Document No: D/D&P/PRO/G/06 rev 1 w.e.f. 16/9/98
GUIDELINES FOR BASIC ENGINEERING
(Incl. Design Calculations)
Basic engineering includes the following activities:
a) Study the DOB b) Prepare the scheme
c) Batch time cycle finalization / Material balance d) Prepare the PFD
e) Prepare the Conceptual layout f) Prepare the equipment specifications g) Prepare the P&ID
h) Prepare the IPDS (Incl. ON-OFF valves), RD/SRV data sheet
i) Prepare the other documents ( Linelist, service list, Drive list, Effluent data, RM / Utility calculation etc)
j) Prepare the Operation manual and Logic k) Commissioning of the plant
1. STUDY THE DOB.
Study the DOB. Witness the experiments carried out in the Lab and/or and discuss with R&D / Customer to confirm the understanding.
Check for any mismatch between the data in the entire DOB and get it corrected by R&D.
Check for any missing information as required for engineering and get it from R&D
While preparing appraisal, for costing of Plant / individual equipment / item, LAN data is used.
2. PREPARE THE SCHEME
List out the key equipment. List out the major operations / steps and sub-steps. Allocate these operations to the key equipment.
3. BATCH TIME CYCLE FINALIZATION / MATERIAL BALANCE
- Assume batch time cycle for each key equipment.
- Find batch size considering Plant capacity and Yield ( Overall and per pass) and Batch time cycle.
- Batch size change from one key eqpt to next should be studied carefully. (Check whether such # size change will be practical, e.g. slurry
intermediate splitting from 3 batches to 4 batches.) If necesssary correct / increase the batch size accordingly.
- Calculate the equipment size for the batch size from the DOB data and select appropriate size from the standard equipment sizes available (Refer equipment specification section for standard sizes). Select the required size in case standard is not available /suitable.
- Calculate the actual time required for Batch taking into account 10 % margin. ( for human factor, batch to batch performance variation etc.)
- Check whether the assumed and calculated time cycle is matching. If not, repeat the above steps till these match.
- In case the equipment size is too high, more equipment in parallel can be considered.
- These calculations should preferably be done in EXCEL to facilitate check of any other alternative.
- CONTINUOUS PROCESS.
In case of a continuous plant a detailed flowsheet with stream nos added is to be used and material balance (Component wise) is to be prepared.
4. PREPARE THE PFD
Prepare the PFD as per the guideline and following information:
After the key equipment and their sizes are decided, prepare the PFD. Show all equipment around these key equipment taking into account the following: - Raw material supply packaging
- Product physical specs
- Collection, storage , handling and recycle/ disposal of all streams ( Try to use gravity than pumping/ pressure or vacuum transfer)
- Solid handling ( Keep slurry / solid handling to the minimum by re-slurrying, dissolving in situ after isolation, extraction etc)
- Vent scrubbers, condensers, high level venting / dilution
- Required catch pots / seals to avoid mixing / contact with eqpt with different MOC.
- Intermediate cuts
- Storages for countercurrent washes - Traps for the effluent
- Weighing / accounting requirements - Dump reactor / tanks
5. CONCEPTUAL LAYOUT
Prepare the conceptual layout as per the guideline.
6. PREPARE THE EQUIPMENT SPECIFICATIONS
Process Data Sheets of various equipment to be prepared taking following into account :
- Flexibility as required for any probable change in the DOB
- Uniformity in various equipment e.g. Pump model, GCL std reactors /tanks to facilitate common inventory for easy replacement at a later date. - System balancing for maximum possible batch size in key equipment - Corrosion data as available from the DOB and following literature :
Corrosion Resistance Tables, Third Edition, Part A and B, Philip A. Schweitzer.
Corrosion guide by Erich Rabald
- Physical properties data as given in DOB and following literature : a) Lange’s Handbook of Chemistry, Eleventh Edition, John A. Dean. b) Perry’s Chemical Engineers’ Handbook, Perry & Green.
c) Vapor Liquid Equilibrium Data at Normal Pressures, First Edition, E. Hala, I. Wichturle, J. Polak & T. Boublin.
d) Azeotropic Databook , Lee Horsely.
e) The Properties of Gases & Liquids , Fourth Edition, Robert C. Reid, John M. Prausnitz, Bruce E. Poling.
f) Computer Aided Databook of Vapor Liquid Equilibrium , Hirata Mitsuho, Ohe Shuzo & Naga Hama.
In case data is not available then study/ use the experimental data from Lab. Design of various equipment should be done as per the reference / guideline mentioned below :
TYPE OF EQPT REFERENCE / GUIDELINE FORMAT No Reactors Annexure I D/D&P/PRO/F/05 Columns Annexure II
Centrifuges Annexure III D/D&P/PRO/F/04 Filter Annexure IV
Vacuum pump / Ejector Annexure V
Exchangers and Process Heat transfer D/D&P/PRO/F/06 Other Heat transfer calculations by D Q KERN
Pumps Annexure VI D/D&P/PRO/F/07 Tanks Use 80-90% filling max D/D&P/PRO/F/08 Other eqpt As per literature data D/D&P/PRO/F/04 Refer explanatory notes for the format also.
7. P&ID PREPARATION
P&ID should be prepared as per the guideline and after taking into account the following :
- Legends document should be prepared for each project showing the symbols used in the P&IDs. A legend drawing prepared for BP0095 to be generally followed.
- MOC of piping etc should be judiciously decided to keep the cost to the minimum and taking into account the corrosion data as available from the DOB and Corrosion Resistance Tables, Third Edition, Part A and B, Philip A. Schweitzer
Corrosion guide by Erich Rabald
- One system should preferably be provided with single MOC, however vapor / vent equalisation lines can have a different spec.
- Take into account effect of moisture ingress by any means on MOC selected.
- Piping specs should be selected from the available std specs and used. In case of non-availability of suitable spec, FLUID service list should be issued to PIP for their recommendation on suitable spec to be used. - Refer MSDS and properties data to decide the Conductivity strip ( for
Static electricity problem) and Flange guard ( For hazardous / hot fluids) requirement.
- Provide various safety gadgets like Flame arrestor, RD, SRV, TRV, Gas detectors etc (Smoke and heat detectors, Fire alarm system are to be provided but not to be mentioned in the P&ID)
- Provide adequate piping connections to take care of any probable abnormal operation and recycle / reprocessing of the off spec materials.
- Provide adequate local / panel instruments, sight glasses, and proper type of manual valves at proper location so as to facilitate smooth operations from field / panel.
- Local instruments should be provided to crosscheck the Panel instruments and to control of operations from the field in exceptional cases.
- Pressure gauge to be provided on steam in jacket, pressure and return temp of cooling/heating media to assess the performance of heating / cooling operation
- Adequate isolation valves to be provided for each equipment or a group of equipment to isolate them in case of emergency.
- Slurry piping should be considered critically for routing specified, valves and instrument or any other items specified in the piping. Provisions for cleaning should also be considered.
- Adequate drains and vents should be provided to facilitate proper cleaning / draining of the system.
- Jacketting / tracing requirements should be carefully studied / provided. Action plan in case of jacketting / tracing is not used ( by mistake) should also be thought of and incorporated.
- U seals (Incl. inverted) should be specified wherever required to avoid mix-ups / backflow.
- Slopes and non-hold up lines requirement should be critically studied and specified.
- Check valves / catch pots to be provided to avoid reverse flow.
