olumn
http://chemelab.ucsd.edu/CAPE/keyword/column.html
Rigorous Distillation There are three modules modules (IO, SURE SURE and CHEMDIST, CHEMDIST, in a way they t hey are just subroutines) that ccan an perform rigorous rigorous distillation distillation calculations. The modules solve the genera l heat and mass balance s of equil e quilibrium ibrium staged operations operat ions and thus apply to t o gas absorption, stripping operations operations and liquid-li liquid-liquid quid extractions extrac tions as well as distillations. distillations. (Of course, we are ar e only applying the concept c oncept of theoretical theoret ical stages to absorptions or liquid-li liquid-liquid quid extractions extract ions that use a continuous c ontinuous column.) column.) The differences among the modules are mainly in the iterative iter ative algorithms and how they handle the thermodynamic properties. The SURE SURE method uses a Newton-Raphson convergence convergence scheme sche me with matrix matrix partitioning, partitioning, while while the CHEMDIST CHEMDIST method uses uses the modified Naphtali-Sandholm algorithm which involves stage grouping as a variation of the basic NewtonRaphson procedure. Both of these methods and how to handle liquid-liquid liquid-liquid extraction simulations simulations are explained in detailed in Seader and Henley. The The IO (inside-out) (inside-out) alg a lgorithm orithm uses the stripping st ripping factors as the iteration variables. The convergence is fast fa st because beca use the column is always always in mass balance. Simplifi Simplified ed thermodynamic thermodynamic models are also used to solve the column heat balance s in earlier trials (inner loops). Simple guidelines on which one to use: Always try the IO algorithm algorithm first; this is the de fault method in PRO/II. It is fast and supports most conventional fractionators, absorbers, and strippers (Section 72). Except for a pure water decant from the condenser, conde nser, IO does not support two liquid phases. When IO fails fa ils or when you have non-ideal systems, systems that form two liquid phases or azeotropes, or systems that have free water, use CHEMDIST. You will also have to use a thermodynamic method that uses liquid activity coefficients; CHEMDIST does not work with generalized generalized K-value predictors pre dictors such as a s GS or SRK SRK (Section 74). When neither ne ither IO nor CHEMIDST CHEMIDST converges or applies, use SURE. It has better convergence and is more versatile, but slow (Section 73). SURE SURE is the only algorithm algorithm that can ca n handle total tota l pumparounds and water decant dec ant on any tray. Use the LLEX option when you deal with liquid-liqui liquid-liquid d extractions extra ctions (Section 79). A few words on the conventions: Trays are always numbered from top to bottom. bott om. If there is a condenser, it is tray number 1. (Essentially it is the first theoretical stage with an associated cooler c ooler.. See Henley & Seader, Chapter 15.) If there is a reboiler, it is the bottom tray. There is no upper limit on the number of trays, but there must be at least two stages. There is no limit on the number of feed or product streams. The pressure of each e ach tray t ray must be spec ified and is invariant except in CASE CASE STUD STUDIES. IES. The configuration of the column must must be completely defined, including the total t otal number of theoretical trays, and the locations of all the feeds, product draws, pumparounds and heater/coolers. By default, the entire feed is added directly to the liquid portion of the feed tray. Using the SEPARA EPARATE TE key word will flash the feed adiabatically a diabatically and any a ny vapor if exist e xist will will be fed fe d to the tray t ray directly above; this option simulates a typical feed nozzle projecting into the vapor space.
