Thermodynamic performance evaluation of a reverse osmosis and nanofiltration desalination

Thermodynamic Performance Evaluation of a Reverse Osmosis and Nanofiltration Desalination

Y.Aroussy

- M. Nachtane

, D. Saifaoui

-M.Tarfaoui

and M. Rouway

1

Renewable Energy Laboratory and dynamic systems, FSAC, Morocco.

2

Office Chérifien des Phosphates (OCP) Morocco.

3

ENSTA Bretagne, FRE CNRS 3744, IRDL, F-29200 Brest, France.

Abstract

Nanofiltration membranes (NF) and Reverse osmosis have applications in several areas. One of the main applications has been in water treatment for drinking water production as well as wastewater treatment. The Reverse osmosis (RO) and nanofiltration (NF) plants use a semi permeable membrane that allows water to pass but not salts. The thermodynamic analysis of desalination processes along with actual plant operational data can be useful to put various desalination techniques in perspective, and the best analytical tool to do this is the second law analysis. Thus the analysis in this article starts from a basic separation process which is the main objective of all desalination process, and conducts a comprehensive analysis using actual operation data from major desalination processes.

In this work we made a comparative study between two desalination systems with application of thermodynamic balance sheets more precisely the First and Second thermodynamic laws programmed on the EES and ROSA software.

Keywords: Nanofiltration, Reverse osmosis, thermodynamic analysis

INTRODUCTION

The nanofiltration (NF) membrane is a type of pressure-driven membrane with properties in between reverse osmosis (RO) and ultrafiltration (UF) membranes. NF offers several advantages such as low operation pressure, high flux, high retention of multivalent anion salts and an organic molecular above 300, relatively low investment and low operation and maintenance costs. Because of these advantages, the applications of NF worldwide have increased [1 ]. The history of NF dates back to the 1970s when RO membranes with a reasonable water flux operating at relatively low pressures were developed. Hence, the high pressures traditionally used in RO resulted in a considerable energy cost. Thus, membranes with lower rejections of dissolved components, but with higher water permeability, would be a great improvement for separation technology. Such low-pressure RO membranes became known as NF membranes [2]. By the second half of the 1980s, NF had become established, and the first applications were reported [3, 4].

Reverse osmosis is a very appealing process of saline water desalination, and is becoming a leading method in the

commercial desalination industry. A number of factors have affected the advance of the reverse osmosis process. The foremost is the lower energy consumption of the reverse osmosis plants compared to distillation plants. The reverse osmosis is a semi-permeable membrane process in which pure water from pressurized saline water is separated from the dissolved salts by forcing it to flow through the membrane.

Generally speaking, it is the transport of the preferential material through the membrane against the osmotic pressure.

Therefore, the reverse osmosis does not deal with heating or phase change which is the chief Feature of distillation and freezing desalination processes. Several theories for the mechanism of the mass transport through membranes have been proposed. The simplest and the most obvious theory is that the membrane functions as a molecular sieve. Sodium and chloride ions are slightly larger than the water molecules.

However, the ions would be able to pass through the membrane rather well due to the small difference in size

NF AS PRETREATMENT FOR DESALINATION The introduction of NF as a pretreatment will lead to a breakthrough in the application of RO and MSF because it has implications for the desalination process itself, and not only on quantity of the feed water [5]. MF can remove suspended solids and lower the silt density index (SDI) while in UF, not only suspended solids and large bacteria are retained, but also (dissolved) macromolecules, colloids and small bacteria.

Redondo [6] strongly preferred MF or UF as a pretreatment over conventional pretreatment to treat difficult effluent water. Bou-Hamad et al. [7] tested three pretreatment methods (conventional, a beachwell seawater intake system, and MF) prior to RO.

According to the results of SDI, COD and BOD removal and the total unit cost, the beachwell seawater intake and MF systems were identified as promising techniques to replace conventional

Pretreatments. However, all of the pretreatments mentioned

did not lower the TDS value. Several studies have also been

carried out on the use of UF membrane as pretreatment [6, 7].

RO DESALINATION SYSTEM

The membrane desalting systems include reverse osmosis (RO), where seawater is pressurized against a semipermeable membrane that allows almost pure water to permeate. and not salt. It is mechanically driven by pumping energy. The main problem of the SWRO is the membrane’s fouling that required extensive pretreatment to avoid or decrease that fouling. The feed water pressure (Pf) to the membranes is in the range of 60–80 bar (depending on the feed water salinity, and membranes characteristics). The brine leaving the membranes is at about 2 or 3 bar only less than Pf. The energy of this brine can be recovered by energy recovery device (ERD), (e.g. reversed centrifugal pump working as a turbine, Pelton wheel, and pressure exchanger). The specific consumed energy is in the range of 4–6 kW h/m3 depending on the feed water salinity and the type of ERD. The share of RO membrane process is rapidly increasing with the time compared to distillation processes as it consumes much less energy, and thus less cost, see Fig. 1.

