1 /171 /17
Introduction to Petroleum Refinery(Static
Equipment)
2 /142 /14
Contents
Towers (Distillation) Heat Exchangers (Shell & Tube) Boilers & Furnace Separators Valves
3 /143 /14
ATMOSPHERIC DISTILLATION
Eng. Ahmed Deyab Fares
4 /144 /1404/07/2023 4
1. Fundamentals of Separation
3. Crude Distillation
4. Operation
Contents
2. Tower Internal
5 /145 /1404/07/2023 5
1.FUNDAMENTALS OF SEPARATION IN TOWERS
6 /146 /1404/07/2023 6
Distillation
We separate many things by detecting a difference in a physical properties
Separation by distillation implies a
difference in boiling points of two
or more materials
color, size, weight, shape
7 /147 /1404/07/2023 7
The components or compounds making up crude oil are numbered in thousands
Many of these components have similar physical properties including boiling points that may differ by only a few degrees. Therefore, it is difficult to separate some pure compounds from the complex mixture of components in crude oil by distillation alone
There are other methods of separation used in a refinery for example, extraction with a solvent, crystallization, and absorptionFortunately, rarely need pure compounds and it is often enough to separate groups of compounds from each other by boiling range
8 /148 /1404/07/2023 8
If we separate many compounds in crude oil into groups we find that these groups have characteristics that make them considerably more valuable than the whole crude oilSome of these groups are products Some may be feedstock to other processing units where they are chemically changed into more valuable products
These products, in turn, are usually separated or purified by distillation
9 /149 /1404/07/2023 9
The basic principle of distillation is simple
When a solution of two or more components is boiled
The lighter component ( the one most volatile or the one with the greatest tendency to vaporize )vaporizes preferentially
Principles Of Distillation
10 /1410 /1404/07/2023 10
Tow component mixture is contained in a vessel
When heat is add until the more volatile material ( red dotes ) start to vaporize. Now the vapor contains a higher proportion of red dots than dose the original liquid
11 /1411 /1404/07/2023 11
It is important to note that an equilibrium in
composition will be established
at a given temperature and
pressure
By equilibrium we mean there is a given
concentration as “red dots" in the vapor and in the
liquid depending upon the original concentration of
each component in the liquid and their respective
properties in relation to each other
12 /1412 /1404/07/2023 12
Now, let's develop this simple distillation concept into a practical operation as it is used in the refinery
First , let’s separate and remove the product
This results in the vapor above the liquid being relatively rich in the lighter ( more volatile material )
And the liquid is left with proportionately more of the less volatile ( heavier liquid )
Thus a separation, to some degree, has taken place
13 /1413 /1404/07/2023 13
By cooling the over head vapor, we condense and remove it from the original mixture
Thus to have made a partial separation, partial because you will note that there are a few “blue dotes" in the distillate product
14 /1414 /1404/07/2023 14
This has occurred because at the
temperature and pressure we are
conducting the distillation, the heavier
component still vaporizes to some
extent
This is because the components of
interest in a given distillation usually
have fairly close boiling points
15 /1415 /1404/07/2023 15
Therefore, to purify the distillate product, we may have to conduct a second distillation as shown
Obviously, we can continue to cascade these simple distillations until we achieve the desired purity of product
16 /1416 /1404/07/2023 16
The distillations depicted so far are those
we call patch, and normally practical in the
refinery, although it is done frequently in
the laboratory
Let us make our distillation equipment
look more like refinery pieces of
equipment and let us make continuous
instead of patch operation
17 /1417 /1404/07/2023 17
This is called Flash Vaporization As shown.
The liquid is pumped continuously through a heater and into a drum where the pressure is lower
The lighter material flashes instantaneously (vapor and liquid flow from the drum continuously)
The same system is shown diagrammatically in the in the lower section of the Figure.
18 /1418 /1404/07/2023 18
Suppose we have 50% of the charge
taken overhead
That is, we set the temperature and the
pressure of the system in such a way that
half the charge is boiled off
And further, suppose the resulting
overhead product does not contain the
desired concentration of the lighter
product
As we have seen before, we can increase
the purity by adding a stage of
distillation
19 /1419 /1404/07/2023 19
Suppose we add two more stages of distillation
Although this is accomplishing our goal of increasing the purity of the light friction, we are also making large amounts of the intermediate product, each of which contains the same light friction
20 /1420 /1404/07/2023 20
An obvious simplification in
equipment can be made if we allow
the hot vapor from the stage above
the next higher (the intermediate
product)
This eliminates the need for the
intermediate condensers and heaters
Now we have the continuous,
multi-stage distillation
21 /1421 /1404/07/2023 21
Tower SectionsWe have described staging for the purpose of concentrating the lighter component in the overhead
The same principles apply to concentrating the heavier component in the bottom product
The upper stages are called rectifying stages
These below the feed are called stripping stages
22 /1422 /1404/07/2023 22
The upper rectifying section increases the
purity of the overhead product.
