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Optimizing the Efficiency of Reverse Osmosis

Seawater Desalination

Uri Lachish, guma science

Abstract: A way is considered to achieve efficient reverse osmosis

seawater desalination without use of energy recovery or pressure exchange

devices.

Contents

1. Introduction

2. Basic scheme of desalination by reverse osmosis

3. Improving desalination: I. Modules in series

4. Improving desalination: II. Energy recovery

5. Summary of the energy balance

6. A spiral membrane module

7. Cyclic flow operation

8. Desalination energy, salinity and cycle time in cyclic flow operation

9. Comparison of continuous flow to cyclic flow

10. Difficulties with cyclic flow operation

11. Utilization of the energy accumulated within concentrated salt water

12. Summary and conclusions

1. Introduction

Seawater desalination requires minimal energy consumption equal to the

osmotic pressure times the volume of desalinated water [1]. The osmotic

pressure is nearly proportional to the salt concentration in the water. For a

seawater osmotic pressure of 27 bar the minimal energy is about 0.75 kW

hour / cubic meter and it varies according to the water salinity. This

minimal energy, derived by thermodynamic considerations, is general and

true to all desalination technologies and not only reverse osmosis.

Advanced reverse osmosis systems apply energy recovery or pressure

conversion devices and report higher energy consumption of above 2 kW

hour / m3 [2, 3]. Curiously, the energy may be easily reduced and approach

the theoretical minimum. Why this is not done?

Producing one volume of desalinated water with nearly minimal

consumption of energy requires the use of several volumes of seawater that

mostly go back to the sea. These volumes are prepared prior to desalination

by chemical treatment and filtering operations. The cost of the pre osmosis

water is then higher than the cost of the energy saved in the process, so

there is no advantage doing that.

The ratio of the desalinated water volume to the seawater volume used to

produce it is called the recovery ratio. High recovery ratio saves on the

cost of seawater preparation prior to the osmosis process, and low recovery

ratio saves on the energy cost of desalination. The optimal recovery ratio

depends on the relative costs of these operations and may vary under

different conditions.

The purpose of these pages is to consider a way to achieve efficient

seawater desalination by reverse osmosis in a system that does not apply

energy recovery or pressure conversion devices.

2. Basic scheme of desalination by reverse osmosis

Figure-1 shows the basic scheme of desalination by reverse osmosis:

Figure-1: Basic scheme of desalination by reverse osmosis.

High-pressure pump pumps seawater into a module separated by a semi

permeable membrane into two volumes. The membrane lets water flow

through it but blocks the transport of salts, so the water in the volume

beyond the membrane, called permeate, is desalinated, and the salt is left

behind in the volume in front of the membrane. The concentrated salt

water in this volume leaves the module via a pressure control valve.

The osmotic pressure Ps is given by van't Hoff equation:

Ps = c∙R∙T (1)

Where c is the ionic molar concentration, R = 0.082 (liter bar / degree

mole) is the gas constant, and T is the absolute temperature in Kelvin units.

T is equal to the Celsius temperature + 273.17. Thus, T = 300 K for 27o C.

Typical ionic salt concentration of seawater is: c = 1.1 mole / liter, and the

corresponding osmotic pressure is:

Psea = 1.1 x 0.082 x 300 = 27 bar.

The flow rate of water through the membrane Frate is given by:

Frate = Kf∙(Ppump - Ps) (2)

The membrane properties and its area determine the flow rate factor Kf.

Ppump is the pressure generated by the pump and controlled by the pressure

control valve. Ps is the osmotic pressure of the concentrated salt water in

the module.

The pump pressure must be higher than the osmotic pressure in order to

force seawater flow through the membrane and permeate water out of the

module. The flow rate is proportional to the difference between the two

pressures. When they are equal water does not flow through the membrane,

and if the pump pressure is lower than the osmotic pressure, permeate

water will flow back towards the concentrated salt water.

