GB2050192A - Monitoring sodium content of condensates during polishing - Google Patents

Abstract

In a method of condensate polishing in which substantially all sodium is removed from cation resin during regeneration so that residual sodium cannot be displaced by ammonium ions during service by exhaustion of cation resins by ammonium ions derived from ammonia conditioning of the boiler feedwater, or from any other source there is diverted from the mainflow 10 a pilot flow of feedwater 16 which is continuously fed to a pilot cation resin exchanger 18 containing strongly acidic cation resin. The total flow leaving the pilot exchanger is continuously monitored as is the conductivity of the flow and the results are integrated. A value is derived indicative of the total of sodium ions which have passed in the main service flow. Thus the correct amount of acid required to elute that sodium from the cation resin in the polisher for regeneration can be derived. <IMAGE>

Description

SPECIFICATION Condensate polishing method and apparatus The invention relates to methods and apparatus for the polishing of boiler condensate using ion exchange materials.

In the treatment of boiler condensate using ion exchange units known as polishers mixed cation and anion ion exchange resins are used. The cation resin is initially in the hydrogen form and the anion resin in the hydroxyl form.

It is common practice to condition the boiler feedwater with ammonia and to continue to exhaust the ion exchange material until the cation material has been exhausted to the ammonium form by the ammonium ions derived from such ammonia conditioning.

When this procedure is adopted, it is preferable during regeneration of the cation material to ensure that all or substantially all sodium is removed from the cation material. Unless this is done, the residual sodium will be displaced by ammonium ions during the continued period just mentioned. The resulting leakage of sodium ions into the boiler can cause extensive damage.

The sodium ions are derived normally from two sources namely, from improperly treated raw water used for boiler make-up or from the ingress of untreated cooling water, especially brackish water or seawater, owing to a leak in the condensor unit oP the boiler system.

The sodium ions from those sources can attach themselves to the cation material while it is in the hydrogen or the ammonium form.

In order to eliminate or at least minimise adverse effects from the leakage of sodium ions into the boiler system it is necessary that the operating staff know how much sodium is present on the cation resin before regeneration commences so that an adequate amount of regenerant is used and also to ensure that regenerant is not wasted by the use of excessive amounts.

It has been proposed in British patent specification No. 6;71307 continuously to measure the conductivity of decationised water prior to its entry into an anion unit, continuously measuring the rate of flow of water through the anion unit, automatically combining the indications thus obtained to produce a continuous indication of the degree of exhaustion of the anion exchange material, and determining the quantity of the regenerant chemical to be employed in accordance with the valve of the last-mentioned indication at the time when the anion unit is to be regenerated.

The purpose was to enable a cation and an anion demineraliser unit to be regenerated simultaneously. The question of break-through of sodium ions was not considered and the patent was not concerned with water polishing.

It is an object of the present invention to provide a method and apparatus by which the break- through of sodium ions is eliminated or substantially eliminated.

A method of condensate polishing according to the invention using service cation and anion exchange materials is characterised by continuously diverting from the main service flow of water to be treated a pilot flow and passing the pilot flow through a pilot amount of cation material in the hydrogen form to remove at least ammonium and sodium ions, monitoring the conductivity of the resultant pilot flow, monitoring the rate of said main flow combining the monitor results and converting the resultant to represent the total amount of sodium present in said main flow regenerating the service cation exchange witl an appropriate amount of regenerant determined in accordance with said total amount of sodium and regenerating the service anion material.

Apparatus for performing the method according to the invention is characterised by a pilot cation ion exchanger connected to receive a pilot flow from the! main flow of water to be treated, a conductivity-responsive instrument arranged to monitor the cconductivity of flow leaving the pilot exchanger, a flow-rate monitor arranged to monitor said rriain flow and an integrator arranged to receive outputs from said instrument and said monitor and to produce an output representing the total amount of sodium present in the total of said main flow of water before treatment.

A form of method and apparatus will now be described by way of example to illustrate the invention with reference to the accompanying drawings, in which: Figure 1 is a schematic diagram showing the apparatus arranged in conjunction with a mixed bed, boiler condensate polishing unit; and Figure 2 is a graph showing curves relating the amount of sodium on the cation resin as a percentage of the total ion content (plotted as ordinate) to the! concentration in units of 100 grams per litre of 98% sulphuric acid regenerant (plotted as abscissa) needed for the elution of sodium ions from 12% divinyl-benzene (DVB) macroporous cation resin in the sodiurn or mixed sodium ammonium form using normal sulphuric acid solution at a rate of 5 bed-volumes per hour.

Fig. 1 is self-explanatory. Boiler condensate water is fed along the line 10 as a main flow of water to be treated in a condensate polisher unit 1 2 containin!g a mixed bed (not shown) of cation and anion ion exchange resin materials.

The treated water passes along the line 14 to the boiler feed pump.

A pilot flow of water is fed along the pilot line 1 6 to a pilot cation ion exchanger 1 8 containing strongly acidic cation ion exchange resin material. The pilot flow leaving the exchanger 18 passes to waste at 20.

