Several Forms of Combined Application of Strong and Weak Resins

There are four main forms of combined application of strong and weak resins: ① double-layer bed ② double-chamber double-layer bed ③ double-chamber double-layer floating bed ④ series co-current.

(1) Double-layer bed: The double-layer bed was the initial form used in combined applications. It involves placing two resins, strong and weak, with specific requirements for gravity and particle size, in a single exchanger. The weak resin is placed on top of the strong resin. During operation, water flows from top to bottom, first passing through the weak resin and then into the strong resin. During regeneration, the regenerated liquid flows from bottom to top, first passing through the strong resin and then through the weak resin. The regeneration operation is the same as that of a single-layer countercurrent regeneration fixed bed, and its internal structure is also basically the same as that of a single-layer countercurrent regeneration fixed bed. The double-layer bed is used in Jinzhou Ferroalloy Plant and Bajiaotai Power Plant. Its advantages are simple system, fewer equipment, and low investment. Its disadvantages include limited resin layer height, strict requirements on resin performance and stratification operation, complex regeneration operation, and low flow rate. The biggest disadvantage is the easy mixing of resin layers.

(2) Double-chamber double-layer bed: The double-chamber double-layer bed is designed to overcome the resin mixing problem in double-layer beds. A perforated plate is added at the boundary between the weak and strong resins in the double-layer bed, relying on the perforated plate to separate the weak and strong resins.

Its operation and regeneration methods are the same as the double-layer bed, but there are differences: Firstly, the top-pressure operation is omitted during regeneration. When the double-chamber double-layer bed is about to fail, the strong resin in the lower chamber shrinks in volume, resulting in a water cushion layer on top. Therefore, the top-pressure method used in the double-layer bed process cannot be used for compaction regeneration. An appropriate amount of plastic white balls must be filled on top of the strong resin to prevent layer disorder during regeneration. The weak resin is mostly in a suspended state during regeneration. Even if layer disorder occurs, it is easy to regenerate and does not affect its working exchange capacity. This allows for pressure-free operation; secondly, an external resin cleaning tank is required because the strong resin chamber is filled with resin and white balls, so the resin cannot be cleaned internally and a special external cleaning device is needed; thirdly, the double-chamber double-layer bed has no special requirements for the resin and can use resin with ordinary particle size and specific gravity.

The disadvantages of the double-chamber double-layer bed are mainly as follows: ① If the plastic white balls in the upper part of the strong resin are not filled properly, the regenerated strong resin is prone to layer disorder; ② During operation, the water flows from top to bottom, resulting in high resistance and limiting the output of the equipment; ③ It requires additional external cleaning equipment; ④ It has strict requirements for the turbidity of the influent.

(3) The most commonly used double-chamber double-layer floating bed is to install three perforated plates in one exchanger. The perforated plates are equipped with filter caps, and the three perforated plates divide the exchanger into upper and lower chambers. The lower chamber contains weak resin, and the upper chamber contains strong resin. During operation, the water flows from bottom to top, and during regeneration, the regenerated liquid flows from top to bottom, which is the opposite of the double-layer bed. Therefore, the double-chamber double-layer floating bed not only possesses the advantages of a double-layer bed but also offers advantages such as simple floating bed regeneration operation, high operating flow rate, and large equipment output. It is therefore widely used, for example by Qilu Company, Yangzi Company, Yanshan Petrochemical, and Jinzhou Petrochemical.

The disadvantages of the double-chamber double-layer floating bed are as follows: ① High resin layer resistance, mainly due to the high operating flow rate. It is also related to the resin particle size, layer height, and water temperature; ② The perforated plate is prone to clogging; during operation, water flows from bottom to top, pressing the resin against the upper perforated plate. Small particles and worn resin concentrate in the upper part, easily clogging the water cap, leading to increased operating pressure differential and even affecting equipment output. The anion-containing double-chamber bed is most prone to clogging, mainly because the domestically produced D301FC resin is easily broken. The operational experience of the above-mentioned units shows that: the inlet pressure of the double-layer floating bed must be controlled at 0.20-0.25 MPa, and a certain height of inert resin with low specific gravity and high buoyancy should be filled in the upper part of the resin layer to intercept fine resin. ③ The resin needs to be cleaned externally. ④ Strict requirements are placed on the performance of the resin. In order to adapt to the high flow rate and external cleaning characteristics of the double-layer floating bed, the resin is required to have high strength, low uniformity coefficient, and slightly larger particle size. ⑤ The requirements for influent turbidity are very strict, and the inlet turbidity is generally less than 1 FTU.

(4) Series System The series system is the earliest combined application method, which is divided into equal diameter series system and variable diameter series system, and is suitable for water quality with high salt content. In this process, strong and weak resins are placed in two exchangers respectively. The first exchanger is filled with weak resin and the second exchanger is filled with strong resin. During regeneration, the strong resin is regenerated first, and then the weak resin is regenerated with the regeneration waste liquid. Operational experience at Jinxi Chemical Plant shows that this process has advantages such as simple internal equipment structure, reliable operation, and suitability for water with high salinity and high suspended solids content. Its disadvantages include numerous external pipelines and valves, a complex system, and a large footprint. Under normal water quality conditions, the equipment utilization rate is low, the flow rate is low, and the output is low.