Take Home Exam (1)

ROSON, Carla Marie L. Take Home Exam-Equipment

Design

BSChE-V Engr. Fritzie Baldovino

I. EQUIPMENT DESIGN

A. DISTILLATION COLUMN

DISTILLATION

Distillation is the process of purification of compounds based on their

volatility. It is a physical method of assorting mixtures depending upon the

difference in the boiling point of the component substances. The working principle

of distillation is to heat a mixture at a specific temperature, collect the hot vapors

and condense to separate the component substance. In simpler terms, a highly

volatile compound is separated from a less-volatile or non-volatile compound by

using distillation. As per evidences, the principle of distillation has been used since

ancient times. It is believed that the ancient Arab chemists applied distillation for

the first time to separate perfumes. Today, it is one of the most popular technique

implemented for purification and separation of a mixture.

Different types of distillation are simple, fractional, steam, vacuum and short

path distillation.

Types of Distillation

There are several types of distillation depending on the procedure and the

instrument setup. Each of the distillation type is used for the purification of

compounds having different properties. Following are the common types of

distillation:

1. Simple Distillation

Simple distillation is practiced for a mixture in which the boiling point of the

components differ by at least 70°C. It is also followed for the mixtures

contaminated with nonvolatile particles (solid or oil) and those that are nearly

pure with less than 10 percent contamination. Double distillation is the process

of repeating distillation on the collected liquid in order to enhance the purity of

the separated compounds.

2. Fractional Distillation

Those mixtures, in which the volatility of the components is nearly similar or

differs by 25°C (at 1 atmosphere pressure), cannot be separated by simple

distillation. In such cases, fractional distillation is used whereby the constituents are

separated by a fractionating column. In the fractionating column, the plates are

arranged and the compound with the least boiling point are collected at the top

while those with higher boiling point are present at the bottom. A series of

compounds are separated simultaneously one after another. Fractional distillation is

used for the alcohol purification and gasoline purification in petroleum refining

industries.

3. Steam Distillation

Steam distillation is used for the purification of mixtures, in which the

components are temperature or heat sensitive; for example, organic compounds. In

the instrument setup, steam is introduced by heating water, which allows the

compounds to boil at a lower temperature. This way, the temperature sensitive

compounds are separated before decomposition. The vapors are collected and

condensed in the same way as other distillation types. The resultant liquid consists

of two phases, water and compound, which is then purified by using simple

distillation. Steam distillation is practiced for the large-scale separation of essential

oils and perfumes.

4. Vacuum Distillation

Vacuum distillation is a special method of separating compounds at pressure

lower than the standard atmospheric pressure. Under this condition, the compounds

boil below their normal boiling temperature. Hence, vacuum distillation is best

suited for separation of compounds with higher boiling points (more than 200°C),

which tend to decompose at their boiling temperature. Vacuum distillation can be

conducted without heating the mixture, as usually followed in other distillation

types. For the separation of some aromatic compounds, vacuum distillation is used

along with steam distillation.

5. Short Path Distillation

Thermal sensitive compounds can also be separated by following short path

distillation. In this technique, the separated compounds are condensed immediately

without traveling the condenser. The condenser is configured in a vertical manner

between the heating flask and the collecting flask. Similar to vacuum type, the

pressure is maintained below the atmospheric pressure. Short path distillation is

used for the separation of organic compounds with high molecular weight,

especially in the pharmaceutical industries.

Selection of Distillation Equipment

Separation of components from a liquid mixture via distillation depends on

the differences in boiling points of the individual components. Also, depending on

the concentrations of the components present, the liquid mixture will have different

boiling point characteristics. Therefore, distillation processes depends on the vapour

pressure characteristics of liquid mixtures.

Vapor Pressure and Boiling

The vapor pressure of a liquid at a particular temperature is the equilibrium

pressure exerted by molecules leaving and entering the liquid surface. Here are

some important points regarding vapor pressure:

energy input raises vapor pressure

vapor pressure is related to boiling

a liquid is said to ‘boil’ when its vapor pressure equals the surrounding

pressure

the ease with which a liquid boils depends on its volatility

liquids with high vapor pressures (volatile liquids) will boil at lower

temperatures

the vapor pressure and hence the boiling point of a liquid mixture depends on

the relative amounts of the components in the mixture

distillation occurs because of the differences in the volatility of the

components in the liquid mixture

Trays and Plates

The terms "trays" and "plates" are used interchangeably. There are many types

of tray designs, but the most common ones are :

Bubble cap trays

A bubble cap tray has riser or chimney fitted over each hole, and a cap that

covers the riser. The cap is mounted so that there is a space between riser and

cap to allow the passage of vapour. Vapour rises through the chimney and is

directed downward by the cap, finally discharging through slots in the cap, and

finally bubbling through the liquid on the tray.

Valve trays

In valve 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 than is possible in sieve trays.

Sieve trays

Sieve trays are simply metal plates with holes in them. Vapour passes straight

upward through the liquid on the plate. The arrangement, number and size of

the holes are design parameters.

