July 12, 2010

MVR - Use it for Higher Benefits

In my last post, I mentioned some background about mechanical vapor re-compression.

In this post I am going to illustrate some benefits out of MVR system.

In general they must be seen as energy saving schemes in the current scenario of high energy costs in most of the processes. The need to recover or re-utilize energy is essential in thermal separation processes such as evaporations, distillation etc.

Following are the general options available in the industry out of which MVR is the most unutilized or neglected one while being the most efficient one.

1. Multi Effect evaporation (MEE)
2. Vapor re-compression
a. Thermal recompression
b. Mechanical recompression

I am not covering other options as they are well known as my focus here is to mainly emphasize on the benefits of MVR. However, for comparison purpose we will start with MEE only.

In a multi effect evaporation plant, the vapors produced in the first effect are utilized as the heating medium of the second effect and so on. This effectively reduces steam consumption in proportion to the number of effects. Ideally unit mass of vapor on condensation can evaporate unit mass of liquid. The vapors generated at the first effect are condensed in the second stage to further evaporate the liquid from the second stage and so on. A temperature gradient of about 7-10°C is maintained between stages for maximum efficiency.

So a triple effect evaporator would consume only 35-36% of the energy in comparison to a single effect system. Thus for each kg of steam input to first effect we can theoretically produce 3 kg of evaporation, practically it would be as below because of efficiency of evaporation in the subsequent systems. So let us work on some nos.

The efficiency of 95% is assumed in all evaporation systems through out this comparison

Feed to first effect (say) – 1 Kg steam
The evaporation from first effect – 0.95 Kg steam
The evaporation from second effect – 0.90 Kg steam (i.e. 0.95 x 0.95)
The evaporation from third effect – 0.86 Kg steam (i.e. 0.90 x 0.95)

So total evaporation possible in a 3 stage MEE is = 0.95+0.90+0.86 = 2.71

This is generally the overall economy ratio of a 3stage MEE system. This means for every kg of live steam fed to the system you can remove 2.71 kg of water from the MEE system. Thus, the so called COP is 2.71.

Thermal recompression
Now let us see thermal recompression system which is nothing but an ejector system which boosts the pressure of first effect evaporation by using higher pressure steam so that it can be utilized in the next effect. The major difference is that it requires high pressure steam to elevate the pressure of evaporated steam and may or may not require pressure gradient in the system which is essential in MEE.

Initially, heating steam is used to initialize evaporation. The vapors evaporated are compressed to higher pressure and temperature by steam jet ejector, condensed back for heat recovery and the residual vapors are taken to second stage for condensation / heat recovery. The amount of surplus energy contained in the residual vapor corresponds to the amount of energy supplied for steam jet ejector operation. This is taken as additional heat input / work done for recovery of large heat content of the evaporated vapors

So by theory practically the economy ratio is 1 for thermal compression system as whatever energy we put into the system in terms of motive steam is recovered back in the condenser of the next stage.

So now it’s a turn of MVR system. The process is that the initial vaporization is done using steam or electrical heater to initiate the process & then system may balance itself if you run only one stage of evaporation (by inherent nature of the process & economy ratio you do not need more than one stage practically unless there is a huge requirement or some mechanical issue is involved). So let us now see the calculations.

Every kg of steam initiated from the system is already having its latent heat. Let us assume that the process is near atmospheric condition. So the latent heat is ~2257 KJ/Kg at 100 C & 1 bar.

Now this steam is taken by the compressor & compressed to bit higher pressure. The energy required for compression is given as electrical energy which is increased by the efficiency terms over & above the theoretical requirement. This compression energy depends on pressure ratio. Generally MVR are currently designed to a pressure ratio limit of 1.5. So if the compressor is increasing the pressure to say 1.5 bar then the work done by compressor is

W = (P2 – P1) x Vs x 100

W = specific work done = KJ/Kg
P2 = final pressure = 1.5 bar
P1 = initial pressure = 1.0 bar
Vs = specific volume of steam = 1.67 M3/Kg

So the energy required is

W = (1.5 -1.0) x 100 x 1.67 = 83.5 KJ/Kg

Now let us assume overall efficiency of compression process including electrical, transmission efficiencies is 60%. Then total actual energy requirement is

W = 83.5/0.6 = 139 KJ/Kg

This means by putting an energy of 139 KJ/Kg, we can recover the latent heat of 2257 KJ/Kg out of steam. Therefore by definition of COP of the process, it is

COP = 2257 / 139 = 16.2

This means that one stage of MVR is equal to 16 stages of MEE in terms of energy efficiency. This also means that energy equal to one kg of steam put into an MVR will or can evaporate 16 Kg of steam from the system.

Now in terms of cost of energy, theoretically the power cost is double than steam cost. So effectively the ratio will be 16.2/2 = 8. Thus, effectively the operation cost can be reduced by 8 times in comparison to MEE or any other option.

Following are the other advantages

  1. Low specific energy consumption i.e. energy consumed per kg of product.

  2. Unmatched Higher Performance co-efficient i.e. the overall economy of the operation.

  3. Gentle evaporation of the product due to low temperature differences and hence useful for temperature sensitive operation. In fact it is never a problem for any material because it operates only at very low delta T.

  4. No load or reduced load on cooling towers as there is no CW condenser in the system.

  5. Simplicity of process, operation & maintenance.

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Anonymous said...

It's indeed a good alternative..and best among other mentioned.

Though I would like to add that As the normal compression ratio is 3 to 4 ..which will limit the saturation pressure of the steam...

Thus this temperature will limit the process side outlet temp...

And also the steam will have an icreased moisture content.. so the means to achieve a dry steam should be incorporated in the system from safety, ecocnomy & design point of view.

But the COP of at leat 8 is to loafty to miss...


Moiety said...

The cost of electricity over steam as a 2:1 ratio is absurdly optimistic; a ratio closer to 8:1 is more likely.

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