NPSH - Net Positive Suction Head

Low pressure at the suction side of a pump can encounter the fluid to start boiling with
reduced efficiency, cavitation, damage of the pump as a result. Boiling starts when the pressure in the liquid is reduced to the vapor pressure of the fluid at the actual temperature.

To characterize the potential for boiling and cavitation, the difference between the total head on the suction side of the pump - close to the impeller, and the liquid vapor pressure at the actual temperature, can be used.
Suction Head
Based on the Energy Equation - the suction head in the fluid close to the impeller can be expressed as the sum of the static and the velocity head:
hs = ps / γ + vs2 / 2 g (1)
where
hs = suction head close to the impeller
ps = static pressure in the fluid close to the impeller
γ = specific weight of the fluid
vs = velocity of fluid
g = acceleration of gravity
Liquids Vapor Head
The liquids vapor head at the actual temperature can be expressed as:
hv = pv / γ (2)
where
hv = vapor head
pv = vapor pressure
Note! The vapor pressure in a fluid depends on temperature. Water, our most common fluid, starts boiling at 20 oC if the absolute pressure in the fluid is 2.3 kN/m2. For an absolute pressure of 47.5 N/m2, the water starts boiling at 80 oC. At an absolute pressure of 101.3 kN/m2 (normal atmosphere), the boiling starts at 100 oC.
Net Positive Suction Head - NPSH
The Net Positive Suction Head - NPSH - can be expressed as the difference between the Suction Head and the Liquids Vapor Head and expressed like
NPSH = hs - hv (3)
or, by combining (1) and (2)
NPSH = ps / γ + vs2 / 2 g - pv / γ (3b)

Available NPSH - NPSHa
The Net Positive Suction Head made available the suction system for the pump is often named NPSHa. The NPSHa can be determined during design and construction, or determined experimentally from the actual physical system.
The available NPSHa can be calculated with the Energy Equation. For a common application - where the pump lifts a fluid from an open tank at one level to an other, the energy or head at the surface of the tank is the same as the energy or head before the pump impeller and can be expressed as:
h0 = hs + hl (4)
where
h0 = head at surface
hs = head before the impeller
hl = head loss from the surface to impeller - major and minor loss in the suction pipe
In an open tank the head at surface can be expressed as:
h0 = p0 / γ = patm / γ (4b)
For a closed pressurized tank the absolute static pressure inside the tank must be used.
The head before the impeller can be expressed as:
hs = ps / γ + vs2 / 2 g + he (4c)
where
he = elevation from surface to pump - positive if pump is above the tank, negative if the pump is below the tank
Transforming (4) with (4b) and (4c):
patm / γ = ps / γ + vs2 / 2 g + he + hl (4d)
The head available before the impeller can be expressed as:
ps / γ + vs2 / 2 g = patm / γ - he - hl (4e)
or as the available NPSHa:
NPSHa = patm / γ - he - hl - pv / γ (4f)

