CLASSIFICATTION OF PUMPS

Sunday, 29 May 2011


Classifications of Pumps:

Selecting between Centrifugal Pumps and Positive Displacement Pumps

Pumps are in general classified as Centrifugal Pumps (or Roto-dynamic pumps) and Positive Displacement Pumps.

Centrifugal Pumps (Roto-dynamic pumps)

The centrifugal or roto-dynamic pump produce a head and a flow by increasing the velocity of the liquid through the machine with the help of a rotating vane impeller. Centrifugal pumps include radial, axial and mixed flow units.
Centrifugal pumps can further be classified as 
  • end suction pumps
  • in-line pumps
  • double suction pumps
  • vertical multistage pumps
  • horizontal multistage pumps
  • submersible pumps
  • self-priming pumps
  • axial-flow pumps
  • regenerative pumps 

Positive Displacement Pumps

The positive displacement pump operates by alternating of filling a cavity and then displacing a given volume of liquid. The positive displacement pump delivers a constant volume of liquid for each cycle against varying discharge pressure or head.
The positive displacement pump can be classified as: 
  • Reciprocating pumps - piston, plunger and diaphragm
  • Power pumps
  • Steam pumps
  • Rotary pumps - gear, lobe, screw, vane, regenerative (peripheral) and progressive cavity

Selecting between Centrifugal or Positive Displacement Pumps 

Selecting between a Centrifugal Pump or a Positive Displacement Pump is not always straight forward. 

Flow Rate and Pressure Head 

The two types of pumps behave very differently regarding pressure head and flow rate: 
  • The Centrifugal Pump has varying flow depending on the system pressure or head
  • The  has more or less a constant flow regardless of the system pressure or head. Positive Displacement pumps generally gives more pressure than Centrifugal Pump's.

Capacity and Viscosity

Another major difference between the pump types is the effect of viscosity on the capacity:
  • In the centrifugal pumpthe flow is reduced when the viscosity is increased
  • In the positive displacement pump the flow is increased when viscosity is increased
Liquids with high viscosity fills the clearances of a Positive Displacement Pump causing a higher volumetric efficiency and a Positive Displacement Pump is better suited for high viscosity applications. A Centrifugal Pump becomes very inefficient at even modest viscosity.

Mechanical Efficiency

The pumps behaves different considering mechanical efficiency as well.
  • Changing the system pressure or head has little or no effect on the flow rate in the Positive Displacement Pump
  • Changing the system pressure or head has a dramatic effect on the flow rate in the Centrifugal Pump

Net Positive Suction Head - NPSH

Another consideration is the .
  • In a centrifugal pump NPSH varies as a function of flow determined by pressure
  • In a positive displacement pump , NPSH varies as a function of flow determined by speed. Reducing the speed of the Positive Displacement Pump pump, reduces the NPSH.

AMMONIA PROCESS


 Synthetic Ammonia process:

Synthetic ammonia (NH3) refers to ammonia that has been synthesized (Standard Industrial Classification 2873) from natural gas. Natural gas molecules are reduced to carbon and hydrogen. The hydrogen is then purified and reacted with nitrogen to produce ammonia. Approximately 75 percent of the ammonia produced is used as fertilizer, either directly as ammonia or indirectly after synthesis as urea,ammonium nitrate, and monoammonium or diammonium phosphates. The remainder is used as rawmaterial in the manufacture of polymeric resins, explosives, nitric acid, and other products.Synthetic ammonia plants are located throughout the U. S.andCanada.Syntheticammonia isproduced in 25 states by 60 plants which have an estimated combined annual production capacity of 15.9million megagrams (Mg) (17.5 million tons) in 1991. Ammonia plants are concentrated in areas with abundant supplies of natural gas. Seventy percent of U. S. capacity is located in Louisiana, Texas, Oklahoma, Iowa, and Nebra ska.

Process Description:

Anhydrous ammonia is synthesized by reacting hydrogen with nitrogen at a molar ratio of 3 to 1, then compressing the gas and cooling it to -33°C (-27°F). Nitrogen is obtained from the air, while hydrogen is obtained from either the catalytic steam reforming of natural gas (methane [CH4]) or naphtha,or the electrolysis of brine at chlorine plants. In the U. S., about 98 percent of synthetic ammonia isproduced by catalytic steam reforming of natural gas.
 general process flow diagramof a typical ammonia plant:


Six process steps are required to produce synthetic ammonia using the catalytic steam reformingmethod: 
 (1) natural gas desulfurization
 (2) catalytic steam reforming
 (3) carbon monoxide (CO) shift
 (4) carbon dioxide (CO2) removal
 (5) methanation and
 (6) ammonia synthesis.
The first, third, fourth, and fifth steps remove impurities such as sulfur, CO, CO2and water (H2O) from the feedstock, hydrogen, and synthesis gas streams. In the second step, hydrogen is manufactured and nitrogen (air) is introduced into this 2-stage process. The sixth step produces anhydrous ammonia from the synthetic gas. While all ammonia plants use this basic process, details such as operating pressures, temperatures, and quantities of feedstock vary from plant to plant.