Because of their efficiency, wide operating range, ease of maintenance and cost

factors, sieve and valve trays have replaced the once highly thought of bubble

cap trays in many applications.

Liquid and Vapour Flows in a Tray Column

Each tray has 2 conduits, one on each side, called ‘downcomers’. Liquid falls

through the downcomers by gravity from one tray to the one below it.The flow

across each plate is shown in the above diagram on the right.

A weir on the tray ensures that there is always some liquid (holdup) on the tray and

is designed such that the the holdup is at a suitable height, e.g. such that the

bubble caps are covered by liquid.

Being lighter, vapour flows up the column and is forced to pass through the

liquid, via the openings on each tray. The area allowed for the passage of vapour on

each tray is called theactive tray area.

The tops of the 4 bubble caps on the tray can just be seen. The downcomer in this

case is a pipe, and is shown on the right. The frothing of the liquid on the active tray

area is due to both passage of vapour from the tray below as well as boiling.

As the hotter vapour passes through the liquid on the tray above, it transfers

heat to the liquid. In doing so, some of the vapour condenses adding to the liquid on

the tray. The condensate, however, is richer in the less volatile components than is

in the vapour. Additionally, because of the heat input from the vapour, the liquid on

the tray boils, generating more vapour. This vapour, which moves up to the next

tray in the column, is richer in the more volatile components. This continuous

contacting between vapour and liquid occurs on each tray in the column and brings

about the separation between low boiling point components and those with higher

boiling points.

General Design Considerations

As mentioned, distillation columns are designed using VLE data for the

mixtures to be separated. The vapour-liquid equilibrium characteristics (indicated

by the shape of the equilibrium curve) of the mixture will determine the number of

stages, and hence the number of trays, required for the separation. This is

illustrated clearly by applying theMcCabe-Thiele method to design a binary column.

McCABE-THIELE DESIGN METHOD

The McCabe-Thiele approach is a graphical one, and uses the VLE plot to

determine the theoretical number of stages required to effect the separation of a

binary mixture. It assumes constant molar overflow and this implies that:

o molal heats of vaporization of the components are roughly the same

o heat effects (heats of solution, heat losses to and from column, etc.)

are negligible

o for every mole of vapor condensed, 1 mole of liquid is vaporised

The design procedure is simple. Given the VLE diagram of the binary mixture,

operating lines are drawn first.

Operating lines define the mass balance relationships between the liquid and

vapor phases in the column.

There is one operating line for the bottom (stripping) section of the column,

and on for the top (rectification or enriching) section of the column.

Use of the constant molar overflow assumption also ensures the the

operating lines are straight lines.

Operating Line for the Rectification Section

The operating line for the rectification section is constructed as follows. First

the desired top product composition is located on the VLE diagram, and a vertical

line produced until it intersects the diagonal line that splits the VLE plot in half. A

line with slope R/(R+1) is then drawn from this instersection point as shown in the

diagram below.

R is the ratio of reflux flow (L) to distillate flow (D) and is called the reflux ratioand is

a measure of how much of the material going up the top of the column is returned

back to the column as reflux.

Operating Line for the Stripping Section

The operating line for the stripping section is constructed in a similar manner.

However, the starting point is the desired bottom product composition. A vertical

line is drawn from this point to the diagonal line, and a line of slope Ls/Vs is drawn

as illustrated in the diagram below.

Ls is the liquid rate down the stripping section of the column, while Vs is the vapour

rate up the stripping section of the column. Thus the slope of the operating line for

the stripping section is a ratio between the liquid and vapour flows in that part of

the column.

Equilibrium and Operating Lines

The McCabe-Thiele method assumes that the liquid on a tray and the vapour

above it are in equilibrium. How this is related to the VLE plot and the operating

lines is depicted graphically in the diagram on the right.

A magnified section of the operating line for the stripping section is shown in

relation to the corresponding n'th stage in the column. L's are the liquid flows while

V's are the vapour flows. x and y denote liquid and vapour compositions and the

subscripts denote the origin of the flows or compositions. That is 'n-1' will mean

from the stage below stage 'n' while 'n+1' will mean from the stage above stage 'n'.

The liquid in stage 'n' and the vapour above it are in equilibrium, therefore, xn and

yn lie on the equilibrium line. Since the vapour is carried to the tray above without

changing composition, this is depicted as a horizontal line on the VLE plot. Its

intersection with the operating line will give the composition of the liquid on tray

'n+1' as the operating line defines the material balance on the trays. The

composition of the vapour above the 'n+1' tray is obtained from the intersection of

the vertical line from this point to the equilibrium line.

Number of Stages and Trays

Doing the graphical construction repeatedly will give rise to a number of

'corner' sections, and each section will be equivalent to a stage of the distillation.

This is the basis of sizing distillation columns using the McCabe-Thiele graphical

design methodology as shown in the following example.

Given the operating lines for both stripping and

rectification sections, the graphical construction

described above was applied. This particular example

shows that 7 theoreticalstages are required to achieve

the desired separation. The required number of trays (as

opposed to stages) is one less than the number of

stages since the graphical construction includes the

contribution of the reboiler in carrying out the separation.