Available NPSHa - the Pump is above the Tank
If the pump is positioned above the tank, the elevation - he - is positive and the NPSHa decreases when the elevation of the pump increases.
At some level the NPSHa will be reduced to zero and the fluid starts to evaporate.
Available NPSHa - the Pump is below the Tank
If the pump is positioned below the tank, the elevation - he - is negative and the NPSHa increases when the elevation of the pump decreases (lowering the pump).
It's always possible to increase the NPSHa by lowering the pump (as long as the major and minor head loss due to a longer pipe don't increase it more). This is important and it is common to lower the pump when pumping fluids close to evaporation temperature.
Required NPSH - NPSHr
The NPSHr, called as the Net Suction Head as required by the pump in order to prevent cavitation for safe and reliable operation of the pump.
The required NPSHr for a particular pump is in general determined experimentally by the pump manufacturer and a part of the documentation of the pump.
The available NPSHa of the system should always exceeded the required NPSHr of the pump to avoid vaporization and cavitation of the impellers eye. The available NPSHa should in general be significant higher than the required NPSHr to avoid that head loss in the suction pipe and in the pump casing, local velocity accelerations and pressure decreases, start boiling the fluid on the impeller surface.
Note that the required NPSHr increases with the square capacity.
Pumps with double-suction impellers has lower NPSHr than pumps with single-suction impellers. A pump with a double-suction impeller is considered hydraulically balanced but is susceptible to an uneven flow on both sides with improper pipe-work.
Example - Pumping Water from an Open Tank
When increasing the the elevation for a pump located above a tank, the fluid will start to evaporate at a maximum level for the actual temperature.
At the maximum elevation NPSHa is zero. The maximum elevation can therefore be expressed by (4f):
NPSHa = patm / γ - he - hl - pv / γ = 0
For optimal theoretical conditions we neglect the major and minor head loss. The elevation head can then be expressed as:
he = patm / γ - pv / γ (5)
The maximum elevation or suction head for an open tank depends on the atmospheric pressure - which in general can be regarded as constant, and the vapor pressure of the fluid - which in general vary with temperature, especially for water.
The absolute vapor pressure of water at temperature 20 oC is 2.3 kN/m2. The maximum theoretical elevation height is therefore:
he = (101.33 kN/m2) / (9.80 kN/m3) - (2.3 kN/m2) / (9.80 kN/m3)
= 10.1 m
Due to the head loss in the suction pipe and the local conditions inside the pump - the theoretical maximum elevation is significantly decreased.
The maximum theoretical elevation of a pump above an open water tank at different temperatures can be found from the table below.

Chlorine

Industrially, elemental chlorine is usually produced by the electrolysis of sodium chloride dissolved in water. Along with chlorine, this chloralkali process yields hydrogen gas and sodium hydroxide, according to the following chemical equation:
2 NaCl + 2 H2O → Cl2 + H2 + 2 NaOH

Production

Gas extraction
Chlorine can be manufactured by electrolysis of a sodiumchloride solution (brine). The production of chlorine results in the co-products caustic soda (sodium hydroxide, NaOH) and hydrogen gas (H2). These two products, as well as chlorine itself, are highly reactive. Chlorine can also be produced by the electrolysis of a solution of potassium chloride, in which case the co-products are hydrogen and caustic potash (potassium hydroxide). There are three industrial methods for the extraction of chlorine by electrolysis of chloride solutions, all proceeding according to the following equations:
Cathode: 2 H+ (aq) + 2 e− → H2 (g)
Anode: 2 Cl− (aq) → Cl2 (g) + 2 e−
Overall process: 2 NaCl (or KCl) + 2 H2O → Cl2 + H2 + 2 NaOH (or KOH)

Mercury cell electrolysis
Mercury cell electrolysis, also known as the Castner-Kellner process, was the first method used at the end of the nineteenth century to produce chlorine on an industrial scale.The "rocking" cells used have been improved over the years.Today, in the "primary cell", titanium anodes (formerly graphite ones) are placed in a sodium (or potassium) chloride solution flowing over a liquid mercury cathode. When a potential difference is applied and current flows, chlorine is released at the titanium anode and sodium (or potassium) dissolves in the mercury cathode forming an amalgam. This flows continuously into a separate reactor ("denuder" or "secondary cell"), where it is usually converted back to mercury by reaction with water, producing hydrogen and sodium (or potassium) hydroxide at a commercially useful concentration (50% by weight). The mercury is then recycled to the primary cell.
The mercury process is the least energy-efficient of the three main technologies (mercury, diaphragm and membrane) and there are also concerns about mercuryemissions.
It is estimated that there are still around 100 mercury-cell plants operating worldwide. In Japan, mercury-based chloralkali production was virtually phased out by 1987 (except for the last two potassium chloride units shut down in 2003). In the United States, there will be only five mercury plants remaining in operation by the end of 2008. In Europe, mercury cells accounted for 43% of capacity in 2006 and Western European producers have committed to closing or converting all remaining chloralkali mercury plants by 2020