Figure 1: The trend of increasing use of RO compared to distillation processes.

GEOMETRIC AND MATHEMATICAL FORMULATION Desalination technology can be regarded as a system whose input is the saline water. This water requires an amount of energy to separate it into brine (often called the concentrate) and in pure water. This amount of energy depends on the

characteristics of the saline water and on the technology used [8].

Figure 2: Desalination system modeling

The membrane desalting systems include reverse osmosis (RO), where seawater is pressurized against a semipermeable membrane that allows almost pure water to permeate, and not salt. It is mechanically driven by pumping energy. The main problem of the SWRO is the membrane’s fouling that required extensive pretreatment to avoid or decrease that fouling. The feed water pressure (Pf) to the membranes is in the range of 60–80 bar (depending on the feed water salinity, and membranes characteristics). The brine leaving the membranes is at about 2 or 3 bars only less than Pf. The energy of this brine can be recovered by energy recovery device (ERD), (e.g. reversed centrifugal pump working as a turbine, Pelton

- -M5 = M6 = M7 = M: the salt water feed rate;

- -M10 = M11: the flow of rejected brine;

- -M9=M10 = M: e the flow of water produced;

- r1 = M9 / ṁ: permeate recovery rate;

- r2 = M10 / ṁ: the rejection rate of the brine;

- -r3 = M / m: the mass ratio between the pure water flow rate in -the Rankin

Wheel and pressure exchanger). The specifically consumed energy is in the range of 4–6 kW h/m3 depending on the feed water salinity and the type of ERD.

During this study will be considered the following assumptions [9]:

- -The change in kinetic energy and potential energy flows are negligible;

- -The fluid at the inlet of the hydraulic pump must be a compressed liquid which confirms the selection of the heat supplied to the boiler;

- -The temperature T5 and pressure P5 of saline water at the entrance of the system are considered the reference conditions, and are considered also as ambient conditions;

- -The losses have been neglected.

- -m1 = m2 = m3 = m4 = M: the flow rate of fluid circulating in the Rankin cycle; cycle and the saline feed water flow rate;

- -The temperature of the sea water is assumed to equal to 15 ° C;

- -For lack of data at the output of the reverse osmosis system, they were considered the following values:

T9=T10=20°CandT11=15°

NUMERICAL RESULTS

EES uses a solving technique to solve a number of equations and an equal number of unknown variables [10]. In case of an explicit solution of a poorly earthed matrix, the system tends to be rather unstable and can therefore not solve the initial guess values and is too far from the solution. This means that for complex solutions, changes need to be done in small steps updating new guess values after each iteration. If this is not done, the program has some calculation instabilities.

Properties of pure water are readily available in tabulated or computerized forms. We will use water properties evaluated by the built-in functions of the EES software at mixture temperature and pressure.

Figure 3: Efficiency of Rankin cycle vs thermal heat of the RO at different pressure and flow rate of steam

The figure 3 present a parametric study of the reverse osmosis system, by setting the pressure P

of outlet condenser, and also the mass flow rate of steam M entering turbine. The efficiency η of system increase exponentially with thermal heat of evaporator. The green curve increase from 1,87 % to 89,58 %, red one from 4,07 % to 74,36 % and the blue from

3,02 % to 51,65 %, as observation the form of all curves have the same shape, because of linearity of equation system.

Figure 4: Efficiency of Rankin cycle vs pressure of condensation of the Reverse Osmosis at different thermal combustion and flow rate of steam

The figure 4 describes the efficiency vs pressure by fixing the thermal heat of evaporator q

three times, each curve have a minimum and maximum extremity of variation. The green curve rises from 31,29 % to 80,44 %, red one from 8,045 % to 90,79 % and the blue one from 3,025 % to 80,9 %.

Figure 5: Efficiency of Rankin cycle vs mass flow rate of the Reverse Osmosis at different pressure and thermal heat

the variation of efficiency vs flow mass rate in three curves

have the same shape, by taking pressure and heat energy

constant, with a little growth in the beginning of each curve,

then it became constant. The green, red and blue curves have a

maximum efficiency of 44,76 %, 30,18 % and 8,308 %

respectively.

COMPARISON BETWEEN RO AND NF

Study of this pilot was calculated from the Rosa software that facilitates us the design of the membrane and the pilot installation diagram.

This pilot scheme is a system called "di-stage" .The principle of this system is tried again to put more concentration output that has been filtered by the system and have a circulation of permeate a module another to decrease the concentration of permeate to the system output. The advantage of this system is to have a concentration of permeate that meets the drinking water quality following Moroccan standard of drinking water.