The actual number of trays required is given by the

formula:

(number of theoretical trays)/(tray efficiency)

Typical values for tray efficiency ranges from 0.5 to 0.7 and depends on a number

of factors, such as the type of trays being used, and internal liquid and vapour flow

conditions. Sometimes, additional trays are added (up to 10%) to accomodate the

possibility that the column may be under-designed.

Using Operating Lines and the Feed Line in McCabe-Thiele Design

If we have information about the condition of the feed mixture, then we can

construct the q-line and use it in the McCabe-Thiele design. However, excluding the

equilibrium line, only two other pairs of lines can be used in the McCabe-Thiele

procedure. These are:

• feed-line and rectification section operating line

• feed-line and stripping section operating line

• stripping and rectification operating lines

This is because these pairs of lines determine the third.

[see Flash tutorial on Distillation Basics written by Jon Lee]

Overall Column Design

Determining the number of stages required for the desired degree of

separation and the location of the feed tray is merely the first steps in producing an

overall distillation column design. Other things that need to be considered are tray

spacings; column diameter; internal configurations; heating and cooling duties. All

of these can lead to conflicting design parameters. Thus, distillation column design

is often an iterative procedure. If the conflicts are not resolved at the design stage,

then the column will not perform well in practice. The next set of notes will discuss

the factors that can affect distillation column performance.

EFFECTS OF THE NUMBER OF TRAYS OR STAGES

Here we will expand on the design of columns by looking briefly at the effects of the

number of trays, and the position of the feed tray, and on the performances of

distillation columns.

Tray Designs

A tray essentially acts as a mini-column, each accomplishing a fraction of the

separation task. From this we can deduce that the more trays there are, the better

the degree of separation and that overall separation efficiency will depend

significantly on the design of the tray. Trays are designed to maximisevapour-liquid

contact by considering the liquid distribution and vapour distribution on the tray.

This is because better vapour-liquid contact means better separation at each tray,

translating to better column performance. Less trays will be required to achieve the

same degree of separation. Attendant benefits include less energy usage and lower

construction costs.

Column Reboilers

There are a number of designs of reboilers. It is beyond the scope of this

set of introductory notes to delve into their design principles. However,

they can be regarded as heat-exchangers that are required to transfer

enough energy to bring the liquid at the bottom of the column to boiling

boint. The following are examples of typical reboiler types.

SAMPLE DESIGN COMPUTATIONBINARY FRACTIONATING COLUMN

A fractionating column or fractionation column is an essential item used in

the distillation of liquid mixtures so as to separate the mixture into its component

parts, or fractions, based on the differences in their volatilities. Fractionating

columns are used in small scale laboratory distillations as well as for large-scale

industrial distillations.

Fractional distillation is one of the unit operations of chemical engineering.

Fractionating columns are widely used in the chemical process industries where

large quantities of liquids have to be distilled. Such industries are the petroleum

processing, petrochemical production,natural gas processing, coal tar processing,

brewing, liquified air separation, and hydrocarbon solvents production and similar

industriesб but it finds its widest application in petroleum refineries. In such

refineries, the crude oil feedstock is a complex, multicomponent mixture that must

be separated, and yields of pure chemical compounds are not expected, only

groups of compounds within a relatively small range of boiling points, also called

fractions. That is the origin of the name fractional distillation or fractionation. It is

often not worthwhile separating the components in these fractions any further

based on product requirements and economics.

Distillation is one of the most common and energy-intensive separation

processes. In a typical chemical plant, it accounts for about 40% of the total energy

consumption. Industrial distillation is typically performed in large, vertical cylindrical

columns known as "distillation towers" or "distillation columns" with diameters

ranging from about 65 centimeters to 6 meters and heights ranging from about 6

meters to 60 meters or more.

Industrial distillation towers are usually operated at a continuous steady

state. Unless disturbed by changes in feed, heat, ambient temperature, or

condensing, the amount of feed being added normally equals the amount of product

being removed.

It should also be noted that the amount of heat entering the column from the

reboiler and with the feed must equal the amount heat removed by the overhead

condenser and with the products. The heat entering a distillation column is a crucial

operating parameter, addition of excess or insufficient heat to the column can lead

to foaming, weeping, entrainment, or flooding.

Industrial fractionating columns use external reflux to achieve better

separation of products. Reflux refers to the portion of the condensed overhead

liquid product that returns to the upper part of the fractionating column.

Inside the column, the downflowing reflux liquid provides cooling and condensation

of upflowing vapors thereby increasing the efficacy of the distillation tower. The

more reflux and/or more trays provided, the better is the tower's separation of

lower boiling materials from higher boiling materials.

The design and operation of a fractionating column depends on the

composition of the feed and as well as the composition of the desired products.

Given a simple, binary component feed, analytical methods such as the McCabe-

Thiele method or the Fenske equation can be used. For a multi-component feed,

simulation models are used both for design, operation, and construction.