Diaphragm cell electrolysis
In diaphragm cell electrolysis, an asbestos (or polymer-fiber) diaphragm separates a cathodeand an anode, preventing the chlorine forming at the anode from re-mixing with the sodium hydroxide and the hydrogen formed at the cathode This technology was also developed at the end of the nineteenth century. There are several variants of this process: the Le Sueur cell (1893), the Hargreaves-Bird cell (1901), the Gibbs cell (1908), and the Townsend cell (1904) The cells vary in construction and placement of the diaphragm, with some having the diaphragm in direct contact with the cathode.
The salt solution (brine) is continuously fed to the anode compartment and flows through the diaphragm to the cathode compartment, where the caustic alkali is produced and the brine is partially depleted.
As a result, diaphragm methods produce alkali that is quite dilute (about 12%) and of lower purity than do mercury cell methods. But diaphragm cells are not burdened with the problem of preventing mercury discharge into the environment. They also operate at a lower voltage, resulting in an energy savings over the mercury cell method,but large amounts of steam are required if the caustic has to be evaporated to the commercial concentration of 50%.

Membrane cell electrolysis
Development of this technology began in the 1970s. The electrolysis cell is divided into two "rooms" by a cation permeable membrane acting as an ion exchanger. Saturated sodium (or potassium) chloride solution is passed through the anode compartment, leaving at a lower concentration.[23] Sodium (or potassium) hydroxide solution is circulated through the cathode compartment, exiting at a higher concentration. A portion of the concentrated sodium hydroxide solution leaving the cell is diverted as product, while the remainder is diluted with deionized water and passed through the electrolysis apparatus again.
This method is more efficient than the diaphragm cell and produces very pure sodium (or potassium) hydroxide at about 32% concentration, but requires very pure brine.
Other electrolytic processes
Although a much lower production scale is involved, electrolytic diaphragm and membrane technologies are also used industrially to recover chlorine from hydrochloric acid solutions, producing hydrogen (but no caustic alkali) as a co-product.
Furthermore, electrolysis of fused chloride salts (Downs process) also enables chlorine to be produced, in this case as a by-product of the manufacture of metallic sodium or magnesium.

Sodium hydroxide (NaoH)

Methods of production
Sodium hydroxide is produced (along with chlorine and hydrogen) via the chloralkali process. This involves the electrolysis of an aqueous solution of sodium chloride. The sodium hydroxide builds up at the cathode, where water is reduced to hydrogen gas and hydroxide ion:
2Na+ + 2H2O + 2e− → H2 + 2NaOH
To produce NaOH it is necessary to prevent reaction of the NaOH with the chlorine. This is typically done in one of three ways, of which the membrane cell process is economically the most viable.
Mercury cell process (also called the Castner-Kellner process) – Sodium ions are reduced to sodium metal, which forms an amalgam with a mercury cathode; this sodium is then reacted with water to produce NaOH. There have been concerns about mercury releases, although modern plants claim to be safe in this regard.[2]
Diaphragm cell process – uses a steel cathode, and the reaction of NaOH with Cl2 is prevented using a porous diaphragm, often made of asbestos fibers. In the diaphragm cell process the anode area is separated from the cathode area by a permeable diaphragm. The brine is introduced into the anode compartment and flows through the diaphragm into the cathode compartment. A diluted caustic brine leaves the cell. The sodium hydroxide must usually be concentrated to 50% and the salt removed. This is done using an evaporative process with about three tonnes of steam per tonne of sodium hydroxide. The salt separated from the caustic brine can be used to saturate diluted brine. The chlorine contains oxygen and is purified by liquefaction and evaporation.[3][4]
Membrane cell process – similar to the diaphragm cell process, with a Nafion membrane to separate the cathode and anode reactions. Only sodium ions and a little water pass through the membrane. It produces a higher quality of NaOH. Of the three processes, the membrane cell process requires the lowest consumption of electric energy and the amount of steam needed for concentration of the caustic is relatively small (less than one tonne per tonne of sodium hydroxide).[5][6]
An older method for sodium hydroxide production was the Leblanc process, which produced sodium carbonate, followed by roasting to create carbon dioxide and sodium oxide. This method is still occasionally used. It helped establish sodium hydroxide as an important commodity chemical.
The LeBlanc process was superseded by the Solvay process in the late 19th century.