- Strainer requirement to be studied to avoid foreign material ingress from MS eqpt to SS / MSGL assemblies OR to trap solids.
- Venting locations should be properly specified for safety.
- Blinds should be provided on various nozzles / valves to avoid any leakage to atmosphere of Toxic material.
- Nitrogen blanketing / Vacuum connections to be provided as required.
- Line sizing in a batch plant is not so critical due to cushion available in the time cycle from other activities. However, it should be done as per ANNEXURE VIII and the comments given below :
All lines (except steam) should preferably be 25 NB size minimum. Transfer lines to be sized as per the timing available in time cycle and 2-3 M/sec (liquid) velocity. Gravity transfer and slurry transfer to be at ~0.6 M/sec velocity. Transfer time should generally be kept below 1 hr. Steam lines to be sized at 10-15 M/sec velocity.
Vent equalisation / vent lines can be of 25/ 40 NB size. Steam lines to limpets for temp maintenance to be of 15 NB / 25 NB.
8 PREPARE IPDS / RD/SRV DATA SHEET
RD/SRV data sheet to be prepared as per the procedure given in. Annexure VII IPDS to be filled in the prescribed formats as per the explanatory notes.
9 PREPARE THE OTHER DOCUMENTS ( Linelist, service list, Drive list, . . Effluent data, Energy balance, RM / Utility calculation etc)
Prepare these documents in prescribed formats as applicable as per explanatory notes.
RM calculations should have reference to DOB data. (Preferably to be done in EXCEL to check . . the variations in DOB easily) . Following norms to be used for solvent losses calculations :
TYPICALSOLVENT > Operation involved Methanol Toluene / Xylene General handling 1 1 Vac distillation 5 5 Atm distillation 5 3 Filteration 5 3 Extraction 2 2
10 PREPARE OPERATION MANUAL AND LOGIC : To be prepared as per the guideline. Logic requirement should be in line with the Operation manual.
11 COMMISSIONING OF THE PLANT : To be done as per the guideline 12 LIST OF ADDITIONAL IMPORTANT REFERENCES
( Available in Library)
a) Applied Process Design for Chemical & Petrochemical Plants Vol. 1/2/3, Second Edition, Ernest E. Ludwig.
b) Mass Transfer Operations, Second Edition, Robert Treybal. c) Coulson & Richardson’s Chemical Engineering Vol. 6 ( Design ) ,
Second Edition, R. K. Sinnott.
d) Encyclopedia of Chemical Technology, Fourth Edition, Kirk Othmer. e) Ullmann’s Encyclopedia of Industrial Chemistry, Fifth Edition.
ANNEXURE I
(Prepared by KSS)REACTOR
MS & SS reactors
MS/GL reactors MS/LB, MS/LB/TL, MS/FRVE/TL, MS/CTL reactors MS & SS reactorsa) Capacity For calculation click Capacity chart
Based on the batch size & reaction mass volume the capacity of the reactor is decided. The R.M volume is available from R & D in the DOB. The # size is decided based on the capacity of the plant.
For a given # size the volume of R.M, lit. = Max R.M volume (lit/km) x # size (km).
From this volume of the reaction mass the suitable reactor is selected from the GCL standard. Standard reactor in GCL
4400 lit. 5750 lit.
9400 lit. Nominal capacity 15700 lit.
19000 lit. 25000 lit.
After selecting a particular reactor the % filling is checked. Vol. R.M *100
% Filling = Nominal cap. of reactor
The % filling chosen is 80 % to start with but it can be as high as 90 % based on the GCL standard reactor. For reaction involving gas sparging the % filling should be more 75-80 % for gas space. For washing / extraction it can be as high as 95 % even in some cases.
b) MOC selection
MOC is selected based on the information given in the DOB. If the information is ambiguous the actual corrosion study should be done by corrosion lab (Activities to be co-ordinated by R & D ) & actual corrosion data to be furnished. The reaction mixture contains so many components in it that the reference can not be taken from books also in most of the cases. So corrosion study is must. If the process is to be fitted in the existing plant which is having some equipment then the corrosion study should be done for the MOC of the said equipment to certify its suitability.
c) Agitator design
The design of the agitation system involves: Selection of type of agitator
Deciding sweep Deciding RPM
Calculation of fluid power
Calculation of motor HP / Motor selection Sizing / Selection of gear box
Selection of pulleys & belt
Selection of type of agitator
The type of agitator is decided based on the type of application. Various types of agitator & their combinations are used.
Paddle:
2 bladed paddles are used mainly for blending operations. The sweep is normally 60-80 %. Paddle may be used with or without baffles. If baffles are provided it will give good intense agitation & vortex formation will be minimized. Either 2 or 4 baffles are used. Paddles may be straight or pitched (45° pitched). Pitched paddles are used for solid suspension.
Gate anchor:
It may be single or double gate anchor. When the height of the vessel is less single gate anchor is used. If height is more staggered double gate anchor is used. Gate anchors are used mainly for crystallization / Distillation operation. Sweep of these agitators is 80-90 %. These agitators provide very good scraping action on the wall thereby preventing the deposition of solid on the wall (Preventing caking). This results in good heat transfer which is the main criteria for
crystallization / Distillation. Baffles are not used because baffle will help in solid deposition on the wall of the vessel. This type of agitator operate at low RPM of the order of 20-40 depending upon the size of the vessel.
3 bladed curved paddle:
These are used in chemical reaction, distillation etc. This type of agitator is available on glasslined vessels operating at very high speed of the order of 96 RPM. It provides very intense agitation. The sweep is 55-60 %.
Turbine:
There are 2 types of turbine agitator: Open turbine and Disc turbine.
In open turbines 6 / 4 bladed the blades are connected to the hub directly. In disc turbine the blades are connected to disc which in turn is connected to the hub. As per the mounting of the blades the turbines can be flat blade or pitched blade. In pitched turbine the blades are at an angle of 45°. Due to this angle the flow pattern becomes axial in pitched turbine whereas it is radial in flat blade turbines. The sweep is 33-43 %. These operate at very high speed. Agitation is very vigorous. Normally baffles are used with turbines ( 2 or 4 baffles). Pitched turbines are used for solid suspension. Disc turbines are used for 2-phase reaction, gas-liquid reaction.
Multiple agitator:
In this various kind of agitation is possible depending upon the method of installation. In the reactors used in GCL following combination can be achieved by proper fixing of impellers. 60 % & 70 % paddle, no baffle.
60 % & 70 % paddle, 2 baffles or 4 baffles. 80 % staggered double gate anchor, no baffles. 40 % disc turbine, 4 baffles.
70 % paddle + single gate anchor, 2 baffles. Examples of Some of the Typical Agitators used are:
6 bladed flat blade disc turbine is used in GCL for IP condensation. The DMA gas is sparged in p-cumidine. 4 baffles are used.
6 bladed flat blade disc turbine is used for Cypermethrin condensation, Temephos condensation, Napropamide condensation reaction in GCL. These reactions are between aqueous & organic phase with the help of phase transfer catalyst. Very intense agitation is required which disc turbine is quite capable of providing.
80 % staggered double gate anchor is used for 2 CB crystallization in CMAC process, IP crystallization etc.
Paddles are used for distillation operation.
Combination of flat blade disc & pitched turbine is used in hydrogenation of vegetable oil. This reaction is done using raney nickel catalyst. Hydrogen gas is sparged into the vegetable oil. The flat blade disc turbine is used for effective dispersion of hydrogen in oil. The pitched turbine is used for suspending the raney nickel catalyst solid. The bottom pitched turbine is upward flow type & top one is of downward flow type. The reaction is 3 phase reaction involving gas-liquid-solid.
