Memoranda On Front-end Crude Fractionation.

Transcription

1 VtÜÜV{xÅ \ÇvA Tel, Fax Memoranda On Front-end Crude Fractionation. ATMOSPHERIC STILL PETROLEUM SEPARATION GLOBAL MATERIAL BALANCE & OPTIMAL UTILITY (HEATER AND STEAM CONSUMPTION ) BALANCE. In earlier issue, we specified crude distillation as the petroleum refining process which lightens the crude, producing light-ends (sweetened by Hydro-treating and Reforming), light oil, heavy oil, atmospheric gas oil (AGO) and crude residue that feeds vacuum distillation and coking/cracking operations. In this issue, we will attempt to address in detail, atmospheric distillation as complicated by trace elements (sulfur, nitrogen, etc.), with process optimization that reduces associated utility consumption and toxic discharges. The assay of atmospheric still feed range from light crude with high API (low density) to heavy crude with low API (high density), the range of 28 API to 10 API. Ultimate analysis of petroleum fuel indicates trace elemental sulfur and nitrogen inherent in bulk hydrocarbon. The feed to the still, typically from tank farm, is released via preheat exchangers at ambient temperature and atmospheric pressure. Preliminary heating to 260 o F occurs prior to desalting, which removes minerals, metals and siliceous gangues. Further preheating to 400 o F with a pump-around recycle stream drawn from the still mid section, is attained before passing through a fuel-fired heater. Crude exiting the heater at about 600 o F is hold-up in a drum from where it feeds the atmospheric tower. Light-ends are drawn from tower overhead rectifying section and crude residue passes out the tower bottom. Cuts (petroleum fractions) are drawn from the still inter-stage sections. All the streams interveining the atmospheric distillation, is depicted in picture below and

2 illustrated in Figure 1.0. Inlets and their descriptions are shown here subsequently. Outlets Streams in the figure are yellowed and their cut temperatures are indicated. Stream 1., Light-ends fuel gas (butane-thrumethane) to separation. Stream2., Gasoline lightend to reforming and hydro treating, which deprives the hydrocarbon of smelly trace elements (sulfur, etc.). Stream3., Sour water to water-treatment. Stream 4., Naphtha to upgrading., Stream5, Kerosene Stream6., Diesel fuel. Stream7., Atmospheric gas oil (distillate fuel). Stream8., Crude residue. Stream9., Pump around. Feed Stream., Crude oil Utility Stream., Intermediate Pressure (IP) Steam to stripping Temperature gradient to and from all system s unit operations, are reflected in the figure. Room for optimization in the atmospheric distillation process abounds in utility consumption efficiency. Pre-heat duty requirement is accomplished by drawing from downward tray to tower s feed stream and returned to tray, upward to the feed stream. The ratio of feed crude oil to recycle pump around stream is optimum at 1.5 with temperature spread as shown in figure. Material balance approach for streams crossing the atmospheric-distillationstill boundary is addressed subsequently, followed by optimal utilities requirement considerations. Steam versus Heater load calculations, as depicted in the respective segment below in this literature, is geared towards ensuring process energy conservation and safe performance of distillation tower and steam stripper. CarrChem Inc. engineering excellence

3 Crude Oil Refining Process Operation

4 Figure 1.0 Atmospheric Fractional Distillation Still STRPPER 210 o F STILL OVERHEAD CONDENSER SURGE DRUM CW o F 350 o F o F o F 630 o F o F IP STEAM 77 o F o F 400 o F CRUDE 570 o F 85 o F STAGED PRE- HEATING WITH INTERSTAGE DESALTING 9 HEATER WITH HOLD-UP DRUM CarrChem Inc. engineering excellence

5 ATMOSPHERIC DISTILLATION: MATERIAL BALANCE API 160 o F Aliphatic Alkanes 54.4 API o F Gasoline 41.8 API 420 o F Crude Oil Feed 10 API 77 o F Distillation Tower 32.8 API 550 o F Diesel Oil Light Gas Oil 28.5 API 630 o F Atmospheric Gas Oil 2 API 550 o F Reduced Crude (Bottom) Basis Feed is 10 API (62.43 lbs/ft 3 ) Calculation assumptions: Cut API Lbs/Ft 3 ABP (Actual Boiling Point) AGO o F LGO o F Diesel o F Gasoline o F Gases (Alkanes) o F Bottom (liquid) NA CarrChem Inc. engineering excellence

6 For Material Balance at Steady State, Ins = Outs or 100(Lbs) = Sum of Effluents = [ sigma d i V i ], where d i is density of cut(i) and V i (volume i) Assume all column tower nozzles have same cross sectional diameter and uniform flow velocity. Therefore, 100 Lbs = V*(Sigma d i ) or 100 = V[ ] Divide through by 100 to get, 1 = V*{ } = V*(3.1089)...Equation (1) From Equation (1) = V*(3.0243), Solve for V to get V = Substitute for V in Equation (1) to get roughly: 1 = Tabulated, cuts percentages from the atmospheric still (distillation) are as follows, Cuts Yield (wt%) Atmospheric Gas Oil (AGO) 18 Light Gas Oil (LGO) 18 Diesel (Kerosene) 18 Gasoline (Straight-run) 16 Gases (Alkanes) 10 Bottom (Reduced Crude) 20 Feed 100 Scale-up of feed rate can be by prorating above feed/yields proportions.