Electrolysis of ALUMINIM

Although aluminium is the most abundant metallic element in the Earth's crust (believed to be 7.5 to 8.1 percent), it is rare in its free form, occurring in oxygen-deficient environments such as volcanic mud, and it was once considered a precious metal more valuable than gold. Napoleon III, emperor of France, is reputed to have given a banquet where the most honoured guests were given aluminium utensils, while the other guests had to make do with gold.[11][12] The Washington Monument was completed, with the 100 ounce (2.8 kg) aluminium capstone being put in place on December 6, 1884, in an elaborate dedication ceremony. It was the largest single piece of aluminium cast at the time. At that time, aluminium was more expensive than silver, gold, or platinum. Aluminium has been produced in commercial quantities for just over 100 years.

Bauxite
Aluminium is a strongly reactive metal that forms a high-energy chemical bond with oxygen. Compared to most other metals, it is difficult to extract from ore, such as bauxite, due to the energy required to reduce aluminium oxide (Al2O3). For example, direct reduction with carbon, as is used to produce iron, is not chemically possible, since aluminium is a stronger reducing agent than carbon. Aluminium oxide has a melting point of about 2,000 °C. Therefore, it must be extracted by electrolysis. In this process, the aluminium oxide is dissolved in molten cryolite and then reduced to the pure metal. The operational temperature of the reduction cells is around 950 to 980 °C. Cryolite is found as a mineral in Greenland, but in industrial use it has been replaced by a synthetic substance. Cryolite is a chemical compound of aluminium, sodium, and calcium fluorides: (Na3AlF6). The aluminium oxide (a white powder) is obtained by refining bauxite in the Bayer process of Karl Bayer. (Previously, the Deville process was the predominant refining technology.)
The electrolytic process replaced the Wöhler process, which involved the reduction of anhydrous aluminium chloride with potassium. Both of the electrodes used in the electrolysis of aluminium oxide are carbon. Once the refined alumina is dissolved in the electrolyte, its ions are free to move around. The reaction at the cathode (negative electrode) is
Al3+ + 3 e− → Al
Here the aluminium ion is being reduced (electrons are added). The aluminium metal then sinks to the bottom and is tapped off.
At the anode (positive electrode), oxygen is formed:
2 O2− → O2 + 4 e−
This carbon anode is then oxidized by the oxygen, releasing carbon dioxide.
O2 + C → CO2
The anodes in a reduction cell must therefore be replaced regularly, since they are consumed in the process.
Unlike the anodes, the cathodes are not oxidized because there is no oxygen present, as the carbon cathodes are protected by the liquid aluminium inside the cells. Nevertheless, cathodes do erode, mainly due to electrochemical processes and metal movement. After five to ten years, depending on the current used in the electrolysis, a cell has to be rebuilt because of cathode wear.

Synthesis and production

Because of its many uses, ammonia is one of the most highly produced inorganic chemicals. Dozens of chemical plants worldwide produce ammonia. The worldwide ammonia production in 2004 was 109 million metric tonnes.[15] The People's Republic of China produced 28.4% of the worldwide production followed by India with 8.6%, Russia with 8.4%, and the United States with 8.2%.[15] About 80% or more of the ammonia produced is used for fertilizing agricultural crops.[15]
Before the start of World War I, most ammonia was obtained by the dry distillation[16] of nitrogenous vegetable and animal waste products, including camel dung, where it was distilled by the reduction of nitrous acid and nitrites with hydrogen; in addition, it was produced by the distillation of coal, and also by the decomposition of ammonium salts by alkaline hydroxides[17] such as quicklime, the salt most generally used being the chloride (sal-ammoniac) thus:
2 NH4Cl + 2 CaO → CaCl2 + Ca(OH)2 + 2 NH3
(Two molecules of ammonium chloride plus two calcium oxide yields calcium chloride and calcium hydroxide and two molecules of ammonia)
Today, the typical modern ammonia-producing plant first converts natural gas (i.e., methane) or liquified petroleum gas (such gases are propane and butane) or petroleum naphtha into gaseous hydrogen. The processes used in producing the hydrogen begins with removal of sulfur compounds from the natural gas (because sulfur deactivates the catalysts used in subsequent steps). Catalytic hydrogenation converts organosulfur compounds into gaseous hydrogen sulfide:
H2 + RSH → RH + H2S(g)
The hydrogen sulfide is then removed by passing the gas through beds of zinc oxide where it is absorbed and converted to solid zinc sulfide:
H2S + ZnO → ZnS + H2O
Catalytic steam reforming of the sulfur-free feedstock is then used to form hydrogen plus carbon monoxide:
CH4 + H2O → CO + 3 H2
In the next step, the water gas shift reaction is used to convert the carbon monoxide into carbon dioxide and more hydrogen:
CO + H2O → CO2 + H2
The carbon dioxide is then removed either by absorption in aqueous ethanolamine solutions or by adsorption in pressure swing adsorbers (PSA) using proprietary solid adsorption media.
The final step in producing the hydrogen is to use catalytic methanation to remove any small residual amounts of carbon monoxide or carbon dioxide from the hydrogen:
CO + 3 H2 → CH4 + H2O
CO2 + 4 H2 → CH4 + 2 H2O
To produce the desired end-product ammonia, the hydrogen is then catalytically reacted with nitrogen (derived from process air) to form anhydrous liquid ammonia. This step is known as the ammonia synthesis loop (also referred to as the Haber-Bosch process):
3 H2 + N2 → 2 NH3
Hydrogen required for ammonia synthesis could in principle be obtained from other sources, but these alternatives - apart from the electrolysis of water into oxygen + hydrogen - are presently impractical. At one time, most of Europe's ammonia was produced from the Hydro plant at Vemork, via the electrolysis route. Various renewable energy electricity sources are also potentially applicable.

