Osmotic Pressure

8 Key excerpts on "Osmotic Pressure"

• 

  eBook - PDF

  Separation Processes in the Food and Biotechnology Industries

  Principles and Applications

  • A S Grandison, M J Lewis(Authors)
  • 1996(Publication Date)
  • Woodhead Publishing

    (Publisher)

  On the other hand. proteins and other macromol- ecules do not produce high Osmotic Pressures. There will only be small increases during their concentration as well as small differences in Osmotic Pressure between the feed and permeate in ultrafiltration. Values for Osmotic Pressures are not easy to find in the literature and a selection of values is given in Table 3.1. A further complication with foods and other biological systems is their complexity, with not just one but many components. In reverse osmosis the applied pressure must exceed the Osmotic Pressure, and the driving-force term in reverse osmosis is normally the difference between the applied pressure and the Osmotic Pressure. It could be that Osmotic Pressure is one of the factors that limits the extent of concentration. One suggested experimental method for measuring Osmotic Pressure is to determine the pressure that would give zero flux, by extrapolation. In ultrafiltration and microfiltration, there is little Osmotic Pressure differ- ence over the membrane as the low molecular weight components are almost freely permeating (see equation (3.8)). Table 3.1. Osmotic Pressures of some solutions Solution Osmotic Pressure (bar) Sugar beet Tomato paste Apple juice Citrus juice Sucrose Coffee extract Sea-water Milk Whey Lactose 20 Brix 33 Brix 15 Brix 10 Brix 34 Brix 44 Brix 28% TS 3.5% salt 15.0% salt 1% w/v 34.1 69.0 20.4 14.8 69.0 69.0 34.0 23.2 138.0 6.9 6.9 3.7 Compiled from data in Cheryan (1986) and Lewis (1982). Some equations for Osmotic Pressure are given by Cheryan (1986). As the membrane pore size increases, the membrane becomes permeable to low molecular weight solutes in the feed; even the transport mechanisms are likely to change. Lower pressure driving forces are required as Osmotic Pressure differences between the feed and permeate are reduced. However, molecules of a larger molecular weight are still rejected by the membrane.

  Sign up to read

  Learn more about book
• 

  eBook - PDF

  Water Renovation and Reuse

  • Hillel Shuval(Author)
  • 2012(Publication Date)
  • Academic Press

    (Publisher)

  If one n o w imposes a n applied pressure greater t h a n the Osmotic Pressure at osmotic equilibrium o n the brine solution, w a t e r will pass through the m e m b r a n e from the brine to the freshwater reservoir. This process is called reverse osmosis (or hyperfiltration). This discussion is schematically presented in Fig. 4. 6. Pressure-Driven Membrane Processes 141 (a) OSMOSIS W A T ER F L O WS I N TO B R I NE (b) OSMOTIC EQUILIBRIUM N O W A T ER F L OW (c) R E V E R S E OSMOSIS W A T ER F L O WS F R OM B R I NE APPLIED PRESSURE OSMOTIC I :=.·.'·.· PRE S SURE I :•'··..: Water transport Water transport Sem ipermeable membrane Fig. 4. Principles of normal and reverse osmosis. Thus, to attain reasonable w a t e r fluxes [J ~ 10 gal/(ft 2 , day)], the brine solution has to be pressurized well above the equilibrium Osmotic Pressure as estimated from the brine concentration. In practice, reverse osmosis systems are usually operated from about 4 to 20 times the equilibrium Osmotic Pressure π 0 , for sea a n d brackish water, respectively. For seawater, the operating pres-sure is of the order of 100 atm, while for brackish w a t e r a n d w a s t e w a t e r it is about 40 atm. Typical Osmotic Pressures can be obtained for dilute solutions from Van't Hoff's equation: = ^-RT v m (6) w h e r e η is the n u m b e r of moles of solute, v m the m o l a r v o l u m e of water, R the universal gas constant, a n d Τ the absolute temperature. For m o r e concentrated solutions, the Osmotic Pressure coefficient φ is used to modify Eq. (6): » —RT (7) The Osmotic Pressure coefficients of m a n y pure solutions are tabulated a n d available in the literature (Sourirajan, 1970). A useful rule of t h u m b for estimat-ing the Osmotic Pressure of a natural water is 1 psi (pounds per square inch)/ (100 mg/liter).

