Contents Chemical Design Selection Of Column Type Engineering Essay

There are two sorts of column, tray or packed, to take from. In this undertaking, the type of column is chosen after taking a few considerations into history ( Peters, Timmerhaus, and West 2004, 772 ) , for case, the flow rate, the diameter and force per unit area bead. The flow rate of the provender is rather big ; hence, a tray column is preferred. When the flow rate is big, the diameter of the column will increase excessively. Normally, a tray column is non smaller than 0.67 metres and random jammed column is non larger than 1.5 metres ( Peters, Timmerhaus, and West 2004, 777 ) . Therefore, tray column is preferred. As for the force per unit area bead, it would be best that it is every bit low as possible. From the book, Plant Design and Economics for Chemical Engineers, it is preferred that a structured jammed column is used to cut down the force per unit area bead ( Peters, Timmerhaus, and West 2004, 772 ) . As a decision, tray column is chosen due to the high flow rate and the big diameter of the column.

Determination of Actual Number of Stages

The figure of equilibrium phase is taken from Hysys. There is other method to find the figure of equilibrium phases such as Kremser Equation. To utilize this method, the equilibrium line must be known. The equilibrium line can be obtained utilizing the equation 1.1. The equilibrium line can non be obtained due to the Henry ‘s invariable is non gettable ; hence, the figure of equilibrium phase is decided to be taken from the Hysys simulation. From Hysys, the existent figure of equilibrium phase is 6.

Where,

YB = mole ratio of solute B in gas stage

XB = mole ratio of solute B in liquid stage

HB = Henry ‘s invariable for solute B

PT = entire force per unit area

Determine the Column Efficiency

In existent industrial pattern, it is impossible for the mass transportation through soaking up to hold 100 % efficiency. From Hysys, the existent figure of equilibrium phase is 6.25. By utilizing the equation suggested by Sinnott and Towler the efficiency of the column is calculated. Therefore, the equation is shown in equation 1.2.

The deliberate efficiency is 80.0 % .

Choice of Plate Type

Chief factors such as cost, public presentation, and operating scope are considered in choosing home base type, which are between sieve home base, bubble-cap, and valve home base. Sieve home base is chosen as it is comparatively cheaper than bubble caps and valve home base since sieve home base does non hold traveling portion. Besides, sieve home base gives lowest force per unit area bead among the three home base types. Therefore, sieve home base is selected as it is suited for most application and cheapest in cost ( Sinnott and Towler 2009, 732 ) .

Approximate Column Sizing

Choice of Tray Spacing

The normal spacing for a tray column is in between of 0.15 m to 1 m. Therefore the chosen spacing is the 0.5m spacing. The home base spacing will be the finding of the overall tallness of the column. Close spacing is merely used when the diameter of the column is little ( Sinnott and Towler 2009, 730 ) .

Determination of Column Diameter

The key to the column diameter is the vapour flow-rate. This flow-rate should non be excessively little or excessively big. If the vapor flow-rate is excessively little, it will do inordinate liquid entrainment, as for excessively big, it will do hard-hitting bead across the column. By utilizing the Souders and Brown equation ( Sinnott and Towler 2009, 730 ) , the superficial speed can be found therefore the country and diameter can be found. Below is the equation for Souders and Brown.

Where, & A ; ucirc ; v = maximal allowable vapor speed, m/s

lt = home base spacing, m

?L and ?v = denseness of liquid and vapor severally, kg/m3

Before the net vapor speed can be calculated, the liquid-vapour flow factor should be calculated as in equation 1.4, so that the Souders and Brown factor can be obtained from Figure 15-5 of Peters, Timmerhaus and West ( 2009, 778 ) .

Next the existent vapor speed need to be calculated. The existent vapor speed is calculated by presuming it is 80 % of net vapor speed. Then the net column country is calculated utilizing equation 1.5. The cross-sectional country of the column is the summing up of the net column country and downcomer country. Downcomer country is assumed that it occupies 15 % of the cross-sectional country as can be calculated utilizing equation 1.6.

Where, m’v = volumetric flow rate of the vapor, m3/s

Vn = existent vapor speed, m/s

Where, An = net column country, M2

With the known cross-sectional country of the column, Ac, the column diameter, DC can be calculated utilizing the expression below.

Where, Vw = maximal vapor rate, kg/s

The deliberate value of diameter is 1.322 m.

Determination of Column Height

With the known home base spacing, the column tallness can be calculated. The column tallness is of import to be calculated as it is needed in the mechanical design of the column. The column tallness is calculated as below based on the equation from Plant Design and Economics for Chemical Engineers ( 2009, 779 ) .

Where, N = figure of trays

Hs = home base spacing, m

?H = extra tallness required for column, m, assume dome caputs are 20 % higher than the home base spacing.