Before we go through the key words, here is a sample COLUMN COLUMN input from Section Section 71: S3, S3, NAME=MOONSHI NE PARAMETER TRAY PARAMETER TRAY = 21 $u $use I O, 21 t heoret i cal cal st age ages $ FEED F1, F1, 17, SEPARATE $f $f l ashi shi ng t he f eed at at st age 17 17 PROD OVHD = D3, 153, 153, BTMS =B3 $2 $2 pr pr oduct oduct s; t op est i mat ed at 15 153 3 $ COND TYPE =BUBB BUBB,, PRES=65 $t ot al con condenser enser wi wi t h bubbl e pt . l i q. PRESSURE 2, 2, 70/ 21, 75 $def i nes pr pr essur e i n r est of col col umn DUTY 1, 1, 1/ 2, 21 $def i nes l ocat cat i on of heat er / coo cool er VARY DUTY=1, 2 $l et PRO/ I I var var y t he he heat dut i es COLUMN
1 f6
UID=DI
25 04 2014 12 27
Column
http://chemelab.ucsd.edu/CAPE/keyword/column.html
TEFF (
MURPHREE) 2, 0. 9/ 20, 0. 9 $90% Mur phr ee t r ay ef f i ci ency
$ PRINT PROP =BRI
EF, RECOVERY $pr i nt opt i ons ESTIMATE MODEL =CONV $l et PRO/ I I m ake i ni t i al guesses TSIZE SECTION( 1) =2, 20, SIEVE $si ze t he col um n $ SPEC STREAM =D3, COMP=5, 6, RATE, VALUE=0. 3 SPEC STREAM =B3, RVP( API N) , VALUE=12 $t wo col umn specs; t he f i r st speci f i es t he f l ow r at es of component s 5 pl us 6 at t he $t op; t he second speci f i es t he Rei d vapor pr essur e at t he bot t om
Now, we'll try to go through the key words with special focus on the IO method. These notes will get you started, but you really have to read the Input Manual to see all the other fancy options and items specific to CHEMDIST and SURE. Section 71 of the Manual covers features general to all methods. Sections 72 to 74 contain information specific to each method. COLUMN
Just like all other modules, define the 4 alphanumeric UID and the optional 12 alphanumeric NAME.
PARAMETER
Defines the tray number, the calculation method and the maximum number of iterations. Examples: PARA TRAY=32,
IO=20,
DAMP =0.
PARA TRAY=32,
CHEMDIST
PARA TRAY=32,
SURE
7
You have to get the tray number from a shortcut calculation first. Count the condenser and the reboiler in the TRAY number. For IO, the default number of iterations is 15. The example above reset it to be 20. When convergence appears to be difficult, say for heavy-ends refinery fractionators and non-ideal mixtures, lower the relaxation factor DAMP to 0.7 or 0.8 (default is 1). You can also set ERRINC slightly larger than unity to allow inner loop errors to increase during iterations. For CHEMDIST, the default number of iterations is 20. Calculation will stop if there is no improvement after 5 successive iterations (STOP=5); it can be reset with STOP=value. Use of DAMP is same as IO, but the default value for ERRINC is 100. For SURE, the default number of iterations is 10. Calculation will stop if there is no improvement after 5 successive iterations (STOP=5). DAMP and ERRINC do not apply here. FEED
Identify the feed streams and the feed tray numbers. Example: FEED
F1, 3/ F2, 6/ ...etc.
Adding the SEPARATE key word will flash each of the feed stream adiabatically into its vapor and liquid
2 f6
25 04 2014 12 27
Column
http://chemelab.ucsd.edu/CAPE/keyword/column.html
phases. PRODUCT
Identify the product streams and the flow rates. You must use the key words OVHD, BTMS, LDRAW and VDRAW Example with top and bottom products plus two liquid sidedraws at stages 6 and 9 and a vapor sidedraw at stage 4: PROD OVHD =OV,
121, BTMS=BT, & LDRAW =LD1, 6, 100/ LD2, 9, 120, & VDRAW =VD1, 4, 60
Like SHORTCUT, you must provide the estimated flow rates to all but one product streams. Output usually will try to stick to the molar flow rates given unless they are specified as variables. � CHEMDIST uses different key words for the
sidedraws. Check the Manual if you have to use them. CONDENSER
Just like the SHORTCUT module COND TYPE =PART,
MI XED, BUBB, TFI X, or DTBB
If the PRESSURE statement does not specify the pressure of tray 1, make sure you specify the condenser pressure here. DUTY
Identifies the number and the location of condenser, reboiler and side heaters or coolers. You must provide the heat duty if it is not going to be varied by the VARY statement. The default dimension is millions of energy unit per time unit. Example of a 30-plate column with a condenser, a side cooler ( �ve duty) and a reboiler: DUTY
VARY
1, 1/ 2, 12, �6/ 3, 32
Define the variables. For simple distillation columns, we vary the heat duties: VARY DUTY=1,
32
For say a gas absorber, we vary the liquid feed rate: VARY FEED=2
$say we' ve def i ned f eed number
2 as l i qui d PRESSURE PSPEC
The column pressure profile must be defined. If only the pressures at the top and bottom are given, a linear profile is assumed for the plates in between. The follow forms are equivalent: COND TYPE =.
. . , PRES=65 PRESSURE 2, 70/ 21, 75
or
3 f6
25 04 2014 12 27
Column
http://chemelab.ucsd.edu/CAPE/keyword/column.html
COND TYPE =.
