COMPUTER PROGRAM ENHANCES GUIDELINES FOR GAS-LIQUID SEPARATOR DESIGNS

A.K. Coker
A.K.C. Technology
Sutton Coldfield, U.K.

Designers are often required to size separators or knockout drums for removing liquids from process gas streams. A Fortran computer program called "Vessel" has been developed to size both horizontal and vertical separators for known fluids' rates and physical properties.

A review of the design of horizontal and vertical separators precedes the program listings.

PROCESS VESSELS

Two principal kinds of processing vessels are used in the chemical process industries: those without internals and those with internals.

Empty separators (without internals) are drums that provide intermediate storage or surge of a process stream for a limited or extended period. Alternatively, they provide phase separation by settling.

The second category consists of equipment such as reactors, mixers, distillation columns, and heat exchangers.

In some cases, it is important to separate liquid and gas flowing simultaneously through a pipe. This simultaneous separation is necessary because the conditions of the flowing mixture and the efficiency of separation may vary widely. A separator for such duty therefore must be adequate.

In addition, constraints of space or weight often affect the choice of separators, the need to handle solids or effect a three-phase separation, and the requirements for liquid holdup. In practice, most separation problems are solved by knockout or surge drums or demister separators.

KNOCKOUT DRUMS

A knockout drum is suitable for bulk separation of gas and liquid, particularly when the liquid-volume fraction is high with stratified or plug flow in the pipe (Figs. 1 and 2).

A knockout drum is also useful when vessel internals are required to be kept to a minimum; for example, in relief systems or in fouling service. It is unsuitable if a mist is being separated or if high separation efficiency is required.

DEMISTER SEPARATORS

A demister separator is fitted with either a vane demister package or a wire-mesh demister mat. The mat-type is preferred, although it is unsuitable for fouling service.

The wire mesh demister is a widely applied separator type 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 horizontal vessel.

A vertical vessel is generally preferred because its efficiency does not vary with liquid level. Alternatively, a horizontal vessel is chosen when it offers a clear size advantage when headroom is restricted or when a three-phase separation is required.

Knockout drums and cyclones are recommended for waxy and coking feeds. Demister mats are not suitable with these feeds because of the danger of plugging. Vane demister packages are used as alternatives, but cleaning provisions should be made.

SEPARATOR SIZING

Vertical liquid-vapor separators are used to disengage a liquid from a vapor when the volume of liquid is small compared with the vapor volume.

The maximum allowable vapor velocity in a vertical separator that reduces the liquid carry-over depends upon:

Liquid and vapor densities
A constant, K, based on surface tension, droplet size, and physical characteristics of the system (see Nomenclature).

The proportionality constant, K, is 0.35 for oil and gas systems with at least 10 in. 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, K decreases with pressure, having an approximate value of 0.30 at 250 psig and 0.275 at 800 psig.

Watkins developed a correlation between the separation factor and K. 1 Fig. 3 illustrates Watkins' vapor velocity factor chart, based on 5% of the liquid being entrained with the vapor. Blackwell developed a polynomial equation using Watkins' data to calculate the K value for a range of separation factors between 0.006 and 5.0.2

Watkins proposed a method for sizing reflux drums based on several factors, as illustrated in Tables 1 and 2.

Table 1 gives the recommended design surge times; Table 2, the multiplying factors for various operator efficiencies.

The operating factor is based on the external unit and its operation, its instrumentation and response to control, the efficiency of labor and chronic mechanical problems, and the possibility of short or long-term interruptions. 3

The multiplying factors F1 and F2 represent the instrument and labor factors. A multiplying factor F3 is applied to the net overhead product going downstream. F4 depends on the kind and location of level indicators.

It is recommended that 36 in. plus one half the feed nozzle OD (48 in. minimum) be left above the feed nozzle for vapor. Below the feed nozzle, 12 in. plus one half the feed nozzle OD is required for clearance between the maximum liquid level and the feed nozzle (minimum of 18 in.).

At some value between L/D ratios of 3 and 5, a minimum vessel weight will occur, resulting in minimum costs for the separator. Fig. 4 shows the dimensions of a vertical separator.

CALCULATION METHOD

The following steps are used to size a vertical drum.

Calculate the vapor-liquid separation factor using Equation 1 (see Equations).
From Blackwell's correlation, determine the design vapor velocity factor Kv, and the maximum design vapor velocity (Equations 2-4).
Calculate the minimum vessel cross sectional area (Equations 5 and 6).
Set a vessel diameter based on 6-in. increments and calculate cross sectional area (Equations 7 and 8).
Estimate the vapor-liquid inlet nozzle based on the velocity criteria given in Equations 9 and 10.
From Fig. 4, make a preliminary vessel sizing for the height above the center line of a feed nozzle to top seam, use 36 in. + one half the feed nozzle OD or 48 in. minimum. Use 12 in. + one half the feed nozzle OD or 18 in. minimum to determine the distance below the center line of the feed nozzle to the maximum liquid level.
From Tables 1 or 2 select the appropriate full surge volume in seconds. Calculate the required vessel volume (Equations 11 and 12). The liquid height is calculated as shown in Equation 13.
Check the geometry. The result of (HL + 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 greater than 3. If the required liquid surge volume is greater than that obtained in a vessel having L/D less than 5, a horizontal drum must be provided.

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 KH for horizontal separators is generally set at 1.25 Kv.

