21.2.3 Technical questions

1.  Can the steam generated in the waste heat recovery boiler be sufficient to pre-heat the air if a principle of steam reuse is adopted ?

Ans: From the basic principle of thermodynamics, some where heat needs to be added to the system. If we presume that the feed pre-heater heats the feed to 650°C (the desired temperature of the reactor) and the catalyst is regeneration energizes the catalyst by 100°C (from 550 to 650°C), the flue gas should be able to generate some steam. However, it can be speculated that the steam generated will not be enough to meet the total air pre-heater requirements. Therefore, some make up steam will be always required.

2. Why is ammonia allowed to get absorbed into cuprous ammonium acetate ?

Ans: Ammonia absorption into cuprous ammonium acetate facilitates maximum absorption of butadiene. This is because ammonia in many cases enhanced absorption factors by allowing chemical interaction between molecules which is better than just physical absorption.

3. Why is the fluid recirculated in the quench tower ?

Ans: We are targeting gas quenching using a liquid in this process. Quenching a liquid with liquid is instantaneous but quenching a gas with a liquid needs considerable amount of time, and hold up time is very important. Therefore, fluid internal recirculation is required in the vapour quenching tower.

4. Why a cooler is used in the quenching tower ?

Ans: The quenching operation increases the temperature of the fluid that is used for quenching. And we can see that the fluid is always circulated. Therefore, the purpose of quenching is to just cool the gas and don't absorb any of the components into it self. Therefore, to do so, the fluid shall be cooled in between.

5. Why compressor and cooler are at all required after quenching process ?

Ans: The unit following quench tower is absorption. Absorption is favoured at higher pressure and lower temperature. Therefore both compressor and cooler are required to achieve these conditions of favourable absorption.

6. Why cooler is followed after compression but not vice versa ?

Ans: In general, compression enables enhancement in the temperatures of the gas/vapour. This is because we don't usually follow adiabatic compression but we follow polytropic compression in reality. And polytropic compression will enable heating the gas/vapour. Therefore, cooling is used after compressor but not vice-versa. If cooler is used before compressor then the gas entering absorber will be at a higher temperature and this is not favourable for absorption.

7. Explain in a more elaborate way what happens in the Butadiene purification fractionators ?

Ans: The butadiene purification fractionator is fed with ammonia + Butadiene stream in which somehow ammonia should be recovered. Therefore, this unit is part of the azeotropic distillation that separates butadiene + ammonia as one of the products and eventually butadiene and ammonia are separated using water, as water absorbs ammonia instantaneously to form ammonium hydroxide. Eventually, it is also easy to strip the ammonia gas from aqueous ammonia solution.

8. Why we cannot separate butadiene-butane mixture using ordinary fractionation ?

Ans: Their boiling points are very close and distillation requires large differences between boiling points of the components to be separated.