Seawater Design By Osmosis

Seawater design by osmosis

Introduction

The man has sailed the seas and oceans for centuries, and one of his fears on Altamar has been to die of thirst. Navigators seemed a contradiction to suffer the lack of water being surrounded by it, so they have always wondered how to adapt it to their needs. Throughout history, different techniques have been developed to treat brackish waters and seawater, but their application in the supply of drinking water in maritime vessels was not entirely effective until recently.

This made the transport of drinking water on board essential, which is a series of problems. Formerly, the ships were loaded with drinking water but it suffered deterioration within a few days due to the lack of adequate containers. This problem was solved with the development of containers that retained water better, such as mud amphorae or wood boots covered with special materials. 

Currently, more modern tanks are used, normally integrated into the ship, but that suppose an important load on board and occupy a lot of space. In recent years, desalination techniques have become more effective, culminating recently with the discovery of the reverse osmosis process. This made its application viable aboard a ship, which allows us to navigate with more autonomy, security and comfort.

Project purpose

The objective of this project is to design a seawater desalination installation that supplies drinking water aboard a fishing ship.

First, the different desalination techniques of today will be raised and the inverse osmosis process will be defined, as well as the different elements involved. It will be necessary to estimate the amount of drinking water demanded by the ship and analyze the different factors that influence its production. Through the equations that define the process, the necessary calculations will be carried out to dimension the equipment that makes up the installation. To conclude the project, its environmental impact and its economic viability will be analyzed.

Justification

Long maritime navigations make the drinking water supply aboard a ship essential. Depending on the circumstances, people need a certain daily amount of water to maintain an optimal level of health. In addition, some height fishing vessel.

This project is motivated to alleviate the problems of carrying the water stored on board. Thanks to the advance of desalination technologies, especially the inverse osmosis process, we can obtain drinking water supply on a ship without the need for traditional tanks. This option provides more autonomy and savings in space and weight. In addition, navigation becomes more comfortable and safe thanks to the unlimited healthy fresh water supply.

Developing

Drinking water

The surface of our planet is covered around 70% by water, but most of us is not suitable for human consumption. 97% are in the seas and oceans and it is salt water. Only 3% is fresh water and is mostly in deep aquifers, in the moisture of the soil and trapped in the form of ice in the polar caps. About 1% of fresh water is accessible in rivers, lakes and other surface water bodies, although not all is suitable for human consumption.

Standards to define fresh water can vary in each country;However, the standard used by the World Health Organization defines it as an aqueous solution that contains less than 500 ppm parts per million of the T.S.D. Total dissolved solids. Drinking water can be considered as fresh water with the following qualities: toilet, colorless, tasteless and free of polluting bacteria.

According to WHO, drinking water is defined to anyone who is suitable for human consumption. This includes not only water to drink, but to cook, to see, wash clothes, hospital uses, recreational uses and cleanliness. In the European Union, regulations 98/83/EU establishes maximum and minimum values for mineral content and different ions, in addition to pathogenic germs. As defined in the Guides for the Quality of Potable Water of the WHO, drinking water will not represent any significant health risk during its life consumption.

Desalination, or desalination, is the process by which sea water and brackish waters become water suitable for human consumption and industrial or agricultural use, reducing the total solids dissolved in it. Seawater contains about 35.000 ppm parts per million total solids of dissolved, t.D.S. and the brackish waters of 5.000 to 10.000 ppm of t.D.S. According to the standard used by the World Health Organization, fresh water will be considered to be less than 500 ppm of T.D.S.

Historical evolution of desalination

Desalination is a technology with a wide history. His first references date from the era of ancient Greece where geniuses such as Miletus or Aristotle addressed issues related to the nature of seawater and the possibility of its desalination by filtration through the Earth. In the Middle Ages, several authors dealt with the issue of desalination, such as Dr. John Gadden who described four methods for the desalination of seawater in his treaty on medicine. 

In the Modern Age, due to geographical discoveries and trade expansion, desalination experienced an important advance, especially in the supply of ships. With the arrival of the Industrial Revolution, the first saline water distillation towers in Great Britain and EE was built and built. UU. 

Although its industrial use had a slow development, World War II ships already had desalination facilities. In the 60s, the reverse osmosis system is discovered, a technique that revolutionizes the entire process of desalination for its effectiveness and high performance, so it is the most used in the world today today.

Desalination processes

Desalination can be carried out in different ways, the most used worldwide and inverse osmosis being used worldwide. There are several variants of the distillation process, but all are based on the same operation principle. A certain amount of salt water evaporates to condense later in fresh water. 

