Thermosiphon Reboiler 1. Introduction: Thermosiphon Thermosiphon Reboilers are heat exchangers exchangers used to provide stripping section vapor for fractional distillation distillation columns. This type of reboiler reboiler is very popular for use within plants. The reasons for the popularity of the thermosiphon unit are several. First, this type of exchanger minimizes piping and ground area and does not introduce undue undue probl problem emss of tubesi tubeside de access access for clean cleaning ing.. Secon Second d is the relat relative ively ly low low equipment cost associated with this type of exchanger. These reboilers also offer excellent rates of heat transfer. 2. Working Principle: Thermosiphon Thermosiphon reboilers is basically a shell and tube heat exchanger, requiring requiring no pumps to pump the vapor into the column bac. These reboilers reboilers wor on a simple principle based on difference of densities of liquid and vapor. !elow is shown a typical diagram of thermosiphon reboiler.
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Fig#$.%, Thermosiphon Reboiler The above shown reboiler is a typical recirculating reboiler. Recirculation of these systems is driven by the density difference between the outlet and inlet line. &n the system as the total driving force for flow should be equal the total resistance to flow, so we can write" 'riving force ( Resist force against flow )r *%+density% ( ' pipe in - ' pipeout- 'reboiler -*$+average density$ So as seen from the relation written above, the gravitational potential of the liquid boot is responsible for sending the mixture of liquid and vapor bac into the column. So in thermosiphon reboilers we dont need to use any pump for pumping the vapor. &n this reboiler first liquid comes into the reboiler, where it come in contact of hot fluid flowing through the reboiler, due to which it gets heated and most of its part is vaporized. /nd then due to reduction in density, it raises itself and this mixture goes bac to column.
3. Types of Thermosiphon Reboilers: There are mainly two inds of thermosiphon reboiler" • *orizontal, shell#side boiling reboilers 0Figure 1.%2 • 3ertical reboilers 0Figure 1.$2
Horiont!l" shell#side boiling reboilers: There are two types of *orizontal, shell#side boiler. )ne is recirculating and another is once through. &n recirculating reboilers, liquid comes out from the liquid boot at the bottom of the column, gets heated and then goes bac to column. 4hile in case of once through, feed to the reboiler comes from the last tray of the column instead of the liquid boot in the bottom. The liquid gets heated while passing through the shell side and then this heated mixture of vapor and liquid goes bac.
$igure 3.1.1
$igure: 3.1.2
%ertic!l reboilers: These are also of two types" 3ertical, tube side boiling, once through and vertical, shell#side boiling, recirculating. !esides the difference stated earlier between once through and recirculating, there is one more difference that in once through liquid is heated while passing through shell and in recirculating liquid is heated while passing through tubes.
$igure: 3.2.1
$igure: 3.2.2
The table shows the choice of reboiler based on different factors. The table tells which type of reboiler should be chosen as well as which should not be chosen. Factor Favored types 'isfavored types *igh bottoms product )nce#through Recirculating fraction compared to boilup 5ow relative volatility Recirculating systems *igh relative volatility )nce#through Recirculating systems 5arge exchanger size or *orizontal 3ertical high duty requirements Tight plot plan 3ertical *orizontal /mple plot plan *orizontal
&i'uid (&i'uid#%!por )ep!r!tor Introduction: / liquid#liquid#vapor separator is one which first separates vapor from the liquid mixtures and then separates the mixture of two liquids. &t first uses a vertical separator to separate vapor followed by a horizontal separator to separate the two liquids from each other.
)cope: The analysis presented here is a case study to chec whether the design of the separator separating a mixture caustic, 'S) and vapor is appropriate or not.
Principle: &n vertical separators, for separations the velocities of both the continuous phases should be less than the terminal velocities of their dispersed phases respectively. /nd the same principle is in the case of horizontal separator.