 Natural Gas Desulfurization :

In this step, the sulfur content (as hydrogen sulfide [H2S]) in natural gas is reduced to below 280 micrograms per cubic meter (µg/m3) (122 grams per cubic feet) to prevent poisoning of the nickel catalyst in the primary reformer. Desulfurization can be accomplished by using either activated carbon or zinc oxide. Over 95 percent of the ammonia plants in the U. S. use activated carbon fortified with metallic oxide additives for feedstock desulfurization. The remaining plants use a tank filled with zinc oxide for desulfurization. Heavy hydrocarbons can decrease the effectiveness of an activated carbon bed. This carbon bed also has another disadvantage in that it cannot remove carbonyl sulfide. Regeneration of carbon is accomplished by passing superheated steam through the carbon bed. A zinc oxide bed offers several advantages over the activated carbon bed. Steam regeneration to use as energy is not required when using a zinc oxide bed. No air emissions are created by the zinc oxide bed, and the higher molecular weight hydrocarbons are not removed. Therefore, the heating value of the natural gas is not reduced.

Catalytic Steam Reforming :

Natural gas leaving the desulfurization tank is mixed with process steam and preheated to 540°C (1004°F). The mixture of steam and gas enters the primary reformer (natural gas fired primary reformer) and oil fired primary reformer tubes, which are filled with a nickel-based reforming catalyst. Approximately 70 percent of the CH4is converted to hydrogen and CO2An additional amount of CH4 . is converted to CO. This process gas is then sent to the secondary reformer, where it is mixed with compressed air that has been preheated to about 540°C (1004°F). Sufficient air is added to produce a final synthesis gas having a hydrogen-to-nitrogen mole ratio of 3 to 1. The gas leaving the secondary reformer is then cooled to 360°C (680°F) in a waste heat boiler.

 Carbon Monoxide Shift :

After cooling, the secondary reformer effluent gas enters a high temperature CO shift converter which is filled with chromium oxide initiator and iron oxide catalyst. The following reaction takes place in the carbon monoxide converter:
CO+ H 2O → CO2+H2
The exit gas is then cooled in a heat exchanger. In some plants, the gas is passed through a bed of zinc oxide to remove any residual sulfur contaminants that would poison the low-temperature shift catalyst. In other plants, excess low-temperature shift catalyst is added to ensure that the unit will operate as expected. The low-temperature shift converter is filled with a copper oxide/zinc oxide catalyst. Final shift gas from this converter is cooled from 210 to 110°C (410 to 230°F) and enters the bottom of the carbon dioxide absorption system. Unreacted steam is condensed and separated from the gas in a knockout drum. This condensed steam (process condensate) contains ammonium carbonate ([(NH42CO3· H2O]) from the high-temperature shift converter, methanol (CH3OH) from the lowtemperature shift converter, and small amounts of sodium, iron, copper, zinc, aluminum and calcium. Process condensate is sent to the stripper to remove volatile gases such as ammonia, methanol, and carbon dioxide. Trace metals remaining in the process condensate are removed by the ion exchange unit.
 Carbon Dioxide Removal:
In this step, CO2
in the final shift gas is removed.
CO2removal can be done by using 2 methods:
monoethanolamine (C2H4NH2OH) scrubbing and hot potassium scrubbing. Approximately 80 percent ofthe ammonia plants use monoethanolamine (MEA) to aid in removing CO2. The CO2 gas is passedupward through an adsorption tower countercurrent to a 15 to 30 percent solution of MEA in waterfortified with effective corrosion inhibitors. After absorbing the CO2, the amine solution is preheated andregenerated (carbon dioxide regenerator) in a reactivating tower. This reacting tower removes CO2 bysteam stripping and then by heating. The CO2gas (98.5 percent CO2) is either vented to the atmosphereor used for chemical feedstock in other parts of the plant complex. The regenerated MEA is pumped backto the absorber tower after being cooled in a heat exchanger and solution cooler.
 Methanation :
Residual CO2in the synthesis gas is removed by catalytic methanation which is conducted over a nickel catalyst at temperatures of 400 to 600°C (752 to 1112°F) and pressures up to 3,000 kilopascals (kPa) (435 pounds per square inch absolute [psia]) according to the following reactions:

CO +3H 2 → CH4+H2O
CO (3)+2H2 → CO +2H2O
2CO +4H2 → CH4+2H2O
Exit gas from the methanator, which has a 3:1 mole ratio of hydrogen and nitrogen, is then cooled to
38°C (100°F).
 Ammonia Synthesis :
In the synthesis step, the synthesis gas from the methanator is compressed at pressures ranging from 13,800 to 34,500 kPa (2000 to 5000 psia), mixed with recycled synthesis gas, and cooled to 0°C (32°F). Condensed ammonia is separated from the unconverted synthesis gas in a liquid-vapor separator and sent to a let-down separator. The unconverted synthesis is compressed and preheated to 180°C (356°F) before entering the synthesis converter which contains iron oxide catalyst. Ammonia from the exit gas is condensed and separated, then sent to the let-down separator. A small portion of the overhead gas is purged to prevent the buildup of inert gases such as argon in the circulating gas system. Ammonia in the let-down separator is flashed to 100 kPa (14.5 psia) at -33°C (-27°F) to remove impurities from the liquid. The flash vapor is condensed in the let-down chiller where anhydrous ammonia is drawn off and stored at low temperature.

emissions and controls:
Pollutants from the manufacture of synthetic anhydrous ammonia are emitted from 4 process steps: (1) regeneration of the desulfurization bed, (2) heating of the catalytic steam, (3) regeneration of carbon dioxide scrubbing solution, and (4) steam stripping of process condensate. More than 95 percent of the ammonia plants in the U. S. use activated carbon fortified with metallic oxide additives for feedstock desulfurization. The desulfurization bed must be regenerated about once every 30 days for an average period of 8 to 10 hours. Vented regeneration steam contains sulfur oxides (SOx) and H2S, depending on the amount of oxygen in the steam. Regeneration also emits hydrocarbons and CO. The reformer, heated with natural gas or fuel oil, emits combustion products suchas oxides of nitrogen, CO, CO2, SOx, hydrocarbons, and particulates. Emission factors for the reformer may be estimated using factors presented in the appropriate section in Chapter 1, "External Combustion Source". CO2is removed from the synthesis gas by scrubbing with MEA or hot potassium carbonatesolution. Regeneration of this CO2scrubbing solution with steam produces emission of water, NH3, CO,CO2  and MEA. Cooling the synthesis gas after low temperature shift conversion forms a condensate containing NH3, CO2, CH3OH, and trace metals. Condensate steam strippers are used to remove NH3 and methanolfrom the water, and steam from this is vented to the atmosphere, emitting NH3, CO2 andCH3OH. Some processes have been modified to reduce emissions and to improve utility of raw materials and energy. One such technique is the injection of the overheads into the reformer stack along with the combustion gases to eliminate emissions from the condensate steam stripper.


ALUMINIUM PROCESS

Saturday, 28 May 2011


Bayer process chemistry

Extraction

The aluminium-bearing minerals in bauxite - Gibbsite, Böhmite and Diaspore - are selectively extracted from the insoluble components (mostly oxides) by dissolving them in a solution of sodium hydroxide (caustic soda):
Gibbsite:
Al(OH)3 + Na+ + OH- ---> Al(OH)4- + Na+
Böhmite and Diaspore:
AlO(OH) + Na+ + OH - + H2O ---> Al(OH)4- + Na+
Depending on the quality of the ore it may be washed to beneficiate it prior to processing. The ore is crushed and milled to reduce the particle size and make the minerals more available for extraction. It is then combined with the process liquor and sent in a slurry to a heated pressure digester.
Conditions within the digester (concentration, temperature and pressure) are set according to the properties of the bauxite ore. Ores with a high Gibbsite content can be processed at 140oC. Processing of Böhmite on the other hand requires between 200 and 240°c. The pressure is not important for the process, as such but is defined by the steam pressure during the actual process conditions. At 240°c tl the pressure is approximately 35 atmospheres (atm).
Although higher temperatures are often theoretically advantageous there are several disadvantages including corrosion problems and the possibility of oxides other than alumina dissolving into the caustic liquor.
Clarification – Along with the supersaturated caustic solution, there is a large amount of impurities from the bauxite, which is removed through the use of settling tanks and filtration vessels. Some of the caustic solution will be lost here, and can be recovered by washing the bauxite waste (aka red mud) with caustic soda. The red mud is then sent to a waste management area for disposal, while the supersaturated solution contains through the plan
After the extraction stage the insoluble bauxite residue must be separated from the Aluminium-containing liquor by a process known as settling. The liquor is purified as much as possible through filters before being transferred to the precipitators. The insoluble mud from the first settling stage is thickened and washed to recover the caustic soda, which is then recycled back into the main process. 

