Formaldehyde and Chloromethanes | Chemical Technology - Chemical Engineering PDF Download

14.1 Introduction

• In this lecture, we present the production technology for formaldehyde and chloromethanes.
• Formaldehyde is produced from methanol
• Chloromethanes are produced from methane by chlorination route.

14.2 Formaldehyde production

14.2.1 Reactions

a) Oxidation: CH₃OH + 0.5 O₂→ HCHO + H₂O

b) Pyrolysis: CH₃OH → HCHO + H₂

c) Undesired reaction: CH₃OH + 1.5 O₂→ 2H₂O + CO₂

In the above reactions, the first and third are exothermic reactions but the second reaction is endothermic. The reactions are carried out in vapour phase.
Catalyst: Silver or zinc oxide catalysts on wire gauge are used.
Operating temperature and pressure: Near about atmospheric pressure and 500 – 600 ^oC

14.2.2Process Technology  ( Figure 14.1 ): 

• Air is sent for pre-heating using reactor outlet product and heat integration concept.
• Eventually heated air and methanol are fed to a methanol evaporator unit which enables the evaporation of methanol as well as mixing with air.  The reactor inlet temperature is 54 ^oC.
• The feed ratio is about 30 – 50 % for CH₃OH: O₂ 
• After reaction, the product is a vapour mixture with temperature 450 – 900 ^oC
• After reaction, the product gas is cooled with the heat integration concept and then eventually fed to the absorption tower.
• The absorbent in the absorption tower is water as well as formaldehyde rich water.
• Since formaldehyde rich water is produced in the absorption, a portion of the rich water absorbent solution from the absorber is partially recycled at a specific section of the absorber.
• From the absorber, HCHO + methanol rich water stream is obtained as the bottom product.
• The stream is sent to a light end stripper eventually to remove any light end compounds that got absorbed in the stream.  The vapors from the light end unit consisting of light end compounds can be fed at the absorption unit at specific location that matches with the composition of the vapors in the absorption column.
• Eventually, the light end stripper bottom product is fed to a distillation tower that produces methanol vapour as the top product and the bottom formaldehyde + water product (37 % formaldehyde concentration).

Figure 14.1 Flow sheet of Formaldehyde production

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 absorber?

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.

14.4 Chloromethanes 

Chloromethanes namely methyl chloride (CH₃Cl), methylene chloride (CH₃Cl₂), Chloroform (CHCl₃) and Carbon Tetrachloride (CCl₄) are produced by direct chlorination of Cl₂ in a gas phase reaction without any catalyst,

14.4.1 Reactions

CH₄ + Cl₂→CH₃Cl + H₂

CH₃Cl + Cl₂→ CH₂Cl₂ + H₂

CH₂Cl₂ + Cl₂→ CHCl₃ + H₂

CHCl₃ + H₂→ CCl₄ + H₂

• The reactions are very exothermic.
• The feed molar ratio affects the product distribution.  When CH₄/Cl₂ is about 1.8, then more CH₃Cl is produced.  On the other hand, when CH4 is chosen as a limiting reactant, more of CCl₄ is produced. Therefore, depending upon the product demand, the feed ratio is adjusted.

14.4.2Process Technology

• Methane and Cl₂ are mixed and sent to a furnace
• The furnace has a jacket or shell and tube system to accommodate feed preheating to desired furnace inlet temperature (about 280 – 300 ^oC).
• To control temperature, N₂ is used as a diluent at times.
• Depending on the product distribution desired, the CH₄/Cl₂ ratio is chosen.
• The product gases enter an integrated heat exchanger that receives separated CH₄ (or a mixture of CH₄ + N₂) and gets cooled from the furnace exit temperature (about 400 ^oC).
• Eventually, the mixture enters an absorber where water is used as an absorbent and water absorbs the HCl to produce 32 % HCl.
• The trace amounts of HCl in the vapour phase are removed in a neutralizer fed with NaOH
• The gas eventually is compressed and sent to a partial condenser followed with a phase separator.  The phase separator produces two streams namely a liquid stream consisting of the chlorides and the unreacted CH₄/N₂.
• The gaseous product enters a dryer to remove H₂O from the vapour stream using 98% H₂SO₄ as the absorbent for water from the vapour.
• The chloromethanes enter a distillation sequence.  The distillation sequence consists of columns that sequentially separate CH₃Cl, CH₂Cl₂, CHCl₃ and CCl₄. 

Figure 14.2 Flowsheet of Chloromethane production

14.4.3Technical questions

1. Why compressor is used before partial condenser?

Ans: The compressor increases the pressure of the system which is beneficial to increase the boiling points of the mixtures. Note that the boiling points of chloromethanes are -97.7, -97.6, -63.5 and -22.6 ^oC for CH₃Cl, CH₂Cl₂, CHCl₃ and CCl₄ respectively.  On the other hand, the boiling point is -161.6 ^oC.  For these boiling point mixtures, when the system pressure is increased substantially, the boiling points of the compounds increase and could reach close to those of the cooling water (20 – 30 ^oC).  Cooling water is required in the partial condenser and if it is not used, a refrigerant needs to be used which requires an additional refrigeration plant. Therefore, the system pressure is increased.

2. Why water is removed using the dryer?

Ans: Water enters the vapour system due in the absorption column where solvent loss to the vapour will be a common feature. Water molecule can react with the highly active intermediate chloromethanes to form oxychlorides, which are highly undesired.

3. Will there be any difficulty in separation by increasing boiling points of the chloromethanes in the distillation sequences?

Ans: Definitely yes.  This is because the relative volatility of compounds atleast slightly increases with reducing pressure and viceversa.  But due to cooling water criteria in the distillation sequences also, there is no other way economical than doing distillation at higher pressure.

4. Since the boiling point of CH₃Cl and CH₂Cl₂ are very close, what do you expect for the production of CH₃Cl from the first column?

Ans: It is indeed difficult to separate CH₃Cl and CH₂Cl₂ and therefore, good number of separation trays be used.  Or structured packing be used to reduce the height of the first column.

5. When the reactions are highly exothermic, why is the feed pre-heated?

Ans: Irrespective of the reactions being exothermic or endothermic, the reaction rate always increases with temperature for non-equilibrium reactions.  Therefore, feed is pre-heated to the desired temperature so as to fastly convert the reactants to products.