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Industrial water production by utilization of a reverse osmosis and an evaporation plant
DESALINATION ELSEVIER
Desalination
108 (1996) 231-245
Industrial water production by utilization of a reverse osmosis and an evaporation plant R. Borsani*, M. Fazio, and B. Ferrando FISIA-ZTALIMPIANTI
SPA, Via Di Francis Received
I, 16126 Genova, Italy.
Tel.: +39-10-64091;
Fax. +39-lo-6409991
28 July 1996; accepted 3 August 1996
Abstract
ITALIMPIANTI has realized the steel making plant in Woljsky (on 1989), situated in the Volgograd area between the Caspian Sea and the Black Sea, with a steel output of 150,000 t/y from scrap, 720,000 t/y of seamless pipes, and 210,000 t/y of blooms. In the ambit of the operation system of water necessary for the production cycles, for ancillary equipments and in compliance with the former USSR rules, it has been chosen a treatment system to bring to “zero” the waste water discharge from the steel plant complex. In this point of view, the “conventional” treatment stages of the make-up recycling water and steam generators feeding is as follows: flow rate 300 ms/h, softening line, gravity sand filters, two lines of demiwater production for two different purposes, final storage, and sand filter backwash treatment and sludge dewatering lines. A “nonconventional” process is added to the previous one to treat different effluents of backwashing water of resins used for the demineralizing system for which the following scheme is foreseen: flow rate max. 600 ms/d, neutralization, and two lines of RO having each three stages in series. The aim of this treatment plant is to produce industrial water with characteristics as above mentioned. The concentrated makes up the feeding of a multi-flash evaporator with a production of mixed crystals and condensate used as industrial water, Keywords: Zero discharge water treatment; RO system; RO membrane; Concentration; Crystallization
1. Introduction The steel shop having a total production capacity of 720,000 t/y of pipes and 210,000 t/y of blooms has started in 1989. The iron and steel cycle takes in the following steps: Presented at the Second Annual Meeting of the Euro ean Desalination Society (EDS) on Desalination an dp the Environment, Genoa, Italy, October 20-23, 1996. *Corresponding
author.
OOll-9164/97/$17.00 Copyright PZZ SO01 l-9164(97)00031-3
- Steel production starting from scrap-iron by two electric arc furnaces having a capacity of 150,000 t, - Production of round and square section blooms by three continuous casting machines, - Pipe shop for seamless pipes having diameters included between 159 mm and 426 mm and having length from 8 m to 13.5 m, and - Pipe finishing by thermal and mechanical treatment.
e 1997 Elsevier Science B.V. All rights reserved.
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Within the steel complex construction, settled down close to Voljski, in the district of Volgograd located in an area between Caspian Sea and Black Sea, various systems for water treatment and recycle have been foreseen in order to comply with environmental protection context and with full respect of Soviet law norm. Furthermore, the a.m. Soviet law foresees that waste waters discharged into superficial water courses do not increase the dissolved salts concentration already existing; from this rises the necessity to foresee water balance suitable to utilize as well as possible the cycles in order to obtain a water discharge aiming to zero. In the iron and steel cycle of Voljski steel shop water, indispensable for the production, is utilized for the cooling of various equipment making in the production plants, for the cooling and the washing of steel products and gases, for the cleaning and removal of scales and, as steam, for various heatings as well as for energy production. Water supply to feed the Voljski steel shop is drawn from an existing network fitted for industrial services with a flow rate of 300 m3/h. The whole amount of drawn water is collected to a pre-treatment plant in order to reduce the hardness till a max. value of 250 mg/l as CaCOs and a minimum value of 100 mg/l, obtaining in this way the water named type “B” (see Table 1). The water pre-treatment plant is mainly composed by the following steps:
Table 1 Industrial
108 (1996) 231-245
- Flocculation: where lime milk is added to water together with a coagulant to agglomerate fine suspended solids. - Clarification: that allows the settlement of particles suspended in the water obtained by previous flocculation step. - Filtration: that allows to eliminate the particles even having the smallest dimensions, - Storage: in which pre-treated water accumulation and pumping to the network are carried out. The water pre-treatment plant is also equipped with a clarification step relevant to waste waters rising from sand filters backwash step and a thickening and a dewatering step fitted to treat sludges coming from the plant itself. A part of type “B” water is then treated by a demineralization plant suitable to eliminate completely the salts dissolved in the water. From the demineralization plant two types of water are obtained. First type, named “C2” (see Table 2), rises from a double stage demineralization plant on a strong cationic resin column in acid cycle and a following anionic resin column in alkaline cycle. The second water type, named “Cl” (see Table 3), with very high purity characteristics, is obtained by collecting the “C2” type water into a mixed bed column in which the strong cationic resin is mixed together with the strong anionic one and therefore the simultaneous cationic/anionic exchange is Table 2 Industrial
water type “C2”
water type “B” Characteristics
Characteristics Hardness Alkalinity Suspended solids Dry residual content Fe Co2 PH
Concentrations, 100-140 35-60
Concentrations,
mg/l Total dissolved solids Dissolved oxygen SiO2 Fe Hardness CO2 Organic substances PH
max. 2 l-2 0.4 0.1 0 0 0 7.5-8.5
mg/l
R. Borsani et al. /Desalination Table 3 Industrial
Concentrations,
mg/l
0.05 0.007 0.02 0.01 0 0 0 8.5-9.2
Total dissolved solids Dissolved oxygen SiO2 Fe Hardness CO2 Organic substances PH
233
obtained. “Cl” type water is utilized to feed the steam generators and for the water makeup of three indirect cooling circuits relevant to cooling of ingot moulds of continuous casting machines and to water cooled elbows and ducts on the fume outlet from electric arc furnaces. Aiming to reach a “zero” discharge and to get a technical and economical optimization of remaining nine water recycle systems (see Table 4) in service on production plant of Voljski steel shop, it has been carried out an early circuit filling with type “B” water and the relevant make-up with type “C2” in such a way to make not necessary the water blowdown from evaporative towers.