Consider an example where the water recovery ratio is 0.5. That is, for

every two volumes of seawater pumped into the module one volume will

come out as permeate water and one as doubly concentrated salt water. The

high-pressure pump consumes energy equal to the pump pressure times the

volume of water that it pumps. Since the pump has to pump two V

volumes of seawater in order to produce one V volume of permeate water,

the consumed work is:

W = P∙2∙V (3)

Since the osmotic pressure of the concentrated salt water is twice as much

as that of seawater, Ps = 2∙Psea, the required pump pressure will be:

P = 2∙Psea + ΔP (4)

ΔP is the overpressure, above the osmotic pressure, that drives water flow

through the membrane. The work then becomes:

W = (4∙Psea + 2∙ΔP)∙V (5)

It is, therefore, more than four times higher than the minimal theoretical

desalination energy (Psea∙V).

In summary, the practical desalination energy is higher than the theoretical

minimum for two reasons.

a. The feed volume of seawater is higher than the volume of permeate-

water.

b. The osmotic pressure of concentrated salt water within desalination

module is higher than that of seawater.

3. Improving desalination: I. Modules in series

Figure-2 shows a desalination system where a number of modules are

connected in series. In practical systems there are six or seven modules in

series.

Figure-2: Connecting membrane modules in series.

Seawater flows into a first module where about 10% penetrate through the

membrane and become permeate water. The rest more concentrated water

flows to a second module where again part of it penetrates through the

membrane and part of it continues to the next membrane.

The salt concentration and therefore also the osmotic pressure increase at

each consecutive module, while the overall pump pressure is nearly the

same in all of them. The flow rate through the membrane is proportional to

the difference between the pump pressure and the osmotic pressure

(equation 2). Therefore, the pressure difference and the flow rate through

the membrane are highest at the first module. They decrease at each

consecutive module, and are lowest at the last module.

In this system there is no need of overpressure to drive water through the

membranes if sufficient number of modules are connected in series. Most

of the permeate-water comes from the first modules and little water comes

from the membrane of the last module, where the osmotic pressure is

slightly below the pump pressure. For 50% water recovery the work of

desalination thus becomes (by equation 5):

W = 4∙Psea ∙V (6)

The semi permeable membrane is not perfect and about 0.5% - 1% of the

salt in the water penetrates through it. Series connection of modules is

advantageous since most of the water comes from modules with lower salt

concentration, resulting in lower salt concentration in the permeate water.

4. Improving desalination: II. Energy recovery

The mechanical energy consumed by the high-pressure pump is

transformed into heat within the desalination system. Part of the heat is

generated by dissipate water flow through the membrane and part by water

flow through the pressure control valve. Part of the (free) energy is

accumulated within the concentrated salt water that leaves the system. This

energy is not lost and in principle can be utilized, returned back to the

system and improve its efficiency. Is it worth doing? The question will be

discussed in a next section.

The energy loss within the pressure control valve can be avoided by

application of a variety of energy recovery devices. Figure-3 presents a

system where the pressurized salt water, that leaves the membrane

modules, drives a rotary turbine [4]. The turbine drives an auxiliary high-

pressure pump that supplies seawater to the membrane modules and

reduces the water supply and energy consumption of the first pump.

Figure-3: Energy recovery with a turbine and an auxiliary pump.

Water is practically incompressible and therefore cannot accumulate

energy. This property is the basis of a class of devices that exchange

pressurized concentrated salt water within the modules with outside

seawater [5 - 12]. There are many specific designs but they all operate with

the same "rotating door" principle presented in figure-4.

Figure-4: Exchange of pressurized concentrated salt water with seawater.

The "rotating door" has two compartments, one filled with pressurized

concentrated salt water, and one filled with seawater. The "door" rotates

180 degrees and exchanges the positions of the two compartments (as seen

on the left of figure-4). By that it introduces seawater to the high-pressure

line of the modules and relieves pressurized concentrated salt water to the

seawater line. The seawater in the right compartment now flows towards

the membranes and is replaced by another dose of concentrated salt water.