Based on Dutta ( 2009, 205 ) , it specifies the extra tallness required for the column as below:

Table. Additional Height Specification

Specification

foot

m

Excess infinite at provender tray

1.5

0.46

Excess infinite at tray manholes

1.5

0.46

Bottom infinite

9

2.74

Top infinite

4

1..22

Entire extra tallness, ?H

4.88

Using equation 1.8, the deliberate tallness is 7.5m.

Table: Summary of Column Design

Plate spacing ( m )

0.5

Diameter ( m )

1.322

Height ( m )

7.5

Internal Column Design

Deluging Fraction Calculation

In any column of a works, the flooding per centum must be known. Therefore, the flooding per centum of the absorber must be known with the operating conditions and flow-rate. The equation to utilize for the computation is shown in equation 1.9.

Where, un = existent speed based on the net country, m/s

ufa = a per centum of deluging speed, m/s

From equation 1.9, the existent speed based on the net country and the flooding speed must be found before the implosion therapy per centum can be found. The equation used for deluging speed is besides the same as the equation for the net vapor speed.

The value of FLV is 0.0033 therefore the K is 0.065. With all the values known, the deluging speed can be calculated. The deliberate value for the implosion therapy speed is 1.747 m/s. For the design intent, a 80 % of the implosion therapy speed, Ufa is considered at 1.398 m/s. Next the cross-sectional country of the column which will besides be the country of tray is to be calculated with merely utilizing the country expression. With the tray country of 1.373m2, the existent speed is 1.188 m/s as calculated below.

Where, vi = inlet vapour rate, m3/s

Ac = country of tray/column, M2

Downcomer Area

The downcomer country is to be assumed of 15 % of the cross sectional country of the column at Ad = 0.206 M2. The downcomer country is the country needed for the liquid to flux downwards to the last phase where it flows to the regenerator column.

Net Area

The net are for the home base must be determined as the values will be needed in the undermentioned computation.

Active Area

Active country is the country where the mass transportation happens on the home base. Active country is besides known as bubbling country.

Hole Area

Hole country is the country where the vapor base on balls through the holes to let mass transportation and heat transportation. By presuming the hole country is 6 % of active country, the hole country is 0.058 M2.

Weir Height

In this bomber chapter, the weir tallness demands to be chosen. Based on Sinnott and Towler ( 2009, 747 ) , it is recommended that 50 millimeters weir tallness to be chosen. In taking a weir tallness, the efficiency and force per unit area demands to be considered. The higher the weir, the better efficiency of the tray, but it will besides increase the force per unit area bead and frailty versa. Therefore 50 millimeter is chosen due to in the procedure, the force per unit area bead must be low and it is runing at high force per unit area. It is non possible to take the weir tallness of 6 to 12 millimeter as the column is non runing at vacuum status.

Weir Length

Based on the Figure 11.39 in Sinnott and Towler ( 2009, 748 ) , the weir length can be determined. Since the assume downcomer country is 15 % of the country tray, therefore the lw/Dc is 0.81 from Figure 11.13. Therefore, the weir length, lw is 1.071 m.

Weir Liquid Crest

After the weir length is calculated, weir liquid crest can be estimated utilizing the Francis expression in equation 1.10

Where, lw = weir length, m

how = weir crest, millimeter liquid

Lw = liquid flow rate, kg/s

The estimated weir crest is 24.41 millimeter liquid.

Weep Point

Weep point is the point where the liquid leaks through the home base holes overly. This happens when the column operates at the lowest scope of operating scope. During the weep point, the vapor speed is the lower limit of the stable operation. Therefore, weep point must be determined to avoid crying occurs in the column. The undermentioned equation 1.10 is the equation to find the minimal vapour speed at weep point.

Where, uh = minimal vapor speed through the holes ( based on the hole country ) , m/s

dh = hole diameter, millimeter

K2 = a changeless, dependant on the deepness of clear liquid on the home base

Based on Figure 11.37 ( Sinnott and Towler 2009, 746 ) , K2 is 28.6. The deepness of the clear liquid is 100 millimeter. For the hole size, there is a scope of 2.5 to 19 millimeters to be chosen from. Based on Sinnott and Towler ( 2009, 748 ) , the recommended hole size is 5 millimeter. With the known values, the minimal vapour speed is calculated as below.

The following measure is to look into whether the absorber column working above or below the weep point. To make that, merely merely split the volumetric flowrate of vapor with the hole country. The computation is shown below.

Where, Vi = volumetric flowrate of vapor, m3/s

Ah = hole are, m2

This proves that the absorber column is runing above the weep point.

Perforated Area

Plate Pressure Drop

Downcomer Head Loss

Downcomer Residence Time