Process flow diagram

A process flow diagram (PFD) is a diagram commonly used in chemical and process engineering to indicate the general flow of plant processes and equipment. The PFD displays the relationship between major equipment of a plant facility and does not show minor details such as piping details and designations. Another commonly-used term for a PFD is a flowsheet.
Typically, process flow diagrams of a single unit process will include the following:
Process piping
Major equipment items
Control valves and other major valves
Connections with other systems
Major bypass and recirculation streams
Operational data (temperature, pressure, mass flow rate, density, etc.), often by stream references to a mass balance.
Process stream names
Process flow diagrams generally do not include:
Pipe classes or piping line numbers
Process control instrumentation (sensors and final elements)
Minor bypass lines
Isolation and shutoff valves
Maintenance vents and drains
Relief and safety valves
Flanges
Process flow diagrams of multiple process units within a large industrial plant will usually contain less detail and may be called block flow diagrams or schematic flow diagrams.
Single process unit
The process flow diagram below depicts a single chemical engineering unit process known as an amine treating plant:

Unit process

In manufacturing, a unit process is single component part of the end-to-end manufacturing process that transforms raw materials into finished goods.
Chemical engineering unit processing consists of the following important processes:
Oxidation
Reduction
Hydrogenation
Dehydrogenation
Hydrolysis
Hydration
Dehydration
Halogenation
Nitrification
Sulfonation
Ammoniation
Alkaline fusion
Alkylation
Dealkylation
Esterification
Polymerization
Polycondensation
Catalyze

Unit operation

In chemical engineering and related fields, a unit operation is a basic step in a process. For example in milk processing, homogenization, pasteurization, chilling, and packaging are each unit operations which are connected to create the overall process. A process may have many unit operations to obtain the desired product.
Historically, the different chemical industries were regarded as different industrial processes and with different principles. In 1923 William H. Walker, Warren K. Lewis and William H. McAdams wrote the book The Principles of Chemical Engineering and explained the variety of chemical industries have processes which follow the same physical laws. They summed-up these similar processes into unit operations. Each unit operation follows the same physical laws and may be used in all chemical industries. The unit operations form the fundamental principles of chemical engineering.
Chemical engineering unit operations consist of five classes:
Fluid flow processes, including fluids transportation, filtration, solids fluidization
Heat transfer processes, including evaporation, condensation
Mass transfer processes, including gas absorption, distillation, extraction, adsorption, drying
Thermodynamic processes, including gas liquefaction, refrigeration
Mechanical processes, including solids transportation, crushing and pulverization, screening and sieving
Chemical engineering unit operations also fall in the following categories:
Combination (mixing)
Separation (distillation)
Reaction (chemical reaction)
Chemical engineering unit operations and chemical engineering unit processing form the main principles of all kinds of chemical industries and are the foundation of designs of chemical plants, factories, and equipment used.