Ramnad Desalination Specs 3.80 MLD.pdf
-
Upload
hemantaq2308 -
Category
Documents
-
view
229 -
download
1
Transcript of Ramnad Desalination Specs 3.80 MLD.pdf
-
15
CHAPTER 2
EXPERIMENTAL STUDIES OF THE RO
DESALINATION PROCESS
The scenario for the demand of portable water in the world and in
India, the process of reverse osmosis desalination, objective of the present
study and the scope of the present work have been described in previous
chapter. Based on the outcome, as described in chapter 1, it is proposed to
carryout experimental work for setting up scaled up plants of higher capacity
to meet the demand for water. The experimental facility is described in this
chapter.
2.1 INTRODUCTION
Potable water is produced from sea / brackish water, using
desalination with the RO method. Domestic and industrial waste waters are
also treated by this technique. RO has the potential to remove TDS, organic
compounds and all classes of pathogens. The raw feed (sea/brackish water)
enters the mixing tank and is pumped through the RO membranes. The
resulting product is potable water, part of which is stored in the permeate
tank. The other by product is brine, part of which is recycled, and the rest is
concentrated for reuse and generation of value added salt-products. The
entire system consists of the integration of the mixing tank, pump, RO
membranes and permeates / brine tanks. The main objective of this chapter is
to report the experiments conducted to collect data from the different units
-
16
that comprise the RO desalination system, and to develop simple
experimental models which may be used to predict the performance of each
unit of the RO desalination system. The purpose of the modelling is to scale
up/down the process for the design, the calculation of the optimal operating
conditions, output predictions and for the fabrication of commercial units. An
experimental facility (3.8 MLD Desalination plant at Ramanathapuram, TN)
consisting of the mixing tank, dual media filter, polished dual media filter,
disc filter, cartridge filters, high pressure pump, RO brine tank and the
permeate tank, has been chosen to carry out the experiments to characterize
the system. Figure 2.1 shows the 3.8 MLD Reverse Osmosis Plant set up in
Ramanathapuram.
Figure 2.1 Experimental desalination plant set up with reverse osmosis
in Ramanathapuram (3.8MLD)
It is required to build two types of models for this study: steady
state and transient. The experimental set-up / plant operates at optimal steady
state conditions. The transient characteristics of the output can be obtained by
introducing external excitation / perturbations in the input variables. During
-
17
the operation of the RO system, the conditions, such as pressure, temperature
and feed water quality, can cause variations in the product water quality and
productivity. The dynamic and steady state performance operation of the RO
was obtained from the experimental work. The experimental runs were
performed on different units of the RO plant, such as a mixing tank, RO
membrane, brine tank and permeate tank. The inlet and outlet parameters of
each unit of desalination process, such as the flow rate, pH and total dissolved
solids (TDS) were recorded. The inlet and outlet flows are measured by the
Paddle wheel flow meter. At the end of each test, samples were taken from
each unit, and the TDS was measured using the conductivity meter and the
pH by using the pH meter. The main aim of the experiment is to enhance
higher diffusion of the solute away from the membrane, to reduce the
magnitude of the salt concentration at the membrane wall, cw , relative to the
concentration in the bulk of the feed cF .This leads to a higher permeate flux
(Jw ) , decrease in the salinity of the product water, increased recovery, and
an acceptable rejection rates.
2.2 DESCRIPTION OF EXPERIMENTAL SET UP
A sea water based desalination plant has been installed in
Narippaiyur village of Ramanathapuram district, Tamil Nadu, India, with a
capacity of 3.80 MLD of drinking water. This is the first Desalination plant
for the production of potable water in South Asia. The arrangement of the
experimental set up is shown in Figure 2.2.
-
18
Figure 2.2 Schematic of the experimental set-up for the desalination of
sea water
2.2.1 Filters and Pumps
A well with an inner diameter of 1000 mm is dug in the sea and is
covered with a basket with small holes and mesh (100x15 mm GRP material)
so that fish and algae are not allowed to enter the well, which is connected to
an equalisation tank of 70sq m x 7m by two pipes (GRP material) of an inner
diameter of 450 mm and a length of 450 meters. GRP is a composite
consisting of two different materials such as glass fibre-reinforced plastic.
