Sustainable Technologies for Greener Environment

Over the years, all parts of a commercial refrigerator, such as the compressor, heat exchangers, refrigerant, and packaging, have been improved considerably due to the extensive research and development efforts carried out by academia and industry. However, the achieved and anticipated improvement in conventional refrigeration technology are incremental since this technology is already nearing its fundamentals limit of energy efficiency is described is ‘magnetic refrigeration’ which is an evolving cooling technology. The word ‘green’ designates more than a colour. It is a way of life, one that is becoming more and more common throughout the world. An interesting topic on ‘sustainable technologies for a greener world’ details about what each technology is and how it achieves green goals. Recently, conventional chillers using absorption technology consume energy for hot water generator but absorption chillers carry no energy saving. With the aim of providing a single point solution for this dual purpose application, a product is launched but can provide simultaneous chilling and heating using its vapour absorption technology with 40% saving in heating energy. Using energy efficiency and managing customer energy use has become an integral and valuable exercise. The reason for this is green technology helps to sustain life on earth. This not only applies to humans but to plants, animals and the rest of the ecosystem. Energy prices and consumption will always be on an upward trajectory. In fact, energy costs have steadily risen over last decade and are expected to carry on doing so as consumption grows.

Keywords: Energy Saving; Energy Efficiency, Sustainable Technologies; Heat Exchangers; Refrigerant; Future Prospective

This section describes the different methods and techniques for providing energy for heating and cooling systems. It also, covers the optimisation and improvement of the operation conditions of the heat cycles and the performance of the ground source heat pump systems (GSHPs).

With the improvement of people’s living standards and the development of economies, heat pumps have become widely used for air conditioning. The driver to this was that environmental problems associated with the use of refrigeration equipment, the ozone layer depletion and global warming are increasingly becoming the main concerns in developed and developing countries alike. With development and enlargement of the cities in cold regions, the conventional heating methods can severely pollute the environment. In order to clean the cities, the governments drew many measures to restrict citizen heating by burning coal and oil and encourage them to use electric or gas-burning heating. New approaches are being studied and solar-assisted reversible absorption heat pump for small power applications using water-ammonia is under development [1].

An air-source heat pump is convenient to use and so it is a better method for electric heating. The ambient temperature in winter is comparatively high in most regions, so heat pumps with high efficiency can satisfy their heating requirement. On the other hand, a conventional heat pump is unable to meet the heating requirement in severely cold regions anyway, because its heating capacity decreases rapidly when ambient temperature is below -10˚C. According to the weather data in cold regions, the air-source heat pump for heating applications must operate for long times with high efficiency and reliability when ambient temperature is as low as -15˚C. Hence, much researches and developments have been conducted to enable heat pumps to operate steadily with high efficiency and reliability in low temperature environments [2]. Similarly, the packaged heat pump with variable frequency scroll compressor was developed to realise high temperature air supply and high capacity even under the low ambient temperature of 10–20˚C [3-4]. Such a heat pump systems can be conveniently used for heating in cold regions. However, the importance of targeting the low capacity range is clear if one has in mind that the air conditioning units below 10 kW cooling account for more than 90% of the total number of units installed in the EU [5].

Conventional heating or cooling systems require energy from limited resources, e.g., electricity and natural gas, which have become increasingly more expensive and are at times subjects to shortages. Much attention has been given to sources subject to sources of energy that exist as natural phenomena. Such energy includes geothermal energy, solar energy, tidal energy, and wind generated energy. While all of these energy sources have advantages and disadvantages, geothermal energy, i.e., energy derived from the earth or ground, has been considered by many as the most reliable, readily available, and most easily tapped of the natural phenomena.

Ground source based geothermal systems have been used with heat pumps or air handling units to satisfy building HVAC (heating, ventilation, and air conditioning) loads. These systems are favoured because geothermal systems are environmentally friendly and have low greenhouse emissions.

The installation and operation of a geothermal system of the present invention may be affected by various factors. These factors include, but are not limited to, the field size, the hydrology of the site the thermal conductivity and thermal diffusivity of the rock formation, the number of wells, the distribution pattern of the wells, the drilled depth of each well, and the building load profiles. Undersized field installations require higher duty cycles, which may result in more extreme water temperatures and lower HVAC performance in certain cases. Oversized field designs, on the other hand, require more wells, pumps and field plumbing and therefore will be more expensive, albeit adequate to handle almost any load circumstances. The detailed knowledge of the field rock (e.g., porosity, permeability, thermal diffusivity, heat capacity, or other aquifer parameters) may facilitate the determination of the appropriate drilling depth for each well, as well as the number and position of such wells needed at that site. Some of this information may be obtained during the drilling operation.

The earth-energy systems, EESs, have two parts; a circuit of underground piping outside the house, and a heat pump unit inside the house. And unlike the air-source heat pump, where one heat exchanger (and frequently the compressor) is located outside, the entire GSHP unit for the EES is located inside the house.

The outdoor piping system can be either an open system or closed loop. An open system takes advantage of the heat retained in an underground body of water. The water is drawn up through a well directly to the heat exchanger, where its heat is extracted. The water is discharged either to an aboveground body of water, such as a stream or pond, or back to the underground water body through a separate well. Closed-loop systems, on the other hand, collect heat from the ground by means of a continuous loop of piping buried underground. An antifreeze solution (or refrigerant in the case of a direct expansion ‘DX’ earth-energy system), which has been chilled by the heat pump’s refrigeration system to several degrees colder than the outside soil, circulates through the piping, absorbing heat from the surrounding soil.