water type “Cl”
Characteristics
IO8 (1996) 231-245
Table 4 Water recycle plant description
Flow rate
SRl: Electric arc furnace recycle indirect cooling circuit
1.500
20
Cooling,
lifting
SR2: Electric arc furnace recycle indirect cooling circuit
1.500
20
Cooling,
lifting
SR3: Steel shop recycles indirect cooling circuit
1,100 550
1120
15 7
Cooling,
lifting
SR4: Degassing recycle direct cooling circuit
1,100
2140
10
Filtration, cooling, lifting
SR5: Mould recycle of continuous casting machine, indirect circuit
2,340
51
10
Cooling,
lifting
SR6: Continuous casting machine recycle, indirect cooling circuit
1,040
2120
15
Cooling,
lifting
870
1140
20
Settlement, filtration cooling, lifting
SR8: Pipe shop recycle indirect cooling circuit
1,100
5120
8
SR9: Pipe shop recycle direct cooling circuit
2,700
2140
10
500
Cl20
7
Cooling,
3,000
5140
5
Settlement, filtration cooling, lifting
760
1140
10
SR7: Spraying recycle casting, direct cooling
of continuous circuit
SRlO: Pipe finishing recycle indirect cooling circuit SR 11: Hardening recycle direct cooling circuit SR 13: Coolers recycle indirect cooling circuit
Water quality in cycle
Thermal drop
Treatment
Cooling,
steps
lifting
Settlement, filtration cooling, lifting
Cooling,
lifting
lifting
234
R. Borsani et al. /Desalination
Infact, the salt concentration remains constantly on initial value due to the fact that the minimum salt content in “C2” water compensates the entrainment losses taking place in evaporative towers. Within the a.m. water balance inside the steel shop, the only one effluent is the draw off relevant to the resins regeneration step, showing a very high concentration of salts. According to soviet law limits, that do not allow to discharge water having salt concentration greatest that the ones existing in the receiving superficial waters, dirty waters rising from resins regeneration are treated in a proper plant operating by reverse osmosis principle. In this osmosis plant the action due to the pressure allows the permeation of the pure water across the membranes. Pure water obtained is then recycled to the demineralization plant feeding step. Impurities and mineral salts contained in feed water to the reverse osmosis plant are increased in concentration and then discharged to a evaporation plant multi stage type and finally to a crystallization plant complete of centrifugal equipment fitted for salts dehydration. Dirty water coming from centrifugal decanter is sent upstream to the concentration fraction is plant, while the concentrate discharged suitable for shovelling as it contains a max water content of 20%. As well as the salt concentration results the only discharge, the objective of zero discharge is therefore obtained. 2. Technological cycle Water rising from industrial water network, named type “A”, has the following characteristics (in mg/l): Flow rate, Qm, m3/h 300-350 6.9-8.2 PH Temperature,“C O-26 Oxidability(perrnanganate) 7-13 Dry substances Calcinated residue Total iron
300-600 100-300 0.2-1.5
108 (1996) 231-245
Hardness, act. to CaCO3 150-250 Alkalinity, act. to CaCOs 95-175 Sulphates 70-200 Chlorides 4&120 Free CO2 2-10 Magnesium lo-27 Total suspended solids 30 For short periods during up to 50 Aluminium 0.33 Nitrites 0.043 Nitrates 0.32 Silicon 2.9 Mineral phosphorus 0.05 Dissolved oxygen 8.0 Dichromatic oxidability, mg 02/l 29.6 Biochemical oxygen demand (BOD5), mg 02/l 1.90 Phenol 0.004 Petrolif. productsðer-sol. substance 1.64 Surface-active synthetic substances 0.032
Fluorides Chrome Zinc Copper Nickel
0.002 0.002 0.006 0.005 0.004
Together with this effluent, another effluent rising from a lagoon adds with the following characteristics : Average flow rate, mVh 25-50 TSS, mg/l 2,000 The a.m. contributions form raw water supplying the feeding of the various circuits. The first treatment cycle is composed by a chemical additive with lime and aluminium sulphate plus polyelectrolyte solution as flocculation helper. In this step, it is also dosed sulphuric acid to control the pH value and chlorine as commercial solution of sodium hypochlorite derived from other water treatment sections. Downstream the clariflocculation step it is carried out a filtration step on gravity type sand filters. Chemical additives in the step of clariflocculation are the following: Chemicaladditive Concentration, ppm Polyelectrolyte 1 lo-40 A12(S04) 75-150 O-5 5
Ca(Of-02 H2S04 NaOCl
R. Borsani et al. /Desalination
235
108 (1996) 231-245
HYP
r
To sludge
Fig. 1. Make-up
water treatment.
drying
Flow diagram.
See the flow diagram of the make-up water treatment (Fig. 1). Produced water has the characteristics above listed and is named type “B”. Type “B” water is collected to two demineralization plants fitted to produce water suitable for two different purposes; more in detail water type “Cl” for steam generators feeding and water type “C2” for the make-up of leakages of various cooling circuits. The plant is split in two sections: - The first one, which product is demineralized water with type “C2” characteristics, is composed by no. 3 lines operating in parallel, each line including a cationic and an anionic unit. Normally the plant operates on two lines simultaneously without excluding the possibility to run with only one line or, extraordinarily, with three lines (within the limits due to the regeneration necessities). - The second section, supplying demineralized water type “Cl”, is composed by no.
2 lines operating in parallel, each including a mixed bed type unit. The plant can operate both with 1 and 2 lines according to production requirements. The plant is mainly composed by: 3 (three) cationic exchangers, having following characteristics and dimensions: Diameter, nun Cylindrical vessel height, mm Unit total height, mm Body thickness, mm Bottom thickness, mm Construction material Filling resins - Strong cationic capacity, 1 - Inert resin capacity, 1
2,100 4,000 5,600 10
10 Steel 8,000 1,400
and by no. 3 anionic exchangers: 2,500 Diameter, mm Cylindrical vessel height, mm 4,600 5,800 Unit total height, mm
the
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R. Borsani et al. / Desalinahon
12 Body thickness, mm 12 Bottom thickness, m steel Construction material Ebonite, 3 mm Lining, mm Filling resins 7,700 - Strong anionic capacity, 1 1,800 - Inert resin capacity, 1 In order to get water having characteristics as per “Cl” type, no. 2 mixed bed type ion exchangers are utilized, each having the following characteristics: Diameter, mm Body height, mm Body thickness, mm Bottom thickness, mm Strong anionic capacity, 1 Strong cationic capacity, I
1,300 3,000 7 7 1,200 800
An automatic and sequential regeneration system operates on a.m. demineralization lines. Regeneration eluate is collected and stored in a tank having a capacity of 240 m3 in order to make the osmosis plant run at a flow rate not exceeding 16-17 m3/h with reference to three lines operating extraordinarily per 24 h. See the flow sheet (Fig. 2). Resins regeneration is carried out by means of solutions of HCl and NaOH. In order to get the objective of zero discharge, the eluate is sent to a neutralization step and finally to a reverse osmosis plant for the production of type “B” industrial water and the simultaneous getting of a concentrate to send to a plant evaporation and final crystallization.
3. Reverse osmosis plant The characteristics of the feeding water to the RO plant are: Nominal flow rate, m3/h Max. flow rate, m3/h Min. flow rate per 1 line, mVh (with a conversion factor = 56%) Nominal temperature, “C Temperature max.-min. values, “C
16.6 25 7.75 16 2-26
108 (1996) 231-245
Chemical analysis as ppm of CaC03: Ca2+ g/m3 Mg2; g/m3 Na+, i/m3 HC0j2-, g/m3 SOd2-, g/m3 C12-,g/m3 C032-, g/m3 PH
476 122.4 10.193 171.3 2,910.4 8,482.0 304.7 8-10
The characteristics of the product water outcoming from the RO plant are the following: Nominal flow rate, my/h Max. flow rate, m?h Pressure, bar g Hardness, m/l CaCO3 TLX, g/l The characteristics
outcoming following:
from
9.3 14 0.7 <140 950
of the concentrate (brine) the RO plant are the
Nominal flow rate, m3/h Max. flow rate, my/h Min. flow rate per 1 line, m3/h (with a conversion factor = 56%) Pressure, bar g TDS, g/l
7.3 11 4.35 2 33
4. Plant description (see Fig. 3)
The reverse osmosis plant has the purpose to treat the discharge coming from the demineralization plant after the resin regeneration. Quantity of aqueous solution discharged by demi plant is 100 m3 each regeneration with 2 regeneration per day each demi line and 2 demi lines normally operating (no. 3 exceptionally). Accordingly, the total quantity of discharge solution results to be 400 m3/day normally and 600 m3/d exceptionally. The solutions are discharged into a tank where they are neutralized and then pumped to a close storage tank. The storage tank has the purpose to allow a constant feed to the RO plant. Three centrifugal vertical pumps (two operating plus
R. Borsani et al. /Desalination
I
I
??
Fig. 2. Demineralization
108 (1996) 231-245
237
I
plant. Flow sheet.
one stand-by) suck from the tank and send through a common header the solution to the high pressure pumps. Before reaching the H.P. pumps, the solution is treated with some chemicals and then filtered by 2 sand filters and 2 cartridge filters. The high pressure pumps increase the pressure of the feed water up to the required one and pump it to the permeators. The product outcoming from each stage is collected in a common header, while the concentrate outcoming from one stage enters the next stage and so on up to the outlet of the last stage. The product water is recovered as “Industrial Water type B” and sent to the relevant storage tank. The concentrate is sent to a further treatment plant (evaporation and crystallization plant). The RO plant is also equipped with a
membrane cleaning unit composed by a tank with a mixer, a centrifugal pump, a cartridge filter and relevant connecting piping. Connection between the cleaning unit and the process piping is achieved by means of flexible house and quick coupling. 5. Topics of the RO plant design Considering the main characteristics of the feed water to the RO plant, the requirements of the steel plant and of the client and moreover the construction/erection aspects, the following choices has been made: - type of membranes; - pretreatment system; - plant configuration and regulation system; - material choices; - construction/erection choices.