In some of the applications the particular use is playing significant role in the very selection of the agitator. For example in 2 CB preparation in CMAC process previously multiple agitator was sued where disc turbine was extended with blades to make paddle. These blades were connected with verticals to make gate anchor. In the reaction some tarry material was generated. Before starting next batch the vessel needs to be washed very thoroughly. The tar used to stick on the disc turbine & washing became real problem. For this reason the idea of multipurpose agitator was dropped for this particular application. Standard double gate anchor was made & installed. So in this case the very process is playing very significant role in governing the agitator type.
Terminology’s used in agitation Sweep:
It is the ratio of outside diameter of impeller to the inside diameter of the vessel expressed as the percentage. If the vessel dia is 1500 mm and impeller dia is 600 mm the sweep is 40 %.
RPM:
It is the revolution per minute the shaft makes. Power per unit volume:
Power number:
It is a constant & characteristic of a given type of impeller. The power number chart is given in the calculation program.
P X gc X 75
= ---, where X 1000 X (N/60)3 X D5
F = Power number
P = Input fluid power, HP
gc = Acceleration due to gravity, 9.8 m/sec²
= Density of liquid, gm/cc
N = RPM
D = Sweep of the impeller, m
Having known the power number for particular agitator the power required can be calculated. L/D ratio:
It is the ratio of the liquid height to the dia of the vessel.
Deciding sweep
The sweep is decided by the agitator type.
Deciding RPM
It is decided using various scale up scale down criteria, type of application. Some of the scale up criteria are:
Constancy of power per unit volume.
This is the criteria very useful & used in more than 90 % of the cases. If we know the type of agitator in one vessel & its RPM the RPM of other geometrically similar vessel can be determined.
P P --- = --- V 1 V 2 From power number formula P X gc X 75 = X 1000 X (N/60)3 X D5 . . . P N3 D5 . . . N13D15 N23 D25 --- = V1 V2 1/3 N13 D15 V1 N2 = V1D25
Constancy of tip speed.
This is used mainly for reaction. To start with some value of tip speed is taken such as 800-900 ft/min. for reaction involving gas sparging in liquid. Based on this the RPM is calculated. From this RPM scale down is done to lab scale. Experiments are conducted in lab varying some parameters. The best results obtained in lab are again scaled up to plant scale. V = rwhere
V = Tip speed in ft/min. r = Sweep/2 = Angular speed 2 = ---- = 2 T . . . V = 2rN = DN, D = Sweep N = RPM V1 = V2 . . . D1N1 = D2N2 D1 N2 = ---- X N1 D2
Constancy of Reynolds number
This criteria is used mainly for heat transfer purpose such as in applications like crystallization, distillation etc. The overall heat transfer coefficient consists of inside & outside h.t.c. The outside h.t.c is constant depending on the flow rate of utility fluid in the jacket / limpet of the vessel. The inside h.t.c is directly proportional to some power of Reynolds number. D2 N Re = --- , where Re = Reynolds number D = Sweep, cm
N = RPS ( revolutions per second ) = Density, gm/cc = Viscosity, poise. Re1 = Re2 . . . D12 N1 D22 N2 --- = . D12 N11 2 . . N2 = --- RPM D22 2
Calculation of Fluid Power For calculation click---- Agitator calculation Having calculated the required RPM based on the appropriate scale up criteria power can be calculated. Pgc X 75 = --- X 1000 X ( N/60)3 D5 . f X r X 1000 X (N/160)3 X D5 . . P = --- HP , where 75 X 9.81 P = Fluid power, HP = Power number = Density, gm/cc N = RPM D = Sweep of impeller, m
Calculation of motor HP / motor selection
To the fluid power calculated various losses added such as frictional loss in bottom guide, shaft transmission losses, gear box loss, belt friction loss etc. The calculation is shown in the calculation program.
After calculating the motor HP suitable motor is selected for the required duty. The HPs of the standard motor’s are:
0.5, 1.0, 1.5, 2, 3, 5, 7.5, 10, 12.5, 15, 20, 25, 30, 40,50, 60 etc.
The nearest higher size motor should be selected. For example if the motor HP comes to 13.5 by calculation select 15 HP motor. The RPM of the motors used for agitators is normally 1440. If 2900 RPM motor is used lot of reduction will be required to attain the required RPM. Some times where there is a chance of agitator jamming deliberately higher size motor is selected than calculated to overcome the initial torque.
Sizing / selection of gear box
Having selected the motor, gear box is selected by referring to the gear catalogue. Two criteria should be satisfied by the gear selected.
1. The actual output torque is lesser than the allowable input torque to gear. 2. The actual input HP is lesser than the allowable input HP to gear.
HP X 63000 KW X 9550 Torque = - or Torque = ---(lb-inch) RPM (Nm) RPM
For turbine agitators power is more but RPM is also more so the output torque may not be that high. For anchor agitators where RPM is very low, the output torque becomes very high. The gear in such cases may be suitable from HP point of view but may not be o.k from torque carrying capacity point of view. So such gears should not be selected.
The reduction ratio of gear should be selected such that the ratio of pulleys sizes required is <= 1.7. The standard reduction ratios of gears are:
Output RPM of gear Reduction ratio of gear = Input RPM to gear
In GCL most of the gears used are of Radicon make. Planetary gears have also been used of Breveni make. The are high speed gears which give very high efficiency compared to worm gear. Selection of pulleys & belt
From pulleys chart the power carrying capacity of pulleys is given. Number of grooves is decided based on the power carrying capacity per belt after applying various correction factors such as slippage etc. Based on power carrying capacity various sections of pulleys come.
Upto 10 HP : ‘A’ section pulley
10-20 HP : ‘B’ section pulley
20 HP & above : ‘C’ section pulley
Fenner make pulleys are very common & have got lot of flexibility. It has got two parts pulleys & bush. By changing the bushes same pulley can be used for various applications. In ‘C’ section pulley minimum 4 grooves come.
Pulleys are normally designated as: 200B3, 236C4, 280C4 etc. Here 200, 236, 280 etc--- PCD of pulleys
B, C--- Sections
3, 4 --- Number of grooves.
If the ratio of pulley on gear & motor is > 1.7 lot of slippage will occur.
Depending upon the HP of motor, its shaft dia is fixed & for that shaft dia some minimum size of pulley will fix and not less than that. So this should be borne in mind while selecting the pulleys. The calculation program shows power calculation, motor, gear, pulleys design / selection.
MS/GL reactors For calculation & chart click---- Glass lined vessels
For corrosive applications glass lined reactors are used. The design is totally by supplier. For various capacities the details of GMM glass lined reactors are shown in the chart. The reaction mass volume for the given # size is calculated as per the procedure mentioned for MS-SS reactors.
In conventional GMM glass lined vessels three bladed curved paddle is typical impeller used but now various other types of agitators are also available such as turbine, anchor etc. Some of the typical uses of MS/GL reactors in GCL are: Chlorination, Bromination, Acidification to low pH of the order of <=2 etc. MS/LB, MS/LB/TL, MS/FRVE/TL, MS/CTL reactors
For corrosive applications these MOC reactors are also used. On the parent material which in MS bonding / lining is done of lead, FRVE etc. Tiles are acid resistant, carbon etc. Care should be taken while deciding the capacity of these lined vessels. Because of lining the actual ID becomes lower than the parent material which is MS & hence there is reduction in the volume.
