R&D Project 9 – Rate of saltwater evaporation model on post reverse-osmosis desalination brine

The Desalination Brine Project

Both alt NaCl and water are of great importance. They are used on large scale for variety of industrial and culinary purposes. So if we have saline water that is obtained from sea then it is processed through different stages to obtain pure water and NaCl. The different stages to obtain pure water is post reverse osmosis of desalinated brine and after that dissolved salts are passed through salt evaporation model to obtain NaCl. 

It is a treatment to remove brine from the saline water since saline water contains high concentration of many dissolved salts. Therefore Desalination process is done on it to remove these largely concentrated dissolved salts so that we are left with only high concentration of NaCl in water that is brine. 

Brine is a high concentration of NaCl in water. 

Then after getting this brine treatment is done on it to get water and salt separated from it. Technology treatment done here is reverse osmosis. 

Reverse osmosis 

Reverse osmosis is opposite of osmosis where osmosis is just like diffusion i.e. movement of molecules from high concentration region to low concentration region it is natural tendency of a liquid the liquid what we have in this case is desalinated brine. Therefore osmosis allows the movement of solvent molecules i.e. salt from brine to pass through the semi permeable membrane and get through the pure water on the other side. This would continue until an equal concentration of salt on both sides. But in reverse osmosis hydrostatic pressure greater than osmotic pressure is applied to reverse the tendency of flow. This flow allows the flow of water from brine through the semi permeable membrane and get through the pure water leaving the all dissolved salts behind. 

Post reverse osmosis 

After the water obtained through reverse osmosis, still it has acidic nature and a large number of total dissolved solvents therefore this water is not suitable for various uses. For example drinking, irrigation and several other purposes. If we require water for drinking then it should have NaCl < 450mg/L and hardness 6-10oD. 

Then the post treatment done in this case is Post Reverse osmosis so that after this reverse osmosis we have better quality water that is free from any odor and also boron, magnesium and further NaCl concentration is reduced after post reverse osmosis. 

Salt Water Evaporation: 

After getting this post reverse osmosis desalinated brine it is passed through salt water evaporation to get salts. This salt water evaporation method could be different. For example natural solar evaporation technique. When water is evaporated from the salt it results in precipitation and ultimately crystallization of salts which greatly reduces the evaporation rate. Therefore it is very important to determine and calculate these factors which affect salt water evaporation rate. 

Factors affecting salt evaporation rate: 

Following are the factors that affect water vapor evaporation: 

External Factors: 

  • Saturated vapor pressure above its surface 
  • Humidity
  • Wind 
  • Temperature 
  • Radiation 

Internal Factor: 

  • Precipitation of salt crystals 

Saltwater Evaporation Model

The model used for salt water evaporation is natural solar evaporation technique and since we have to incorporate the effects of these external factors and modelling is done on these basis since it would help in managing and controlling the effects of these factors over salt evaporation rate. 

Wind 

Wind affects salt water evaporation .To calculate this parameter win sensor will be used. The wind speed sensor 1733 sensor used in this case.it will calculate wind speed directly in ms-1. Also the advantage of using this sensor, it is hazardous free. 

Radiation 

Solar radiation is another important aspect which affects the rate of evaporation. Solar radiations can be calculated using number of day of year, duration of sunshine during the day, albedo of surface and site height from sea level. 

To calculate all these parameters: 

Duration of sunshine during the day can be calculated using GY 30 light intensity sensor. 

Site height from sea level can be obtained from altimeter app on Google Play. 

Albedo of surface can be found from SRA01 albedo meter. 

Humidity 

Humidity can be calculated using capacitive type humidity sensor DHT22 since it requires low power and no additional extra components to work. 

Temperature 

DHT22 will also be used to measure temperature sensor. 

Saturated vapor pressure 

Saturated vapor pressure is determined by modified Penman equation which is as follows: 

Where: E= evaporation 
λ=latent heat of vaporization (MJKg-1) 
Δ= Vapor pressure-temperature curve gradient. 
ϒ=psychometric constant (location specific) 
Rn=net solar radiation (MJm-2day-1) 
F (u) =function of wind speed (m/s) 
e s=saturation vapor pressure (function of temperature and salinity) 
e s =0.6108awexp 17.27T17.27 T where: T=mean temperature 237.3+T 237.3 +Taw=-0.0011m2-0.0319m+1, m is salinity of brine in mole per liter) 
e= ambient water vapor pressure (function of relative humidity =H r e s /100 where H r=relative humidity) 

Since vapor pressure also incorporate the effects of Temperature, Humidity, Solar Radiation, Wind speed which have already been computed. 