Bubble-cap "trays" or "plates" are one of the types of physical devices, which

are used to provide good contact between the upflowing vapor and the downflowing

liquid inside an industrial fractionating column. Such trays are shown in Figures 4

and 5.

The efficiency of a tray or plate is typically lower than that of a theoretical

100% efficient equilibrium stage. Hence, a fractionating column almost always

needs more actual, physical plates than the required number of theoretical vapor-

liquid equilibrium stages.

In industrial uses, sometimes a packing material is used in the column

instead of trays, especially when low pressure drops across the column are

required, as when operating under vacuum. This packing material can either be

random dumped packing (1–3" wide) such as Raschig rings or structured sheet

metal. Liquids tend to wet the surface of the packing, and the vapors pass across

this wetted surface, where mass transfer takes place. Differently shaped packings

have different surface areas and void space between packings. Both of these

factors affect packing performance.

DESIGN DESCRIPTION

A binary distillation column is a type of distillation column in which the feed

contains only two components. It is a type of continuous column. In this column,

sieve trays are used to hold up the liquid to provide better contact between vapor

and liquid, hence, promoting better separation of the desired products.

DESIGN SELECTION

A binary distillation column using sieve trays will be used to separate the

mixture of benzene and toluene.

DESIGN DESCRIPTION

This equipment is a pressure-type filter consists of plates and frames

assembled alternatively with a filter cloth over each side of the plates. It is widely

used for batch processes and is very effective in separating coarse to fine materials

from mother liquor. They are simple to operate, very versatile and flexible in

operation.

DESIGN SELECTION

Plate-and-frame filter press was selected among the other types because the

process could be used whether the filtrate or the cake is the product.it is also the

cheapest and has low floor space.

DATA AND ASSUMPTIONS

1. The rectification column is to be fed with 100 kmol/h of 50 mol% benzene and 50

mol% toluene at 101.325 kPa abs.

2. The feed is liquid at its boiling point.

3. The distillate is to contain 90 mol% benzene and the bottoms 10 mol% benzene.

4. MW benzene=78.114kgmol

5. MW toluene=78.114kgmol

6. Consider a reflux ratio of 4.52:1

7. Take reboiler as equivalent to one stage.

8. Assume 100 mm water, pressure drop per plate (Towler, Principles, Practice and

Economics of Plant and Process Design)

9. Take plate spacing as 24 inches =0.6096 m.

10. Assume a weir height of 50 mm, hole diameter., hole pitch of 12.5 mm and

plate thickness equivalent to 5 mm.

DESIGN CONSIDERATIONS

1. The material of construction is carbon steel. (Plant Design and Economics for

ChE, Peters and Timmerhaus)

2. The preferred tube arrangement is triangular pitch since it provides maximum

flow of the liquid. (Unit Operations by Brown)

3. Sieve plates are to be used.

4. Assume column efficiency of 60%.

5. Use 85% flooding at maximum flow rate.

6. Use sieve trays for simple distillation systems (binary).

DESIGN REQUIREMENTS

1. Number of Theoretical Plates

2. Feed plate location

3. Actual Number of Trays

4. Column Diameter

5. Column Height

6. Plate I.D.

7. Number of Active holes

DESIGN CALCULATIONS

By McCabe-Thiele Method:

1. Number of theoretical plates = 5 trays + reboiler

2. Feed Plate = 3rd plate

Slope of the top operating line= 0.8188

Slope of the bottom operating line = 1.18

Top composition = 90 mol% benzene

Bottom composition = 10 mol% benzene

Reflux ratio = 4.52

FLOW RATES

Overall Material Balance:

F = D + W 100 kmol/h = D + W

Solute balance:

xFF = xDD + xWW ; 0.50 (100 kmol/h) = 0.90D + 0.10W

By simultaneous solving,

D = 50 kmol/h W = 50 kmol/h

For the vapor rate:

V = D (1 + R) = 50 kmol/h (1 + 4.52) = 276.0 kmol/h

For the vapor flow below feed (Vm+1) and the liquid flow below feed (Lm):

Using the slope of the bottom operating line (LmV m+1

=1.18)

Lm=1.18V m+1

And

V m+1=Lm+W

V m+1=1.18V m+1−50kmolhr

Thus,

V m+1=277.78kmolhr

Lm=1.18V m+1=1.18(277.78 kmolhr )=327.78 kmolhr

PHYSICAL PROPERTIES

Number of Actual Stages=6−10.60

=8.33≈9

ColumnPressure Drop=(100 x10−3 ) (1000 ) (9.81 ) (9 )=8829 Pa

TopPressure=101325Pa

Estimated Bottom Pressure=101325 Pa+8829 Pa=110154 Pa=110.154 kPa

Employing the use of Physical Properties Data Service

pv=0.6417kg /m3

pL=956.6552kg/m3

Surface Tension = 66 x 10-3

At 90 % top Composition, the corresponding top temperature is 82.33OC

pv=1.7113kg /m3

pL=810.98kg/m3

MW=78.114 kg /mol

Surface Tension = 76 x 10-3 N/m

3. Column Diameter

FLV bottom=1.18√ 0.6417956.6552

=0.0306

FLV top=0.8188√ 1.7113810.98=0.0376

From Figure 11.29 (Towler, Principles, Practice and Economics of Plant and Process

