Coker A.K. Fortran Programs for Chemical Process Design, Analysis, and Simulation

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Equipment Sizing 257

minimum, e.g., in relief systems or in fouling service. It is unsuitable if

a mist is being separated or if high separating efficiency is required.

Cyclones (Figure

4-3). These are robust and not susceptible to foul-

ing or wax. Multicyclones are compact and are reasonably efficient for

foam, but not for slugs. The efficiency of a cyclone decreases with

increasing diameter, and cyclones are not applicable above 5 ft. diam-

eter. They possess a good efficiency for smaller flow rates. However,

cyclones are expensive to operate, especially under vacuum conditions,

because they have a higher pressure drop than either a knockout drum

or a demister separator.

Demister Separators.

A demister separator is fitted either with a

vane demister package or with a wire mesh demister mat. The latter

type is much preferred, although it is unsuitable for fouling service. The

wire mesh demister is a widely applied type of separator, and is adequate

for all gas-liquid flow regimes over a wide range of gas flow rates.

A knockout drum or demister separator may be either a vertical or a

horizontal vessel. A vertical vessel is generally preferred because its

efficiency does not vary with the liquid level. Alternatively, a horizontal

T L L (>I 2.5D)

b'! I

Vortex breaker

Note:

TL = Tangent line

Figure

4-1. Horizontal knock-out drum.

A-A

0.g D

(rain. 0.g m)

Tangent

line

0.3 D I

(rain. 0.3 m)

~, i ~

--_~--

Vortex br

~l ,-----.. '

LZA (HH)

Figure 4-2. Vertical knock-out drum.

258

Equipment Sizing

259

Dc ,.J

-t

' S

-..I B ! -

Figure 4-3. A cyclone: design configurations.

vessel is chosen when it offers a clear size advantage, if the headroom is

restricted or if a three-phase separation is required. Knockout drums

and cyclones are recommended for waxy and coking feeds. Demister mats

are not suitable because of the danger of plugging. Vane demister packages

are used as alternatives, but provision should be made for cleaning.

SIZING OF VERTICAL AND HORIZONTAL SEPARATORS

Vertical Separators

Vertical liquid-vapor separators are used to disengage a liquid from a

vapor when the volume of liquid is small compared with that of vapor

volume. The maximum allowable vapor velocity in a vertical separator

that reduces the liquid carryover is dependent on the following:

9 liquid and vapor densities.

9 a constant K based on surface tension, droplet size, and physical

characteristics of the system.

260

Fortran Programs for Chemical Process Design

The proportionality constant, K, is 0.35 for oil and gas systems with

at least 10-inch disengaging height between the mist eliminator bottom

and gas liquid interface. For vertical vessels, K can vary between 0.1

and 0.35, if mist eliminators (demisters) are used to enhance

disentrainment. The value of K also depends on the operating pressure

of the vessel. At pressures above 30 psig, the K value decreases with

pressure with an approximate value of 0.30 at 250 psig and 0.275 at 800

psig. Watkins [1] has developed a correlation between the separation

factor and K. Figure 4-4 illustrates Watkin's vapor velocity factor chart,

based on five percent of the liquid being entrained with the vapor.

Blackwell [2] has developed a polynomial equation using Watkin's

data to calculate the K value for a range of separation factors between

0.006 and 5.0. Watkins proposed a method for sizing reflux drums based

upon several factors, as illustrated in Tables 4-1 and 4-2. Table 4-1 gives

the recommended design surge times. Table 4-2 gives the multiplying

factors for various operator efficiencies. The operating factor is based

upon the external unit and its operation, its instrumentation and response

to control, the efficiency of labor, and chronic mechanic problems, and

the possibility of short or long term interruptions [ 1 ].

The multiplying factors F~ and

F 2

represent the instrument and the

labor factors.

0.6

0.4 ~r

0.?.

0.1

0.08 -

0.06

0.04,

O.OZ I L ~ ....