The lower stripping section increases the
recovery of the overhead product.
In many cases, the bottom product is the
one of primary interest
For the bottom, or heavy, product the
rectifying section improves recovery
23 /1423 /1404/07/2023 23
Equilibrium StageA stage, or more specifically, an equilibrium stage, is defined as :
Any portion of the distillation column such that the liquid and vapor leaving it have composition in equilibrium with
each other.
By definition, then, a stage should be designed in such a way as to provide intimate contact, or mixing, of the rising vapor and the descending liquid. The concept of an equilibrium stage is converted to an actual mechanical separation tray by using an efficiency factor which is less than one and depends on the tray design.
24 /1424 /1404/07/2023 24
25 /1425 /14
Column Internals :The plates or trays are contacting devices
and used to hold up the liquid to provide better contact between vapour and liquid, hence better separation .
1. Trays /Plates 2. Packings
VALVE TRAYS
SIEVE TRAYS
BUBBLE CAP TRAYS
TRAYS W ITH DO W NCO M ER
DUAL FLO W TRAYS
BAFFLE TRAYS
DISC & DO NUT TRAYS
RIPPLE TRAYS
TRAYS W ITHO UT DO W NCO M ER
PRO PRIETARY DESIG NS(M VG , SUPERFRAC)
TRITO N, PRO VALVE
NYE TRAYS
M ULTI DO W NCO M ER TRAYS
NO N- FRACTIO NATIO N TRAYS
CO LLECTO R/ CHIM NEY TRAYS
TRAYS
R A S C H IGL E S S IN GC R O S S P A R TITIO N R IN GB E R L S A D D L E S
I G E N E R A TIO N
P A L L R IN GH Y P A KIM TPC M RN U TTE R R IN G
II G E N E R A TIO N
R A N D O M
G E M P A KM E L L A P A KIN TA L O XP A R L P A K
III G E N E R A TIO N
S TR U C TU R E D
P A C K IN G
26 /1426 /14
27 /1427 /14
Sieves
28 /1428 /14
Sieve Tray This tray is a sheet of light metal
with a large number of holes drilled through it.
Vapor rising through the holes keeps the liquid on the tray and bubbles up through it.
The overflow weir keeps a constant depth of liquid on the tray.
29 /1729 /1729
A sieve tray is:
Inexpensive, Easy to clean, and Maintains good liquid and vapor contact
as long as it is operated at its design load.
Because the sieve tray has fixed openings and does not have covers over the holes, it does not perform well if tower loads are constantly changing.
30 /1430 /14
31 /1431 /14
32 /1432 /14
33 /1433 /14
Bubble Cap / Valve traysIn valve trays / Bubble Cap trays,
perforations are covered by liftable caps.
Vapour flows lifts the caps, thus self
creating a flow area for the passage of
vapour. The lifting cap directs the vapour to
flow horizontally into the liquid, thus
providing better mixing
Valve Trays
Sieve Trays
Bubble Cap
34 /1734 /1734
TraysBubble Cap TrayThe vapor is broken into small bubbles which increases the surface area for vapor-liquid contact.
The bubble cap sits on top of a riser.
The riser channels vapors into the bubble cap.
35 /1435 /14
36 /1436 /1436
37 /1437 /14
38 /1438 /14
39 /1439 /14
40 /1440 /14
41 /1441 /14
42 /1442 /14
43 /1743 /1743
Valve Tray
A valve tray has a variable opening for vapors to flow through.
The hole has a cover that consists of a cap held in place by guides which go down through the plate, or tray and hook underneath it.
44 /1744 /1744
When there is no vapor flow, the caps sits over the hole and close it.
Under low pressure the cap start to rise.
As the flow of vapors increases, the cap rise until it is stopped by the guide tabs.
45 /1745 /1745
The valve tray is similar to the babble cap tray.
Both are more adaptable to variations in tower loads than a sieve tray.
The valve trays and bubble cap trays are designed to perform well with variable tower loads.
