Sea water desalination: thermal desalination vs membrane

Transcription

1 Sea water desalination: thermal desalination vs membrane Gazzani Matteo,

2 Water Desalination Desalination: process of removing dissolved solids, such as salts and minerals, from water. Historical background: : sailors produced fresh water from seawater using shipboard distillation during the journeys 1843: Rillieux successfully patented, built and sold multi-effect evaporators 1900: first distillation facilities developed on the Island of Curacao and in the Arabian Peninsula 1980: exponential growth in the construction of desalination facilities Today: > plants worldwide > 80 million cubic meters per day > 300 million people around the world rely on desalinated water Matteo Gazzani 20/11/2015 2

3 Why desalination The increased demand for fresh water by population growth in arid climates and other geographies with limited access to high-quality, low salinity water The per capital increase in demand for fresh water due to industrialization and urbanization that out paces availability of highquality water The effects of climate change, which increase the number of drastic events such as drought Desalination is a viable technique for generating fresh water from water of relatively low quality Matteo Gazzani 20/11/2015 3

4 Water availability in the world Source: Desalination, Kucera, Wiley Source: UN world water development report 2014 Source: Le Monde diplomatique, February 2006 Matteo Gazzani 20/11/2015 4

5 Water standard Source water Total dissolved solids (ppm) Classification Drinking Water < 500 Fresh Fresh < 1000 Fresh Brackish Mildly Brackish Moderately Brackish Heavily Brackish/Seawater Seawater Standard average seawater Seawater Matteo Gazzani 20/11/2015 5

6 Desalination today Overall installed capacity As of 2013: 80 Mm 3 /day Installed capacity by source River water, 9% Pure water, Wastewater 5%, 6% Installed capacity by type of user Tourism, 2% Militay, 1% Irrigation, 2% Other, 1% Power station, 6% Brackish water, 21% seawater, 59% Industry, 26% Municipal, 62% Matteo Gazzani 20/11/2015 6

7 Main technologies for water desalination Installed capacity per technology - Process Main Technology Electrodialysis, 3% Other, 2% Hybrid, 1% Multieffect distillation, 8% Thermalbased separation Multi-stage flash (MSF) Multiple-effect distillation (MED) Desalination Membranes Reverse osmosis Electrodialysis Multieffect Flash, 23% Reverse Osmosis, 63% Hybrid Matteo Gazzani 20/11/2015 7

8 Thermal vs Membrane desalination Source: Desalination, Kucera, Wiley Matteo Gazzani 20/11/2015 8

9 Thermal desalination Vapor pressure curve as function of temperature for different substances Clausius-Clapeyron equation dp dt = Δh v l TΔv v l For ideal gas: dp dt = pδh v l T 2 R NaCl: the vapor pressure is very low in the range of temperature for water desalination Due to the large difference in the vapor pressures (H 2 O vs NaCl), a single stage process is sufficient to separate water and salt Matteo Gazzani 20/11/2015 9

10 Single stage evaporation Three mass flow entering-leaving Entering: feed F (seawater) Exiting: brine B and distillate D Two energy fluxes: Heat supply for the production of the vapor Q H Heat extraction for the condensation of the pure vapor Q C Assumptions: Pure water is evaporated Temperature of the feed equal to the temperature of the evaporation chamber No heat losses Matteo Gazzani 20/11/

11 Single stage evaporation 1. Mass balance m F = m B + m D 2. Salt balance m F x F = m B x B 3. Energy balance Q H = Q C Q H = m D Δh v 100 C Q H m D = Δh v l = 2257 kj/kg Maximum salt content determined by the solubility of the salts. CF: Concentration Factor CF = x B = m F x F m B Recovery Rate φ = m D = CF 1 m F CF Matteo Gazzani 20/11/

12 Single stage evaporation 1. Mediterranen Sea x F = mg/l x B = mg/l CF = 1.7 φ=0.4 40% of the water is recovered as pure, 60% is wasted 2. Arabian Gulf x F = mg/l x B = mg/l CF = 1.4 φ=0.3 30% of the water is recovered as pure, 70% is wasted Concentration factor and yield should be chosen as high as possible Matteo Gazzani 20/11/

