The pilot plant study of cleaner production (CP) process of a dyes intermediate, Fast Bordeaux GP Base (2-nitro-p-anisidine), is presented in this work. The existing production process with acetic anhydride as raw material generates highly polluting (high chemical oxygen demand), huge-volume effluent, and thus the treatment is expensive. In the proposed process, raw material acetic anhydride in acetylation step is replaced with acetic acid. The reaction conditions like reaction time (3 h), temperature (120∘C) and molar ratio of p-anisidine and acetic acid (1 : 3.58) are optimized in the laboratory scale and implemented in pilot plant. The extent of conversion has been monitored by nitrite value test, and purity of product mixture is measured with thin-layer chromatography (TLC). The acidic wastewater quantity is dramatically reduced by incorporating recycling in washing scheme, and thus water consumption is reduced. Reduction in wastewater generation and reduction/elimination of treatment cost is also observed. Chemical oxygen demand (COD) of the effluent stream is reduced by the recovery of by-products sodium acetate and spent acid. The technoeconomical feasibility studies show that the proposed CP options are highly economical and environmental friendly.
Progress in the understanding of the phenomena of pollution and modifications to the ecosystem has revealed that environmental impacts must be seen from a product and process design point of view [
The high wastewater production of different industrial processes sustains high treatment cost and requires large effluent treatment systems. To reduce the impact of high water consumption, industries have been searching for alternative methods by adopting systems and technologies to control and treat (clean-up or end-of-pipe) the pollutant emissions generated by production processes. These pollution techniques require additional energy and materials, and disposal of resulting waste has negative influence on the society [
Fast Bordeaux GP base (2-nitro 4-methoxy aniline) is an intermediate dye used for yarn dyeing as raw material for pigments and for printing of cotton. The details of Fast Bordeaux GP Base are given in Table
Details of fast bordeaux GP base.
Molecular structure | Uses | ||
---|---|---|---|
Trade name | Fast Bordeaux GP Base | (i) Cotton and yarn dyeing | |
Chemical name | 2-nitro-4-methoxyaniline | (ii) Intermediate for pigments | |
Molecular formula | C7H8N2O3 | (iii) Printing for cotton. | |
Molecular weight | 168 | ||
Appearance | Amorphous, reddish orange coloured. | ||
Melting point | 129°C | ||
CAS no. | 96-96-8 |
The schematic diagram of conventional production process of Fast Bordeaux GP Base is presented in Figure
Conventional prodn. Process of fast bordeaux GP base (batch 400 kg of p-anisidine).
The detailed in-plant assessment was carried out, and various waste sources were evaluated from the theoretical consideration and actual material balance. The excess raw materials like acetic anhydride, nitric acid, caustic flakes, and soda ash as well as the by-product like sodium acetate were going into effluent treatment plant (ETP) as waste stream. Therefore, various CP options were generated for the waste minimization.
The use of acetic anhydride as an acetylating agent for acetylation of p-anisidine produced one mole of acet-p-anisidine and one mole of sodium acetate with soda ash. Excess of acetic anhydride was used to make the reaction mixture slurry. By-product sodium acetate and excess raw material acetic anhydride were discharged into the waste stream and in turn contributes high COD.
To solve this problem, a new method of acetylation using acetic acid in the presence of sulphuric acid (catalyst) was proposed, and the reaction conditions like reaction time, temperature and molar ratio of reactant were investigated. The chemical reaction involved acetylation with acetic acid as shown in Scheme
Reaction scheme for acetylation of p-anisidine with acetic acid.
The lab experiments were carried out in a round-bottom conical flask equipped with stirring facility. Molar ratio of 1 : 3.58 glacial acetic acid and p-anisidine was mixed to prepare homogeneous solutions. 1 mL of concentrated sulphuric acid was used as a catalyst. Samples of about 2 mL were withdrawn from the reactor at different intervals of time and analyzed. The extent of conversion was monitored by nitrite value test (Standard Methods, AWWA, 1995), and purity of product mixture was measured with thin-layer chromatography, (TLC) having 0.25 mm thickness silica gel G coated Al plates (Merck). The spots were visualized by exposing the dry plates in UV light using toluene:methanol (19 : 1 v/v, upper layer) as a solvent. After the complete conversion, excess of acetic acid was distilled out by distillation and the residue was taken for nitration stage. The distillate (acetic acid) was recycled back for acetylation in the next batch. The reaction conditions investigated were reaction time, temperature, and molar ratio of reactants.
The reaction was initially carried out at different time interval to know the equilibrium time for this proposed reaction. The reaction was carried out at different temperatures 80, 90, 100, 110, and 120°C for 3 h to incur the impact of temperature on the proposed reaction. The extent of conversion at various molar ratios of 1 : 1.75, 1 : 2, 1 : 3, 1 : 4, and 1 : 5 at a fixed time of 3 h and temperature of 120°C was also investigated. After completion of acetylation reaction, excess of acetic acid present in the reaction mass was recovered by distillation at 120°C.