*i!gr!m:
$ormul!e +sed: For terminal velocity of vapor bubble in liquid phase" ut ( 0ρv # ρl2+g+'$60%7+µl2 For terminal velocity of liquid droplets in vapor phase" ut ( 8.%91+0ρv # ρl28.:%+g8.:%+'%.%;60ρl8.$<+µl8.;12 For terminal velocity of 'S) droplet in caustic phase" ut ( 0ρv # ρl2+g+'$60%7+µl2 'min ( 0;+=6π+ ut28.9 µv
( viscosity of vapor µl ( viscosity of liquid ρv ( density of vapor ρc ( density of liquid ' ( diameter of bubble or droplet
,bser-!tion !nd !lcul!tion: =v 0vapor volumetric flow rate2 =c0?austic volumetric flow rate2 =d0'S) volumetric flow rate2 µv0viscosity of vapor2 µc0viscosity of caustic2 µd0viscosity of 'S)2 ρv0density of vapor2 ρc0density of caustic2 ρd0density of 'S)2
( >8 m16h ( %8 m16h ( 8.19 m16h ( 8.8%7 c ( %.$ c ( 8.1 c ( %.$< @g6m1 ( %8:8 @g6m1 ( <:8 @g6m1
$or %ertic!l sep!r!tor: For liquid phase" ' ( %:9 micron 0taen from A) data2 So, ut 0bubble2 ( 8.8%9%% m6s 'min 0vapor side2 ( 8.;<$ m ' ( %98 micron 0taen from A) data2
So,
ut 0bubble2 ( 8.88<:8 m6s 'min 0vapor side2 ( 8.>888 m For vapor phase" ' ( $98 micron 0taen from A) data2 So, ut 0droplet2 ( 8.7>:;9 m6s 'min 0liquid side2 ( 8.898 m
$or Horiont!l sep!r!tor: ' ( %$9 microns ut ( 8.888:87 m6s For minimum diameter, 5630continuous phase2 ( '60terminal velocity2
'min ( ;+=560π+5+ut2 *ere 5 ( 7.> B8.:9 ( :.79 m 'min ( 8.>> m
*iscussion: /s seen from the above calculation, in liquid phase of vertical separator minimum diameter belonging to a droplet size of %:9 microns is 8.9. This default value for diameter of droplets has been suggested by A). !ut if diameter of droplet reduces to %98 microns, minimum diameter becomes 8.> m, which is the diameter in our case. 4hich shows that all the droplets smaller than %98 microns will go down with liquid. *ence a diameter of 8.> m is inadequate for our case. &t needs to be increased, as we always have to tae more diameter than the minimum diameter. *owever, diameter of 8.> m is adequate for all other separations as seen from the results calculated.
onclusion: 'iameter of vertical separator requires to be increased in order to avoid vapor bubbles to go with liquid.
&!bor!tory Tests The various tests performed on crude as well as products are briefly described below.
$l!sh point: The minimum temperature, at which the sample produces sufficient vapor, which can flash on applying a flame. &t is associated with the safety during storage. For products, which are stored should have high flash points in order to avoid accidents. &n this experiment a spar is ept being given to sample with gradual increase in temperature. The temperature, at which vapor starts flashing, is flash point. This test is carried out for heavier ends starting from diesel.
Pour Point: The maximum temperature, at which oil becomes immobile due to settling of wax and hence cant be pumped, is nown as our point. &f pour point is less than there will be difficulties in transport of oil. This is mainly done for heavier ends. For *S', it should be 1 deg.? in winter and %9 deeg.? in summer. 4hile for 5') , it is %$ and $1 deg.? in summer and winter respectively.
old filter plugging point: &t is the maximum temperature at which liquid fails to pass through a standard orifice of diameter 8.7mm in % minute. &n this test, time taen for the liquid to pass the orifice is noted, and hence cold filter plugging point can be measured. This is mainly done for heavier ends. &t should generally be less than 8 deg.?.
opper corrosion: This test indicates whether the sample is corrosive towards copper containing alloys or not, because if it is corrosive then it can cause corrosion in equipment while storage and transport. This is applicable to all fuels.
)ulphur: This test measures the content of sulphur in the sample. Sulphur is one of the main pollutant as well as corrosive to the fuel systems. So its quantity in the sample is to be checed. The sulphur content is measure through C#ray test, in which electrons in sulphur atoms are excited. )ther way is use of furnace in which sulphur is first oxidised to S) $, then its quantity is measured 0this test is done to measure sulphur content in
catalyst used for polymerisation2. &ts maximum allowed value is 8.89 D by weight. This test is done for all fuels.
