EOR Feed Water Treatment and Softening Systems

 

      REGENERATION PRINCIPLES AND METHODS

 

During the service or softening phase of water softener, the throughput of raw water exchanges calcium and magnesium ions (“Hardness elements”) for sodium ions, while flowing through the sodium zeolite resin beds. After a metered volume of water has passed through the system, the available sodium ions become depleted, and the resin is considered exhausted for efficient softening purposes. Whenever feasible, vessel sizes and resin volumes are selected to allow a minimum of 8 hours service flow before primary vessel resin exhaustion is approached.

 

In the absence of a preceding raw water filtration system, the primary vessel resin bed tends to collect any solids present in the raw water flow, eventually creating a flow restriction and reducing softener throughput. Such a “filtering” tendency is not a design consideration, but an inherent characteristic that must be tolerated in certain instances.

 

Also, during the service flow, smaller resin particles tend to “pack down” among the larger ones, and must be periodically redistributed, or “fluffed”, to retain maximum resin bed efficiency.

With the exception of very large systems, all components are unitized in a self contained package, either skid or trailer mounted, with optional mounting configuration available to suit any application.

 

In certain systems, raw water filtration vessels may precede the softener banks, either unitized with the system, or operating as an add-on. Filter maintenance controls are often integrated with those of the softener system.

 

The two vessel “series”, or primary/ polisher flow design allows maximum utilization of resin in the primary vessel between regeneration cycles, while still maintaining “zero” hardness effluent water throughout the service flow duration. Allowable raw water throughput and regenerant brine calculations are based on the volume of resin in the primary vessel only, leaving the polisher vessel to function as its name implies, removing the normally increasing hardness leakage from the primary vessel, as the primary resin approaches exhaustion. The polisher also serves as a back-up softening vessel, in the event raw water hardness increases for some reason exhausting the primary vessel resin before the selected throughput is reached.

 

Careful consideration is given to the construction and placement of vessel internals, with inlet distributor and outlet collector/strainer manifolds designed for mechanical strength and minimal flow restriction. Depending on anticipated operating conditions, stainless steel or PVC strainers are used, which are sized, and spaced across the vessel to prevent “channeling” flow through the resin beds, at design service flow rates.

 

External support requirements for the water softener systems include a consistent source of raw water, with known constituents, and preferably delivered to the softener charge pump at 2 psig minimum suction head pressure. In most cases, the only additional support requirement is a reliable and adequate source of three phase electrical power.

 

In certain softener systems, a reliable supply of 70 psig minimum compressed air is also required, where pneumatic flow measuring instruments and controls are supplied, usually at customer’s request. The standard softener flow control valves can either by operated on system charge pump water pressure, or compressed appropriately.

With the above conditions, the primary vessel resin bed must be cleansed of solids, expanded or “fluffed” to redistribute resin particles, and both primary and polisher resin beds must be regenerated to original sodium form. Complete and effective water softener regeneration must include the following functions:

 

A. Cleanse the resin beds of solids.

 

B. Redistribute the resin particles.

 

C. Regenerate the resin to sodium form through contact with brine solution.

 

D. Rinse the brine solution out of the system.

 

The first two functions are accomplished with a Backwash phase, where raw water is pumped in a reverse direction through the primary vessel resin bed, and out to drain. Backwash flow rate is adjusted so as to create a 50% minimum resin bed “expansion”, and by design, preferably between 70% and 100%. However, the flow rate should not be set so high as to cause resin carryover out of the system. Duration of the Backwash phase is determined by monitoring the cleanliness, or turbidity of the effluent backwash water. In certain softener systems, soft water from the adjacent softener is used for backwash, but raw water use is the more common practice.

 

Since the polisher vessel resin bed should rarely collect any significant volume of solids, backwash of that vessel is not usually included in the regeneration cycle. For this reason, cleanliness of the regenerated brine and soft brine displaced water is an absolute necessity.

 

The third function, regenerating the resin to original sodium form is accomplished by “soaking” the resin beds in a brine solution, at a relatively slow flow rate “adjusted to provide a 20 minute recommended contact time). Brine Injection allows the re-exchange of sodium ions for calcium and magnesium ions absorbed by the resin during the softening, or service flow.

 

The brine solution flows upward through the polisher vessel resin bed, with a certain amount of resin particle redistribution, into the primary vessel, and downward through that resin bed, to drain. Brine injection reverse flow through the polisher vessel insures maximum resin regeneration near the soft water outlet.

 

Volume and solution strength of regeneration brine is influenced by several factors, primarily the hardness of the raw water, flow rate through the system, total dissolved minerals, amount of sodium chloride in the raw water, and volume of resin in the primary vessel. Ion Exchange resin can be regenerated to various levels of hardness absorbing capability, depending on brine solution volume. More salt used per cubic foot of resin results in higher capacity, but only to a point where declining increases in resin capacity no longer justify the greater salt costs.

 

Normally, 10% by weight brine solution based on 10 pounds of salt per cubic foot of resin will result in both satisfactory salt economy, and resin hardness removal capacity. However, if all the above factors are excessive, or the raw water constituents fluctuate, the normal ion exchange demand on the resin will change, and salt percentage may be increased or decreased.

 

The fourth function, flushing the brine solution out of the system, occurs during Brine Displace and Rinse sequence.

 

Soft water is used for Brine Displace, and flows through the resin beds in the same direction and at the same flow rate, as during Brine Injection. Brine Displace flow volume is adjusted to displace all the polisher vessel brine into the primary vessel, for maximum brine/resin contact in that vessel, prior to the Rinse phase.

 

Rinse phase employs raw water flowing at a considerably faster rate, in the normal service direction through the softening vessels, to drain. Recommended Rinse flow rate is 1 ½ GPM/FT3 of resin, with 40-60 U.S. gallons/FT3 normally required for complete vessel Rinse. More precise duration of the Rinse flow is established by checking chlorides content of the primary vessel effluent water, with Rinse considered sufficient when effluent chlorides are no more than 50 ppm above those in the influent raw.

 

In certain softener systems, the primary and polisher vessels are rinsed in simultaneous series flow, with a portion of primary rinse effluent diverted to drain, while the remainder flows on down through the polisher vessel, to drain. The more common procedure is to thoroughly Rinse the primary vessel to drain, then divert the soft primary effluent water through the polisher vessel, with the soft Rinse water insuring maximum softening capacity in the Polisher vessel resin, upon return to service flow.

 

At the end of the Rinse phase, automatic controls open and close the necessary sequence of valves to return the regenerated series softener bank to service flow operation. At this time, if another softener bank has been “waiting” to regenerate, interlock circuitry will not allow regeneration to begin in that bank.