FLASH TANK DESIGN

To determine the Flash temperature, we’ll have to determine the Dew Point and the Bubble Point Temperatures. To calculate the Bubble Point Temperature, we assume that all of our feed is saturated liquid. We assume a temperature and at that temperature, the K value for the component is determined. Then by multiplying this K value with the liquid weight fraction, we get the vapor fractions. The sum of these fractions should be unity in order to have the correct Bubble Point Temperature. So we see that it’s a hit-and-trial method. Similarly, we assume a temperature and at that temperature we determine the K value for the component. Then we assume that all of our feed is saturated vapor. So dividing these fractions with the K values, we get liquid fractions; whose sum should be unity. If the sum of liquid fractions is unity then our assumed Dew Point is correct. Taking the arithmetic average of this Bubble Point and Dew Point Temperatures, we get the Flash Temperature. K values for Iodomethane and Acetic Acid has been determined directly from the Himmelblau Software. The equation that this software uses is V.P = A- {B/(T + C)} Here A,B and C are empirical constants while T is the assumed temperature. By dividing this Vapor Pressure (V.P) by the total pressure, we get the K value. K values for Water and Methyl Acetate have been determined by using the empirical relation given in Perry’s Chemical Engineering Handbook. The relation is:

V.P = exp [C1+C2/T + (C3*lnT) + (C4*TC5)] * 9.869233E-06 atm

Here C1, C2, C3, C4 and C5 all are empirical constants and there value is given in Chemical Engineer’s Handbook by Perry. The Vapor Pressure thus obtained is divided by the total pressure to get the K value at the assumed temperature. The process of calculating Bubble Point and the Dew Point Temperature is given below:

Compound	Xi	K at 101 oC	K*Xi
Acetic Acid	0.632	0.416	0.263
Methyl Acetate	0.215	2.673	0.574
Iodomethane	0.018	3.682	0.065
Water	0.136	0.736	0.100
	1.000		1.002

This employs that our Bubble Point Temperature is 101 ^oC.

Compound	Yi	K at 119 oC	Yi/K
Acetic Acid	0.632	0.739	0.855
Methyl Acetate	0.215	4.163	0.052
Iodomethane	0.018	5.469	0.003
Water	0.136	1.348	0.101
	1.000		1.011

This determines our Dew Point Temperature which comes out to be 119 ^oC. Now:

                        Flash Temperature = (101 + 119)/2

                                                       = 110 ^oC         

Determining the Vapor and Liquid Flows:-

                Determination of Vapors going out and the liquid draining the drum is a result of some lethal calculations. These calculations are explained over here. First we make a material balance for a single component. It yields:

                                    Fx_fi = Vy_i + Lx_i ………….Eq. I

From Henry’s Law, we have:            x_i = y_i /K

Putting this value in Eq.I, we get:

                                    Fx_fi = Vy_i + L (y_i/K)……….Eq. II

ð                 y_i = Fx_fi / (V + L/K)

Since F = V + L which employs that V = F – L, therefore;

                                    y_i = Fx_fi / (F- L + L/K)

                                    y_i = x_fi / {1 – L/F (1 – 1/K)}

Also from Eq. II, we can write that

                                    y_iV = Fx_fi / (1 + L/VK)………..Eq. III

Which employs that:    y_i = (Fx_fi / V) / (1 + L/KV)……..Eq. IV

Now after determining y_i’s, we can calculate x_i’s by using the K values from the expression:           

y_i = K_i x_i where K_i is determined by using the relation K_i = V.P/P

V.P stands for Vapor Pressure at the specified Flash Temperature.

