressure of the effect, some flashing of this vapour would occur.
But this is going to be a problem because if it occurs, the quantity of vapour condensed vapour that is going to go to back and need to support our plan will be less and we cannot tamper with that we cannot play with this and that’s why we normally do not put a flash box there and also those who do not put a preheater there, the reason of not putting the preheater there is simply saying that let’s make use this as the 1st effect and this is the first quantity of vapour that we have generated.
Feedwater Preheaters (n-2)
Let’s make use of all of it so that we can generate additional vapour. But maybe somebody would say no, I want to put up a preheater and this portion of the vapour there and as it condenses, it may join the same stream as it’s coming in. So, to be some sort of an awkward design, but maybe somebody can try it and say fine, I’m coming up with something which makes sense.
Flash Boxes (n-1)
Otherwise, there’s no point of doing that because basically the condensate which leaves the preheater is going to the flash box and as we said that is no flash box. So, effect number 1 normally or naturally we do not put a preheater, or a flash box associated with it. So, flash boxes, if you have 4 effects, you have on 3 flash boxes, but 2 preheaters because we cannot put a preheater there. There’s not the vapour which is leaving there is going to condense.
So, is it worth to put a preheater there and make use of this steam or part of it, which is leaving this part in additional heat exchanger, where the temperature difference between this one and that one is not substantially high? So, you may need either a very large heat exchanger or the benefit in terms of heating the water is not worth.
This is a recommendation, if you decided to put it, put it there, make the analysis. So, the point again is that there’s no rigid rules, except maybe for the first effect that you do not put a preheater and do not put a flash box other than that, you can put whatever that you want, but you have to show that it is resulting in a better performance
Whatever condenser at the end of the venting system, as the CO2 is evaporating and condensing the or non-condensable gases like carbon dioxide, you need to get rid of it. So, you have to make a venting system and the venting system is normally a thermal vapour compressor a small injector would be connected not necessarily to every effect, but it should several of them need to be there in the system. So that you need to get rid of these gases otherwise, they are going to spread in the vapour region and they are basically going to affect the performance of the unit either by making like a layer of insulation that would cover the tubes from outside and reduce the heat transfer rate or they might travel with the vapour formed from one effect to the other they are not condensing then inside the tubes you’re going to have some sort of vapour which is not needed. So, try to vent it out as much as you can that’s why the venting system is there.
- In a formal feed arrangement, the direction of heat flow and the flow direction both are from left to right
- Pressure decreases in the direction of the flow.
- Each effect has heat exchanger tubes normally it is the form of tube bonds.
Accordingly, lower heat transfer rate would be that’s why the horizontal tubes bundle is the best option for having a higher heat transfer coefficient and higher heat transfer rate.
The disadvantages normally associated with a multiple effects desalination unit is that we are worried or concerned about scale and fouling on the outside surface of the tubes. We cannot use ball cleaning as we do in the MSF. So, if you need to clean a system, you have to shut it down, pull the tube bundle outside, clean it and put it to service again and that’s something actually it’s a downtime for the plants.
Losses in each effect would add thermal resistance to the flow of the heat between the condensing vapour and the boiling brine. The losses could be boiling point innovation, non-equilibrium allowances, which are basically related to the non-condensable gases that would be there and in the associated with the evaporation and condensation losses can be associated with friction losses across the domestic major and minor friction losses in the pipelines and there could be some losses associated with the condensation process as well.
Then, you need to account for this by slightly increasing the heat transfer area so that you will account for losses that can take place. The amount of vapour which is formed in every effect is less than the amount of vapour formed in the previous effect and just a little reminder that at when you are going from higher to lower pressure and temperature.
Equilibrium allowances as it’s some sort of a measure of how the flashing process is efficient. When you are having a kind of vapour which is being flashed off while spraying or even being sprayed and evaporating. The seawater has many ions not only water, it has not only water and sodium and chloride, but it has other for example some calcium carbonates, some hydroxides that are there, which means that while the evaporation and the condensation occurs, you can find some vapours that are released in upon evaporation, these vapours will not condense accordingly, if these vapours pile up, then it might affect the efficiency or the efficacy of the transfer process which is occurring.
The Effect Balance
We’re talking about multiple effect distillation system or desalination system forward feed, then talking about the first effect or basically an MED system, the first effect is treated in a different way where all the others are treated. Why because the first effect is the only one which is receiving heat from an external source of steam.
Nothing else is receiving steam, it’s only the first effect from effect number 2 to effect number n, they can be having this symbol i and (i +1). So, it means that you can put it in a loop write the equations once, put it in a loop and then it will be calculated once but for the first effect it is the one which is number one having external source of steam.