The pipe is connected to the equalisation tank with a certain slope, so that the
height of water in the equalization tank is equal to the height of the sea. The
tank is equipped with three vertical turbines and two horizontal centrifugal
pumps (capacity 450 500 m3/hr) that deliver water to the filters section with
a pressure of 3 - 3.5 kg/cm2. Three rough dual media filters (RDMF) of 3m
diameter and 7m height, with a capacity of 120 170 m3/hr filled with sand
and gravel, are employed in a series as the primary filters to remove coarse
particles and suspended solids from the water. The dirt and turbidity are also
reduced to some extent at the first and second stages of filtering. Water then
-
19
enters the third stage of filtering, viz, the polished dual media filters (PDMF)
where suspended solids, dirt and turbidity are reduced to yield colourless and
odourless water. Figure 2.3 shows the different layers of the PDMF
Figure 2.3 Schematic diagram of the typical PDMF filter
After PDMF, water enters the disc filter to eliminate particles larger
than 5 microns. Figure 2.4 shows the cross sectional view of the disc filter.
Figure 2.4 Cross sectional view of a disc filter
Water enters through the inlet pipe at the bottom of the disc filter
housing through the hydraulic valve way 1&2 and way 3 is closed. As the
water enters at the base of the filter chamber it passes through fin plate blades
-
20
which force the water to enter the filter chamber with a vortex effect. As the
water swirls within the filter chamber, some of the dirt particles are forced
outwards and upward and collect on the wall of the upper part of the filter
chamber, where they remain until the next flushing cycle. The water flows
from outside of the disc stack to the inside, passing along the specially
designed grooves on the surface of the disc which catch the suspended
material. The stack of discs is held compresses by the piston at its top. Clean
filtered water flows out of the filter from the centre outlet through the
hydraulic valve way 2 & 3 and way 1 is closed.
Then the feed water enters the cartridge filter, which will remove
particles sized more than 3 microns. Fig 2.5 shows the front view of the
cartridge filter.
Figure 2.5 Front view of a cartridge filter
The feed water is pumped (discharge pressure > 50 Kg/cm2) to the
RO sections, using a high pressure centrifugal pump.
-
21
2.2.2 RO Section
RO operates over a pH range of 2 to 11 with excellent performance,
in terms of flux, salt, organic rejection, and microbiological resistance, with a
free chlorine tolerance of less than 0.1 ppm. The water and solute
permeability characteristics depend on the performance of the membrane. The
feed water is allowed to enter the innermost radius of the spiral RO section.
The permeate comes out through the outermost layer of the RO. The ions are
attracted by the polyamide material of the membrane. The TDS reduces from
40000 ppm to 500 ppm for a running (operation) time of 12 hours through
168 ROs (spiral bound) of 1m length each. One RO consists of 30 membrane
leaves. Each leaf is made up of two membrane sheets glued together back to
back with a permeate spacer between them. The consistent glue line of about
1.5 inches wide seals the inner (permeate) side of the leaf against the outer
(feed/concentrated) side. The leaves are rolled up with a sheet of feed spacer
between each of them (shown in Figure 3), which provide the channel for the
feed and concentration to flow. The permeate (p) and brine (b) from all the
ROs are collected and passed to the opposite direction of the feed water
entering section of the RO. The brine from the RO section is collected, that
amounts to approx 50% of the feed. A schematic process flow sheet
(Figure 2.6) describes the flow rates in m3/hr, pressure in kgf/cm2 and the
TDS in ppm of different streams in the entire plant.
-
22
Figure 2.6 Process flow scheme of the desalination plant with streams
under operating conditions in (m3/hr, kgf/cm2, ppm)
About 70% of the brine is recirculated to the feed-mixing tank.
The rest of the brine can be used for the recovery of salts. 10% of the feed
precipitates or forms scales that get adhered to membrane. About 35-40% of
the feed goes to the permeate tank, and can be used for potable purposes.
