A method for evaluating membrane fouling in nanofiltration systems with different pressure vessel arrangements

Abstract

The present invention relates to a method for evaluating the membrane fouling degree of a nanofiltration system with different pressure vessel arrangements. The method is based on the Bernoulli equation and comprises the following steps: (1) establishing an assumption; (2) selecting a pressure vessel of each section of the system as a pipeline section and establishing the Bernoulli equation on the selected section. (3) calculating the changes in kinetic energy and pressure energy during membrane system fouling to obtain the system membrane fouling energy loss. The present invention can characterize the membrane fouling degree of the system through the membrane fouling energy loss. The present invention can simply and effectively analyze the membrane fouling degree of a nanofiltration system with different pressure vessel arrangements, which is beneficial to the optimal design and operation of the system.

Description

Technical Field

[0001] The invention belongs to the field of membrane water treatment, and in particular relates to a method for evaluating the membrane pollution degree of a nanofiltration system with different pressure vessel arrangements. Background Art

[0002] Nanofiltration membrane technology has been widely used in many water treatment fields such as seawater desalination, brackish water treatment and sewage reuse due to its high efficiency in removing inorganic ions and soluble organic small molecules. However, membrane fouling has always been an important bottleneck problem restricting the efficient operation of nanofiltration membrane technology. The occurrence of membrane fouling will cause serious attenuation of membrane flux, affect the physical and chemical properties of the membrane surface, reduce the service life of the membrane, and increase the operating cost of the system.

[0003] At present, the main method for judging the degree of membrane system contamination is to measure it by analyzing the changes in the system operating conditions, such as the decline in standardized water production, or the changes in standardized salt permeability, or the changes in the standardized pressure difference between the feed water and the concentrate. The above methods are not only unable to fully evaluate the degree of membrane contamination, but also unable to estimate the actual losses caused by the system contamination. Therefore, there is an urgent need for effective methods and means to quantitatively evaluate the degree of membrane contamination in nanofiltration systems.

[0004] The arrangement of membrane system pressure vessels is an important part of membrane system design. It not only affects the effective performance of the membrane system, but also causes changes in the operating environment of the system membrane elements, thereby affecting the degree of membrane contamination of the system.

[0005] In summary, the purpose of the present invention is to establish an effective method for quantitatively evaluating the degree of membrane contamination for nanofiltration membrane systems with different pressure vessel arrangements, thereby providing an important basis for optimizing the design and operation of nanofiltration membrane systems. Summary of the invention

[0006] The technical problem to be solved by the present invention is to provide a method for evaluating the degree of membrane contamination of nanofiltration systems with different pressure vessel arrangements. The method is based on the Bernoulli equation. Under reasonable assumptions, the Bernoulli equation is established for the membrane elements in each pressure vessel of the membrane system, and the system energy loss caused by membrane contamination is calculated to quantitatively evaluate the degree of membrane contamination of the system.

[0007] In order to solve the above technical problems, the technical solution adopted by the present invention is:

[0008] A method for evaluating the degree of membrane fouling of a nanofiltration system with different pressure vessel arrangements comprises the following steps:

[0009] (1) Establish hypothetical conditions.

[0010] (2) Select the pressure vessel of each section of the system as the pipeline section and establish the Bernoulli equation on the selected section.

[0011] (3) The basic Bernoulli equation is

[0012]

[0013] Among them, P in and P out Indicates inlet pressure and outlet pressure (MPa), V initial represents the initial flow velocity (m / s), V fouling represents the flow rate after membrane fouling (m / s), ρ represents the liquid density (kg / m 3 ); g represents the acceleration due to gravity (m / s 2 );

[0014] Furthermore, in step (1), the assumption is that the mechanical energy of the entire system is conserved and adiabatic.

[0015] Furthermore, in step (1), the assumption is that the membrane components connected in series inside the pressure vessel are regarded as a straight pipe, and the fluid flowing inside is the produced water of the membrane elements.

