Double pipe heat exchangers are the simplest exchangers used in industries. On one hand, these heat exchangers have low cost for both design and maintenance, making them a good choice for small industries. But on the other hand, low efficiency of these exchangers beside high space occupied for such exchangers in large scales, has led modern industries to use more efficient heat exchanger like shell and tube or other modern heat exchangers. But yet, since double pipe heat exchangers are simple, they are used to teach heat exchanger design basic to students and as the basic rules for modern and normal heat exchangers are the same, students can understand the design techniques much easier.
Heat exchangers (HEX) are one of the most important parts of industries like chemical, medical, or many other industries. These systems are used to transfer or “exchange” thermal energy (heat) between two fluids which can be gas or liquid. Heat exchangers can be found in different types such as double pipe, shell & tube, spiral tube, plate, etc. but the two first types are and specially shell & tube exchangers are more common in industries.
Phosphoric acid fuel cell:
In Phosphoric acid fuel cells phosphoric acid is used as electrolyte. fuel cells use hydrogen as fuel with oxygen provided from air. Hydrogen is channeled to the cell and faces to platinum catalyst layer on anode. In this section, hydrogen molecules split to protons (hydrogen positive ions ) and electrons.
H2 + 2Pt ——> 2 (Pt – H)
2 (Pt – H) ——> 2Pt + 2H+ +2e–
Electro chemical reactions always consist of two half reactions taking place in anode and cathode electrodes. In polymer electrolyte membrane fuel cells or PEFCs, hydrogen ions produced in anode electrode travel through the ion conductive membrane and reach to the cathode electrode and the released electrons in anode, will go to cathode through an external circuit. These electrons and hydrogen ions react with oxygen producing heat and water in cathode.
In today’s world, one of the most important problems of humanity is the energy. Due to irregular use of fossil fuels, the environment is getting polluted more and more. Therefore, it’s crucial to replace the fossil fuels with modern clean energies. Fuel cells are one of the most promising technologies that can be used with any electrical system. Polymer electrolyte membrane or proton exchange membrane (PEM) fuel cells are currently one of the most efficient systems designed to convert chemical energy of hydrogen and oxygen to electricity directly. These systems can be used anywhere from cars to power plants.
There are several types of fuel cells. These are categorized in 5 main parts as below:
- Solid oxide fuel cell (SOFC)
- Alkaline fuel cell (AFC)
- Phosphoric acid fuel cell (PAFC)
- Molten carbonate fuel cell (MCFC)
- Polymer electrolyte fuel cell (PEFC or PEM)
In addition to the above categories, there is one more type named direct methanol fuel cell or DMFC. But this type is commonly included in 5th group. The working temperature of fuel cells can vary between 80 degrees Celsius for PEFCs and 1000 degrees for SOFCs. Low temperature cells have H+ and OH– carriers that exchange ions through electrolyte and electrons through an external circuit. In high temperature cells like MCFCs and SOFCs, electric current is carried by CO32- and o22- .
Fuel cells have several advantages that make them a suitable replacement for ordinary power production devices. These advantages are high efficiency, wide range for design parameters, environment friendly, Wide range for selecting fuel, less energy loss, low maintenance,etc. These advantages are discussed below . . .
Fuel cell is a new technology to produce high amounts of energy using direct oxidation of fuel by oxygen, without any environmental or noise pollution. As fuel cells have very high efficiencies with lower thermal energy loss, they can be a considerable replacement for Carnot cycle to produce electricity.
Activated Carbon is the most important sorbent in ANG process. There are several types of activated carbon and main factors to choose the suitable one are total surface area, density, and pore volume. Total surface area is the most important one among these factors and is calculated through nitrogen adsorption on carbon in square meter per grams of carbon. This parameter can have values between 500 – 1900.
Natural gas comprising more than 20 percent of methane has been used as a fuel for vehicles and that’s because of the low cost and less environmental effects relative to gasoline. Methane, the main component of natural gas, is in gas phase at STP condition. As it has very low density, for unit volume of methane, low amount of energy will be produced, requiring high amounts to be stored in tanks.