According to theoretical calculation, 1 : 1 molar ratio (p-anisidine to acetic acid) was required for the conversion of p-anisidine to acet-p-anisidine, but this was not producing homogeneous solution; therefore, the molar ratio of 1 : 3.58 was selected for this reaction, which produced homogeneous solution. The reaction was carried out at different temperatures: 80, 90, 100, 110, and 120°C for 3 h. The extent of conversion at various reaction temperatures shown in Figure
Conversion versus time at different temperature with molar ratio of 1 : 3.58 in 3 h.
Conversion versus molar ratio of p-anisidine and acetic acid at 120°C in 3 h.
Distillate quantity versus time for distillation of excess acetic acid.
In the conventional production process, nitric acid was used in excess, and thus sodium acetate was produced during the nitration and hydrolysis stage, respectively, which contributed acidity and COD to the effluent. Huge quantity of fresh water was used for the washing of wet cake throughout the process. Stage-wise washing scheme was suggested to overcome these drawbacks for batch operation. The newly developed washing scheme with recovery and recycling options was as shown in Figure
Modified washing scheme with recovery and recycling option.
In a typical run, residue from acetylation stage was subjected to nitration with nitric acid, water, and sodium nitrite at temperature of 40°C–45°C. The progress of reaction was monitored by TLC. Upon completion of nitration, the reaction mass was filtered into vacuum filter. The mother liquor was collected in a separate collection tank. The wet cake was washed with fresh water for four times. The water of washings was collected separately and recycled in the next batches. The filtrate (mother liquor) and washed water of different washings was analyzed for the acidity of effluent. The washed cake was hydrolyzed with caustic flakes and water, at 76-77°C. The progress of reaction was monitored by TLC. Upon completion of the reaction, the reaction mass was filtered in vacuum filter. The mother liquor was collected separately in collection tank. The wet cake was washed with fresh water for four times, and the water of washings was collected separately and recycled in the next batches. The filtrate (mother liquor) and washed water of different washings was analyzed for the sodium acetate content. The percentage of acidity and sodium acetate in filtrate and washed water of three batches of nitration and hydrolysis was shown in Table
Acidity and sodium acetate in filtrate and washed water in nitration and hydrolysis step.
Acidity (% w/v as HNO3) of nitration | Sodium acetate (% w/v) of hydrolysis | |||||
Batch-1 | Batch-2 | Batch-3 | Batch-1 | Batch-2 | Batch-3 | |
Filtrate | 20 | 20.1 | 19.8 | 19 | 19.2 | 19.7 |
Washed water I | 7.2 | 8.4 | 9 | 7.5 | 8.1 | 8.3 |
Washed water II | 2.8 | 4 | 4.4 | 2.5 | 3.7 | 4.0 |
Washed water III | 0 | 1.2 | 1.6 | 0 | 1 | 1.2 |
Washed water IV | 0 | 0.3 | 0.5 | 0 | 0.3 | 0.4 |
The modified process flow diagram after accounting the technical viable options is shown in Figure
The comparison of existing and proposed practice.
Sr. no. | Process steps | Water consumption in kl | Effluent generation in kl | Recovered by-product |
---|---|---|---|---|
Existing process: Reaction temperature was maintained to 70°C by the addition of ice and reaction time of 3.5 h (3 h for acetylation and 30 min for neutralization with soda ash); yield was 90%. | ||||
(1) | Acetylation | Nil | 76 | — |
(2) | Nitration | 225 | 258 | — |
(3) | Hydrolysis | 275 | 280 | — |
(4) | Neutralization | 10 | 116 | — |
Total | 510 | 729 | — | |
Proposed process: Reaction temperature was 120°C, in reaction time of 3 1/2 h (3 h for acetylation and 30 min for distillation of excess acetic acid); yield was 95%. | ||||
(1) | Acetylation | Nil | Nil | Spent acid (15%) |
(2) | Nitration | 75 | Nil | Sodium acetate (15%) |
(3) | Hydrolysis | 125 | Nil | — |
(4) | Neutralization | 10 | 116 | — |
Total | 210 | 116 | — |
Proposed process flow diagram for the manufacturing of fast bordeaux GP base after CP.
Cleaner production techniques of raw material substitution and recovery and recycling operation have been used in the current study for the manufacturing of Fast Bordeaux GP Base. The following conclusions can be drawn. The existing production process is highly polluting and expensive due to the use of raw material acetic anhydride and also due to the disproportionate addition of raw materials. The proposed raw material substitution of acetic anhydride with acetic acid in acetylation step can reduce (from 76 kl to nil, refer to Table The proposed recovery and recycling operation in nitration and hydrolysis steps not only harvest the byproducts of spent acid (in nitration) and sodium acetate (in hydrolysis) but also eliminate the effluent generation in the process step and the selling of by-products can increase the economic benefits. The proposed production process further eliminates the requirement of effluent treatment of neutralization step due to the decrease in pollutant amount in effluent. The proposed options further contribute in saving compared to the existing practice besides increasing the % yield from 90 to 95. In view of the above findings, it can be concluded that the proposed cleaner production options are technoeconomically viable and ecofriendly.