Number two, for the first effect here we’re getting steam from an external plant a potted plant for example, we’re upon condensation, we’re back again to the plant.
If we’re going to talk about forward fields case, then the seawater which is coming to be sprayed here is normally a temperature less than the saturation temperature effect number 1. So that is sensible heating plus latent heating and the first effect for the rest of the effects. The brine which is leaving effect number 1, it will be sprayed in effect number 2 and when it displayed here, it is displayed as saturated vapour. So, the kind of energy and it will be late and too late. But there is a sense of a component, which is only found in effect number 1.
This flash box is at the same condition of this effect. In some cases, even they make the flash box as integrated port to the effect, we’re separating them here just for the sake of illustration. So, you have saturated liquid coming into a box, which is at a lower pressure and temperature than that stream.
What would happen you have saturated liquid coming into a lower pressure area, what will happen is that it will flash. So, part of the vapour is going to form and the form of vapour is going to leave this flash box going upward and it will get into the preheater where it will condense some of the vapour leaving the effect by evaporation or boiling is going to leave there both streams of vapour are going to join they will condense and the condensate is going to leave this going to join that stream as you can see downstream or maybe it can join the box itself it depends on the design.
This is the other layout only difference is going to be in the whole the quantity of the vapour plus the flesh or vapour are going to join the feed heater and after that the remainder is going to go to the effects that is basically the kind of elements that we have to consider in our analysis, like the flow rates, temperatures as well as salinities for the effect itself and that the balance for the feed heater and that the balance will the flash box for that specific arrangement that we have.
Multi-effect desalination system with thermal vapour compression MED-TVC
Desalination has now become one of the most important water treatment procedures in a few nations where lack of water is a major issue. Because traditional desalination procedures demand a substantial quantity of energy, which is costly, energy consumption is a critical economic factor in determining the type of desalination process. Because of its low energy consumption, the multi-effect desalination system with thermal vapour compression (MED-TVC) is more appealing than other thermal desalination systems. The MED-TVC is distinguished by
- Its great performance-to-cost ratio
- Ease of operation
- Low maintenance requirements
- Straightforward geometry
These appealing characteristics set MED-TVC apart from other well-known desalination systems such as reverse osmosis and multi-stage flash desalination.
The need for high-quality water has expanded dramatically in recent decades because of fast population growth, rising living standards, and the expansion of industrial and agricultural operations. Although seawater covers over 97% of the globe, it is a truth that seawater is unfit for human consumption as well as industrial and agricultural uses.
As a result, the growing need for fresh water drives the development of various desalination technologies to generate purified water for utilities. Several countries, particularly those in the Gulf Cooperation Council (GCC), are currently adopting desalination processes to provide potable and industrial water for their needs. Saudi Arabia, the United Arab Emirates (UAE), and Kuwait, for example, use desalination technology to purify water on a huge scale.
Because it uses less energy to heat water, the MED method is more energy efficient than other desalination procedures like MSF. During the 1960s and 1970s, MSF desalination plants were erected in the Middle East. Because MSF has a large capacity but low efficiency, adding thermal vapour compression (TVC) makes the MED more competitive and energy efficient. Because of the huge volume of MED development with over 15 MIGD (million imperial gallon per day), the MED process has recently gained study interest. Furthermore, the end of MSF’s life in the Middle East is raising MED demand.
The most appealing MED configuration among several types of MED systems has been horizontal tubes falling-film evaporation (HT–FF–MED) with TVC. Because of features including ease of operation with any heat source, low scale development, and a high-performance ratio, the HT–FF–MED with TVC has been a popular choice in the Middle East. If a low-cost energy source is available, the optimum method for producing demineralized or drinkable water is to use a MED system.
As compared to other common desalination technologies, such as MSF and RO, MED need far less energy in terms of heating steam and/or electrical energy. Many MED systems are currently in use in the Middle East region, using waste heat energy from thermal or gas turbine power facilities. For example, Iran’s Kish Island heat recovery steam generator (HRSG) produces 2,000 m3 per day of heating steam to MED-TVC systems.
A MED-TVC system with n effects is represented schematically. The condenser, steam jet ejector (which works as a thermal compressor) and falling film evaporators (also known as effects) are the primary components of the system. Motive steam S is injected into the steam jet ejector at a reasonably high-pressure Ps from an external source such as a boiler or power plant.