The concentrated salt-water in the left compartment flows away and is

replaced by fresh seawater. The "door" then rotates 180 degrees again. The

operation involves pressurizing seawater and depressurizing concentrated

salt water. Since water is incompressible, these processes do not involve

consumption or waste of energy.

Many practical systems do not look like figure-4 at all, but rather apply

mechanisms of moving pistons and valves to achieve this mode of a

"rotating door" operation [5 - 11]. One company has developed a

continuously rotating high-speed rotor for this purpose [3, 12].

In the limit of a 100% efficient energy recovery device, the externally

powered pump will supply a volume V of seawater equal to the volume V

of the delivered permeate water. The rest of the required seawater comes

from the energy recovery device. The work of desalination, P∙V, is far

lower than in systems that do not apply energy recovery because now V is

the volume of permeate water and not of seawater supply. For the case of

50% water recovery the pressure is P = Ps = 2∙Psea and the desalination

work is:

W = Ps∙V = 2∙Psea∙V (7)

It is half of the work required by a system without energy recovery,

calculated in equation 6. In practical systems an energy recovery efficiency

of 70 - 80% is reported for turbine type devices, and over 90% for rotating

door type devices. The work of desalination is then higher than the ideal

value of equation 7.

The osmotic pressure at the system exit for any water recovery ratio α(α =

output volume of permeate water / input volume of seawater) is Ps = Psea /

(1 - α), and the corresponding work of desalination is:

W =Psea∙V / (1 - α) (8)

The calculation for energy recovery efficiency below 100% is given in the

appedix. Figure-5 shows the minimal desalination energy as a function of

the water recovery ratio for the energy recovery efficiencies 0, 0.85, 0.9,

0.95, and 1.

Figure-5: Dependence of the minimal desalination energy on the water

recovery ratio for the energy recovery efficiencies 0 (1), 0.85 (2), 0.9 (3),

0.95 (4), and 1 (5).

One energy unit in the figure is the theoretical limit Psea∙V, equal to 0.75

kWatt Hour / cubic meter for osmotic pressure of 27 bar. The work of

desalination decreases and approaches the theoretical limit as α is reduced.

The optimal α value is determined by the energy cost compared to the cost

of pre-osmosis water, as discussed in the introduction.

5. Summary of the energy balance

The work consumed by the pump is equal to P∙V where P is the pump

pressure and V is the volume of seawater that it pumps. All this work is

transformed into heat.

Ps is the osmotic pressure of the concentrated salt solution within a

membrane module and ΔP is the over pressure that drives water flow

through the membrane. The pump pressure P is equal to their sum, P = Ps +

ΔP. When modules are connected in series Ps and ΔP change from module

to module but their sum is nearly the same. Ps is lowest at the first module

and it increases with each successive module.

Vpermeate is the volume of desalinated water produced by the process and

Vconcentrate is the volume of concentrated salt solution that returns to the sea.

In systems that do not apply energy recovery devices the overall pumped

volume is equal to the sum V = Vpermeate + Vconcentrate. In systems that apply

energy recovery devices of 100% efficiency the volume pumped by the

pump is equal to the volume of desalinated water, V = Vpermeate.

Table 1 summarizes the energy losses, how they can be reduced, and for

what price.

Loss

Type

Reduce by

The price

Ps∙Vpermeate

Thermodynamic

transformation of

mechanical energy

into heat.

Decrease the

osmotic pressure Ps

by reducing the

water recovery

ratio.

Higher

consumption of

seawater.

ΔP ∙Vpermeate

Dissipate heat of

water flow through

the membrane

Lower water

throughput.

Lower utilization

of the desalination

plant

P∙Vconcentrate

Dissipate heat of

water flow through

the pressure control

valve.

Application of

energy recovery or

pressure exchange

devices.

More equipment

Note:

W = P∙V

Where W is the pump work, P is the pump pressure, and V is the volume of

pumped water.

W = P∙V ∙ 100

for W in Joules (Watt seconds), P in bars, and V in Liters.

