CRUDE OIL DISTILLATIONThe feed to a refinery often changes every few days. An existing refinery has received an Indonesian crude oil and you must simulate the atmospheric distillation column to establish the correct operating conditions for a throughput of 120,000 barrels/day. The crude unit configuration and typical operating data are shown in Figure R3.1. The products must meet the specified D86 and TBP points in Table R3.1, and the yields are to be calculated.
Figure R3.1 Crude Distillation Column
Table R3.1 ASTM D86 (F) LV % 0 5 95 100*
Naphtha 340 370
Kerosene 330 380 520 550
Diesel 460 530 665 705
Gas Oil 555 635 (885) (955)* *
Topped Crude (695) *
Figures in parentheses are TBP points rather than ASTM.
Process DataR3.1 are used.
For this simulation, the stripping stream and pumparound cooling shown on Figure
The crude oil has been analysed as an atmospheric TBP curve and defined lightend components as shown in Table R3.2. Table R3.2 CRUDE OIL DATA TBP @ 760 mm Hg LV% 5 10 20 30 40 Temp (F) 149 208 330 459 590 LV% 50 60 70 80 100 29.2 Temp (F) 690 770 865 980 1600 Lightends Component Ethane Propane i-Butane n-Butane i-Pentane n-Pentane Average API gravity Total LV% 0.1 0.2 0.3 0.7 0.5 1.2 3.0
The laboratory data for the TBP curve are supplied only up to the 80% point. A typical value is assumed for the 100% point to assist the extrapolation of the curve.
Methods and Data
The GS thermodynamic system is recommended for calculating K-values and enthalpies in crude and vacuum systems. The BK10 system may also be used, but it tends to predict slightly less vaporization. Column temperatures can generally be matched with either system. If not, the column pressure data are probably in error. Accurate characterization of the crude oil is vital for good simulation results. The greatest errors often appear in the residue stream and can lead to poor results when simulating the downstream vacuum column. The characterization may be improved by supplying gravity and molecular weight data if they are available. The program automatically generates a number of petroleum fractions and calculates the molecular weight, gravity, and boiling point for each. These are then used in standard petroleum correlations to generate all the other required properties.
Simulation Model (Shortcut)
Viscosity and thermal conductivity data are generated for the streams for use in other calculations. The API data book method is selected for the liquid viscosities. For crude oil streams, these data are more accurate than the PETRO option used for the other transport properties. The first step in the simulation is to ensure that it is possible to produce the specified products from the crude oil feed and establish the material balance. The multidraw shortcut distillation model (MODEL = REFINE) is a quick and easy tool for these calculations. This model uses the Fenske calculation method and considers the column to be a series of two-product columns as shown in Figure R3.2. The condenser is simulated with an external flash drum. The specifications chosen for the shortcut model define the separation in each section. For this simulation, the 95% distillation points and 5-95% gaps are used. The data are taken from Table R3.1. 5% and 95% points should always be used instead of initial and end points, as they are generally more accurate. The simulated end points are affected by the number of petroleum cuts, the parameter TBPEP (on the ASSAY statement in Component Data) which defaults to 98%, and the Fenske model itself, which tends to overfrac- tionate on the product tails. In addition, the shortcut simulation ignores the side- strippers which also affect the ends of the curves.
Figure R3.2 Shortcut Crude Unit Model
In the shortcut model, all the stripping steam is added with the feed. The main effect of this on the simulation results is that the influence of the steam on the product temperatures is not modeled, and the calculated temperatures will be high.
Input Data (Shortcut)
The simulation units are defined as English but with pressure as psig and liquid volumes in barrels. As crude oil has no defined components, the component flow rate output is suppressed, and only the stream summary is printed. The PRINT TBP option gives the TBP and ASTM curves for all hydrocarbon streams. It is not known whether or not there will be any vapor product from the column. As there may be vapor, the condenser is defined as mixed phase at the defined temperature. The MODEL=REFINE option specifies no reflux between sections in the column. The default would assume total reflux between the sections. The product rate estimates are supplied as volume percent cutpoints on the crude oil feed.