46 /1446 /14
Valve Tray
47 /1447 /14
48 /1448 /14
49 /1449 /14
50 /1450 /14
51 /1451 /14
52 /1452 /14
53 /1453 /14
54 /1454 /14
55 /1455 /14
56 /1456 /14
57 /1457 /14
58 /1458 /14
59 /1459 /14
60 /1460 /14
61 /1461 /14
62 /1462 /14
63 /1463 /14
64 /1464 /1464
Selection of Tray Type
The principal factors to consider when comparing the performance of bubble-cap, sieve and valve trays are:
Cost,
Capacity,
Operating range,
Maintenance and
Pressure drop.
65 /1465 /1465
Cost:Bubble-cap trays are appreciably more expensive than sieve or valve trays.
The relative cost will depend on the material of construction used;
For mild steel the ratios, bubble-cap: valve: sieve, are approximately
3.0 : 1.5 : 1.0
66 /1466 /1466
Capacity:
There is little difference in the capacity rating for the three types (the diameter of the column required for a given flow-rate).
The ranking is:
sieve, valve, and bubble-cap
67 /1467 /14
68 /1468 /1468
Operating range:
This is the most significant factor.
By operating range is meant the range of vapour and liquid rates over which the plate will operate satisfactorily (the stable operating range).
Some flexibility will always be required in an operating plant to:
Allow for changes in production rate, and Cover start-up and shut-down conditions.
69 /1469 /1469
Bubble-cap trays have a positive liquid seal and can therefore operate efficiently at very low vapour rates.
70 /1470 /1470
Sieve trays rely on the flow of vapour through the holes to hold the liquid on the tray and cannot operate at very low vapour rates, but, with good design, sieve trays can be designed to give a satisfactory operating range;
Typically, from 50 to 120 % of design capacity.
Valve trays are intended to give greater flexibility than sieve trays at a lower cost than bubble-caps.
71 /1471 /1471
Maintenance:
For dirty services, bubble-caps are not suitable as they are most susceptible to plugging. Sieve trays are the easiest to clean.
72 /1472 /1472
Pressure Drop:
The pressure drop over the trays can be an important design consideration, particularly for vacuum columns.
The trays pressure drop will depend on the detailed design of the tray but.
In general,
sieve plates give the lowest pressure drop, followed by valves, with bubble-caps giving the highest.
73 /1473 /1473
Summary
Sieve trays are the cheapest and are satisfactory for most applications.
Valve trays should be considered if the specified turn-down cannot be met with sieve trays.
Bubble-caps should only be used where: Very low vapour (gas) rates have to be handled and A positive liquid seal is essential at all flow-rates.
74 /1474 /14
Typical Tray SectionTypical Tray Section
Outlet Weir
Inlet Weir
Seal Pot
DowncomerTrays
75 /1775 /1775
Inlet Weirs
These contribute to the uniform distribution of liquid as it enters the tray from the down comer.
It is not recommended for fluids that are dirty or tend to foul surfaces.
If a more positive seal is required at the downcomer at the outlet, an inlet weir canbe fitted or a recessed seal pan used.
76 /1476 /14
Downcomer sealing can be achieved primarily by 2 means:
(1) inlet weirs (2) recessed seal pan. These devices provide a positive seal on the tray. The disadvantage is
that they create a pocket of stagnant liquid where dirt, sediments, etc can build up. A large amount of such build up can restrict the downcomer outlet area and lead to premature flooding. Thus, the use of these devices is not recommended in fouling or corrosive services.
77 /1477 /14
Outlet WeirsThe function of a weir is to
maintain a desired liquid level on the tray , thus insuring bubbling of vapors through liquid. Typical weir height is between 2 - 4 inches. Low weirs are frequently used in low pressure or vacuum columns. Notched (rectangular or V-shaped) weirs are commonly used for low liquid loads
78 /1478 /14
79 /1479 /14
80 /1780 /1780
DowncomersReflux flows down from one tray to the next through downcomers.
Downcomers must be large enough to allow for drainage from one tray to the next or flooding might occur on some of the trays.
Downcomers can be designed in several different ways to providesmooth flow from tray to tray.
81 /1481 /14
The straight, segmental, vertical
downcomer is widely used as it
provides good utilization of column
area for downflow and has cost and
simplicity advantage. Sloped
downcomer can be used if vapour-
liquid disengagement is difficult (e.g.
due to foaming). Sloped downcomer
also provide a slightly larger active
area for vapour-liquid contact, but it
is also more expensive.
82 /1482 /14
A downcomer must be sufficiently large to allow liquid to flow smoothly without choking. Sufficient time must also be provided in the downcomer to allow proper vapour disengagement from the down-flowing liquid, so that the liquid is relatively free of vapour by the time it enters the tray below. Inadequate downcomer area will lead to downcomer choking, whereby liquid backs up the downcomer into the tray above and eventually flood the column.