Design, p.720)

bottomK 1=9.0 x10−2

topK 1=8.75 x10−2

Correction for Surface Tension

BottomK1=( 6620 )0.20

(9.0 x10−2)=11.43 x10−2

TopK 1=( 7620 )0.20

(8.75 x 10−2 )=11.43 x10−2

bottomu f=11.43 x10−2√ 956.6552−0.64170.6417

=4.4116 ms

topu f=11.43 x10−2√ 810.98−1.71131.7113

=2.4856 ms

At 85% flooding

bottomu f=4.4116ms

(0.85 )=3.7499ms

topu f=2.4856ms

(0.85 )=2.1128 ms

Maximum Volumetric Flow rate:

bottom=277.78 x92.1410.6417 x3600

=11.0788 m3

s

top= 276x 78.1141.7113 x3600

=3.4995m3

s

Net Area required

bottom=11.07883.7499

=2.9544m2

top=3.49952.1128

=1.6564m2

Take the downcomer area as 12% of the total. Thus,

bottom=2.95440.88

=3.3573m2≈3.5m2

top=1.65640.88

=1.8823m2≈2.0m2

Column Diameter

bottom=√ 4 (3.3573m2 )π

=2.0675m≈2.5m

top=√ 4 (1.8823m2 )π

=1.5481m≈2.0m

Column Height

columnheig h t for trays=9 x0.6096m=5.4864m

columnheig h t for trayswit hallowance for top∧bottome=(1.15 )5.4864m=6.3094m

5. Liquid Flow Pattern

Maximumvolumetric liquid rate= 327.78 x 92.1413600 x 956.6552

=8.8x m3

s

Thus, a single pass plate can be used.

PROVISIONAL PLATE DESIGNS

Column Diameter (DC) = Plate ID = 2.5 m

Column Area (Ac) = 3.5 m2

Downcomer Area (Ad) = 0.12 x 3.5 m2 = 0.42 m2

Net Area (An) = 3.5 m2 – 0.42 m2 = 3.08 m2

Active Area (Aa) = 3.5 m2 – 2(0.42) m2 = 2.66 m2

Hole Area(Ah) = 0.1x 2.66 = 3.5 m2


Design),

lwDc

=0.76

Weir length (lW) = 0.76(2.5m) = 1.9 m

7. Number of Active Holes

Areaof a Hole=¿ π4

¿

Number of Active Holes= 0.066

1.963 x10−5=3361.36≈3362

WEEPING

Maximum Liquid Rate=327.78x 92.1413600

=8.3894 kgs

MinimumLiquid Rate ,70% turndown=0.70 x8.3894 kgs

=5.8726 kgs

Maximumhow=750¿

Minimumhow=750¿

Atminimumrate hw+how=50+16.3911=66.3911mm


Design), K2 = 30.4

h(min )=30.4−0.90 (25.4−5 )

0.6417.5=15.0296m

s

Actualminimumvapor density=0.70 (11.0788 )

0.266=29.1546 m

s

Thus, the minimum operating rate is far greater than the weep point.

PLATE PRESSURE DROP

Dry plate drop

Maximum vapor velocity through holes:

h=11.07880.266

=41.6495 ms

From Figure 11.36 (Towler Principles, Practice and Economics of Plant and Process

Design), CO=0.84

hd=51¿

For the Residual head:

hd=(12.5 x 103956.6552 )=13.0664mmliquidTotal Pressure Plate Drop

hd=84.1071+50+56.8694+13.0664=204.0429mmliquid

The total plate pressure drop is acceptable because the base pressure used is 100

mm liquid.

DOWNCOMER LIQUID BACK-UP

Downcomer pressure loss (hap)

hap=50−10=40mm

Area under apron

Aap = 1.9 x 0.040 = 0.076 m2

Since Ad=0.42m2∧¿ Aap<Ad , use equation11.92 (Towler )

hdc=166¿

Back-up in downcomer

hb=(50+56.8694 )+204.0429+3=0.3139m

0.3139m< 12

( plate spacing+weir heig ht )

0.3139m< 12

(0.6096+0.05 )m

0.3139m<0.6096m

Therefore, plate spacing is acceptable.