238
R. Borsani et al. /Desalination
108 (1996) 231-245
/
Fig. 3. RO plant. Flow sheet.
5.1. Choice of the type of membrane Taking into account the limited flow rate of the RO plant and the chemical analysis of the feed water, it has been considered convenient to use membrane “spiral wound type” that, among others, show less sensibility to fouling and scaling problems. The membrane chosen for desalination of water by reverse osmosis must be highly permeable to water to achieve high rates of production while being highly impermeable to salts to obtain acceptable product water quality. It must also be extremely thin to maximize water flow but strong enough to withstand high pressure. Changes in the membrane’s transport and mechanical properties should be minimal after long exposures to high pressure. Likewise, resistance of the membrane to chemical and biological attack is necessary. For the subject plant the selected
membrane was the B-15 type by Permasep DuPont. The membrane used in the B-15 spiralwound cartridge is a flat film made from an aromatic polyamide polymer cast on a fabric support backing. The film is asymmetric, meaning it consists of a very thin dense “skin”, the actual membrane, on the surface, and a thick, porous layer of the same polymer underneath that provides support for the “skin”. The thin membrane “skin” inhibits the passage of salts, but readily allows water to pass. The support layer, being highly porous, does not impede the flow of water into the product channel and has no salt rejection properties. Flat film membrane is made into leaves with each leaf consisting of two sheets of membrane with a sheet of polyester tricot between to act as the product water-collection channel. Three sides of the leaf are glued to form an envelope. Tricot extending from the
R. Borsani et al. /Desalination
Product Tube \
Brine Seal Cay
239
108 (1996) 231-245
Anti-Telescoping Oev?
PRODUCT+ V FEE0
-4
Tricot Product Water Channel
Membrane Support Backing &
’
Adhesive Membrane
Fig. 4. Membrane.
open end is attached to a perforated PVC tube containing holes through which the product water will exit (see Fig. 4). Plastic netting between the leaves serves as the feed channel spacer enhancing mixing which minimizes concentration polarization. The leaves are wound around the product tube to give a spiral configuration. Identical plastic spooked devices are attached to the ends of the rolled leaves. One acts as an antitelescoping device (ATD) and the other serves as a brine seal carrier. In a pressure vessel the ATD is located on the brine discharge end at each cartridge and a brine seal is installed on the feed end. A fiberglass reinforced plastic (FRP) cover protects the cartridge. All cartridge are 40 inches in length. The &inch diameter models have 18 leaves with about 400 ft2 of surface area. As shown in Fig. 1, pressurized feedwater enters the cartridge and flows axially through the feed channel spacer to the opposite end where it exits as the brine. Water from the feed solution under the influence of the elevated pressure which raises its chemical potential, passes through the flat membrane
Surface /
into the product water collection channel. The desalinated product water flows spirally to the central product tube and exits the cartridge.
5.2. Pretreatment system 5.2.1. General considerations For optimum RO performance, water fed to the membranes should be treated to remove gross amounts of solids and prevent formation of solid matter inside the RO cartridge by precipitation or biological growth. Therefore, upstream of the cartridge, various unit operations (collectively called “pretreatment”) must be carried out on most natural waters. Typically, pretreatment consists of filtration to remove large particles and one or more of the following: 1) Adjustment of solubility parameters to prevent precipitation of sparingly soluble salts (“scaling”) as a result of the concentrating action of the RO process; 2) Coagulation of colloidal matter; 3) Chemical treatment to prevent biological growth.
240
R. Borsani
et al. /Desalination
The importance of pretreatment for an RO plant cannot be overemphasized. If cleaning of the cartridges is required frequently, pretreatment is deemed inadequate. With proper pretreatment, the frequency of cleaning is usually once every three to four With an adequate months or longer. pretreatment system, periodic cleaning will restore most of the productivity. If pretreatment is inadequate, cleaning will be less effective in restoring the RO performance and the need for cleaning will increase. In this case, taking into account the characteristics of the water feeding the RO plant the following pretreatment have been foreseen: 5.2.2. Acid injection (H2SO4) The acid injection is used for scale control (especially scaling caused by the precipitation of CaC03). The quantity of acid to be injected is based on the calculation of LSI (Langelier Saturation Index). If the LSI is negative, CaC03 will tend to dissolve; if the LSI is positive, CaCOa will tend to precipitate. LSI calculation is used to determine the approximate quantity of HzS04 required to obtain a negative LSI of the brine stream and to size the acid injection pumps. At start-up, the LSI of the brine stream must be determined. For an operating RO plant, the LSI of the brine stream must be determined periodically and pH adjustments made if necessary. The injection pump is driven automatically by a pH analyzer/controller. 5.2.3. Sodium exametaphosphate
injection
(NaHMP)
NaHMP can be injected as a precipitation inhibitor. For negative LSI values (LSIcO) NaHMP is not required. In case of LSIIl, 10 mg/l NaHMP shall be injected (since NaHMP is an inhibitor, usually 10 mg/l rather than stoichiometric quantity is used in the RO brine stream). However, since NaHMP only delays precipitation, it is necessary to flush the cartridge within four hours at shutdown. Operation at an LSI of 1.0 with NaHMP
108 (1996)
231-245
results in reduced acid consumption. The choice of whether to use an LSIIl.0 with NaHMP or a negative LSI without NaHMP is based on such factors as ion concentrations, and availability and cost of chemicals. In case of LSI>l, the plant cannot be work even using NaHMP injection. 5.2.4. Pressure medium filtration Every RO pretreatment system must consider how to prevent coagulation of colloids. The concentration of colloids is generally expressed as the Silt Density Index”(SD1) of the water. For the membranes used in this RO plant the max allowable SD1 is 5. Considering the characteristics of the feed water, and the previous treatment of the same water, a sand filtration seamed to be not necessary. Anyhow to assure a more reliable performance of the plant a further filtration phase has been foreseen. The filtration system was designed as follows: no. 2 pressure dual media filters (sand-anthracite) normally both working, but each one able to treat the whole flow rate during the backwashing operation of the other filter. The filtration is achieved using the in-line coagulation/flocculation process. Accordingly, a polyelectrolyte dosing unit and an aluminium phosphate dosing unit have been supplied. The feed water is added with the above two solution, then properly mixed through a static mixer before entering the dual media filters, This filtration system assures a very low LSI value saving the membrane life and furthermore the cartridge filter life. 5.3. Plant system
configuration
and
regulation
Since no. 2 demi lines are generally working (with relevant feed flow to RO plant = 8.3 m3/h each) and only exceptionally there are no. 3 demi lines working, the RO plant has been designed with two lines each one able to treat the nominal flow coming from one demi line but designed in such a way that it can
241
R. Borsani et cd. I Desalination 108 (1996) 231-24.5
treat also a flow rate of 1.5 times the nominal discharge of one demi line. Accordingly with 2 RO lines it is possible to treat both the discharge coming from 2 demi lines (nominal flow rate) and from 3 demi lines (max flow rate). In this manner, it has been avoided the installation of a third RO line generally not working, avoiding consequently all the problems that in the RO plants make difficult the stand-by use (cleaning, sterilization, flushing, etc.). In order to get what above, each RO line has been designed in a brine staging way with 3 stages (see Fig. 5). Each stage is composed by 1 permeator with no. 3 B-l 5 cartridges, High pressure pumps (one each line plus one stand-by) are centrifugal multistage type. The working pressure at the membrane inlet depends by the temperature (see Figs. 6-8). The regulation system of the plant is realized with a flow control valve upstream of the membranes and a pressure control valve on brine discharge (see Fig. 9). To avoid hunting of control system the response times of the two regulation loops have been selected different between themselves with priority to the pressure one.
