Jones D.S.J., Pujado P.R. Handbook of Petroleum Processing
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876 CHAPTER 17
Therefore, the average amount paid out as interest every compounding period is:
Average interest payment = A −
C
n
or, on a percentage basis relative to C, we obtain:
Average rate of interest =
A
C
−
1
n
This rate is per compounding period; it should be annualized by multiplying it by the
number of compounding periods in one year.
What is the effect of using interest averaging? Just to give an example, a 10 year
loan at 10% compounded annually ( A = 0.1627454, if C = 1) represents an average
annual interest of only 6.27%.
Chapter 18
Process equipment in petroleum refining
D.S.J. Jones
Introduction
This chapter deals with the items of equipment normally met with in the petroleum
refining industry. Indeed, many of the items that will be described and discussed here
are also common to many other process industries. Knowledge of these equipment
items are essential for good refinery design, operation, and troubleshooting when
necessary. The equipment described here falls into the following categories, and will
be presented in the following parts:
Part 1 Vessels
Part 2 Pumps
Part 3 Compressors
Part 4 Heat Exchangers
Part 5 Fired Heaters
These sections will include a description of the various types, an in depth discussion,
and design features. They will also provide an example of the data sheet usually
forwarded to manufacturers for the items required. Invariably in refinery technical
libraries these data sheets are included as part of the ‘Mechanical Catalogues’and
supported by narrative specifications which give details of metallurgy and fabrication
codes etc. These catalogues are provided by the equipment supplier and are part of
all the information dossier on each item. Included also are such items as installation
details, start-up procedures, routine maintenance procedures and the like. In most
refineries today the catalogues are kept on computer discs or microfilm.
18.1 Vessels
This section address the pressure vessels that are common to most refineries. These
include:
877
878 CHAPTER 18
r
Columns and Towers
r
Knock out Drums and Separators
r
Accumulators and Surge vessels
Storage tanks have been dealt with in Chapter 13 of this Handbook.
Fractionators, trays, and packings
Trayed towers
Columns normally constitute the major cost in any chemical process configuration.
Consequently it is required to exercise utmost care in handling this item of equipment.
This extends to the actual design of the vessel or evaluating a design offered by others.
Normally columns are used in a process for fractionation, extraction or absorption
as unit operations. Columns contain internals which may be trays, or packing. Both
types of columns will also contain suitable inlet dispersion nozzles, outlet nozzles,
instrument nozzles, and access facilities (such as manholes or handholes). This item
deals with the trayed towers.
Tray types
There are three types of trays in common use today. These are:
r
Bubble cap
r
Sieve
r
Valve
Bubble cap trays
This type of tray was in wide use up until the mid to late 1950s. Their predominance
was displaced by the cheaper sieve and valve trays. The bubble cap tray consists of
a series of risers on the tray which are capped by a serrated metal dome. Figure 18.1
shows two types of caps. One is used in normal fractionation service while the other
is designed for vacuum distillation service. Vapor rises up through the risers into the
bubble cap. It is then forced down through the serrated edge or, in some cases, slots
at the bottom of the cap. A liquid level is maintained on the tray to be above the slots
or serrations of the cap. The vapor therefore is forced out in fine bubbles into this
liquid phase thereby mixing with the liquid. Mass and heat transfer between vapor
and liquid is enhanced by this mixing action to effect the fractionation mechanism.
Capacity. Moderately high with high efficiency.
Efficiency. Very efficient over a wide capacity range.
Figure 18.1. Bubble cap design.
880 CHAPTER 18
Entrainment. Much higher than perforated type trays due to the “Jet”action that
accompanies the bubbling.
Flexibility. Has the highest flexibility both for vapor and liquid rates. Liquid heads
are maintained by weirs.
Application. May be used for all services except for those conditions where coking or
polymer formation occur. In this case Baffle or Disc & Donut trays should be used.
Note: Because of the relatively high liquid level required by this type of tray it incurs
a higher pressure drop than most other types of trays. This is a critical factor in tray
selection for vacuum units.
Tray Spacing. Usually 18
to 24
. For Vacuum service this should be about 30
to 36
.
Sieve trays
This is the simplest of the various types of trays. It consists of holes suitably arranged
and punched out of a metal plate. The vapor from the tray below rises through the
holes to mix with the liquid flowing across the tray. Fairly uniform mixing of the liquid
and vapor occurs and allows for the heat/mass transfer of the fractionation mechanism
to occur. The liquid flows across a weir at one end of the tray through a downcomer
to the tray below.
Capacity. As high as or higher than bubble cap trays at design vapor/liquid rates.
Performance drops off rapidly at rates below 60% of design.
Efficiency. High efficiency at design rates to about 120% of design. The efficiency
falls off rapidly to around 50–60% of design rate. This is due to “weeping”which is
the liquid leaking from the tray through the sieve holes.
Entrainment. Only about one-third of that for bubble cap trays.