Or,

W = P∙V / 36

for W in kWatt hours, P in bars, and V in cubic meters.

For example, the energy required to pump a volume of V = 1 cubic meter

of salt water with osmotic pressure of

Ps = 27 bar, through a semi permeable membrane , is:

W = 27 ∙ 1 / 36 = 0.75 kWatt hour.

6. A spiral membrane module

Figure-6 shows the water flow within a spiral membrane module

Figure-6: Spiral membrane module.

The membrane is shaped into a spirally wound flat sleeve (green)

contained in a high-pressure cylinder. The sleeve is closed at the spiral

sides and outer end, and the inner end is connected to a central pipe.

Pressurized seawater (red arrows) flows in the direction of the cylinder

axis along the external surface of the membrane sleeve. Some water

penetrates through the membrane and leaves the salt behind, thus it turns

into permeate water within the sleeve. Permeate water (blue arrows) flows

within the spiral sleeve towards the central pipe that leads it out of the

module.

Water that penetrates through the membrane leaves behind it a locally

highly concentrated salt at the external surface of the membrane. This

concentrated salt immediately stops any further water flow through the

membrane unless it is removed fast enough by a lateral seawater flow

along the surface.

The membrane sleeve is supported from its inside with a porous spacer that

prevents sleeve collapse by the osmotic pressure. Another porous spacer

surrounds the sleeve and stabilizes the space of seawater flow.

The module manufacturer supplies the testing conditions of the membrane

module. For example, the following data is given for "FILMTEC 8"

Seawater RO Elements" (SW30HR-380) by "DOW" [13].

Module size: Length 1016 mm, diameter 201 mm, diameter of central pipe

29 mm.

Operating pressure: 55.2 bar (800 psi), (max 70 bar (1015 psi).

Max Feed Flow: 14 m3 / hour.

Product water flow rate: 23 m3 / day at 25o C.

Single element recovery (Permeate Flow to Feed Flow): 0.08 (max 0.15 at

lower feed flow).

Salt (NaCl) concentration: 32000 ppm (32 gram / liter).

These numbers give:

Feed water flow of 200 (max 233) liter / minute.

Permeate water flow of 16 liter / minute.

Osmotic pressure of 27 bar (390 psi), calculated by van't Hoff formula

(equation 1).

Flow rate factor (equation 2):

Kf = Frate / (P - Ps) = 16 / (55.2 - 27) = 0.57 (liter / minute) / bar (9)

7. Cyclic flow operation

Sections 2 - 6 describe the reverse osmosis technology of seawater

desalination. The rest of these pages are theoretical considerations and

calculations by the author.

Semi permeable membranes favor operation with continuous water flow

and permanent operating pressure. Flow disturbances and unstable pressure

stress the membranes and increase their wear. However, the continuous

flow mode requires application of energy recovery devices for efficient

operation.

An operation mode of cyclic flow may achieve, in principle, energy

efficiency comparable to continuous flow and there is no need of energy

recovery devices. Therefore, this possibility may not be ignored, even for

a price of modifying the semi permeable membrane or the membrane

module.

The system described in figure-7 includes a low pressure circulating pump

and a two-state valve.

Figure-7: Seawater desalination in a cyclic water flow.

At one state of the valve the salt-water compartment of the module is

closed. The high-pressure pump pumps seawater into the membrane

module and all the water penetrates through the membrane and turns into

permeate water since there is no other water exit. The low-pressure pump

circulates the water in the module at a flow rate required by the module

manufacturer for proper operation. Since there is no exit for the salt it will

accumulate within the module and steadily increase the osmotic pressure.

At a pre determined osmotic pressure the valve revolves and relieves the

pressure within the module.

At this state of the valve the two pumps drive the concentrated salt water

out of the module and replace it with fresh seawater. The valve then

revolves again and the operation is repeated.

Pressure release of concentrated salt water by valve revolution does not

waste energy, similarly to the case of the "rotating door" (section 4), since

water is incompressible and does not accumulate energy. However, there

are other energy-losses that will be considered later.