The heat extracted in condensation is used to heat the water to distill. The process is carried out in several stages, in each of which the pressure and temperature is reduced, achieving concentration of the resulting brine. The most used processes in distillation are Multieteapa instant evaporation, multi -effect evaporation and mechanical steam compression.

The inverse osmosis process arises from natural osmosis. The phenomenon of natural osmosis occurs when, through a semipermeable membrane, water flows from the solution of less salinity to another of greater concentration of salts in order to reach the balance.

Inverse osmosis is to invest this natural process, applying a pressure on the side of the solution with more salts, making water flow from the most concentrated solution to the most diluted solution. It was not until the 80s in which the first membrane capable of working with those concentrations appeared. From then on the method has evolved and becoming more efficient thanks to the improvement of membranes, the application of pretreatments and the development of equipment with greater performance.

Table 1 shows the energy required by each process to obtain 1m3 of fresh water. Distillation processes m.S.F. and m.AND.D. They need heat energy to heat and evaporate salt water and electricity to drive the pumps and compressor that the process carries out. The inverse osmosis process does not require heat energy to be carried out;Only electric power to drive pumps that move salt water to membranes.

Salt water composition

Seawater is a water -based solution characterized by the concentration of dissolved mineral salts that it contains. The proportions of the components of seawater are almost constant anywhere in the world, although together the concentration of all of them, known as salinity, is enormously variable according to what places. 

In areas with a more intense evaporation of tropical latitudes or in warm places and with little water renewal such as the Mediterranean Sea, salinity is greater than in more open places such as some oceans or river moods areas. Salinity can be estimated easily by measuring a single parameter. Some such as electrical conductivity or the water refractive index depend directly on the concentration of dissolved salts.

Seawater pollution

About 80% of the pollution of the marine environment comes from the continents. Much of this pollution is the result of the runoff of pollutants that produce non -specific sources such as motor vehicles or waste from industrial plants that open the sea through river contributions. You also have to take into account the pollution from the air of the atmosphere or maritime vessels, which especially affects deep water. The pollutants that go to the sea to the sea are the following:

• Hydrocarbons.
• Organic matter, nutrient substances and microorganisms provided by urban and livestock wastewater.
• Heavy metals.
• Organohalogenated compounds.
• Solid substances.

The various phases of the desalination process are very conditioned by contribution water pollution, so it is essential to know the quality of the area previously. The coastal strip is the one that supports the greatest marine pollution, an aspect to take into account in the water treatment aboard a low fishing ship.

On a height fishing vessel, as in the case of this project, the collection will be made on the high seas, away from the coastal areas where the majority of pollutants are. There the presence of hydrocarbons is the main one to consider to treat water properly aboard the ship.

Desalinated Water Quality

The quality of desalinated water depends on the use that will be given. In the case of a fishing vessel, it will be destined mainly to human consumption and the production of ice for fish conservation. You could accept a lower quality water for ice or other services such as laundry, kitchen, etc. But other desalination systems, apart from the reverse osmosis or fresh water generators, would not be viable aboard a ship. The WHO also recommends that, as far as possible, a single supply system to be drink and the one destined for other purposes will be installed.

The phenomenon of osmosis

Osmosis is a physical phenomenon that occurs when two solutions with different concentrations are put in contact through a semipermeable membrane, which allows the passage of solvent through it preventing it from the solute. It consists of the movement of the solvent through the membrane, from the solution with the lower concentration of solute to the most concentrated area. This behavior is a simple diffusion, without energy expenditure, which tends to match the concentration of both solutions. This phenomenon of osmosis occurs naturally and has a crucial role in the cellular metabolism of living beings.

The diffusion of water stops when the pressure generated by the increase in the counter -recruited level that tends to make the water spread and pass through the membrane. This difference in heights that exists between the two compartments when reaching balance is called osmotic pressure. The osmotics of pure water is considered null by agreement.

Definition of the industrial process of reverse osmosis

Considering the previous system, suppose that a pressure higher than the osmotics is applied on the highest concentration side. In that case, as shown in Figure 4, it can be verified that the diffusion of the solvent occurs in reverse following the solute without being able to cross the membrane. This phenomenon is known as reverse osmosis.

To develop the industrial process of reverse osmosis, it is necessary for a pump to send the solution to treat one of the sides of the semipermeable membrane, maintaining a high pressure permanently. Both the solution that crosses the membrane and the one that is rejected are evacuated from its compartments continuously. The rejection will present a high concentration of dissolved substances, the one of the permeated product being low. The percentage of solution that is permeated is controlled by a regulation valve located in the rejection pipe.

