Consider the tank car unloading of a hazardous chemical, as shown in Fig. 5.5. A pump is used to empty the tank car and fill the storage tank. Then the chemical is transferred to a chemical reactor. Describe the discrete steps necessary to carry out such an operation and discuss any safety issues

Question

Consider the tank car unloading of a hazardous chemical, as shown in Fig. 5.5. A pump is used to empty the tank car and fill the storage tank. Then the chemical is transferred to a chemical reactor. Describe the discrete steps necessary to carry out such an operation and discuss any safety issues associated with the transfer of the hazardous chemical; that is, what instrumentation and alarms would you recommend installing?

Accepted Answer

In batch processing, a sequence of one or more steps is performed in a defined order, yielding a finished product of a specific quantity. Equipment must be properly configured in unit operations in order to be operated and maintained in a reasonable manner.

The discrete steps necessary to carry out this operation could be:

.- Open exit valve in tank car.

.- Turn on pump 1

.- Empty the tank car by using the pump and transfer the chemical to the storage tank (assume the storage tank has larger capacity than the tank car)

.- Turn off pump 1

.- Close tank car valve (to prevent backup from storage tank)

.- Open exit valve in storage tank.

.- Transfer the chemical to the reactor by using the second pump

.- Close the storage tank exit valve and turn off pump 2 .

.- Wait for the reaction to reach completion.

.- Open the exit valve in the reactor.

.- Discharge the resulting product

Safety concerns:

Because a hazardous chemical is to be handled, several safety issues must be considered:

.- Careful and appropriate transportation of the chemical, based on safety regulation for that type of product.

.- Appropriate instrumentation must also be used. Liquid level indicators could be installed so that pumps are turned off based on level.

.- Chemical leak testing, detection, and emergency shut-down

.- Emergency escape plan.

Therefore, care should be exercised when transporting and operating hazardous chemicals. First of all, tanks and units should be vented prior to charging. Generally, materials should be stored in a cool dry, well-ventilated location with low fire risk. In addition, outside storage tanks must be located at minimum distances from property lines.

Pressure, level, flow and temperature control could be utilized in all units. Hence, they must be equipped with instrumentation to monitor these variables. For instance, tank levels can be measured accurately with a float-type device, and storage temperatures could be maintained with external heating pads operated by steam or electricity. It is possible for a leak to develop between the tank car and storage tank, which could cause high flow rates, so a flow rate upper limit may be desirable.

Valves and piping should have standard connections. Enough valves are required to control flow under normal and emergency conditions. Centrifugal pumps are often preferred for most hazardous chemicals. In any case, the material of construction must take into account product chemical properties.

Don't forget that batch process control often requires a considerable amount of logic and sequencing for their operation. Besides, interlocks and overrides are usually considered to analyze and treat possible failure modes.

Question 22.8: Consider the tank car unloading of a hazardous chemical, as ...

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