In cyclic operation the high-pressure pump pumps a volume of seawater

equal to the volume of delivered permeate water. In this respect it is

equivalent to continuous operation with an energy recovery device. Only

here there is no such a device. Efficient continuous operation without

energy recovery is achieved with deep sea deslination by reverse osmosis

[14 - 15].

8. Desalination energy, salinity and cycle time in cyclic flow operation

Since the pressure increases with the salt concentration of salt-water within

the module, the work of pumping water through the membrane will be:

W = ∫p∙dV = ∫(Ps + ΔP) ∙dV (10)

where Ps is the increasing osmotic pressure. The over pressure ΔP is

determined by the flow rate of the high-pressure pump ΔP = Frate / Kf.

The salt concentration cs within the module is given by:

cs = csea∙ (V + V0) / V0 (11)

where csea is the salt concentration of seawater, V0 is the salt-water volume

within the module, and V is the delivered permeate water. Since the

osmotic pressure is proportional to the salt concentration it is given by a

similar equation:

Ps = Psea∙(V + V0) / V0 (12)

The work of desalinating a volume V of permeate water will be (by

inserting equation 12 into equation 10 and integration):

W = Psea∙(0.5∙V2 / V0 + V) + ΔP∙V (13)

Or:

W = (Psea∙(0.5∙V / V0 + 1) + ΔP)∙V (14)

Or:

W = (Psea∙(1 - α / 2) / (1 - α) + ΔP)∙V (15)

Where α = V' / (V' + V0) is the recovery ratio. V' is the volume of permeate

water delivered in one cycle.

In cyclic operation there is no need to connect modules in series. This is an

advantage that leads to higher permeate water throughput.

Salinity, the salt concentration in permeate-water for 1% salt penetration

through a semi permeable membrane, is:

Salinity = 0.01∙ [∫cs∙dV] / V (16)

where cs is the salt concentration of salt water within the module and V is

the volume of permeate water.

cs =(csea / Psea)∙Ps by using equations (10) - (12), therefore:

Salinity = 0.01∙ (csea / Psea)∙ [∫Ps∙dV] / V = 0.01∙ csea ∙ (1 - α / 2) / (1 - α) (17)

The cycle time in cyclic operation depends on the seawater volume within

the module. Using the module dimensions in section 6 its internal volume

is estimated to be 32 liter. Assuming that half of this volume is solid

material, membrane and spacers, and the rest is divided to equal volumes

of salt-water and permeate-water, the salt-water volume will be V0 = 8

liter. This is a coarse estimate.

The permeate-water recovery ratio is α = V' / (V' + V0) , where V' is the

permeate-water delivered per cycle.

V' = Frate ∙ t, where Frate is the permeate-water flow rate and t is the cycle

time. Therefore, the cycle time in seconds is:

t = 60 ∙ (V0 / Frate) ∙ α / (1 - α) (18)

Calculated values of the desalination energy, salinity, cycle time and water

throughput are given in the next section.

The cycle time may be increased by connecting an auxiliary tank in series

to the salt water side of the membrane module. It is also possible to

alternately connect two tanks so that in one tank pressurized water

circulates with increasing salt content while the other tank is flushed with

seawater and vice versa. In this case the membrane module may be loaded

under permanent pressure. Such a system, however, requires the operation

of more valves.

9. Comparison of continuous flow to cyclic flow

The comparison is done for the testing parameter values mentioned in

section 6. Higher values may be applied to practical operation, though they

should not exceed the operating limits.

a. Continuous flow system equipped with 6 modules connected in

series, and with a 100% efficient energy recovery device.

Table 2 summarizes the operation parameters.