J
a 9 0
I
I xl0
330
400
?? M
33
30
600
b>O
Prcrrurc
(,“ig,
Fig. 6. Operating version = 56%.
curves
at temperature
Fig. 7. Operating version = 56%.
curves
at temperature
of 16T;
con-
FEED \\ .ATER
IST STAGE
4
I
I
I
3RD STAGE
V PRODL’CT
COSCESTIUTE JI
Fig. 5. Stage arrangement
of 2°C; con
In this way, the system always finds automatically its working point over the pump curve everytime a change of working condition is required (eg. change from 2 to 3 demi lines). Considering possible future extension of the plant (eg. installation of a further demi line with a consequent a 3rd RO line) all the part of the RO plant from the feed water tank up to the high pressure pumps have been already designed for no. 3 RO lines with the max flow rate.
R. Borsani et al. /Desalination
242
Fig. 8. Operating version = 56%.
curves
;o~
Fig. 9. RO plant regulation
at temperature
of 26°C; con-
,_--_- ~----~
system.
5.4. Material choices 5.4. I. General considerations
Metal corrosion can be a serious problem in many water treatment processes. All RO devices are particularly sensitive to metal If corrosion is not corrosion products. controlled, premature failure of RO system will result. The degree to which corrosion occurs, and the effect it will have on RO system, must be carefully considered when designing the plant. Generally, the following criteria should be followed: - Non metallic materials such engineering plastics, plastic composites, glass, rubber, etc are desirable materials of construction for RO plans and they should be used
108 (1996) 231-245
wherever they practical and economical. - Stainless steel. Stainless steel have been successfully used in many RO plants for piping, pumps and vessels. Particularly type 316 stainless steel has proven good corrosion resistance. Type 304 stainless steel has also been successfully used, but it is less corrosion resistant than type 316. When using stainless steel stagnant flow zone should be avoided. Moreover, experience has shown that corrosion can be observed at threaded connection. Therefore these connections should be minimized when using stainless steel - Non ferrous materials. In general, copper, bronze and brass should be avoided if pH is below about 6.5. Aluminium should also be avoided because of its sensitivity to high and low pH and high TDS. - Non metallic liners and coatings. Nonmetallic liners and coatings are frequently used to protect pipe and equipment from corrosion and chemical attach. In selecting and applying the liner or coating the following factors relative to both base material and process water have to be considered: thermal expansion and contraction, chemical compatibility, temperature rating, and mechanical integrity. 5.4.2. Material selected Based on the above general criteria and the operating condition of the subject RO plant, the following main materials have been selected. It must be observed that, due to the severe temperature condition imposed by the client during the erection and storage period (-25”(J), plastic materials have not been selected also if in some cases, considering the characteristics of the process water and the economical reasons, they should have to be chosen. Vertical pumps Sand filters Cartridgefilters Low press. proc. pip.
AISI 316 Carbon steel epoxy painted AISI 3 16L / polypropilene Feed water AISI 3 16L Product AISI 304L
R. Borsani et al. /Desalination
Sodium exametaphosphate dosing system AISI 316 AISI 316 Mixer AISI 316 Pumps AISI 316 Piping
Tanks
Sulphuric acid dosing system INCOLOY 825 Pumps ALLOY 20 / PTFX In line mixer Piping PVC I GRP Aluminium sulphate dosing system AISI 316L Tanks AISI 3 16 Mixer AISI 316 Pumps AISI 3 16L Piping Polyelectrolyte dosing system AISI 304L Tanks Mixer AISI 304 AISI 304 Pumps Piping AISI 304L
In line mixer
AISI 304
High pressure pumps High pressure piping
AISI 316 Feed water AlSI 3 16L Brine AISI 316L Perrneators (press. vessels) GRP Polyamide Membranes Cleaning system Tanks Mixer Cartridgefilter Pump Piping
AISI 316L AISI 316 AISI 316L / polypropilene AISI 316 AISI 304L
5.5. Construction/erection choices Considering the construction and storage problems of the steel work (particularly the very low temperature problem) and the peculiarity of the RO plant and likewise taking into account the dimension of the subject RO plant, it has been choien to supply the plant preassembled on skids. Starting from the foreseen final lay-out, it has not been possible to assembly all the components above one skid; accordingly it was decided to proceed as follows:
243
108 (1996) 231-245
Feed water pumps (vertical ones): supplied loose Common header: supplied in several prefabricated spools No. 1 skid for sulphuric acid dosing system No, 1 skid for sodium exametaphosphate dosing system No. 1 skid for polyelectrolyte and aluminium sulphate dosing system No. 1 skid for dual media filters No. 1 skid for the balance of the RO plant (cartridge filters, high pressure pumps, permeators, cleaning system, etc.). Consequently it has been possible to proceed testing all the plant section both as pressure test (where required) and as functional test (as far as possible). Each skid has been supplied complete with its electro/instrumental components and relevant control panel. All connecting parts (piping) has been supplied as prefabricated spools to allow a quick connection among the skids and with the other plants. In this matter, all erection operations resulted extremely simplified and fast. 6. Concentration and crystallization plant
The concentrate outcoming from the reverse osmosis plant is collected to a threestages type evaporation plant. Permeatecharacteristics Nominal flow rate, kg/h Max. flow rate, m3/h Dissolved solids flow rate, kg/h Water only flow rate, kg/h Averageconcentration Mg2+, C2+, % Na+, % so&, % Cl-, % HC03, % Suspended solids, % Separatedcrystalscharacteristics Flow rate, kg/h
9,000 11 285415 8,500-8,700
3 33 16-28 3547 1
300-500
244
Residual humidity, % ‘be
R. Borsani et al. /Desalination
5 Mixed salts
1.4-1.6 Apparent specific gravity, kg/dm” 800 (‘) Centrifugaldecanter sizing, kg/h (*)sizingsuitable to drain on two duties the crystals producedon three daily duties. Characteristicsof the fluids outcoming from the plant 2,650 Pure condensate,kg/b 8,000 impure condensate,kg/b 1 Cooling water, m3/h (max. overheating 7°C) max. 180,000 Saturatedwet air, m3/b
Materialsbalance (at nominal conditions, temperature lO”C,relative humidity 80%) Salts Hz0 Total Feeding 285 8,715 9,000 Servicewater 150 150 15 Separatedsolids 285 300 Evaporation 8,850 8,850 Power consumption (at the following conditions) Externaltemperature,“C 10 80 Relative humidity, % 10 Feed temperature,“C Live steam (10 bar, 2OO”C),kg/h 3,100 Service water, m3/b 3 200 Electric energy, kWh/b 8 Compressed air, Nm3/h
108 (1996) 231-245
6. I. Process description (see Fig. IO)
The process on continuous operation includes a concentrator-crystallizator equipment at triple effect and at forced circulation, fed with low pressure steam, and is completed with a final stage of crystallization evaporative by counter current injection of air. Mixed salts are separated with a residual humidity of about 5% via a centrifugal decanter suitable to discharge them on a receiving box fit to be emptied by a bucket. Condensates of live steam are returned to the steam generator and condensates of vapours extracted during the concentration step are recycled to the industrial water storage tank. Saturated air outcoming from the plant is sent directly to the atmosphere. Pure condensate - It returns to the steam generator without any pollution, having a residual pressure of 4 bar at battery limit. Impure condensate - It is collected to the industrial water storage tank, as it contains a salt concentration of about 50 ppm. Its pressure at battery limit is 2 bar. Incondensables extraction - The heat exchanger is maintained under vacuum and incondensable are extracted by the pump. The assembly of mentioned plants forms the main chain of plants built for Voljski steel shop, are running since the date of the start of steel shop itself and are still accomplishing the scope for they have been constructed.
