00:00:04.440 --> 00:00:10.020 In this screencast I am going to describe the operation of a triple effect evaporator 00:00:10.020 --> 00:00:16.289 where we have backward feed. There is a separate screencast that describes a triple-effect 00:00:16.289 --> 00:00:23.000 evaporator with the feed in the same direction that the steam is fed. And we will see exactly 00:00:23.000 --> 00:00:31.060 what that means for this backward feed system we are going to feed into the third effect, 00:00:31.060 --> 00:00:37.989 so we will call this the 3rd, the 2nd and the 1st. And we are going to feed a dilute 00:00:37.989 --> 00:00:47.589 solution. For example, a solid in water. And then, our objective is to create a concentrated 00:00:47.589 --> 00:00:55.429 solution out of the first effect. And the idea of using a triple-effect evaporator is 00:00:55.429 --> 00:01:02.079 that we use significantly less energy than a single effect. We can essentially, say for 00:01:02.079 --> 00:01:12.120 water, evaporate 3 kilograms of water for feeding in 1 kilogram of steam. So the way the evaporation 00:01:12.120 --> 00:01:18.830 is going to work in the various stages, what we are going to do is feed into the heat exchanger 00:01:18.830 --> 00:01:27.360 coil, steam, and the outlet of this is going to be condensate, namely the steam that is 00:01:27.360 --> 00:01:34.290 condensed liquid water. We use the energy from the steam to evaporate out this first 00:01:34.290 --> 00:01:42.520 effect. What we are going to do with this vapor is use it now as the heating source 00:01:42.520 --> 00:01:49.580 for the second evaporator. So this means we are going to feed in, now, this steam again 00:01:49.580 --> 00:01:55.840 to the heat exchanger coil. Again, coming out it is going to be liquid, as we condense 00:01:55.840 --> 00:02:03.530 the steam. And that steam of course is going to evaporate. So we are going to have vapor, 00:02:03.530 --> 00:02:11.810 call that vapor 2. Well that vapor is then going to be fed into the heating coils for 00:02:11.810 --> 00:02:19.340 the third effect and coming out of the third effect again is liquid water. So that is part 00:02:19.340 --> 00:02:24.440 of the steps of how we are getting evaporation and of course in the third step means, we 00:02:24.440 --> 00:02:32.980 are again forming vapor which is the solvent. Well we fed into this third effect a dilute 00:02:32.980 --> 00:02:39.590 solution. Because we are evaporating some of the liquid of the solvent we have made 00:02:39.590 --> 00:02:44.950 a more concentrated solution so that means we have more concentrated solution leaving 00:02:44.950 --> 00:02:52.130 the third effect. So we want to feed that now to the second effect. However, one consequence 00:02:52.130 --> 00:02:58.780 of running this system in the direction we are running it is that temperature 2 for this 00:02:58.780 --> 00:03:07.000 system, namely that is the temperature here, that is the temperature of the more concentrated 00:03:07.000 --> 00:03:16.690 material that is leaving here. So T2 is going to be higher than T3. So if we are going to 00:03:16.690 --> 00:03:24.420 feed the liquid coming out of the third effect into the second effect And it becomes the 00:03:24.420 --> 00:03:32.530 feed for the 2nd effect, we actually have to raise the pressure because P3 is less than 00:03:32.530 --> 00:03:38.790 P2 because we are talking about saturation temperature and of course bonding pressure. 00:03:38.790 --> 00:03:45.349 So we can't feed it in without raising the pressure. So that means we take out this more 00:03:45.349 --> 00:03:54.489 concentrated liquid, we have to use a pump to raise the pressure and then we can feed 00:03:54.489 --> 00:04:02.129 the concentrated liquid into the second effect. And then the same thing, the outlet from the 00:04:02.129 --> 00:04:11.459 second effect, because, again, temperature 1 is greater than T2. Which means P1 is greater 00:04:11.459 --> 00:04:20.709 than P2. So the liquid leaving the second effect also needs a pump to raise its pressure 00:04:20.709 --> 00:04:28.940 so it can be fed into the first effect where it is concentrated to form our final concentrated 00:04:28.940 --> 00:04:36.900 solution. And so this describes a triple-effect evaporator. A couple more things to point 00:04:36.900 --> 00:04:46.840 out. The mole fraction now in the liquid of our solute, x1, is greater than x2 is greater 00:04:46.840 --> 00:04:59.240 than x3. x3 is what is leaving here, x2, here, x1, here, and this would be x of the feed 00:04:59.240 --> 00:05:05.540 and feed mole fraction is even lower. So this describes the operation where we take advantage 00:05:05.540 --> 00:05:12.660 of a vapor that we form in one effect as the heat source, and that is why T1 must be greater 00:05:12.660 --> 00:05:20.590 than T2, in order for it to be able to evaporate the liquid in the second effect. Likewise 00:05:20.590 --> 00:05:27.720 the T2 must be greater than T3 to have a temperature difference such that we can evaporate the 00:05:27.720 --> 00:05:35.400 vapor 3. There is a couple reasons why we might run the triple effect evaporator with 00:05:35.400 --> 00:05:43.070 this backward feed. One is if our fresh feed that is coming in here, this is our dilute 00:05:43.070 --> 00:05:49.250 solution. If our fresh feed is cold. The reason being now, it will take a smaller amount to 00:05:49.250 --> 00:05:55.580 heat it to our lowest temperature rather than heating it to our highest temperature. The 00:05:55.580 --> 00:06:02.900 second would be that as we concentrate the solute the liquid becomes more viscous. So 00:06:02.900 --> 00:06:10.820 the viscosity increases as x increases, the mole fraction of the solute. And therefore 00:06:10.820 --> 00:06:18.680 we want the hottest temperature to be when we have the highest concentration of the solute 00:06:18.680 --> 00:06:24.199 because the heat transfer is going to be higher when the material is less viscous and at a 00:06:24.199 --> 00:06:30.560 higher temperature it is going to be less viscous. Other aspects of this are the same 00:06:30.560 --> 00:06:38.139 as the forward feed, and an important aspect is of course that we can get a lot more evaporation 00:06:38.139 --> 00:06:44.990 for the same amount of feed of the steam. So this is the big cost in the process separate 00:06:44.990 --> 00:06:49.979 from the capital costs is this steam being provided to the system. And the more we can 00:06:49.979 --> 00:06:54.700 evaporate the steam the more economical the process is.
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