Checking the residence time,

t r=0.42 x0.3139 x 956.6552

8.3894=15.03 s≈15 .03

DESIGN SPECIFICATIONS

Binary Distillation ColumnNumber of required Units: 1 Materials handled: Benzene – Toluene Mixture Function: To separate benzene from the mixture. Materials of Construction: Carbon Steel DESIGNPARAMETERS Proposed Design Existing Design Mass flow rate 1.1744 -- Feed Plate 3rd plate 6th plate No. of Plates Theoretical Actual

5 9

--- 13

Column Sizing Diameter (ID) Height Area

2.067 m 6.3094 m 3.3573 m2

0.75 m 9 m ---

Plate Specifications Plate I.D. Hole diameter Hole pitch Active holes Plate spacing Plate thickness

2.5 m 5 mm 12.5 mm Δ 3362 0.6096 m 5 mm

--- 5 mm 15 mm Δ 1385 0.6 m 3 mm

Source: Bioinformatics, Design of a Sieve Plate Distillation Column

B. ABSORTION TOWER

ABSORTION TOWER

Gas absorption is a unit operation in which soluble components of a gas

mixture are dissolved in a liquid. The inverse operation, called stripping or

desorption, is employed when it is desired to transfer volatile components from a

liquid mixture into a gas. Both absorption and stripping, in common with distillation

make use of special equipment for bringing gas and liquid phases into intimate

contact. Absorption is usually carried out in vertical, cylindrical columns or towers in

which devices such as plates or packing elements are placed. The gas and liquid

normally flow counter currently, and the devices serve to provide the contacting

and development of interfacial surface through which mass transfer takes place.

The absorption process requires the following steps:

(1) Diffusion of the solute gas molecules through the host gas to the liquid boundary

layer based on a concentration gradient

(2) Solvation of the solute gas in the host liquid based on gas-liquid solubility

(3) Diffusion of the solute gas based on concentration gradient, thus depleting the

liquid boundary layer and permitting further solvation

Absorption Equipment

Absorption and stripping are conducted in tray (or plate or stage) columns,

packed columns, spray towers, bubble columns, and centrifugal contactors.

1. Tray Column

The types of trays used in absorption include: sieve tray, valve tray and

bubble-cap trays.

2. Packed Column

Both random and structured packings had been used.

3. Spray Column

The gas flows upward continuously through an open chamber in which

scrubbing liquid droplets falls from spray nozzles through the gas. The gas pressure

drop is small, but separation is not as good as the bubble column. This column is

widely used for its simplicity, low pressure drop, and resistance to scale deposition

and plugging.

4. Bubble Column

The gas is forced under pressure through perforated pipes submerged in the

scrubbing liquid. As such the gas phase is dispersed and the liquid phase is

continuous. As the bubbles rise through the liquid, absorption of the gas occurs.

This type of device suffers from the high pressure drop due to the liquid hydrostatic

head.

Comparison of Absorption Equipment

Packed Towers

Packed towers or column is a hollow tube, pipe, or other vessel that is filled

with a packing material. The packing can be randomly filled with small objects like

Raschig rings or else it can be a specifically designed structured packing.

The purpose of a packed column is typically to improve contact between two

phases in a chemical or similar process. Packed beds can be used in a chemical

reactor, a distillation process, or a scrubber, but packed beds have also been used

to store heat in chemical plants. In this case, hot gases are allowed to escape

through a vessel that is packed with a refractory material until the packing is hot.

Air or other cool gas is then fed back to the plant through the hot bed, thereby pre-

heating the air or gas feed.

Steps in Designing A Packed Column

Step 1: Selecting a Type and Size of Column Packing

Below are charts showing both English and Metric unit packing factors.

The most common random packing types are shown here:

Step 2: Determine the Column Diameter

Most methods for determining the size of randomly packed towers are

derived from the Sherwood correlation. A design gas rate, G, can be

determined with the help of the figure below which is based on correlation

from the Sherwood equation:

Guidelines are as follows:

Moderate to high pressure distillation = 0.4 to 0.75 in water / ft

packing

= 32 to 63 mm water / m packing

Vacuum Distillation = 0.1 to 0.2 in water / ft packing


Absorbers and Strippers = 0.2 to 0.6 in water / ft packing


Step 3: Determine the Column Height

Perhaps the most interesting step in designing a packed column is

deciding how tall to build it. You should first ask yourself "What stage of the

design are we currently working on?" If the design is preliminary, the general

HETP (Height Equivalent to a Theoretical Plate) will work well. If the design

requires a higher degree of accuracy, please consulting

the column packings manufacturer or a book entitled Distillation Design by

Henry Kister (McGraw-Hill, ISBN 0-07-034909-6). Distillation Design contains

an exhaustive list of HETP values based on the components of the system

and the type of packing used. As for preliminary estimates, the following

HETP values should be used:

SETUP HETP expressed as ft (meters)Method Packing Size

(in)Distillation 1.0 1.5 (0.46)

1.5 2.2 (0.67)2.0 3.0 (0.91)

Vacuum Distillation 1.0 2.0 (0.67)1.5 2.7 (0.82)2.0 3.5 (1.06)

Absorption/Stripping

All Sizes 6.0 (1.83)

SAMPLE DESIGN COMPUTATION

Gas Absorption Packed Column

Design Description

Packed towers or column is a hollow tube, pipe, or other vessel that is filled with a

packing material. The packing can be randomly filled with small objects like Raschig

rings or else it can be a specifically designed structured packing.The purpose of a

packed column is typically to improve contact between two phases in a chemical or

similar process.

