14.3Technical questions

1. Why water + HCHO + methanol stream is sent to a specific section of the absorber but not the top section of the absorber ?

Ans: This is to maximize the removal efficiency of both water and formaldehyde rich solution.  If both are sent from the top, then formaldehyde rich solution will be dilute and not effective in extracting more HCHO + methanol from the gas phase stream.

2. Explain Why light end stripper is used after absorbe ?

Ans: Water + HCHO + Formaldehyde solution may absorb other light end compounds which are not desired for absorption.  This is due to the basic feature of multicomponent absorption where absorption factors for various absorbing components is not biased sharply and other undesired components also get absorbed.  Therefore, the light end stripper would take care of removing these unwanted components by gently heating the same. 

3. Suggest why pure formaldehyde is not produced in the process ?

Ans: Pure formaldehyde is not stable and tends to produce a trimer or polymer.  Formaldehyde is stable in only water and therefore, 37% formaldehyde solution with 3 – 15 % methanol (stabilizer) is produced as formalin and sold.

4. What type of process design is expected for the air preheater ?

Ans: Since we have a problem of vapour and air, we should use extended surface area heat exchanger or finned heat exchanger.