In the realm of industrial and pharmaceutical production, the generation of significant amounts of organic waste, primarily composed of ethanol and water, poses environmental challenges. Conventional methods of treating ethanol waste, such as incineration, often prove inefficient, costly, and environmentally unfriendly.

As a leading supplier in process and control equipment, Greatwall Process and Control understands the critical importance of utilizing cutting-edge methods and equipment for purifying industrial ethanol.

This blog post will delve into the innovative technologies that are reshaping the ethanol purification landscape, providing invaluable insights for processing companies aiming to elevate their production processes.

The Importance of Ethanol Recycling

The ethanol industry stands as a cornerstone in the economic fabric of industrial societies, finding extensive applications in chemical plants, pharmaceutical process, the food industry, personal care products, healthcare and additive ingredients, synthesis processes and beyond.

Beyond its role as a foundational material, ethanol plays a crucial part in various sectors, serving as a base for alcoholic beverages, extraction agents, solvents, detergents, and surfactants. Recycling ethanol waste emerges as a viable solution due to its versatile solvent properties.

Ethanol can dissolve a wide range of inorganic and organic substances, making it a popular choice for dissolving plant pigments, medicinal components, and serving as a solvent in various reaction process.

The efficient purification of ethanol is crucial, especially when considering its applications in the pharmaceutical, chemical, and food industries.

The production of industrial ethanol requires meticulous attention to detail, with impurities posing significant challenges.

Greatwall Process and Control recognizes the intricate nature of this process and offers tailored solutions that go beyond conventional methods.

Challenges in Ethanol Waste: Characteristics and Composition

1. High Suspended Solids Content

Industrial ethanol waste is characterized by a high suspended solids content, reaching an average of 40,000 mg/L. Addressing this challenge requires a systematic approach to gradually eliminate impurities from waste stream.

2. Elevated Temperatures

The temperature of ethanol waste is notably high, averaging around 70°C, with distillation vessel discharge temperatures reaching as high as 100°C. Managing such high temperatures is a key consideration in the purification process.

3. High Concentrations of COD (Chemical Oxygen Demand)

The COD of ethanol waste can range from 20,000 to 30,000, comprising suspended solids, soluble COD, and colloids. Organic compounds constitute 93%-94%, including carbohydrates, nitrogen-containing compounds, and residual products like butanol and unreacted ethanol.

4. Presence of Organic Acids

Ethanol waste contains approximately 500mg/L of organic acids, rendering the wastewater acidic. Balancing the acidity through alkali addition or sludge recirculation is a consideration during initial operations.

5. Inorganic Impurities

Inorganic elements, primarily dust and impurities from raw materials, contribute to the overall inorganic content in ethanol waste.

Purification Techniques: Tailored Solutions for Ethanol Recycling

The principles behind solvent recovery and purification involve regenerating contaminated organic solvents into clean ones, achieving theoretical recovery rates above 95% and practical recovery rates exceeding 85%.

The process not only prevents environmental pollution but also saves over 85% of solvent usage, thereby reducing production costs significantly.

Pre-purification methods such as steam distillation, vacuum distillation, and extraction are often employed before pervaporation membrane type dehydration process, followed by recrystallization treatment.

1. High-Concentration Ethanol Recycling

High-concentration ethanol wastewater holds considerable recovery value. Through processes like vacuum distillation and process kettle thermal heating and condensing techniques, solvents, by-products, and water can be efficiently removed, resulting in a higher purity ethanol concentration.

Additionally, ethanol recovery often targets concentrations ranging from 95% ethanol to anhydrous ethanol (99.7%).

Achieving such high ethanol concentrations may involve the use of packing type distillation towers, but managing water content is crucial, necessitating processes like zeolites membranes and molecular sieves.

2. Low-Concentration Ethanol Recycling

When dealing with lower ethanol concentrations, a phased approach is recommended. Separation process based on processing volume and internal components is conducted initially.

Solid waste separation through initial methods like filtration, impurity and salt removal through packing type distillation tower and kettle processes, and ethanol recovery can significantly reduce liquid waste discharge.

Advanced techniques like membrane bioreactors (MBRs) can be employed for microbial degradation to meet discharge standards.

Customized Solutions for Varied Industries

Industries, including pharmaceuticals, chemicals, and food production, often encounter diverse ethanol waste compositions.

Achieving optimal purification requires a tailored approaches based on material composition, scale, and site-specific conditions.

Currently, there is no one-size-fits-all solution, and the integration of multiple specialized processes is often necessary for effective treatment.

When changing the type of materials in the alcohol extraction tank or alcohol settling tank, ethanol in the tank needs to be recovered through the ethanol recovery tower under vacuum conditions.

