The heat transfer eEciency of sea water desalters is very aften imptied by the deposition of scale of various compositions on the heat tmnsf~ surfaces, Four major classifications of such scales can be recog- nised - cunosion produ& such a5 iron and copper oxides, ;;tfk*e SC& Ca(OE& and M@3, additive sea&~ consisting of calcium and magnesium combined with additive matmi& that a~ usu&ly soft and easily removed, iind calcium sulphate.

Acid cleaning techniques have been developed and success3Mly appli4 ti the pmblrrm of removing ti the i%st 3 cabgoties of ~de’and these are reviewed, However for calcium sulphate scale there is zu yet na widely acceptxxl chemical cie&ing process and mechanical methads are mu&y adopt&. These aIs0 me bri&ly cansidemd and shuwn to be inad- equacte ta deal effectively with evrzry im.V!xmce af cal&ux& sul~hafx? SC&g. The need therefore for chemical descaling techniques able to adequately remove cn3cium sulplmte is slmwn to be cleas.

A number 0% ch&ting 2&$%&s which can diW&e c&&.m sUlphate and therefore are potential candidates far the form&&ion of a &table reagent have been reviewed and 4xvo such reqenk taken to pilot scale tials. These tx4.s imluded cumusion tests on a typical range of desa.lter constxuctio~ materials and in la&x work used actual scale removed &om operational plants.

Fmm the data obtained an economic assessment of the use of these reagenti for the descaling of a typical dm&x heat input vessel was made, fium which it was cuncluded that the once %hmugh application tf either reagent chid only be considered feasible for limited treatment. S’o aver- come this econarnic problem, processes for the regenemtion of &e spent reagents have been dmmloped to the pilut plant stage. From this data one resent haf been chasm and following continuing development, a design fur f% mobile chemiml descaIin& tit has been mmlved. Tl&i unit* which

also has the capability of carrying out a wide range of descaling operations is described.

INTRODUCTION

The heat transfer efficiency of sea water disfillation pIen& is very ofkn impaired by the deposition of foulling films deposited on heat tram- fer surfaces, Although fouling can occur on either side of the tubes it is much more usuai in our experience for deposits to form on tie brine side since this carrks the neater burden of both d&oh& and suspended material, md this paper considers brine side deposits only.

Fouling can be caused by factors external to the disiAler such as silt, marine @owth or water poUution_ Such problems are a function of the plant site and as such are usually specific to the particular sit@, requiring the formulation of a special treatment. The more general causes of fouling are related to probhns with brine chemistry control which results in plant corrosion and build-up of corrosion product material or chemical scale deposits of magnesium hydroxide, calcium carbonate or calcium sulphate. It is this latter and more general problem, and particuY@Iy with methods of removing such fifms, with which W paper is concerned.

In our experience deposits can be ditided into four categories: (a) Iron rich, resulting from plant construction mat&al corrosion

product;s c&ed both in suspension and solution, The mechanism whereby this material is caused to deposit on heat transfer surfaces is not My understood. Usually some basic scale, mainly magnesium otide, deposits at the same We. These can become ixrcarporated info the copper oxide f&n growing in situ to give a highly adherent and dense fihn which resists on-line mechanical cleaning such as &he Taprogge system.

(b) Alkaline scale, predominanG3y magnesium hydroaide with some cakium, found in acid dosed plants, let is caused by Zusufficient acid dosing tu reduce residual alkaliMy to a low enough fevell. The film is generally t;hin and strongly adherent and again resists mechanical cleaning systems,

(c) Carbonate sludges, formed in additive dosed plants Which use either polyphosphate or polycarbaxylic additives. Their composition is mainly calcium carbonate with some magnesium hydroxide and cabana& They are soft, tind to be thick and porous and can be removed by in situ mechanical methods up to a point.

