New process for synthesizing epichlorohydrin

Thread Content

New Process for the Synthesis of Epichlorohydrin Using Glycerol and Industrial Hydrochloric Acid 1 Introduction Epichlorohydrin, also known as chlorohydrin (chemical name: 1-chloro-2,3-epoxypropane, abbreviated as ECH), is an important organic chemical raw material with wide-ranging applications and promising future prospects, as well as a key intermediate in the fine chemicals industry. Before the 21st century, the global industrial processes for synthesizing epichlorohydrin relied mainly on the high-temperature chlorination of propylene using propylene as the starting material, and the acrylate method. Subsequently, as the reserves of non-renewable fossil fuels dwindled, crude oil prices kept rising. Since propylene is primarily derived from petroleum refining, the pressure to increase costs in these two production processes grew ever greater. On the other hand, biodiesel, as one of the renewable energy sources, is developing rapidly; and for every 10 tons of biodiesel produced, more than 1 ton of glycerin is generated as a by-product. This has led to an increase in global efforts to develop new technologies for utilizing glycerin. Against this background, a process using glycerol as the starting material has become the preferred option for building new or expanding epichlorohydrin production facilities. According to statistics, since 2006, about seven or eight new, under-construction, or planned epichlorohydrin production facilities in China with an annual output of over 10,000 tons have adopted the glycerol method. All these glycerol methods employ a batch process in which hydrogen chloride gas is introduced into glycerol (hereinafter referred to as the gas absorption process). Their common feature is that, in the glycerol chlorination step, the amount of water in the reaction system is controlled (using various methods) in order to shift the equilibrium reaction in favor of the formation of products. Compared with the traditional process that uses propylene as a raw material, this process offers advantages such as easy access to raw materials and sustainability, no consumption of propylene or chlorine, no need for expensive catalysts, mild operating conditions, low costs, and reduced emissions of waste gases, solids, and liquids. However, it also has the following drawback: hydrogen chloride gas is difficult to store and transport ； The gas absorption concentration is low, resulting in a slow reaction rate ； Only a batch intermittent process can be used ； Multi-stage series absorption is required, which results in high equipment investment; yet the utilization rate of hydrogen chloride remains low, and the equipment utilization rate is also low ； Some processes use desiccants, increasing costs associated with wear and separation operations. To address the shortcomings of the existing glycerol-based gas absorption process, the author developed suitable catalysts and additives; by continuously adding a mixture of glycerol and hydrochloric acid, dichloropropanol and water were vaporized to drive the equilibrium reaction forward toward the formation of products. It not only retains the original advantages of the gas absorption process while overcoming its shortcomings, but is also easy to operate, results in lower costs, generates less waste, and thus improves the existing glycerol-based process. 2 Experimental Section 2.1 Instruments and Reagents Instruments: Agitent 6820 gas chromatograph (capillary column DB-10.53 mm×15 m). Reagents: Glycerin, catalyst, additives, sodium hydroxide (all of the above reagents are CP) ； Hydrochloric acid (AR and industrial grade). 2.2 Experimental Principle 1) Glycerol reacts catalytically with hydrochloric acid to yield dichloropropanol. The specific ratio of α-dichloropropanol to β-dichloropropanol varies depending on the type and amount of catalyst as well as other process conditions, but this ratio does not affect the subsequent synthesis of epichlorohydrin. 2) Dichloropropanol (structural isomers) reacts with a base to remove HCl, yielding epichlorohydrin. 2.3 Experimental Procedures 2.3.1 Chlorination reaction of glycerol with hydrochloric acid In a 250 mL four-necked reaction flask equipped with a distillation unit, a constant-pressure addition funnel, and a thermometer, 184 g of glycerol, 10 g of catalyst No. 5, 12 g of additive, 50 mL of industrial hydrochloric acid, and 50 mL of water were added. Once the mixture reached boiling point and distillates began to flow out, 300 mL of hydrochloric acid was added gradually, while keeping the temperature of the reaction mixture at 115 ± 2°C. After all the hydrochloric acid had been added, a premixed solution of glycerol and hydrochloric acid (with a molar ratio of glycerol:hydrogen chloride = 1:2.1) was added. The reflux ratio of the distillation column is controlled at about 1:3; the dichloropropanol produced as a reaction product vaporizes along with water. The distillate is separated