RU2817786C1 - Method of producing highly concentrated salt solutions by reverse osmosis - Google Patents

Abstract

FIELD: technological processes.

SUBSTANCE: invention relates to membrane reverse osmosis plants and can be used in treatment of filtrates of solid domestic wastes, in heat engineering, ferrous metallurgy, chemical and other industries, especially for technological processes where a high degree of concentration and further processing of the concentrate are required. Method of producing highly concentrated salt solutions by reverse osmosis involves a high-pressure pump, three reverse osmosis stages arranged in series along the concentrate, providing at all stages a constant flow rate over the membranes, and a throttle at the concentrate outlet, which creates the required pressure, and is characterized by that at third stage differential pressure measurement is provided, and part of concentrate after the second stage, which is 0.1–0.5 of the inlet flow of the third stage, is returned to the suction line of the pump through an automatic control valve.

EFFECT: reduced operating costs due to increased interval between chemical washes.

1 cl, 2 dwg, 1 tbl

Description

The invention relates to membrane reverse osmosis units and can be used for the purification of filtrates from solid waste landfills, in thermal power engineering, ferrous metallurgy, chemical and other industries, especially for technological processes that require a high degree of concentration and further processing of the concentrate.

There is a known method for treating highly mineralized waters with reverse osmosis, which includes a high-pressure pump, two reverse osmosis stages located in series with the concentrate (the first stage has two membrane apparatuses, the second one has one apparatus) and a control valve at the outlet of the second stage [1]. This method is the most common in reverse osmosis desalination technology, because provides high-quality chemical cleaning of membrane elements, uniform flow rate over the membranes and minimal cost of demineralized water.

This method has a drawback: it is impossible to obtain a concentrate with the maximum salt content. With this configuration of the hydraulic circuit, the degree of permeate selection can be no more than 70% (the degree of concentration is no more than 3.3 times) while maintaining the minimum permissible flow rate over the membranes.

The closest in technical essence and achieved technical result to the proposed one is a method for producing highly concentrated saline solutions by the reverse osmosis method, which includes a high-pressure pump, three reverse osmosis stages located in series across the concentrate and a throttle at the outlet of the concentrate from the installation, creating the necessary operating pressure [2]. With this configuration of the distribution of membrane devices in the installation, almost the same flow rate over the membranes is maintained at all stages, i.e. along the entire length of the hydraulic circuit. Increasing the number of stages in the hydraulic circuit makes it possible to obtain a concentrate with a higher salt content at the output, which makes it possible to reduce its quantity and the cost of its disposal. This method has a number of disadvantages:

1. The mode of chemical washing of membrane elements is not optimally carried out due to the low speed of the washing solution at the first and second stages. This is due to the absence in the hydraulic circuit of a concentrate recirculation line after the second stage and the large ratio of the number of membrane devices in stages 5:3:1.

2. During prolonged operation of a membrane installation, a “self-extinguishing” effect of membrane performance occurs. Initially, at the third stage, due to the highest salt content in the feed water, a gradual decrease in permeate productivity and an increase in pressure drop are observed. Because of this, the hydrodynamics in the first and second stages deteriorate, which leads to increased sedimentation in the pressure channels and a decrease in permeate consumption in these stages. Taken together, negative factors at all stages lead to even more severe clogging of the membrane elements and a decrease in the permeate productivity of the entire membrane installation as a whole.

3. During long-term operation of a membrane installation, quite often the pressure drop at the third stage exceeds the maximum permissible, which leads to “telescoping”, i.e. shift of membrane packages relative to each other, and subsequent failure of membrane elements.

The purpose of the invention is to reduce the cost of obtaining highly concentrated salt solutions by reducing operating costs.

This goal is achieved by proposing a method for producing highly concentrated saline solutions using the reverse osmosis method, which includes a high-pressure pump, three reverse osmosis stages located in series across the concentrate, providing a constant flow rate over the membranes at all stages, and a throttle at the outlet of the concentrate, creating the necessary pressure, characterized in that at the third stage the differential pressure is measured, and part of the concentrate after the second stage is returned through an automatically regulating valve to the suction line of the pump.

Multi-stage reverse osmosis installations are characterized by a gradual increase in the content of salts and mineral compounds along the entire length of the hydraulic circuit. At the first stage, the salt content in the feed water in front of the membrane elements is minimal, at the last (third) stage it is maximum.

