SUBJECT MASS TRANSFER AND EQUIPMENTS REPORT ABSORPTION

Calculation of column diameter for packed columns this is usually based on flooding conditions, and for plate columns is based on the optimumgas velocity or the liquid handling capacity

HO CHI MINH CITY UNIVERSITY OF TECHNOLOGY

AND EDUCATION

FACULTY OF CHEMICAL AND FOOD TECHNOLOGY

SUBJECT: MASS TRANSFER AND EQUIPMENTS

I Definition 3

II Process 4

III Equipment 6

3.1 Packed column 6

3.2 Plate columns 11

3.3 Packed vs plate tower comparison 12

IV Calculation 14

4.1 Design and performance equations - Packed columns 14

4.1.1 Liquid Rate 14

4.1.2 The column diameter 18

4.1.3 The column height 22

4.1.4 Pressure drop 25

4.2 Design and performance equations - Plate columns 25

REFERENCES 32

I Definition [1]

Many gaseous materials are used in the chemical industry, as are manyproducts obtained in gaseous form As a result, if we want to keep processinggas mixtures, we must separate them into their constituents

There are 3 methods to separate the gas mixture

 Suction separation method

 Physicochemical method

 Chemical method

This report focuses solely on the suction separation method

The suction separation method is understood as the reception of onesubstance into another through their phase separation surface We call itabsorption if we use a liquid to absorb it, and adsorption if we use a porous solid

Figure 1 The difference of adsoption and absoption

The process of absorption conventionally refers to the intimate contacting

of a mixture of gases with a liquid so that part of one or more of the constituents

of the gas will dissolve in the liquid This contact usually takes place in sometype of packed or plate column In this case, the gas that is absorbed is referred

to as the absorbent, the liquid that absorbs is referred to as the solvent, and thegas that is not absorbed is referred to as the inert gas

The absorption process is used to

 Recovery of precious components

1 Gas solubility - a high gas solubility is desired since this increases theabsorption rate and minimizes the quantity of solvent necessary;generally, a solvent of a chemical nature similar to that of the solute to beabsorbed will provide good solubility

2 Volatility - a low solvent vapor pressure is desired since the gas leaving

an absorption unit is ordinarily saturated with the solvent and much willtherefore be lost

3 Corrosiveness

4 Cost (particularly for solvents other than water)

5 Viscosity - low viscosity is preferred for reasons of rapid absorption rates,improved flooding characteristics, lower pressure drops, and good heattransfer characteristics

6 Chemical stability - the solvent should be chemically stable and, ifpossible, nonflammable

2 Equilibrium data evaluation.

3 Estimation of operating data (usually obtained from a mass and energybalance, where the energy balance determines whether the absorptionprocess can be considered isothermal or adiabatic)

4 Column selection (should the column selection not be obvious orspecified, calculations must be carried out for the different types ofcolumns, and the final selection based on economic considerations)

5 Calculation of column diameter (for packed columns this is usually based

on flooding conditions, and for plate columns is based on the optimumgas velocity or the liquid handling capacity of the plate)

6 Estimation of the column height or the number of plates (for a packedcolumn, the column height is obtained by multiplying the number oftransfer units, obtained from a knowledge of equilibrium and operatingdata by the height of a transfer unit; for plate columns, the number oftheoretical plates, often determined from the plot of equilibrium andoperating lines, is divided by the estimated overall efficiency to give thenumber of actual plates, which in turn allows the column height to beestimated from the plate spacing)

7 Determination of pressure drop through the column (for packed columns,correlations dependent on packing type, column operating data, and

physical properties of the constituents involved need to be available toestimate the pressure drop through the packing; for plate columns, thepressure drop per plate is obtained and multiplied by the number ofplates).

III Equipment [2]

The principle types of gas absorption equipment may be classified asfollows:

- Packed columns (continuous operation)

- Plate columns (stage operation)

Principle operation of packed column: The gas stream moves upwardthrough the packed bed against an absorbing or reacting liquor (solvent-scrubbing solution), which is introduced at the top of the packing This results inthe highest possible efficiency Since the solute concentration in the gas streamdecreases as it rises through the column, there is fresh solvent constantlyavailable for contact This provides the maximum average driving force for themass transfer process throughout the packed bed

Figure 2 Typical counter-current packed column

Figure 3 Absorption column

The packing is the heart of this type of equipment Its proper selectionentails an understanding of packing operational characteristics and the effect ofperformance of the points of significant physical difference between the varioustypes Some examples of packing include:

Figure 4 Some typical packings

The main point to be considered in choosing the column packing include:

- Durability and corrosion resistance (the packing should be chemicallyinert to the fluids being processed)

- Free space per unit volume of packed space (this controls the liquorholdup in the column as well as the pressure drop across it; ordinarily, thefractional void volume, or fraction of free space, in the packed bed should

be large)

- Wetted surface area per unit volume of packed space (This is veryimportant since it determines the interfacial surface between liquid andgas; it is rarely equal to the actual geometric surface since the packing isusually not completely wetted by the fluid.)