In some EESs, a heat exchanger, sometimes called a “desuperheater”, takes heat from the hot refrigerant after it leaves the compressor. Water from the home’s water heater is pumped through a coil ahead of the condenser coil, in order that some of the heat that would have been dissipated at the condenser is used to heat water. Excess heat is always available in the cooling mode, and is also available in the heating mode during mild weather when the heat pump is above the balance point and not working to full capacity. Other EESs heat domestic hot water (DHW) on demand: the whole machine switches to heating DHW when it is required.

Hot water heating is easy with EESs because the compressor is located inside. Because EESs have relatively constant heating capacity, they generally have many more hours of surplus heating capacity than required for space heating. In fact, there are sources of energy all around in the form of stored solar energy, which even if they have a low temperature, can provide the surroundings with enough energy to heat the soil, bedrock and ground water as a heat source for domestic dwellings as shown in Figure 1, for example. Some emphasis has recently been put on the utilisation of the ambient energy from ground source and other renewable energy sources in order to stimulate alternative energy sources for heating and cooling of buildings. Exploitation of renewable energy sources and particularly ground heat in buildings can significantly contribute towards reducing dependency on fossil fuels.

The collector liquid (cooling medium) is pumped up from the borehole in tubing and passed to the heat pump. Another fluid, a refrigerant, circulates in the heat pump in a closed system with the most important characteristic of having a low boiling point. When the refrigerant reaches the evaporator, which has received energy from the borehole, and the refrigerant evaporates. The vapour is fed to a compressor where it is compressed. This results in a high increase in temperature. The warm refrigerant is fed to the condenser, which is positioned in the boiler water. Here the refrigerant gives off its energy to the boiler water, so that its temperature drops and the refrigerant changes state from gas to liquid. The refrigerant then goes via filters to an expansion valve, where the pressure and temperature are further reduced. The refrigerant has now completed its circuit and is once more fed into the evaporator where it is evaporated yet again due to the effect of the energy that the collector has carried from the energy source (Figure 2).

Efficiencies of the GSHP systems are much greater than conventional air-source heat pump systems. A higher COP (coefficient of performance) can be achieved by a GSHP because the source/sink earth temperature is relatively constant compared to air temperatures. Additionally, heat is absorbed and rejected through water, which is a more desirable heat transfer medium because of its relatively high heat capacity. The GSHP systems rely on the fact that, under normal geothermal gradients of about 0.5˚F/100 ft (30˚C/km), the earth temperature is roughly constant in a zone extending from about 20 ft (6.1 m) deep to about 150 ft (45.7 m) deep. This constant temperature interval within the earth is the result of a complex interaction of heat fluxes from above (the sun and the atmosphere) and from below (the earth interior). As a result, the temperature of this interval within the earth is approximately equal to the average annual air temperature [6]. Above this zone (less than about 20 feet (6.1 m) deep), the earth temperature is a damped version of the air temperature at the earth’s surface. Below this zone (greater than about 150 ft (45.7 m) deep), the earth temperature begins to rise according to the natural geothermal gradient. The storage concept is based on a modular design that will facilitate active control and optimisation of thermal input/output, and it can be adapted for simultaneous heating and cooling often needed in large service and institutional buildings [7]. Loading of the core is done by diverting warm and cold air from the heat pump through the core during periods with excess capacity compared to the current need of the building [8-10]. The cool section of the core can also be loaded directly with air during the night, especially in spring and fall when nights are cold and days may be warm.

The installation can additionally be fitted with fan convectors, for example, in order to allow connections for free cooling (Figure 3). To avoid condensation, pipes and other cold surfaces must be insulated with diffusion proof material. Where the cooling demand is high, fan convectors with drip tray and drain connection are needed.

The pressure (ps) is a function of how rapidly vapour can be removed through suction or formed through pressure. At equilibrium, the rate at which vapour is formed (determined by Q) equals the rate at which it is removed. Therefore, both the heat transfer rate into the liquid (Q) and the vapour removal rate (suction pump capacity) determines the pressure and hence Tsat(s) (Figure 4). This is governed by the following set of equations.

Both hfg and vg depend on the saturation temperature (or pressure) as assumed in Figure 5, which describes the relationship represented by eqn. 4.

The RHS of the Figure 6 is the ‘converse’ of the LHS, and constitutes a heat pump. Heat is ‘pumped’ from the LHS to the RHS. The main difference is that the vapour, after compression, will almost certainly be superheated and must cool to Tsat(c) before condensing will occur. The same reasoning (in converse) applies to the RHS as previously applied to LHS. Obviously, with the above system, the entire refrigerant would eventually end up on the RHS, and the heat pumping (& refrigeration) effect would cease.

Clearly, to ensure that the system can operate continuously liquid refrigerant needs to be fed from the RHS back to the LHS. This can be achieved by simply allowing it to flow back under its natural pressure difference. In this way a continuous closed circuit refrigeration (Or heat pump) system is obtained (Figure 7).

Control of the liquid flow rate is needed to ensure that it equals the vapour formation rate, and an appropriate balance of liquid quantities in the evaporator and condenser is maintained. When the liquid passes through the expansion valve it experiences a sudden drop in pressure, which causes instantaneous boiling (known as flashing). Vapour is formed using the liquid’s sensible heat, which causes the liquid to drop in temperature to Tsat(s). A saturated liquid/vapour mixture will enter the evaporator. Figure 8 explains this cycle in practice.

