(569ad) Systematic Approach for Sustainable Process Design: A Case Study of the Thermal Chlorination of Methane

Kasper Torbensen & Lasse Hansen
empap@kt.dtu.dk

The chlorination of methane is a well-known and studied chemical process, which has supplied chloromethanes for a range of uses since its first industrialization in the first part of the 20^th century, and continues to be relevant. The process itself has undergone mechanical and structural changes, as technology and the physical and chemical sciences have progressed over the years. Today, chloromethanes are mainly used as solvents and as precursors for Teflon. This project evaluates this process, with regards to its overall structure, as well as its associated expenses, mechanical and material. In this work, which was part of the MSc course 28350: “Process Design: Principles and Methods” at DTU, a thermal methane chlorination plant was systematically synthesized using the hierarchical decomposition method to produce 90,000 tons chloroform per year.

The overall chlorination process itself is divided into four main parts; reaction, hydrochloric acid (HCl) removal by adsorption, product recovery, and product refining, the latter two by distillation. During the reaction part of the process the four different chloromethanes are produced; methyl chloride (CH₃Cl), methylene chloride (CH₂Cl₂), chloroform (CHCl₃), and tetrachloride carbon (CCl₄). The latter three are considered products, while methyl chloride is recycled. The reactor is followed by an HCl-absorption unit, along with a number of preparation steps, before the chloromethanes are recovered in the distillation columns. Recovery of excess, unreacted methane and methyl chloride is recovered by means of distillation, along with trace amounts of chlorine. To avoid accumulation of chlorine in the reactor, a gas-membrane system is implemented as a purging unit.

As proven by a number of studies, the previous use of chloromethanes as refrigerants have been connected to the depletion of the ozone layer, which is why heavy precautions are to be implemented in the plant to prevent these gases to enter the atmosphere, as is common practice when dealing with highly toxic substances. Therefore, high product separations efficiency is necessary and the purge stream cannot contain any chloromethanes, which also is why a gas-membrane is applied in the recycle and purge stream. After obtaining the base case process design, this was optimized, especially with regards to heat integration, which reduces the annual expenses for utilities needed to control the temperature flow through the process. Additionally, to further improve the base case design, sustainability and LCA analysis was performed, to identify the process hotspots.

This environmental aspect, as well as a thorough energy integration of the process, adds a new perspective to this conventional, industrial process. To further elaborate on this, performing hazard analysis of the plant according to the Seveso II directive could provide intel on key aspects of the process, based on the chemical inventory of the plant.