Design Data and Assumption

1. Liquid water stream flowrate is 8000 kg/h

2. Chlorine gas stream flowrate is 10000 kg/h

3. The inlet gas contains 2.6 mole chlorine

4. The outlet gas contains 0.5 mole chlorine

Design Considerations

1. At 25°C: the viscosity of water is 9.638 centistokes; density: 1000

kg/m3(Geankoplis, pp. 960)

2. At 25°C: air Density is 2 kg/m3 (Geankoplis, pp. 961)

3. Use 15% safety Factor for gas absorption column (Perry’s Chemical

Engineering Handbook, 14-20)

4. HETP Values for Absorption Column tower is 6 ft (1.83 m) (Distillation Design

by Kister, Henry)

5. Use Pall Plastic Rings for packing

Design Requirements

1. Number of Theoretical Stages

2. Type and size of packings

3. Column diameter

4. Specific gas rate

5. Height of Column

Computation

L out x out + G out y out = L in x in + G in y in

(8000) x out + (10000)(0.005) = (8000)(0)+(10000)(0.026)

x out = 0.02625

The equilibrium and operating lines are constructed as follows: Just as in the

McCabe-Thiele analysis of distillation, the equilibrium stages are stepped off

between the two lines. Note that for stripping, the operating line would be on the

other side of the equilibrium line.

Once the theoretical numbers of stages have been determined, you can proceed

with the design of the column by following the three steps that we'll outline below.

Number of Theoretical Stages : 3 Stages

Generally, the column diameter

to column packing size ratio should be greater

than 30 for Raschig rings, 15 for ceramic

saddles, and 10 for rings or plastic saddles. The

geometry of your column packing will typically be a function of the needed surface

area and/or allowable pressure drop. If several column packings meet your

8000 kg/h

8000 kg/h

10000 kg/h

10000 kg/h

requirements, you'll typically choose the least expensive so long as it has an

acceptable operating life. For our example, we'll choose Pall rings (plastic). For

columns over 24 inches in diameter, No. 2 or 2 inch packing should be examined

first. By looking at our flowrates, the chances of our column having a diameter of at

least 24 inches are good, but we'll verify this later. For now, we'll settle on 2 inch

plastic Pall rings for our initial analysis.

Most methods for determining the size of randomly packed towers are

derived from the Sherwood correlation. A design gas rate, G, can be determined

with the help of the figure below which is based on correlation from the Sherwood

equation:

Each line on the graph is marked with an acceptable pressure drop in inches of

water per foot of packing (numbers in parentheses are in mm of water per meter of

packing). Guidelines are as follows:

Moderate to high pressure distillation = 0.4 to 0.75 in water / ft

packing


Vacuum Distillation = 0.1 to 0.2 in water / ft packing


Absorbers and Strippers = 0.2 to 0.6 in water / ft packing


These guidelines are designed around "flooding pressure drops" documented

in literature. In other words, for most cases, designing with these pressure drops

should help you avoid flooding. In the later stages of design, you may want to

perform a thorough flooding calculation. Perry's Chemical Engineers'

Handbook covers this topic well. Since we are designing an absorber, we will design

for 42 mm water / m packing (you could design for a lower pressure drop, but the

column will increase in diameter and most likely cost). First, we'll evaluate the x-

axis of the graph above:

(L/V)(vapor density/liquid density)0.5 = (8000/10000)(1.7/1000)0.5 = 0.033

Note that 1.7 kg/m3 was used for the vapor density. The average vapor

density was given as 2 kg/m3. However, at the top of the column, the vapor will be

less dense and at it's highest velocity. This is what you should design for. As a rule

of thumb, I reduce the average vapor density by about 15% for design, however if

you can get real data from a similar tower, certainly do so! Reading the

intersection of the 42 mm water/m packing line and 0.03 on the axis, we find a

value of 1.7 for the y-axis.

From the previous charts, we read a column packing factor of 24 for 2 inch

plastic Pall rings. All other information is know so we can solve for G as shown on

the y-axis of the graph:

G = [1.7 [(1.7)(1000-1.7)]/[(10.764)(24)(9.638)0.1]]0.5 = 2.9839 kg/m2 s

Now, we solve for the column cross sectional area:

Ax = Vapor Flow / G = 8000 kg/h / [(2.9839 kg/m2 s)(3600 s/hour)] = 0.7447 m2

And the column diameter is calculated by:

Diameter = [Ax / (PI/4)]0.5 = [0.7447/(PI/4)]0.5 = 0.9737 m or 3.1947 ft

Using bigger Plastic Pall rings (3.5 in.) with Packing factor of 16,

G = [1.7 [(1.7)(1000 - 1.7)]/[(10.764)(16)(9.638)0.1]]0.5 = 3.6545 kg/m2 s

Ax = Vapor Flow / G = 8000 kg/h / [(3.6545 kg/m2 s)(3600 s/hour)] = 0.6081 m2

And the column diameter is calculated by:

Diameter = [Ax / (PI/4)]0.5 = [0.6081/(PI/4)]0.5 = 0.8799 m or 2. 8868 ft

It would be practical to use bigger Pall rings in this design in order to obtain a

smaller column Diameter to lessen the cost of construction.