The system balance requires the overall work done to be equivalent to the net energy used by the system. Hence,

For operation as a refrigerator, a measure of system performance is the amount of heat absorbed per unit work supplied to drive the system. This is known as the Coefficient of Performance [11].

For operation as a heat pump, a measure of system performance is the amount of heat delivered per unit work supplied to drive the system. This is known as the Coefficient of Performance [12].

It follows that (for the same system):

The term “vapour compression refrigeration” is somewhat of a misnomer, it would be more accurately described as ‘vapour suction refrigeration’. Vapour compression is used to reclaim the refrigerant and is more aptly applied to heat pumps. Vapour compression refrigeration exploits the fact that the boiling temperature of a liquid is intimately tied to its pressure. Generally, when the pressure on a liquid is raised its boiling (and condensing) temperature rises, and vice-versa. This is known as the saturation pressure-temperature relationship.

In practice, the choice of a refrigerant is a compromise, e.g., Ammonia is good but toxic and flammable while R12 is very good but detrimental to the Ozone layer. Figure 9 shows some commonly used refrigerants and their typical ranges of usability.
Ideally, a refrigerant will have the following characteristics.
• Non-toxic - for health and safety reasons. • Non-flammable - to avoid risks of fire or explosion. • Operate at modest positive pressures - to minimise pipe and component weights (for strength) and avoid air leakage into the system. • Have a high vapour density – to keep the compressor capacity to a minimum and pipe diameters relatively small. • Easily transportable - because refrigerants are normally gases at SSL conditions they are stored in pressurised containers. • Environmentally friendly - non-polluting & non-detrimental to the atmosphere, water or ground. • Easily re-cycleable, and relatively inexpensive to produce. • Compatible with the materials of the refrigeration system - non-corrosive, miscible with oil, and chemically benign.

The pilot plan is used to obtain optimal conditions for the semi-continuous extraction of PA and BA from their combination with benzoic acid, maleic acid, phthalic acid, phthalic anhydride, aldehydes, phthalimide, Toufic acid, and some minor impurities [37].

Variables such as pressure (220-260 bar), temperature (160-140 °C), dynamic time (5 to 45 minutes) and flow velocity (0.2-1 mL/min) were used. The Molten phthalic acid sample contains PA, toluic acid, BA, phthalimide, maleic acid (MA), aldehydes, phthalic acid and some minor impurities.

MPA solid and hard samples were broken to 40 to 60 sizes (particle diameter about 0.3-0.55 mm) to increase the contact surface and mass transfer of the Analyte in the PHWE process. The prepared MPA sample (~ 3 g) is transferred to a 10 ml cylindrical stainless steel extraction chamber. The PHWE method is dynamically performed by passing the water at different flow rates, temperatures, pressures and time through the extraction chamber. The remaining water in the chamber, the pipe and the recycle pressure regulator is eliminated by cleaning the PHWE system with CO₂ at the end of each extraction. Ethanol is injected into the tube so that it washes the tube each time the pipe system is blocked. The extraction solvent is evaporated at 80 °C and volumes were made to 10 ml with a solution of 2.50 g per naphthalene liter in a volumetric flask, in acetonitrile. [38-40].

The results showed that at 140 °C, the pressure, extraction time and solvent flow rate were 118 bar, 27 min, and 0.2 ml/min respectively for 100% yield of PA and these values are 118 bar, 29 min and 0.9 ml/min for 98% extraction of BA. Moreover, the maximum selection of PHWE at 100 °C was obtained as 220 bar, 5 min, and 0.2 ml/min.Figure 3

Pressurized hot water extraction (PHWE), herein is described as water extraction and steam extraction (PCDFs) in the sand at various conditions water at 10 or 50 atmospheres (excluding 50 atmospheres and 200 °C) is in the gaseous phase [41]. And in 250 atmospheres at all temperatures, except at 400 °C which is located at the supercritical point, is liquid. The extraction was started by the first water pumping through the extractor chamber at a constant speed of about 1 milliliter per minute and the pressure regulator was adjusted to match the desired pressure. The total extraction time was about 30 minutes, with a volume of about 30 ml (28 to 35 ml). ml).volume of about 30 ml (28 to 35 ml).Figure 4

The results showed that optimal extraction conditions for these compounds were found to be between 300 and 350 °C. The highest extraction efficiency was achieved in the gas phase at 50 atmospheres. With Steam at 300 °C and 50 horsepower extraction (compared to Soxhlet extraction), polychlorinated naphthalenes (PCNs) were achieved from the industrial soil. Additionally, compared to the values obtained from an international comparative study, Soil toxicity by PCDFs is reduced by at least 90% using PHWE.

Restoration of PAH-contaminated soils by extraction using subcritical water: The correction of soil contaminated with polycyclic aromatic hydrocarbons (PAHs) has been investigated through extraction using subcritical continuous flow water [42]. (Islam et al. 2012). Water temperature from 100 to 300 °C, extraction time from 15 to 60 minutes and flow rate in the range of 0.5 to 2.0 ml/min were investigated to specify their effect on removal efficiency of PAHs.