Feed Flow

Permeate Flow (liter / min)

Pressure

Feed (L/min)

1st Module

Water Recovery

Energy

Salt

bar

Pump

Recovery

%

V1

α (%)

V

kWh/m3

mg/L

55.2

78

155

7

16

33.5

78

1.53

377

55.2

74

136

8

16

37

74

1.53

385

55.2

50

50

16

16

50

50

1.53

414

45.4

50

150

5.2

10.5

25

50

1.26

360

The table is calculated by the equations:

Ps(1) = Psea (19)

Permeate(i) = Kf ∙ (Ppump - Ps(i)) (20)

Supply(i) = Σ(j = 1 to i) Permeate(j) (21)

Ps(i + 1) = Psea ∙ Feed / (Feed - Supply(i)) (22)

W = Ppump∙V (23)

Salinity = 0.01 ∙ csea ∙ (Σ Permeate(i) ∙ Ps(i) / Psea) / Supply(6) (24)

Ps(i) is the osmotic pressure in the i'th module.

Permeate(i) is the permeate water flow of the i'th module.

Supply(i) is the sum of permeate water flows of the first i modules.

Psea = 27 bar is the osmotic pressure of seawater at 300 K (27o C).

csea = 32 gram / liter is the salinity of seawater.

Ppump is the pump pressure in bars, given in the table.

Kf = 0.57 (liter / minute) / bar is the flow rate factor.

α is the permeate-water recovery ratio.

V is the volume of delivered permeate-water.

V1 is the volume of permeate-water delivered by the first module.

Feed, given in the table, is the water feed flow through a module. Feed is

the same for all modules since they are connected in series.

The work of desalination per 1 m3 of permeate-water is:

W/V = Ppump∙100 (Joule / liter = Watt second / liter) = Ppump∙100 / 3600

(kW hour / m3) (25)

Salinity, the amount of salt in permeate-water is calculated for %1 salt

penetration through semi permeable membrane.

Notes:

I. The calculation is somewhat inaccurate since it assumes uniform salt

concentration within each module while the concentration does change

within each one.

II. The desalination energy calculated in the table assumes 100% energy

recovery. In practical systems, with lower energy recovery, the

desalination energy will be higher than the table values, and the difference

will increase as the water recovery ratio decreases.

III. Comparison of lines 1 - 3 demonstrates the effect of increasing the

water recovery ratio by reducing the overall feed rate of seawater. Higher

ratio saves pre-osmosis seawater but reduces the throughput of permeate-

water.

IV. Comparison of lines 2 - 4 demonstrates the effect of pump pressure on

the system performance. Higher pressure saves pre-osmosis seawater and

increases the throughput of permeate-water, but also increases the energy

of desalination.

b. Cyclic flow system equipped with 6 modules connected in parallel.

In a cyclic system there is no need to connect modules in series. The

modules are connected in parallel and the flow in each module is 1 / 6 of

the overall flow.

Table 3 summarizes the operation parameters for permeate water supply

similar to table 2.

Feed Flow

Permeate Flow (liter / min)

Pressure (bar)

Feed (L/min)

1 module

Recovery

Energy

Salt

cycle

ΔP

Pstart

Pend

Pump

Flush

%

V1

α( )

V

kWh/m3

mg/L

sec

22.8

49.8

63.4

78

155

5.6

12.8

33.5

78

1.57

401

20.5

21.6

48.6

64.5

74

136

6.2

12.3

37

74

1.57

414

25.1

14.6

41.6

68.6

50

50

8.3

8.3

50

50

1.53

480

63.4

14.6

41.6

50.6

50

150

4.2

8.3

25

50

1.28

373

21.1

28.2

55.2

70

96

173

8.3

16

35.7

96

1.74

409

16.6

The table is calculated for the pumping period only. The period required to

flush the concentrated salt water in the module and replace it with fresh

seawater is about 10% of the pumping period. Therefore, the overall cycle

is about 10% longer than the table values, and the flow rates per overall

cycle are about 10% lower than the table values.

The Feed and Water Recovery columns are identical to table 2 (except the

last line), so that the two processes are compared for the same permeate-

water recovery-ratio and throughput.