Perhaps the most interesting step in designing a packed column is deciding

how tall to build it. You should first ask yourself "What stage of the design are we

currently working on?" If the design is preliminary, the general HETP (Height

Equivalent to a Theoretical Plate) will work well. If the design requires a higher

degree of accuracy, please consulting the column packings manufacturer or a book

entitled Distillation Design by Henry Kister (McGraw-Hill, ISBN 0-07-034909-

6). Distillation Design contains an exhaustive list of HETP values based on the

components of the system and the type of packing used (Chapters 10 and 11). As

for preliminary estimates, the following HETP values should be used:

SETUP HETP expressed as ft (meters)Method Packing

Size (in)Distillation 1.0 1.5 (0.46)

1.5 2.2 (0.67)2.0 3.0 (0.91)

Vacuum Distillation

1.0 2.0 (0.67)1.5 2.7 (0.82)2.0 3.5 (1.06)

Absorption/Stripping

All Sizes 6.0 (1.83)

To determine the height of the absorption tower in our example, we multiple

the 3 theoretical stages by 6 ft or 1.83 m. We estimate the height of the tower to

be 18 ft or about 5.49 meters.

Design Specifications

Packed TowerIdentification

Item: Packed TowerNumber of required Units: 1Materials handled: Air Stream with chlorine gasFunction: To absorb the chlorine Gas in the air Stream using water as the solventMaterials of Construction: Stainless Steel Type 405

Design DataPARAMETERS Proposed Design Existing DesignColumn Diameter: 0.88 m 0.6-1.2 mColumn Height 5.49 m 5-10 mTheoretical Stages 3 ---------------------Packing Plastic Pall rings 3.5 in Plastic Pall rings (2,

3, 3.5 in)

Design Configuration

II. SELECTING A PUMP

In selecting a pump, there are four things to consider. First is the total head

pressure against which it must operate. Second, the desire flow rate. Third, the

suction lift; and lastly, the characteristic of the fluid.

The total head, suction lift and flow rate are dependent upon the piping

system and the pump’s characteristics. The piping system and the pump interact to

determine the operating point of the pump flow rate and pressure. The pump

cannot independently control these parameters. As flow rate increased the work to

move each unit of water or total dynamic head the pump must produce increases. A

pump will typically have reduced the capacity as the pressure or head it is pumping

against increases. In order to obtain a pumping system that will meet the

requirements in efficient manner, the pump must be match with the piping system

and the required flow rate.

A cost analysis of pumping will consider initial cost of capital investment,

annual fixed cost and operating cost. All three cost are somewhat dependent on

each other. The type of pumping equipment, size of pipelines, size of pumps and

type of water supply affect not only the initial cost but also the fixed cost as well as

the operating cost . for example, piping system using large pipes may cost more but

could allow the use of smaller horsepower pumps which cost less, requires smaller

power sources and cost less to operate than a piping system with small diameter

pipe.

The lowest priced system is not always the best especially if the lower price

means the less efficient pumps. In getting the most efficient pump, an analysis

should be made all pumping requirements. The key points to consider are the

following:

Net positive suction head (amount of energy in the water at the

pump’s inlet).

Priming

Flexibility

Corrosion

Useful life

Maintenance

Quality pumped

Pumping head

Power source

Economics

Types of Pumps

Some types of pumps and their uses.

Centrifugal Pump

There are two basic types of centrifugal pumps, the horizontal and the

vertical pump. As the name implies, centrifugal pumps use centrifugal force to

move water from one point to another and to overcome resistance to its flow. In its

simplest form, this pump consists of an impeller fixed on a rotating shaft within a

volute-type (spiral) casing.

Jet Pump

A jet pump is often used for every low capacity requirements (5 to 20 gpm),

such as home water system this pump consist of a small centrifugal pump located

at ground level connected to a jet installed below the water level in the well.

Axial Flow Propeller Pump

Axial flow propeller pumps are designed to operate efficiently for

aquacultural, irrigation or drainage pumping at low head and high volume (more

than 500 gpm). Their efficiency is high especially when the total head is in the

range of 8 to 20 feet.

Deep Well Vertical Turbine Pump

Vertical Centrifugal pump, commonly refer as deep well vertical turbine

pump, is designed to be installed in a well.

Choice of Impellers

The impeller of the pump has a wear ring that must match with a similar

wearing surface in the bowl of the pump. It is necessary to maintain the proper

clearance between these two surfaces and to allow for the starch of the drive shaft

within the pump. Periodic adjustment of the impeller clearance is essential for high

efficiency operation.

Basic Consideration

Surface sources of water usually require much less lift than pumping from

wells. Two common types of pumps designed primarily for low lift operations are the

propeller axial flow pump and the horizontal centrifugal pump.

Pump Efficiency

All segments of the economy, including aquaculture and agriculture, must

make the most efficient use of available energy sources. Selecting a correct

pumping plant not only will conserve valuable energy supplies but also will reduce

total annual pumping cost. Inefficient pumping plants can increase cost

dramatically.

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