5 replies to this topic

[Merek Roman]

Hello everyone, I just got out of a presentation about a bio-based ethylene plant, and part of the project included getting DDGS from the leftover mash from the ethanol-water distillation. Anyways my group used a series of 4 centrifuges and then 3 evaporators; because centrifuges are very expensive. Anyways the professor roasted me alive because I should have had 1 triple-effect evaporator; I used timmerhaus for price of an evaporator. I guess do any professionals have any suggestions when to use a single effect versus a multi-effect evaporator?

I understand the concept, here is a pretty animation for those who dont understand.

http://rpaulsingh.co...ures/fig8_1.htm

Thanks in advance

[katmar]

It's all about steam economy. In a multi-effect evaporator you can use the energy in the initial steam over and over. In the sugar industry it is not uncommon to have 5-effect evaporators. There is of course a temperature drop from each effect to the next, so one requirement will be that you can operate over a range of temperatures. This would rule out temperature sensitive products, unless you could employ deep vacuum.

In terms of capital cost the multi-effect unit will probably be a bit more expensive than separate steam driven units (or one very large unit) because you are constrained on the temperature driving force per unit, and also because the vacuum units have to be larger to keep vapour velocities reasonable.

[riven]

It's all about steam economy. In a multi-effect evaporator you can use the energy in the initial steam over and over. In the sugar industry it is not uncommon to have 5-effect evaporators. There is of course a temperature drop from each effect to the next, so one requirement will be that you can operate over a range of temperatures. This would rule out temperature sensitive products, unless you could employ deep vacuum.

In terms of capital cost the multi-effect unit will probably be a bit more expensive than separate steam driven units (or one very large unit) because you are constrained on the temperature driving force per unit, and also because the vacuum units have to be larger to keep vapour velocities reasonable.

Going to the basics of multi effect evaporator (MEV) gives the answer.

In an MEV water is boiling off in a sequence of vessels that are held at lower temperatures. Therefore the steam from evaporator can provide some or all of the heating duty and temperature difference for evaporator B... when designed correctly and depending on how deep in dryness you want to go.
Therefore there is a large potential for steam savings compared to a series run of single evaporators all at the same pressure. For the MEV you will need to provdie fresh steam to the first stage and for subsequent stages, considerablly less fresh steam will be required. For the sseries run units fresh steam will be required for each unit. For 3 stages that could be quite a saving.

Do not worry so much about the capital cost; the MEV in food ethanol plants and fuel ethanol plants is always the most expensive single unit operation on site and generally the biggest too.

[katmar]

Do not worry so much about the capital cost; the MEV in food ethanol plants and fuel ethanol plants is always the most expensive single unit operation on site and generally the biggest too.

It is precisely because the evaporator is an expensive piece of kit that you do have to be concerned with its cost.

[riven]

Do not worry so much about the capital cost; the MEV in food ethanol plants and fuel ethanol plants is always the most expensive single unit operation on site and generally the biggest too.

It is precisely because the evaporator is an expensive piece of kit that you do have to be concerned with its cost.

Should have been clearer; don't worry if your MEV seems to be very expensive compared to other items; this is what we would expect.

[kkala]

I agree with members, a multiple effect evaporator (MEV) is the proper choice because of the resulting steam economy. My understanding is that steam produced in one effect goes to the next effect without any external steam addition. External steam is added to the first effect only.

If the external steam used in a single evaporator is S, for the n effects this is somehow more than S/n. But how many effects n? One way is to find this number in the literature of the relevant process and adopt it. For instance, Alumina plants concentrate NaOH solution in three effect evaporators (evidently optimum between savings in steam and extra investment, compared to single effect). There must be a proper n for every process to avoid the rather long procedure of optimization.

If no information can be found, following rough procedure may be a guide, subject to criticism:
Suppose capital cost of one effect is Q. Total cost of all effects is nQ.
One single effect evaporator would have capacity n times that of Q (actually more), so its capital cost would be Qn^0.6 (six tenth factor sizing). So the two options, for payout time estimate, are:
Single evaporator: Capital Qn^0.6, external steam consumption S (operating cost)
n effects : Capital Qn, external steam consumption S/n (actually more)
So n is the one giving the minimum payout time.
Above has not been applied in practice, it is only theoretical and comments could improve it; and does not take boiling point elevation into account.