Subcritical water (SCW) extruder containing a 10-mL reactor, consisting of a water tank, a high-pressure pump, a preheater reactor, a thermometer, a pressure regulator and a separator were used.Figure 5

Distilled water is used as a solvent for extraction. Before using distilled water, helium gas was purged for 30 minutes to remove the dissolved oxygen because the water-soluble oxygen causes the oxidation of PAH in water at subcritical conditions [43]. 8 g of highly contaminated soil is transferred to the reactor and purified distilled water flows through the preheater and the reactor at a rate of 2.0-0.5 ml/min. Pressure ranges from 50 to 100 bar at room temperature to reach the desired temperature and maintain a constant pressure of 100 times. The water is transferred through the extraction reactor to the post-flow, purge vertically to prevent filter clogging and lighter materials extraction. At high temperatures, water acts as an organic solvent which can dissolve PAHs.

The results showed that extraction efficiency depends mainly on the temperature and extraction time. There is also a significant dependence on flow rates. The results showed that increasing water temperature causes an increase in the phenanthrene, fluoranthene, and pyrene adsorption. More than 95% of extracted phenanthrene, fluoranthene, and pyrene from contaminated soil are done at 300 °C for 30 minutes and 250 °C for 60 minutes with a constant pressure of 100 bar. However, naphthalene was almost exclusively extracted at a relatively low temperature of 150 °C and extraction time of over 30 minutes and a pressure of 100 bar. The subcritical water flow rate of 0.5 ml/min is recommended. The extraction of PAHs has been strongly dependent on water temperature since rising the temperature the dielectric constant (polarity) of water. These results indicate that contaminated soils with chemicals such as PAHs can be easily purified using pure, high-temperature exhaust without modification.

Liquid water at 250 or 300 °C efficiently extracts PAHs but does not extract high molecular weight alkanes (eg, CO₂). Little extraction of high-molecular-weight alcohols was achieved only with super-heated steam (250 and 300 °C at 5 atmospheres). The extraction of phenols, alkylbenzenes, PAHs, and alkanes is simply done by a ch the water temperature (300-50 °C) or pressure (5-100 amperes) [44].Figure 6

The soil sample contained PAHs and alkanes, petroleum sewage sludge including phenols and alcoholic phenols and water sludge include BTEX (benzene, toluene, ethylene benzene and xylene) and a very high level (6% w / w) of branched hydrocarbons (C10 C17). First, the extraction chamber was completely filled with sand and then targeted analytes were located on the sand. The water was dissolved in nitrogen for 1-2 hours to remove its oxygen. The pump operates at constant pressure mode (or in a steady flow for steam extraction) to supply water to the extraction chamber. The water flow rate is 1 ml/ minute. The solvent evaporates at temperatures above 200 to 300 °C during the extraction. So, the bottles are cooled with ice water to prevent evaporation of the solvent. Both the inlet and outlet valves return from the chamber while the GC oven is heated to the desired temperature. Therefore, during the heating time, typically the water is maintained for 1, 2, 3, 4, 5, and 7 minutes at 50, 100, 150, 200, 250 and 300 °C. The wastewater temperature reaches 250 °C for only 1.5 minutes after the oven. A set of extracted analytes is transferred to a glass bottle of 22 ml containing 3 ml of collector solvent (methylene chloride) by placing the output flow limiter (flow control device). The results indicate the recovery of all target analytes by extraction of subcritical waters (generally 90 to 120 percent).

A specific amount of water (4.5 liters, water to soil ratio being 3: 1) was transferred to the extraction chamber. The most suitable temperature for removing oil from soil was 260 °C in this study. The reactor pressure was kept above the vapor pressure (at the respective temperature) being 8 MPa and the water was in a liquid state and its extraction time was 90 minutes. The water was quickly pumped at a rate of 15 ml/min until the desired temperature reaches the extraction cell (60 minutes). Afterward, the flow rate was increased to 40 ml/min and kept constant during the experiment (90 minutes). After full water extraction, the remaining TPH in the soil was measured by the remaining soil extraction with dichloromethane. The oil and wastewater phases achieved from the oil separation tank and the wastewater reservoir were separated by a separation funnel and then the oil separated from the oil layer was measured and analyzed as recycled oil. The concentration of TPH in the oil layer and the aqueous phase were analyzed.

The SWE method presented in this study is known as a desirable candidate for the treatment of solid fatty wastes. After 90 minutes of extraction at 260 °C in a water-to-oil ratio of 3: 1, the removal efficiency was about 86% and oil recovery was about 39%. After 90 minutes extraction at 260 °C in a water-to-oil ratio of 3: 1, the Recovered oil had a higher level of density and specific gravity than commercial crude and had similar levels of elements and amounts of heat. After the extraction, there was a significant decrease in TPH content in soil residues. In summary, this research has found notable results in the field of oil removal and recovery and in dicated the feasibility of using the SWE technique, which can be an effective and plausible way to restrain hazardous, highly moist, contaminated soils.Figure 8

The present study shows the static-dynamic and dynamic state of subcritical water extraction [45]. Extraction of subcritical liquid at high temperature and pressure uses liquids which are still in a liquid state and kept under the critical point. Subcritical water extraction (SCWE) is a green technology that uses heated water (100 to 374 °C) at a pressure above 22.1 MPa to maintain it in liquid form [25]. At high temperatures, hydrogen bonding forces between water molecules weaken and the dielectric constant and polarity are reduced [46]. So, subcritical waters approve the hydrophobic nature and act as organic solvents (Gan et al., 2009). Lubricant oil materials were used at relatively low temperature using SCWE [47]. The SCWE process is also used to clean infected PHC soils. The effect of temperature and time on SCWE has been investigated on lubricating oil from contaminated soil, where the extraction efficiency depends on temperature [48]. The soil sample was weighed and transferred to the extraction chamber located inside the thermal chamber. Afterward, the extraction chamber was closed and connected to the flow of water and pollution. Subsequently, the water was pumped out with a flow for a given period of time between the preheating coil and the extraction chamber.Figure 9