(d) Calcium sulphate scales, formed when the brme concentration Emits or top temperature of 120°C are exceeded. Three forms of caMum suiphak s&e are known - CaSU4 @nhydMe), C&U4 -@-I20 (hemi- hydrrite) and &SO4 - 2H2 0 (gypsum). The parGcular form found ixa a plant

CHEMICAL I3ESCALIW.Z OF ACID DOSED PZLANTS 275

depends un the brine conditions under which it was formed, anhydrite being Itid down at the highest temperature and being the must difficult t0 redissulve.

ti the cafe of acid dosed plants an acid reagent is really av&ble and is used in the routine operation of the plant, this is usually sulphuric acid since this is normally the cheapest acid available, In roufrine operation acid is added to reduce the residual a&&r&y tu between 5 and 10 ppm, i-e,, a_pH of 73 to 7_5_ These conditiorrs result in -a slow deposition of an alkaline scale, chiefly magne&m hydroxide, but tith some iron oxide also, Routine exha additions of acid sufficient to reduce the brine pH tu 4.0 for periods of 2 to 4 hours are used a$ frequencies considered necessary by the fall off in @ant efficiency, This treatment &fectively remuves the aMAine scale but is nut entire& succes&W with the irun oxide ~~&ing in a slow but steady duwnward m in. over& operM.ng effi&ncy, To correct this a more vigorous acid clezm is required on occasions. This can be achieved using sulphtic acid, tltis time reducing the pH to 2.5, but this gives rise ti excessive general plant corrosion. The addition of corrosion inhibitors such as stannine helps tu rduce this corrosion, or the use of milder organic acids such as citric, but this is more expensive.

In i&ese plants the heat transfer tubes become fouled with a carbon- ate sludge which must be removed fYequent.Iy, On-line mechanical methods are reasonably satisfactory but are not usu;lfly 100% successM, and acid cleaning is necessaq from time to time. sufphuric acid, pr&erabIy inhibited, is a commonly used reagent when tile operation is cz%rried out tith the plant still on Iine but at reduced output, but hydruchIoric is sometimes used. Alternatively, if the plant can be shut down for this operation, a number of proprietary reagents which are based an organic acids have been successfully used which minimise plant corrosion.

CALCIUM SULPHATE SCALE

Calci~ sulphate scale is usually found in acid dosed planti because these operate at the highest temperature. However, mure recently, additive dosed p&&s wing BeIgard are nuw able $o reach temperabre~ af 120% and are therefore a&o Iikely tu experience calcium sulphate scaling.

Calcti~m sulphate dues not react with simple acids since it is the relatively &iotubZe salt of a sti0ng anion. The treatmen& which zire effect&e for etcher types of fouling agmts are tkrefure ineffedve with. calcium suiphate.

Calcium sulphate is not totally insoluble in water - one litre dksohres 2 vs at 30°C - and circulation of clear, water can remove it; this has been used in some situations. However, this process is not very efficient since 1 ton of calcium sulphate requires 100,000 gallons of fresh water to dissolve it or 200,000 gallons af sea water. Also the rate of dissolution is slow and although ticrea.Gng the temperature will help a httle, the solu- ISty decreases so that a compromise must be worked out.

Because simple ciremical, methods of removing c&iMrn sulphate scale have not been available it has been usual to resort to rn~h~&~al methods. In-line methods such as Taprugge are qui& ineffective because of the hard, st;rongly adherent nature of this scale, part;icularlg the anhydrite fun-n. Off-line methods using high pressure water jets at 5UUO-10,000 psi have been used to remove calcium sulphate scales, this metiad being more generally successful with the gypsum and hemihydrite than with anhydrite- Anhydrit~ usually requires drilling which is time consuming. Wire brush pigs are not usually effective.

With modem sea water distiller design moving towards operation at higher temperatures the hkelihoad of calcium sulphate scaling increases,

p&icula#y the form&ion of the more intractable anhydrite Eon-n- The need for a system of removing such scales which does not require exten- sive plant dismantig and cunsequentiy extended down-times is clearly S%!Xl.

CALCIUM SULPHATE IXSSULUTION

In order to dissolve substantial amounts of calcium sulphate it is necessq tx, use a reagent which can effectively compete with sulphati ions for any calcium ions present in solution. There are a number of such reagents of which EDTA is perhaps the most well documented, These reagents, known as chelatig agenti, can dissolve calcium sulphate because they form a sulubl~ complex w&h c;xlcium which has a smd dissociatiqn constant, i.e., the calcium ions are 53 strongly held that the calcium ion concentration in solution is lower than the sdubility cunsbt of calcium suJpha&

Ch&&ng reagents react similarly with other divalent and some tri- valent ions so that copper and iron oxides may also be dissolved. This can be either desirable or undesirable, since it effectively reduces the capacity of the reagent to dissolve calcium sulphate, but it can also be used to remove fouling oxide films of Type (a) (iron rich). However, since the typical d%iSHer also contains lmge amounts of iron oxide on non heat

Q

277

transfer sur&ces such as chamber walls, it is preferable to fon[nulate the descaling reagent to preferentially dissalve calcium sulphate.