The system’s efficiency is improved by using a steam jet ejector. Using the idea of gas dynamics, motive steam is utilised to compress a portion of the vapour created in the last effect. The compressed vapour from the ejector, coupled with expanded motive steam (S + Dn), is entrained as a heat source in the first effect, where it condenses and releases its latent heat inside the tubes.
Feed that has been pre-heated Each evaporation effect is sprayed with seawater, which drips down as a liquid layer. Due to the condensation of motive steam inside the tubes, the film warms up to boiling point, and a portion of it evaporates. A portion of the condensate is returned to its source, while the remainder is added to the desalinated water product. The first effect’s created vapour D1 is directed to the second effect. The vapour travels through the wire mesh demister to separate entrained brine droplets before proceeding to the second effects.
At a lower pressure and temperature, the produced vapour D1 acts as a heat source in the next effect. The hot brine from the first effect flows into the second, which has a lower temperature and pressure. The second action produces vapour D2 by flashing brine B1 and evaporating feed seawater F2. All effects go through this iterative process.
The ejector is used to continually evacuate non-condensable gases (NCG) that have accumulated in the condenser. NCG has the potential to disrupt system operation and performance. They can lower heat transfer efficiency and condensed vapour partial pressures.
The problem with mechanical compression is that you have vapour that is leaving in the last effect. We can see it here in the figure, the last effect vapour is taken out and all the vapour is routed into a mechanical vapour compressor, the compressor compresses the vapour from the last effect pressure to the first effect a little bit more than the first effect pressure so, that superheated vapour is going to enter into the tubes and heat the seawater which is distributed, it is distributed in two heat exchangers
- A brine feed heat exchanger preheater and
- A distillate preheater.
The seawater has been distributed in both of the sides to be and then the feed is taken out of these units and is distributed in parallel is coming in it heats the seawater to the saturation temperature and part of it will evaporate. The vapour will be taken out it will be used for as a source of heat and the next effect where it will condense eating there and the issue here or the process continues until the last effect where the vapour leaving the last effect is taking all of it as an input to the mechanical compressor.
The mechanical compressor can be connected into a diesel engine, or it can be connected it can be connected just to an electric motor can be operated by a photovoltaics system. This superheated vapour is taken as the source of heat and the socket impingement and terms that it is self-contained, you don’t need to connect it to a power plant all you have to take it is that you have a unit that you can move it anywhere as long as you have mechanical work input to the unit in terms of diesel engine or whatever or even electric mode.
Wind Turbine is another option, but you don’t connect it directly to the wind turbine because though interpret does not operate in continuously whenever there is when it operates this speed will increase if you have a strong wind the speed will decrease a wind turbine that’s connected to a generator to produce electricity and then the electricity runs the compressor.
The problem with this system is vapour compression requires a lot of energy because you’re compressing vapour and the as you have seen in the equation that you have, that we looked at in terms of the power calculations for the compressor have a single effect system or about the effect system that the equation is a function of the suction pressure and specific volume.
The specific volume of vapour is large and accordingly the work required to operate this large. So, in terms of power consumption, this is an energy consumer it consumes a lot of energy compared to other desalination systems. Therefore, people have thought of other ideas instead of using a compressor others distance. So, the whole thing here remains the same except for the compressor itself is going to operate with a different systems absorption for example is one of them vapour absorption or adsorption.
Absorption is a source so that you have some sort of strong liquid desiccant like calcium chloride or lithium bromide or lithium chloride, it absorbs the water vapour at a low pressure. So that the water vapour is as if it dissolves in that solution and then you pump the solution to a high pressure through a pump and a pump requires much less power than a compressor and then at high pressure you heated by solar energy. So that the water vapour is separated from the solution, and you take the water vapour here as a source of heat and then the solution is taken as a strong solution.
You pass to a certain amount of vapour to it and then the cycle of the absorption is repeated. One of them is a cooling effect for air conditioning and the other one is desalination effect.
For a mechanical vapour compression system, one of the effects is going to have a vapour space mist eliminator condenser or evaporator tubes, spray nozzles for the seawater calming and play is for collecting the brine that does not evaporate.
The vapour moves from left to right in our case here because as you can add additional amount of vapour that goes to the second effect, and then it will condense and meanwhile it will generate vapour that will go to the third effect. So, vapour is moving from effect to one, two to three to four, it’s moving basically in this direction.
Whereas the seawater is going perpendicular from up to down through the sprayers. So, we’re talking about here parallel flow system where horizontal flow of the vapour from left to right, vertical flow of the seawater which is being sprayed it after being distributed equally between the effect or in this unit the vapour leaving the last effect is going to the compressor right away. But what we do will preheat the seawater by the brine which has been rejected.