The table is calculated by the equations:

ΔP = (V / 6) / Kf (26)

Pstart =Psea + ΔP (27)

Pend = Ps +ΔP = P sea / (1 - α) + ΔP (28)

W / V = (Psea∙ (1 - α / 2) / (1 - α) + ΔP) / 36 (29)

ΔP is the over pressure that drives water flow through the membrane.

V is the delivered volume of permeate-water.

V1 is the volume of permeate-water delivered by one module.

Kf = 0.57 (liter / minute) / bar is the flow rate factor.

Pstart is the pressure at the start of the pumping cycle.

Psea = 27 bar is the osmotic pressure of seawater.

Pend is the pressure at the end of the pumping cycle.

α = V' / (V' + V0) is the permeate-water recovery ratio.

V' is the volume of permeate water delivered in one cycle.

V0 = 8 liter is the volume of salt water within a module.

W / V is the desalination energy per 1 m3 of permeate-water (equation 15,

section 8).

The permeate water salinity is calculated by equation 17, section 8.

csea = 32 gram / liter is the salt concentration of seawater.

c. Conclusion

Comparison of the two tables indicates that the energy of desalination in

the two processes, operated at similar permeate-water recovery ratios and

throughputs, is practically the same. However, the two processes have

further energy losses not considered in the tables.

In the continuous flow process there is a full permeate-water flow only at

the first module and the flow drops at each successive module. Therefore

the capacity of permeate-water flow is not fully utilized. Compared to that,

in the equivalent cyclic process the modules are connected in parallel and

the permeate-water flow per module is lower than the permitted limit

value. Alternatively (line 5 of table 3), the cyclic process can operate at the

highest permitted permeate-water flow and achieve higher permeate-water

throughput per module, though, at a cost of a higher desalination energy.

10. Difficulties with cyclic flow operation

Apart from variable pressure operation that might wear or even damage the

membrane, other factors should be considered as well. Any part of the

system that accumulates energy will waste it in the cyclic process.

Consider a possible expansion of the high-pressure cylinder that contains

the membrane unit by the pressurized water in it. If the 201 mm diameter

cylinder expands by one millimeter its inner volume will increase by V =

0.4 liter. The energy accumulated in the cylinder is equal to p∙ V / 2 and it

is lost when the pressure is relieved. Inserting

p = Psea = 27 bar, and V = 0.4 ∙ 10-3 m3, the energy will be E = (27 / 36) ∙

0.4 ∙ 10-3 / 2 = 0.15 ∙ 10-3 kW hour per cycle. If a cycle delivers about 8

liters of permeate-water, the energy loss will be 0.15 ∙ 10-3 ∙ 1000 / 8 = 0.02

kW hour per one m3 of permeate water.

Similar loss might come from pressure squeezing of the permeate-water

spacer within the membrane sleeve, and the loss can be calculated in a

similar way. A more rigid spacer material, and possibly, mechanically pre

squeezing the membrane unit within the cylinder, may reduce the loss.

When a number of modules are connected in parallel to one pump it is

important to have similar water flow in each of them to within tight

tolerance. Otherwise, in some modules the replacement of concentrated

salt water with seawater will not be complete, while in other modules there

will be excessive flow and loss of seawater.

The concentrated salt water within the membrane module is replaced by

fresh seawater when the pressure is relieved. During this time permeate

water will start to flow back through the membrane towards the salt-water.

According to specs, the flow rate of salt-water, in parallel to the

membrane, is at least ten times higher than the flow rate of permeate-water

through the membrane. Therefore, the time of seawater replacement will

be about ten times shorter than the time of permeate-water pumping, and

the permeate water loss will be less than 10%. The back flow of permeate-

water is not completely negative since it automatically flushes the

membrane during each cycle.

11. Utilization of the energy accumulated within concentrated salt

water

Figure-8 presents a scheme for utilizing energy from concentrated salt

water.

Figure-8: Utilizing energy from concentrated salt water.