REAGENT FOFtMULATI0NS

A number of chelating agenti which are available in commercial quantities were formulated into rewents suitable for dissolving c&zium sulphate but with minimum @x&e of iron and copper oxides and COP rusion afplants4xucM materi~.Tk;leser@~e~tswerescreened~ labora- tory tests folc effectiveness w%h spectiens of calcium su_Iphate ~0w-n k operating phnts. The results were assessed against theoretical capacity for dissolution of calcium sulpha&e, uptake of iron and copper both from the calcium sulphak scale and corrosion coupons, and the rate at which the scale was dissolved.

Table I shows the resulh obtained with the five reagents judged to be most useful from these screening *sts. All these reagents were shown to be effective in dissolving even the?: most difficult calcium sulphate deposits but variations in reaction rates were noted. This factor, coupled with the relative costs of the basic chemicals and the environmental effects of the discharge waste sufution, govern the choice of reagent for a pa&icular application.

Two reagent-s were taken to pilot scale trials in which whale tubes were descaled in a test loop having fnst been fouled by driving the heat input section of the loop to temperatures above l!X°C until a thick coating of anhydrite had been produced.

The reagents fez&d were: one based on EDTA being the most effective agent but expensive, and one bred on glycollic acid because this was the cheapest of the useful agents. Sat& reagents contained a corrosion inhibitor to lirrrit the attack on plant materials, The tests were run at 80°C

The results frum a number of pilot plant tests confinned the obser- vatiuns given in Table 1, in that the EDTA rewent removed the scale most, effectively but that sodium glycollate perEomed adequateiy, A si@ficant drawback tith sodium &moUate was the lack of cold solubility which caused the solution to solidify an cooling. Although this problem could be overcome by correct operating procedures it would be a majar disad- vantage on 8 plant, where operational problems are always likely,

Corrosion data obtained using caupons during these pilot tists are given in Table II. This shows that bath reqents aze satisfactory but EDTA is less aggressive to metals. However, neither reagent is as c0rrusive as normal sea water, The average corrosion rates for an acid dosed sea water distiller are given for comparison_

CHEbW2U DFSCALTNG OF ACID DQSBD PLANTS 279

TABLE XI

CURROSZON OF PLANT ~MATEMALS BY THE DESCALZi!+lG REAGENTS

Reagent Cottosiun nzte (mg/dm2/dtzy)

sudluln 333 1.5 18.0 0s 29.4 1.0 26.4 glycouate

N0md* 6.0 11.4 320 0.85 402 5.2 402 plant

COMPARISON OF COSTS

Using the restdt~ from the pilot scale tti&s the economics of using these two reagents for remutig scak fmm the Jersey MSF piant were cornpa&, Xtwas zmxu~~ed that theheat~putvesselc~n~~onetonpf

cakG.un sulphate; the reagent costs to remove this were estimated to be $2.5K for the EDTA system and BLSK for sodium gIycoUat;e. To the cost of chemicals must be added those for effluent disposd, for temporary pku& and manpower. Relatively large volumes of solution are required and this makes the cost of the plant rather highs

Comparison of the costs of chemical cleaning using the prucess outlined above with that of mechanical cleaning showed the balance to favourmechanicaf. clleaning, even though this irx;volved some risk of damage to the phmt. Two aspects of the chemical process were responsible for its poor cost perfmmxnce, firstly, the high cost of the reagenti and secondly &he Iarge vdums of reagents needed for a one-step operation. Ry carrying out the operation in small steps this second cost factor could be reduced but oinly by extending the time of the operation atlcI thereby increasing mmpawer costs.

To solve the two major problems, i.e, rduce resent costs ad keep the volume of the reagent small, the concept of w-using the same reagent

280 K. HARDXNG, D, A. BRIDLE AND F:THORNE

Waste regeneront

Fig_ 1, Fluw diagram of typical descahg operation.

was considered. In such a process a re’Iative& small volume of reagent, enough to fill the plant and a small auxtiary heating and header tank, is used. The reagent is circulated through the system md dissolves scale; a~ it approaches safuration, part of the circulating flow is dive&cl through WI ion-exchange column containing a resin which can compete with the reagent fur the c&ium_ This regenerates part of the reagent which @an then di~sulve more scale_ The net effect is to transfer the scale from the plant to the ion-e~change material_ When the ion exchanger is saturated with calcium it is vtived out of the circuit and regenerated whilst a second column takes over. Fig. 1 shows the flow dim for a typical chemical desc&ng operation using this concept.