We also studied a nanofiltration membrane, to get an idea of the mass transfer mechanisms involved during use; we worked with solutions containing solutes (brackish water).

Tab 1: Permeability values of membranes tested.

Membranes Permeability of the membrane by the Rosa Software (L/h.m2.bar)

NF90 5.33

NE90 4.11

BW30 1.6

BLF 3.8

The results show that the total permeates flux increases with the pressure applied to each membrane tested. In the pressure range studied, the order of total permeates flux for the three membranes are as follows:

NF90 > NE90 > BLF > BW30

a. Variation of pore radius vs λ transfer coefficient

It is observed in the previous scheme the linear increase of the pore radius size (RP) with (1/ λ) for different NaCl solute radius sizes.

- Note that if 1/λ> 1 when the pore radius is greater than the radius of the solute (Rs> Rp) implies a transfer of the solute. This is against the desired objective

- if 1/λ <1 while the pore radius is less than the radius of the solute (Rs<Rp) implies a solute lock.

b. Rejection rate variation with RP.

Note in the above diagram, the rejection rate decreased according to the pore radius size for different sizes of solutes rays. This result satisfies the importance of the small pore radius of filtration.

In this context a comparative study was made on the two reverse osmosis membranes (BW30, BLF) and nanofiltration (NE90 and NF90). Initially, a Casablanca brackish water salinity calculated is performed for each of the tested membranes.

This result showed the usefulness of the nanofiltration

membrane, which is permeable with a TDS rate consistent

with Moroccan standards of water compared to that of the

reverse osmosis, which are denser modules and therefore

leaves not spend more TDS. Thereby still make a

pretreatment. The following figure shows the results of the 5

modules tested by nanofiltration membranes following the pilot scheme.

CONCLUSIONS

The recovery of the sensible heat of Sulfuric workshop and now dissipated thermal power plant in seawater can be used to desalinate heat a considerable amount of seawater.

The investment for recovering heat from circulating sulphuric acid enhances profits and reduces dependence on external distributors of high quality water.

 The production of drinking water through seawater desalination system is a viable and effective solution to address the problem of lack of drinking water in some regions of the Kingdom and also cope with the pollution water in others.

 The economics of energy at the exchanger by reusing waste energy to preheat the seawater before entering the reverse osmosis system.

In this context to avoid the problem of Cadence must study the feasibility of integrating of renewable energy as a source of free and why not the recovery of kinetic energy using turbines to generate electricity.

REFERENCE

[1] X. Lu, X. Bian and L. Shi, Preparation and characterization of NF composite membrane, J.Membr.Sci., 210 (2002) 3-11.

[2] B.Van der Bruggen and C. Vandecasteele, Removal of pollutants from surface water and groundwater by nanofiltration: overview of possible applications in the drinking water industry, Environ. Poll., 122 (2003) 435--445.

[3] W.J., Conlon and S.A McClellan, Membrane softening:

treatment process comes of age, J. AWWA,81 (1989) 47-51.

[4] J. Schaep, B. Van der Bruggen, S. Uytterhoeven, R.

Croux, C. Vandecasteele, D. Wilms, E. Van Houtte and F. Vanlerberghe, Removal of hardness from

groundwater by nanofiltration, Desalination,119 (1998) 295-302.

[5] B. Van der Bruggen and C. Vandecasteele, Distillation vs. membrane filtration: overview of process evolutions in seawater desalination, Desalination, 143 (2002) 207- 218.

[6] S. Bou-Hamad, M. Abdel-Jawad, S. Ebrahim, M. AI- Mansour and A. A1-Hijji, Performance evaluation of three different pretreatment systems for seawater reverse osmosis technique, Desalination, 110 (1997) 85-92.

[7] AI-Ahmad and F. Adbul Aleem, Scale formation nd fouling problems effect on the performance of MSF and RO desalination,plants in Saudi Arabia, Desalination, 93 (1993) 287-310.

[8] Y. Aroussy, M. Nachtane, D.Saifaoui, M.Tarfaoui, Numerical investigation of a reverse osmosis desalination system with cogeneration and renewable energy integration , International Journal of Scientific

& Engineering Research Volume 7, Issue 7,July-2016 , ISSN 2229-5518

[9] Bouzayani, N., Galanis, N., & Orfi, J. (2009).

Thermodynamic analysis of combined electric power generation and water desalination plants. Applied Thermal Engineering, 29(4), 624-633.

[10] Klein, S. A., & Alvarado, F. L. (1992). EES:

Engineering equation solver for the Microsoft

Windows operating system. F-Chart software.