.. PRESSURE 1, 65/ 2, 70/ 21, 75
or COND TYPE =.
..,
PRES =65
$PTOP i s f or t r ay
PSPEC PTOP =70,
DPCOL=5
$pr essur e dr op
PRES =65
$pr ess ur e dr op per
no. 2 across col .
or COND TYPE =.
..,
tray PSPEC PTOP =70,
DPTRAY =0.
26 $i s
5/ ( 21�2) =0. 263 ESTIMATE
All column calculations are inherently iterative. ESTIMATE is an initial estimate generator for the temperature and vapor flow profiles. A Fenske calculation will be used under the option MODEL=CONV. If the column is difficult to converge, use MODEL=CHEM; a multiflash technique will bring the profiles closer to the final solution before the column algorithm takes over. (Chapter 15 of Henley & Seader uses linear profiles; MODEL=SIMPLE) The default reflux ratio is 3. So if you know of a better value, you can specific it here. Example: ESTIMATE MODEL =CHEM,
RRATIO =1.
1
You can provide other estimates or your own profiles, but they are generally not necessary. SPECIFICATION
The number of SPECIFICATIONS must match the number of variables in the VARY statement. Just like the shortcut, purity or recovery specifications under most circumstances (See Manual for all the other options if you need to). You can also specify the reflux ratio or the heat duty. Example: SPEC RRATIO , VALUE=2.
0
SPEC DUTY ( 2) ,
DUTY ,
RATIO ,
FLASH =F1,
VALUE=�1 VARY DUTY=2
$Thi s speci f i es t he heat dut y 2 ( say r eboi l er ) as mi nus t he val ue of t he $dut y of FLASH dr um F1. DUTY 2 must be a var i abl e i n t hi s case.
Other options: TEFF
4 f6
Tray efficiency. The example below provides the simplest scenario which is a constant Murphree efficiency for all the plates in the column.
25 04 2014 12 27
Column
http://chemelab.ucsd.edu/CAPE/keyword/column.html
TEFF (
MURPHREE) 2, 0. 9/ 20, 0. 9 $say 21 i s r eboi l er PRINT
Print options to get extra information or to help figure why column does not converge. Options: $pr i nt s i ni t i al est i mat es and br i ef i t er at i on r epor t ITER =ESTI $pr i nt s r eport of most usef ul col umn pr of i l es PROP =BRI EF $pr i nt s t he per cent r ecover y of each component i n $each pr oduct st r eam RECOVERY
$pr i nt s t r ay by t r ay composi t i on r epor t by M or WT COMP =M $pr i nt s t r ay by t r ay K- val ues of al l component s KVALUE
TSIZE
PRO/II can do tray sizing and rating for trayed columns, and HETP and pressure drop calculations for packed columns. We'll only provide the simplest example for tray sizing, TSIZE, i.e., the topic covered by Chapter 13 in Henley & Seader. Example: We have a column with TRAY=21. From trays 2 to 9, we want valve trays spaced 20 inches apart. From trays 10 to 20, we want sieve trays, default spacing (which is 24") and a system factor of 0.8 to account for foaming (Default is SF=1). TSIZE SECTION(
1) =2, 9, VALVE, SPACE( TRAY, I N) =20 TSIZE SECTION( 1) =10, 20, SIEVE,
SF=0.
8
Choices of trays are VALVE, SIEVE and CAP. You can define the number of flow paths with PASSES=1, 2 or 4. The default is 1. The default spacing is 24 inches. The system factor to account for foaming is tabulated in Section 77. Or refer to Henley & Seader or Perry's. By default, sizing is done after convergence. Using the DPCALC=value key word will force the calculations to be performed during flowsheet solution. This enables the pressure drop to be reflected in the column solution and gives CONTROLLER access to the flooding factor. The value is used to adjust for tray efficiency. For example, a column is modeled as having 8 theoretical trays but in fact has 10 actual trays. Set DPCALC=0.8.
5 f6
25 04 2014 12 27
Column
http://chemelab.ucsd.edu/CAPE/keyword/column.html
You can set the flooding factors with FF=value. The default is in Section 77: 70% for tray diameters 0-2 ft, 75% for trays 2-4 ft, 78% for trays 4-10 ft and 80% for trays 10-50 ft. Minimum column diameter DMIN is 15 inches. DEFINE
6 f6
Other than to replace SPEC, column parameters like DUTY, PTOP, or PCOND can be defined. (See Section 71 for details.)
25 04 2014 12 27