R. Borsani et al. /Desalination
108 (1996) 231-24.5
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-
t
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245
Desalination
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Industrial water production by utilization of a reverse osmosis and an evaporation plant R. Borsani*, M. Fazio, and B. Ferrando FISIA-ZTALIMPIANTI
SPA, Via Di Francis Received
I, 16126 Genova, Italy.
Tel.: +39-10-64091;
Fax. +39-lo-6409991
28 July 1996; accepted 3 August 1996
Abstract
ITALIMPIANTI has realized the steel making plant in Woljsky (on 1989), situated in the Volgograd area between the Caspian Sea and the Black Sea, with a steel output of 150,000 t/y from scrap, 720,000 t/y of seamless pipes, and 210,000 t/y of blooms. In the ambit of the operation system of water necessary for the production cycles, for ancillary equipments and in compliance with the former USSR rules, it has been chosen a treatment system to bring to “zero” the waste water discharge from the steel plant complex. In this point of view, the “conventional” treatment stages of the make-up recycling water and steam generators feeding is as follows: flow rate 300 ms/h, softening line, gravity sand filters, two lines of demiwater production for two different purposes, final storage, and sand filter backwash treatment and sludge dewatering lines. A “nonconventional” process is added to the previous one to treat different effluents of backwashing water of resins used for the demineralizing system for which the following scheme is foreseen: flow rate max. 600 ms/d, neutralization, and two lines of RO having each three stages in series. The aim of this treatment plant is to produce industrial water with characteristics as above mentioned. The concentrated makes up the feeding of a multi-flash evaporator with a production of mixed crystals and condensate used as industrial water, Keywords: Zero discharge water treatment; RO system; RO membrane; Concentration; Crystallization
1. Introduction The steel shop having a total production capacity of 720,000 t/y of pipes and 210,000 t/y of blooms has started in 1989. The iron and steel cycle takes in the following steps: Presented at the Second Annual Meeting of the Euro ean Desalination Society (EDS) on Desalination an dp the Environment, Genoa, Italy, October 20-23, 1996. *Corresponding
author.
OOll-9164/97/$17.00 Copyright PZZ SO01 l-9164(97)00031-3
- Steel production starting from scrap-iron by two electric arc furnaces having a capacity of 150,000 t, - Production of round and square section blooms by three continuous casting machines, - Pipe shop for seamless pipes having diameters included between 159 mm and 426 mm and having length from 8 m to 13.5 m, and - Pipe finishing by thermal and mechanical treatment.
e 1997 Elsevier Science B.V. All rights reserved.
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R. Borsani et al. /Desalination
Within the steel complex construction, settled down close to Voljski, in the district of Volgograd located in an area between Caspian Sea and Black Sea, various systems for water treatment and recycle have been foreseen in order to comply with environmental protection context and with full respect of Soviet law norm. Furthermore, the a.m. Soviet law foresees that waste waters discharged into superficial water courses do not increase the dissolved salts concentration already existing; from this rises the necessity to foresee water balance suitable to utilize as well as possible the cycles in order to obtain a water discharge aiming to zero. In the iron and steel cycle of Voljski steel shop water, indispensable for the production, is utilized for the cooling of various equipment making in the production plants, for the cooling and the washing of steel products and gases, for the cleaning and removal of scales and, as steam, for various heatings as well as for energy production. Water supply to feed the Voljski steel shop is drawn from an existing network fitted for industrial services with a flow rate of 300 m3/h. The whole amount of drawn water is collected to a pre-treatment plant in order to reduce the hardness till a max. value of 250 mg/l as CaCOs and a minimum value of 100 mg/l, obtaining in this way the water named type “B” (see Table 1). The water pre-treatment plant is mainly composed by the following steps:
Table 1 Industrial
108 (1996) 231-245
- Flocculation: where lime milk is added to water together with a coagulant to agglomerate fine suspended solids. - Clarification: that allows the settlement of particles suspended in the water obtained by previous flocculation step. - Filtration: that allows to eliminate the particles even having the smallest dimensions, - Storage: in which pre-treated water accumulation and pumping to the network are carried out. The water pre-treatment plant is also equipped with a clarification step relevant to waste waters rising from sand filters backwash step and a thickening and a dewatering step fitted to treat sludges coming from the plant itself. A part of type “B” water is then treated by a demineralization plant suitable to eliminate completely the salts dissolved in the water. From the demineralization plant two types of water are obtained. First type, named “C2” (see Table 2), rises from a double stage demineralization plant on a strong cationic resin column in acid cycle and a following anionic resin column in alkaline cycle. The second water type, named “Cl” (see Table 3), with very high purity characteristics, is obtained by collecting the “C2” type water into a mixed bed column in which the strong cationic resin is mixed together with the strong anionic one and therefore the simultaneous cationic/anionic exchange is Table 2 Industrial
water type “C2”
water type “B” Characteristics
Characteristics Hardness Alkalinity Suspended solids Dry residual content Fe Co2 PH
Concentrations, 100-140 35-60
Concentrations,
mg/l Total dissolved solids Dissolved oxygen SiO2 Fe Hardness CO2 Organic substances PH
max. 2 l-2 0.4 0.1 0 0 0 7.5-8.5
mg/l
R. Borsani et al. /Desalination Table 3 Industrial
Concentrations,
mg/l
0.05 0.007 0.02 0.01 0 0 0 8.5-9.2
Total dissolved solids Dissolved oxygen SiO2 Fe Hardness CO2 Organic substances PH
233
obtained. “Cl” type water is utilized to feed the steam generators and for the water makeup of three indirect cooling circuits relevant to cooling of ingot moulds of continuous casting machines and to water cooled elbows and ducts on the fume outlet from electric arc furnaces. Aiming to reach a “zero” discharge and to get a technical and economical optimization of remaining nine water recycle systems (see Table 4) in service on production plant of Voljski steel shop, it has been carried out an early circuit filling with type “B” water and the relevant make-up with type “C2” in such a way to make not necessary the water blowdown from evaporative towers.