The lined vessels should not be used for some materials where contamination of foreign particles is not acceptable at all. For example in polymer no impurity is allowed so tile lined vessels are out of question for final polymer at least. In tile lined vessels teflon lined agitators are also possible to install. The biggest disadvantage of these tile lined vessels is the heat transfer. In MS/LB the heat transfer is possible lead being good conductor of heat. In AR tile lined vessels it is not possible. In carbon tile lined vessels it is possible
carbon being good conductor of heat. In some of the tile lined vessels where heat transfer is real problem heat is removed by reflux condenser.
---ANNEXURE II
(Prepared by VR and PGB)
Design of Fractionation Systems
Fractionation systems represent a major component of chemical process plants .Since large volumes of published data are available design of distillation systems seldom requires pilot plant studies.
Design of fractionation columns involves the following :
Calculation of number of stages
Calculation of height of column
HydrodynamicsI) Calculation of number of stages:
Determination of number of stages required to perform the required separations is first step in design .Accuracy of design to a large extent is dependent on vapour-liquid equilibrium data on which calculations are based.
A) Sources of vapour-liquid equilibrium (VLE) data:
1) Reported VLE data are available in books & Journals . A few selected referances are given below :
a) Vapour Liquid equilibrium data by Hala b) DECHEMA chemistry data series c) Journal of chemical & Engineering data d) Fluid Phase Equilibria
2) If reported VLE data is not available ,either determine VLE by experimental methods or design based on relative volatility () calculated from vapour pressure data. vapour pressure data can be obtained from books/literature or can be determined experimentally .
The following points are to be kept in mind while designing columns based on
from vapour pressure data .
Confirm that azeotrope is not formed. Aspen ; Azeotropic data book by Lee Horsley Perry, Lange gives extensive data on azeotropes.
For highly polar compounds calculation of from vapour pressure data gives large errors & is not to be used.
Vapour pressure in absence of reported data , can be be experimentally determined . Vapour pressure apparatus is available in pilot plant which is suitable for measurement of vapour pressure < 760 mm Hg .For measurement of vapour pressure first determine the range in which vapour pressure -temperature data are to be made .
Ensure that,
i) Data is be collected at several points (minimum five points) . ii) For measurement of vacuum ,correct for barometric pressure.
iii) If we are measuring vapour pressure of two compounds with <1.5 , ensure that temperature & pressure measuring devices have accuracy of < 1 % of full scale .
Standard mercury manometers do not give required accuracy .The following devices improve the accuracy of pressure measuring devices
Oil filled absolute pressure manometer for measuring pressure below 150 mm HgA. Kathetometer which magnifies the scale reading & gives level up to 0.01 mm . 3) Prediction of VLE data from thermodynamic phase equilibrium models using Aspen etc.
4) VLE data from experiment methods : The method followed in GCL till date is distillation method in which small quantity of distillate is collected .This system gives large errors due to condensation of vapours on cold walls of flask. Apparatus called ‘Jones still’ has been fabricated & is available in pilot plant . In this system the returning vapours are bubbled through the liquid to ensure adequate mixing & thus eliminate errors due to condensation of vapours on cold walls of flask .
II) Calculation of height of column :
A)First step is Calculation of Number of Theoretical Stages ( N ) & Reflux Ratio ( R R) For batch fractionations use programme -Batch
For continous fractionations use programme -Continous These programme is based on fixed value of .
Batch fractionations :
i) For fitting in existing column ‘N’ is fixed & R R is to be adjusted accordingly. The programme has three alternatives :
Alternative 1 - for main cut : Input desired composition of distillate .Programme calculates distillate & bottoms quantity ;final bottoms composition .
Alternative 2 - for inter cut : Input desired composition of bottoms .Programme calculates distillate & bottoms quantity ; average composition of distillate .
Alternative 3 - : Input desired quantity of distillate .Programme calculates distillate & bottoms composition ;quantity of bottoms .
ii ) For new column both N & R R is varied to obtain desired results.
Continous fractionation : Input data - Feed flow rate ,composition,condition ; ; R R ; desired distillate & bottom composition
Guidelines :
a)Take at end conditions of the cut.This gives conservative design. b) Check minimum R.R & minimum number. of plates .
Operating R.R = 1.25 to 1.50 times minimum . N =1.5 to 2 times Nmin.
B) Column Height =N * HETP :
HETP is a function of packing size & type ,physical properties(surface tension & viscosity ), distribution & capacity.For calculation of HETP of IMTP packing use Norton Formula (see appendix 1 ).
Norton has given formula for system base HETP which assumes uniform distribution of vapour & liquid in the column.This concept is useful because it isolates the system’s effect on HETP away from distribution conditions.The formula is valid for for systems which are non aqueous , non ionising and have < 3
C)GCL has standardized height of column section for distillation = 4270 mm . Even if calculated height is less ,take column of 4270 mm height. The packed height is 3900 mm and ‘N’ is to be taken accordingly.
Note : 1)M/S Kevin Ind. the manufacturer of IMTP has recommended to dry pack IMTP.
2) Thickness of IMTP is 0.5 mm & hence corrosion should be carefully checked .MOC of packing should be superior to MOC of column thus providing galvanic protection .
III) Hydrodynamics :
A) Packing used are IMTP (available in SS316,SS316 L) and Ceramic intalox saddles Ceramic preferentially wets aq. systems while metal preferentialy wets
organic systems.
IMTP is prefered except in cases where ceramic shows better performance.For e.g aqueous NH3/DMA fractionation.
Prefered size of IMTP is 15 mm .As per Norton IMTP is effective at minimum wetting rate of 0.5 M3/M²/hr. For other packings minimum wetting rate is 1.25 M3/M²/hr .
B) For Columns with IMTP packing ,Column Diameter is calculated as per procedure & capacity/pressure drop correlations given in Norton catalouge. (See appendix 2 )
Columns are to be designed for capacity factor of 60 to 70 % at top.
For columns with other packing ,column diameter is calculated as per generalized correlation of Leva & Eckert given in a)Treybal page 160 (in FPS units )
b) Perry (VI th edition ) page no 18-22 fig 18-38. (in SI units ) Column are to be designed for capacity factor of 60 % at top.
Pressure drop (P)permitted is 10 % of flow the minimum head (h) required are as under : P across column =geometric mean of P at top & P at bottom .
P across column ranges from 0.3 to 1.0 inch water/ft of packing .Flooding can occur when P exceeds 1.3 inch water/ft of packing . For fractionation below 15 mm Hg ,pressure drop across column is critical & is calculated at several points over the height of column.
C) Design of column internals :
1)Liquid distributors Liquid distributor is the most important internal of packed column. Liquid distributors are either gravity type or pressure type. Pressure type distributors consist of perforated pipes and are used in absorbers where head available is much higher . Liquid distributors should be positioned 4 to 6 inch above the packed bed .