Latent heat of evaporation can be calculated using mean temperature. 

Δ will be computed using saturation vapor pressure and temperature. For computation of Saturation vapor pressure, salinity would be used which would be found using DFR0300 sensor to measure conductivity of brine and DS18B20 to measure temperature of solution. 

Model for saltwater evaporation of post reverse-osmosis desalination brine based on a variety of external factors: 

Now all the parameters are found. These parameter values would be displayed on LCD to monitor the values and further monitoring is done so that to control the effects of these parameters on salt water evaporation rate. 

Specific Milestone Objectives

The project comprises 3 milestones, where: 

Milestone 1 is divided into 3 milestones: 

Milestone 1A – Research, identify and document existing reverse osmosis technologies 

Milestone 1B – Evaluate scalability and applicability of the identified reverse osmosis technologies in a developing-world environment 

Milestone 1C – Investigate the ability to accept intermittent power on each of the identified technologies 

Milestone 2 is divided into 3 milestones: 

Milestone 2A – Laboratory testing of chemical trace elements within desalinated brine 

Milestone 2B – Investigate the effect of sulphur isotopic variations and fractional crystallization consequences during seawater evaporation yields within shallow land-based salt-traps 

Milestone 2C – Develop a model for scaling of salt and other trace element harvesting from salt evaporation pans 

Milestone 3 is divided into 3 milestones: 

Milestone 3A – Investigate salt evaporation techniques for commercial harvesting of salt and trace elements from brine 

Milestone 3B – Investigate techniques for constructing evaporation pans 

Milestone 3C – Investigate thermal absorption of solar irradiation on high salt content pans 

Analysis of the effect of Erosion on Seabed 

THE SEABED SEDIMENT EROSION RESEARCH PROJECT 

The erosion of sediment is the detachment of particles or aggregations from earth surface. Erosion occurs when sediment is subject to the natural force of flowing water, wind, gravity and ice, etc. Human activities can also accelerate erosion condition such as agriculture, urbanization, mining operations, infrastructure construction. In marine environments, offshore facilities may cause a series of local influence on accelerating seabed sediment erosion. The presence of structures gives rise to an increase of both flow velocity and turbulent intensity in the vicinity of the structure. established pipelines are susceptible to scour and the generation of free spans which develop from a small scour hole and then extend to a prolonged unsupported gap beneath the pipeline. As a result, the pipeline is likely to sag or fail due to the loss of support. A lot of reported and unreported pipeline failures occurred since the placement on the seabed. Sediment is a loose material broken down from rock mass or shell remnants by processes of weathering and erosion and is subsequently transported or deposited by the hydrodynamic action of fluids. The natural seabed sediments are normally classified as non-cohesive sediments and cohesive sediments based on electrochemical bond between sediment particles. While in the field situation, sediments are widely found as a mixture of both types of particles in the form of homogeneous or laminated bed. 

The threshold of motion of sediment is regarded to reach when the flow velocity increases to a critical value that, if increases even slightly will bring the grain into motion. considered this phenomenon as critical stage of initiation of sediment transport, and surface and uppermost bed layer can have either an “instantaneous” or a “statistical” (average) meaning. Further increase of flow velocity will subsequently lead to mass erosion. For non-cohesive sediments, the resistance to erosion is mainly provided by gravitational friction force which is predominantly influenced by grain particle size and density. To parameterize the critical criterion, Shields (1936) has originally provided a curve based on the experimental data in the form of a dimensionless Shields parameter θcr versus grain Reynolds number Re*. The Shields curve has supported by many experimental data on threshold of motion of non-cohesive sands. Moreover, some efforts have also been contributed to modify the Shields diagram to a practical application, e.g. a dimensionless grain diameter D* was proposed by Van Rijn (1993) and then modified by Soulsby (1997) to resolve the iteration of friction velocity u* which appear on both axes. The validation of modified method to evaluate the initial motion of sediment has been justified by a large set of experimental data on non- cohesive sediment, as shown in Figure 3 below. 

Figure 3 – Shields curve proposed by Soulsby with reference of previous data set (After Soulsby, 1997) 

Table 1 – Typical soil classification on the west coast of south Africa is as follows: 

Sediment can be characterized by particle size distribution (PSD), composition of particles in different size categories, and plasticity. 