into an organic layer at the bottom, and a sample from this layer is analyzed by gas chromatography, showing that the total content of dichloropropanol is 99.2%, allowing it to be used directly as a raw material for the subsequent epoxidation process. The upper aqueous layer is collected together and used to prepare an alkaline solution for the epoxidation step (this aqueous solution contains approximately 11% dichloropropanol at 19°C). By adding the reagents in this manner continuously for 30 days, a total of 2021 g of glycerol was used, and a total of 3,850 mL of 37% industrial hydrochloric acid was used. 4,557 g of distilled water was obtained, along with 2,261 g of crude dichloropropanol (with a total dichloropropanol content of 98.9%). Based on the amount of glycerol used, the yield of dichloropropanol is 93.9%. Since 238 g of residual reaction mixture remains in the reaction flask and has not been taken into account, the yield for this step can be increased further if the reaction time is extended. 2.3.2 Epoxidation of dichloropropanol: 26.5 g of sodium hydroxide was weighed and dissolved using 112 g of the aqueous solution obtained from the first-step reaction, which was used as the base solution for addition. Into a 250 mL four-necked round-bottom reaction flask equipped with a constant-pressure addition tube, a thermometer, a steam introduction tube, and a distillation water separation device, 65 g of the crude dichloropropanol obtained from the previous reaction was added. The mixture was heated to 90°C using an oil bath, and then the alkaline solution was added drop by drop, while steam was introduced to facilitate the vaporization of the epichlorohydrin produced as a byproduct out of the reaction flask. Add the alkali solution completely within 15 minutes, and then continue steaming for 2 minutes to complete the process. At this point, 53.9 g of the organic layer at the bottom was separated from the splitter, and a sample was analyzed by gas chromatography, showing an epichlorohydrin content of 91.4% and a total dichloropropanol content of 8.6% ； The 29 g of water in the upper layer of the water separator was extracted twice using extractor 20g x 2. The extracts were refined under atmospheric pressure, and 36e of the extractor was recovered. The residue was combined with the organic phase separated by the water separator and further refined under atmospheric pressure; 47.0 g of the main fraction at 115–116°C was collected. A sample of this fraction was analyzed by gas chromatography, showing a content of 99.8% epichlorohydrin. 4.5 g of the combined distillation residue and initial fraction is recycled as raw material for the next batch of epoxidation reaction. Based on the amount of dichloropropanol used, the yield of ECH in the first reaction is 85.3%; taking into account the distillation residues and the mixture from the initial distillation for a second reaction, the overall yield of ECH for this step is 90.3%. Based on glycerol as the starting material, the overall yield is 84.6%. 3 Results and Discussion 3.1 Glycerol chlorination reaction 3.1.1 Selection of catalysts for glycerol chlorination The activation energy for the reaction of glycerol chlorination to produce dichloropropanol is very high; this value is even higher for the second chlorination step. Catalysts must be used to reduce this activation energy and increase the reaction rate in order for this process to be viable for industrial application. Carboxylic acids are commonly used as catalysts. Initially, glacial acetic acid, a traditional catalyst, was used in the experiments. However, it was found that although glacial acetic acid provides good catalytic performance and is readily available, its boiling point (118.1°C) is close to that of water, which makes it easy to be carried away during dehydration distillation, resulting in significant losses. Furthermore, during the distillation and purification of the dichloropropanol intermediate, acetic acid can also easily mix into the product, making its separation and purification more difficult. The author investigated 6 entirely new catalysts that differ from existing patents. The experimental results are shown in the table below. Table 1 Performance of different catalysts in the reaction for preparing glycerol dichlorohydrin using glycerol and hydrochloric acid. Table 1: Behavior of various catalysts in the production of glycerol dichlorohydrin from glycerol and hydrochloric acid. Catalyst number, Catalytic efficiency*, Amount carried away by distillation*, Impurity content in the resulting glycerol dichlorohydrin/%, Degree of cross-linking and gelation of the reaction mixture: 1# – Good; no carry-away; 10-12%; gelation occurs after 2 days. 2# – Good; significant amount carried away; 5-6%; gelation occurs after 10 days. 3# – Insoluble; poor performance; no carry-away. 4# – Fair; some amount carried away; 3-6%; gelation occurs after 10 days. 5# – Good; no carry-away; ≤0.1%; gelation occurs after ≥30 days. 