7 ATMOSPHERIC DISTILLATION: GLOBAL UTILITY BALANCE. <I1> DDDD <E1> <E2> <I2> <I3> Distillation Systems Boundary Limit <E3> <E4 > <E5> < E6> <E7>

8 No: Inlet Enthalpy Stream Effluent Energized Stream Flow (lb) Thermo-State ( o F) I1 Feed None I2 Non-Adiabatic Heater Duty Charge None Heat 600 I3 Steam None 4 (see Calc.) 750 E1 None Gases E2 None Gasoline E3 None Diesel (Kerosene) E4 None Light Gas Oil (LGO) E5 None Atmospheric Gas Oil (AGO) E6 None Bottom (Reduced Crude) E7 None Cooling Water Duty Various

9 SUMMARY OF STREAM S HEAT CONTENT Inlet Streams: Feed; Stream I1, Mass Flow is 100lbs, Temperature is ambient (Datum) and referenced heat content is zero. Calculation; 1000 (lbs) * (77 77) o F * 0.9 (Btu / lb o F) = 0 Btu. Stripping Steam; Stream I3. Assumption, Hydrocarbon humidification is 5% or Steam rate = 0.05 * 100 = 5 lbs Mass Flow is 5lbs (per 100lbs crude feed), Condition is intermediate pressure steam, superheated to 700 o F, and back-pressured in stripping to atmospheric steam. Heat Content of steam is 545 Btu. Calculation; Heat Content = Latent Heat + Superheat i.e., 5 (lbs) * ( ) (Btu / lb) + 5 (lbs) * ( ) o F * 0.4 (Btu / lb o F) = 545 Btu **To arrive at non-adiabatic heat across system s boundary from the Heater, we will first estimate effluents heat. Outlets Streams: Gases; Stream E1 Mass Flow is 10 lbs. Temperature is 160 o F and heat content is 332 Btu. Calculation; 10 lbs * 0.4 (Btu / lb o F) * ( ) o F = 332 Btu. Gasoline; Stream E2 Mass Flow is 16 lbs. Temperature is 160 o F and heat content is 2,848 Btu. Calculations; [[16 lbs * (Btu / lb)] + [16 lbs * 0.4 (Btu / lb o F) * (160 77)]] o F = 2,848 Btu. Diesel; Stream E3 Mass Flow is 18 lbs. Temperature is 420 o F, crossing the boundary with heat content of 4,558 Btu. Calculations; 18 lbs *[116 (Btu / lb) (Btu / lb o F) * (420 77) o F] = 4,558 Btu.

10 Light Gas Oil (LGO); Stream E4 Mass Flow is 18 lbs. Temperature is 550 o F crossing, with heat content of 5,202 Btu. Calculations; 18 lbs *[99.8 (Btu / lb) (Btu / lb o F) * (550 77) o F] = 5,202 Btu. Atmospheric Gas Oil (AGO); Stream E5 Mass Flow is 18 lbs. Temperature is 630 o F crossing, with heat content of 5,422 Btu. Calculations; 18 lbs *[80.4(Btu / lb) (Btu / lb o F) * (630 77) o F] = 5,422 Btu. Bottom; Stream E6 Mass Flow is 20 lbs. Temperature is 550 o F crossing, with heat content of 568 Btu. Calculations; 20 lbs *[1.2 (Btu / lb o F) * (550 77) o F] = 568 Btu. Cooling Duty; Stream E7 Naphthalene cooling, 350 o F 160 o F, 10 lbs *[0.4 (Btu / lb o F) * ( ) o F] = 760 Btu. C 5 + cooling, 210 o F 160 o F, 6 lbs *[0.4 (Btu / lb o F) * ( ) o F] = 120 Btu. Gases cooling, 210 o F 160 o F, 10 lbs *[0.4 (Btu / lb o F) * ( ) o F] = 200 Btu. Net duty = 1,080 Btu. Total heat effluent from system is: , , , , ,080 = 20,010 Btu System s heat Balance (Ins = Outs), enables estimation of Heater s heat input (X Btu). Therefore; 20,010 Btu = (Feed heat + Stripping Steam heat + X) Btu i.e. X = 20, = 19,465 Btu Thus, per 100 lbs crude oil feed to fractional distillation, total utility requirement are; 20,010 Btu. Bulk of non-adiabatic heat is (19,465 Btu) is required from Heater and the balance (545 Btu), is compensated by stripping steam to fractionating tower and the stripper column