Inverse osmosis modules

In the industry it is necessary that the membranes be placed in such a way that support the different work pressures. Inverse osmosis modules are a membrane grouping, with a certain configuration, which constitute the elementary production unit. The main objectives pursued in the manufacture of these modules are:

• Get maximum membranes performance.
• Get a system as compact as possible.
• Minimize the phenomena of membranes polarization.
• Facilitate the replacement of deteriorated membranes.
• Improve the cleaning of dirty membranes.

Frequently the objectives pursued are opposed, so the manufacturer will adopt the technical solution that makes its product more competitive. Inverse osmosis membranes are designed in such a way that by integrating them into the modules, water flow is maximized, minimizing load losses. “Concentration polarization” should be avoided, which is the accumulation of salts on the surface of the membrane, dirtying them and causing low flows and passing of salts.

 Plate modules

They were the first to be used in reverse osmosis technology. The membranes are arranged on both sides of a plate by which the permeated produced is collected. They have a lower membrane packaging density, resulting at a higher price per module and makes them more expensive compared to more modern configurations. They are still using in the food industry.

Tubular modules

In this configuration the membranes are located inside a PVC tube that supports the pressure. The membrane is fed inside, obtaining the permeated by the outside of the same. The tube collects that permeated and deposits it in the container in which it is located. Its packaging density is also low, more expensive the price of the module.

Hollow fiber modules

They are a set of very thin fibers encapsulated in a pressure box. This is fed by the center of one of its ends and distributes the flow radially. In this way the permeated penetrates the walls of the fibers and is collected at the opposite end of the pressure box. They have the highest packaging of all configurations, which provides greater production by occupied water. However, the possibilities of membrane obstruction are very high because the passage through the fine fibers is more difficult, thus being able to reduce the performance of the same. They are used in water treatments with low suspended solid content.

Spiral modules

They are the most used type in the current inverse osmosis facilities. This is a configuration in which flat membranes are spirally rolled around a central tube. Each module consists of a semipermeable membrane folded in half so that the active layer is outside. Between the two halves a fabric provided with tiny distribution channels is placed to collect the permeated that crosses the membrane. The flow is driven in the central tube and goes outside by one of its ends. Food water circulates axially, in parallel to the central one. The rejection follows this same direction until it leaves the tube at the other end.

The modules in this provision have both the feeds and exits of permeated and rejection connected to each of the corresponding general pipes, as shown in Figure 11. This type of assembly is used to produce a "N" flow sometimes higher than supplied by a module, being "n" the number of modules installed in parallel.

 The load losses is the same in all modules, that is, that the flow of contribution of each will depend on its degree of clogging and the permeated flow that produces. For that reason it is important that the modules that work in paralle.

Rejection recirculation

In this group you can exceed the maximum 50% recovery percentage established for a single stage. It uses recirculating all or part of the rejection by mixing it with the feed water, as we can see in Figure 14. With this assembly, high conversions and concentrations can be achieved in the rejection much higher than those of the contribution solution. This alternative presents a specific consumption of energy and a salinity of the permeated major in installing two stages with serial rejection. For this reason it is only used when you seek to concentrate the contribution solution.

Membranes dirty

In an inverse osmosis installation it is essential that the solution that reaches the membranes has previously suffered an adequate pre-treatment. It is necessary to eliminate all substances that can be deposited on the membranes by carrying a decrease in productivity and an increase in the passage of salts.

The main sources of dirty are salts or large particle deposit. There is an index called SDI that estimates the degree of dirty membranes due to contribution water pollution. This value is determined by measuring the flow drop in a 47mm diameter membrane and with a 0.45 -micro pore size.

conclusion

The installation will be carried out in the stern zone of the ship’s machine room. The process begins with the aspiration of seawater from the shots that the ship has in the stern zone. The water that enters goes through an initial thick filter to avoid the entry of large objects, such as shells or algae. 

A centrifugal pump pumps it to the sand filter, where pretreatment begins in order to eliminate suspended solids that can damage the membranes. To ensure clean contribution water in the membranes, it is necessary to reduce the 5. To do this, continuing the pretreatment, the water will be subjected to microfiltration using two fine cartridge filters.

After pretreatment, high pressure is pumped at the entrance of the reverse osmosis modules, where two currents are produced: the permeated and the rejection. The rejection will be returned to the sea and the permeated is sent to a tank where it will be stored and chlorinated. A pressure group will be available that will send the permeated drinking water to the different service points.