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The capacity of atmospheric liquid storage tanks should be

at least 1.5 times the size of the transportation equipment,

typically 4,000 to 7,500 gal for tank trucks and up to 34,500

gal for tank cars. A shipment by barge may be as large as

420,000 gal. The maximum size for a single cone-roof or

floating-roof tank is approximately 20,000,000 gal, which

corresponds to a diameter of about 300 ft and a height of

about 50 ft. Storage of liquid feeds, products, and interme-

diates may also be provided on-site in so-called surge tanksor day tanks, which provide residence times of 10 min to

one day. Equations for estimating the f.o.b.purchase costs of

open, cone-roof, and floating-roof tanks are included in

Table 22.32.

For liquid stored at pressures greater than 3 psig or under

vacuum, spherical or horizontal (or vertical) cylindrical

(bullet) pressure vessels are used. Vertical vessels are not

normally used for volumes greater than 1,000 gal. Horizontal

pressure vessels for storage are at least as large as 350,000

gal. Spherical pressure vessels are also common, with more

than 5,000 having been constructed worldwide. For liquid

storage, spheres as large as 94 ft in diameter (3,000,000 gal)

have been installed. The design and costing of cylindrical

pressure vessels is considered in detail in Section 22.5.

Purchase costs are plotted in Figure 22.13. For spherical

pressure vessels, Eq. (22.60) for cylindrical pressure vessels

is revised to:

PdDi (22 72)

In the rough region, the available vacuum systems in-

clude: (1) one-, two-, and three-stage ejectors driven with

steam and with or without interstage surface or barometric

(direct-contact) condensers, (2) one- or two-stage liquid-ring

pumpsusing oil or water as the sealant, and (3) dry vacuum

pumpsincluding rotary lobe, claw, and screw compressors.

Although the first two systems have been the most widely

used dry vacuum pumps are gaining attention because they

Table22.30 Lower Limits of Suction Pressure and Capacities

of Vacuum Systemsa

System Type

Lower Limit

of Suction

Pressure (torr)

Volumetric Flow

Range at Suction

Conditions (ft3/min)

Steam-jet ejectors 101,000,000

One-stage 100

Two-stage 15

Three-stage 2

Liquid-ring pumps 318,000

One-stage water sealed 50

Two-stage water sealed 25

Oil-sealed 10

Dry vacuum pumps

Three-stage rotary-lobe 1.5 60240

Three-stage claw 0.3 60270

Screw compressor 0.1 501,400

aReprinted with permission from Ryans and Bays (2001).

22.6 Purchase Costs of Other Chemical Processing Equipment 589


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PdDi (22 72) used dry vacuum pumps are gaining attention because they

equipment operating under a vacuum, the air leakage leaving

the equipment will be accompanied by volatile process

components. To partially recover these components and

reduce the load on the vacuum pump, the exiting gas should

first pass through a precondenser before proceeding to the

vacuum system. Theflow rates of process components still in

the gas leaving the precondenser with the air can be deter-

mined by a flash calculation as illustrated in the example

below.Note, in Table 22.30, that steam-jet ejector systems can

handle a very wide range of conditions. They have no moving

parts and are inexpensiveto maintain, but arevery inefficient

because of the high usage of motive steam. The maximum

compression ratio per stage is approximately 7.5. The re-

quired motive steam rate for each stage depends on the ratio

of suction pressure-to-discharge pressure, steam pressure,

temperature, gas properties, and ejector nozzle-to-throat

ratio. A reasonably conservative range for the total motive

steam requirement for all stages, when using 100-psig steamto evacuate mostly air, is 510 lb of steam per pound of gas

being pumped. A detailed procedure for designing an ejector

vacuum system is given by Sandler and Luckiewicz (1987).

Liquid-ring pumps are limited to a suction pressure of

10 torr with a moderate capacity and efficiency (2550%).

Dry vacuum pumps can achieve very low pressures at higher

efficiencies, but only for low capacities. Since vacuum

pumps are actually gas compressors, a tendency exists for

the gas temperature to increase in an amount corresponding

EXAMPLE 22.18

A vacuum distillation column produces an overhead vapor of

1,365 kmol/hr of ethylbenzene and 63 kmol/hr of styrene at 30

kPa. The volume of the column, vapor line, condenser, and reflux

drum is 50,000 ft3. The overhead vapor is sent to a condenser

where most of the vapor is condensed. The remaining vapor at

508C and 25 kPa is sent to a vacuum system. Determine the air

leakage rate in the distillation operation and the flow rate to the

vacuum system. Select an appropriate vacuum system and deter-

mine its f.o.b. purchase cost at a CE cost index of 550.

