The urea route involves two reaction steps: 1) synthesis of urea from NH
3 and CO
2; 2) production of DMC from urea and methanol, while methylcarbamate is formed as intermediate component. The operating conditions are milder than the previously reported processes, yet the purification of DMC by pressure swing distillation is a very energy-intensive process [
4]. For the EC route, ethylene oxide (which is a toxic compound) is required as raw material and the EC reactor requires extreme reaction conditions (588 °C, 125 bar) to produce it, though the process is more efficient [
4]. Lately, a new improved pathway has been discovered to produce DMC by indirect alcoholysis of urea (PC route). The production of DMC by indirect alcoholysis of urea is an environmentally friendly method, due to the use of CO
2 captured from the air, the use of two reactants that are regenerated in the process (propylene glycol (PG) and NH
3), and rather mild operating conditions. The process requires three reaction steps: 1) urea synthesis from NH
3 and CO
2; 2) PC synthesis from urea and PG; 3) DMC synthesis from PC and methanol (as illustrated in Fig. 1). In the reaction between urea and PG, one of the products (NH
3) has a low boiling point and can be easily removed from the reaction mixture. Therefore, the chemical equilibrium does not impose any serious constraint and high conversions can be achieved. However, the DMC synthesis is a reversible reaction which can be performed using an excess of methanol or an excess of PC to enhance the conversion. High conversion of PC can be obtained when a reactive distillation column (RDC) is used [
3]. However, the drawback of using an excess of methanol is that only a DMC-methanol mixture with azeotropic composition can be obtained from RDC, and new energy-intensive separation steps are required to obtain high DMC purity by extractive distillation or pressure swing distillation [
7,
8,
12]. More recently, a novel reaction-separation-recycle (RSR) process using conventional technology was developed, where the DMC synthesis takes place at an excess of PC, allowing to obtain high purity DMC, while recycling the DMC-methanol azeotrope [
8]. However, that process using conventional operating units could be significantly improved further by employing RD as an effective process intensification technology. Shi et al. [
7] developed such a process where reactive distillation technology and heat integration are used to reduce the total annual cost (
TAC). Indeed, reactive distillation technology and heat integration can offer significant advantages, but the drawback of that process is the use of an excess of methanol for the DMC synthesis, which leads to an azeotropic composition of DMC-methanol product. Further, the separation of this mixture by extractive distillation or pressure swing distillation is an energy-intensive process and makes the use of an excess of methanol unfavourable [
12,
13].