Precipitation

Crystalline aluminium trihydroxide (Gibbsite), conveniently named "hydrate", is then precipitated from the digestion liquor:
Al(OH)4- + Na+ ---> Al(OH)3 + Na+ + OH-
This is basically the reverse of the extraction process, except that the product's nature is carefully controlled by plant conditions, including seeding or selective nucleation, precipitation temperature and cooling rate. The "hydrate" crystals are then classified into size fractions and fed into a rotary or fluidised bed calcination kiln. Undersize particles are fed back into the precipitation stage.

Calcination

"Hydrate", is calcined to form alumina for the aluminium smelting process. In the calcination process water is driven off to form alumina:
2Al(OH)3 ---> Al2O3 + 3H2O
The calcination process must be carefully controlled since it dictates the properties of the final product.
FLOW DIAGRAM:



Environmental problems in mining and transporting the bauxite
Think about:
  • Loss of landscape due to mining, processing and transporting the bauxite.
  • Noise and air pollution


    USES:

    aluminium is used forbecause
    aircraftlight, strong, resists corrosion
    other transport such as ships' superstructures, container vehicle bodies, tube trains (metro trains)light, strong, resists corrosion
    overhead power cables (with a steel core to strengthen them)light, resists corrosion, good conductor of electricity
    saucepanslight, resists corrosion, good appearance, good conductor of heat










BIO-DIESEL PROCESS


Biodiesel Production and Quality

The production processes for biodiesel are well known.  There are three basic routes to biodiesel production
from oils and fats:
        *  Base catalyzed transesterification of the oil.
        * Direct acid catalyzed transesterification of the oil.
        * Conversion of the oil to its fatty acids and then to biodiesel.  
Most of the biodiesel produced today is done with the base catalyzed reaction for several reasons:
       * It is low temperature and pressure.
       * It yields high conversion (98%) with minimal side reactions and reaction time.
       * It is a direct conversion to biodiesel with no intermediate compounds.
       * No exotic materials of construction are needed.
The chemical reaction for base catalyzed biodiesel production is depicted below.  One hundred pounds of fat 
or oil (such as soybean oil) are reacted with 10 pounds of a short chain alcohol in the presence of a catalyst to 
produce 10 pounds of glycerin and  100 pounds of biodiesel.  The short  chain alcohol, signified by ROH (usually methanol, but sometimes ethanol) is charged in excess to assist in quick conversion.  The catalyst is usually sodium or potassium hydroxide that has already been mixed with the methanol.  R', R'', and R''' indicate the fatty acid chains associated with the oil  or fat which are largely palmitic, stearic, oleic, and linoleic acids for naturally occurring oils and fats. 

The Bio-diesel Reaction

CH2OCOR'''      CH2OH  R'''COOR
|  Catalyst  |
CH2OCOR''      +     3 ROH        ------> CH2OH + R''COOR
 |       |
CH2OCOR'     CH2OH  R'COOR
100 pounds 10 pounds  10 pounds  100 pounds
 Oil or Fat  Alcohol (3)      Glycerin       Biodiesel (3)