Eq. III can be written in the form as:           

 _{i=c             i=c}

Σ_i=1 (y_i V) = Σ_i=1 {Fx_fi / (1 + L/KV)}……..Eq. V

 Procedure to be followed for Flash Calculations:-

            So simplifying all the procedure, we come to know that if we are to calculate V, L, y_i’s and x_i’s then we’ll have to follow these steps:

1.                  Assume V.

2.                  Calculate L = F – V

3.                  Calculate L / V

4.                  Look up for K values at Flash Temperature and Total Vessel Pressure

5.                  Substitute values in;

           _i=c

V = Σ_i=1 {Fx_fi / (1 + L/KV)}

If equality is obtained between the assumed V and the calculated V, then the assumed value is satisfactory.

6.           Calculate y_i’s from Eq. IV

7.           Calculate x_i’s from y_i = K_i x_i

Now using this procedure the values of V, L, y_i’s and x_i’s have been calculated for the Flash Drum. 

Getting started with Design of Flash Drum:-

                Since our calculations are based upon an hour of operation, so we have the following amount of vapor and liquid flow rates;

            F_L = 1480.196 kg/hr                        p_L = 961.55 kg/m³

                F_v = 4664.308 kg/hr                        p_v = 2.654 kg/m³

Vapor liquid separation factor, which is equal to (F_L/F_v) / (p_v/p_L) ^½; comes out to be 0.017. Using the graph, we notice that the Vapor Velocity Factor is equal to 0.35 m/sec.

Maximum design vapor velocity is obtained by multiplying the vapor velocity factor with {(p_L – p_v)/ p_L} ^0.5. The value of velocity comes out to be

                                    U_v = K_v* {(p_L – p_v)/p_L} ^0.5

                                    U_v = 6.653 m/sec

If we divide the vapor mass flow rate by the density, we get the volumetric flow rate. So

                                    V_L = 0.488 m³/sec

Dividing volumetric flow rate by the vapor velocity, we get the minimum cross sectional area of the drum. Hence

                                    A_min = V_L/U_v

                                    A_min = 0.073 m²

From this minimum cross sectional area, we can calculate the minimum diameter for the vessel. The minimum diameter is:

                                    D_min = 0.306 m

Actual internal diameter is obtained by adding 6in to this minimum diameter. Therefore

                                    D = 0.458 m

For a vertical Flash Drum the surge time is in the range of 4 to 7 min and that for a horizontal vessel, it ranges between 7 to 12 min. Flash Drum used in Cativa Process has a surge time of 5 min. So multiplying this time with the liquid volumetric flow rate, we get the liquid volume held in the flash drum.

                                    Liquid Volume = V_L * 300

                                    Liquid Volume = 0.128 m³

Since the vessel is cylindrical, therefore its volume is equal to 3.145*(radius) ²*height. Using this relation, we can determine the liquid height in the vessel. The liquid height comes out to be:

                                    Liquid Height = 0.779 m

Now both H. Silla and Coulson have suggested the following formula for determining the vapor height in the vessel. This formula is:

                                    Vapor Height = 1.5* D + 0.4

                                    Vapor Height = 1.087 m

Now by adding the liquid and vapor heights, we can determine the total internal height of the vessel. Thus

                                    Total Height = 1.866 m

The L/D ratio for the Flash Drum comes out to be 4.072 which is a satisfactory value. This ratio actually determines the type of vessel. It tells us that whether we should go for a horizontal vessel or a vertical one. If the value of L/D ratio is between 3 and 5, then a vertical flash drum is used. If its value exceeds 5, then a horizontal vessel should be employed.

Material of construction:-

                Though material of construction is the part of mechanical design of the equipment but we can predict about it. Since we are dealing with acidic, corrosive fluid; therefore we’ll have to look for a material that is corrosion resistant. We come across two important choices that are corrosion resistant as well as economical. The flash drum can either be manufactured from Stainless Steel or we may make use of Aluminium. We can use either of the materials. Both have good mechanical strength, quite resistant to corrosion and are also cheap. Most of the heat transfer equipment in industry is made up from Aluminium Alloys. We are not that concerned with the heat transfer over here, so stainless steel is recommended as the priority material of construction.