6# – Good; little amount carried away; ≤0.1%; gelation occurs after 5 days. Note*: With a material ratio of n(glycerol):n(catalyst) = 100:5, a good performance is indicated when glycerol dichlorohydrin is distilled out within ≤12 hours; a fair performance is indicated when it takes 12-17 hours ； 17-22h is normal ； 22-27h is poor ； ≥27h is poor. Note **: Refers to the percentage of the total amount of catalyst in the aqueous and organic phases evaporated over 24 hours, relative to the initial amount of catalyst added; a value of ≤0.1% indicates no carryover ； 0.1%-1% indicates low carry-out ； 1%–5% is the carryout ； ≥5% is too much to carry out. As can be seen from the results in the table above, except for catalyst No. 5*, the other catalysts are not suitable; therefore, catalyst No. 5* is chosen here. 3.1.2 The impact of different hydrochloric acids on the process 1) Differences between reagent-grade hydrochloric acid and industrial hydrochloric acid Once a suitable 5* catalyst was found, the experiments using reagent-grade hydrochloric acid progressed smoothly. However, reagent-grade hydrochloric acid is expensive, which is unfavorable for industrial production costs. To this end, the author conducted experiments with industrial hydrochloric acid; by the 9th day of continuous addition, the reaction rate slowed down, and on the 10th day, cross-linking and gelation occurred in the reaction mixture, with no product evaporating and the reaction being unable to proceed any further. To address this, the author used an additive; after adding it to the reaction mixture and continuing the addition over 30 days, no cross-linking or gelation occurred, nor was there any decline in the reaction rate or product quality. 2) Effect of hydrochloric acid concentration on the process The effect of hydrochloric acid concentration on the process is shown in Table 2 (reaction temperature: 115±2°C, reflux ratio: 1:3). Table 2 Effect of hydrochloric acid concentration
Concentration/% Drop rate/(mL/8h) Distillate Organics/g/8h Chlorodipropyl alcohol in organics/% Water/g/8h Hydrogen chloride in water/% Total amount of chlorodipropyl alcohol distilled/g
10 245 17.1 99.3 253.3 0 (pH=7) 45.0
15 156 26.6 99.2 172.3 0 (pH=7) 45.5
20 134 28.7 99.2 145.8 3 44.7
25 110 31.7 99.3 118.9 5 44.8
30 99 33.3 99.5 108.7 9 45.3
37 78 36.6 99.7 83.6 12 45.8

Note*: The total amount of chlorodipropyl alcohol distilled refers to the sum of the 11% chlorodipropyl alcohol contained in the aqueous phase of the distillate and the chlorodipropyl alcohol contained in the organic phase of the distillate; all other process conditions are the same as those used in the experimental examples. As can be seen from the table above, at constant reaction temperature and reflux ratio, different hydrochloric acid concentrations only affect the dripping rate and the amount of organic matter distilled per unit time, having little impact on the overall reaction rate (measured by the total amount of dichloropropanol distilled per unit time). Therefore, as long as there are no organic impurities, this process can be applied to recycled hydrochloric acid of various concentrations as well as crude glycerin with a purity of ≥80%. However, when the concentration is too low, the resulting dichloropropanol mainly dissolves in the distilled aqueous phase, which has an impact on the yield of the subsequent epoxidation reaction and on the amount of wastewater that needs to be treated (but this has little effect on manufacturers who need to absorb and recycle the hydrochloric acid produced or the hydrogen chloride gas generated as a by-product). It can also be seen from the table above that when a hydrochloric acid concentration of over 20% is used, the water distilled contains a certain amount of dilute hydrochloric acid. However, this dilute hydrochloric acid can be returned to the reaction system; by controlling the dripping rate and the reflux ratio, the water distilled again can reach a neutral pH of 7 and contain almost no hydrochloric acid, thus allowing for full utilization of the hydrochloric acid. 3.1.3 Temperature and reflux ratio: An increase in temperature accelerates the reaction rate and raises the distillation rate of dichloropropanol; however, if the temperature is too high, the content of impurities such as monochloropropylene glycol in the distilled product increases significantly. Based on comprehensive experiments, a temperature of 115±2°C is appropriate. A high reflux ratio results in high purity of the distilled dichloropropanol, but it leads to a low production rate and high energy consumption ； With a low reflux ratio, the production rate of dichloropropanol is high and energy consumption is low, but the content of impurities such as monochloropropylene glycol increases. Therefore, while ensuring the purity of the produced dichloropropanol, the reflux ratio should be reduced as much as possible; comprehensive experiments have shown that a reflux ratio of around 1:3 is appropriate. 