Rectification under Vacuum Conditions

Rectification columns running below atmospheric conditions are referred to as vacuum rectification columns [3]. For industrial applications, pressure levels from several hundred millibar(a) down to 2-10 mbar(a) or even lower are applied. The reasons to perform the process below atmospheric conditions result from the effects of 

The second main effect of decreasing the rectification pressure is the increase in average relative volatilities, which is defined as the ratio of the vapour mole fraction over the liquid mole fraction of two components  

The average relative volatility can be understood as a measure for the ease of separation. With an increase in average relative volatility, as a rule, the separation gets easier and the number of theoretical stages required to achieve the target fractionation cut is reduced respectively, at a given number of theoretical stages, the same 

In contrast, it is highly relevant that with the same number of theoretical stages, the reflux ratio to achieve the specified fractionation split is lowest for the 

In the heat-integrated two-column system, the total heat consumption is further decreased. As the vacuum column is heated using vapours from the pressurised column, only llOOkW is consumed in total. Thus, compared to the one-column system, the heat integration of the two-column system leads to a very significant and cost competitive 40% reduction in heat consumption. 

---

Packed towers are used for rectification under vacuum conditions, for gas scrubbing and absorption, for vaporization and in liquid-liquid extraction. Applications are also direct heat transfer between gas and liquid phases and/or between gas and liquid in heterogeneous reaction processes (often with parallel phase flow). 

Table 16.1 illustrates several characteristic effects from running rectification columns under vacuum conditions. For example, naturally, the bottoms temperature is lowest at vacuum conditions and it increases with pressure, but for this application this effect is of no practical relevance as none of the species is corrosive or subject to thermal degradation at the applied temperature conditions. 

The use of lower reflux ratios for the separation of mixtures under vacuum conditions results in a lower consumption of heating steam, compared to rectification under normal pressure. 

The influence of the system on the resistance coefiflcient i[f is sufiflciently described for practical appHcations by the kinematic viscosity, expressed in the modified Reynolds number. In addition, it is sufficient to determine the function i/r = / (Rey) based on simulation tests with air under ambient conditions, in order to then calculate the pressure drop Apo/H of the dry packing, using Eq. (3-8), accurately enough for rectification systems under vacuum and normal pressure and for the high pressure range, as well as for any type of absorption process. 

For practical reasons, key importance has been attached to the turbulent flow range, ReL > 2, as this is the range mostly used for operating packed columns containing large packing elements in rectification processes under normal pressure and vacuum conditions and/or in pressure rectification and in absorption processes at moderate liquid loads. 

Processing of petroleum under the above-specified conditions results in a vacuum residue of 585°C cut point. The combined rectification yields the following cuts ... 

The preparation of polycarbonates is carried out in two stages. In the first stage a polycarbonate of lower MW is synthesised, at 150-200 °C, by distillation of EG resulting from reaction 8.35 under moderate vacuum (6.6-26.6 MPa). As for all polyesterification processes, a rectification column assures the elimination of EG and ethylene carbonate and the glycol used as the starter are returned to the reactor. In the second stage, the low MW polycarbonate is heated up to 250 °C under conditions of high vacuum (13-1333 Pa), and is condensed to a higher MW polycarbonate [11]. 

---