83 /1783 /1783
Weep HolesHoles for drainage must be adequate to drain the tower in a reasonable time, yet not too large to interfere with tray action.
Draining of the tower through the trays is necessary before any internal maintenance can be started.
The majority of the holes are placed adjacent to the outlet or downcomer weir.
84 /1784 /1784
Bottom Strainer
During the operation of a tower:
The bubble caps, Bolts, and Other foreign objects
may be dislodged and carried along with bottom stream.
85 /1785 /1785
To prevent these objects leaving the tower and damaging pumps, a strainer is installed in the bottom outlet line.
Strainers must have openings small enough to catch small objects, but large enough not to hinder the flow of liquid, or product, or oil out of the tower.
The holes in the strainers must be kept open so that the flow of liquid out of the tower will not be stopped, or hindered.
86 /1786 /1786
Reflux distributor
Reflux entering the top of the tower should be spread evenly across the top tray to avoid dead spots.
One way to disperse reflux is to place a reflux distributor in front of the inlet line.
87 /1787 /1787
A reflux distributor is simply a plate or baffle that prevents liquid from spraying across the tray.
Reflux entering the tower is forced to flow under the baffle so that the liquid is distributed evenly across the tray.
88 /1488 /14
89 /1789 /1789
Top Tower DemisterSometimes small drops of liquid suspended in vapor are carried up from one tray to the next or into the overhead vapor line.
This is called entrainrnent.
When the overhead product must be a dry vapor or gas, entrainment is a more serious problem.
Entrainment between trays can usually be prevented by controlling vapor velocity.
90 /1790 /1790
Entrainment at the top of a tower can be cut down by placing a demister on the vapor outlet line.
Demisters are constructed of fine-gauge wire knitted into mesh.
Demisters must be kept clean of dirt and foreign matter, or the flow of vapor will be restricted, or stopped.
91 /1491 /14
92 /1492 /14
93 /1493 /14
Classification of Trays
Based on the Number of liquid paths
2. Two Pass
3. Three Pass
4. Four Pass
1. Single Pass
94 /1494 /14
Packings :
PALL RING
RASCHIG RING
INTALOX SADDLE
1. Random Packing
2. Structured Packing
95 /1495 /14
96 /1496 /14
97 /1497 /14
98 /1498 /14
Structured PackingStructured packings are considerably
more expensive per unit volume than random packings. They come with different sizes and are neatly stacked in the column. Structure packings usually offer less pressure drop and have higher efficiency and capacity than random packings.
99 /1499 /14
100 /14100 /1404/07/2023 100
RefluxThe word reflux is defined as
"flowing back“
Applying it to distillation tower, reflux is the liquid flowing back down the tower from each successive stage
Reflux & Reboiling
101 /14101 /1404/07/2023 101
Kinds of Reflux
Cold Reflux
Cold reflux is defined as liquid that is supplied at temperature a little below that at the top of the tower
Each pound of this reflux removes a quantity of heat equal to the sum of its latent and sensible heat required to raise its temperature from reflux drum temperature to the temperature at the top of the towerA constant quantity of reflux is recirculated from the reflux drum into the top of the tower
It is vaporized and condensed and then returns in like quantity to the reflux
102 /14102 /1404/07/2023 102
103 /14103 /1404/07/2023 103
Hot Reflux
It is the reflux that is admitted to the tower at the same temperature as that maintained at the top of the tower
It is capable of removing the latent heat because no difference in temperature is involved.
104 /14104 /1404/07/2023 104
Internal Reflux
It is the liquid that overflow from one plate to another in the tower, and may be called hot reflux because it is always substantially at its boiling point
It also capable of removing the latent heat only because no difference in temperature is involved.