13 Single stage evaporation Entering water must be heated up to evaporation temperature Salt concentration change evaporation temperature Q H = Q eva + Q sensible Q H = m D Δh v l + m F c p ( T pre h + T boiling_salt ) Energy demand for a single stage evaporator: Q H = Δh m v l + CF D CF 1 c p( T pre h + T boiling_salt ) Therefore: T sea = 20 C, T eva =100 C, ΔT boiling =1K CF=1.4 Q H m D =3391 kj/kg=942 kwh/ton For production of m 3 /day: Q H =3391*50E6/24/3600/1000 = 1962 MW th Q LHV = Q H 0.9 = kg/s_coal Matteo Gazzani 20/11/

14 Reduction of the energy amount Reduction of the preheating step recovering the heat of condensation (figure below) Reduction of the evaporation energy demand using multiple-effects (Reduction of the evaporation demand using vacuum conditions) Balance of the condenser: m D Δh v l = m sw c p (T outcond T SW ) = (m rej +m feed )c p (T outcond T SW ) m rej m D = Δh v l c p (T outcond T SW ) CF CF 1 Matteo Gazzani 20/11/

15 Reduction of the energy amount Energy demand for a single stage evaporator: Q H = m D Δh v l + m F c p T final + T boil_salt Q H = Δh m v l + CF D CF 1 c p( T final + T boil_salt ) Final temperature difference = 3K: Q H m D = /(1.4-1)*4*(3+1)= 2313 Need of further reduction! kj/kg Matteo Gazzani 20/11/

16 Multi-effect distillation (MED) Multi-effect is required to reduce the energy demand and not to achieve the separation specifications as in distillation. The MED evaporator consists of several consecutive cells maintained at a decreasing level of pressure (and temperature) from the first (hot) to the last (cold). Each cell contains a horizontal tube bundle. The top of the bundle is sprayed with sea water make-up that flows down from tube to tube by gravity Matteo Gazzani 20/11/

17 Multi-stage flash (MSF) Feed flows through the preheaters and enters the first stage where the flash process starts. The flash progresses stage to stage. The vapor generated condenses on each of the pre-heaters. Only one external pre-heater is needed to heat the brine before entering the first stage. Matteo Gazzani 20/11/

18 Multi-stage flash (MSF) Matteo Gazzani 20/11/

19 Multi-stage flash (MSF) energy requirement Q H = m F c p T stage + T terminal + T boil_salt + T losses T stage : stage temperature difference in order to have a driving force for the flash (correspond to the pressure decrease) T terminal : terminal temperature difference between brine and evaporation temperature T boil_salt : because of the salt content T losses : losses The difference in the highest temperature at the inlet of the first stage (T TOP ) and the brine temperature of the final stage is designated as the Overall Temperature Difference T Overall = T TOP T B,N T stage = T Overall N Matteo Gazzani 20/11/

20 Multi-stage flash (MSF) energy requirement Q H = m F c p T stage + T terminal + T boil_salt + T losses The mass of distillate produced can be calculated from the mass flow of boiling liquid, the temperature difference from stage to stage and the heat of evaporation: m F c p T Overall = m D Δh v l m D m F = c p T overall Δh v l Which lead to the specific energy consumption: Q H m D = Δh v l c p T overall c p T stage + T terminal + T boil_salt + T losses = = Δh v l N 1 + N T terminal + T boil_salt + T losses T overall Matteo Gazzani 20/11/

21 MSF with brine recycle Part of the brine flow is recirculated: only the make-up flow has to be pretreated with significant cost savings. Also higher plant flexibility. On the other hands: more complex design, higher pump consumption Matteo Gazzani 20/11/

22 Number of stages and area required The optimal number of stages can be derived by analyzing the total costs given by the energy consumption and by the capital cost. The latter depends on the area required for the heat exchange. Matteo Gazzani 20/11/

