Chapter 6, Problem 6.91P

An aqueous waste stream leaving a process contains 10.0 wt% sulfuric acid and l kg nitric acid per kg sulfuric acid. The ?ow rate of sulfuric acid in the waste stream is 1000 kg/h. The acids are neutralized before being sent to a wastewater treatment facility by combining the waste stream with an aqueous slurry of solid calcium carbonate that contains 2 kg of recycled liquid per kg solid calcium carbonate. (The source of the recycled liquid will be given later in the process description.)

The following neutralization reactions occur in the reactor:

   $\begin{array}{l}{\text{CaCO}}_{\text{3}}{\text{+H}}_{\text{2}}{\text{SO}}_{\text{4}}\to {\text{CaSO}}_{\text{4}}{\text{+H}}_{\text{2}}{\text{O+CO}}_{\text{2}}\\ {\text{CaCO}}_{\text{3}}{\text{+2HNO}}_2\to \text{Ca}{\left({\text{NO}}_3\right)}_2{\text{+H}}_{\text{2}}{\text{O+CO}}_{\text{2}}\end{array}$

The sulfuric and nitric acids and calcium carbonate fed to the reactor are completely consumed. The carbon dioxide leaving the reactor is compressed to 30 atm absolute and 40°C and sent elsewhere in the plant. The remaining reactor ef?uents are sent to a crystallizer operating at 30°C, at which temperature the solubility of calcium sulfate is 2.0g CaSO₄/1000 g H₂O. Calcium sulfate crystals form in the crystallizer and all other species remain in solution.

The slurry leaving the crystallizer is ?ltered to produce (i) a ?lter cake containing 96% calcium sulfate crystals and the remainder entrained saturated calcium sulfate solution, and (ii) a ?ltrate solution saturated with CaSO₄, at 30°C that also contains dissolved calcium nitrate. The ?ltrate is split, with a portion being recycled to mix with the solid calcium carbonate to form the slurry fed to the reactor, and the remainder being sent to the wastewater treatment facility.

1. Draw and completely label a ?owchart for this process.

Speculate on why the acids must be neutralized before being sent to the wastewater treatment facility.

Calculate the mass ?ow rates (kg/h) of the calcium carbonate fed to the process and of the ?lter cake; also determine the mass ?ow rates and compositions of the solution sent to the wastewater facility and of the recycle stream. (Caution: If you write a water balance around the reactor or the overall system, remember that water is a reaction product and not just an inert solvent.)

Calculate the volumetric ?ow rate (L/h) of the carbon dioxide leaving the process at 30 atm absolute and 40°C. Do not assume ideal-gas behavior.

The solubility of Ca(NO₃)₂at 30°C is 152.6 kg Ca(NO₃)₃per 100 kg H₂O. What is the maximum ratio of nitric acid to sulfuric acid in the feed that can be tolerated without encountering difficulties associated with contamination of the calcium sulfate by-product by Ca(NO₃)₂?