To prove this concept in practice and op&r~ise the canf!&&ing pxacess requirements, experimental wark, again in our pilot plant, was car&d utz*t. The object of thi work was to develop the best reagent composition to dissolve calcium sulphate effectively, stih with minimum plant material corrosian, but able to exchange the complexed calciuncl with an ion exchanger and to select an ion exchanger which earl regenerate the reagent ad can in turn be regenerate& using 8 cheap ~~~~BVZZUZ~, Data on opti- mum temperatures and fluw rates in the varicms plant components were also obtained to enable an operational system to be designed for plant trials.

REAGENT REGENERATION TESTS

The requirement to remove calcium from the descaling soIutiun c& for modified properties to those needed far the simple dissolution of calcium sulphate. Experiments with a range of mixtures of complexing

Cacti DESCALZNG.OF ACID DO_SED PLANTS 281

Fig. 2. Capacity of ion exchange resin for calcium from descaling solution successive regenerations.

Fig. 3, Regeneration of ion-exchange resin - comparison of tegenerant sotutions.

a@nts of the EL)TA and NTA type were carried out using our pilot plant for ca2cium sulphate dissolution and with a range of commercia22y ava& able cation-exchange resins for subsequent regeneration of the spent reagent. F’rom these &i&s a reagent fornndation has been selected which represents the best compromise beizween the various confhcting require- ments,

The effectiveness of reagent regeneration using ion-exchange resin

can be seen brom Fig. 2, which shows graphically the capacity of a column of the sefected resin for ca2cium exchanged with the descaling solution. The initial fa22 in capacity after the first regeneration is checked in sub- sequent cycles and a steady useful capacity of sround 85% of maximum is obGned.

A feature of major importance to the success of this process is the effective regeneration of the ion-exchange resin. A number of regenemnt

solutions have been tied, the aiin Being tic& find the cheapest practical oale,

Fig. 3 shows the results obtained using four solutions* So&:um chl0tide is the reagent used and when in the furm of salt s&&iion ~r~tides theoretical results. However, an a sea watir distillation plant salt W&W is avaSable in abundance. The use of acidified sea water is not very affective but the blawdown brine which is approximately twice cuncentra&zd is q&e an effective regenerant. On a multi-plant station where waste brine would be avdable, this could be used with only a small reduction in ove~4.I efficiency,

IFruns the data prutided by the abme pi.lc& pIant studis, a nx.abiIe chemical deschg unit has been designed, This consists of a heated vessel in which the descaEng reqent is made up, a pump tu &cuIate the hat reagent through the vessel to be desc&d, and ~JIWO ion-exchange vess& thruugh which part of the flow can be passed bo regenerate the reagent Once a signif%cant concentration of calcium has been reached, These items, together with a smaller tank in which the xresin-regeneration solution is prepwed, are dil mountid on a single unit izs a transportable item,

For operation this unit is connected via temporary pipework to the plant item to be descakd. ?E‘he reagent mixing vessel is B.&d with water and circ%.&ted to check c:onnecti;ions and f~emove air locks, The reagent is made up in the mixing tank, heated, and circtiati~~~ is start&L If the vo’fume of the circuit is toa great tu alloslrr the initial resent mix to achieve the correct concentration, then Further chemicti are added later. When the calcium concentratian in the reagent reaches t;fie optimum value the ion exchangers are brought into aperation. Adjustment of the solution pH is achieved by adding sadium hydroxide as ne3c;~sary. Corrosion probes and corrusion coupans are also used t;o indicati corrosion rates cont&%uousIy anG pruvide integr&& corrosIun data for the fWl optm&i~~~,

Estimakd c&s for d&s&&g a ton of calcium s&&ate using this plz~t, yuith sodium chloride as a regenerant reagent, is 6450 fur chemicals but if concentxated W&E brine is available this can be reduced to E200, With costs reduced to these fev& there are tidespread petential app& catians fox: thii process.