3.2 Synthesis of epichlorohydrin from dichloropropanol 3.2.1 Process selection The saponification of dichloropropanol to epichlorohydrin involves numerous side reactions; primarily, the resulting epichlorohydrin further reacts with alkalis, leading to hydroxylation (including hydroxyl substitution of chlorine and hydroxyl ring-opening addition). Therefore, to prevent further hydroxylation of epichlorohydrin and thus reduce its yield, it is necessary to shorten the contact time between ECH and the base, so as to remove it from the reaction mixture as quickly as possible. The author chose a process that involves raising the temperature of the epoxidation reaction and using steam to promptly remove the generated epichlorohydrin from the reaction system. Here, the high temperature ensures that the epoxidation reaction proceeds rapidly, thereby reducing the contact time between the starting material, dichloropropanol, and the base ； Timely stripping ensures that the generated epichlorohydrin no longer comes into contact with the alkaline solution, thereby preventing side reactions and resulting in a higher yield. 3.2.2 Influence of process parameters on ECH yield The main process parameters that affect the ECH yield include temperature, time, and alkali concentration. Since the concentration of the alkaline solution is determined by the concentration of hydrochloric acid used in the previous step, when industrial concentrated hydrochloric acid is used, the concentration of the resulting alkaline solution is around 20%, which is thus taken as a fixed parameter. The experimental results on the effect of temperature and reaction time on the ECH yield are shown in Table 3 below. Table 3 Effect of epoxidation reaction parameters on the yield of ECH. Temperature/°C, Time/min, Initial yield of epichlorohydrin/%: 70, 15 – 67.6; 80, 15 – 79.5; 90, 15 – 88.2; 100, 15 – 70.0. For temperatures of 90°C, the times were 5 min, 25 min, and 35 min respectively, with yields of 70.8%, 82.0%, and 77.6% accordingly. As can be seen from Table 3, a temperature of 90°C is appropriate when starting the addition of the alkaline solution to the reaction mixture. At low temperatures, the ECH produced at the start of the reaction continues to react with the base due to the temperature not reaching the azeotrope temperature with water (88°C), resulting in a decrease in yield ； When the temperature reaches 100°C or higher, boiling over occurs easily, and the solubility of ECH in water increases, resulting in a decrease in yield. A reaction time of around 15 minutes is appropriate; if the time is too short, the rate at which the alkaline solution is added will be too fast. As it is an exothermic reaction, the reaction becomes intense and temperature control becomes difficult ； The time taken is too long, resulting in significant amounts of unreacted dichloropropanol being vaporized, and thus the yield per batch is not high. 4 Conclusion From the above experimental results, it can be seen that compared with the gas absorption process, the author’s new process has the following advantages. 1) The source of raw materials is more convenient. Since various concentrations of hydrochloric acid are suitable in the new process, it is also applicable to the large quantities of crude 80% aqueous glycerol produced as a by-product of biodiesel; moreover, the price of 80% glycerol is often only half that of 98% glycerol. Furthermore, compared to hydrogen chloride gas, liquid hydrochloric acid is more convenient for storage and transportation. Additionally, the cost of recovering hydrochloric acid free of organic impurities is low, all of which provide significant potential for reducing the costs of raw materials used in new processes. 2) Gas-based processes can only use batch processing, whereas the new process allows for continuous automatic control; its simple operation and stable quality control make it more suitable for industrial application ； 3) The utilization rate of hydrochloric acid is improved without the need for multiple absorption reactors, reducing equipment investment. 4) Since it is a homogeneous reaction and the hydrochloric acid concentration is high, the reaction rate is fast. 5) For manufacturers that produce hydrochloric acid either as a primary product or as a by-product, the hydrogen chloride gas can be recycled by reusing the epoxidation wastewater. This not only improves the utilization rate of glycerol but also eliminates wastewater discharge, further reducing costs related to raw materials and wastewater treatment. 6) No desiccant is required, eliminating the costs associated with desiccant consumption and separation processes. Furthermore, since the product quality and overall yield (based on glycerin) of the new process are high, and the catalysts and additives used are the result of experimental exploration with no intellectual property issues involved, this new process is feasible for industrial application. Last edited by brucehan on 2008-12-20 09:09.]