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PAK ARAB REFINERY LTD
Department: Process
Visbreaking & Gas Concentration Unit
Area: 200 (U – 130 & 411)
Significance of Debutanizer Reflux Ratio 1. INTRODUCTION Debutanizer column is used to separate C3/C4 from stripped unstabilized naphtha and to produce LPG as overhead product and stabilized naphtha which is feed of NHT as bottom product. The basic principle behind this separation is relative volatility of components to be separated. 411-V7 contain 40 flexi valve trays and equipped with reboiler 411-E11, overhead condenser 411-EA2C1~C6, and overhead accumulator 411-V7. 2. DESIGN CONDITIONS
Stream
Temperature oC
Pressure kg/cm2
Feed
171.7
Overhead Vapour
109.8
Condensed Vapour
56.7
74536
Reflux
56.7
62220
Overhead Product
37.8
Bottom Product
192.5
311580
Bottom Reboiler Flow
203
25723
Mass Flow Rate kg/hr 248200
11.11
14.49
74536
16259 (52.46MW, 29.35m3/hr, 554kg/m3)
3. REFLUX RATIO Reflux ratio is ratio between flow rates of stream return to column as reflux to product draw or feed introduce. R = Ln/D ; Ln = Liquid return to column top tray ; D = Distillate Design Reflux Ratio of Debutanizer 411-V7 = (120 m3/hr)/(22.23 m3/hr) = 5.4 OR 62220/16259 = 3.83
PAK ARAB REFINERY LTD
Department: Process
Visbreaking & Gas Concentration Unit
Area: 200 (U – 130 & 411)
Based on Feed of Debutanizer = 62220/248200 = 0.251
The most critical reflux stream is the top reflux. The amount of this reflux will determine how good the fractionation will be between two compounds or two fractions. If there is too much top reflux, there will be too much internal reflux. Each tray will have a greater flow of liquid across it and the turbulence of the liquid on the tray will increase. This will result in greater and greater liquid entrainment in the vapor as it passed from one tray to the next. The liquid composition on each tray will change because of this entrainment and the fractionation will no longer be as precise. This phenomena is sometimes referred to as flooding. Flooding can be restricted to only one section of the column or the whole column. On the other hand, too low a reflux rate will result in too little internal reflux resulting in a very low flow of liquid across each tray. The contact between the rising vapors and the liquid on each tray will not be as good. The vapors may not even have time enough to establish an equilibrium with the liquid and less vapors will condense. The liquid composition on each tray will change and again the fractionation will be a sloppy one. This phenomena is sometimes referred to as drying up a tray or section of the column.
4. EFFECT OF REFLUX RATIO ON DEBUTANIZER PERFORMANCE (OVERHEAD PRESSURE REMAIN SAME) 1.
OVERHEAD PRODUCT LPG SPECIFICATIONS
A. C3/C4 Ratio: As reflux ratio increase, heavier contents of overhead vapour decrease because they starts to condense giving latent heat and C3/C4 ratio increase and vice versa. Design C3 = 33.33mole% : iC4 B. Weathering Temperature: As reflux ration increase tray 10th temperature start to decrease and C4 contents in LPG start to decrease and vice versa. Design LPG weathering temperature is +4.4oC. C. RSH & H2S: Reflux ratio has very minimum effect on RSH & H2S of LPG product. D. Vapour Pressure: Vapour pressure of LPG is indicator of C3/C4+ ratio, higher Vapour pressure higher will be C3/C4 ratio. As reflux ratio increase C3/C4 ratio increase and hence vapour pressure also increase and vice versa. 2.
BOTTOM PRODUCT STABALIZED NAPHTHA SPECIFICATIONS
Bottom product Stabilized naphtha production is normally controlled through circulating diesel flow through reboiler 411-E11. A. Specific Gravity: SG is based upon concentration of light components higher light components lesser will be SG. As reflux ratio increase light ends start to condense and begin to accumulate in bottom product. (Design : 0.707) B. ASTM D-86 Distillation: IBP is an indicator of lightest component boiling point and FBP is heaviest component. As reflux ratio increase C4 starts to condense and start lowering IBP of naphtha if heating remain same. C. Saybolt Color Test: an empirical definition of the color of a clear petroleum liquid based on a scale of − 16 (darkest) to + 30 (lightest). Reflux ratio increase means increase in color of naphtha. (Design: +30) D. C4 Contents: C4 contents has similar effect as above mentioned.
