[behnam]

Dear frinds

I want to evaporate about 70% of 5000 liters solution (acid formic 80% + other chemicals) in batch process and in a vaccum reactor and by lp steam that it can go to jacket or coil in the reactor. reactor has an impeller.
I d like to know your suggestions that how can i calculate the evaporation rate approximately?? thanks

[kkala]

behnam, on 11 March 2010 - 01:48 PM, said:

I want to evaporate about 70% of 5000 liters solution (acid formic 80% + other chemicals) in batch process and in a vaccum reactor and by lp steam that it can go to jacket or coil in the reactor. reactor has an impeller.
I d like to know your suggestions that how can i calculate the evaporation rate approximately??

Having experience only from phosphoric acid evaporation using a water ring vacuum pump, my opinion could be as follows.
A1. Estimate max evaporated liquid rate that can keep vacuum at an acceptable level. This may be known from previous reactor operation. Capacity of heat exchangers is expected to be higher than this rate.
A2.1 Estimate liquid rate evaporated at acceptable pressure as above by using LP steam in the coil to its max capacity. If this liquid rate is higher than the above, using only heating coil is adequate. If not, similarly estimate evaporated liquid rate by using LP steam in the jacket. You may need LP steam to both jacket and coil (manual regulating e.g. the LP steam to coil).
A2.2 Heat transfer calculations for A2.1 could be based on "Heat transfer in agitated vessels" in the chapter of "Heat-exchange equipment", of the book "Unit Operations of Chemical Engineering" by McCabe, Smith & Harriot; or on other similar references. Controlling resistance is that of liquid, so assume a constant high value for heat transfer coefficient of steam (e.g. 1500 Btu/(ft2h oF))(Note 1).
However max heat flux (depending on vacuum level) can limit heat transfer below the rate indicated by above heat transfer coefficients. See "Maximum flux and critical temperature drop" in the chapter of "Heat transfer to fluids with phase change" of mentioned book by McCabe, Smith & Harriot, or similar references. "Pool boiling of saturated liquid" is expected. So heat transfer (and subsequent evaporation) will be the least among the two calculated (overall coefficient or max heat flux).
B. After having estimated liquid rate evaporated, you can start the real action by gently introducing LP steam to the agitated vessel filled with liquid. Probably you will not verify estimates totally, but you will be more aware of the development. As evaporation proceeds batch wise, remaining liquid will be more concentrated, so vacuum is expected to be higher.
I am not aware of formic acid and may neglect safety issues on its handling. It may be good not to operate to the limits (e.g. max evaporation rate), but leave a margin. I remember having done "experiments" in a fertilizer plant; even though we were cautious and never in a hurry, unexpected accidents occurred (fortunately without victims).
Note 1: For partial coefficient of liquid side h=200 Btu/(ft2h oF), order of magnitude estimate by Branan (Pocket Guide to Chemical Engineering, film resistances, organic solvents). Relevant fouling factor is probably close to 1/h.

This post has been edited by kkala: 28 March 2010 - 01:44 AM

[behnam]

Thanks for your suggestion.