It’s at relatively high temperature, we’ll make use of that for preheating the seawater and the district that we are connected, it’s also making use of in preheat.
Description of the Process
- Compressed vapor is introduced into the tube side in the first effect; while, on the shell side feed seawater is sprayed on the tube’s top rows.
- The brine spray forms a thin falling film on the succeeding rows within the evaporator.
- In the first effect, the brine falling film absorbs the latent heat of the compressed vapor. As a result, the brine temperature increases to saturation, where evaporation commences and a smaller amount of vapor forms.
- This vapor is used to heat the second effect, where it condenses on the tube side and releases its latent heat to the brine falling film.
- This process is repeated for all effects, until effect n.
- Removal of the down condenser is a result of routing the entire vapor formed in the last effect to the mechanical vapor compressor.
- In the compressor, vapor is superheated to the desired temperature and pressure.
- At the other end, the feed preheaters recover part of the sensible heat found in the rejected brine and distillate product streams.
- This improves the system thermal efficiency and maintains production at the design levels.
- The main difference of the MEE-P and MEE-PC is that in the later system, the brine leaving effect (i) is introduced into the brine pool of effect (i+1).
- This occurs as a result of the positive temperature difference for the brine of effects (i) and (i+1) and a small portion of the feed brine flashes off as it is introduced into effect (i+1).
- The flashed off vapours improves the system productivity and thermal efficiency.
- In effect (i+1), the flashed off vapours are added to the vapor formed by boiling within the same effect.
- As for the MEE-P process, the brine leaving each stage is directly rejected to the sea.
Basic assumptions for modelling include
- Constant heat transfer area in each effect
- Negligible heat losses to the surroundings
- Salt free distillate product.
Parallel stream MEE systems is sub-classified to:
- MEE-P MVC only parallel stream
- MEE-PC MVC parallel-cross stream
I just mentioned that our dough and it’s been repeated since we talked about that earlier and flashing good be there the system that I showed a while ago has flashed boxes. You can see it so that the distillate that condenses inside the tubes, it goes into a flush box the flush box normally is connected to that or it’s at the same condition in terms of pressure as the effect that it’s connected to will have compressed superheated vapour the vapour will come in it will condense it will the superheat and then condense at a constant pressure which is that pressure of steam coming.
This is higher than the pressure of perfect number one. So that when you get that liquid, distil it to a flash box of a lower pressure sound flash and will occur, you take the flash of vapour in addition to the vapour which is coming by boiling from the effects and both of these are going to be the source of heat for the next effect and the things be repeated here.
The difference that we have compared to the previous installations of MED systems is that the number of flash boxes here is equal to the number of effects because for this effect, the first for this system, the first effect has a flash box in the previous ones we did not have flash box with the first effect. Because in the previous installation, we had vapour coming from a power plant. So, it’s not our vapour, it’s not our product, you condense it, and you turn it into the power plant.
But here the vapour that we’re using as a source of heat is basically our product, which is coming from the last effect that So, we talked about the process already. Which is basically is given in sort of summary in this slide and a modification to the system is being done through what not having a parallel effect, but a parallel cross effect and the parallels cross away.
Seawater Desalination Frequently Asked Questions
1) What is Multiple Effect Distillation (Med) in power plant?
The Multiple Effect Distillation (MED) evaporator is made up of numerous parallel cells, each of which is maintained at a lower pressure (and temperature) than the preceding cells (cold).
2) How does Multiple Effect Distillation MED desalination work?
Differential Distillation MED is a desalination technique that uses both heat and electricity to provide drinkable water. This sort of system has numerous chambers. The pipes transport the hot steam from the nuclear power plant into the chamber while repelling the seawater.
3) What is the difference between MSF and MED desalination?
Evaporation occurs from a seawater film in contact with the heat transfer surface in a Multiple Effect Distillation (MED) plant, but only convective heating of seawater takes place inside the tubes in an MSF plant, and evaporation occurs from a flow of brine “flashing” to produce vapour in each stage.
4) What is thermal Vapour compression?
MED with thermal vapour compression, often known as TVC, is the distillation technique that is growing the quickest. The process combines the basic MED system with a steam jet ejector in order for the plant to utilize the enthalpy and kinetic energy from motive steam.
5) What is Med TVC?
Using low to medium pressure steam and energy, the Multi-Effect Distillation (MED) thermal desalination technique creates distilled-quality water right from seawater. The technology is known as MED-TVC because it combines a Thermo Vapour Compressor with MED.