Additionally, when the ethanol concentration in the alcohol extraction tank and alcohol settling tank falls below 30%, the ethanol in the tank should be recovered through the ethanol recovery tower.

Ethanol from the extraction process enters the recovery tower distillation kettle, where about 95% of the ethanol can be recovered.

The resulting low-concentration ethanol residue, which cannot be reused, serves as wastewater from the ethanol recovery tower. After treatment at the wastewater treatment station, it meets the standards for discharging.

Non-condensable and volatile gases are collected, washed in a scrubber, and the treated absorption liquid is discharged to the centralized wastewater treatment station. The recovered high-concentration ethanol, after cooling, awaits reuse.

Case Illustration: Ethanol Recovery in the Esterification Process

In the esterification reaction process, the produced “esterification water” undergoes separation in a distillation tower to recover benzene and ethanol, with wastewater discharge.

During the esterification reaction, the esterification water formed is a ternary solution of ethanol, benzene, and water.

Distillation begins the separation process, revealing two phases in the condenser: an “oil phase” containing a small amount of water and a “water phase” containing a small amount of benzene.

The phases mix to form a milky white solution, collected in the benzene measuring tank. After settling, the lower layer, containing water, is discarded, leaving the upper layer as benzene.

Once benzene is completely distilled from esterification water, the rising gases in the distillation tower consist of a mixture of ethanol and water vapor.

Material transfer occurs between the ascending gas and the reflux liquid cascading down. This transfer leads to a concentration gradient within the tower, resulting in higher ethanol content at the top and higher water content at the bottom.

The distilled vapor at the tower top, rich in ethanol, is condensed to recover ethanol. A portion is drawn off and refluxed into the tower as “reflux liquid.”

The ethanol content in the top vapor is primarily influenced by the reflux amount. A larger reflux amount yields higher ethanol content, while lower reflux results in lower ethanol content.

However, a larger reflux amount means less ethanol is recovered as a product, elongating the time needed to distill the same quantity of esterification water.

Controlling the reflux ratio (the ratio of reflux amount to product quantity) is crucial, initially using full reflux to ensure acceptable ethanol content, followed by gradual reduction.

The duration of distillation (under constant equipment, esterification water quantity, and cooling water conditions) depends on steam pressure and reflux ratio.

Insufficient steam pressure significantly prolongs distillation time, while excessive pressure compromises ethanol content. Maintaining appropriate steam pressure allows for both adequate ethanol recovery and efficient distillation.

1.2 Process Control Indicators

1. Each esterification water batch: 1500–2000kg.
2. Tower top temperature: <80°C (78°C).
3. Tower bottom temperature: 95–98°C.
4. Residue temperature: <99°C (end of distillation).

1.3 Quality Indicators

• Ethanol recovery content: ≥95.5%.
• Weight of benzene and ethanol recovery: >60% of the esterification water input weight.

2. Operation Procedure

1. Inspect water, steam, vacuum pipelines, valves, and instruments in the station for normal operation.
2. Start the pump and introduce esterification water into the distillation kettle.
3. Heat the distillation column with steam, initiate cooling water for the condenser, and when the liquid temperature reaches approximately 67°C, boiling begins, with the tower top temperature rising to around 67°C after 10 minutes.
4. After a few minutes, a milky white “benzene liquid” appears in the rotor flowmeter; all of it enters the benzene measuring tank.
5. After benzene is extracted, when the tower top temperature exceeds 80°C, go to full reflux, close the rotor flowmeter valve for discharge, and the benzene measuring tank inlet valve. When the tower top temperature drops below 80°C, open the ethanol measuring tank inlet valve slowly and adjust the rotor flowmeter valve to maintain the tower top temperature below 80°C.
6. In the initial stage of ethanol liquid discharge, the tower top temperature is unstable. After 3–5 minutes, take a sample from the sampling port and measure ethanol content with a hydrometer. If it does not meet the specifications, increase the reflux ratio appropriately.
7. As the tower top temperature gradually approaches 80°C, gradually increase the reflux ratio to “depress” the tower top temperature and distill more qualified ethanol.
8. When the tower top temperature exceeds 80°C, indicating lower ethanol content in the distilled vapor, close the ethanol measuring tank inlet valve, open the low-proof measuring tank valve, and combine the low-proof ethanol with the esterification water for recovery in the next distillation.
9. Increase the reflux ratio appropriately when discharging “low-proof liquid.”
10. When the tower temperature reaches 95–98°C and the residue temperature is 99°C, conclude the distillation of the batch. Close the steam, open the exhaust valve, open the bottom valve, and after draining wastewater, close the valve.
11. During content measurement in the control area, slight deviations in individual batches are allowed, but the ethanol content in the recovery ethanol measuring tank must be within specifications.
12. Monitor the level of the large esterification water storage tank before leaving each day. If the level is high, promptly initiate recovery. If the quantity of esterification water is excessive for distillation, report to the workshop promptly to take preventive measures and avoid overflow.
13. Clean the condenser every two months, or monthly in the summer. Open the end cap, use a wire brush, and clean the pipes one by one.