Dynamic method: 12 g of soil sample was placed inside the extraction chamber. A laboratory device was used for this method. After assembly, the extraction chamber was filled with water at a flow rate of 1 ml/min (thus the system was pressurized at 6 MPa), and the heater was turned on. When the temperature inside the chamber reached 100 °C, the outlet valve with the flow rate of 0.3 ml/min was activated to increase the temperature inside the cell to the desired level (275 °C). When the temperature in the chamber reaches the set temperature, the extraction time counting begins and then the dynamic extraction is performed by pumping water at 1 ml/min during the desired extraction time (120 min).

Dynamic-static Method: The conditions are as follows for laboratory and pilot tests:
12 g of soil was placed in the extraction chamber for dynamic extraction (360 g for extraction with a 30-fold scale), the device is filled with water when the outlet valve is closed (thus the system is under pressure of 6 MPa) and the heater is turned on, When the internal temperature of the cell reached 100 °C, the outlet valve is opened and flowed at a flow rate of 0.3 ml/min (9 ml/ min for extraction at a 30-fold scale) to increase the temperature inside the chamber to the desired value; when The temperature inside the enclosure reached 275 °C and the system pressure reached 6 MPa, the static extraction stage was carried out for 15 minutes by closing the inlet and outlet valves. After this time period, both the outlet and inlet valves are opened and the high-pressure pump is turned on. Water flowed through the system at a rate of 1 ml/min (30 ml/min, if extracted at 30 times scale to maintain water residence time inside the Extraction chamber).

The study indicates that the static-dynamic mode has a significant effect on extraction efficiency. The time and volume required for a dynamic static mode are much lower than those required for the dynamic mode. These results are practical in the development of subcritical water extraction technology for the extraction of lubricating oils widely used in soils contaminated with hydrocarbons. The work of Islam et al (2013) shows an increase in the SCWE temperature from 200 to 275 °C in MPa 6, which reduces the TPH level in the oil contaminated soil by 3.5 times. This is related to the decrease in the dielectric constant and the surface tension of the subcritical water, which reduces polarization of water and increases oil dissolution. The optimal subcritical water extraction has been reported at 275 °C [49]. The Further works Company improved the efficiency of removing the lubricating oil by more than 98% at 275 °C by using static dynamic extraction in 120 minutes in a laboratory and also extracting 150 minutes in 30 times larger scale test by the same author the next year [48]. The change from static to static-dynamic mode increases the internal diaphragm distribution and mass transfer, which causes further increase in the efficiency of the elimination. Extraction of PAH-contaminated sand by subcritical water at different temperatures has been studied [50]. An efficiency of about 80% was reported regardless to extraction time, with the exception of naphthalene, with an improvement of about 35% when the extraction time increased from 1 to 20 hours. Decreasing dielectric constant and surface tension of subcritical water with increasing temperature will cause significant dissolution of non-polar organic compounds in water. Improved oil removal from contaminated soil is also dependent on time. The results of the effect of the influential parameters on extraction indicated that the extraction of lubricant from soil is strongly dependent on temperature and time. Comparing the two modes of SCWE (dynamic and static-dynamic) represents the 100% extraction efficiency of the TPH in the static-dynamic approach and 52% efficiency for a dynamic approach. Therefore, we can point out that 99% of lubricating oils Can be extracted using the process introduced in this study using subcritical water.

The present study aimed to investigate the extraction of subcritical water at temperatures of 250 °C in 8 kg of soil contaminated with 70-70 mg/kg of each of trifluralin, atrazine, cyanazine, pendimethalin, alachlor, metolachlor and pesticides, which removes them to an unrecognizable degree [49].

Laboratory-scale system: Water is pumped through a low-flow preheater cycle (0.1 mL/min) to fill the cavity from bottom to top. Toluene is pumped through the second pump for collecting extracted organic substances at a flow rate of 0.3 ml/min. The pressure is adjusted to 50 bar (for extraction at 250 °C or lower) or 100 bar (for extraction at 275 °C) and the oven is heated to the desired temperature. The flow is kept constant through the pump at 0.1 ml/min until the oven reaches the desired temperature and the system pressure is manually controlled. After reaching the desired temperature, the water flow rate increases to the desired amount (0.5 or 1 ml/min) and the toluene stream begins. The Analyte/toluene water mix flows and exits through the cooling coil and is collected in 20 ml glass bottles. During extraction, both the water and toluene pumps operate in steady flow mode and the system pressure (usually at a favorable 5 bar pressure) is maintained by manual adjustment of the output value of the system.