0.006 8 0.01 Z

I I I I

4 6 8 0.1

2 4 6 8 1.0 Z 4 6

(.,/.v)( e v/e,) ~ s

Figure 4-4. Design vapor velocity factor for vertical vapor-liquid separators at 85%

of flooding.

Equipment Sizing

Table 4-1

Design Criteria for Reflux Distillate Accumulators

261

Operation

FRC

LRC

TRC

Instrument Factor

w/Alarm

w/o Alarm

1 •

Minutes

Labor Factor

Good

Fair

1.5

Poor

Table 4-2

Operation Factors for External Units

Operating Characteristics

Under good control

Under fair control

Under poor control

Feed to or from storage

Board mounted level recorder

Level indicator on board

Gauge glass at equipment only

Factor

1.25

Factor

1.0

1.5

2.0

A multiplying factor,

F3,

is applied to the net overhead product going

downstream.

F 4

depends on the kind and location of level indicators.

It is recommended that 36 inches plus one-half the feed nozzle, OD,

(48 inches minimum) be left above the feed nozzle for vapor. Below the

feed nozzle, allowance of 12 inches plus one-half the feed nozzle, OD,

is required for clearance between the maximum liquid level and the feed

nozzle (minimum of 18 inches). At some value between L/D ratios of 3

and 5, a minimum vessel weight will occur resulting in minimum costs

for the separator. Figure 4-5 shows the dimensions of a vertical separator.

262 Fortran Programs for Chemical Process Design

H V

H L

k36" * 1/2 (FEED NOZZLE .O.D)

48" MIN.

FEED

NOZZLE

12"+ 1/2 (FEED NOZZLE .O.D)

18" MIN.

MAX.

LEVEL

4------- D ----.-~

Figure

4-5. Dimension of a vertical separator.

Calculation Method for a Vertical Drum

The following steps are used to size a vertical drum.

Step 1. Calculate the vapor-liquid separation factor.

/0.5

S. Fac- WL 9v (4-1)

w--V

Step 2. From Blackwell's correlation, determine the design vapor

velocity factor, K v, and the maximum design vapor velocity.

X - In (S. Fac) (4-2)

K v - exp(B + DX

+ EX 2 + FX 3 + GX 4)

(4-3)

where B =-1.877478

D = -0.814580

E = -0.187074

F = -0.014523

G = -0.001015

Equipment Sizing 263

)0.5

V max __

PL - Pv

Step 3. Calculate the minimum vessel cross-sectional area.

(4-4)

Qv = ww

, ft 3/s

(4-5)

3600. Pv

Av _- ~Qv, ft 2 (4-6)

V max

Step 4. Set a vessel diameter based on 6-inch increments and calcu-

late cross-sectional area.

4 Av) ~

- " , ft (4-7)

min

D mi n tO next largest 6 inch

]--)2

/1;.

Area

- ~-" min

(4-8)

Step 5. Estimate the vapor-liquid inlet nozzle based on the following

velocity criteria.

100

(Umax)nozzle -- 05 ft/s

(4-9)

Pmix

(gmin)nozzle -- 05 f[/s

(4-10)

Pmix

where

WL + WV

lb m

(4-11)

9 mix --

WL WV ~,,----5-

+--

PL Pv

Step 6. From Figure 4-5, make a preliminary vessel sizing for the

height above the center line of a feed nozzle to top seam. Use 36 inches

+ 1/2 feed nozzle, OD, or 48 inches minimum. Use 12 inches + 1/2 feed

264

Fortran Programs for Chemical Process Design

nozzle, OD, or 18 inch minimum to determine the distance below the

center line of the feed nozzle to the maximum liquid level.

Step 7. From Table 4-1 or 4-2, select the appropriate full surge vol-

ume in seconds. Calculate the required vessel volume.

QL = WL ft3/s (4-12)

3600. p L

V - (QL)(design time to fill), ft 3

= (60.0)(QL)(T), ft 3

(4-13)

The liquid height is:

"L V(4)

rt'D 2 , ft (4-14)

Step 8. Check geometry. (H E + Hv)/D must be between 3 and 5. For

small volumes of liquid, it may be necessary to provide more liquid

surge than is necessary to satisfy the L/D > 3. If the required liquid

surge volume is greater than that obtained in a vessel having L/D < 5, a

horizontal drum must be provided.

