Methylene chloride recovery process

Methylene chloride recovery process.

The Methylene chloride (Dichloromethane, CH2Cl2) production is the typical example of organic aqueous stream separation. Recovering the methylene chloride from wastewater has two goals:

• From the environmental requiments, the emission specification of methylene chloride should be less than 150 ppm in water stream.
• From point of view of economy, the minimizing the amount of steam injection is reduced the recovery cost.

Inlet streams

Equipment list of the process:

• Flash units
• mixer
• heat exchange (condenser).

Streams S2 and S4 are steam saturate vapor. Two vapor streams are mixing and condensed at a Temperature of 75 ⁰F and pressure of 14.7 psi. This stream is fed at a liquid-liquid decanter to separate the condensate into a methylene–chloride-rich stream and a water-rich stream. The simulation flowsheet in the program PRO II (release 8.0) is shown in Fig. 1.

Fig. 1. Preliminary Process Flow Diagram in the program PRO II

for the Recovery Methylene chloride from wastewater.

To calculate physical properties of the substances in the considered task, the NRTL thermodynamic model with two liquid phases calculations was used.

Objectives.

• To Recover of Methylene chloride in the wastewater.
• To do sensitivity analysis to obtain Methylene chloride concentration of 150 ppm in the bottom of the Flash Tower2 by changing the flow rate of the steam inlet to the Flash Tower1. 
• To do sensitivity analysis to obtain Methylene chloride concentration of 150 ppm in the bottom of the Flash Tower2 by changing the flow rate of the steam inlet to the Flash Tower2. 
• Using the controller to obtain Methylene chloride concentration of 150 ppm in the bottom of the Flash Tower2 by controlling the flow rate of the steam inlet to the Flash Tower1.
• Using the controller to obtain Methylene chloride concentration of 150 ppm in the bottom of the Flash Tower2 by controlling the flow rate of the steam inlet to the Flash Tower2.
• To optimize the total stream injections in the process. 

Results

The results of simulation with the initial condition are shown in Table below. The concentration of residual Methylene chloride in the liquid outlet of the Flash Tower2 (stream S7) is 220 ppm. In these conditions, it is necessary to supply 555.094 and 277.547 lb-mol/hr of steam in the Flash Tower1 and Flash Tower2 respectively. The total steam supplied in the process is 832.641 lb-mol/hr.

Table. Results obtained for Methylene chloride simulation at the initial data.

Sensitivity analysis

Taking into account the environmental restrictions (Lower organic content to less than 150 ppm in water stream) and the cost of energy in this process is necessary to carry out sensitivity analysis in the main variables (streams S2 and S4) to determine the influence of their variation.

Sensitivity analysis 1.

As a dependent variable was chosen the composition of component CH2Cl2 on a Wet basis in Mole ppm. As an independent variable was chosen the stream S2 flowrate in lb-mol/hr. It was specified, the stream S2 flowrate changes from cycle 1 through cycle 7 with starting value = base case value * 0.7 and step size = 0.1 * base case value. The base case value of stream S2 flowrate is 550. Another words, the changing of independent variable corresponds to the program code in Visual Basic below:

BaseCase = 550

S2F = BaseCase * 0.7

Step = 0.1 * BaseCase

For I = 1 to 7

S2F = S2F + Step

Next i

In the Fig. 1, strong variations of the methylene cloride concentration it is observed when the flow rate of steam in the first tower is varied in a (450 -700 lb-mol/hr) range.

Fig. 1. Sensitivity analysis for S2 stream flow rate.

Sensitivity analysis 2

Similar behavior (Fig. 2) is observed when the flow inlet on the second tower is varied in a (150 - 400 lb-mol/hr) range (Sensibility analysis 2).

Fig. 2. Sensitivity analysis for S4 stream.

The next question is to determine which variable exercises bigger influence in the methylene chloride recovery. A measure of the quantity of steam consumed in each stream maintaining the concentration of Methylene chloride in 150 ppm of the variables is achieved by installing a controller.