[0016] Furthermore, in step (1), the assumption is that all pipelines are on one plane (ignoring potential energy changes).

[0017] Furthermore, in step (1), the assumption is that the energy variation inside each section of the pipeline is uniform and consistent.

[0018] Furthermore, in step (1), the assumption is: ignoring the longitudinal resistance and local resistance loss in the pipeline.

[0019] Furthermore, the Bernoulli equation for the energy loss of membrane contamination established for any section of the system is:

[0020]

[0021] Where ξ represents the pollution energy loss (J / kg).

[0022] The membrane resistance equation is:

[0023]

[0024] Among them, J (L / s·m 2 ) represents membrane flux; ΔP (MPa) represents applied pressure; η is dynamic viscosity;

[0025] R is the total membrane resistance.

[0026]

[0027] The flow rate Q and membrane resistance R can be linked through the membrane flux J, that is:

[0028]

[0029] Among them, A(m 2 ) is the effective surface area of ​​the membrane, Q (L / s) represents the flow rate, and d (m) represents the effective diameter of the water production channel of the membrane element.

[0030] Using flow rate instead of flow velocity, the membrane fouling energy loss is calculated as:

[0031]

[0032] As membrane fouling occurs, the membrane resistance R increases, causing the value of the kinetic energy term to decrease. At the same time, membrane fouling also causes the applied pressure of the membrane component to drop, reducing the value of the pressure energy term. When the system energy remains unchanged, the value of the pollution energy loss ξ will show an increasing trend from zero to one. In fact, as the membrane fouling resistance R increases, it directly leads to a decrease in the membrane water production flow rate. Therefore, in the calculation process, it is possible to avoid directly calculating the membrane fouling resistance R, and the change in the membrane element water production flow rate Q can be used to represent the change in the membrane fouling resistance R.

[0033] Furthermore, when the selected pipeline section is the first section of the system, the system membrane fouling energy loss is:

[0034]

[0035] Among them, subscripts 1 to k represent data information recorded at different times.

[0036] Therefore, the degree of membrane fouling of the system can be evaluated by the changes in the produced water flow rate and the inlet and outlet pressure difference during the operation of the membrane system.

[0037] The membrane contamination degree assessment method provided by the invention is applicable to nanofiltration and reverse osmosis rolled membrane systems with different pressure vessel arrangements.

[0038] Compared with the prior art, the present invention has the following advantages and technical effects:

[0039] The present invention provides a simple and effective means for quantitatively evaluating the degree of membrane contamination, and can also predict the actual energy loss caused by system membrane contamination, thereby providing evaluation indicators for the optimal design of nanofiltration systems with different pressure vessel arrangements. BRIEF DESCRIPTION OF THE DRAWINGS

[0040] Figure 1 Cross-section selection for Bernoulli equation

[0041] Figure 2It is a two-stage 3-2 nanofiltration membrane system

[0042] Figure 3 It is a three-stage 2-2-1 nanofiltration membrane system

[0043] Figure 4 The total water flux and water flux changes of each section of the nanofiltration system with different pressure vessel arrangements after pollution occurs

[0044] Figure 5 Energy loss of membrane fouling in nanofiltration systems for different pressure vessel arrangements DETAILED DESCRIPTION

[0045] The technical solution of the present invention is further described below in conjunction with the accompanying drawings, but the scope of protection claimed by the present invention is not limited thereto.

[0046] Example 1

[0047] like Figure 2 As shown in the figure, a two-stage 3-2 system was built using 15 NF90-4040 membrane elements, with artificial simulated seawater as the inlet water (TDS of 35000 mg / L) and constant pressure (3.5±0.1MPa) operating conditions, and it was operated for 450 hours respectively. During the operation, the changes in the system water production and the changes in the inlet and outlet pressures of each section were recorded in real time.