105 /14105 /1404/07/2023 105
Circulating Reflux
It is also able to remove only the sensible heat which is represented by its change in temperature as it circulates
The reflux is withdrawn and is returned to the tower after having been cooled
106 /14106 /1404/07/2023 106
Reflux Ratio
It is defined as the amount of actual reflux divided by the amount of top product
It is denoted by R which
equals L/D
107 /14107 /1404/07/2023 107
The Importance of Reflux Ratio
In general, increasing the reflux Improves overhead purity and Increases recovery of the bottom product
The number of stages required for a given separation will be dependent upon the reflux ratio used
108 /14108 /1404/07/2023 108
1. A minimum number of plates (stages) required at total reflux
2. There is a minimum reflux ratio below which it is impossible to obtain the desired enrichment however many plates are used
Two points to consider
109 /14109 /1404/07/2023 109
Total Reflux
Total reflux is the conclusion when all the condensate is returned to the tower as reflux, no product is taken off and there is no feed
At total reflux, the number of stages required for a given separation is the minimum at which it is theoretically possible to achieve the separation
Total reflux is carried out at :
1. Towers start-up 2. Testing of the tower
110 /14110 /1404/07/2023 110
Minimum Reflux
At minimum reflux, the separation can only be achieved with an infinite number of stages
This sets the minimum possible reflux ratio for the specified separation
111 /14111 /1404/07/2023 111
Optimum Reflux RationPractical reflux ratio will lie between
The minimum for the specified separation and Total reflux
The optimum value will be the one at which the specified separation is achieved at the lowest annual cost
For many systems, the optimum value of reflux ratio will lie between
1.2 to 1.5 times the minimum reflux ratio
112 /14112 /1404/07/2023 112
Reboiling
In all distillations processes
Heat been added by Means the feedMeans of a reboiler
113 /14113 /14
COLUMN REBOILERS
114 /14114 /14
Types of reboilers
The most critical element of reboiler design is the selection of the proper type of reboiler for a specific service. Most reboilers are of the shell and tube heat exchanger type and normally steam is used as the heat source in such reboilers. However, other heat transfer fluids like hot oil or Dowtherm (TM) may be used. Fuel-fired furnaces may also be used as reboilers in some cases.
115 /14115 /14
Many factors influence reboiler type selection. In the end, all these factors reduce to economics. Every plant will weight the trade-off between these factors differently. No one-size fits all selection exists. Major factors include:
Plot space available Total duty required Fraction of tower liquid traffic
vaporized Fouling tendency Temperature approach available Temperature approach required
116 /14116 /14
Kettle reboilers
117 /14117 /14
Thermosyphon reboilers
118 /14118 /14
Fired reboiler
119 /14119 /14
Forced circulation reboilers Forced circulation reboilers are used
for reboiler duties where viscous and/or heavily contaminated media are to be expected in the bottom product.
High liquid velocities in the tubes and the resulting shearing forces ensure that this type of heat exchanger is operated within its optimum performance range, while keeping fouling to a minimum. Pump selection influences performance and efficiency. Forced circulation reboilers can be designed for either horizontal or vertical installation.
120 /14120 /14
1 = Column
2 = Trays
3 = Downcomer
4 = Reboiling
circulation line
5 = Manhole
6 = Forced
circulation
reboiler
7 = Steam inlet
8 = Baffles
9 = Heating tubes
121 /14121 /14
122 /14122 /1404/07/2023 122
The Reboiler is a heat exchanger through which the bottom liquids circulate
Heat is transferred to the bottom materials which cause vaporization of the lighter components
This vapor travels up the column to provide
The stripping action and The additional heat necessary to vaporize the down coming reflux
123 /14123 /1404/07/2023 123
3. CRUDE DISTILLATION
124 /14124 /1404/07/2023 124
The purpose of crude oil distillation
is primarily to split the crude into several distillate fractions of a certain boiling range
Sharpness of fractionation is of secondary importance
A crude distillation tower, producing 6 fractions has 40 to 50 trays
125 /14125 /1404/07/2023 125
Crude can be separated into
gasoline,
naphtha,
kerosene,
diesel oil,
gas oil, and other products, by
distillation at atmospheric
pressure
126 /14126 /1404/07/2023 126
127 /14127 /1404/07/2023 127
Crude is generally pumped to the unit directly from a storage tank, and it is important that charge tanks be drained completely free from water before charging to the unit
If water is entrained in the chargeIt will vaporize in the exchangers and in the heater, and cause a high pressure drop through that equipment
Process Description
128 /14128 /1404/07/2023 128
Desalting
Most crude contain traces of salt which can decompose in the heater to from hydrochloric acid and cause corrosion of the fractionator's overhead equipment
In order to remove the salt, water is injected into the partially preheated crude and the stream is thoroughly mixed
129 /14129 /1404/07/2023 129
The mixture of oil and water is separated in a desalter, which is a large vessel in which may be accelerated by the addition of chemicals or by electrical devices
