Ft

Solvent: 10 Parts

Reaction

Distillation

Reactant 2 Reactant 2

Figure 7.26 Comparison of liquid and gas phase phosgenation of isocyanates.

Distillation

Product

Reactant 1 ^

Reaction

Distillation

Product

_►

Oxygen Depolarized Cathode Process 2 NaCI + H2O + 1/2 O2 CI2 + 2 NaOH

2H20

NaCI

I H2

k NaOH

2H20

NaOH

NaCI

'h9o

k NaOH

j/l!

20H" 2NaV

>

1\ r

r /

ST

2CI

H20|

NaCI1

'h9o

Energy consumption: 2.5 MWh /t Cl2

Energy consumption: 1.8 MWh /1 Cl2

Figure 7.27 Comparison of conventional membrane and oxygen depletion cathode process for chlorine production.

ity is produced during synthesis of hydrogen with oxygen. Oxygen is supplied to the cell and is reduced on the cathode to hydroxide (OH"). Thanks to this principle, the co-product hydrogen is used in the electrolysis to recover electricity, which consequently reduces the specific energy consumption by another 30% to 1.8kWht_1 chlorine. Of course, hydrogen is no longer a product of such a cell. The technology can be applied both for HC1 electrolysis as well as conventional chlorine-alkali electrolysis [27, 31].

Microstructured Equipment

Microstructured process equipment, such as reactors, mixers or heat exchangers, have been introduced in recent years as a novel approach to improve the efficiency of chemical processes. The dramatic increase of specific surface areas in micro-structured equipment enables a more precise control of process conditions, which is often the key to significant increases in yield and selectivity of chemical reactions, reduced solvent and reactant excesses or improved heat transfer rates and recovery of waste heat streams. For example, an improved temperature control in a microstructured reactor can result in significantly reduced side reactions and less pronounced hot-spots for rapid, exothermic reactions. As the separation of unwanted side products often is the major cause for energy-intensive downstream purification steps, this can offer a tremendous potential for improved energy efficiency [34].

In addition, intensified heat exchanger/reactor concepts (HEX reactor) are the subject of process development [35]. The primary aim is to improve the temperature control leading to higher yield and selectivity or to directly use the heat of an exothermal reaction in an endothermal reaction to improve the energy efficiency.

7.11 Advanced Process Control and Performance Monitoring | 265

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