Fundamental Process: Ethanol Recovery Distillation System for Pervaporation and High Concentration

The Ethanol Recovery Distillation System as a continuous or intermittent device serves as a foundational process to ensure the further purification and subsequent processing, particularly through pervaporation membrane for high concentration, to achieve anhydrous ethanol with a purity of 99.7%.

The operation adheres to the fundamental distillation principle, leveraging the varying boiling points of original liquid ethanol and other mixtures, such as water. The process initiates with the heating of ethanol, causing partial evaporation of both ethanol and water.

The resulting vapors, comprising a mixture of ethanol and water, ascend through the distillation tower.

Distillation Tower Mechanism

1. Rising Process:
   • As the vapor ascends through the tower, it encounters filler material, inducing a gradual decrease in temperature.
   • This temperature reduction facilitates internal reflux within the tower, allowing some of the vapor to condense and return downward.
2. Separation and Condensation:
   • Simultaneously, the majority of ethanol steam continues its ascent through the tower’s top connecting pipeline.
   • The vapor then enters the condenser for the crucial phase of condensation, resulting in qualified ethanol.
3. Quality Assurance:
   • The condensed ethanol undergoes sampling and testing to ensure its quality.
   • Verified material is directed to ethanol storage tanks for further processing.

Recycling and Reflux

In cases where the material does not meet the desired specifications, natural reflux or pump reflux may be employed. This involves returning the material to the tower for additional distillation.

The Ethanol Recovery Tower, operating seamlessly with distillation principles, acts as a continuous or intermittent device, providing an essential step in the process chain.

Through careful control of temperatures and reflux mechanisms, it ensures the efficient separation and recovery of high-purity ethanol, setting the stage for subsequent steps in achieving anhydrous ethanol with a purity level of 99.7%.

Risk Analysis and Control in Ethanol Recovery: Distillation Concentration Process

Distillation concentration, a common unit operation in the process engineering, serves as a vital process for separating components based on their varying volatilities. Widely employed in the pharmaceutical and chemical sectors, this process aims at product purification, recovery, or raw material refinement.

While theoretically grounded in physics, the operational nuances often lead producers to overlook potential risks, resulting in frequent accidents.

Physical and Chemical Risks in Distillation Concentration

Physical Risk Considerations

In the precise realm of fine chemical production, distillation concentration introduces multiple hazardous elements, primarily stemming from the flammable and explosive nature of the solvents involved.

During the vaporization process induced by heating, attention must be given to the oxygen content in the gas phase space.

Leaks can lead to the formation of flammable mixtures with air, posing a risk of combustion or explosion. Beyond direct contact with air, risks also emerge during depressurization in the concentration distillation process, increasing the potential for combustion or explosion if air infiltrates the vacuum.

From a physical standpoint, the primary consideration involves the risk of overpressure. As liquids evaporate into gases during the distillation concentration process, system pressure increases.

Inadequate venting and insufficient cooling capacity elevate the possibility of system overflow, causing a surge in distillation system pressure.

This surge can result in mechanical seal failure, material ejection, or even physical explosions. It is imperative, from a process design perspective, to comprehend the material properties involved in the process and adopt rational distillation methods and heating mediums.

The selection of distillation methods should align with the solvent’s boiling point range, considering factors such as vacuum distillation for high-boiling-point substances (boiling point above 150℃ at atmospheric pressure) and atmospheric distillation for moderately volatile materials (boiling point around 100℃ at atmospheric pressure).

Chemical Reactivity Perspective

Chemical reactivity risks in the distillation concentration process primarily focus on the thermal decomposition of system materials (including intermediates) and impurities.

As the solvent diminishes during the distillation process, and impurity and residual material concentrations increase, the stability of system materials (including impurities) becomes a critical consideration.

Some solvents, such as ether, tetrahydrofuran, isopropanol, and dioxane, may generate peroxides upon exposure to air, posing an explosion risk when heated at elevated concentrations.

For instance, prolonged exposure of ether to air, especially in sunlight, results in gradual oxidation into aldehydes, acids, and peroxides.

When peroxide concentrations reach a certain level, the distillation process risks explosion during the enrichment phase.

Even relatively stable solvents like DMSO may undergo decomposition reactions in the presence of certain impurities during solvent recovery.

It is advisable to minimize the number of recovery cycles in solvent recovery processes, reducing the number of distillation cycles for distillation residues.