If the oil entering the desalter is not enough heated, it may be too viscous to permit proper mixing and complete separation of the water and the oil, and some of the water may be carried into the fractionators
130 /14130 /1404/07/2023 130
If, on the other hand, the oil is too hot, some vaporization may occur, and the resulting turbulence can result in improper separation of oil and water
The desalter temperature is therefore quite critical, and normally a bypass is provided around at least one of the exchangers so that the temperature can be controlled
131 /14131 /1404/07/2023 131
The optimum temperature depends upon
The desalter pressure and quantity of light material in the crude
but is normally about 120ºC 10ºC being lower for low pressure and light crudes
The average water injection rate is 3-5% of the charge
132 /14132 /1404/07/2023 132
Heat Exchange
In order to reduce the cost of operating a crude unit
As much heat as possible is recovered from the hot streams by heat exchanging them with the cold crude charge
The number of heat exchangers within the crude unit and cross heat exchange with other units will vary with unit design
133 /14133 /1404/07/2023 133
Crude Flashing (Furnace)
Desalted crude is heat exchanged against what ever other heat sources are available to recover maximum heat before crude is charged to the heater, which ultimately supplies all the heat required for operation of the crude unit
The heater transfer temperature is merely a convenient control, and the actual temperature, which has no great significance, will vary from 325ºC to as high as 430ºC, depending on The type of crude and The pressure at the bottom of the fractionating tower
134 /14134 /1404/07/2023 134
Fractionation
Crude entering the flash zone of the fractionating column flashes into:
The vapor which rises up the column and The liquid residue which drops downwards
This flash is a very rough separation
The vapors contain appreciable quantities of heavy ends, which must be rejected downwards into reduced crude, while the liquid contains lighter products, which must be stripped out
135 /14135 /1404/07/2023 135
Flashed vapors rise up the fractionating column countercurrent to the internal reflux flowing down the column
The lightest product, which is generally gasoline passes overhead and is condensed in the overhead receiver
The temperature at the top of the fractionators is a good measure of the endpoint of the gasoline and this temperature is controlled by returning some of the condensed gasoline as reflux to the top of the column
Increasing the reflux rate lowers the top temperature and results in the net overhead product having a lower endpoint
136 /14136 /1404/07/2023 136
External reflux which is returned to the top of the fractionators passes downwards against the rising vapors. Lighter components of the reflux are revaporized and return to the top of the column while the heavier components in the rising vapors are condensed and return down the column. We have then an internal reflux stream flowing from, the top of the fractionators all the way back to the flash zone and becoming progressively heaver as it descends
137 /14137 /1404/07/2023 137
Temperature of the drawoff decks is a fair indication of the endpoint of the product drawn at that point
The degree of fractionation between cuts is generally judged by measuring the number of degrees centigrade between the 95% point of the lighter product and the 5% point of the heaving product
Some people use IBP and FBP
But the IBP varies with stripping
138 /14138 /14
Recommended Gab between products
142 /14142 /1404/07/2023 142
Product Stripping (Side Strippers)
The flashed residue in the bottom of the fractionators and the sidecut products have been in contact with lighter boiling vapors
These vapors must be removed to meet flash point specifications and to drive the light ends into lighter and more valuable products
Steam, usually superheated steam, is used to strip these light ends
143 /14143 /1404/07/2023 143
Generally only enough steam is used to meet a flash point specification
While further increases in the quantity of steam may raise the IBP of the product slightly
144 /14144 /1404/07/2023 144
All the stripping steam is condensed in the overhead receiver and must be drained off
Refluxing water will upset the fractionators
If the endpoint of the overhead product is very low, water may not pass overhead, and will accumulate on the upper trays and cause the tower to flood
147 /14147 /1404/07/2023 147
Product Disposal
All products are cooled before being sent to storage
Light products should be below 60ºC to reduce vapor losses in storage, but
Heavier products need not be as cold
148 /14148 /1404/07/2023 148
If a product is being charged to another unit, there may be an advantage in sending it out hot
A product must never leave a unit at over 100ºC
If there is any possibility of it entering a tank with water bottoms
The hot oil could readily boil the water and blow the roof off
149 /14149 /1404/07/2023 149
The composition of a distillation product is determined by performing laboratory tests on samples of that product
These test results are then compared with product specifications or standards that have been set for the product
Product Specifications
150 /14150 /1404/07/2023 150
If the product is meeting specifications, column operations do not have to be adjusted