MIXING TANK
HP PUMP
BRINE TANKENERGY
RECOVERYDEVICE
RO UNIT PERMEATETANK
SEA WATER
PRODUCTTANK
(80, 1, 850)
(80, 0.75, 850)CAKE
(65, 8, 60000)
(180,60, 41350)
(115, 3, 40000)(180, 11, 41350)
(10, 60000)
( 90, 59, 60000)
( 90, 59, 60000)
(25, 51, 60000)
SEA
(25, 0.25, 850)
-
23
To boost the feed water pressure from 3 kg/cm2 to 60 kg/cm2,
(centrifugal type of energy recovery devices, electromechanically operated
butterfly dump valve and a flow control valve) are used by taking (energy
conservation /recovered) the energy from the brine stream.. The percentage of
salt rejection is found to be 97.5%. The designed recovery is calculated as 50
%. Figure 2.7a and 2.7b show the cross sectional view of one RO vessel and
one RO (from Filmtech) . One RO vessel consists of six RO modules. The
plant consists of 28 numbers of RO vessel for one bed. The plant has two
such beds.
The total feed flow rate of one RO bed is 180 m3/hr. Feed
flow/vessel=180/28=6m3/hr; Permeate flow/vessel= 4m3/hr; Brine
flow/vessel= 2m3/hr
Figure 2.7a. Schematic flow of feed, brine, permeate in one RO
membrane module (TWAD board, Ramanathapuram)
-
24
Figure 2.7b. Cross sectional view of a typical RO membrane
Table 2.1 shows the technical details of the RO Plant. From
Figure 2.7a it is seen that the permeate (due to the radial component of
velocity) comes out along the surface of the membrane while the
concentrated brine (due to the axial component of velocity) flows axially
through the membrane. The RO process can be thought of as comprising a
grey box model (Figure 2.8) with one input or manipulated variable, several
disturbance and design variables, and output or control variables. Due to the
transverse diffusion across the walls of the RO tubes (cylindrical), the
permeate comes out and gets accumulated in the product tank. The axial
stream flows out as rejection or brine. Part of the brine flow is recycled with
raw feed that enters to the mixing tank.
-
25
Pre-treated Feed Water Permeate FP, CP PP
FF,CF,PF
Brine
Fb,Cb,Pb Brine
Figure 2.8 Representation of the RO process as a grey box model
Table 2.1 Important technical details of the RO plant (TWAD Board)
SerialNo Components Description Specification
1. Sea water intakemouth
Intake mouth covering meshsize
10015 mm slots
2. Connecting pipefrom intake mouth toEqualization tank
Inner diameter and outsidediameter
450mm, 490 mm
3. Feed Pump motorSquirrel Gaugeinduction type
SpeedRating
2900 rpm30KW,415 V, 3 phase
4 Feed Pump(Vertical CentrifugalType)
Head 49/39 m of watercolumn (wc)
5 RDMF(Horizontal) i. Sand mesh size 14 heightii. Gravel 1/81/16 heightiii. Gravel1/41/8 heightiv. Gravel 3/81/4v. Gravel 3/43/8vii Diameterviii Length
400 mm150mm150mm150mm600mm3000 mm7000 mm
6 PDMF(Horizontal) i. Anthracite heightii. Gravel 1/81/16 heightiii. Gravel1/41/8 heightiv. Gravel 3/81/4v. Gravel 3/43/8vi. Refilling freqvii. Diameterviii Length
300 mm100 mm100 mm100 mm600 mm5 year once3000 mm7000 mm
Kw
Ks
-
26
Table 2.1 (Continued)
SerialNo
Components Description Specification
7. Disc Filter:2ADF i. Disc element: Polyethyleneii. Flushing cycle timeiii.Max flowrateiv. Max working pressure
15-25 secs25 m3/hr10 bar
8. Cartridge Filter i. Material: Polypropyleneii. Filter element sizeiii. No of Catridge elements/Filter
2.6OD1.1ID40Length
30 nos
9. Energy RecoveryDevice
i. Material:Duplex stainless steelii. Feed flowiii. Brine flow
158 m3/hr79 m3 /hr
10. HP Pump i. Centrifugal pumpii. Total discharge headiii. Suction pressure Min
634 m of WC5 m of WC
11. HP pump motorrating
Sq. cage motor 450KW, 2980rpm,50 HZ, 6.6 KV,3 phase,
12. RO Membrane i. Type:Filmtech,SW30HR380 Outer material:Polyamide Center material:Polysulphoneii. Lengthiii. Outer diameteriv. One vessel length(6 ROS)v. Vessel inner diavi. Top most vessel pressurevii Feed flow/vesselviii Permeate flow/ vesselix. Brine flow /vessel
1 meter86.6 meter210 mm2 kgf/cm2
6 m3/hr5m3/hr1.5m3/hr
-
27
2.3 UNIT OPERATIONS IN THE DESALINATION PROCESS
2.3.1 Pre- treatment and Post- treatment (or) Demineralization
As the feed water comes from the sea / brackish source, it contains
clay, dirt and suspended materials that need to be separated . Hence pre and
post treatment using clarifiers are done in the factory. Chlorination and de-
chlorination are done to remove bacteria & pathogens; turbidity is removed
during pre-treatment. Calcium and bi-carbonate ions are removed from the
feed during post treatment when the pH is raised slightly above 7. The details
of these methods are not under the scope of this research.
