Contraction-expansion Helical Mixer And Its Application In Continuous Preparation Of Azo Dyes

With the full color range, easy synthesis, superior color fastness and good compatibility, azo dyes have become one of the most widely used synthetic dyes. However, traditional route adopted to manufacture azo dyes is the batch method under ice bath, which brings about the difference in color strength and color shade. Along with mass and heat transfer deficiency in the batch method, the yield and the quality of azo dyes cannot reach the peak. Unlike the batch method, the continuous coupling process allows the reactant particles to collide and react instantaneously, which enhance the mass transfer rate and eliminate the difference between batches. Thus, the study of continuous coupling reaction is of great theoretical and practical value.In this study, a contraction-expansion helical mixer was designed especially for coupling reaction. With the computational fluid dynamics (CFD) simulation and the segregation index method, the influences of radius of curvature, ratio of the length of the expansion part to contraction part (Le/Lc), and the pitch of helical mixer on mixing efficiency were investigated The numerical and experimental results indicate that:the optimum mixing performance is obtained when the radio of Le to Lc is1to4, screw pitch of helical mixer is5mm, and the radius of the curvature is10.5mm. The results also demonstrate that the mixing performance of contraction-expansion helical mixer is superior to the common helical mixer.In this paper, the contraction-expansion helical mixer was utilized as the central mixing and reacting unit for the continuous coupling reaction. Aniline, p-nitroaniline and the sulfanilic acid were the diazo components, and1-(4-sulfophenyl)-3-methyl-5-pyrazolone,1-benzoyl-8-hydroxy-3,6-naphthalenedisulfonicacid,1-amino-8-hydroxy-2-(4-sulfophenylazoyl)-3,6-naphthalenedisulfonic acid acted as the coupling components. Thus nine azo dyes were synthesized by the continuous method. These continuous coupling reactions of the nine azo dyes were set as the model reactions, and the influences of the initial pH value of the coupling components, temperature, flow rate and residence time on the yield of the azo dyes were investigated. The results reveal that:the yield of the nine coupling reactions is higher than95%when the flow rate ranges from0.509m/s to0.764m/s and the initial pH value of the coupling component is in the range of9.8-10.0at ambient temperature with the absence of a heat exchanger. The reaction conditions of the model dyes were used to guide the continuous synthesis of the commercial dye-C.I. Reactive Black5, whose yield turns out to be95.5%.The effects of the structure of the diazo and the coupling components on the residence time in the contraction-expansion helical mixer were investigated. The results imply that:the coupling components of1-(4-sulfophenyl)-3-methyl-5-pyrazolonerequiredthe longest residence time, while as for the diazo components, the residence time ofl-benzoyl-8-hydroxy-3,6-naphthalenedisulfonic acid is the shortest for the complete reaction. It was also discovered that among the diazo components, the p-nitroaniline diazonium salt demanded the shortest residence time while the residence time for phenylamine diazonium salt is the longest because the residence time of the continuous coupling reaction can be reduced by the electron withdrawing group of the diazo components.In our study, the yield of azo dyes synthesized by the continuous method and the batch method were studied, and the azo dyes were also applied to silk dyeing to study the color and fastness properties of the dyes. The results showed that the yield of the azo dyes synthesized by continuous method was increased by2.0%-20.1%compared with the batch method, the exhaustion was increased by0.4%-12.9%, the K/S value was increased by1.1%-5.7%, the lightnesswas increased by0.7%-16.0%, the chroma value was increased by0.9%-52.3%, and the dry and wet rub fastness of the azo dyes synthesized by continuous method was ascendant by more than half grade than the corresponding azo dyes synthesized by the batch method. The light fastness of the yellow azo dyes synthesized by the continuous method was comparable with the corresponding yellow azo dyes synthesized by the batch method, while the light fastness of red and blue azo dyes was half grade and one grade higher than the corresponding azo dyes synthesized by the batch method respectively.The influence of the amplification effect on the continuous synthesis of C.I. Reactive Black5was investigated.The study found that the purity of C.I. Reactive Black5which were continuously synthesized in2-3contraction-expansion helical mixer(inner diameter of the thinner pipe and of the thicker pipe is2and3mm respectively),4-6contraction-expansion helical mixer (inner diameter of the thinner pipe and of the thicker pipe is4and6mm respectively) and6.5-10contraction-expansion helical mixer (inner diameter of the thinner pipe and of the thicker pipe is6.5and10mm respectively) was individually81.5%,66.4%and56.7%, higher than the purity (30.8%) of C.I. Reactive Black5synthesized by the industrial batch method.To achieve the same color depth, the dose of C.I. Reactive Black5synthesized in4-6and6.5-10contraction-expansion helical mixer only covers53.9%and62.9%of industrial product which indicates that the novel synthesis method or the synthesis in4-6and6.5-10contraction-expansion helical mixer can save46.1%and37.1%of dyes in dying process.