A roll-type reverse osmosis membrane element (the drawing is shown in Fig. 1) consists of a perforated permeate tube onto which membrane packages are spirally wound. Each bag has an adhesive seam around the perimeter and consists of two membranes and a drainage material located between them. The bags are separated from each other by a turbulator mesh, which is a pressure channel for the passage of source water. Source water under pressure enters the end of the element, passes over the surface of the membrane and is divided into two streams: permeate (filtrate) and concentrate, saturated with salts. The permeate flows through the drainage material into the permeate tube.

During concentration by reverse osmosis, dissolved components retained by the membrane accumulate in the boundary layer near the surface of the membrane, which leads to concentration polarization and precipitation of poorly soluble salts (CaCO ₃ , CaSO ₄ , CaF ₂ , BaSO ₄ , etc.). As a rule, poorly soluble salts precipitate intensively in the pressure channels of water movement, i.e. between the membrane surface and the cells of the turbulator mesh. Thus, sedimentation reduces the height of the pressure channels, impedes the passage of water over the membranes, leads to an increase in pressure drop and a decrease in permeate productivity.

In industrial membrane installations, one membrane apparatus (housing) usually contains from one to seven membrane elements. Manufacturers of reverse osmosis membrane elements allow a maximum pressure drop (P _add ) on one element of no more than 0.1 MPa. Thus, the total pressure drop ( _Ptot ) at one stage should not exceed:

P _total =N*P _add

In connection with the decrease in permeate consumption at the third stage, it is proposed to return part of the concentrate flow after the second stage to the inlet of the first stage via the recirculation line in order to prevent a further decrease in permeate consumption at the first and second stages and, thereby, ensure normal operation of the membrane installation as a whole . Moreover, it is advisable to regulate the flow rate of the concentrate discharged through the recirculation line after the second stage automatically depending on the value of the pressure drop at the third stage.

As the concentrate recirculation flow rate decreases, the flow rate above the membranes at the first and second stages decreases, which leads to deterioration of hydrodynamic conditions at all three stages, a decrease in the total permeate consumption and the need for unscheduled chemical cleaning.

It has been experimentally established that the minimum permissible concentrate recirculation flow rate after the second stage is 0.1 of the input flow of the third stage.

By increasing the concentrate recirculation flow rate, more optimal performance indicators of the first and second stages can be achieved, but at the same time, the permeate consumption in the third stage can be significantly reduced due to a sharp decrease in the input flow and it will be necessary to carry out a complex chemical cleaning of the entire installation in order to wash the third stage from poorly soluble sediments salts

It has been experimentally established that the maximum permissible concentrate recirculation flow rate after the second stage is 0.5 of the input flow of the third stage.

In addition, achieving the permissible feed flow to the third stage and at the same time increasing the concentrate recirculation flow rate after the second stage requires an increase in the productivity and power of the feed pump, which will lead to an increase in capital and operating costs.

In fig. 2 shows diagrams reflecting the known and proposed methods for producing highly concentrated saline solutions by reverse osmosis, where:

1 - high pressure pump;

2 - throttle at the concentrate outlet;

3 - automatic control valve;

4 - device for measuring flow in the recirculation line.

The results of experiments in the technology of producing highly concentrated saline solutions using the known and proposed methods are presented in Table 1.

Thus, the proposed method for producing highly concentrated salt solutions using reverse osmosis allows one to reduce operating costs for chemical washing of membrane elements by half.

Bibliography

1. Frog B.N., Pervov A.G. Water treatment. Textbook for universities: - M.: ASV Publishing House, 2015, p. 445-454.

2. Mulder M. Introduction to membrane technology: Transl. from English - M.: Mir, 1999, p. 441-444. - prototype.

BRIEF DESCRIPTION OF THE DRAWINGS

1 - source water flow

2 - permeate output

3 - concentrate output

4 - recycle flow

5 - anti-telescoping disk

6 - perforated pipe

7 - turbulator mesh

8 - membrane

9 - drainage

10 - permeate flow

11 - sedimentation zone

Claims (2)

1. A method for producing highly concentrated saline solutions by the reverse osmosis method, including a high-pressure pump, three reverse osmosis stages located sequentially throughout the concentrate, ensuring a constant flow rate over the membranes at all stages, and a throttle at the outlet of the concentrate, creating the necessary pressure, characterized in that the third stage provides measurement of the pressure drop, and part of the concentrate after the second stage, amounting to 0.1-0.5 from the input flow of the third stage, is returned through an automatically regulating valve to the suction line of the pump. 2. The method according to claim 1, characterized in that the pressure drop at the third stage is no more than N*P _add , where N is the number of membrane elements in the stage, P _add is the permissible pressure drop across one membrane element, which is no more than 0, 1 MPa.