- Resistance to the flow of gas (this effects the pressure drop over thecolumn)

- Packing stability and structural strength to permit easy handling andinstallation

- Weight per unit volume of packed space

- Cost per unit area of packed space

Packed columns may also operate in a cross-flow mode (see Fig 5)where the air stream moves horizontally through the packed bed and isirrigated by the scrubbing liquid which flows vertically down through thepacking Cross-flow designs are characterized by low water consumptionand fairly high air flow capacity at a low pressure drop Where highlysoluble gases are to be recovered, the cross-flow packed scrubber hasseveral advantages over the counter-current scrubber For example, whenoperating with the same liquid and gas mass flow rates, a cross-flowscrubber has a lower pressure drop Besides reducing water consumptiondrastically, the cross-flow principle also reduces pump and fan motorsizes Other advantages include less piping, less plugging from solidsdropout at the packing support plate, and the possible use of higher gasand liquid rates because of the extremely low pressure drop On the other

hand, liquid entrainment from these systems is rather high and misteliminators are usually required downstream.

Figure 5 Cross-flow operation in a packed column

Packed columns are characterized by a number of features to which theirwidespread popularity may be attributed

- Minimum structure — the packed column usually needs only a packingsupport and liquid distributor approximately every 10 feet along its height

- Versatility — the packing material can be changed by simply discarding

it and replacing it with a type providing better efficiency

- Corrosive-fluids handling — ceramic packing is used and may bepreferable to metal or plastic because of its corrosion resistance When packingdoes deteriorate, it is quickly and easily replaced; it is also preferred whenhandling hot combustion gases

- Low pressure drop — unless operated at very high liquid rates wherethe liquid becomes the continuous phase as the flowing films thicken and merge,the pressure drop per lineal foot of packed height is relatively low

- Range of operation — although efficiency varies with gas and liquidfeed rates, the range of operation is relatively broad

- Low investment — when plastic packings are satisfactory or when thecolumns are less than about 3 or 4 feet in diameter, cost is relatively low

3.2 Plate columns

Plate columns (also commonly referred to as “tray columns”) areessentially vertical cylinders in which the liquid and gas are contacted instepwise fashion (staged operation) on plates or “traps,” as shown schematicallyfor one type in Figure 6

Principle operation: The liquid enters at the top and flows downward viagravity On the way, it flows across each plate and through a downspout to theplate below The gas passes upward through openings of one sort or another inthe plate, then bubbles through the liquid to form a froth, disengages from thefroth, and passes on to the next plate above

The overall effect is a multiple counter-current contact of gas and liquid

Figure 6 Typical bubble-cap plate column.

Each plate of the column is a stage since the fluids on the plate arebrought into intimate contact, interface diffusion occurs, and the fluids areseparated The number of theoretical plates (or stages) is dependent on thedifficulty of the separation to be carried out and is determined solely frommaterial balances and equilibrium considerations The diameter of the column,

on the other hand, depends on the quantities of liquid and gas flowing throughthe column per unit time The actual number of plates required for a givenseparation is greater than the theoretical number because of plate inefficiencies.3.3 Packed vs plate tower comparison

Of the various types of gas absorption devices, packed columns and platecolumns are the most commonly used in practice Although packed columns areused more often, both have their special areas of usefulness, and the relativeadvantages and disadvantages of each are worth considering In general:

- The pressure drop of the gas passing through the packed column issmaller

- The plate column can stand an arbitrarily low liquid feed and permits ahigher gas feed than the packed column It can also be designed to handle liquidrates that would ordinarily flood the packed column

- If the liquid deposits a sediment, the plate column is more advisable Byfitting the column with manholes, the plate column can be cleaned ofaccumulated sediment that would clog many packing materials and warrantnecessary costly removal and refilling of the column Packed columns are alsosusceptible to plugging if the gas contains particulate contaminate(s)

- In mass transfer processes accompanied by considerable heat effects,cooling or heating the liquid is much easier in the plate column A system of

pipes immersed in the liquid can be placed on the plates between the caps, andheat can be removed or supplied through the pipe wall directly to the area inwhich the process is taking place The solution of the same problem for apacked column leads to the division of this process into a number of sections,with the cooling or heating of the liquid taking place between these sections.