  Sign up to read

  Learn more about book
• 

  eBook - PDF

  Efficient Desalination by Reverse Osmosis

  A guide to RO practice

  • Stewart Burn, Stephen Gray(Authors)
  • 2015(Publication Date)
  • IWA Publishing

    (Publisher)

  In membrane technology, flux is defined as the ratio of the permeate flow and surface area of the membrane. It is expressed as: J Q A A dV dt w = = 1 ⋅ (2.2) Flux ( J ) is the permeate flow through a membrane surface area ( Q w / A ) (m 3 /m 2 · h or L/m 2 · h). Then, J A dV dt P K w = = − ( ) 1 ⋅ Δ Δ ⋅ π (2.3) EXAMPLE 2.1: Flux and permeate flow of spiral wound membrane elements Assuming: Pressure is 10 bar; permeability of membrane is: a ) 1 L/m 2 /h/bar and b ) 5 L/m 2 /h/bar; membrane surface area is 35 m 2 ; and no Osmotic Pressure. Question: Calculate the flux and permeate flow under the given conditions. Answer: a ) 10 bar × 1 L/m 2 /h/bar = 10 L/m 2 · h then 10 L/m 2 /h × 35 m 2 = 350 L/h b ) 10 bar × 5 L/m 2 /h/bar = 50 L/m 2 · h then 50 L/m 2 /h × 35 m 2 = 1,750 L/h 2.2 Osmotic Pressure For Reverse osmosis to work, a pressure greater than the Osmotic Pressure needs to be applied to the seawater side of the membrane. Osmotic Pressure is the pressure The process of reverse osmosis 7 which needs to be applied to a solution to prevent the inward flow of water across a semipermeable membrane (Voet et al. 2001). Osmotic Pressure is also defined as the minimum pressure needed to cancel out osmosis. Figure 2.2 illustrate the process of reverse osmosis in which the applied pressure needs to overcome the Osmotic Pressure head to force water to pass through the semipermeable membrane. Figure 2.2 Illustration of reverse osmosis principle. The Osmotic Pressure reduces the effect of hydraulic pressure, as a consequence, the effective pressure or net driving pressure is equal to the hydraulic pressure minus the Osmotic Pressure. NDP P = − Δ Δ π (2.4) where: Δ P = differential hydraulic pressure (pressure feed – pressure permeate) (bar) Δ π = difference Osmotic Pressure (Osmotic Pressure feed – Osmotic Pressure permeate) (bar) In membrane filtration, the Osmotic Pressure hinders the water flow as illustrated in Figure 2.3.

  Sign up to read

  Learn more about book
• 

  eBook - PDF

  Stantec's Water Treatment

  Principles and Design

  • John C. Crittenden, R. Rhodes Trussell, David W. Hand, Kerry J. Howe, George Tchobanoglous(Authors)
  • 2022(Publication Date)
  • Wiley

    (Publisher)

  17-8b: (1) the concentration gradient and (2) the pressure gradient. These opposing forces are exploited in RO. Consider a new experiment using the apparatus on Fig. 17-8‚ modified so that it is possible to exert an external force on the left side‚ as shown on Fig. 17-8c. Applying a force equivalent to the Osmotic Pressure places the system in thermodynamic equilibrium‚ and no water flows. Applying a force in excess of the Osmotic Pressure places the system in nonequilibrium‚ with a Concentration, g/L Concentration, g/L 0 20 40 60 80 100 120 0 0 20 20 40 40 60 60 80 80 100 100 120 1.10 1.05 1.00 0.95 0.90 0.85 0.80 Osmotic coefficient (dimensionless) Osmotic Pressure, bar NaCl (Eq. 17-7, φ = 1) NaCl (measured) Seawater (measured) Seawater NaCl (a) (b) Figure 17-9 (a) Osmotic Pressure of aqueous solutions of sodium chloride� (b) Osmotic coefficients for sodium chloride and seawater (osmotic coefficient for seawater with the van’t Hoff equation is based on a concentration of NaCl equal to the TDS of the seawater)� 1357 17-5 Reverse Osmosis Fundamentals pressure gradient exceeding the chemical potential gradient. Liquid would flow from left to right‚ that is‚ from the concentrated solution to the dilute solution. The process of causing water to flow from a concentrated solu- tion to a dilute solution across a semipermeable membrane by the appli- cation of an external pressure in excess of the Osmotic Pressure is called reverse osmosis. Models have been developed to describe the flux of water and solutes through RO membranes using two basic approaches. The first approach relies on fundamental thermodynamics and does not depend on a physical description of the membrane. The other approach uses physical and chemical descriptions of the membrane and feed solution‚ such as mem- brane thickness and porosity.