/inem!tic -iscosity: &t is defined as resistance to flow. This property of fuel oils is very crucial for pump selections for transporting. This test is also done for fuel oils. For *S' it should be maximum $.9 cStoe and for 5') it should be max. 8.% cStoe. &n this test, the liquid is passed through a bubble, which is attached to the tubes from both sides. The time taen by the liquid to pass through the bubble is noted and then inemetic viscosity is calculated. The two instruments used for it are ?annon fiensi and Abelhood.
,ct!ne number 0Ron !nd ,: &t is defined as the percent volume of iso#octane in a mixture of iso#octane and normal heptane that gives the same nocing as the fuel tested. &t is a measure of nocing. &n this test the mechanical shocs in the engine are converted into electrical shocs and octane no. is measured by comparing these shocs with that of the standard fuel. &t should be between 7;#77 for the gasoline used in &ndia and above <9 for the gasoline exported. There are two types of )ctane numbers" Research octane no. 0R)E2 and =otor octane no. 0=)E2. The only difference in these two numbers is that these are measured in different test conditions.
4nti /nock Inde5: /@& is defined as the average of R)E and =)E. /@& is regarded as more critical for engine performance than R)E alone.
et!ne umber: &t is defined as the percent volume of n#cetane in a mixture of n# cetane and alpha =ethylnaphthalene that would give the same nocing as that of the fuel under test. &t is also a measure of nocing. This test is performed for 'iesel. &t should be more than ;7. &n this test also, mechanical shocs are converted into electrical shocs and compared with the standard fuel.
)moke Point: &t is defined as the maximum length of the flame, which does not produce smoe when, tested under prescribed condition. This test is mainly done for @erosene. Flame cant be elongated after a certain length, when erosene is burnt, because light decreases due to the formation of smoe. &t should be minimum %7 mm.
Reid -!por pressure: &t is the vapor pressure generated by the vapor, occupying four times of volume as the liquid is occupying, with which it is in equilibrium. This test is done for naphtha and gasoline. This is used to predict the maximum pressure applied to the fuel tan. &t should be around :8 and 98 @pa for Eaphtha and asoline respectively.
6enene content: !enzene content is measured through this test. This test is mainly done for gasoline. !enzene is carcinogenic in nature. S) this test shows how harmful the fuel is. &n &ndia its maximum value is 9D by volume.
4cidity: This test measures acidity of solution. &t is measured by titrating the solution with alc. @)*. This test is performed for all products as well as crude. =ore acidic the sample more corrosive it will be. So acidity of the ample should be less. &t should be less than 8.89 mg@)*6gm.
erc!pt!ne sulphur: This test measures the content of mercaptane sulphur in the sample. &t is measures by titrating the sample with /gE) 1. This test is done also done for all the products as well as crude. This is responsible for pollution.
6romine umber: &t is defined as the grams of bromine reacted with %88 grams of sample. &t gives the olefinity of sample and hence the reactivity. =ore the olefins in the sample more it will react with environment and more gummy product will be formed, which create problem in storage. This test is done for all products. &t should be less than 99.
4)T *istill!tion: etroleum oils are mixtures of hydrocarbons. So we have a range of boiling points instead a boiling point. &t should have a particular range so that it can be used for a particular application. For an example, the fuel to be used in engines should have its boiling point range such that the fuel mixture should burn through out the burning stroe and not Gust at the beginning only. For =otor asoline the specification are as follows" Recovery at :8H? in Dv %8#;9 Recovery at %88H? in Dv ;8#:8 Recovery at %78H? in Dv <8 min Final !oiling oint $%9 max.
&n /ST= distillation, we have a column, which has one theoretical plate and hence acts lie a flash chamber. The distillate samples are taen at different temperatures from this column. From the composition of the distillate samples, the fraction of liquid vaporized at that temperature can be calculated. Thus we can find out &nitial and final boiling point as well as fraction of liquid vaporized at any temperature.