A low-pressure pump flushes one compartment of a membrane module

with seawater, while a medium-pressure pump pumps concentrated salt

water via the other compartment. The pressurized water drives a turbine

that supplies mechanical energy. The pressure difference that drives water

through the membrane is:

ΔP = Ppump + Psea - Ps (30)

where Ppump is the pump pressure, Psea is the osmotic pressure of seawater

and Ps is the osmotic pressure of the concentrated salt water. If the pressure

difference is negative, ΔP < 0, or, Ppump < Ps - Psea, water will flow from the

seawater side of the membrane towards the concentrated water side. The

volume of water that drives the turbine is then equal to the sum of a

volume V delivered by the pump, and a volume V1 that flows through the

membrane. The work consumed by the pump is Ppump∙ V and the work that

drives the turbine is Ppump∙ (V + V1). Therefore there is a net energy profit

of Ppump∙ (V + V1) - Ppump∙ V = Ppump∙ V1 that comes from dilution of the

concentrated salt water.

The size of a membrane module for utilizing concentrated salt water is

similar to that of a desalination module, and, as seen in figure-8, it has four

different water outlets instead of three. Therefore, addition of salt utilizing

ability to a desalination plant practically means using two different types of

membrane modules and doubling their number. In addition to that the

energy utilizing process consumes more seawater.

Apart from investing in more membrane modules of a type that doesn't

exist yet, the consumption of extra seawater makes energy utilization of

concentrated salt water a non-beneficial process. The same amount of extra

seawater may alternatively be added to a standard desalination system and

save more energy by the reduction of the water recovery ratio. The use of

more seawater in a desalination system reduces the osmotic pressure

within it, and the reduced pressure saves energy consumption in systems

equipped with an energy recovery device, as discussed in section 4.

In summary of this section, there is no benefit in utilizing the (free) energy

accumulated within the concentrated salt water. The same amount of

seawater, required to dilute the concentrated salt, will achieve higher

energy saving by adding it into a standard desalination system, without the

need to invest in extra equipment.

12. Summary and conclusions

A cyclic operated system that does not apply energy recovery devices is

suggested for seawater desalination by reverse osmosis. The desalination

energy, product water salinity and system throughput are comparable to

that of continuous water flow systems that do apply energy recovery

devices.

Appendix: Energy recovery efficiency below 100%

Consider a system operating with a water recovery ratio α and with an

energy recovery device of efficiency Ef.

V = α ∙ Vsea is a volume of permeate water and Vsea is the overall volume

of seawater used to produce it. Out of the volume Vsea, a volume V is

delivered by the pump, and the rest of the seawater volume

Vsea - V = V∙ (1 / α - 1) is delivered by the energy recovery device.

The work done by the pump is P ∙ V where P is the pump pressure. For the

volume V ∙ (1 / α - 1) delivered by the energy recovery device there is a

need to add an energy (1 - Ef) ∙ P ∙ V ∙ (1 / α - 1) to compensate for the

incomplete efficiency. Adding together the work of the pump and the

energy added to the recovery device yields:

W = P ∙ V ∙ [1 + (1 - Ef) ∙ (1 / α - 1)] (31)

For example, the work for efficiency Ef = 0.95 and water recovery ratio α

= 0.1 is

W = P ∙ V ∙ [1 + 0.05 ∙ 9] = P ∙ V ∙ 1.45, compared to P ∙ V, for the

efficiency Ef = 1. Therefore, for a recovery ratio of 0.1, a system with 95%

efficient energy recovery device consumes 45% more energy than a system

without any recovery loss.

The minimal desalination energy for recovery without loss is given by

equation 8, P ∙ V = Psea ∙ V / (1 - α). Therefore the minimal desalination

energy for a system including the energy recovery loss will be:

Wmin = Psea ∙ V ∙ [1 + (1 - Ef) ∙ (1 / α - 1)] / (1 - α) (32)

See:

Osmosis Reverse Osmosis and Osmotic Pressure what they are

Desalination machine

Energy of Seawater Desalination

A Pipe of Fresh Water instead of "Canal of the Seas"

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