2.3.2 Data Collected across Each Unit of the Experimental Setup
The experiments were performed on the RO desalination process
equipped with spiral membranes. First, the data for the feed flow rate, TDS,
pH and the HP (high pressure) pump were collected across the mixing tank,
for the verification of the dynamic and steady state operation of the mixing
tank during the starting of the process, after the power shut down of the plant
for nearly15 minutes. The data are listed in Table 2.2. The corresponding
graphs are shown in Figure 2.9. It can be seen from Figure 2.9(a) that after
1000 sec of startup time, the feed flow rate attains steady value of 0.05
m3/sec. Figure 2.9(b) reveals that after 1000 sec of startup time, the
concentration in mixing tank attains steady value of 4.176104 ppm, pH
becomes 12.83 (Figure 2.9c) and the pressure of HP pump after 150 sec of
startup time changes to 6105 pascals (Figure 2.9d)
-
28
Table 2.2 Data to describe the dynamic characteristics of the feed in
mixing tank
SerialNo Time in sec
HP pump pressurein Pascal*10000
Feed flowratein m3/sec
Feed TDSin ppm
Feed pH
1. 1 3 0 41400 12.9382. 10 3.3 0 41444 12.9343. 20 3.5 0.0055 41500 12.904. 30 3.7 0.00833 41580 12.895. 40 4 0.0111 41600 12.896. 50 4.2 0.01388 41700 12.877. 60 4.5 0.01944 41760 12.8658. 70 4.8 0.0222 41765 12.859. 80 5 0.025 41780 12.84
10. 90 5.2 0.02666 41800 12.8311. 100 5.4 0.029166 41820 12.82512. 150 5.6 0.031944 41840 12.8213. 200 5.8 0.0388 41845 12.8114. 250 5.9 0.04388 41825 12.80515. 300 6 0.04666 41805 12.81216. 350 6 0.04833 41790 12.81517. 400 6 0.04888 41780 12.8218. 450 6 0.04944 41770 12.8319. 500 6 0.049722 41767 12.83220. 550 6 0.05 41767 12.83521. 600 6 0.050277 41760 12.83622. 650 6 0.0501388 41760 12.8423. 700 6 0.05 41760 12.8424. 750 6 0.05 41760 12.83725. 800 6 0.05 41762 12.83726. 850 6 0.05 41761 12.83627. 900 6 0.05 41762 12.83528. 950 6 0.05 41473 12.83529. 1000 6 0.05 41761 12.83530. 1050 6 0.05 41760 12.83431. 1100 6 0.05 41760 12.83432. 1150 6 0.05 41761 12.83433. 1200 6 0.05 41760 12.834
-
29
(a) Feed flowrate vs time (b)Feed TDS vs time
(c).Feed pH vs time (d) Feed Pump pressure vs time
Figure 2.9 Dynamic characteristics of the feed in mixing tank
Then, the data are collected for the brine flow rate, pressure, TDS
and pH of the brine tank, for studying the dynamic performance of the brine
tank during the start-up of the process, after the power shut down of the plant
for nearly15 minutes. The data are listed in Table 2.3. The graphs are shown
in Figure 2.10. It can be observed from Figure 2.10 that, after 1200 sec of
start uptime, the brine flow rate becomes 0.0258 m3/sec, the brine TDS
settles at 6.366 104 ppm and the brine pH attains a pH of 12.65.