23 Heat exchange area In the MSF, the area required for the heat exchange is given by the preheater/condenser where the brine is heated (inside) and the vapor is condensed (outside). We can use the following heat balance equation: 1. Q stage i = m D,i Δh v l,i for the steam condensation 2. Q stage i = US i T ln,i for the heat exchange across the tube It follows: S i = m D,iΔh v l,i U Tln,i S tot = N i=1 m D,i Δh v l,i U T ln,i T ln,i = (T steam i T Bin,i ) (T steam i T Bout,i ) ln (T steam i T Bin,i ) (T steam i T Bout,i ) T stage = ln ( T stage + T terminal ) = Nln T overall T overall N T terminal + 1 T terminal T steam i T brine out,i T stage T steam i T brine in,i Matteo Gazzani 20/11/

24 Heat exchange area We therefore obtain the total area of the MSF: S tot = N i=1 m D,i Δh v l,i U T ln,i = NS i = Nm D,iΔh v l,i U Nln T overall N T terminal + 1 T overall We have two equations for the specific energy consumption and the specific area required to distillate a given amount of water: Q H 1 = Δh m v l + T terminal+ T boil_salt + T losses D N T overall S tot = Δh Nln v l,i m D U T overall N T terminal +1 T overall Matteo Gazzani 20/11/

25 Thermal desalination - MED Matteo Gazzani 20/11/

26 Thermal desalination multi stage desalination Matteo Gazzani 20/11/

27 Thermal desalination Matteo Gazzani 20/11/

28 Thermal desalination Matteo Gazzani 20/11/

29 Membrane desalination: Reverse Osmosis Reverse osmosis is a special case of hyperfiltration. The general transport equation in this case is (see lecture 1 notes): J H2O = P H2O L (p feed p perm ) c δ H2O,feed c H2O,permeate exp v H2O RT Assuming that we are at the point of osmotic equilibrium (the point at which the applied hydrostatic pressure balances the water activity gradient and the water flux across the membrane is zero) we can rewrite the equation introducing the osmotic pressure difference Δπ: At osmotic equilibrium J i = 0, thus: J i = P H2O L δ π Rearranging: c H2O,permeate = c H2O,feed exp v i RT Substituting this in the general flux equation: J H2O = P H2O L δ c H2O,feed c H2O,permeate exp v i π RT = 0 c H2O,feed 1 exp v H2O p feed p perm π RT Matteo Gazzani 20/11/

30 Membrane desalination: Reverse Osmosis J H2O = P H2O L δ c H2O,feed 1 exp v H2O p feed p perm π RT This equation can be simplified in: J H2O = P H2O L δrt c H2O,feedv H2O p π = A p π Where A is the water permeability constant. This equation is reliable for highly selective reverse osmosis membranes at pressures above the osmotic pressure (region where Δπ is constant) A similar simplified expression can be derived for the salt flux: J s = P s L δ c (p feed p perm ) s,feed c s,permeate exp v s RT = P s L δ c s,feed c s,permeate P L s δ c s,feed = Bc s,feed Under these assumptions, the flux of solute is independent of pressure. Matteo Gazzani 20/11/

31 Membrane desalination: Reverse Osmosis Δπ= ρ g Δh Δh p L, c L = 0 p R, c R > c L Δp = (p feed -p perm ) = Δπ no driving force no solvent flux Equilibrium Increased pressure on the right, in order to counterbalance the concentration difference (condition Δµ = 0) p 0, c L = 0 p 0, c R > c L Δp = (p feed -p perm ) = 0 (or < Δπ) driving force towards high conc. solvent flux from c L to c R Osmosis (as in plant roots) The solvent flux will be positive, i.e. from the left to the right side or, more generally, from the pure solvent towards the more concentrated solution Matteo Gazzani 20/11/

32 Membrane desalination: Reverse Osmosis p L, c L = 0 p R >> p L c R > c L Δp = (p feed -p permeate ) > Δp driving force towards low p solvent flux from c R to c L Reverse Osmosis (as in seawater desalination) ΔP - Π is positive, and hence the solvent flux is negative, i.e. from the concentrated solution towards the pure solvent Matteo Gazzani 20/11/

