Flash drum design

EQUIPMENT DESIGN: BASIC CONCEPTS

GOALS:  u D ef i ni ti ons and Con Con cepts  u T ypes ypes of D esi gn  u D esi gn M ethod th odol olog ogy  y 

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Equipment Design

EQUIPMENT CHARACTERIZATION

Input  Streams 

Operating  Parameters  24/99

Output  Streams 

Size  Parameters  Equipment Design

DEFINITIONS EQUI PM ENT RATI NG:  ð Gi ven the i nput str eams, oper ating par ameter s, and  size parameter s, deter mine th e equipment  per formance and output str eams 

EQUI PM ENT SI ZI NG:  ð Given the i nput str eams and the per formance  specifi cati ons, esti mate the economi cally impor tant  size parameter s usi ng approxi mate methods 

EQUI PM ENT DESI GN:  ð Produce a design in enough detail to gui de the  fabr i cation of the item of equi pment  24/99

Equipment Design

Equipment Design Methodology

Set Performance Specifications

Determine Operational Constraints

Choose Equipment Type and Design Parameters

Try First

Perform Rating Calculations

N o  I terate 

Satisfy Specs & Constraints? Yes 

Done

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Equipment Design

EXAMPLE #1: REFLUX DRUM ROBL EM : Design a r efl ux dr um for a di stil l ation col um  F or a residence time of 15 minutes when half full, That has a length to diameter r atio L /D = 3.0, and  I s hori zontal. PERATI NG CONDI TI ONS:  F low r ate out of dr um i s 5,000 Kg/hr  F l ow is 100% Ethanol (SG = 0.789) 

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Equipment Design

REFLUX DRUM

From Overhead Condenser

D 

Distillate

Reflux to Column

L

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Equipment Design

DESIGN CALCULATIONS

H oldup = (5000 kg/hr )(0.25 hr ) = 1250 kg  3  3  Densi ty = (0.789)(1000 kg/m   ) = 789 kg/m  3  3  Vol ume = (2)(1250 kg)/(789 kg/m   ) = 3.17 m 

(To simplify the size cal culations, neglect the holdup in t  dished heads of the drum.)  2  Vol ume = 3.1416 (D   ) L

(but L = 3.0 D, so) 

3  D  = 3.17/(3)/(3.1416) So,

D = 0.695 m and L = 2.09 m. 24/99

Equipment Design

EXAMPLE #2: FLASH DRUM ROBL EM : Design a flash dr um that i  To fl ash a str eam that is 60 mol % ethylene and 40  mol% water at a fl ow r ate of 800 kmol/hr , To produce a vapor that i s 99% ethylene  At a fr acti onal r ecover y of ethylene of 99 % The flash drum i s a ver ti cal pr essur e with an L /D = 4.0. PERATI NG CONSTRAI NTS:  L ow entr ai nment of water i n the vapor  10 mi nu te holdup of l iqui d when dr um is 1/2 ful l  24/99

Equipment Design

FLASH DRUM VAPOR 

FEED 

Heating  or  Cooling 

LIQUID 

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Equipment Design

DESIGN PROCEDURE BA SI C DE SI GN VARI ABL ES: Pressur e P and H eat  Duty Q.. Guess val ues for P and Q. Do the adiabati c flash  calculation to deter mi ne V, L and their compositi ons an  temperature  I f the per f ormance specifi cati ons are satisf ied, proceed  to size the drum. I f not iter ate on P and Q. Size drum to satisfy entr ainment, li qui d holdup and L /  constraints.

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Equipment Design

SUMMARY F or ver y simple situations, the design calculations are  explicit. F or most situations, the design calcul ations involve a  tr i al-and-er r or search for those val ues of key design  var iables that satisfy the per f ormance specificati ons and  constraints. The detail s of the design methodology wil l dif fer fr om  one equi pment type to another . The details of the desi gn methodology may al so differ  for dif fer ent per formance specif icati ons for the same typ  of equi pment. 24/99

Equipment Design