A closed-loop Mini O-tube (MOT) facility comprises of a closed circular channel of water driven by a turbine, with an enlarged rectangular test section in which experiments were conducted. The MOT can measure the erosion threshold and erosion rate of non- cohesive and cohesive sediment. 

Figure 4 -Hydraulic regimes in oscillatory flow (after Jonsson, 1996 and Kamphuis, 1975) 

For the case of oscillatory flow, the amplitude bed shear stress τw is applied to estimate the stability of bed sediment, which is obtained from the ‘quadratic stress law. 

The influence of combined waves and currents is of great importance in the transport of seabed sediments. In shallow waters near the coasts, progressive waves may encounter relatively strong currents and consequently vary the distribution of velocities and bed shear stress 

The time-invariant eddy viscosity model assumes that the eddy viscosity ε varies linearly with the distance z from the sheared boundary (Grant and Madsen, 1979). Then the maximum combined shear stress τwc associated with wave and current motion is defined by: 

The erosion of cohesive sediment deposit is complex due to turbulent flow boundary layer near the bed, the particle size distribution, mineralogy and organic content of bed material; the effects of the chemistry and electrochemical bond of flocs or aggregates. Therefore, the threshold bed shear stress τcr has been correlated to several measurable soil properties, e.g. particle size distribution, plasticity, bulk density, etc 

Several erosion experiments have demonstrated that the erosion characteristics can change dramatically when small amounts of mud are added to a sand bed. The effects of fraction of fine materials with d50<62.5μm on erosion characteristics were tested by Dunn (1959) . It was found that the capacity of cohesive sediment to resist erosion increases with clay content (Pcl) and plasticity index (PI). 

Since field measurements, Allersma (1988) empirically correlated the dry density of bed ρd with mass fraction of sand component (Psa) 

The correlation of threshold erosion of sediment with Plasticity Index PI has been considered by Smerdon and Beasley (1959), and an expression was given as 

The parameter that defines bed levels (nautical and hydrodynamic depths) is essentially related to density. The nautical bed has been specified at a bulk density between 1.1 and 1.2 g/cm3, based on the acceptable levels of ship hull resistance in muddy waters. Although the hydrodynamic bed depends on bed resistance, it is defined in a very approximate way by bed density. 

Figure 5 -Definitions of bed level and profiles of erosion shear strength vs. bulk density (after Mehta, et al., 1989) 

The coefficient A is correlated with grain Reynolds number Re*=u*d/ν and the results are classified by different particle size category. Fine grained sediment including silt and fine sand are mainly located in the hydrodynamic smooth flow, while for median sand and coarse sand, the flow type in the boundary layer is transitional flow. About the relationship between A and threshold shear stress, it is found that the coefficient A generally decreases with the threshold shear stress τcr for all the test results. It means that sample which has larger threshold shear strength is even harder to be eroded where ρd is dry density of sediment, αc is the consolidation coefficient which ranges from 0 (fresh deposits) to 2.4 (old deposits). 

Specific Milestone Objectives

The project comprises 3 milestones, where: 

Milestone 1 is divided into 3 milestones: 

Milestone 1A – Calculate and correlate threshold sheer stress with soil properties including particle size, mud content, plasticity index and bulk density 

Milestone 1B – Establishment of relationship between threshold sheer stress and median particle size as well as understanding fundamental erosion characteristics of seabed sediments on which pipelines and other pumped-storage structures are laid 

Milestone 1C – Examine relationship between the initial motion of the selected sand/clay mixtures and the threshold of motion as a result of Oscillatory flow 

Milestone 2 is divided into 3 milestones: 

Milestone 2A – Identify sample collection locations as well as Identifying sand to clay ratios based on field samples on the West coast of South Africa 

Milestone 2B – Field sample collections 

Milestone 2C – Laboratory testing of collected samples to identify sample composition 

Milestone 3 is divided into 3 milestones: 

Milestone 3A – Testing impacts of unidirectional currents and wave induced oscillatory flow and combined waves and currents in mini O-tube testing facility 

Milestone 3B – Create multi-variant regression equation based on particle size, mud content, grain Reynolds number and bed sheer stress 

Milestone 3C – Develop sediment-specific coefficient of erosion using soil and fluid parameters, based on test result