Increasing demand for green chemistry
Epoxy resins are synthetic resins that can be processed into thermosetting plastic and are used for many different purposes. Epichlorohydrin (ECH) is an important component in the production of epoxy resins. Due to its toxicity, ECH is, however, an unappealing chemical. It is essential for the production of epoxy resin. To minimise the risk to which people and the environment are exposed, it seems sensible to keep epichlorohydrin away from roads and public places and to manufacture it on a decentralised basis, directly at the location where it is processed. A question that also needs to be answered is whether the classic approach involving the use of fossil oil-based propylene is not outdated nowadays, since natural glycerine (which is produced in the course of biodiesel manufacturing) is available today.
Epoxy resin is a viscous polymer that is used for many different purposes, e.g. as a component in adhesive and fibre-reinforced composite materials, in inks/paints and coatings. When a curing agent is added to it, epoxy resin forms rugged, resistant plastics, without which our world would not be the same as we know it. The special thing about epoxy resin is the special compound it contains. Epichlorohydrin (ECH) is needed to produce epoxy resin. This is a colourless liquid that smells like chloroform and has tremendous chemical appeal because of what is at the heart of the molecular structure: epichlorohydrin has an epoxy ring that opens under appropriate conditions and in the presence of a suitable reaction partner such as bisphenol A and that forms stable, chlorine-free epoxy resins.
The road to the future: from fossil oil chemistry to glycerine chemistry
Epichlorohydrin was produced synthetically in a laboratory for the first time in 1854. According to German hazardous substances regulations, this chemical is combustible, caustic, very poisonous and a danger to health. In spite of this unappealing property profile and due to a lack of suitable alternatives, epichlorohydrin is in greater demand than ever before all over the world – and not just in the plastics industry. Epichlorohydrin is also used in the paper and textile industry as well as, for example, in the production of active substances for medical drugs, insecticides and tensides.
Until a few years ago, epichlorohydrin was produced mainly from propylene, a colourless gas that is obtained petrochemically from fossil oil via the thermal division of short-chain hydrocarbon compounds. In this laborious manufacturing process, chlorine is used to turn propylene into allyl chloride, which reacts with hypoclorous acid in the course of the reaction to form 1,3-dichloropropane-2-ol and 2,3-dichloropropane-1-ol (dichlorohydrin). When sodium hydroxide (NaOH) is added, another reaction takes place, with the formation of epichlorohydrin; what is left over is a mixture of water, salt and organic residue, which has to be disposed of.
Biodiesel production is an important source of raw glycerine – basis of sustainable solvents.
Demand for synthetic glycerine has decreased with the boom in biodiesel production; industry has discovered a different source to satisfy its raw material needs that is better because it is more sustainable: “fuel from fields”. Large quantities of raw glycerine are a by-product when biodiesel is manufactured. Glycerine is used in the pharmaceutical and cosmetic industry, e.g. as an ingredient in soap and toothpaste. It acts as a sweetener in beverages and bakery products, as a moistening agent in tobacco and as an important additive in the production of alkyd resins and polyether polyols. The range of different applications is wider than this, however.
Isopropylidene glycerine, glycerine formal and glycerine carbonate are some of the products that can be made from biogenic glycerine. “According to the literature, all three glycerine derivatives are excellent, high-boiling solvents that can be blended with all standard solvents, such as water, alcohols, ketones, acetals, esters, ethers, aromates, benzene hydrocarbons, chlorocarbons, turpentine, essential and other oils”, explains the chemist Dr Michael Charwath. He adds that practical process engineering experience has demonstrated that epichlorohydrin can also be manufactured in a very efficient and sustainable way from biogenic glycerine.
The alternative to conventional and comparable processes
Before the biodiesel industry developed into what is now considered to be a major source of raw material for the production of epichlorohydrin, glycerine was manufactured primarily from synthetic, i.e. petrochemical, epichlorohydrin. Experts are convinced that the use of biogenic glycerine obtained from sustainable agricultural sources is more environmentally and economically sensible and beneficial in the long run, since it helps not only to protect the environment but also to increase the user’s efficiency and added value.
There is nothing new about using glycerine in the production of epichlorohydrin, as the German patent 197308 filed by the Böhringer company in 1906 shows. In 1960, Leuna Werke presented its “process for the continuous production of epichlorohydrin from glycerine” (DE Patent 1,075103), which such companies as Solvay, Dow and Spolchemie evidently took as the basis for filing patents of their own that only differed to a minor extent from the Leuna Werke patent when examined more closely, however. Following an analysis of the progress made to date, which led to the conclusion that all of the above-mentioned processes had optimisation potential as far as energy efficiency and value for money were concerned, KVT – a company from Austria – also started to develop a process of its own.
Epoxide resins are important to produce wind energy.