  Sign up to read

  Learn more about book
• 

  eBook - ePub

  Membrane Technology for Osmotic Power Generation by Pressure Retarded Osmosis

  • Tai-Shung Chung, Chun Feng Wan, Tai-Shung Chung, Chun Feng Wan(Authors)
  • 2020(Publication Date)
  • CRC Press

    (Publisher)

  3 to desalinate seawater (UNWWAP 2014). There are growing opportunities for the joint development of water and energy technologies that maximize co-benefits and minimize negative trade-offs. A wide range of opportunities exists to coproduce energy and water and to harvest the benefits of synergies. This book aims to explore such opportunity where wastewater streams can be used to generate clean energy, which in turn can be used to compensate the energy consumption of desalination processes.

  1.2 Osmotic Energy

  Nature has the greatest mechanism for plants to take up water from soils and transport it across the cell membranes by forward osmosis (FO) (McElrone et al. 2013). In the past decade, FO has received increasing attentions in various water, energy, and food applications. In an FO process, as shown in Figure 1.1 (a) , a semipermeable membrane is placed between two solutions with different salinities – a draw solution with a higher salinity and a feed solution with a lower salinity. Water spontaneously diffuses across the membrane, driven by the chemical potential difference that arises from the salinity difference. The Osmotic Pressure difference (∆π ) between the feed and draw solutions is defined as the hydraulic pressure that has to be applied on the draw solution to stop the spontaneous water flow (Awad et al. 2019, Chung et al. 2012a, 2012b, Shaffer et al. 2015). The term “Osmotic Pressure (π )” implies the potential of a solution to generate power. Osmotic Pressure can be calculated as follows (Van’t Hoff 1901):

  π = i c R T

  (1)

  where c is the molar concentration, i is the van’t Hoff factor, R is the universal gas constant, and T

  Sign up to read

  Learn more about book
• 

  eBook - PDF

  Engineering Technology, Engineering Education and Engineering Management

  Proceedings of the 2014 International Conference on Engineering Technology, Engineering Education and Engineering Management (ETEEEM 2014), Hong Kong, 15-16 November 2014

  • Deyao Tan(Author)
  • 2015(Publication Date)
  • CRC Press

    (Publisher)

  When the water osmosis in the two solutions is in a dynamic balanced state, m D 0 = . The osmotic pres-sure of the two solutions is 1 π and 2 π respectively, and the differential pressure D p equals the differential Osmotic Pressure D 2 1 π π π = − . According to equa-tion (7), there is D D ( ) , 1 2 c dc V p V c c p T m m ∫ µ π ∂ ∂ = − = − (8) Based on the result of equation (8), (7) can be expressed as Dm D D ( ) V p m π = − (9) Therefore, according to whether formula (9) is positive or negative, the direction of water molecule osmosis can be determined. When the differential pressure of a solution is lower than the osmotic pres-sure difference, the water molecule will pass through the semipermeable membrane from the weak solution to the strong solution, which is positive osmosis; oth-erwise, the water molecule will pass through the sem-ipermeable membrane from the strong solution to the weak solution, which is negative osmosis. 3 EXPERIMENTAL RESEARCH Sea water desalinization requires the separation of water molecules from the sea water. Analysis on the chemical potential and Osmotic Pressure of sea water reveals that to separate the water molecule, the Osmotic Pressure difference must be smaller than the differential pressure of the solution. To achieve this, a pressure larger than the Osmotic Pressure must be applied on one side of the sea water, which means the chemical potential of sea water will be higher than that of freshwater to push the water flow into the freshwater through the semipermeable membrane. 387 That is the so-called reverse osmosis, which is the most commonly used method in sea water desalini-zation. At present, this method is applied in most of the sea water desalinization projects, and during the process, there is no water phase change, with no need for heating, so the power consumption is quite small. 3.1 Experimental system A tubular reverse osmosis membrane module is used in the sea water desalinization experiment.