water type “Cl”
Characteristics
IO8 (1996) 231-245
Table 4 Water recycle plant description
Flow rate
SRl: Electric arc furnace recycle indirect cooling circuit
1.500
20
Cooling,
lifting
SR2: Electric arc furnace recycle indirect cooling circuit
1.500
20
Cooling,
lifting
SR3: Steel shop recycles indirect cooling circuit
1,100 550
1120
15 7
Cooling,
lifting
SR4: Degassing recycle direct cooling circuit
1,100
2140
10
Filtration, cooling, lifting
SR5: Mould recycle of continuous casting machine, indirect circuit
2,340
51
10
Cooling,
lifting
SR6: Continuous casting machine recycle, indirect cooling circuit
1,040
2120
15
Cooling,
lifting
870
1140
20
Settlement, filtration cooling, lifting
SR8: Pipe shop recycle indirect cooling circuit
1,100
5120
8
SR9: Pipe shop recycle direct cooling circuit
2,700
2140
10
500
Cl20
7
Cooling,
3,000
5140
5
Settlement, filtration cooling, lifting
760
1140
10
SR7: Spraying recycle casting, direct cooling
of continuous circuit
SRlO: Pipe finishing recycle indirect cooling circuit SR 11: Hardening recycle direct cooling circuit SR 13: Coolers recycle indirect cooling circuit
Water quality in cycle
Thermal drop
Treatment
Cooling,
steps
lifting
Settlement, filtration cooling, lifting
Cooling,
lifting
lifting
234
R. Borsani et al. /Desalination
Infact, the salt concentration remains constantly on initial value due to the fact that the minimum salt content in “C2” water compensates the entrainment losses taking place in evaporative towers. Within the a.m. water balance inside the steel shop, the only one effluent is the draw off relevant to the resins regeneration step, showing a very high concentration of salts. According to soviet law limits, that do not allow to discharge water having salt concentration greatest that the ones existing in the receiving superficial waters, dirty waters rising from resins regeneration are treated in a proper plant operating by reverse osmosis principle. In this osmosis plant the action due to the pressure allows the permeation of the pure water across the membranes. Pure water obtained is then recycled to the demineralization plant feeding step. Impurities and mineral salts contained in feed water to the reverse osmosis plant are increased in concentration and then discharged to a evaporation plant multi stage type and finally to a crystallization plant complete of centrifugal equipment fitted for salts dehydration. Dirty water coming from centrifugal decanter is sent upstream to the concentration fraction is plant, while the concentrate discharged suitable for shovelling as it contains a max water content of 20%. As well as the salt concentration results the only discharge, the objective of zero discharge is therefore obtained. 2. Technological cycle Water rising from industrial water network, named type “A”, has the following characteristics (in mg/l): Flow rate, Qm, m3/h 300-350 6.9-8.2 PH Temperature,“C O-26 Oxidability(perrnanganate) 7-13 Dry substances Calcinated residue Total iron
300-600 100-300 0.2-1.5
108 (1996) 231-245
Hardness, act. to CaCO3 150-250 Alkalinity, act. to CaCOs 95-175 Sulphates 70-200 Chlorides 4&120 Free CO2 2-10 Magnesium lo-27 Total suspended solids 30 For short periods during up to 50 Aluminium 0.33 Nitrites 0.043 Nitrates 0.32 Silicon 2.9 Mineral phosphorus 0.05 Dissolved oxygen 8.0 Dichromatic oxidability, mg 02/l 29.6 Biochemical oxygen demand (BOD5), mg 02/l 1.90 Phenol 0.004 Petrolif. productsðer-sol. substance 1.64 Surface-active synthetic substances 0.032
Fluorides Chrome Zinc Copper Nickel
0.002 0.002 0.006 0.005 0.004
Together with this effluent, another effluent rising from a lagoon adds with the following characteristics : Average flow rate, mVh 25-50 TSS, mg/l 2,000 The a.m. contributions form raw water supplying the feeding of the various circuits. The first treatment cycle is composed by a chemical additive with lime and aluminium sulphate plus polyelectrolyte solution as flocculation helper. In this step, it is also dosed sulphuric acid to control the pH value and chlorine as commercial solution of sodium hypochlorite derived from other water treatment sections. Downstream the clariflocculation step it is carried out a filtration step on gravity type sand filters. Chemical additives in the step of clariflocculation are the following: Chemicaladditive Concentration, ppm Polyelectrolyte 1 lo-40 A12(S04) 75-150 O-5 5
Ca(Of-02 H2S04 NaOCl
R. Borsani et al. /Desalination
235
108 (1996) 231-245
HYP
r
To sludge
Fig. 1. Make-up
water treatment.
drying
Flow diagram.
See the flow diagram of the make-up water treatment (Fig. 1). Produced water has the characteristics above listed and is named type “B”. Type “B” water is collected to two demineralization plants fitted to produce water suitable for two different purposes; more in detail water type “Cl” for steam generators feeding and water type “C2” for the make-up of leakages of various cooling circuits. The plant is split in two sections: - The first one, which product is demineralized water with type “C2” characteristics, is composed by no. 3 lines operating in parallel, each line including a cationic and an anionic unit. Normally the plant operates on two lines simultaneously without excluding the possibility to run with only one line or, extraordinarily, with three lines (within the limits due to the regeneration necessities). - The second section, supplying demineralized water type “Cl”, is composed by no.
2 lines operating in parallel, each including a mixed bed type unit. The plant can operate both with 1 and 2 lines according to production requirements. The plant is mainly composed by: 3 (three) cationic exchangers, having following characteristics and dimensions: Diameter, nun Cylindrical vessel height, mm Unit total height, mm Body thickness, mm Bottom thickness, mm Construction material Filling resins - Strong cationic capacity, 1 - Inert resin capacity, 1
2,100 4,000 5,600 10
10 Steel 8,000 1,400
and by no. 3 anionic exchangers: 2,500 Diameter, mm Cylindrical vessel height, mm 4,600 5,800 Unit total height, mm
the
236
R. Borsani et al. / Desalinahon
12 Body thickness, mm 12 Bottom thickness, m steel Construction material Ebonite, 3 mm Lining, mm Filling resins 7,700 - Strong anionic capacity, 1 1,800 - Inert resin capacity, 1 In order to get water having characteristics as per “Cl” type, no. 2 mixed bed type ion exchangers are utilized, each having the following characteristics: Diameter, mm Body height, mm Body thickness, mm Bottom thickness, mm Strong anionic capacity, 1 Strong cationic capacity, I
1,300 3,000 7 7 1,200 800
An automatic and sequential regeneration system operates on a.m. demineralization lines. Regeneration eluate is collected and stored in a tank having a capacity of 240 m3 in order to make the osmosis plant run at a flow rate not exceeding 16-17 m3/h with reference to three lines operating extraordinarily per 24 h. See the flow sheet (Fig. 2). Resins regeneration is carried out by means of solutions of HCl and NaOH. In order to get the objective of zero discharge, the eluate is sent to a neutralization step and finally to a reverse osmosis plant for the production of type “B” industrial water and the simultaneous getting of a concentrate to send to a plant evaporation and final crystallization.
3. Reverse osmosis plant The characteristics of the feeding water to the RO plant are: Nominal flow rate, m3/h Max. flow rate, m3/h Min. flow rate per 1 line, mVh (with a conversion factor = 56%) Nominal temperature, “C Temperature max.-min. values, “C
16.6 25 7.75 16 2-26
108 (1996) 231-245
Chemical analysis as ppm of CaC03: Ca2+ g/m3 Mg2; g/m3 Na+, i/m3 HC0j2-, g/m3 SOd2-, g/m3 C12-,g/m3 C032-, g/m3 PH
476 122.4 10.193 171.3 2,910.4 8,482.0 304.7 8-10
The characteristics of the product water outcoming from the RO plant are the following: Nominal flow rate, my/h Max. flow rate, m?h Pressure, bar g Hardness, m/l CaCO3 TLX, g/l The characteristics
outcoming following:
from
9.3 14 0.7 <140 950
of the concentrate (brine) the RO plant are the
Nominal flow rate, m3/h Max. flow rate, my/h Min. flow rate per 1 line, m3/h (with a conversion factor = 56%) Pressure, bar g TDS, g/l
7.3 11 4.35 2 33
4. Plant description (see Fig. 3)
The reverse osmosis plant has the purpose to treat the discharge coming from the demineralization plant after the resin regeneration. Quantity of aqueous solution discharged by demi plant is 100 m3 each regeneration with 2 regeneration per day each demi line and 2 demi lines normally operating (no. 3 exceptionally). Accordingly, the total quantity of discharge solution results to be 400 m3/day normally and 600 m3/d exceptionally. The solutions are discharged into a tank where they are neutralized and then pumped to a close storage tank. The storage tank has the purpose to allow a constant feed to the RO plant. Three centrifugal vertical pumps (two operating plus
R. Borsani et al. /Desalination
I
I
??
Fig. 2. Demineralization
108 (1996) 231-245
237
I
plant. Flow sheet.
one stand-by) suck from the tank and send through a common header the solution to the high pressure pumps. Before reaching the H.P. pumps, the solution is treated with some chemicals and then filtered by 2 sand filters and 2 cartridge filters. The high pressure pumps increase the pressure of the feed water up to the required one and pump it to the permeators. The product outcoming from each stage is collected in a common header, while the concentrate outcoming from one stage enters the next stage and so on up to the outlet of the last stage. The product water is recovered as “Industrial Water type B” and sent to the relevant storage tank. The concentrate is sent to a further treatment plant (evaporation and crystallization plant). The RO plant is also equipped with a
membrane cleaning unit composed by a tank with a mixer, a centrifugal pump, a cartridge filter and relevant connecting piping. Connection between the cleaning unit and the process piping is achieved by means of flexible house and quick coupling. 5. Topics of the RO plant design Considering the main characteristics of the feed water to the RO plant, the requirements of the steel plant and of the client and moreover the construction/erection aspects, the following choices has been made: - type of membranes; - pretreatment system; - plant configuration and regulation system; - material choices; - construction/erection choices.