A) Gravity type liquid distributors:
Three types available are Orifice ; Weir flow and V notch Weir. Orifice type distributors are less than sensitive to variation of head across the the distributor caused by improper levelling as explained by the relation between flow & head as under :
Orifice : flow (h)^ 0.5 Weir : flow (h)^ 1.5 V notch : flow (h)^ 2.5
If there is 10 mm variation of liquid head & maximum variation Orifice distributor : h = 47.6 mm
Weir distributor : h = 152.3 mm V notch distributor : h = 257.3 mm:
Liquid distributors in use in GCL are of orifice type .These comprise of flat plate having risers for vapour flow & orifices for liquid flow.
a)Location of orifices ; See appendix 3.
b) Diameter of orifice : Diameter of orifice is calculated using equation W = K*A**(2*g*h)^0. where W = flow rate in gms/sec
= density of liquid in gm/cm3 h = height of liquid above orifice cms g = 980 cm/sec²
K = Orifice coeff. =0.7 for punched orifice The diameter of orifice is selected such that height of liq. above orifice (h) is
i)@ TR is does not exceed 120 mm (see note) ii)@ Operating reflux is not below 15 mm
iii) We take minimum diameter of orifice= 5 mm . Orifices smaller than 5 mm may result in clogging .(However orifice of diameter < 5 mm are specified by vendors of internals such M/S Kevin Enterprises. )
The actual liquid head (H) will be increased due to pressure drop(P ) across the distributor & the aeration factor .
H= (h+P ) (density of liq.) --- *
aeration factor (density of liq-density of gas ) The increase due to above 20 mm of water .
Note : The height of gas riser is usually 150 mm . Hence liquid head at TR should not exceed 120 mm .
c)Location/Diameter of gas risers :Locate of gas risers such that
i) gas riser area is distributed equally between inside and outside of 50 % of column area .( riser area inside is 40-60 % of column cross-sectional area.)
ii)Area of gas risers is minimum 15 % of column cross-sectional area . 2) Packing support plate :
In packing support plate gas distribution is more important than liquid distribution .
Separate passages are provided for gas & liquid by locating gas inlets to packed beds above those points from where liquid flows from bed.Open area for gas flow is equal or exceeds 100 % of column cross-sectional area .Gas injection type packing support plates supplied by M/S Kevin have maximum thickness of 3 mm (due to limitation of punching machine ) . Thickness of packing support plates fabricated as per GCL design is 10 mm
.
For fabricated Packing support plates :
a) Gas riser area is distributed equally between inside and outside of 50 % of column area .Gas riser area inside the 50 % of column cross-sectional area should be 45-55 % of column area .This is the most important criteria in design of packing support plates .Number of gas risers ,diameter ,pcd of gas risers are varied to arraive at optimum design .
b) cross-sectional area of gas riser is minimum 25 % of column area. Gas risers are either circular pipes or rectangular channels with cap at the top. The cap is supported on the riser by 5 mm stripes such that flow area betwen riser & cap is equal to column cross-sectional area.
c)Liquid orifice location is not critical .Diameter of liquid orifice is selected such that liquid head above orifice is minimum. Since IMTP 15 is widely used ,inorder that packings should not pass through orifice select 10 mm diameter .
3) Redistributors :
Redistributors comprise of separate packing support plate & liquid distributors . The packing support plate design is similar to the packing support plate at the bottom of column.
In the liquid redistributor , the orifice location ,number & diameter is same as in liquid. distributor at top. The gas risers number & diameter is same as in liquid distributor at top except that gas flow is from sides similar to gas risers on packing support plate.
Appendix 1 Calculation of system base HETP :
System base HETP = A(20/)^0.16 *(1.78)^ for 0.4 cp
= B(20/)^0.19 *(/0.2)^0.21 for > 0.4 cp Where is surface tension of liquid in dynes/cm .For >27 use =27 = Liquid viscosity in CP
Values of ‘A’ and ‘B’ depends on size of packing and are as under Values of A’ and ‘B’ are in mm .
Packing size A B #15 272 296 #25 351 383
#40 412 452 Norton’s standard designs use operating HETP = 13 % above system base HETP
Rule of thumbs :
Appendix 2
Calculation of capacity & pressure drop in column .
Column C 3401
Function
Cut Toluene Toluene
Packing IMTP 15 IMTP 15
Vapour rate: V Kg/hr 1400.000 750.000 Reflux ratio: R 0.500 0.500 Liquid rate L= (R/R+1)*V Kg/hr 466.667 250.000 M3/M2/HR 5.401 2.893 Liquid density: l Kg/M3 870.000 870.000 Molecular weight: M 92.000 92.000
Surface tension S Dynes/cm 21.00 21
Pressure: P MM Hg 760.000 200.000 Temperature:T Deg C 50.000 50.000 Vapour density: g= (M*P*273)/ (760*(t+273)*22.4) Kg/M3 3.471 0.914 Liquid. viscosity: Cp 0.450 0.450
Column Diameter :D Inch 14.000 14.000
Area: A M2 0.099 0.099
Superficial velocity: U M/Sec 1.128 2.296
Capacity factor Cs M/Sec 0.071 0.074
Parameter X 0.021 0.011
Kinematic viscosity: =/l CS 0.517 0.517
Capacity Co: From graph given in Noton
catalouge M/Sec 0.115 0.120
Efficient. capacity
Csc=Co*(S/20)^0.16*(/0.2)^-0.11
M/Sec 0.105 0.110
Capacity Rating: (Cs*100)/Csc % 67.875 67.829
Ordinate value (from graph ) Y 2.620 2.849
Pressure drop:from graph given in Norton catalouge
MM H2O/M 55.000 55.000
Inch H2O/ft 0.660 0.660
APPENDIX 3 LIQUID DISTRIBUTORS:
The design is based on article by F MOORE & F RUKOVENA presented in 36 th Canadian conferance of chemical engineers .
This article describes design of liquid & gas distributors based on concept of distribution quality `DQ'. DQ is a measure of uniformity of liquid flow at the top of the bed. Each distribution point is representated by a point circle, whose centre is located where liquid strikes the top of bed. Sum of area of point circles is equal to Cross Sectional area of column.
DQ = 0.4 (100-A) + 0.6B - [0.33 * (C-7.5)]
where A = Cross sectional tower area not covered by point circles in %. B = Least point circle area in 1/12th of tower area x 100
tower area / 12
OR tower area / 12 x 100 most point circle area in 1/12th tower area
whichever is less
C = area of overlap of point circles x 100 tower area
An optimum distributor evaluated by this method will have all tower area on top covered by point circles. It is geometric fact this is not possible & there will be overlap between point circles and/or point circle overlap outside the tower area. It is also geometric fact, with this method of evaluation, tower area not covered by point circle (A) is equal to point circle area which overlaps tower wall plus overlapping area of adjacent point circles.
Both ‘A’ and ‘C’ values are a measure of uniformity of liquid distributed over the cross sectional tower area at the top of bed .Factor `B' is a measure of deviation of liquid flow from average flow in any relative small area of tower. A 1/12 tower area seems to be an appropriately small area to determine maximum deviation whether flow in that area is high or low. This is most difficult value to determine and requires examination at several locations in tower cross section.Points to be kept in mind while selecting 1/12 th of area are
a) 1/12 th of area should be a continous section.
b) Section can be rectangular or formed from arc of circle at any location across cross-section of distributor.
It is generally found that if liquid hydraulic design will ensure that random flow variation across tower cross section does not exceed 10 % ,each point circle can be assumed to be equal area to simplify distribution quality evaluation procedure.
Norton defines three categories of liquid distributors : High performance distributors : DQ > 90%.
Intermediate performance distributors : DQ = 75 -90 % Standard : DQ = 30 -65 %
Procedure
1) Assume no. of orifices such that there are minimum 65 points per m2 of column Xn Each orifice
corresponds to a point circle.