[0048] After running for 450 hours, if Figure 4 As shown in the figure, the total water flux of the system is 175.32L·m -2 ·h -1 Down to 150.12 L·m -2 ·h -1 The first stage of water production flux is 40.26Lm -2 .h -1 Down to 32.45L·m -2 ·h -1 , the pressure dropped from 3.6MPa to 3.55MPa; the second stage water flux increased from 19.42L·m -2 ·h -1 Down to 13.66L·m -2 ·h -1 , the pressure drops from 3.42MPa to 3.37MPa. Substituting the data into the Bernoulli equation, Figure 5 As shown, the membrane pollution energy losses of the first and second sections of the 3-2 system are 3475 J / kg and 863.5 J / kg respectively, that is, after pollution occurs in the system under this pressure vessel arrangement, an additional 4338.5 J / kg of energy is required to produce 1000L of product water.

[0049] Example 2

[0050] like Figure 3 As shown in the figure, a three-stage 2-2-1 system was built using 15 NF90-4040 membrane elements, with artificial simulated seawater as the inlet water (TDS of 35000 mg / L) and constant pressure (3.5±0.1MPa) operating conditions, and it was operated for 450 hours respectively. During the operation, the changes in the system water production and the changes in the inlet and outlet pressures of each section were recorded in real time.

[0051] After running for 450 hours, if Figure 4 As shown in the figure, the total water flux of the system is 173.52L·m -2 .h -1 Down to 142.2 L·m -2 ·h -1 The first stage water flux is 43.97L·m -2 ·h -1 Down to 30.55 L·m -2 ·h -1 , the pressure dropped from 3.6MPa to 3.55MPa; the second stage water flux increased from 23.13L·m -2 ·h -1 Down to 19.89 L·m -2 ·h -1 The pressure dropped from 3.55MPa to 3.50MPa; the water flux in the third stage increased from 13.97L·m -2 ·h -1 Down to 12.69 L·m -2 ·h -1 , the pressure drops from 3.40MPa to 3.30MPa. Substituting the data into the Bernoulli equation, Figure 5 As shown, the membrane fouling energy losses of the first, second and third sections of the 2-2-1 system are 4729.44 J / kg, 665.74 J / kg and 89.1 J / kg respectively. That is, after the system is fouled under this pressure vessel arrangement, an additional 5484.3 J / kg of energy is required to produce 1000L of product water.

[0052] In summary, the present invention can, in principle, estimate the membrane fouling energy loss generated by the entire system or each section or single membrane element of the system, and can conveniently and quickly evaluate the membrane fouling degree of systems with different pressure vessel arrangements, which is beneficial to the optimal design and operation of the system.

Claims

1. A method for evaluating the degree of membrane fouling in nanofiltration systems with different pressure vessel arrangements, characterized in that: The following steps are involved: (1) Establish hypothetical conditions; (2) Select the pressure vessel of each section of the system as the pipeline section and establish the Bernoulli equation on the selected section; (3) Establish the Bernoulli equation including the energy loss of membrane fouling, calculate the energy loss of membrane fouling to characterize the degree of membrane fouling of the system; The assumptions are as follows: the mechanical energy of the whole system is conserved and adiabatic; the membrane components connected in series inside the pressure vessel are regarded as a straight pipe, and the fluid flowing inside is the produced water of the membrane components; all the pipes are on the same plane, and the potential energy change is ignored; the energy change inside each section of the pipe is uniform and consistent; the along-the-line resistance and local resistance loss in the pipe are ignored; The Bernoulli equation for the energy loss of membrane contamination established for any section of the system is: Among them, P in and P out Indicates the inlet and outlet pressures of the section (MPa), V initial represents the initial flow velocity (m / s), V fouling represents the flow rate after membrane fouling (m / s), ξ represents the pollution energy loss (J / kg); ρ represents the liquid density (kg / m 3 ); g represents the acceleration due to gravity (m / s 2 ); based on Using flow rate instead of flow velocity, the membrane fouling energy loss is calculated as: Wherein, Q (L / s) represents the flow rate, and d (m) represents the effective diameter of the water production channel of the membrane element.

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