System on the pilot scale: the setting is designed for operation at various temperatures from 100 to 300 °C, at a maximum pressure of 105 bar and in a water flow of 0.1 to 1 liter per minute. 8 kg of soil is poured into the extraction cell. The water is pumped at about 570 ml/min to start filling the inner area. After leaving the reactor, the lid was closed, the outlet valve was closed, and the electric heaters installed on the extraction cell are turned on. After 1 hour of preheating (at that time, the internal temperature of the soil was 150 °C, placed by the thermocouples at the center of the radial top and bottom of the soil column), the propane heater was turned on and the water flow began. The flow of water is controlled by the pump and the system pressure is controlled manually by adjusting the outlet valve. Sewage is collected periodically and analyzed after methylene chloride extraction. Afterward, the system was cooled for 4 hours and then three soil samples were collected (bottom, center, and top of soil column) and analyzed to determine the removal efficiency.Figure 10

The results show that experiments on a laboratory scale (8 g of soil) are used to determine the state of the pilot scale (8 kg of soil). The extraction of PAH (2200 ppm total PAHs including naphthalene to benzoperylene) from contaminated soil with 275 °C water, with high and low molecular weights achieve a detectable level (less than 0.5 ppm) in less than 35 minutes.

In this study, a small-scale semi-continuous extraction with and without oxidation solution as the removal factor in the PAHs contaminated soil using subcritical water (for example, liquid water at high temperature and high pressure, but Less than the critical point) is addressed. Experiments were carried out in a 300-ml reactor using a soil sample [51].Figure 11 Healthy natural soil samples that were contaminated with PAHs were screened with mesh 40 (420-micrometer holes). Double distilled water was used as extraction medium, Acceleration solvent extraction (ASE), acetone 99.99% for cleaning equipment, nitrogen gas from compressed cylinders for the initial pressure of the extraction chamber, and diluted aqueous solution of hydrogen peroxide (30%) as the oxidation agent.

10 g of soil sample which was extracted by ASE and analyzed by GC. 50 g of soil is added to the reactor and 200-220 ml of double distilled water is added to the top of the soil to reduce the dead volume. For extraction without oxidation, the reactor is pressurized with nitrogen, while an oxidizing agent (air, oxygen, or hydrogen peroxide) is needed in oxidation extraction. In hot water extraction, the reactor reaches a peak pressure of 450-400 psig of nitrogen. Hot water extraction with oxidation is fundamentally similar to warm water. An oxidizing agent is added to the water/ soil instead of nitrogen. Oxidizing agents include air, oxygen and hydrogen peroxide. As using air or oxygen, the amount of soil in the reactor to create the additional oxygen atmosphere required for oxidation of PAH is slightly lower. In case of hydrogen peroxide, with the same soil/water ratio as before, a portion of water with a volume equivalent to 30% hydrogen peroxide is replaced. All extraction and oxidation experiments are carried out at 250 °C. To find isotherms and oxidation adsorption reactions, semi-continuous experiments with residence times of 1 and 2 hours at 250 °C and hydrogen peroxide as an oxidizing agent was performed. The experiment conditions are in hot water extraction and which combined with the oxidizing agent.

The results showed that in combined extraction and oxidation experiments, the remaining PAHs in the soil after the experiments were almost undetectable. In the combination of extraction and oxidation, after the first 30 minutes of experimentation, no PAH is detected in the liquid phase. Based on these results, hot water extraction, if combined with oxidation, reduces the cost and is considered as an alternative method that can be used to treat contaminated soils. PAH removal from the crushed soil in extraction experiments was only 79% to 99% dependent on the molecular weight of PAH. This composition for extraction and oxidation was 99.1% to higher than 99.99%. While only about 100-28% of extracted PAHs are in the water phase, the oxidation extraction reduces this amount to a maximum of 10%. Based on these results, hot-water extraction, if combined with oxidation, probably reduces the cost of water treatment and can be used as an appropriate alternative method to soil and contaminated sediments treatment.

Restoration of polycyclic aromatic hydrocarbons from soil using supercritical water extraction: The extraction study (SWE) aims to remove polycyclic aromatic hydrocarbons (PAHs) including naphthalene, phenanthrene, anthracene, and pyrene from contaminated soil [52]. Prior to using distilled water, is cleaned for 30 minutes with nitrogen to remove its oxygen.Figure 12

The pump operates at constant pressure. The steel extraction chamber is filled with 5 g of contaminated soil and water. The extraction chamber is located in the oven. The pump is switched on and the pressure is raised up to 20 bar. Then, the pump is switched off and oven temperature is increased to the desired value, the pump is switched on again and the flow is adjusted at the required rate. The water passes through the top-to-bottom in extraction chamber. When the temperature of the extraction chamber reaches the desired point, the extraction time begins. At the end of the extraction time, the oven and the pump are turned off and the pressure is adjusted to atmospheric pressure. For UV analysis, the pure soil is collected to verify the efficiency of SWE.

The results showed that the removal efficiency strongly depends on the water temperature. The effects of the two parameters of water and extraction time in purification efficiency were investigated. The water temperature varied from 100 to 180 °C and the extraction time was from 5 to 20 minutes.

Supercritical water oxidation is an emerging technology that uses the supercritical properties of water as a reactant. The result of the lower product is clean soil with below 200 ppm contamination level, and the top product is a gas stream of CO₂ and water. Oxidation in supercritical water is a relatively new combustion process that uses water in the supercritical state as a reaction of medium and high-pressure oxygen as an oxidizing agent [53].Figure 13

The experiments were started by filling the reactor with 20 g of soil sample. When the temperature and pressure in the oxidation vessel reaches the desired supercritical conditions, 400 °C and 23.4 MPa, the solvent flow rate reduces. When the degradation conditions are achieved, oxygen enters the reactor. Oxygen velocity varied at three levels of 0.09, 0.5 and 1 cm3 / min. After the oxidation reaction, the system is cooled and its pressure is reduced for collecting soil.