Area of each point circle = Cross sectional area of column no. of points
2) Assume pitch and fix liquid orifices on a triangular pitch. It has been seen that DQ for orifices on circular pitch is always less than DQ for orifices on triangular pitch .As a first trial ,pitch can taken = diameter of point circle . Draw point circles with orifice as center .
Ensure that distance orifice from column wall = min 25 mm for column of diameter > 16 “ .
3) Draw point circles starting with either distribution point at centre of column or non central distribution point.
4) Calculate A,B & C . Calculate ‘B’ at several locations across Xn of tower. Calculate DQ.
5) Repeat with various values of pitch and location of distribution point and find out alternative which gives highest value of DQ.
6) Locate gas risers such that
a) X nal area of gas riser is not less than 15 % of tower X nal area .
b) Gas riser area inside 5 0 % of tower cross sectional area is 40 to 60 % of tower area c) Distance between gas riser & orifice is minimum 20 mm.
Table I gives value of DQ of some of GCL columns. Other points to be noted are :
a) Distributor or redistributor must be close to the bed. If there is considerable spacing between distributor & top of bed, the flow stream position on top of packed bed becomes uncontrolled and reduces distribution quality.
b) Bed limiter must not alter flow pattern from distributor. Distribution quality VS tower performance
Sensitivity of tower performance to liquid distribution quality depends only on number of stages each bed of packing could achieve at its system base HETP . Beds of packing designed for many stages will be more sensitive to distribution quality .This is indicated infig 1 .
It has been found that in 1 meter diameter debutanizer number of stages increased from 8 to 15 when distributor with DQ of 36 % was changed to distributor with DQ of 93 %. In 380mm dia. iso octane/toluene system no. of stages increased from 7 to 8.5 when distributor with DQ of 55% was changed to distributor with DQ of 85%.
Conclusion : The importance of liq. distributor increases as number of stages per section of column increases . Liquid & gas flow , packing size and type does not affect the performance of distributor.
TABLE - I
DETAILS OF LIQUID DISTRIBUTORS
Diam
eter
No. of
orifice
DQ
%
Pitch (MM) Dia. of Riser (NB)No. of
Risers
Ratio of X’nal area of riser to column PCD of gas riser (MM) No. of Dist. Point per M² Riser area in side : Outside of 50% Col. area. 14” 3+3+6=12 76 100 65.0 6 20.0 242 125.0 68 : 32 18” 3+3+6=12 72 130 80.0 6 18.6 303 73.5 62 : 38 22” 1+6+12=17 87 126 90.0 1+6 18.0 0;396 80.0 61 : 39 28” 1+6+12+12 = 31 79 140 90.0 1+6+6 20.0 0;380;576 78.0 55 : 45ANNEXURE III
(Prepared by KSS)
CENTRIFUGE
A centrifuge is an equipment utilizing centrifugal force for separation of liquid from solids. It is essentially a development of Gravity Filter wherein the force acting on the liquid, instead of being restricted to gravity, is enormously increased by utilizing centrifugal force. Due to good performance and high cost, centrifuges are often referred to as the Rolls – Royces of solid – liquid separation.
G- level :- Centrifugal acceleration (G) is measured in multiples of earth gravity:
G r b --- = --- g g Where, G : Centrifugal acceleration , m/s2 n) Angular velocity rb : Basket radius, m
g : Acceleration due to gravity, 9.81 m/s2
n : Revolutions per second, s-1
G- force vs. Throughput :- As stated above Centrifugal acceleration is,
G = r
b
The throughput capacity (Q) of a machine, depending on the process need , is roughly proportional to the nth power of basket radius:
Q = C1 (rb)n
Where n is normally between 2 to 3 , depending on the characteristics of slurry to be centrifuged and specifications of centrifuged cake, viz., purity, LOD, absorbent value.
It follows that large centrifuges can deliver high flow rates but separation is @ lower G- force; vice versa, smaller centrifuges can deliver lower flow rate but separation is @ higher G – force.
Centrifuges are classified according to the mechanism used for solids separation: Sedimentation Centrifuges and Filtration Centrifuges
1. Sedimentation Centrifuges :- In these centrifuges the separation is dependent on a difference
in density between the solid and liquid phases (solid heavier). Decanter centrifuge (S-1002; Model No.: S-3400, Make: Pennwalt India Ltd.) used at GCL, Panoli is a type of Sedimentation centrifuge. S-1002 is the only Decanter Centrifuge in GCL.
Decanter Centrifuge:- They are generally applicable to particle size range 1 – 5,000 m. A decanter centrifuge is basically a settling tank of circular form mounted on an axis (horizontal) and spun at high speed to produce separation of solids in decanter bowl. A screw type conveyor carried internally and rotated relative to the bowl provides continuous discharge. The speed with which the cake transports is controlled by differential speed (between bowl and conveyor). High differential speed facilitates high solid throughput where the cake thickness is kept minimum so as not to impair filtrate quality due to entrainment of solids. Also cake de - watering is improved due to reduction in drainage path with smaller
cake height; however, this is offset by fact that the higher differential speed also reduces cake residence time. Therefore, an optimum differential speed is required to balance filtrate clarity and cake dryness.
2. Filtration Centrifuges :- They separate the phases (solid – liquid) by filtration. Such filters
essentially consist of a rotating perforated basket equipped with a filter medium. Similar to other filters, filtration centrifuges do not require a density difference between the solids and the suspending liquid. If such density difference exists sedimentation takes place in the liquid head above the cake. This may lead to particle size stratification in the cake, with coarser particles being closure to the filter medium and acting as precoat for the fines to follow. The capacity of filtration centrifuges is very much dependent on the solids concentration in the feed.
As a general rule, sedimentation centrifuges are used when it is required to produce a clarified filtrate whereas filtration centrifuges are used to produce a pure dry solid.
It is convenient to classify the filtration centrifuges into two broad classes, depending on how solids are removed : fixed bed and moving bed
In the fixed - bed type, the cake of solids remains on the walls of the perforated basket
equipped with filter medium until removed manually, or automatically by means of a knife arrangement. They are essentially cyclic in operation. Top Discharge & Bottom Discharge
Basket Centrifuges and Peeler Centrifuges are fixed – bed centrifuges.
In the moving – bed type, the mass of solids is moved along the basket by a ram.
Washing and drying zones can be incorporated in the moving - bed type. It is essentially
continuous in operation. Pusher Centrifuge is moving – bed centrifuge. Basket Centrifuges ( Top & Bottom
Discharge):-They are applicable to particle size range 10 – 8,000 m.
The basket housing is supported by a three – point suspension called the three – column
centrifuge. Suspension of machine on three columns provides extensive compensation of any
imbalances of the system, thus dispensing with concrete or damper foundations.
The simplest of the fixed – bed centrifuges is the perforated basket centrifuge which has a vertical axis. They are equipped to handle feeding, washing and discharge requirements in a discontinuous filtration process with minimum attrition of the solids. The suspension to be separated enters the machine via a stationary feed pipe. When the basket is filled, the feed valve is closed by automatic control. The subsequent treatment consists of drainage of mother liquor, washing of solids, drainage of the wash liquid, and discharge of the cake.
In the case of bottom discharge, a knife removes the cake towards the open centre, leaving a thin residual layer of cake in the basket. In the top discharge machine, the solids are removed manually ( can be removed pneumatically, mechanically, or by withdrawal of the entire filter bag).