For a typical boiler installation in which the main condensate flow is 10,000 cubic metres per hour (m3/h) the pilot flow would be typically 0.01 m3/h).

The pilot flow is maintained throughout the period of service flow through the unit 1 2.

Service flow continues until the cation resin is totally exhausted to the ammonium form (assuming ammonia is used to condition the feedwater). The service flow can also be continued once the cation is in the ammonium form until such time that a pressure drop across the service vessel indicates that a regeneration cycle including cleaning is required.

The total flow through the unit 1 2 is continuously monitored by a monitor 22. The conductivity of the water leaving the exchanger 1 8 is continuously monitored by an instrument 24 and the outputs from the monitor 22 and the instrument 24 are passed to an integrator device 26.

The integrator device 26 is designed to produce an output indication having a value, at the end of service flow, representing the total amount of sodium ions which passed in the main flow to the unit 1 2. Assuming there was no leakage of sodium ions from the unit 12, all such sodium must have been picked up by the cation resin in the unit 12. The correct amount of regenerant sulphuric acid necessary to elute that sodium from the cation resin in the unit 12, when it is regenerated, can then immediately be found to give any desired degree of removal of sodium ions, as explained below. I The ion exchange reactions which ocur in the exchange 1 8 are as follows, sodium chloride being used to represent all of the impurities present (e.g. Mg, SO4 etc):- 1.RH +NH4OH =RNH4 +H2O Hydrogen Ammonia Ammonium water form cation in solution form of cation resin resin 2. RH + NaCI = RNa + HCI Hydrogen Sodium Sodium Hydrochloric form cation chloride form of resin acid resin From these reactions, it can be seen that the addition of ammonia to the system does not affect the readings of the instrument 24.

The integrator device 26, to give a reading representative of the sodium ion loading of the service unit 12, has to be calibrated to take into account the proportion of sodium present in the cooling water. For example, for sea water the sodium ion content is relatively constant at 77% of the total cation content. For river water, however, the sodium ion content would have to be determined and the device 26 adjusted accordingly. From time to time it may be necessary to recalibrate the device 26 particularly to adjust for possible seasonal variations which may occur in river water.

While the regenerant level is selected to achieve low levels of sodium ions retained by the cation resin, other cations are not necessarily removed to the same low levels. However, as these other ions do not leak from the service unit 1 2 when full conversion of the cation to the ammonia form has occurred, the higher levels of such cations do not present any problems.

The graph in Fig. 2 shows that 12% divinyl-benzene macro-porous cation resin, which typically is used in the unit 12, for example, carrying 5% of sodium and 95% of ammonium ions (ignoring small percentages of other metallic ions for simplicity) requires 450 grams of 98% sulphuric acid per litre of resin (g/l) to reduce the sodium content after regeneration to 0.04% (curve A).

The acid is made up of a solution of normal concentration and used at a flowrate of five bedvolumes per hour to regenerate the cation resin (separated from the anion resin) used in service in the unit 12.

Had the cation resin carried 30% sodium and 70% ammonium ions, 670 g/l of regenerant acid would be required to reduce the sodium content to 0.04% after regeneration (curve B).

A reduction of the sodium content by regeneration of the cation resin, to 0.04% of the total of all ions carried by the cation resin would be amply satisfactory for most boiler applications to ensure that no sodium, or only a negligible small amount, was displaced to leak through into the boiler, even when the resin was allowed to be completely exhausted during service to the ammonium form.

The use of costly sulphuric acid regenerant is minimised to the quantity required to achieve that desirable result.

The service cation and anion materials in the unit 1 2 are regenerated by steps which it is not necessary otherwise to describe here.

The pilot cation material in the pilot exchanger 1 8 is regularly regenerated to ensure that it is never exhausted to the point where it is incapable of removing ammonium and sodium ions from the pilot flow.

Claims (4)

1. A method of condensate polishing characterised by continuously diverting from the main service flow of water to be treated a pilot flow and passing the pilot flow through a pilot amount of cation material in the hydrogen form to remove at least ammonium and sodium ions, monitoring the conductivity of the resultant pilot flow, monitoring the rate of said main flow combining the monitor results and converting the resultant to represent the total amount of sodium present in said main flow regenerating the service cation exchange with an appropriate amount of regenerant determined in accordance with said total amount of sodium and regenerating the service anion material.

2. Apparatus for performing the method claimed in claim 1 characterised by a pilot cation ion exchanger connected to receive a pilot flow from the main flow of water to be treated, a conductivity-responsive instrument arranged to monitor the conductivity of flow leaving the pilot exchanger, a flow-rate monitor arranged to monitor said main flow and an integrator arranged to receive outputs from said instrument and said monitor and to produce an output representing the total amount of sodium present in the total of said main flow of water before treatment.

3. A method according to claim 1 substantially as described herein with reference to the accompanying drawings.

4. Apparatus according to claim 2 substantially as described herein with reference to the accompanying drawings.