33 Operating parameters in RO Salt passage P S = c s permeate c s feed Salt rejection R S = 1 c s permeate c s feed The rejection coefficient is a measure of the ability of the membrane to separate salt from the feed solution. We can rewrite the equation of the water flux including the salt rejections considering the salt concentration as function of the salt/water flux: c s,permeate = J s ρ J H2O H2O R S = 1 c s permeate c s feed = 1 J s J H2O ρ H2O c s feed = 1 Bc s,feed ρ H2O A p π = 1 Bρ H2O c feed s A p π Matteo Gazzani 20/11/

34 Salt rejections R S = 1 Bρ H2O A p π At a pressure equal to the osmotic pressure of the feed (23 bar), the water flux is zero; thereafter, it increases linearly as the pressure is increased. The salt rejection increases accordingly due to the higher water flow rate. Increasing the salt concentration effectively increases the osmotic pressure, consequently, at a constant feed pressure, the water flux falls with increasing salt concentration. Matteo Gazzani 20/11/

35 Concentration polarization in RO desalination Mathematical relations: 0 exp Pe M 1 E exp Pe Input data: E 0 = 1 - R S = =0.007 (typical) J = 0.4 m 3 /(m 2 day) (typical) d = 100µm (hollow fiber membrane) 10 6 fibers/module, L = 1m J Feed = 2 J Resolution: Surface = 10 6 π = 300 m 2 /module Q feed_tot = 2 J A = 240 m 3 /(day mod) m 3 /(s fiber) v 0 = 0.35m/s D Na+ = m 2 /s δ= D / k =. = 45 µm Pe = M = c surface /C bulk = The salt concentration on the membrane is about 13% higher than expected Matteo Gazzani 20/11/

36 Reverse Osmosis desalination: overall plant Matteo Gazzani 20/11/

37 Reverse Osmosis desalination: overall plant General operating conditions: starting from g/l salt bar operating pressure down to mg/l (US or WHO) Desalination of brackish water (5 10 g/l salt) requires lower operating pressures (14 42 bar) Energy total = Pump_water supply + Pump_pressurization + Pump_product + Pump_transfer + Aux = kwh/m 3 Commercial membrane elements are 20 cm in diameter, 100 cm long. Each element contains about 37 m 2 of active membrane area. In field applications seawater element produces m3/day of permeate. Matteo Gazzani 20/11/

38 Module configuration Single stage, one pass membrane Single stage with concentrate recirculation Recovery 50% Double stage membrane 2 pass membrane Recovery > 50% Higher salt conc in the permeate Recovery 75-80% Double stage membrane with recirculation Very high permeate quality Preserve permeate quality at different temperature Matteo Gazzani 20/11/

39 Overall plant Matteo Gazzani 20/11/

40 Steps to design a RO system 1. The system design information (required product flow rate, expected recovery rate, annual water temperature, water source, application, pretreatment, required product water quality, operating pressure limit, etc.) and the feed water analysis should be thoroughly studied and considered in selection of the RO system design. 2. Selection of Element Type: according to the feed water source, pretreatment and feed water salinity, the type of RO membrane element is selected. 3. The average permeate flux (design flux) is chosen according to the membrane manufacturer data 4. Calculation of Number of Total RO Elements: NRE = Product flow rate (Average flux Element Area) 5. Decision of the Recovery Rate (depends on the feed pressure) 6. Decision of Number of Stages: the number of RO stages defines how many pressure vessels are in series in the RO membrane system. Every stage consists of a certain number of pressure vessels in parallel. The number of stages is a function of the system recovery rate, the number of elements per vessel 7. Decision of Number of RO elements per pressure vessel: In a large-scale plant (> 40 m 3 /h), 6-8 elements per pressure vessel are usually adopted, and in a smaller plant, 3-5 elements per pressure vessel. Matteo Gazzani 20/11/

41 Current status of energy consumption in RO Science 5 August 2011: Vol. 333 no pp Matteo Gazzani 20/11/

42 Design data Standard seawater composition Effect of different RO design Number of Stages K.P. Lee et al. / Journal of Membrane Science 370 (2011) 1 22 Matteo Gazzani 20/11/

43 RO real plant: Ashkelon facility m 3 /day 3 pumps each 5.5MW 16 RO banks which contain 40,000 membrane elements multi-stage RO Matteo Gazzani 20/11/