Flexibility. Not suitable for trays operating at variable loads.
Application. In most mass transfer operations where high capacities in vapor and
liquid rates are required. Handles suspended solids and other fouling media well.
Tray spacing. Requires less tray spacing than bubble cap. Usually spacing is rarely
less than 15
although some services can operate at 10
and 12
. In vacuum service
a spacing of 20
to 30
is acceptable.
Valve trays
These trays have downcomers to handle the liquid traffic and holes with floating caps
that handle the vapor traffic. The holes may be round or rectangular and the caps over
the holes are moveable within the limits of the length of the “legs”which fit into the
PROCESS EQUIPMENT IN PETROLEUM REFINING 881
Figure 18.2. Valve unit types—Glitsch Ballast.
holes. Figures 18.2 and 18.3 show the type of valves and valve trays offered by Glitsch
as their “Ballast”trays.
Valve trays are by far the most common type of tray used in the chemical industry
today. The tray has good efficiency and a much better flexibility in terms of turn down
than the sieve or bubble cap trays. Its only disadvantage over the sieve tray is that it
is slightly more expensive and cannot handle excessive fouling as well as the sieve
tray. The remainder of this item will now be dedicated to the sizing and analysis of
the valve tray tower.
882 CHAPTER 18
Figure 18.2. (Cont.)
Trayed tower sizing
The height of a trayed tower is determined by the number of trays it contains, the liquid
surge level at the bottom of the tower, and the tray spacing. The number of trays is
a function of the thermodynamic mechanism for the fractionation or absorption duty
required to be performed. This is described in Chapters 3 and 4 of this Handbook.
PROCESS EQUIPMENT IN PETROLEUM REFINING 883
Figure 18.3. Valve trays—Glitsch Ballast.
The diameter of the tower is based on allowable vapor and liquid flow in the tower and
the type of tray. This section now deals with determining the tower diameter using
valve trays.
The “quickie” method
This method is good enough for a reasonable estimate of a tower diameter which can
be used for a budget type cost estimate or initial plant layout studies. The steps used
for this calculation are as follows:
884 CHAPTER 18
Figure 18.4. Tray spacing versus ‘K’factor.
Step 1. Establish the liquid and vapor flows for the critical trays in the section of
the tower that will give the maximum values. These are obtained by heat balances
(see Chapters 3 and 4 of this Handbook). The critical trays are:
The top tray.
Any side stream draw off tray.
Any intermediate reflux draw off tray.
The bottom tray.
Step 2. Calculate the actual cuft/hr at tray conditions of the vapor. Then using the
total mass per hour of the vapor calculate its vapor density in lbs/cuft.
Step 3. From the heat balance determine the density of the liquid on the tray at tray
temperature in lbs/cuft.
Step 4. Select a tray spacing. Start with a 24
space. Read from Figure 18.4 a value for
‘K’on the flood line. Using the equation:
G
f
= K
((ρ
v
× (ρ
l
− ρ
v
))
PROCESS EQUIPMENT IN PETROLEUM REFINING 885
where
G
f
= mass vapor velocity in lbs/hr sqft at flood,
K = the constant read from the flood curve in Figure 18.4,
ρ
v
= density of vapor at tray conditions in lbs/cuft,
ρ
L
= density of liquid at tray conditions in lbs/cuft.
Step 5. Multiply G
f
by 0.82 to give mass velocity at 82% of flood which is the normal
recommended design figure. Divide the actual vapor rate in lbs/hr by the vapor
mass velocity to give the area of the tray. Calculate tray diameter from this area.
An example calculation now follows.
Example calculation
Calculate the diameter of the tower to handle the liquid and vapor loads as follows:
Vapor to tray Liquid from tray
lbs/hr = 47,700 GPH @ 60 = 119.7
moles/hr = 929.7 Hot GPH = 153.0
ACFS = 7.83 Hot CFS = 0.339
lbs/cuft ρ
V
= 1.69 lbs/hr = 33,273
Temp
◦
F = 167 lbs/cuft ρ
L
= 27.3
Pressure PSIA = 220 Temp
◦
F = 162
Tray spacing is set at 24
and the trays are valve type.
From Figure 18.4 ‘K ’ = 1,110 at flood.
ρ
v
= 1.69 lbs/cuft at tray conditions.
To calculate ρ of vapor at tray conditions use:
ρ =
wt/hr × 520 × pressure (psia)
378 × 14.7 × moles/hr × Temp
◦
R
where
Press is tray pressure.
Temp
◦
R is tray temperature in
◦
F + 460.
ρ
L
= 27.3 lbs/cuft.
Then G
f
= 1,110
1.69 × (27.3 − 1.69)
= 7,302 lbs/hr.sqft
Area of tray @ 82% of flood =
47,700
0.82 × 7,302
sqft
= 7.97 sqft
= 3.18 ft. say 39