True 6oiling Point *istill!tion: This is a laboratory technique defined for distillation. The column used in T! here is paced with steel pacing, which wors lie %9#%7 theoretical plates. &t uses ettle reboiler. The vacuum column used is a hollow column having an operating pressure of %8 mm*g. This is mainly done to chec the quality of crude. Top temperature is increased gradually. Through distillation different samples are taen at different temperature ranges in separate collector and fractional recovery is measured and compared with the standards given by shell. These samples are called cuts. So the yield pattern liely to be obtained from refinery can be estimated. These cuts undergo various other tests. This test is very important as if the crude processed is rough then that can be blended with good crude and then processed and vice#versa.
rude olumn ?rude is first heated from $9I? to %$I?. Then this desalted crude is again heated in two parallel trains up to %<$I? before entering in flash chamber, where it flashes into vapor and liquid. The vapor directly becomes feed to the crude column while the liquid is heated up to 17$I? using furnace and heat exchangers before entering the crude column. From crude column six product streams are taen out having *eavy /tmospheric as )il 0*/)2, *eavy @erosene )il 0*@)2, 'iesel, 5ight @erosene )il 05@)2 and overhead gas which is unstablised naphtha. ?rude column uses 5ow#pressure steam to provide stripping vapor instead of reboiler.
)verhead gases from ?rude columns are sent to Saturated as ?oncetration unit, where stablized naphtha, 5 are obtained. !ottom product of ?rude column is reduced crude which is sent to 3acuum distillation unit for further precessing. */) from crude column goes to */) stripper, from where bottom product goes to 'iesel hydrotreater and overhead gases are refluxed to ?rude column. This stripper uses a reboiler, which is heat exchanger. *@) goes to heavy erosene stripper, from where also overhead gases are refluxed to ?rude column and bottom product goes to 3)*T 0vacuum gas oil hydrotreater2. 5 steam is supplied to this stripper to provide stripping vapor instead of a reboiler. Similarly, 'iesel and 5@) also go to 'iesel and light erosene stripper respectively. 'iesel stripper uses 5 0low#pressure2 steam and light erosene stripper uses reboiler to provide stripping vapor. From here also overhead gases are refluxed while bottom product is used to heat up crude and then stored in tan.
?olumn &nternals 0oints to be focused" type, manufacturer, application, unique features2 SA5JKR =ellapa"
=ellapa is structured pacing having following features and applications" Anique features" Aniversal acing Type suitable for a wide range of applications. ressure drop of 8.1 to %.8 mbar per theoretical stage. Asable over wide range of liquid loads.
/pplication" 3acuum to positive pressure Super fractionators Sour and acid gas absorbers and desorption columns.
=ellapalus" =ellapalus is an improved version of =ellapa. The improvement has been brought out because of problem of flooding in =ellap pacing. =ellaplus is also structured pacing. =ain features" =ellapalus is high#capacity structured pacing. &t has typically $8 to 18D more capacity compared to conventional structured pacings and the useful capacity is boosted up to 98D. &t offers a wide range of technical and coomercial advantages0lower investments and operating costs2" For new columns" smaller column diameter is required. For existing column" higher capacity at the same efficiency. &t can be used from low vacuum up to high#pressure application. Kverything valid for =ellapa is also true with with =ellaplus.
/pplications" &t has same applications as =ellapa has. =ellagrid" =ellagrid has following features" &t is not sensitive to coing and fouling due to its geometrical structure and smooth surface. &t has much better demisting and separation efficiency than traditional grid. &t has mechanically robust structure. The structure and element height allow for easy cleaning.
/pplication" 3acuum tower 0specially in the bottom2. &t is also used in the pump around section with high liquid and gas loadings. F?? main fractionator slurry and wash section. ?oer
*igh erformance Trays Eutter =3 TrayT=" The Eutter =3 tray is a high performance 3#grid tray. &t combines the benefits of the 3#grid tray and smaller fixed valves to achieve these additional features" reater capacity compared to standard sieve or valve trays. *igher efficiency and lower pressure drop than sieve or valve tray. &mproved liquid flow and vapor6liquid contacting through lateral vapor release. *igh turndown ratio maes it useful for varying design loads. &ncreased tray dec stiffness provides added durability and upset resistance.