Input Data File:TITLE PROBLEM=R3S,PROJECT=APPBRIEFS,USER=SIMSCI,DATE=SEP 90 T DIMEN PRESS=PSIG,LIQV=BBL PRINT STREAM=SUMMARY,TBP COMPONENT DATA LIBID 1,H2O/2,C2 /3,C3 /4,IC4/5,NC4/6,IC5/7,NC5 THERMODYNAMIC DATA METHODS SYSTEM=GS,COND=PETR,VISC(V)=PETR,VISC(L)=API STREAM DATA PROP STREAM=1,TEMP=450,PRES=14,RATE(V)=5000,ASSAY=LV TBP STREAM=1,PRES(MMHG)=760,& DATA=3,97/5,149/10,208/20,330/30,459/40,590/& 50,690/60,770/70,865/80,980/100,1600 API STREAM=1,AVG=29.2 LIGHT STREAM=1,PERCENT(V)=3,& COMP(V)=2,0.1/3,0.2/4,0.3/5,0.7/6,0.5/7,1.2 PROP STREAM=WATR,TEMP=600,PRES=60,PHASE=V,COMP(W)=20000 NAME 1,CRUDE FEED/9,NAPHTHA/10,KEROSENE/11,DIESEL/& 12,GAS OIL/20,TOPPED CRUDE UNIT OPERATIONS SHORTCUT UID=SC01,NAME=CRUD TOWER E FEED 1,WATR PROD STREAM=89,PHASE=M,PRESS=8.5,CUTP(V)=25 PROD STREAM=10,PHASE=L,CUTP(V)=40,PRES=11.8 PROD STREAM=11,PHASE=L,CUTP(V)=55,PRES=12.5 PROD STREAM=12,PHASE=L,CUTP(V)=75,PRES=13.3 PROD STREAM=20,PHASE=L,PRES=14.4 COND TYPE=MIXED,TEMP=110 EVAL MODEL= REFINE
SPEC SPEC SPEC SPEC SPEC SPEC SPEC SPEC FLASH PROD ISO END
STREAM=89,D86(95),VALUE=340 STREAM=10,D86(5),DIFF,STREAM=89,D86(95),VALUE=40 STREAM=10,D86(95),VALUE=520 STREAM=11,D86(5),DIFF,STREAM=10,D86(95),VALUE=10 STREAM=11,D86(95),VALUE=665 STREAM=12,D86(5),DIFF,STREAM=11,D86(95),VALUE=-30 STREAM=12,TBP(95),VALUE=885 STREAM=20,TBP(5),VALUE=695 UID=FL01,NAME=CONDENSER FEE D 89 V=8,L=9,W=7 TEMP=110,PRES= 5.3
The shortcut column results are shown on the next page. The shortcut model predicts the minimum number of trays, calculated at total reflux, needed to meet the specified separations. Typical values for crude oil separations for each column section are shown in Table R3.3.
Table R3.3 TYPICAL SHORTCUT TRAYS Column Section Naphtha - Kerosene Kerosene - Diesel Diesel - Gas oil Gas oil -Topped crude Minimum Trays 3.5 - 5.0 2.5 - 3.5 2.0 - 3.0 1.3 - 1.8
As long as the shortcut model reports figures within these ranges, the separations will be feasible. Significant departures from these values mean a difficult, if not impossible, separation. The simulation predicted no overhead vapor at the desired condenser conditions. This is not unusual for crude unit simulations because some lightends may have been lost in sampling the crude oil, or because lightends produced by cracking in the furnace are not included in the simulation model. Once the shortcut model is solved, the material balance is defined. If difficulties occur solving the rigorous model, then the heat balance is likely to be at fault.
SHORTCUT COLUMN OUTPUTUNIT 1, SC01, CRUDE TOWER FEEDS STREAM PHASE ------------ -----------1 WATR PRODUCTS STREAM -------------------- MATERIAL BALANCES -------------------SECTION NO OF LB-MOL/HR LB/HR BBL/HR TRAYS ------------ ------------ --------------------------------------------------------------------------------------------- --------89 MIXED 2878.68 279901.38 1090.22 1 4.49 WATER 1105.36 19913.15 56.89 10 LIQUID 1047.10 184786.42 641.70 2 3.73 11 LIQUID 574.76 144886.42 477.04 3 2.76 12 LIQUID 715.23 236618.47 747.78 4 1.43 20 LIQUID 1337.39 695517.88 2043.50 TOTALS SPECIFICATIONS PARAMETER TYPE -----------------------------------STRM STRM STRM STRM STRM STRM STRM STRM COMP. SPECIFICATION SPECIFIED CALCULATED NO TYPE VALUE VALUE ------------- ------------------------- -------------------89 10 10 11 11 12 12 20 D86 D86 D86 D86 D86 D86 TBP TBP 95 PCT 5 PCT 95 PCT 5 PCT 95 PCT 5 PCT 95 PCT 5 PCT 3.400E+02 4.000E+01 5.200E+02 1.000E+01 6.650E+02 -3.000E+01 8.850E+02 6.950E+02 3.400E+02 4.000E+01 5.200E+02 9.999E+00 6.650E+02 -3.001E+01 8.850E+02 6.950E+02 7658.53 1561623.7 5057.14 12.41 PHASE
Simulation Model (Rigorous)
The number of theoretical trays must be established for the rigorous distillation. This may be obtained by: - applying typical overall tray efficiencies to the number of actual trays - using values from previous simulations of the column - assuming that the number of theoretical trays is about twice the minimum number In this example, typical efficiencies were used to get the configuration of the model shown in Figure R3.3.
Figure R3.3 Rigorous Crude Unit Model
Virtually all the heat in the distillation enters with the crude oil feed, and so it is vital that the feed temperature is correct. For this reason, the furnace and flash zone are included in the simulation as a heat source on the feed tray. The feed is specified as liquid at the flash zone pressure to ensure that no flash is performed and all the feed is placed on this tray. The furnace duty is then calculated to match the specified liquid runback into the flash zone. Only one other specification can be made on the main column, together with one on each sidestripper, and so not all the distillation specifications on the shortcut can be used. The best strategy is to specify one distillation point or gap for each product, except for the topped crude. The topped crude is not usually specified since this has the greatest uncertainty. Specifying the