The Problem: Determining acceptable pollution levels and investigating the proper design for pressure relief

A chemical company with a reactor for producing chlorobenzene via catalytic chlorination of benzene in a liquid phase uses direct discharge for its evacuation during maloperation. The company is investigating whether it needs to change its procedures because the pollution level for chlorobenzene is relatively high (about 100 ppm for air samples taken near the reactor 2 hours after the discharge.)

An engineer is tasked with reviewing the pollution levels and if those are unacceptable, help to design a relief containment system for the reactor. Specifically, she will have to determine the size of an orifice required for evacuating the vapor phase.

The reactor temperature is 498K, the maximum allowable working pressure of the reactor is 3.45 bar, the overpressure is 40% of the maximum allowable working pressure, and the back pressure is at 2.76 bar. The vapor leaves the reactor at 5000 kg/h.

Finding acceptable levels of chlorobenzene in the air and if necessary, finding a suitible orifice area and size for a collecting container

The engineer will start by determining the permissible exposure limits in air. She knows that if the concentration the reactors are releasing into the atmosphere is greater than the acceptable level, then a relief containment system must be designed. She searches ‘permissible exposure limits and chlorobenzene’.

With Sittig's Handbook of Toxic and Hazardous Chemicals and Carcinogens (5th Edition) the engineer finds that the acceptable exposure limit according to OSHA is 75 ppm.

Now that she can confirm that the company's concentration level of chlorobenzene is unacceptable, she must provide requirements for the team to develop an acceptable relief containment system.

Developing Requirements for Design

She starts by identifying the required area of a relief orifice from one of the reactors. She enters ‘orifice area calculations’ into the Knovel search box.

In section ‘9.18 Orifice Area Calculations [42]' of Ludwig's Applied Process Design for Chemical Petrochemical Plants, Volume 1 (4th Edition), she finds an equation that will help her calculate the required orifice area for a vapor.

Based on the equation, she'll need to calculate the value of the area after finding the gas flow constant and molecular weight of chlorobenzene. Reading on, she sees that the back pressure is below 55% of the relieving pressure Kb = 1 and the given information to calculate the relieving pressure is 5.843 bar.

The engineer recognizes that to find the gas flow constant, equation 9-11h will give her the information she needs.

To find the molecular weight the engineer can search for ‘molecular weight chlorobenzene’.

The section called ‘basic physical properties of chemical compounds’ in the title Knovel Critical Tables (2nd Edition) has what she's looking for in an interactive table. The molecular weight of chlorobenzene is .11258 kg/mol.

Click on the image below to work with the interactive table:

Her last step is to calculate the area of the orifice, and move on to the next task of designing the size and type of collecting container.

A relief containment system was needed due to unacceptable pollution levels. The orifice area needed for evacuation of vapor phase is 93.9cm2.

In this solution story, the engineer evaluated the pollution level of toxic substance and performed the calculations necessary for the design of a relief containment system using well-known safety handbooks found on Knovel.

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“Using Knovel, I found critical information early in the design phase, so I didn’t waste time wandering down the wrong path. I’d estimate that checking Knovel first to optimize the process design saved me at least a month of rework time, significantly condensing the project timeline and improving process quality.”

Associate Process/Specialty Engineer
Fluor Corporation