Eutter =3 tray with combination of multiple downcomers forms 3=' trays. 3ortexTrays" Features of 3ortexTrays are as follows"
?ircular downcomers provide optimal use of the downcomer area. 3ortex baffles with chimneys for improved vapor diengagement and reduced downcomer choing. 5onger weirs result in lower tray pressure drop and downcomer bac#up. Eo downcomer seal area maximizes the active tray area. 3ortex downcomers will not partition the inter#tray space to allow vapor and liquid flow equalisation for best efficiency.
@och#litsch *igh erformance Trays !i#FR/?L Tray" !i#FR/? trayshave following features" &t offers more efficient vapor#liquid contact with minimal liquid entrainment. ?ompared to sieve trays, capacity of up to 18D can be achieved with no loss of efficiency. The smaller valves on the !i#FR/? tray provide for a more uniform capability then sieve trays. These trays offer far better turndown capability than sieve trays. The !i#FR/? trayMs bi#directional fixed valve configuration generates a self# cleaning action on the tray dec. This design inhibits fouling and provides longer service life and reduced maintenance.
/pplications" ?oer, /tmospheric ?rude, 3isbreaer and F??A =ain Fractionators
olymerising systems /crylonitrile =oderately fouling systems
SAKRFR/?L Tray" The main features are" Ase of mini#valves 0fixed and floating2, bubble promoters and design techniques has enhanced the effective bubbling activity on the tray and improved the flow of fluid across the tray. 4hich results in improvement of both hydraulic performance and mass transfer efficiency. /dvanced downcomer technology and having longer flow path in these trays maximize the active area available for liquid#vapor contact. &nlet area enhancements provide greater active area for improved capacity, better froth initiation and bubbling activity on the tray.
/pplications"
They are especially beneficial in applications requiring a large number of mass transfer stages, or where mass transfer efficiency is critical to the economics of the operation. Kxamples include superfractionators 0ethylene, propylene, xylene2, light hydrocarbon fractionators, splitters in chemical and petrochemical applications, and aromatics services. ENK TR/NSL" The main feature of ENK Tray is that it uses th tray inlet area under the downcomer as active area for vapor#liquid contacting. Thus it provides more area for vapor#liquid contact as well as decreases the rising vapor velocity for the same vapor flow rate, which helps in increasing the capacity of tower without flooding. /pplication" • Kthylene lants • ropylene lants • Refinery 5ight Knds • 'emethanizers • 'eethanizers • ?$ Splitters • 'epropanizers • ?1 Splitters • 'ebutanizers • 'eisobutanizers • /romatics • Cylene Splitters • /cid as Removal Systems • umparound trays
F5KC&TR/NL 3alve Tray" These valve#type trays have replaced bubble cap and sieve trays as the industry standard. Their main features are" These trays offer uniform vapor distribution for high efficiency and wors over a wide operating range. These trays can bear high loads maintaining same efficiency and low pressure drop. &nitial purchase price, simple installation, and reduced maintenance contribute to cost effective proGects when using F5KC&TR/NL 3alve Trays. Kxcellent turn down ratio.
/n existing tower equipped with F5KC&TR/NL 3alve Trays can often be used in a different application with minimum modifications due to the traysM wide operating range, high capacity, low pressure drop, and excellent efficiency. F5KC&/? L *?T=" F5KC&/? *? structured pacing is actually a pile of corrugated sheets. &ts main features are" Anlie F5KC&/, it has been designed for high capacity to avoid flooding.
&t offers low pressure#drop and high efficiency.
The new pacing can be utilized in any application where increased capacity without loss of efficiency is required. /lternatively, by using a smaller crimp size with lower *KT 0higher ETS=2 of F5KC&/? L *?O structured pacing, a higher column efficiency can be obtained without a loss in capacity. ?ascade =ini Ring 0?=R2" &ts =ain features are" &t has 5ow /spect Ratio 0height to diameter ratio2, which is ey to *ighest erformance for =ass Transfer. &t allows easy vapor flow hence offer low pressure#drop. 5ow pressure drops helps in increasing the capacity. Solid entering the pacing is flushed easily through the pacing matrix by the liquid.
?=RO random pacing has been utilized in a wide variety of distillation, absorption and stripping columns around the world.