Determining the number of recovery cycles should involve conducting thermal safety assessment experiments.

DSC (Differential Scanning Calorimetry) can be employed to test solvent distillation residues. Different cycle numbers exhibit significant differences in the heat flow profiles.

Based on multiple measurements of the scanning heat flow profiles, a foundation for determining the number of recovery cycles can be established.

Critical Considerations for Effective Operation

In pharmaceutical chemical processes, a common description involves “concentration to dryness” or concentration to a “pasty” state.

During solvent recovery in the waste treatment process, improper operation may lead to a “dry pot” scenario.

Transitioning to an overly dry state often results in the worst-case scenario.

The concentrations of unstable active compounds and impurities increase significantly.

For instance, when the concentration of peroxide impurities rises, spontaneous decomposition leading to an explosion becomes a real danger.

Due to the increased concentration, the maximum reaction rate under uncontrolled reaction conditions decreases, making it challenging for operators to respond promptly.

Moreover, due to structural constraints of reaction vessels, in a dried or pasty state, temperature probes often fail to make contact with the material or only make contact with the material surface.

In this scenario, the temperature is measured in the gas phase or at the material surface.

Furthermore, effective stirring cannot be achieved in such situations, leading to localized overheating of material adhering to the vessel wall, creating hotspots and potentially triggering material decomposition.

Risk Control Measures in Distillation Concentration

After evaluating the risks inherent in a process, it is imperative to implement appropriate measures to mitigate these risks.

As emphasized in the discourse on a more fundamentally secure process development framework, eliminating risks at their core necessitates altering synthetic routes to avoid unstable intermediates, strongly exothermic substances, or highly toxic materials.

For reactions carrying risks of losing control, risk mitigation measures should aim at reducing energy to prevent uncontrollable incidents.

Preventive measures should make accidents difficult to occur but may not eliminate them entirely. Strategies such as reducing the quantity and types of hazardous chemicals, adopting continuous processes instead of batch processes to minimize reaction equipment volume (improving heat dissipation), and implementing process automation and routine maintenance fall under this category.

Mitigative measures aim to prevent more severe consequences in the event of an accident, such as accident releases and explosion-resistant design.

The table below illustrates common risk control measures.

For intermittent or continuous distillation concentration, several methods are typically employed to reduce process risks:

• Temperature Control: Choose a heating medium with a maximum temperature below the decomposition temperature (recommended to obtain the decomposition temperature through DSC or ARC tests, with representative samples for the most effective safety operating temperature). Use vacuum distillation instead of atmospheric distillation to lower distillation temperatures.
• Continuous Distillation: Opt for continuous distillation instead of batch distillation when recovering waste liquid or solvent.
• Select Appropriate Distillation Equipment: Some impurities or metal ions catalyze material decomposition (equipment selection and material must meet process requirements).
• Interlocking Systems: Implement interlocks to prevent overheating and “drying up.” High-temperature interlocks shut off the heating medium, while low liquid level interlocks shut off the heating medium.
• Analysis of Distillation Residues and Mother Liquor: Analyze impurity content in distillation residues and mother liquor. Determine the number of recovery cycles and control distillation time based on analysis results.
• Quenching: Liquids containing highly reactive substances must undergo quenching. Regularly clean residual liquids in equipment dead corners to prevent the accumulation and enrichment of unstable impurities in equipment dead corners. For example, before distillation of mother liquor containing peroxide impurities, quench the peroxides. Regularly clean certain intermediate tanks or heat exchanger walls.
• Addition of Inhibitors or Stabilizers: For solvents like DMSO commonly used in pharmaceuticals, pure DMSO is stable at room temperature but decomposes at high temperatures. DSC evaluations indicate decomposition of pure DMSO at around 278°C under nitrogen atmosphere. ARC results show that DMSO’s thermal decomposition can occur near its boiling point of 189°C. The presence of certain substances can significantly lower the starting temperature of DMSO decomposition and exacerbate its thermal decomposition, leading to uncontrollable situations or even explosions. Adding a small amount of DMF as a stabilizer during the recovery process can mitigate DMSO decomposition. Zinc oxide has also been proven to be a good stabilizer for DMSO decomposition.
• Design for Release: The distillation concentration reaction process must undergo release calculations to ensure release capacity meets extreme conditions. Implement safety release measures effectively.

Conclusion

Mastering the purification of industrial ethanol waste is crucial for sustainable production practices. By understanding the unique challenges posed by ethanol waste and implementing customized purification techniques, industries can achieve both environmental compliance and economic efficiency.

As technology continues to advance, the evolution of ethanol recycling processes will undoubtedly contribute to a cleaner, more sustainable industrial landscape.

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