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Publication Date: 2011-Nov-01
Document File: 4 page(s) / 102K

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The integration of dividing wall column technology in the traditional PG process to lower capital investment and energy consumption cost for new production facilities is described.

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The utilization of a divided wall column in the propylene glycol process


 The increasing capital costs and energy prices make it advantageous to improve the traditional PG process for new production facilities. The integration of a dividing wall column (DWC) helps to lower both the capital investment as well as the energy consumption. How this technology is integrated in the PG process and its capability will be described in the following sections.

The conventional PG process

In the conventional PG process, PG is formed by the thermal hydrolysis of propylene oxide (


) in an excess of water in an exothermic reaction. This exothermic reaction can take place in a liquid filled adiabatic plug flow reactor.
In a typical operation, at least a majority of the water is removed, such as by a multistage evaporator system. In the evaporator system the overhead streams are used to heat the reboiler of the next evaporator stage. This heat integration reduces the overall energy consumption of the production facility. In the evaporator train the water content is reduced to about 5 weight percent to 30 weight percent (wt.-%), with the preferred range being 8 to 15 weight percent. The remaining water will be removed in the so called dehydrator reducing the water content in the crude PG mixture down to about 10 ppm to 500 ppm of water.
The crude PG mixture consisting of the three main oligomers mono- (MPG), di- (DPG) and tri-propylene glycol (TPG) and a small fraction of higher oligomers is then separated into the three products MPG, DPG and TPG in three separate distillation columns.
A simplified sketch of the conventional PG process is shown in figure 1. The purification of DPG and TPG are not shown for reasons of simplicity.  In the MPG column, the product stream is taken as either an overhead or side draw.

Figure 1 Sketch of conventional PG process (evap: evaporator) 

The PG process with DWC

Since many studies indicated that separations consume 50 % to 70 % of the capital and energy of a traditional chemical process [1] a dividing wall column is an effective option in reducing both the capital and the energy costs of a separation unit [2]. The installation of a DWC can cut the capital investment and the energy costs by approximately 30 % [2].
A general description of the fundamentals of a DWC can be found in [2].
The majority of the stream leaving the bottom of the third effect evaporator is MPG which is, in turn, the middle boiling component of the following distillation tower.  The dehydrator and the MPG distillation tower work at similar pressure levels, and therefore combining these two towers into one is an excellent application of a DWC.
Inserting the DWC into the PG process does not change the upstream elements of the reactor and evaporator train, and the installation also leaves the purification of DPG and TPG unchanged.
A simplified sketch of the new production process is shown in figure 2 a...