Ultrafiltration Membrane Cleaning Processes

Optimization in Seawater Desalination Plants

Guillem Gilabert-Oriol

• Publisher: De Gruyter
• Year: 2021
• ISBN:
  9783110715163
• Edition: 1
• Topic: Scienze fisiche
• Subtopic: Chimica

1 Introduction

The popularity of membranes is increasing drastically in a broad range of industrial processes, thanks to its ability to control the permeation rate of species through the membrane. This allows the design of different separation processes, where the goal is to allow one component of a mixture to permeate freely through the membrane, while the other elements have difficulties to permeate. This is achieved through different driving forces which drive each different mass transfer across the membrane. These are represented by any combination of a concentration, a pressure, a temperature or an electric potential gradient.

One of the key aspects of membranes is to effectively control the membrane fouling, which decreases the permeability of the membrane. If fouling is not properly controlled, the membrane can irreversibly lose flow.

Ultrafiltration, in particular, is a separation membrane technology based on particle size exclusion. This is achieved thanks to the different small micropores which act as a sieve and prevent the particles which are bigger than the pore diameters to flow freely through the membrane. The use of ultrafiltration as a pretreatment of the reverse osmosis in the seawater desalination application has gained special popularity in recent years. Ultrafiltration is a key factor in reducing fouling to the reverse osmosis. Among its key benefits against the conventional pretreatment are a lower footprint and a better filtrate water quality.

1.1 Membrane filtration

Membranes are classified according to their pore diameter. An overview of each membrane technology regarding its pore diameter is given in the next paragraph. In addition, Figure 1.1 provides a graphical scheme summary [1]. Figure 1.2 details the intersection region between both mass transport models [1]. The pore flow model is represented by ultrafiltration, and the solution diffusion model is represented by reverse osmosis. In the intermediate section, nanofiltration combines both models to describe its behavior. Finally, Table 1.1 illustrates some examples of typical species that are filtrated using one of the described membrane technologies, together with their typical size [1]. Therefore, using Figures 1.1 and 1.2, it is possible to assess which filtration technology will be more suitable to filtrate or concentrate one of the species shown in the table.

Figure 1.1: Membranes classified by their pore diameter.

Figure 1.2: Reverse osmosis, nanofiltration and ultrafiltration membranes classified by their pore diameter.

Table 1.1:Species with their size.

Species	Size
H2O	0.2 nm
Na^+	0.37 nm
Sucrose	1 nm
Hemoglobin	7 nm
Influenza virus	0.1 µm
Pseudomonas diminuta	0.28 µm
Staphylococcus bacteria	1 µm
Starch	10 µm

Reverse osmosis membranes have pore diameters that range from 0.1 to 1 nm [1]. These pores have the particularity that they are so small that discrete pores do not exist. Instead, the pores are formed through unstable spaces between polymer chains, which are created and faded as a result of their molecular thermal motion. These fluctuating pores represent the diffusion of species throughout the dense membranes. In contraposition, the bigger and more stable pores observed in the ultrafiltration porous membranes represent the mass flux through convection described by the pore flow model. The solution diffusion model, which is not covered in this book, makes two assumptions. The first is that the solvents dissolve inside the membrane, and thereafter they diffuse through the dense film according to the present concentration gradient. In the reverse osmosis, separation occurs because of the different solubility and mobility of each species throughout the membrane.

Nanofiltration membranes have pore diameters that range from 0.5 to 1.5 nm [1]. These pores have the particularity of being between truly microporus membranes and clearly dense films. Therefore, mass transfer through nanofiltration membranes is described using both pore flow and solution diffusion models. This happens because if membrane polymer chains are very stiff, the molecular motion of the polymer is...