But, if the products are off-specification, a change in column operations must be made
One can see that the control of the tower is a rather complicated simultaneous solution of material and heat balances
151 /14151 /1404/07/2023 151
Material Balance
At each draw we must draw the quantity of material in the crude that boils within the specified boiling range
If we draw too much, or too little, the product above or below will have to shift by that amount, thereby possibly putting it off specification
To stay on specifications the material balance must be maintained
152 /14152 /1404/07/2023 152
Heat Balance
Must be solved so that the right product appears at the right tray with the proper degree of fractionation
However, crude vary, product requirements vary, and the refinery must manipulate the heat and material balance to draw the right amount of product, with the proper distillation range or other product specifications
155 /14155 /1404/07/2023 155
Initial Boiling Point (IBP)
Is the temperature at which the first drop of condensate is collected during a laboratory distillation test
In a mixture of hydrocarbons, the first molecules to vaporize are the light ones
So, the IBP test is used to check for light hydrocarbons that are present in a product
156 /14156 /1404/07/2023 156
Suppose specifications on the bottom product call for an IBP between 100-110°F
Lab tests show an IBP of 95°F
You know that light material boils at lower temperatures than heavy material
So, the bottom product in this example contains material that is too light
157 /14157 /1404/07/2023 157
In order to raise the IBP of a product, we must make the product heavier
One way is to strip some light components off with steam
Another way is to increase the temperature of the feed or the reboiler temperature so more light components are vaporized
158 /14158 /1404/07/2023 158
End Boiling Point (EP)
Is the temperature at which the last drop of liquid vaporizes during the test
In a mixture of hydrocarbons, the last molecules to vaporize are the heavy ones
So, the EBP (EP) test is used to check for heavy hydrocarbons that are present in a product
159 /14159 /1404/07/2023 159
Specifications call for an overhead product with an EP between 150-160° F
Lab results indicate an EP of 170° F
You know that heavy material boils at higher temperatures than light material
So, the top product does not meet specifications because it contains material that is too heavy
160 /14160 /1404/07/2023 160
In order to lower the EP of a product, we must make the product lighter
One way is to decrease the feed or reboiler temperature so that fewer heavy components vaporize
Another way is to lower the top temperature by increasing the reflux rate
161 /14161 /1404/07/2023 161
Flash Point
Is the temperature at which a petroleum product generates ignitable vapors
Light hydrocarbons tend to flash more easily than heavy hydrocarbons
A sample that contains traces of light hydrocarbons flashes at a lower temperature than a sample without these traces
162 /14162 /1404/07/2023 162
A side draw product carries flash point specifications of 125-130° F
The lab test shows a flash point of 110° FThe sample contains material that is too light
We can bring the product back to specification by
decreasing the reflux rate, orusing more stripping steam, or increasing the reboiler
temperature
163 /14163 /1404/07/2023 163
API GravityIs used to designate the "heaviness"
or "lightness" of products
Kerosene is measured at about 42° API
Gasoline is measured at about 60° API
The lighter the oil, the higher API gravity
164 /14164 /1404/07/2023 164
Suppose specifications call for a product with an API gravity of 30-35°
The product sample tests 28°
The product is too heavy
165 /14165 /1404/07/2023 165
Color
Light hydrocarbons are light colored while Heavy hydrocarbons are dark in color
A light hydrocarbon product that is dark colored probably contains too many heavy molecules
Excessive vapor rates can cause small drops of liquid to become entrained in the vapor and be carried up the tower
166 /14166 /1404/07/2023 166
Entrainment of heavy materials may contaminate the overhead product and make it too dark in color
Hydrocarbons will decompose and change color at very high temperatures
So, an off-color product may indicate that a tower is operating at too high a temperature
167 /14167 /1404/07/2023 167
4. CRUDE DISTILLATION OPERATION
168 /14168 /14168
Reflux Rate Changing
Suppose the reflux rate is increased from 1,000 to 1,200 barrels per hour, and the other tower operating conditions are held constant which of the following will occur
A. Lighter overhead, bottom, and side draw products.
B. Heavier overhead and side draw products.C. Heavier bottom product.D. More overhead product.
QUESTION (1)
169 /14169 /14
This extra reflux flowing down the tower causes the temperature on each tray to decrease
170 /14170 /1404/07/2023 170
Some of the heavier hydrocarbons in the upward
flowing vapors will now condense and fall back
down the tower
The extra reflux flowing down the tower reduces the
temperature of the liquid at the bottom of the
column. When the bottom temperature decreases,
the amount of light material vaporized out of the
liquid at the bottom of the tower is decreased
Because fewer vapors are now going overhead, the
amount of top product formed is decreased, or less.