The results showed that supercritical water oxidation was able to eliminate 99.5% of hydrocarbon contaminants present in the soil. The supercritical water oxidation of PAH-infected soil is a suitable alternative to conventional processes as a full one stage treatment.

Hazardous organic wastes, including chlorinated hydrocarbons, in aqueous media containing salts, can be effectively oxidized by treatment above the critical point of pure water (374 °C, 221 bar). High destruction efficiencies may be achieved at low reactor residence times (approximately 1 minute or less) for temperatures above 550°C. Under these conditions, no NOx.

Compounds are produced. The high solubility of organics and oxygen and the low solubility of salts in supercritical water make it an attractive medium for both oxidation and salt separation.

Oxidation carried out in a supercritical water environment at temperatures above 374 °C and pressures above 221 bar (22.1 MPa) provides a viable method for the efficient destruction of organic wastes in a fully contained system. This patented process has been termed supercritical water oxidation, SCWO, and has been under commercial development for about the past 11 years by MODAR, Inc. [54-60]. The process has also proven suitable for treatment of human metabolic wastes and is being considered for use in life support systems on long-term spaceflights [61-64]. A relatively recent variation of SCWO involves carrying out the process in a deep well utilizing the hydrostatic head of fluid in the wellbore to help provide the necessary pressure level [65].

Economic predictions for SCWO have appeared in literature such as (for example, see Thomason and Modell); Stone and Webster Engineering Corp [66]; and Modell [67]. Due to the evolving nature of the technology and the inherent uncertainties associated with orders-of-magnitude scale-up from pilot-sized units to commercial-sized systems, these studies should be viewed with reservation. Earlier reviews of SCWO technology by Freeman [68], Modell [67] and Thomason et al. [59], while providing important basic process engineering material, need to be updated with respect to current process development and fundamental research on SCWO. A more recent paper by Shaw et al. [69] introduces several aspects of research related to reaction chemistry in supercritical water. In the supercritical water oxidation (SCWO) process water, organics, and oxygen gather together at moderate temperatures (400 °C and above) and high pressures (about 25 MPa). Under these conditions, a single fluid phase reaction system exists where many of the inherent transport limitations of multi-phase contacting are absent. The temperatures are high enough to induce spontaneous oxidation of the organics and the heat of reaction raises the mixture temperature to levels as high as 650 °C. Above about 550 °C, organics are oxidized rapidly and completely to conversions greater than 99.99% for reactor residence times of 1 minute or less. Heteroatomic groups, such as chlorine, sulfur, and phosphorus, are oxidized to acids which can be neutralized and precipitated as salts by adding a base to the feed. For aqueous solutions containing between 1 and 20 wt% organics, supercritical water oxidation could be more economical than controlled incineration or activated carbon treatment and more efficient than wet oxidation [58]. With appropriate temperatures, pressures, and residence times, organics are oxidized completely to CO₂, water, and molecular nitrogen, without formation of NOx compounds or other toxic products of incomplete combustion. Modell and co-workers [70] also destroyed problematic polychlorinated biphenyls (PCBs) and DDT at greater than 99.99% efficiency without formation of dioxins. Cunningham et al. [71] and Johnston et al. [72] oxidized synthetic pharmaceutical and biopharmaceutical wastes and achieved > 99.99% destruction of total organic carbon and > 99.9999% destruction of thermophilic bacteria. Oxidation in supercritical water is not limited to aqueous organics. Biomass, sewage, and soil can be slurried and fed to the reactor as a two-phase mixture. Wastes with large particles or high solids content may need to be homogenized prior to treatment. In principle, any mixture of organic waste that can be pumped to high pressure is treatable by SCWO. Precipitated salts and other solids can be removed from the product stream, providing a clean, high-temperature, high-pressure energy source consisting almost entirely of CO₂, H₂O, and NO. Heat can be recovered from the treated process stream and used to preheat the feed, or the stream can be used to generate steam for direct process use or electric power generation in a high-efficiency Rankine cycle. The heat of the oxidation reaction is thus partially recoverable and can be used to offset the cost of treatment. One of the main attributes of SCWO is its ability to oxidize rapid and complete a wide range of organic compounds in a reaction system that meets the concept of a “Totally Enclosed Treatment Facility”. Dilute aqueous wastes ranging from 1 to 20% organic content are particularly well-suited to SCWO processing. The scale of application is also adaptable, from portable benchtop or trailer-mounted units treating 1 to up to several thousand gallons of toxic waste per day, to large stationary plants capable of processing 10,000 to 100,000 gallons per day of waste with 8 to 10% organic content. In the present designs, additional air pollution abatement equipment is not needed. Nonetheless, solids removal and storage are necessary, and water effluent polishing units incorporating ion exchange may be needed to remove small concentrations of dissolved metal ions. Major drawbacks of SCWO are the high pressures required (>230 bar) and the fact that corrosion at certain points in the process is significant. For some wastes, solids handling can also pose difficulties.

For extraction of PAHs and toluene from sea sand, refined hot, and supercritical water oxidation equipment are made and then oxidation is carried out using hydrogen peroxide [46]. Pressurized hot water extraction (PHWE) and supercritical water oxidation (SCWO) are techniques that use physical and chemical properties of hot water and pressure to treat solid and liquid wastes. For the extraction of (PAHs) from sea sand and a real soil sample and for the breakdown of compounds through oxidation with hydrogen peroxide, the PHWE and SCWO methods were used. Generally, the best extraction recovery was achieved at 300 °C with an extraction time of 20 minutes. Compounds with the highest molecular weight were harder to extract. In contrast to the Soxhlet extraction, PHWE has had a particular improvement, especially for low molecular weight compounds. The oxidation efficiency (PAHs conversion) increases with increasing oxidant concentration and reaction time. The PHWE-SCWO method improves the efficiency.