-
30
Table 2.3 Data to describe the dynamic characteristics of the feed
stream in the brine tank
SerialNo
Timein secs
Brine flowratem3/secs
Brine TDSin ppm
Brine pH
1. 1 0 61000 12.68242. 10 0 62000 12.66653. 20 0.0001 62500 12.6664. 30 0.0006 62700 12.66595. 40 0.0011 62900 12.66586. 50 0.0017 63000 12.66567. 60 0.0022 63050 12.6658. 70 0.0044 63100 12.66499. 80 0.0069 63125 12.6648
10. 90 0.0097 63200 12.664511. 100 0.0181 63250 12.66412. 150 0.0250 63300 12.665813. 200 0.0256 63340 12.665714. 250 0.0256 63355 12.65615. 300 0.0258 63365 12.653516. 350 0.0261 63374 12.65117. 400 0.0256 63420 12.6518. 450 0.0258 63450 12.64919. 500 0.0256 63470 12.64820. 550 0.0253 63480 12.64721. 600 0.0253 63490 12.64622. 650 0.0258 63520 12.64723. 700 0.0258 63550 12.64824. 750 0.0258 63530 12.64925. 800 0.0258 63600 12.6526. 850 0.0258 63600 12.65127. 900 0.0258 63610 12.65228. 950 0.0258 63620 12.65129. 1000 0.0258 63650 12.65130. 1050 0.0258 63650 12.6531. 1100 0.0258 63660 12.6532. 1150 0.0258 63660 12.6533. 1200 0.0258 63660 12.65
-
31
(a)Variation of the brine flowrate with time (b) Variation of the brine TDS with time
(c) Variation of the brine pH with time
Figure 2.10 Dynamic characteristics of the feed stream in the brine tank
The data are collected for the permeate flow rate, pressure, TDS
and pH in the permeate tank, for studying the dynamic performance of the
permeate tank during the starting of the process after the power shut down of
the plant for nearly15 minutes. The data are listed in Table 2.4.The graphs
are shown in Figure 2.11. It can be observed from Figure 2.11 that, after 650
sec of start up period, the permeate flow rate reached a steady value of
0.0229 m3/sec; after 270 min, the permeate TDS drops down to 819.75 ppm
and the pH settled at 7.5163.
-
32
Table 2.4 Data to describe the dynamic characteristics of the feed
stream in the permeate tank
Serial No Time insecs
Permeateflowratem3/sec
Permeate TDS PermeatepHTime in
secs*60Permeate
TDS in ppm1. 1 0 0 860 7.51602. 10 0.0002 5 862.65 7.51603. 20 0.0005 10 883 7.51604. 30 0.0012 20 881.35 7.51605. 40 0.0014 30 850.00 7.51616. 50 0.0022 40 870.70 7.51617. 60 0.0026 50 867.60 7.51618. 70 0.0042 60 864.30 7.51619. 80 0.0069 75 852.75 7.516210. 90 0.0133 90 863.75 7.516211. 100 0.0194 105 871.45 7.516212. 150 0.0217 120 847.80 7.516213. 200 0.0225 135 839.00 7.516214. 250 0.0228 150 846.70 7.516215. 300 0.0233 165 847.80 7.516316. 350 0.0236 180 845.05 7.516317. 400 0.0228 195 839.00 7.516318. 450 0.0233 210 840.00 7.516319. 500 0.0228 225 830.75 7.516320. 550 0.0231 240 828.00 7.516321. 600 0.0231 255 824.70 7.516322. 650 0.0228 270 819.75 7.516323. 700 0.0230 7.516324. 750 0.0230 7.516325. 800 0.0229 7.516326. 850 0.0229 7.516327. 900 0.0229 7.516328. 950 0.0229 7.516329. 1000 0.0229 7.516330. 1050 0.0229 7.516331. 1100 0.0229 7.516332. 1150 0.0229 7.516333. 1200 0.0229 7.5163
-
33
(a)Permeate flow rate with time (b) Permeate TDS with time
(c) Permeate pH with time
Figure 2.11 Dynamic characteristics of the feed stream in the permeate tank
The data are collected for potable water flow rate, pressure, TDS
and pH of the potable tank for the dynamic performance of the potable tank
during the start-up of the process, after the power shut down of the plant for
nearly15 minutes. The data are listed in Table 2. 5. The graphs are shown in
Figure 2.12. It can be seen from the graph ( Figure 2.12) that after 15 mins of
start up/shunt down, the potable water flow rate became 2510 l/min, the
TDS is fluctuating between 1000-1100 ppm, the pH fluctuates around 9.2;
-
34
the recovery comes down to 38% in 240 mins and the rejection becomes 97.4
% after about 200 mins.