Lighter overhead, bottom, and side draw products
are produced by increasing the reflux rate
171 /14171 /1404/07/2023 171
If we decrease the reflux rate from 1,000 barrels to 800 barrels,
QUESTION (2)
172 /14172 /14
The cut point changes are reversed. The temperature on each of the trays increases, and a higher tower temperature mean heavier products. So overhead, bottom, and side draw products become heavier. The amount of overhead product produced increases and the amount of bottom product formed decreases
173 /14173 /14173
Flooding
Occurs when the pressure drop across a tray is so high that the liquid cannot flow down the tower as fast as required.
The pressure drop across the tray increases to very high values, and the tray efficiency drops markedly.
When the froth and foam in the down comer back up to the tray above and begin accumulating on this tray.
174 /14174 /14
Entrainment Flooding - For a constant liquid rate, increasing
the gas rate results eventually in excessive entrainment and flooding.
At the flood point it is difficult to obtain net downward flow of liquid,
and any liquid fed to the column is carried out with the overhead gas.
Furthermore, the column inventory of liquid increases, pressure drop
across the column becomes quite large, and control becomes difficult.
Downflow Flooding - Flooding may also be brought on by
increasing the liquid rate while holding the gas rate constant. Excessive
liquid flow can overtax the capacity of downcomers or other passages,
with the ultimate result of increased liquid inventory, increased
pressure drop, and the other characteristics of a flooded column.
175 /14175 /14
Flooding is brought about by excessive vapour flow, causing liquid to be entrained in the vapour up the column. The increased pressure from excessive vapour also backs up the liquid in the downcomer, causing an increase in liquid holdup on the plate above. Depending on the degree of flooding, the maximum capacity of the column may be severely reduced. Flooding is detected by sharp increases in column differential pressure and significant decrease in separation efficiency.
176 /14176 /14176
Weeping/Dumping
This phenomenon is caused by low vapour flow. The
pressure exerted by the vapour is insufficient to hold up the
liquid on the tray. Therefore, liquid starts to leak through
perforations. Excessive weeping will lead to dumping. That
is the liquid on all trays will crash (dump) through to the
base of the column (via a domino effect) and the column
will have to be re-started. Weeping is indicated by a sharp
pressure drop in the column and reduced separation
efficiency.
177 /14177 /14177
Weeping/Dumping
Occurs at: High liquid rates, and Low vapor loads.
Some of slots or holes will dump liquid instead of passing vapor, resulting in poor tray efficiency.
For towers with conventional down comers, dumping usually occurs at the upstream raw of caps or holes, where the liquid has the largest head and kinetic energy.
178 /14178 /14178
Foaming
Foaming refers to the expansion of liquid due to passage of
vapour or gas. Although it provides high interfacial liquid-
vapour contact, excessive foaming often leads to liquid
buildup on trays. In some cases, foaming may be so bad
that the foam mixes with liquid on the tray above. Whether
foaming will occur depends primarily on physical
properties of the liquid mixtures, but is sometimes due to
tray designs and condition. Whatever the cause, separation
efficiency is always reduced.
179 /14179 /14179
Coning
Occurs at low liquid rate or seals.
The vapor pushes the liquid back from the slots or holes and passes upward with poor liquid contact.
This causes poor tray efficiency.
180 /14180 /14180
Puking
Usually occurs at: High liquid rate and Low gas rate.
At high liquid rate the liquid level on each tray will rise.
As the level rises, the flow of gas up the tower is restricted.
The gas pressure in the bottom of the tower will begin to rise.
It will reach the point that a surge of a gas will suddenly move up the tower with enough velocity to carry the liquid with it.
181 /14181 /14181
Reducing the liquid flow rate will usually eliminate puking.
Puking should not be confused with carryover.
Puking occurs almost instantaneously.
Furthermore, if the liquid rate is not reduced, the tower will puke again when the liquid stacks up.
Carryover is usually caused by a high vapor flow rate (It happens continuously, whereas puking is an intermittent thing).
182 /14182 /14182
Operating Range
Satisfactory operation will only be achieved over a limited range of vapour and liquid flow rates.
183 /14183 /14183
The upper limit to vapour flow is set by the condition of flooding.
At flooding there is:
A sharp drop in plate efficiency and
Increase in pressure drop.
Flooding is caused by either:
Excessive carry over of liquid to the next plate by entrainment, or Liquid backing-up in the down comers.
184 /14184 /14184
The lower limit of the vapour flow is set by condition of weeping.
Weeping occurs when the vapour flow is insufficient to maintain a level of liquid on the plate.
"Coning" occurs at low liquid rates, and is the term given to the condition where the vapour pushes the liquid back from the holes and jets upward, with poor liquid contact.
185 /14185 /14
Thank You