What the experts focussed on was, on the one hand, the principle of using renewable raw materials. On the other hand, the process was supposed to achieve ambitious economic and environmental objectives. They presented what they called the Epiprovit process as the outcome of their efforts.
Let us take a look, first of all, at the general technical details of the obtainment of epichlorohydrin from biogenic glycerine (sorry, but some technical background is essential): with the help of hydrochloric acid and/or hydrogen chloride gas, the biogenic glycerine is converted into dichlorohydrin (DCH) via monochlorohydrin (MCH). Saponification of the DCH with sodium hydroxide (NaOH) leads to epichlorohydrin (ECH). A somewhat more extensive description of the saponification process: fats can be returned to their components, i.e. glycerine and fatty acids. To do this, the fat is heated up with a lye, sodium hydroxide (NaOH), for example. In this process, the fatty acids separate from the glycerine, but are immediately neutralised by the lye. What are produced are water-soluble salts – the soaps. The reaction expressed in a chemical formula is then as follows: fatty acids + lyes = soaps + glycerine. If sodium hydroxide is used as the lye, hard soap is produced, while soft soap is the result when potassium hydroxide lye is used. The epichlorohydrin is then, finally, separated from water, salt and contaminants in the course of a rectification operation involving the application of heat energy.
A look at the process engineering details
The Epiprovit process starts with obtainment of the HCl gas, which is needed to chlorinate the biogenic glycerine. This operation can be carried out without any major technical facilities by installing an appropriate gas supply system (including storage). Although this is said to be the simplest option, the experts are convinced that taking this route leads in the long term to considerable additional costs, while some of the tremendous economic potential available to the operator of the equipment is not exploited. It is claimed that chloralkali electrolysis, which can be carried out close to the process in a cost-effective way, is more advisable.
Stage 1: HCl gas supply
To start up the equipment, HCl gas is formed in the familiar way involving an exothermic reaction between chlorine gas (Cl2) and hydrogen (H2) in which energy is generated. Before it can be fed into the process, the excess H2 that has been used needs to be removed. All of it can, however, be fed into the process too, if the excess hydrogen is subsequently subjected to thermal treatment and is released to the atmosphere in line with German air pollution regulations.
Stage 2: chlorination
The glycerine is chlorinated under the influence of HCl gas and is initially converted into an aqueous solution of dichlorohydrin via an intermediate stage as monochlorohydrin; all that is needed to produce epichlorohydrin is dichlorohydrin (DCH). The blend of MCH, DCH and hydrochloric water is divided up into two phases via (azeotropic) distillation: the DCH in an aqueous solution is saponified immediately after separation of the aqueous phase. A mixture of MCH and DCH that is produced unavoidably as a result is in turn rectified and separated: DCH is saponified and MCH is chlorinated again.
Stage 3: saponification
Saponification of the DCH, i.e. its processing into epichlorohydrin, is carried out with the addition of sodium hydride (NaOH) and/or sodium lye. The mixture that is produced, consisting of ECH and DCH as well as water and salt (NaCl) is separated via rectification. While the epichlorohydrin in an aqueous solution is taken to the next rectification stage and is separated from the water, the salt-water mixture that contains residue goes through another saponification operation. The aqueous epichlorohydrin is rectified, with water and organic residue being removed; the brine produced is in turn purified, so that the salt can, for example, be reused in the process (chloralkali electrolysis).
Stage 4: the process is completed
The result of the final rectification operation, the purpose of which is to remove water and residues from the final product, is extremely pure epichlorohydrin. Notwithstanding other claims, the quality of the glycerine used becomes clear in the course of this process, while the fact that it is very important to use a first-rate raw material is confirmed too. The amount of residue that has to be disposed of properly is impressively small as a result.
Epoxyd resin is a essential ingredient in producing composites using in aerospace applications.
What else needs to be said after all this
The advocates of the use of renewable raw materials and/or biogenic glycerine consider that the production of epichlorohydrin from propylene with the help of conventional process technology is inefficient and thoroughly outdated. Irrespective of this, it cannot be assumed that all the glycerine-based processes available for this purpose on the market reflect the possibilities offered by process technology nowadays. The environmental aspect, the use of renewable raw materials, is one side of the coin; the other side is saving energy and operating costs, while at the same time increasing yield and maximising profits. The questions that are being asked about residue processing need to be answered in the context of efficiency and environmentally responsibility.
The inventors are convinced the process for the production of epichlorohydrin from biogenic glycerine is a convincing solution as regards both economic performance and sustainability. They claim that the final product that is manufactured is of excellent quality – as is the by-product sodium chloride, which can be recovered in a very pure form and can be reused in the manufacturing process to reduce operating costs. GDeußing
A look ahead
The long wait will be over soon: for the first time, the Topic of the Month in August 2013 will be concentrating specifically on K2013 in October. Something to look forward to ...