The base catalyzed production of biodiesel generally occurs using the following steps:
Mixing of alcohol and catalyst:
The catalyst is typically sodium hydroxide (caustic soda) or potassium hydroxide (potash).  It is dissolved in the alcohol using a standard agitator or mixer.
Reaction:
The alcohol/catalyst mix is then charged into a closed reaction vessel and the oil or fat is added.  The system from here on is totally closed to the atmosphere to prevent the loss of alcohol.  The reaction mix is kept just above the boiling point of the alcohol (around 160 °F) to speed up the reaction and the reaction takes place.  Recommended reaction time varies from 1 to 8 hours, and some systems recommend the reaction take place at room temperature.  Excess alcohol is normally used to ensure total conversion of the fat or oil to its esters.   
Care must be taken to monitor the amount of water and free fatty acids in the incoming oil or fat.   If the free 
fatty acid level or water level is too high it may cause problems with soap formation and the separation of the 
glycerin by-product downstream.
Separation:
Once the reaction is complete, two major products exist:  glycerin and  biodiesel.  Each has a substantial amount of the excess methanol that was used in the reaction.  The reacted mixture is sometimes neutralized at this step if needed.  The glycerin phase is much more dense than biodiesel phase and the two can be gravity separated with glycerin simply drawn off the bottom of the settling vessel.  In some cases, a centrifuge is used to separate the two materials faster.
Alcohol Removal:  
Once the glycerin and biodiesel phases have been separated, the excess alcohol in each phase is removed with a flash evaporation process or by distillation.  In others systems, the alcohol is removed and the mixture neutralized before the glycerin and esters have been separated.  In either case, the alcohol is recovered using distillation equipment and is re-used.  Care must be taken to ensure no water accumulates in the recovered alcohol stream.
Glycerin Neutralization:
The glycerin by-product contains unused  catalyst and soaps that are neutralized with an acid and sent to storage  as crude glycerin.  In some cases  the salt formed during this phase is recovered for use as fertilizer.  In most cases the salt is left in the glycerin.  Water and alcohol are removed to produce 80-88% pure glycerin that is ready to be sold as crude glycerin.  In more sophisticated operations, the glycerin is distilled to 99% or higher purity and sold into the cosmetic and pharmaceutical markets. 
Methyl Ester Wash:  
Once separated from the glycerin, the biodiesel is sometimes purified by washing gently with warm water to remove residual catalyst or soaps, dried, and sent to storage.  In some processes this step is unnecessary.  This is normally the end of the production process resulting in a clear amber-yellow liquid with a viscosity similar to petrodiesel.  In some systems the biodiesel is distilled in an additional step to remove small amounts of color bodies to produce a colorless biodiesel.   
Product Quality and Registration:  
Prior to use as a commercial fuel, the finished biodiesel must be analyzed using sophisticated analytical equipment to ensure it meets ASTM specifications.  The most important aspects of biodiesel production to ensure trouble free operation in diesel engines are: 
       * Complete Reaction 
       * Removal of Glycerin 
       * Removal of Catalyst 
       * Removal of Alcohol 
       * Absence of Free Fatty Acids
Property
ASTM
Method Limits
Units
Calcium & Magnesium, combined
EN 14538
5 max
ppm (ug/g)
Flash Point (closed cup)
D 93.
93 min
Degrees C
Alcohol Control (One of the following must be met)
Methanol Content
EN14110
0.2 Max
% volume
Flash Point
D93
130 Min
Degrees C
Water & Sediment
D 2709.
0.05 max.
% vol.
Kinematic Viscosity, 40 C
D 445
1.9 - 6.0
mm2/sec
Sulfated Ash
D 874
0.02 max.
% mass
Sulfur
S 15 Grade
S 500 Grade
D 5453
D 5453
0.0015 max. (15)
0.05 max. (500)
% mass (ppm)
% mass (ppm)
Copper Strip Corrosion
D 130
No. 3 max.

Cetane No.
D 613
47 min.

Cloud Point
D 2500
Report
Degrees C
Carbon Residue 100% sample
D 4530*
0.05 max.
% mass
Acid Number
D 664
0.50 max.
mg KOH/g
Free Glycerin
D 6584
0.020 max.
% mass
Total Glycerin
D 6584
T0.240 max.
% mass
Phosphorus Content
D 4951
0.001 max.
% mass
Distillation, T90 AET
D 1160
360 max.
Degrees C


chemical engineering intrview

Thursday, 26 May 2011

1.What is 3rd Law of Thermodynamics?
2.When salt is added to water, what happens to its freezing point?  
3.Difference between Distillation and Fractionation?
4.Can Bernaulli's theorem be applied on gases?
5.Role of Chemical Engineer in Agricultural Sector?
6.What is corrosion? Which is the most important material used for metallic coating? 
7.You have two layer of material to provide insulation. They have heat transfer co-efficient (k1>k2).How will you obtain better insulation? 
8.Explain the designing of distillation column?
9.What is the difference between unit operation and unit process?
10.What is are the main terms in Unit Operations? and what is its charecteristics?
11.How can a chemical production engineer improve the process with respect to yield & quality?
12.Discuss the important properties of solvent used in LEACHING?
13.How to design distillation column?What is the role of chemical engg?
14.Why the efficiency of multiple effect evaporate is less then single effect evaporater ?
15.what is differance between shell and tube heat exchanger and plate and frame heat exchanger?
16.what is types  reactors?
17.what enthalpy?define?
18.what is entropy?
19.what is differance between absorption and distillation?
20.what is mass transfer and heat transfer?
21.what is fluid?
22.tell types of pumps?
23.what is conduction?
24.what is convection and radiation?
25.what is  differance in conduction and radiation?

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