Filtration media are lifesaving components in an expanding global economy. They help keep the population healthier and safer and the environment pristine. Filter media are used in many diverse industries and applications. They provide safe drinking water in arid areas where water is in short supply. They are also used to purify pharmaceuticals and keep them free of viruses. They also clean process wastewater in the chemical and metal industries before discharge into rivers and streams. The need for filtration media is going to increase many-fold as the world population increases and water shortages become commonplace.
This article primarily focuses on a particular process and type of filtration media: the reverse osmosis (RO) process and thin-film composite (TFC) membranes for purifying seawater into potable water.
RO Process and Design of TFC Membranes
RO is the process of forcing a solvent from a region of high concentration such as salt in seawater to a region of low concentration through a micro-porous membrane. Applying pressure in excess of the osmotic pressure forces transportation of salt-depleted water across the membrane, generating potable drinking water on the low-pressure side.
The thin-film composite membranes used for RO typically have three layers of filtration, each with a very distinct function. In addition, sometimes a layer of protective coatings is applied on top of the fine membrane. The back layer is usually a cast non-woven fabric, made of a material like PET. This back layer provides the coarse filtration and mechanical strength needed against the high pressure of 600 to 1,200 psi for the seawater to overcome the osmotic pressure of around 400 psi.
The middle layer is typically made out of polysulfone and is about 50 microns thick. The polysulfone is one of the progressively more porous layers where the medium-level filtration takes place and additional mechanical support to the thin film is provided.
The third layer is the thin-film layer and is usually only a few microns thick. This layer is created by a reaction between an amine solution and an Isopar solution. This layer has the smallest pores of the three layers and is relatively delicate. It is on the high-pressure side and is supported against the high pressure by the other two layers. Each layer is progressively more porous as it regresses from the high-pressure to low-pressure side. The thin-film layer is sometimes coated with additional coatings such as PVA to further protect the delicate membrane from chemical and microbial attacks and from high mechanical pressure. The three-layer configuration gives the membrane the desired properties of high salt rejection rate, high filtration flux and good mechanical strength.
TFC Membrane Manufacturing
Manufacturing this micro-porous membrane requires specialty coating technology, deep process knowledge and dryers that deliver the performance needed for the process. Years of experience go into the design of these manufacturing lines. The process goes like this.
Stage 1. Amine water is applied to polysulfone coating and dried. The process starts with a PET non-woven fabric with a polysulfone coating. The coated web is dipped in a tank of amine water and partially dried in a flotation dryer.
Stage 2. Isopar solution is applied and solvent stripped. Next, the web is coated with an Isopar-based coating. The amines in the water react with the Isopar solution, which starts the process of generating the thin layer with fine porosity. This coating can be applied by dipping the web into a tank with a certain amount of residence time. The technology has progressed from the dip tank to newer options like curtain coating and more recently to slot-die coating.
The critical factor is the time the Isopar solution has to react with the amines because this determines the size and the distribution of pores.
The longer the two coatings have to react, the larger the holes become and the more uniform their distribution. Thus, a shorter duration for the interaction between the two coatings is better because smaller, randomly distributed holes result in better filtration performance.
The web then moves into an impingement dryer with multiple zones using 99.97 percent HEPA-filtered air. This process, known as solvent stripping, is a very critical process. Here, the rate of drying is a function of the rate of evaporation of the solvent, which determines the level of porosity developed in the membrane.
Stage 3. Water, glycerol and PVA solution are applied and dried. After this solvent-stripping process, the web goes through a series of tanks containing water, glycerol and PVA. The web gets washed in the first tanks containing water, then the second tanks apply glycerol to displace water from the pores. In the third tank, the PVA coating is applied to maintain the pore sizes and provide protection to the membrane. Finally, these coatings are dried using a combination infrared-air dryer followed by a flotation dryer.
Dryer Design Guidelines for TFC Membranes
In all dryers, the interior must be constructed of 304 stainless steel to ease cleaning and to ensure compliance with good manufacturing practices (GMP). HEPA filtration of supply air ensures that no contaminants are present on the web. PLC controls are used to monitor and control all the parameters such as line speed, air temperature, velocity and product temperature. Solvent level monitors (LFL) are required for safe operation in the solvent application zones.
A complete drying system for a 42” wide membrane processing line running at a production speed of 35 ft/min consists of three dryers described below. The energy to heat the air used in the drying can come from a variety of sources. It can be steam, electric or gas. The dryer design must take the energy source into consideration, and appropriate equipment such as steam coils, electric air heaters or gas-fired burners should be integrated into the system.
Stage 1 Dryer. The first stage of the TFC membrane manufacturing process requires a 6’ long floatation dryer to reduce the water in the web to a desired level. The dryer needs to have variable airflow and temperature capabilities for proper control of the residual moisture in the web.
Stage 2 Dryer. In the next stage, the web is coated on top with an Isopar-based coating, which is dried in a two-zone, 20’ long impingement dryer (a vapor-stripping dryer). Each zone has adjustable temperature and velocity to control the drying rate. The web is supported by driven rolls using a slight arch to keep the membrane from curling. The impingement web dryer has nozzles positioned on the top side only, and the web is supported on stainless steel rolls.
The airflow control, especially in the first zone, is gentle to control the reaction and porosity. An alternative drying method has also been successfully used with low-intensity infrared to supply the drying energy and gentle airflow to strip the vapor from the thin micro-porous film being formed in the process, hence the name “stripper dryer.” The dryer uniformity from side to side is paramount, as any non-uniformity will cause different levels of porosity across the width.
Stage 3 Dryer. After processing and drying the Isopar coating, the web then goes through a series of dip tanks containing water, glycerol and PVA. After the glycerol and PVA are added, a short, combination infrared-air pre-dryer is used prior to the web entering a final drying process using a two-zone flotation dryer. The flotation web dryer is 20’ long using two zones of temperature and velocity control. The floater uses dual-slot Coanda nozzles on the top and bottom of the web to provide a sinusoidal web path. The product temperature is monitored and controlled with noncontact infrared thermometers to ensure repeatability and uniformity of the membrane.
As the world requires more and more water, reverse osmosis is a way to make seawater more useful. The RO process and the TFC membrane required for the process are helping to bring about a future where safe drinking water will be available to all.