  Sign up to read

  Learn more about book
• 

  eBook - PDF

  Plant Physiology Theory and Practice

  • Emea, A(Authors)
  • 2018(Publication Date)
  • Agri Horti Press

    (Publisher)

  Cannot be resold/distributed. Osmosis and Osmotic Pressure 111 nor of temperature. It does imply however, an equalization of diffusion pressures and an examination of the circumstances is necessary to determine the exact mechanism by which they have been brought into equilibrium in any given system. Fig. Diagrams illustrating discussion of the vapour pressure theory of osmosis ANALOGIES BETWEEN Osmotic Pressure AND GAS PRESSURE The important fact is that while the Osmotic Pressures of weight molar solutions of non-electrolytes approximate the same value, which is in the neighbourhood of vant Hoffs theoretical value of 22.4 atmospheres, the values for weight molar solutions of electrolytes are much higher. Vant Hoff showed that a remarkable analogy exists between the Osmotic Pressures of dilute solutions and the pressures exerted by gases. Vant Hoff pointed out that the Osmotic Pressure of a dilute solution is equal to the pressure that the solute alone would exert were it present as a gas at the same temperature in the same volume as that occupied by the solution. A dilute solution was considered by vant Hoff to be one in which the volume of solute was small in proportion to the volume of solvent. For example, if we could imagine the complete and instantaneous removal of all of the water in a sucrose solution without any change in the spatial relations of the solute molecules, the residual sucrose molecules would be in a gaseous state. According to vant Hoff the Osmotic Pressure of this solution would be equal to the gas pressure which would be exerted by such a hypothetical sucrose gas. This ebook is exclusively for this university only. Cannot be resold/distributed. 112 Plant Physiology : Theory and Practice Furthermore, he showed that the Osmotic Pressures of dilute solutions obey laws analogous to those which describe the relations of gases to variations in temperature volume and pressure.

  Sign up to read

  Learn more about book
• 

  Available until 4 Dec |Learn more

  Handbook of Membrane Separations

  Chemical, Pharmaceutical, Food, and Biotechnological Applications, Second Edition

  • Anil Kumar Pabby, Syed S.H. Rizvi, Ana Maria Sastre Requena, Anil Kumar Pabby, Syed S.H. Rizvi, Ana Maria Sastre Requena, Anil K. Pabby, Ana-Maria Sastre(Authors)
  • 2015(Publication Date)
  • CRC Press

    (Publisher)

  3.3 REVERSE OSMOSIS TRANSPORT 3.3.1 S INGLE C OMPONENT S YSTEM Currently, the mainstream of RO membrane transport theory is the solution–diffusion model [50]. According to the model, mass transfer occurs in three steps: absorption to the membrane, diffusion through the membrane, and desorption from the mem-brane. The chemical potential gradient from the feed side of the membrane to the permeate side of the membrane is the driv-ing force for the mass transfer. When the difference in hydro-static pressure is greater than the difference in Osmotic Pressure between the upstream and downstream sides of the membrane, a chemical potential difference of water across the membrane drives water against the natural direction of water flow. Thus, the water transport through the membrane can be described by N L p A = -( ) Δ Δ π (3.1) where N A is the water flux through the membrane (subscript A denotes water) L is the water permeability coefficient Δ p is the transmembrane pressure difference Δπ is the difference in Osmotic Pressure between the upstream and downstream sides of the membrane The Osmotic Pressure can be given by an equation similar to the van’t Hoff equation: π = i ϕ CRT (3.2) where i is the dissociation parameter, which is the number of ions produced by the dissociation of the salt ϕ represents a correction factor C is the salt concentration R is the universal gas constant T is the absolute temperature The permeability coefficient can be given by L DSV RTl = (3.3) where D is the diffusivity of water in the membrane S is the water solubility in the membrane V is the partial molar volume of water l is the thickness of the skin layer of the membrane The Osmotic Pressure of seawater is typically 2300–2600 kPa and can be as high as 3500 kPa [51]. Osmotic Pressures of brackish water are much smaller than those of seawater. Corresponding to the concentration range of 2000–5000 mg/L, the Osmotic Pressure ranges from 100 to 300 kPa [51].

  Sign up to read

  Learn more about book