238
R. Borsani et al. /Desalination
108 (1996) 231-245
/
Fig. 3. RO plant. Flow sheet.
5.1. Choice of the type of membrane Taking into account the limited flow rate of the RO plant and the chemical analysis of the feed water, it has been considered convenient to use membrane “spiral wound type” that, among others, show less sensibility to fouling and scaling problems. The membrane chosen for desalination of water by reverse osmosis must be highly permeable to water to achieve high rates of production while being highly impermeable to salts to obtain acceptable product water quality. It must also be extremely thin to maximize water flow but strong enough to withstand high pressure. Changes in the membrane’s transport and mechanical properties should be minimal after long exposures to high pressure. Likewise, resistance of the membrane to chemical and biological attack is necessary. For the subject plant the selected
membrane was the B-15 type by Permasep DuPont. The membrane used in the B-15 spiralwound cartridge is a flat film made from an aromatic polyamide polymer cast on a fabric support backing. The film is asymmetric, meaning it consists of a very thin dense “skin”, the actual membrane, on the surface, and a thick, porous layer of the same polymer underneath that provides support for the “skin”. The thin membrane “skin” inhibits the passage of salts, but readily allows water to pass. The support layer, being highly porous, does not impede the flow of water into the product channel and has no salt rejection properties. Flat film membrane is made into leaves with each leaf consisting of two sheets of membrane with a sheet of polyester tricot between to act as the product water-collection channel. Three sides of the leaf are glued to form an envelope. Tricot extending from the
R. Borsani et al. /Desalination
Product Tube \
Brine Seal Cay
239
108 (1996) 231-245
Anti-Telescoping Oev?
PRODUCT+ V FEE0
-4
Tricot Product Water Channel
Membrane Support Backing &
’
Adhesive Membrane
Fig. 4. Membrane.
open end is attached to a perforated PVC tube containing holes through which the product water will exit (see Fig. 4). Plastic netting between the leaves serves as the feed channel spacer enhancing mixing which minimizes concentration polarization. The leaves are wound around the product tube to give a spiral configuration. Identical plastic spooked devices are attached to the ends of the rolled leaves. One acts as an antitelescoping device (ATD) and the other serves as a brine seal carrier. In a pressure vessel the ATD is located on the brine discharge end at each cartridge and a brine seal is installed on the feed end. A fiberglass reinforced plastic (FRP) cover protects the cartridge. All cartridge are 40 inches in length. The &inch diameter models have 18 leaves with about 400 ft2 of surface area. As shown in Fig. 1, pressurized feedwater enters the cartridge and flows axially through the feed channel spacer to the opposite end where it exits as the brine. Water from the feed solution under the influence of the elevated pressure which raises its chemical potential, passes through the flat membrane
Surface /
into the product water collection channel. The desalinated product water flows spirally to the central product tube and exits the cartridge.
5.2. Pretreatment system 5.2.1. General considerations For optimum RO performance, water fed to the membranes should be treated to remove gross amounts of solids and prevent formation of solid matter inside the RO cartridge by precipitation or biological growth. Therefore, upstream of the cartridge, various unit operations (collectively called “pretreatment”) must be carried out on most natural waters. Typically, pretreatment consists of filtration to remove large particles and one or more of the following: 1) Adjustment of solubility parameters to prevent precipitation of sparingly soluble salts (“scaling”) as a result of the concentrating action of the RO process; 2) Coagulation of colloidal matter; 3) Chemical treatment to prevent biological growth.
240
R. Borsani
et al. /Desalination
The importance of pretreatment for an RO plant cannot be overemphasized. If cleaning of the cartridges is required frequently, pretreatment is deemed inadequate. With proper pretreatment, the frequency of cleaning is usually once every three to four With an adequate months or longer. pretreatment system, periodic cleaning will restore most of the productivity. If pretreatment is inadequate, cleaning will be less effective in restoring the RO performance and the need for cleaning will increase. In this case, taking into account the characteristics of the water feeding the RO plant the following pretreatment have been foreseen: 5.2.2. Acid injection (H2SO4) The acid injection is used for scale control (especially scaling caused by the precipitation of CaC03). The quantity of acid to be injected is based on the calculation of LSI (Langelier Saturation Index). If the LSI is negative, CaC03 will tend to dissolve; if the LSI is positive, CaCOa will tend to precipitate. LSI calculation is used to determine the approximate quantity of HzS04 required to obtain a negative LSI of the brine stream and to size the acid injection pumps. At start-up, the LSI of the brine stream must be determined. For an operating RO plant, the LSI of the brine stream must be determined periodically and pH adjustments made if necessary. The injection pump is driven automatically by a pH analyzer/controller. 5.2.3. Sodium exametaphosphate
injection
(NaHMP)
NaHMP can be injected as a precipitation inhibitor. For negative LSI values (LSIcO) NaHMP is not required. In case of LSIIl, 10 mg/l NaHMP shall be injected (since NaHMP is an inhibitor, usually 10 mg/l rather than stoichiometric quantity is used in the RO brine stream). However, since NaHMP only delays precipitation, it is necessary to flush the cartridge within four hours at shutdown. Operation at an LSI of 1.0 with NaHMP
108 (1996)
231-245
results in reduced acid consumption. The choice of whether to use an LSIIl.0 with NaHMP or a negative LSI without NaHMP is based on such factors as ion concentrations, and availability and cost of chemicals. In case of LSI>l, the plant cannot be work even using NaHMP injection. 5.2.4. Pressure medium filtration Every RO pretreatment system must consider how to prevent coagulation of colloids. The concentration of colloids is generally expressed as the Silt Density Index”(SD1) of the water. For the membranes used in this RO plant the max allowable SD1 is 5. Considering the characteristics of the feed water, and the previous treatment of the same water, a sand filtration seamed to be not necessary. Anyhow to assure a more reliable performance of the plant a further filtration phase has been foreseen. The filtration system was designed as follows: no. 2 pressure dual media filters (sand-anthracite) normally both working, but each one able to treat the whole flow rate during the backwashing operation of the other filter. The filtration is achieved using the in-line coagulation/flocculation process. Accordingly, a polyelectrolyte dosing unit and an aluminium phosphate dosing unit have been supplied. The feed water is added with the above two solution, then properly mixed through a static mixer before entering the dual media filters, This filtration system assures a very low LSI value saving the membrane life and furthermore the cartridge filter life. 5.3. Plant system
configuration
and
regulation
Since no. 2 demi lines are generally working (with relevant feed flow to RO plant = 8.3 m3/h each) and only exceptionally there are no. 3 demi lines working, the RO plant has been designed with two lines each one able to treat the nominal flow coming from one demi line but designed in such a way that it can
241
R. Borsani et cd. I Desalination 108 (1996) 231-24.5
treat also a flow rate of 1.5 times the nominal discharge of one demi line. Accordingly with 2 RO lines it is possible to treat both the discharge coming from 2 demi lines (nominal flow rate) and from 3 demi lines (max flow rate). In this manner, it has been avoided the installation of a third RO line generally not working, avoiding consequently all the problems that in the RO plants make difficult the stand-by use (cleaning, sterilization, flushing, etc.). In order to get what above, each RO line has been designed in a brine staging way with 3 stages (see Fig. 5). Each stage is composed by 1 permeator with no. 3 B-l 5 cartridges, High pressure pumps (one each line plus one stand-by) are centrifugal multistage type. The working pressure at the membrane inlet depends by the temperature (see Figs. 6-8). The regulation system of the plant is realized with a flow control valve upstream of the membranes and a pressure control valve on brine discharge (see Fig. 9). To avoid hunting of control system the response times of the two regulation loops have been selected different between themselves with priority to the pressure one.
