【Daily Question】Chemical Engineering Principles 300: Mixed-bed Regeneration (January 20)

1) Backwashing and stratification, 2) Static settling, 3) Water discharge, 4) Regeneration of anion resin, 5) Replacement of anion resin, 6) Rinsing of anion resin, 7) Regeneration of cation resin, 8) Replacement of cation resin

Mixed-bed regeneration process (two-step method). 1) Before regenerating the mixed-bed, backwash should be carried out first, using a flow rate of 10 m/h, with a backwash control time of 10–15 minutes ； 2) Wait stationary for the resin layer to stratify ； 3) Drain water until the water level is about 10 cm above the resin layer in the exchanger ； 4) Lime is added through the upper lime inlet pipe at a flow rate of 4 m/h; the concentration of the lime solution is 4%, and the lime addition time is more than 15 minutes ； At the same time, acid is introduced through the inlet pipe at the bottom of the exchanger; after flowing through the cation resin layer, the water is discharged together with the waste alkali solution through the intermediate drain pipe located at the interface between the cation and anion resin layers ； 5) Replace the anionic resin following the same procedure, at a flow rate of 4 m/h for more than 15 minutes ； 6) The anionic resin is washed with a flow rate of 15 m/h; the amount of water used for washing is controlled at 10 m3 of water per 1 m3 of resin, until the phenolphthalein alkalinity in the wastewater is below 0.5 mmol/L ； 7) Acid is introduced to regenerate the cation resin through the acid inlet pipe at a flow rate of 4 m/h, with an acid concentration of 5%; the acid introduction time is more than 15 minutes ； At the same time, the upper alkali inlet pipe should continue to supply water ； After flowing through the anion resin layer, the water is discharged together with the waste acid solution through the drainage pipe at the interface between the cation and anion resin layers ； 8) Carry out the replacement and cleaning of the cation resin following the same procedure, at a flow rate of 4 m/h for more than 15 minutes ； 9) The cation resin is washed at a flow rate of 10 m/h, with drainage carried out through the intermediate drain pipe, until the acidity of the discharged water falls below 0.5 mmol/L ； 10) Perform a full forward wash of the exchanger resin, with water entering from the top of the exchanger and wastewater being discharged through the exchanger’s forward wash drain valve; the flow rate should be 15 m/h, and the wash continues until the conductivity of the discharged water is below 1.5 μS/cm ； 11) Drain water until the level in the exchanger is about 10 cm above the resin layer ； 12) Compressed air is introduced to mix the resin; the pressure of the compressed air is 1–1.5 bar gauge, and the duration is 1–5 minutes ； After the resins are mixed, there must be a sufficiently high drainage rate to force the resins to settle quickly, preventing them from separating again. When the resin level drops, feeding water from the top can accelerate its settlement. 13) The mixed resin layer is washed under positive flow at a velocity of 10–20 m/h, until the wastewater meets the required standards, after which it can be put into use for water production.

It means that during regeneration, the acid and alkali regeneration fluids do not enter the exchanger simultaneously, but rather separately. It is further divided into a two-step method involving the flow of alkaline solution through the anion and cation resins, and a two-step method in which acid and alkali pass through the anion and cation resins separately. The specific procedure is as follows: after anti-washing and stratification, water is allowed to reach a level about 100 mm above the resin surface; alkali solution is then introduced from above to regenerate the anion resin, while the waste liquid is discharged through the pipe at the boundary between the anion and cation resins. Subsequently, the anion resin is cleaned using the same process. Until the OH— level in the wastewater drops below 0.5 mmol/L. During the above process, a small amount of water can also be passed through the cation resin layer from below to reduce the contamination of the cation resin by the alkaline solution. Then, acid is introduced from the bottom to regenerate the cation resin, and the waste liquid is also discharged through the middle drain pipe. At the same time, to prevent acid from entering the regenerated anion resin layer, it is necessary to continue washing the anion resin with a low flow rate of water from above. The cleaning process for cationic resins is the same as that during regeneration. Clean until the acidity of the drainage drops below 0.5 mmol/L. Finally, a full forward wash is carried out, with water being introduced from above and drained from the bottom, until the conductivity of the water exiting the system is less than 1.5 μs/cm. During the forward wash process, to improve its effectiveness, a short reverse wash lasting 2–3 minutes may be performed from time to time in order to remove residual liquid from hard-to-reach areas