Calculation Method for a Horizontal Drum

Horizontal vessels are used for substantial vapor-liquid separation

where the liquid holdup space must be large. Maximum vapor velocity

and minimum vapor space are determined as in the vertical drum, except

that K H for horizontal separators is generally set at 1.25K v. The follow-

ing steps are carried out in sizing horizontal separators.

Step 1. Calculate the vapor-liquid separation factor by Equation 4-l,

and K v by Equations 4-2 and 4-3.

Step 2. For horizontal vessels

K H = 1.25K v (4-15)

Step 3. Calculate the maximum design vapor velocity.

Equipment Sizing 265

)O.5

(U) -K.

9L-Pv

v max PV

ft/s (4-16)

Step 4. Calculate the required vapor flow area.

(Av)mi.

-'-

Qv ft 2 (4-17)

(Uv)max

Step 5. Select appropriate design surge time from Table 4-1 or Table

4-2, and calculate full liquid volume by Equations 4-12 and 4-13.

The remainder of the sizing procedure is carried out by trial and error

as follows:

Step 6. When the vessel is full, the separator vapor area can be

assumed to occupy only 15 to 25 percent of the total cross-sectional

area. Here, a value of 20 percent is used and the total cross-sectional

area is'expressed as"

(Atotal)min

( A v )min (4-18)

0.2

4 (A total

) min "~0"5

- " ) (4-19)

Dmin

Step 7. Assume the length-to-diameter ratio of 3 (i.e, L/D = 3). Cal-

culate the vessel length.

L = (3)(D min ) (4-20)

Step 8. Because the vapor area is assumed to occupy 20 percent of

the total cross-sectional area, the liquid area will occupy 80 percent of

the total cross-sectional area.

A L - (0.8)(Atota

1 )min'

ft2 (4-21)

Step 9. Calculate the vessel volume.

VVES -- (Atotal)rain (L), ft 3 (4-22)

Step 10. Calculate liquid surge time.

T- (60"0)(AL)(L)(gL) min. (4-23)

266

Fortran Programs for Chemical Process Design

Liquid Holdup and Vapor Space Disengagement

The dimensions of both vertical and horizontal separators are based

on rules designed to provide adequate liquid holdup and vapor disengag-

ing space. For instance, the desired vapor space in a vertical separator is

at least 1 ~ times the diameter, with 6 inches as the minimum above the

top of the inlet nozzle. In addition, a 6-inch minimum is required

between the maximum liquid level and the bottom of the inlet nozzle.

For a horizontal separator, the minimum vapor space is equal to 20 per-

cent of the diameter or 12 inches.

Wire Mesh Pad

Pads of fine wire mesh induce coalescence of impinging droplets into

larger ones, which then separate freely from the gas phase. No standard

equations have been developed for the pressure drop across wire mesh

because there are no standardized mesh pads. However, as a rule of thumb,

the pressure drop of a wire mesh is AP = 1.0 in

H20.

Every manufacturer

makes a standard high efficiency, very-high efficiency, or high-throughput

mesh under various trade names, each for a specific requirement.

Standards for Horizontal Separators

The following specifications are generally standard in the design of

horizontal separators [3]:

1. The maximum liquid level shall provide a minimum vapor space

height of 15 inches, but should not be below the center line of

the separator.

2. The volume of dished heads is not considered in vessel sizing

calculations.

3. The inlet and outlet nozzles shall be located as closely as practical

to the vessel tangent lines. Liquid outlets shall have anti-vortex baffles.

Piping Requirements

Pipes that are connected to and from the process vessels must not

interfere with the good working of the vessels. Therefore, the following

guidelines should be observed:

9 There should be no valves, pipe expansions or contractions within

10-pipe diameters of the inlet nozzle.