J
a 9 0
I
I xl0
330
400
?? M
33
30
600
b>O
Prcrrurc
(,“ig,
Fig. 6. Operating version = 56%.
curves
at temperature
Fig. 7. Operating version = 56%.
curves
at temperature
of 16T;
con-
FEED \\ .ATER
IST STAGE
4
I
I
I
3RD STAGE
V PRODL’CT
COSCESTIUTE JI
Fig. 5. Stage arrangement
of 2°C; con
In this way, the system always finds automatically its working point over the pump curve everytime a change of working condition is required (eg. change from 2 to 3 demi lines). Considering possible future extension of the plant (eg. installation of a further demi line with a consequent a 3rd RO line) all the part of the RO plant from the feed water tank up to the high pressure pumps have been already designed for no. 3 RO lines with the max flow rate.
R. Borsani et al. /Desalination
242
Fig. 8. Operating version = 56%.
curves
;o~
Fig. 9. RO plant regulation
at temperature
of 26°C; con-
,_--_- ~----~
system.
5.4. Material choices 5.4. I. General considerations
Metal corrosion can be a serious problem in many water treatment processes. All RO devices are particularly sensitive to metal If corrosion is not corrosion products. controlled, premature failure of RO system will result. The degree to which corrosion occurs, and the effect it will have on RO system, must be carefully considered when designing the plant. Generally, the following criteria should be followed: - Non metallic materials such engineering plastics, plastic composites, glass, rubber, etc are desirable materials of construction for RO plans and they should be used
108 (1996) 231-245
wherever they practical and economical. - Stainless steel. Stainless steel have been successfully used in many RO plants for piping, pumps and vessels. Particularly type 316 stainless steel has proven good corrosion resistance. Type 304 stainless steel has also been successfully used, but it is less corrosion resistant than type 316. When using stainless steel stagnant flow zone should be avoided. Moreover, experience has shown that corrosion can be observed at threaded connection. Therefore these connections should be minimized when using stainless steel - Non ferrous materials. In general, copper, bronze and brass should be avoided if pH is below about 6.5. Aluminium should also be avoided because of its sensitivity to high and low pH and high TDS. - Non metallic liners and coatings. Nonmetallic liners and coatings are frequently used to protect pipe and equipment from corrosion and chemical attach. In selecting and applying the liner or coating the following factors relative to both base material and process water have to be considered: thermal expansion and contraction, chemical compatibility, temperature rating, and mechanical integrity. 5.4.2. Material selected Based on the above general criteria and the operating condition of the subject RO plant, the following main materials have been selected. It must be observed that, due to the severe temperature condition imposed by the client during the erection and storage period (-25”(J), plastic materials have not been selected also if in some cases, considering the characteristics of the process water and the economical reasons, they should have to be chosen. Vertical pumps Sand filters Cartridgefilters Low press. proc. pip.
AISI 316 Carbon steel epoxy painted AISI 3 16L / polypropilene Feed water AISI 3 16L Product AISI 304L
R. Borsani et al. /Desalination
Sodium exametaphosphate dosing system AISI 316 AISI 316 Mixer AISI 316 Pumps AISI 316 Piping
Tanks
Sulphuric acid dosing system INCOLOY 825 Pumps ALLOY 20 / PTFX In line mixer Piping PVC I GRP Aluminium sulphate dosing system AISI 316L Tanks AISI 3 16 Mixer AISI 316 Pumps AISI 3 16L Piping Polyelectrolyte dosing system AISI 304L Tanks Mixer AISI 304 AISI 304 Pumps Piping AISI 304L
In line mixer
AISI 304
High pressure pumps High pressure piping
AISI 316 Feed water AlSI 3 16L Brine AISI 316L Perrneators (press. vessels) GRP Polyamide Membranes Cleaning system Tanks Mixer Cartridgefilter Pump Piping
AISI 316L AISI 316 AISI 316L / polypropilene AISI 316 AISI 304L
5.5. Construction/erection choices Considering the construction and storage problems of the steel work (particularly the very low temperature problem) and the peculiarity of the RO plant and likewise taking into account the dimension of the subject RO plant, it has been choien to supply the plant preassembled on skids. Starting from the foreseen final lay-out, it has not been possible to assembly all the components above one skid; accordingly it was decided to proceed as follows:
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Feed water pumps (vertical ones): supplied loose Common header: supplied in several prefabricated spools No. 1 skid for sulphuric acid dosing system No, 1 skid for sodium exametaphosphate dosing system No. 1 skid for polyelectrolyte and aluminium sulphate dosing system No. 1 skid for dual media filters No. 1 skid for the balance of the RO plant (cartridge filters, high pressure pumps, permeators, cleaning system, etc.). Consequently it has been possible to proceed testing all the plant section both as pressure test (where required) and as functional test (as far as possible). Each skid has been supplied complete with its electro/instrumental components and relevant control panel. All connecting parts (piping) has been supplied as prefabricated spools to allow a quick connection among the skids and with the other plants. In this matter, all erection operations resulted extremely simplified and fast. 6. Concentration and crystallization plant
The concentrate outcoming from the reverse osmosis plant is collected to a threestages type evaporation plant. Permeatecharacteristics Nominal flow rate, kg/h Max. flow rate, m3/h Dissolved solids flow rate, kg/h Water only flow rate, kg/h Averageconcentration Mg2+, C2+, % Na+, % so&, % Cl-, % HC03, % Suspended solids, % Separatedcrystalscharacteristics Flow rate, kg/h
9,000 11 285415 8,500-8,700
3 33 16-28 3547 1
300-500
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Residual humidity, % ‘be
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5 Mixed salts
1.4-1.6 Apparent specific gravity, kg/dm” 800 (‘) Centrifugaldecanter sizing, kg/h (*)sizingsuitable to drain on two duties the crystals producedon three daily duties. Characteristicsof the fluids outcoming from the plant 2,650 Pure condensate,kg/b 8,000 impure condensate,kg/b 1 Cooling water, m3/h (max. overheating 7°C) max. 180,000 Saturatedwet air, m3/b
Materialsbalance (at nominal conditions, temperature lO”C,relative humidity 80%) Salts Hz0 Total Feeding 285 8,715 9,000 Servicewater 150 150 15 Separatedsolids 285 300 Evaporation 8,850 8,850 Power consumption (at the following conditions) Externaltemperature,“C 10 80 Relative humidity, % 10 Feed temperature,“C Live steam (10 bar, 2OO”C),kg/h 3,100 Service water, m3/b 3 200 Electric energy, kWh/b 8 Compressed air, Nm3/h
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6. I. Process description (see Fig. IO)
The process on continuous operation includes a concentrator-crystallizator equipment at triple effect and at forced circulation, fed with low pressure steam, and is completed with a final stage of crystallization evaporative by counter current injection of air. Mixed salts are separated with a residual humidity of about 5% via a centrifugal decanter suitable to discharge them on a receiving box fit to be emptied by a bucket. Condensates of live steam are returned to the steam generator and condensates of vapours extracted during the concentration step are recycled to the industrial water storage tank. Saturated air outcoming from the plant is sent directly to the atmosphere. Pure condensate - It returns to the steam generator without any pollution, having a residual pressure of 4 bar at battery limit. Impure condensate - It is collected to the industrial water storage tank, as it contains a salt concentration of about 50 ppm. Its pressure at battery limit is 2 bar. Incondensables extraction - The heat exchanger is maintained under vacuum and incondensable are extracted by the pump. The assembly of mentioned plants forms the main chain of plants built for Voljski steel shop, are running since the date of the start of steel shop itself and are still accomplishing the scope for they have been constructed.
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