Table 2.5 Data to describe the dynamic characteristics of the permeate in
the potable water tank
SerialNo
Timein
mins
Potablewater flow
rateLiters/min
PotablewaterTDSppm
Potablewater
pH
Timein
mins
%Rejection
%Recovery
1. 1 10 1000 9.13 5 97.3 46.65
2. 2 200 950 9.15 10 97 45.933. 3 500 900 9.20 20 97 47.264. 4 750 1050 9.23 30 97.35 46.555. 5 960 1000 9.32 40 97 45.796. 6 1450 850 9.15 50 97.238 45.617. 7 1750 900 9.17 60 97.23 44.898. 8 2000 950 9.23 75 97.32 44.579. 9 2225 1050 9.24 90 97.25 43.85
10. 10 2530 1100 9.12 115 97.22 43.4011. 11 2490 1050 9.14 130 97.34 42.8112. 12 2500 1100 9.15 145 97.4 42.8113. 13 2520 1000 9.12 160 97.36 42.9414. 14 2545 1050 9.16 175 97.37 41.7615. 15 2530 1100 9.21 190 97.36 39.8816 16 2510 1000 9.22 205 97.37 39.17
220 97.40 38.46235 97.44 38250 97.44 37.27265 97.45 36.79280 97.5 36
-
35
(a) Potable water flow rate (b)Potable water TDS
(c)Potable water pH (d) RO bed recovery
(e)RO bed rejection
Figure 2.12 Dynamic characteristics of the permeate in the potable tank
-
36
2.4 SUMMARY
A large scale SWRO Desalination plant has been studied to
evaluate the process development. The preliminary results obtained from the
experimental set up lead to some observations on the individual units, such as
the mixing tank, brine tank, permeate tank and potable tank to predict the
performance of each system. In this study, the flow rate of the feed water in
and out of the mixing tank is 0.05m3/sec, the feed TDS value is around
41,000, the feed pH value is 12.8 and the operating pressure of the HP pump
pressure is 600000 Pascal. During this study the steady operating flow rate of
the brine water in and out of the brine tank is 0.0258 m3/sec, the brine TDS
value is around 61,000, and the brine pH value is 12.6 . The steady flow rate
of the permeate water in and out of the permeate tank is 0.0229 m3/sec. The
steady permeate TDS value is around 800 ppm and the steady permeate pH
value is 7.5163. The results show that the steady flow rate of the potable
water out of the potable tank is 0.04167m3/ sec (2500 litter/min). The potable
TDS steady value is around 1,000 and the potable pH steady value is 9.22.
These experimental results will be helpful in developing step response models
across different units for step changes in inlet feed flow rate, concentration
and pressure exerted by HP pumps using reaction curve method. Based on the
experiment, the following conclusion can be drawn in the present study, viz,
the percentage rejection of salt by the RO is 97.5% and the percentage
recovery is around 40%. With the above results on system investigation and
the quality of the feed water, the modelling, design and simulation of large
scale Seawater Reverse Osmosis process (SWRO) are developed and is
described in the next Chapter.