SOLUTION

The amount of air leakage, W, is estimated from Eq. (22.73).

Using a pressure of 25 kPa 188 torr:

W 5f0:02980:03088ln188

0:0005733ln1882g50; 0000:66 227lb/hr

This is equivalent to 103 kg/hr or 3.6 kmol/hr. Adding this to the

overhead vapor and performing a flash calculation at 508C and

25 kPa (188 torr) gives a vapor leaving the reflux drum and

entering the vacuum system of 3.6 kmol/hr of air and 0.7 kmol/hr

of ethylbenzene. The volumetric flow rate to the vacuum system

is 272 ft3/min. The flow rate in pounds per hour is 394. From

Table 22.30, applicablevacuum systems are a single-stage steam-

jet ejector, a single-stage liquid-ring pump, and a screw compres-

sor. The three-stage claw unit is just out of the range of the

volumetric flow rate

590 Chapter 22 Cost Accounting and Capital Cost Estimation


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Conveyors

Belt Width, W, in. Length, L, ft 1460 in., up to 300 ft CP 21:5WL Does not include motor or drive

Screw Diameter, D, in. Length, L, ft 620 in., up to 300 ft CP 70:5DL0:59 Does not include motor,

drive, lid, or jacket

Vibratory Width, W, in. Length, L, ft 1236 in., up to 100 ft CP 81:6W0:57L0:87 Does not include motor or drive

Bucket elevators Bucket width, W, in. Height, L, f t 6 2 0 i n., 1 5 15 0 ft CP 610W0:5L0:57 Does not include motor or drive

P ne um at ic c onve yo rs S ol id s fl ow r ate ,m, lb/s

Equivalent length, L, feet

330 lb/s, 30600 ft CP 15; 200M0:63L0:20 Includes blower, motor, piping,

rotary valve, and cyclone

Storage tanks

Open Volume, V, gal 1,00030,000 gal CP 18V0:72 Fiberglass

Cone roof Volume, V, gal 10,0001,000,000 gal CP 265V0:51 Carbon steel, pressure to 3 psig

Floating roof Volume, V, gal 30,0001,000,000 gal CP 475V0:51 Carbon steel, pressure to 3 psig

Spherical, 030 psig Volume, V, gal 10,0001,000,000 gal CP 60V0:72 Carbon steel

Spherical, 30200 ps ig Volume ,V, gal 10,000750,000 gal CP 47V0:78 Carbon steel

Gas holders Volume, V, ft3 4,000400,000 ft3 CP 3; 170V0:43 Carbon steel, pressure to 3 psig

Vacuum systems

O ne-stage j et eject or (lb /h r)/(su ct io n p ressure, t orr) 0 .1 1 00 l b/ hr-t orr CP 1; 690S0:41 See Table 22.31 for multistage units

and condensers

Liquid-ring pumps Flow at s uction, ft3/min 50350 ft 3/min CP 8; 250S0:35 Stainless steel with sealant

recirculation

Three-stage lobe Flow at suction, ft3/min 60240 ft 3/min CP 7; 120S0:41 Includes intercoolers

Three-stage claw Flow at suction, ft3/min 60270 ft 3/min CP 8; 630S0:36 Includes intercoolers

Sc rew com pre ssors Flow at s uction, ft3/min 50350 ft 3/min CP 9; 590S0:38 With protective controls

Wastewater treatment

Primary Wastewater rate, Q, ga l/ mi n 7 5 75, 00 0 g al /m in CBM 14; 800Q0:64 Bare-module cost

P ri ma ry + S ec ond ar y Wa st ew at er r ate ,Q, ga l/ mi n 7 5 75, 00 0 g al /m in CBM 43; 000Q0:64 Bare-module cost

Primary + Secondary + Tertiary Wastewater rate,Q, ga l/ mi n 7 5 75, 00 0 g al /m in CBM 88; 000Q0:64 Bare-module cost

595

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