The stable solution of PAH, 99.8% Toluene (to prepare solutions), Dried sea sand (solid with grain size of 0.1-0.3 mm), distilled deionized water (for extraction), aqueous solution of hydrogen peroxide 30 % (As oxidizing agent), 99.8% acetone (for washing equipment), the oxidant concentration varies from 5.6 to 112.6 g / l.

4.5 grams of sand is washed with acid and dried, poured into the tube, and 200 ml of PAH solution is injected.

In the PHWE-SCWO test, the oven 2 was heated to the selected temperature (385 °C or 425 °C) and the oxidant stream (pump 2: v = 1.0 or 2.0 ml/min) is started before the start of the pump 1 (v = 1.0 ml/min) for the water supply. Then the temperature of oven 1 is set at 200 or 300 °C for extraction. In the PHWE method (without SCWO), water is pumped at 385 °C instead of oxidant (pump 2). The extraction time was 20 or 40 minutes at the selected temperature. The total time for extraction at 200 °C was 8 minutes longer, and at 300 °C, it was 18 minutes longer due to warming the oven 1 to the selected temperature.Figure 14

The results showed that the extraction efficiency increases with temperature and time; the best results were obtained at 300 °C with a 40 minute extraction time. In the oxidation stage, the conversion of PAHs increases with the reaction time and oxidant concentration, and the best conversion (97-99.9% depending on the composition) is obtained at 425 °C with a reaction time of 43 seconds and a concentration of 112.6 g / l. Benzaldehyde and benzoic acid are among the most frequent interveners in the oxidation process. In addition, phenol, proxy, and benzyl alcohol were found to be intermediates. Mediators consist mainly of toluene, which is present in the reaction medium at a higher concentration of PAHs.

Supercritical fluid extraction is a fast and efficient way to extract all kinds of organic compounds from a wide range of solids. Due to the highlighted change of temperature in polarity, water is an interesting alternative to CO₂ as an extraction liquid. Many organic compounds are soluble enough to be extracted under subcritical conditions, and therefore problems, such as high corrosion of supercritical water and also technical problems can be avoided [73].

PCBs were filled with water in the extraction vessel along with sea sand, and the actual soil samples were placed in the extraction tank and filled up completely (pure sea sand was added to the 0.5 g soil sample). The water is placed in contact with the sample for 30 minutes at a flow rate of about 1 mL/min with the super and subcritical pressure and temperature. During the extraction, the cooling coil was immersed in water at room temperature. After each extraction, the oven cools down. Where the solid phase valve is used, the valves and tubes are dried with nitrogen prior to washing with n-heptane or other molecules (1 ml/min) inside the GC glass. In liquid duct, tubes are washed with n-heptane and combined with the water extract. The analytes were extracted from aqueous extracts with several parts (5 x 2 ml) of n-heptane, concentrated using nitrogen and analyzed for GC. For each extraction, columns and valves of the column (if used) are washed with 10 ml of nitrogen and 10 ml of acetone and finally dried with nitrogen. The best result from the removal of PCBs in most cases (considering the different conditions of subcritical and supercritical temperatures and the proportional pressure) was more than 85%.Figure 15

The supercritical fluid process is always used to remove pollutants such as PHCs, PAHs, PCBs, dioxins, frances, phenols, chlorophenols, insecticides, metals and radioactive substances. The results have shown that the supercritical fluid method using CO₂ has the ability to effectively eliminate organic and inorganic compounds in different contaminated soils [26].

Moreover, the cost of this method is acceptable compared to other available methods. On the other hand, the use of this method using water is more studied, as it is eco-friendly and has lower cost and better safety. Because of the high power of solubility, by using the PHWE method, various compounds can be extracted from different matrices at an acceptable temperature below the critical water temperature.

Compared to the conventional extraction processes using solvents, in this process, we can finally remove the pollutant from the solvent by changing the pressure and temperature. Also, its extraction time is lower than the conventional method, less solvent is consumed, less waste is produced and less toxic residues are left [22,74,75]. The solvent used in this method is easily recovered and can be used in subsequent extractions. In this method, less energy is consumed and the composition and structure of the soil are retained [76]. Compared to the biological processes, this process is faster and more efficient [76,77].

Compared to the biological processes, this process requires the excavation, which increases the costs. This process can only remove pollutants from soil, and unlike the biological process, does not have the ability to degrade and convert pollutants into materials.

In developing this process, water was also used as an extraction fluid. However, the use of supercritical water is limited because of the high temperature (more than 374 °C) and pressure (more than 218 atmospheres) and equipment corrosion. Therefore, the use of subcritical water extraction (SWE), known as pressurized hot water extraction (PHWE), is easier. In the PHWE method, while the water temperature rises from 100 °C to 274 °C, Hydrogen bonds between water molecules weaken and the dielectric constant of water is reduced. This reduces the polarity of water molecules [78]. Subcritical water, therefore, is more likely to absorb organic compounds than water in environmental conditions. Research has also shown that the SWE method has a better performance in extracting PAHs. The combination of the SWE process with the oxidation process by adding oxidizing agents like air, oxygen, hydrogen peroxide, etc. improves the extraction process in this method [75,78].