After removal of the cake, the centrifuge is ready for another charge. Programming the sequence of events can be accomplished by a fully automatic control unit.
These types of centrifuges are in use in GCL.
Peeler Centrifuges :- They are applicable to particle size range 10 – 8,000 m.
The vertical axis of the basket centrifuge may cause some non-uniformity due to the effects of gravity, with the accompanying problems when cake washing is used. This can be eliminated by making the axis horizontal. This is known as Peeler Centrifuge.
A peeler centrifuge is designed to deal with a wide range of suspensions discontinuously in lots. Each lot is subdivided into the necessary operations – Feeding, Spinning-I to drain off mother liquor, Washing, Spinning-II to drain off wash liquid, Scraping the cake. This adjustable lot cycle is mostly controlled automatically. The various operations within a lot can be performed at constant or varying speed of the centrifuge drum.
The principal application is for high output duties with non-fragile crystalline materials giving reasonable drainage rates which requires good washing and de-watering.
The suspension to be separated is fed to the centrifuge through a feed pipe. In the filtration process the liquid filters through a filter cloth under the effect of centrifugal force. Filtrate is drained through the perforated basket into the filtrate tank. The solid material is retained in the basket by filter cloth and forms a uniform layer. The wash liquid is sprayed on the solid layer through a wash pipe to wash it. The resulting wash liquor leaves the centrifuge in the same manner as the main filtrate and is stored in wash liquor tank. The solids are centrifuged until the desired residual moisture / organic liquid content is reached, and scraped out by a hydraulically operated scraper knife down to a residual layer – which remains on filter cloth. The solid material is discharged by means of a chute or screw. The scraper knife can not be allowed to contact filter medium, a residual layer of solids / products remains in the basket after each unloading. This serves as a precoat to prevent loss of fines to the filtrate through the filter medium during next cycle. The disadvantage is that it also adds resistance to filtration similar to filter medium. The residual layer may become glazed and impervious from the rubbing action of the knife and a rinse may be frequently required to restore the permeability.
Disadvantages & Advantages :- (Based on experience in Polymer project @ GCL,
Panoli). Centrifuge used: 1250 MM dia (Make: ANUP) Filtering area: 2.46 M²
1. Peeler centrifuge has parts rotating at high speeds and require high engineering
standards of manufacture, high maintenance cost, and special foundations or suspensions to absorb vibrations. It is a very sophisticated and critical equipment.
2. Overflow of basket due to malfunctioning of feed-controller can cause loss of
solids to the filtrate as well problems associated with processing of filtrate.
3. It can be operated even @ 200°C of process temperature using suitable lubricating
oil. (It is already established in case of salt filtration from PES Polymer solution @ 200 °C)
4. Cake gets washed thoroughly even with 0.5 Cake Volume wash liquid). Here the
cake mentioned is final product (DCDPS plant).
5. Throughput obtained is 5-7 TPD wet cake with 5-7 % w/w LOD (with 45 – 30
minutes lot cycle & approx. 150 kg lot and 24 hrs operation basis)[DCDPS
plant ]. Whereas throughput in case of centrifugation of PES slurry in water
containing 4-5 % Sulfolane is 11 TPD wet cake with 45 % LOD (with 20 minutes lot cycle & approx.150 Kg lot and 24 hrs operation basis) [PES plant ]. In other
words, throughputs obtainable per unit filtering area for equivalent products are always greater in centrifuges than in conventional filters.
6. Filter cloth fixing / replacement is easier as well as less time consuming. Filter
Following are the details of the Peeler Centrifuges being used in GCL:
Make : ANUP ENGG. LTD., AHMEDABAD
Model : HZ – 125
MOC : SS – 304
NO. OFF : 8 NOS. [5 NOS. at Panoli , 2NOS. at Lote & 1 NO. at Dombivli] Basket I.D. : 1250 MM Basket Height : 625 MM Basket rpm (max.) : 1200 G – level (max.) : 1000 g Filtering Area : 2.46 M²
Nominal volume : 320 litres
Direction of rotation : Clockwise (looking from front side)
Motor : 100 H.P. (1440 rpm; flameproof)
Cost : Rs. 40.0 lacs (including motor & inverter) (12/02/96)
Pusher Centrifuges :- They are applicable to particle size range 100 – 10,000 m.
Pusher centrifuges utilize continuous filtration for separation of suspended, fast – draining crystalline and granular solids from liquids. A pusher ring plate intermittently moves the cake over the screen or filter area in axial direction up to the edge, over which it is discharged. No cloth can withstand the abrasion due to the cake forced on the cloth and pushed over its surface. So the particle size range they are applicable to is generally coarser (larger than 100 m). The material is handled more gently. The pusher plate is usually powered hydraulically. The pusher frequencies are around 60 – 100 per minute. Pusher centrifuges require high feed concentrations to enable formation of a sufficiently rigid cake to transmit the thrust of piston. The capacities of biggest centrifuges are 60 - 80 TPH. They are used mainly in the Potash and salt industries and for other fertilizers.
ANNEXURE IV
(Prepared by KSS)
FILTER
Types of filters Filer cloth & filter aid Calculation of filtration time
Types of filters
Open nutche
Agitated closed nutche (Rosenmund)
Spiral Used in GCL Candle
Sparkler Plate & frame
Cartridge (For polishing filtration) Rotary vacuum drum
Pressure leaf Bag
Horizontal belt Open nutche
In this filter the filtration is by vacuum. The slurry is dumped in the nutche. Vacuum is applied & filtration done. Cake is washed by putting the wash liquid above the cake. For effective washing of the cake the wash liquid is distributed over the entire surface of the cake. For removal of the cake person has to go inside the filter & remove the cake by showel. The design is very simple. This type of filter comes in MS, SS, PP etc. Moc. The washing of cake is problematic as well as discharge of the cake. Also being open this filter can not be used for foul smelling chemicals. Agitated closed nutche
As the name suggests this is also nutche filter but agitation is possible. This filter has got lot many other advantages over conventional nutche filters.
Because of its closed design it can be used for nasty smelling chemicals. The contamination with foreign particles is avoided. Pressure can be applied in addition vacuum & hence higher pressure differential is possible for filtration. Because of agitation the cake can be reslurried in wash liquid & hence the washing is very effective compared to conventional open nutche. The discharge of the filtered cake is possible by rotating the agitator in reverse direction. In one direction the agitator is pressing the cake & in another direction it is discharging the cake. The final cake can be fed directly to dryer for drying or it can be reslurried & transferred to another vessel for further processing. The very popular MOC is SS.
Spiral
This is pressure filter. There is a shaft around which the rod is wound in the form of spiral. Over this spiral filter cloth is wound. The slurry is fed from the side entry of the filter. The filtrate comes out from the bottom outlet & cake is deposited inside the filter around the spiral. After the pressure drop across filter increases the filter is to be opened, the cake discharged & fitted back
again for next filtration. Spiral filters are used where cake volume is small. It comes in MS, SS, Hast C MOC.
Candle
Candle filter is small in nature. This is used after spiral filter & before cartridge filter. The slurry is fed by pressure. Major filtration is done by spiral filter & only small load comes to this filter. As the pressure drop across it increases it indicates that the filter cloth has clogged, filter is opened & cake discharged. Popular MOCs are MS, SS, Hast C etc.
Sparkler
In this filter there are filter plates mounted one above another. The slurry is fed under pressure by pump. The filtrate comes out from bottom hollow shaft & cake is deposited on various filter plates. As the pressure drop across filter increases it indicates that the plate has been loaded with cake. Filter is opened & cake discharged.