The following steps are used to size horizontal separators:

Calculate the vapor-liquid separation factor by Equation 1 and Ky by Equations 2 and 3.
For horizontal vessels, calculate KH by Equation 14.
Calculate the maximum design vapor velocity (Equation 15).
Calculate the required vapor flow area (Equation 16).
Select appropriate design surge time from Tables I or 2 and calculate full liquid volume by Equations 11 and 12.
The remainder of the sizing procedure is carried out by trial and error as follows:
When the vessel is full, the separator-vapor area can be assumed to occupy only 15-25% of the total cross sectional area. Here, a value of 20% is used and the total cross sectional area is calculated by Equation 17 and the minimum vessel diameter by Equation 18.
Assume a length-to-diameter ratio of 3 (L/D = 3). Calculate the vessel length using Equation 19.
Because the vapor is assumed to occupy 20% of the total cross sectional area, liquid will occupy 80% of that area (Equation 20).
Calculate the vessel volume (Equation 21).
Calculate liquid surge time (Equation 22).

LIQUID HOLD-UP

The dimensions of both vertical and horizontal separators are based on rules designed to provide adequate liquid holdup and vapor disengaging space.

For instance, the desired vapor space in a vertical separator is at least 1-1/2 times the diameter, with 6 in. minimum above the top of the inlet nozzle. In addition, a 6-in. 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% of the diameter, or 12 in., whichever is greater.

WIPE-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.

As a rule of thumb, however, the pressure drop (delta P) of a wire mesh is 1.0 in. water. Every manufacturer makes a standard high-efficiency, very high-efficiency, or high-throughput mesh under various trade names, each for a specific requirement.

STANDARD SPECS

The following specifications are generally standard for the design of horizontal separators: 4

The maximum liquid level shall provide a minimum vapor space height of 15 in. but not be below the center line of the separator.
The volume of dished heads is not considered in vessel-sizing calculations.
The inlet and outlet nozzles shall be located as closely as practical to the vessel tangent tines.
Liquid outlets shall have antivortex baffles.

PIPING REQUIREMENTS

Pipes connected to and from the process vessels must not interfere with the proper working of the vessels. Therefore, the following guidelines should be observed:

There should be no valves, pipe expansions, or contractions within 10 pipe diameters of the inlet nozzle.
There should be no bends within 10 pipe diameters (10D) of the inlet nozzle, except for knockout drums and demisters. A bend in the feed pipe is permitted if it is in a vertical plane through the axis of the feed nozzle.
For cyclones, a bend in the feed pipe is allowed if it is in a horizontal plane and the curvature is in the same direction as the cyclone vortex.
A pipe reducer may be used in the vapor line leading from the separator, but it should be no nearer to the top of the vessel than twice the outlet pipe diameter.
A gate or ball-type valve that is fully opened in normal operation should be used where a valve in the feed line near the separator cannot be avoided.
High-pressure drops that cause flashing and atomization should be avoided in the feed pipe.
If a pressure-reducing valve in the feed pipe cannot be avoided, it should be located as far upstream of the vessel as practical.

DESIGN PROBLEMS

Three design problems will illustrate the use of the calculations given previously.

PROBLEM 1:

Size a reflux accumulator for a depropanizer to be installed in an existing large refinery gas plant. Assume that 860 gpm of reflux is pumped back for temperature control, and 400 gpm of product propane is fed to a new ethylene unit. Product flow is on level control with unit alarm.

SOLUTION:

The volume of the drum in gal is given by Equation 23. 3 From Tables 1 and 2, F1 = 2.0, F2 = 1.0 (good labor), F3 = 2.0, and F4 = 1.5. Solving Equation 23 yields Vd = 14,400 gal, full.

The residence time, T, in minutes, is given by Equation 24. Solving Equation 24 gives 6 min, half-full.

PROBLEM 2:

Size a vertical separator under the following conditions:

WL = 5,000 lb/hr
WV = 37,000 lb/hr
PL = 61.87 lb/cu ft
PV = 0. 374 lb/cu ft

SOLUTION:

The surge time, T, calculated by Equation 22, is 5 min.

PROBLEM 3:

Size a horizontal separator for the following conditions:

WL = 56,150 lb/hr
WY = 40,000 lb/hr
rho L = 60.0 lb/cu ft lb
rho V = 1.47 lb/cu ft
Use L/D = 3.0.

SOLUTION:

A computer program called Vessel has been developed to size both vertical and horizontal separators. Table 3 shows the results of both separators. Fig. 5 presents a nomograph of the vapor rise in horizontal vessels. 5 A check of the vapor area of 20% of the total cross sectional area and the calculated diameter of 4.13 ft shows that the vapor rise is approximately 12 in.

REFERENCES

Watkins, R.N., "Sizing Separators and Accumulators," Hydrocarbon Processing, Vol. 46, No. 11, November 1967, pp. 253-56.
Blackwell, W.W., Chemical Process Design On A Programmable Calculator, McGraw-Hill Inc.
Walas, S.M., Chemical Process Equipment-Selection and Design, Butterworths Series in Chemical Engineering, 1988.
Gerunda, A., "How to size liquid-vapor Separators," Chem. Eng., May 1981, pp. 81-84.
Evans, F.L. Jr., Equipment Design Handbook for Refineries and Chemical Plants, Vol. 2, 2nd ed., Gulf Publishing Co., Houston, 1980.