How to Use Melamine Foam to Prepare Gradient Graphene Aerogel Evaporator for High-concentration Brine Desalination?

Solar-driven interfacial evaporation (SIE) is considered one of the most promising technologies to alleviate freshwater shortages due to its ability to efficiently use solar energy to produce fresh water. In addition, solar evaporators have unique advantages in treating concentrated salt wastewater. The evaporation process only produces salt crystals as a byproduct, thus achieving zero liquid discharge. However, the accumulation of salt in the evaporator will hinder the absorption of light and affect the escape of steam, ultimately leading to a decrease in the evaporation performance of the evaporator or even failure.

To alleviate this problem, some researchers have proposed a “salt circulation” strategy. That is, a circulation path is designed for the solution to allow salt ions to flow back into the brine, thereby preventing salt from crystallizing on the evaporation surface, thereby achieving continuous and stable evaporation. However, this method will cause the salinity of the solution to increase, and the problem of salt crystallization is still unavoidable.

Another solution is the “salting out” strategy. That is, the structure is regulated so that the salt is preferentially crystallized in the non-evaporation area, thereby reducing the impact of salt crystallization on evaporation performance, thereby achieving the simultaneous collection of salt and fresh water. However, the “salting out” strategy still faces some severe challenges in practical application.

First, one-dimensional brine transport limits absorptive capacity, significantly reducing the overall evaporation rate. Secondly, the salt crystallization strategy requires regular removal of salt crystals, which are difficult to fall off automatically from the evaporator surface due to their strong adhesion. If the deposited salt is not removed in time, the evaporation performance will inevitably be reduced. In addition, many high-efficiency solar evaporators require dangerous or time-consuming preparation processes, such as high-pressure, low-vacuum, or high-temperature heat treatments.

Therefore, developing a durable solar evaporator to achieve long-term, efficient, and stable water evaporation and salt collection through a simple preparation process for zero liquid discharge desalination is an unachieved scientific goal.

Based on the above challenges, the research team of Shandong First Medical University prepared a three-dimensional gradient graphene spiral aerogel evaporator, which can achieve efficient solar evaporation of high-concentration salt water and achieve zero liquid discharge.

In order to meet the stringent requirements of solar evaporators in terms of efficient solar absorption, full energy utilization, excellent salt resistance and long-term stability, the project design mainly focuses on the following four key points:

Focus on structural design and chemical stability enhancement

The evaporator needs to have a strong network structure and chemical stability, which is crucial to achieve long-term, efficient and stable solar desalination. Therefore, they chose low-cost, super-hydrophilic, high-porosity and high-resilience melamine foam as the supporting skeleton of the solar evaporator.

They then studied the mechanical properties of graphene aerogels with melamine foam as the skeleton in multiple modes, including compression, bending, and tension, to comprehensively evaluate their mechanical properties.

In order to further verify its robustness and durability under actual application conditions, the research team conducted tests under a variety of extreme conditions.

Specifically, they repeatedly bent the sample 20 times in air and water, soaked it in acidic and alkaline solutions, and placed it in harsh environments such as high temperature and ultrasonic stirring.

After these rigorous tests, they found no significant changes in the sample’s appearance and microstructure, which fully demonstrated its excellent durability and structural stability.

Focus on simplification and economic improvement of manufacturing processes

To achieve simplicity and economy in the evaporator manufacturing process and avoid the use of expensive equipment, they designed the following preparation steps:

The first step is hot pressing

Using hot pressing technology, the research team compressed melamine foams of different thicknesses to obtain a pre-pressed melamine film with a thickness of 1mm.

During this process, as the ratio of the original thickness of the sample to the final compressed thickness increases, the pore size of the pre-pressed melamine film is finely regulated from 150 μm to 20 μm, forming a unique three-dimensional gradient network structure.

How to Use Melamine Foam to Prepare Gradient Graphene Aerogel Evaporator for High concentration Brine Desalination?

The second step is surface spraying

Use a spray bottle to spray the graphene oxide suspension evenly on the surface of the pre-pressed melamine film. The uniform distribution of graphene oxide in the film was achieved, which laid the foundation for subsequent solar reduction.

How to Use Melamine Foam to Prepare Gradient Graphene Aerogel Evaporator for High concentration Brine Desalination

The third step is atmospheric pressure drying

After spraying, they placed the film directly in the air for natural air drying. This step avoids the complex, time-consuming and energy-consuming preparation processes such as freeze drying, supercritical drying and solvent replacement.

The fourth step is sunlight reduction

That is, using a magnifying glass to focus sunlight on the sample. Under the action of ultraviolet rays, the color of the sample surface quickly changes from brown to black, indicating that the graphene oxide has been successfully reduced to reduced graphene oxide.

This solar reduction method is not only simple to operate and environmentally friendly, but also does not require the use of chemical reducing agents or high-temperature thermal reduction processes, significantly reducing production costs and environmental pollution.

Focus on efficient solar energy absorption and minimization of energy loss

Solar evaporators must not only absorb and convert a wide range of solar energy into thermal energy, but also minimize energy losses caused by diffuse reflection and thermal radiation, and capture additional energy from the environment. The evaporation process at the solar interface is controlled by three energy fluxes: solar energy input, evaporation energy output, and energy dissipated to the environment.

Unfortunately, conventional two-dimensional solar evaporators face energy dissipation issues due to heat exchange with the surrounding environment, making it difficult to achieve the ideal 100% solar-to-steam conversion efficiency. In contrast, three-dimensional solar evaporators can break the theoretical limit of solar-to-steam conversion efficiency.

To reduce the energy loss caused by diffuse reflection and thermal radiation on the upper surface of the three-dimensional sample, the team designed a three-dimensional spiral concave graphene aerogel.

This structure not only has higher absorbance and lower reflectivity, but its side walls are also able to absorb ambient heat and heat diffusely reflected from the bottom, thereby enabling effective energy recovery.

Focus on salt tolerance optimization and salt crystallization management

That is; by designing a unique transport channel, the radial transport of brine is promoted, which leads to directional crystallization of salt. This design not only effectively prevents salt from depositing and clogging on the evaporator surface, but also realizes the effective collection and utilization of salt, thus significantly improving the efficiency and sustainability of the desalination process.

Specifically, they designed and prepared a three-dimensional gradient graphene network structure based on a pre-pressed melamine porous film. This gradient graphene network, which is sparse inside and dense outside, can form a radial capillary force difference, which promotes the radial transport of brine. As steam continues to be generated, the radial flow of brine increases the brine concentration in the outer surface area, causing the salt to crystallize directional on the outer surface rather than the inner surface. This design ensures that the inner surface of the evaporator maintains sufficient evaporation area, thereby achieving efficient and stable solar evaporation.

It is worth mentioning that the evaporator uses its entire outer surface for salt crystallization, making the crystallization area much larger than other directional crystallization evaporators.

Salt crystals are mainly composed of loosely packed salt crystals with abundant pores, which are beneficial to the transport of brine and the release of steam.

Furthermore, due to the evaporative cooling effect, the surface temperature of the salt layer is lower than the ambient temperature. This characteristic enables the evaporator to absorb more energy from the environment, further improving evaporation performance.

Article source: Deep Tech

The research group: Shandong First Medical University Team Research Group

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