The combination of sparkler, spiral,candle & cartridge is used for fine filtration such as charcoal filtration where the carbon particles are not allowed in the filtrate. In the conventional nutche filter these fine charcoal is very difficult to trap.
The method of operation is that the slurry is circulated through the spiral by pump & filtrate pumped back to the reactor. As the clear filtrate starts coming out the filtrate is paseed through spiral. When the clear filtrate comes out from the spiral it is passed through candle. When the clear filtrate comes out from the candle the filtrate is passed finally through cartridge filter for final polishing filtration. The MOCs are SS, Nickel, Hast C etc.
Plate & frame
As the name suggests this filter consists of plate & frame mounted over a horizontal shaft. The slurry is fed by pump & clear filtrate is collected. Being open in nature it can not be used for foul smelling chemicals. The cake deposits on individual plate. After filtration is over the plates are dismantled & cake removed. The plates & frames can be washed individually & refitted for next filtration. The MOCs available are SS, PP,Hast C etc. In GCL this type of filter has been used only for one or two applications.
Advantages:
Because of its basic simplicity it is versatile & may be used for wide range of materials under varying operating condition of cake thickness & pressure.
Maintenance cost is low.
It provides a large filtering area on a small floor space and few additional associated units are needed.
Most of the joints are external & leakage is easily detected. High pressure operation is usually possible.
It is equally suitable whether the cake or the filtrate is the main product. Disadvantage:
It is intermittent in operation & continual dismantling is apt to cause high wear on the cloths. Despite the improvements mentioned above it is fairly heavy on labour.
Cartridge
Cartridge filters are for very fine filtration at micron level. The cartridge is disposable after it has clogged. For fine charcoal filtration cartridge filter is used at the last stage to trap any carbon
which has passed the candle. The MOCs available are SS, Nickel, Hast C, PP etc. In pharmaceutical industries this type of filters are very common.
In GCL the cartridge filters are used for Oxyclosanide (Veterinary drug) purification with fine charcoal. CPF (Insecticide) purification with fine charcoal.
Rotary vacuum drum
The arrangement consists of a trough in which slurry is fed. There is rotating drum on which filter cloth is fixed. Vacuum is applied within the drum. The drum rotates at very very slow speed. By vacuum the slurry is sucked to the top of drum, it is filtered & cake is deposited on the filter cloth. The deposited cake is cut by knife blade & discharged. This filter is very useful for pasty sticky cake. The disadvantage being that it is not very much air tight so there is smell if some foul smelling chemical is handled. For CPF filtration this filter is used where other types of filters failed to give satisfactory performance.
Other types of filter
Some other types of filters are pressure leaf filter, bag filter, horizontal belt filter etc. For details refer books.
Filter cloth & filter aid
The filter cloths used in GCL are PP, Cotton, Terylene etc. These cloths are available in various mesh sizes. For fine filtration higher size mesh cloth is used. In addition to cloth porous filtration tiles (Grindwell Norton) have also been used. These tiles act as filter cloth. These are very fine in nature.
For fine filtration, say filtration of fine powered charcoal from slurry Hyflo is used. The procedure is to prepare bed of hyflo over filter cloth by circulating slurry of hyflo prepared separately. Once the filter cloth is embedded with hyflo the slurry is fed to the filter. The hyflo bed is easy to prepare. In spiral filter almost always hyflo bed is prepared first & then filtration done.
Calculation of filtration time
By the very nature of the cake two types of cakes are there. Compressible & Non compressible. In filtration two resistances are encountered:
Cake resistance ( )
Filter medium resistance ( Rm )
The pressure drop across filter consists of:
P across cake
P across filter medium
Total pressure drop is the addition of these two pressure drops. If the cake resistance is independent of the pressure drop it is called non-compressible cake. If the cake resistance is dependent on pressure drop it is called compressible cake. The method is different for compressible & non-compressible cake. By nature almost all the cakes are compressible to some extent or other. It is their degree of compressibility that decides whether the cake is compressible or non-compressible.
From lab study time (t) vs filtrate volume (v) data is given. Two methods are available for analyzing these datas.
Method-1
The t/v on y-axis is plotted against v on x-axis. From the slope & intercept the cake resistance & filter medium resistance are calculated respectively. The values are fed along with some other values in the INPUT DATA in program & OUTPUT RESULT is obtained.
Method-2
This is quicker method but it does work for non-compressible cake to the satisfactory level. Here v on y-axis is plotted against t on x-axis. In the lab data the cake thickness is mentioned at the end of filtration. For half of the filtration cake thickness will be half of the total thickness. Time for half filtration is found from the plot. Respective values are put in the INPUT DATA & OUTPUT RESULT is obtained. The filtration factor ‘n’ is calculated which is very close to 2 for most of the non-compressible cakes. Hence filtration is said to follow the square law.
Compressible cake
The filtration data time vs filtrate volume should be collected at various differential pressures in lab. For individual P the plots of t/v vs v are made. From these plots the cake resistance and filter medium resistance Rm are calculated. Table is made of , Rm P.
From these datas two graphs are drawn:
Log () vs Log (P) Rm vs P
The slope of first graph is ‘s’. The equation showing the relationship between cake resistance, & pressure drop P is:
= 0 (P)S
When ‘s’ = ‘0’ the cake is non-compressible. As the value of ‘s’ goes away & away from ‘0’ the cake tends to become compressible. The slope of first plot is ‘s’. From the graph by putting the value of & P into above equation 0 can be calculated.
From Rm vs P graph the value of Rm is taken in the equation for nearest value of P. The cake
ANNEXURE V
( Prepared by KSS )
VACUUM EQUIPMENTS
Introduction
In chemical process industries vacuum is very oftenly used for various purposes e.g.
Vacuum distillation of high boiling organic compounds. At atmospheric distillation, the products may deteriorate due to higher temperature, but in vacuum distillation temperature is lower.
For transferring a material from on place to another, pressure differential can be created by applying vacuum.
In chemical reactions where gases are generated negative pressure that is vacuum is applied for scrubbing gases so that leakage to atmosphere is minimised.
In filtration and drying operations also vacuum is used very frequently. For creating vacuum common devices used are:
Ejectors
Venturi scrubbers Vacuum pumps Ejectors
These are the most common equipments used for creating vacuum because of their simple design, No moving parts.
Basic principle
Type of ejectors
Points to be considered while purchasing steam jet ejector Performance factors
Steam flow through ejector nozzle Information required for ejector selection Cost factors
Basic principles of steam jet ejector
Steam ejectors are pumps without moving parts. Construction and operation are extremely simple in as much as only three main processes are involved.
The main parts are head, the driving nozzle and the diffuser. The main processes are expansion of driving steam in the driving nozzle, mixing of the steam jet thus produced with the medium to be drawn off ( air, gasses or vapors) and the conversion of velocity of this mixture into pressure in the diffusers.
Steam jet ejectors operate at very high velocities. The velocity of the driving steam jet is nearly always many times that of the speed of sound. The large volumes under vacuum can therefore be easily handled. This is the reason why stem ejectors are eminently suitable as vacuum pumps. These are simple is construction, constructed from wide range of material of construction, simple in operation, robust in design, long working life, extremely safe operation.
Type of ejector Single stage
To compress from about 80 torr to atmospheric pressure. It is used generally for compression ration < 10. It is suitable as pre-evacuator.