- The total weight of the plate column is usually less than the packedcolumn designed for the same capacity

- A well-installed plate column avoids serious channeling difficultiesinsuring good, continuous contact between the gas and liquid throughout thecolumn

- Temperature changes are apt to do more damage to the packed columnthan to the plate column

- In highly corrosive atmospheres, the packed column is simpler andcheaper to construct

- The liquid holdup in the packed column is considerably less than in theplate column

- Plate columns are advantageous for absorption processes with anaccompanying chemical reaction (particularly when it is not very rapid) Theprocess is favored by a long residence time of the liquid in the column and byeasier control of the reaction

- Packed columns are preferred for liquids with high foaming tendencies

- The relative merits of the plate column and packed column for aspecified purpose are normally determined only by comparison of the actualcost figures resulting from a detailed design analysis for each type Mostconditions being equal, packed columns in the smaller sizes (diameters up to 2

or 3 ft) are on the average less expensive In the large sizes, plate columns tend

to be the more economical

IV Calculation

4.1 Design and performance equations - Packed columns [2]

For design the packed columns, calculations generally involve thedetermination of three unknown system variables: the liquid rate, the columndiameter, the column height (and Pressure Drop)

4.1.1 Liquid Rate

Most absorption or stripping operations are performed in counter-currentflow procedures, in which the gas flow is injected at the bottom of the columnand the liquid solvent is introduced at the top[3]

Figure 7 Mole balance, counter-current flow (Figure 10.7 - Theodore & Ricci,2011[2])

The overall material balance for the counter-current absorption process is:

��

1+ �� 2 = �� 2+ �� 1

Where:

Gm1is gas rate of Feed gas (kmol/h)

Gm2is gas rate of Treated gas (kmol/h)

Lm1is liquid flow rate of Lean solution (kmol/h)

Lm2is liquid flow rate of Rich solution (kmol/h)

For component A, the mass (or mole) balance becomes:

����1+ ���

�2 = ����2+ ���

�1

Where:

yA1: mol fraction of A in gas phase (Feed gas)

yA2: mol fraction of A in gas phase (Treated gas)

xA1: mol fraction of A in liquid phase (Lean solution)

xA2: mol fraction of A in liquid phase (Rich solution)

Assuming Gm1 = Gm2 = Gm and Lm1 = Lm2 = Lm (suitable for manyapplications where solute concentrations are reasonably small)

This equation is a straight line, which is known as the operational line Itpasses through the points (xA1, yA1) and (xA2, yA2) and has a slope of Lm/Gmon x,

y coordinates, as shown in below Figure:

Figure 8 Operating and equilibrium lines (Figure 10.8 - Theodore & Ricci,2011[2])

The column operation is frequently specified as some factor of theminimum liquid–to–gas ratio For instance, (Lm/G )m act is 1.5(L /G )m m min is atypical situation frequently encountered

Below is an example for find liquid rate:

Example 4.1 (Illustrative Example 10.2 - Theodore & Ricci, 2011[2])Given the following information for a packed counter-current gasscurbber, determine the liquid rate in lbmol/h.ft 2

Gas flux = 18 lbmol/h.ft 2

The mol fractions of the solute in the inlet and outlet gas are 0.08 and0.002, respectively

The mol fractions of the solute in the inlet and outlet liquid are 0.001 and0.05, respectively.

<=> �� 0.05 − 0.001 = 18(0.08-0.002)

=> �� = 27 lbmol/h.ft2

Hence, the liquid rate for the packed counter-current gas scurbber is 27lbmol/h.ft2

4.1.2 The column diameter

Column diameter is often calculated using flooding considerations

Flooding is an undesirable condition when the gas rate is so high that itprevents the liquid from flowing freely down the column, leading to liquidaccumulating and the blockage of the entire cross section for flow Thisprevents the packing from mixing the gas and liquid effectively and increasesthe pressure drop The superficial gas velocity at which it occurs is called theflooding velocity 50–75% of the flooding rate is the usual operating rangebeing taken to calculate the column diameter

The procedure to determine the column diameter is as follows:

Step 1: Calculate the abscissa (mass basis for all terms)

L = lb/s (Liquid rate in mass basis)

G = lb/s (Gas rate in mas basis)

ρG= lb/ft3(Density of gas)

ρ =L lb/ft3(Density of liquid)

Step 2: Proceed to the flooding line and read the ordinate(design parameter):