1) Before regenerating the mixed-bed, backwash should be carried out first, using a flow rate of 10 m/h, with a backwash control time of 10–15 minutes； 2) Wait stationary for the resin layer to stratify ； 3) Drain water until the water level is about 10 cm above the resin layer in the exchanger ； 4) Lime is added through the upper lime inlet pipe at a flow rate of 4 m/h; the concentration of the lime solution is 4%, and the lime addition time is more than 15 minutes ； At the same time, acid is introduced through the inlet pipe at the bottom of the exchanger; after flowing through the cation resin layer, the water is discharged together with the waste alkali solution through the intermediate drain pipe located at the interface between the cation and anion resin layers ； 5) Replace the anionic resin following the same procedure, at a flow rate of 4 m/h for more than 15 minutes ； 6) The anionic resin is washed with a flow rate of 15 m/h; the amount of water used for washing is controlled at 10 m3 of water per 1 m3 of resin, until the phenolphthalein alkalinity in the wastewater is below 0.5 mmol/L ； 7) Acid is introduced to regenerate the cation resin through the acid inlet pipe at a flow rate of 4 m/h, with an acid concentration of 5%; the acid introduction time is more than 15 minutes ； At the same time, the upper alkali inlet pipe should continue to supply water ； After flowing through the anion resin layer, the water is discharged together with the waste acid solution through the drainage pipe at the interface between the cation and anion resin layers ； 8) Carry out the replacement and cleaning of the cation resin following the same procedure, at a flow rate of 4 m/h for more than 15 minutes ； 9) The cation resin is washed at a flow rate of 10 m/h, with drainage carried out through the intermediate drain pipe, until the acidity of the discharged water falls below 0.5 mmol/L ； 10) Perform a full forward wash of the exchanger resin, with water entering from the top of the exchanger and wastewater being discharged through the exchanger’s forward wash drain valve; the flow rate should be 15 m/h, and the wash continues until the conductivity of the discharged water is below 1.5 μS/cm ； 11) Drain water until the level in the exchanger is about 10 cm above the resin layer ； 12) Compressed air is introduced to mix the resin; the pressure of the compressed air is 1–1.5 bar gauge, and the duration is 1–5 minutes ； After the resins are mixed, there must be a sufficiently high drainage rate to force the resins to settle quickly, preventing them from separating again. When the resin level drops, feeding water from the top can accelerate its settlement. 13) The mixed resin layer is washed under positive flow at a velocity of 10–20 m/h, until the wastewater meets the required standards, after which it can be put into use for water production.

Mixed-bed regeneration process (two-step method). Depending on the acid addition, base addition, and rinsing steps, it can be divided into a synchronous method and a two-step method. The synchronous method involves the simultaneous introduction of acid, alkali solution, or cleaning water from both the top and bottom of the exchanger during its regeneration and cleaning processes; these fluids flow through the anion and cation resin layers respectively before being discharged simultaneously through a central drainage device ； The two-step method involves carrying out acid and alkali regeneration as well as cleaning of the cation resin and anion resin in the exchanger separately ；

) Before regenerating the mixed-bed, backwashing is carried out first, using a flow rate of 10 m/h, with a backwashing duration of 10–15 minutes； 2) Wait stationary for the resin layer to stratify ； 3) Drain water until the water level is about 10 cm above the resin layer in the exchanger ； 4) Lime is added through the upper lime inlet pipe at a flow rate of 4 m/h; the concentration of the lime solution is 4%, and the lime addition time is more than 15 minutes ； At the same time, acid is introduced through the inlet pipe at the bottom of the exchanger; after flowing through the cation resin layer, the water is discharged together with the waste alkali solution through the intermediate drain pipe located at the interface between the cation and anion resin layers ； 5) Replace the anionic resin following the same procedure, at a flow rate of 4 m/h for more than 15 minutes ； 6) The anionic resin is washed with a flow rate of 15 m/h; the amount of water used for washing is controlled at 10 m3 of water per 1 m3 of resin, until the phenolphthalein alkalinity in the effluent is below 0.5 mmol/L ； 7) Acid is introduced to regenerate the cation resin through the acid inlet pipe at a flow rate of 4 m/h, with an acid concentration of 5%; the acid introduction time is more than 15 minutes ； At the same time, the upper alkali inlet pipe should continue to supply water ； After flowing through the anion resin layer, the water is discharged together with the waste acid solution through the drainage pipe at the interface between the cation and anion resin layers ； 8) Carry out the replacement and cleaning of the cation resin following the same procedure, at a flow rate of 4 m/h for more than 15 minutes ； 9) The cation resin is washed at a flow rate of 10 m/h, with drainage carried out through the intermediate drain pipe, until the acidity of the discharged water falls below 0.5 mmol/L ； 10) Perform a full forward wash of the exchanger resin, with water entering from the top of the exchanger and wastewater being discharged through the exchanger’s forward wash drain valve; the flow rate should be 15 m/h, and the wash continues until the conductivity of the discharged water is below 1.5 μS/cm ； 11) Drain water until the level in the exchanger is about 10 cm above the resin layer ； 12) Compressed air is introduced to mix the resin; the pressure of the compressed air is 1–1.5 bar gauge, and the duration is 1–5 minutes ； After the resins are mixed, there must be a sufficiently high drainage rate to force the resins to settle quickly, preventing them from separating again. When the resin level drops, feeding water from the top can accelerate its settlement ； 13) The mixed resin layer is washed under forward flow at a velocity of 10–20 m/h; washing continues until the wastewater meets the required standards, after which it can be used for water production ；

Mixed-bed regeneration process (two-step method). Mixed-bed regeneration process (two-step method). Answer: 1) Backwashing and stratification, 2) Static settling, 3) Water discharge, 4) Regeneration of anion resin, 5) Replacement of anion resin, 6) Rinsing of anion resin, 7) Regeneration of cation resin, 8) Replacement of cation resin, 9) Sequential rinsing of anion and cation resins, 10) Water discharge, 11) Mixing of resins, 12) Rinsing, 13) Standby mode.

1) Before regenerating the mixed-bed, backwash should be carried out first, using a flow rate of 10 m/h, with a backwash control time of 10–15 minutes； 2) Wait stationary for the resin layer to stratify ； 3) Drain water until the water level is about 10 cm above the resin layer in the exchanger ； 4) Lime is added through the upper lime inlet pipe at a flow rate of 4 m/h; the concentration of the lime solution is 4%, and the lime addition time is more than 15 minutes ； At the same time, acid is introduced through the inlet pipe at the bottom of the exchanger; after flowing through the cation resin layer, the water is discharged together with the waste alkali solution through the intermediate drain pipe located at the interface between the cation and anion resin layers ； 5) Replace the anionic resin following the same procedure, at a flow rate of 4 m/h for more than 15 minutes ； 6) The anionic resin is washed with a flow rate of 15 m/h; the amount of water used for washing is controlled at 10 m3 of water per 1 m3 of resin, until the phenolphthalein alkalinity in the wastewater is below 0.5 mmol/L ； 7) Acid is introduced to regenerate the cation resin through the acid inlet pipe at a flow rate of 4 m/h, with an acid concentration of 5%; the acid introduction time is more than 15 minutes ； At the same time, the upper alkali inlet pipe should continue to supply water ； After flowing through the anion resin layer, the water is discharged together with the waste acid solution through the drainage pipe at the interface between the cation and anion resin layers ； 8) Carry out the replacement and cleaning of the cation resin following the same procedure, at a flow rate of 4 m/h for more than 15 minutes ； 9) The cation resin is washed at a flow rate of 10 m/h, with drainage carried out through the intermediate drain pipe, until the acidity of the discharged water falls below 0.5 mmol/L ； 10) Perform a full forward wash of the exchanger resin, with water entering from the top of the exchanger and wastewater being discharged through the exchanger’s forward wash drain valve; the flow rate should be 15 m/h, and the wash continues until the conductivity of the discharged water is below 1.5 μS/cm ； 11) Drain water until the level in the exchanger is about 10 cm above the resin layer ； 12) Compressed air is introduced to mix the resin; the pressure of the compressed air is 1–1.5 bar gauge, and the duration is 1–5 minutes ； After the resins are mixed, there must be a sufficiently high drainage rate to force the resins to settle quickly, preventing them from separating again. When the resin level drops, feeding water from the top can accelerate its settlement. 13) The mixed resin layer is washed under positive flow at a velocity of 10–20 m/h, until the wastewater meets the required standards, after which it can be put into use for water production.

Answer: 1) Backwashing and stratification, 2) Static settling, 3) Water discharge, 4) Regeneration of anion resin, 5) Replacement of anion resin, 6) Rinsing of anion resin, 7) Regeneration of cation resin