2014 Spring Meeting & 10th Global Congress on Process Safety
(119e) Steam Stripping of Aniline Process Wastewater -- a Tutorial, Review, and Critique
ROBERT G. KUNZ, Ph.D., P.E.
RGK Environmental Consulting, L.L.C.
Hillsborough, North Carolina, USA
Proposed for Presentation at the 2014 AIChE Spring Meeting
March 30 – April 3, 2014
New Orleans, LA
ABSTRACT
The aniline-water system is a classic illustration of a so-called heterogeneous azeotrope (heteroazeotrope). The liquid phase separates into two immiscible layers, an oily aniline-rich organic layer and a water-rich aqueous layer, in equilibrium with each other and both in equilibrium with a single vapor phase. Aniline-water vapor-liquid equilibrium (VLE) data, including little known results, are reviewed in this presentation.
When aniline is manufactured from nitrobenzene, an understanding of phase equilibria is important in the separation of the aniline product from the water of reaction and their subsequent purification. Soluble water must be removed from the aniline oil, and the aniline soluble in the water phase must be treated prior to discharge to minimize environmental contamination. The present discussion concentrates on treatment of the water phase.
A proposed design and cost estimate for a steam stripper of aniline process wastewater to reduce the load on downstream wastewater treatment facilities is contained in a 1973 US EPA development document. In such a system, the overhead composition must approach the lower-boiling azeotrope, while the higher-boiling bottoms composition approaches pure water. However, the overhead composition in the stripper material balance appears to be incorrectly stated and impossible to achieve regardless of operating conditions. A pressure of 20 psig and the temperatures listed appear to be inconsistent as well. Your author is not aware of any corrections having been issued, and the design as published is called into question.
To attain the same bottoms composition with an overhead composition approaching the azeotrope, one must increase the overhead flow and decrease the bottoms flow. The number of theoretical trays required to achieve the desired separation is then calculated from the Kremser-Souders-Brown Equation. The number of actual trays is estimated using a range of tray efficiencies derived from published correlations plus hard-to-find aniline steam-stripper laboratory data. This analysis confirms the number of trays shown in the EPA diagram to be adequate at atmospheric pressure and 20 psig, with a higher tray efficiency needed at 20 psig.
INTRODUCTION
The subject of this paper started out to be a simple textbook example of a heterogeneous azeotrope – one type of vapor-liquid equilibrium (VLE) diagram that may be encountered in environmental control calculations. This was to join previous examples in the author's book and subsequent paper involving Raoult’s Law, positive and negative deviations therefrom, and maximum- and minimum-boiling homogeneous azeotropes.
For a heterogeneous azeotrope to occur, the liquid phase breaks up into two immiscible solutions of differing composition, each in equilibrium with the other and both in equilibrium with a single vapor phase. The x-y VLE curve is flat in the area of mutual insolubility, and with few exceptions, the y=x 45° line intersects this constant y-value within the insoluble range. Solutions of this type cannot be separated by distillation to obtain two products on opposite sides of the intersection. Distillation of a minimum-boiling azeotrope results in an overhead as close as desired to the azeotropic composition and a bottoms composition as close as desired to either of the pure components, depending on which one is richer in the feed.
The aniline-water system exhibits such behavior. It seemed a likely candidate to illustrate this phenomenon, with a steam-stripper design contained in a 1973 US EPA development document providing a practical application to environmental control. Validation of the design was to be simple and straightforward.
Instead, this exercise turned into an extended project since the operating temperature and pressure of the steam stripper as presented appear to be inconsistent. Furthermore, contrary to expectations, its overhead composition was eventually discovered not to approach an attainable azeotropic composition at any reasonable set of operating conditions.
Nevertheless, once the perceived discrepancies are corrected, the design turns out to be capable of producing the stated aniline bottoms concentration at both atmospheric pressure and 20 psig, the elevated pressure assumed. A higher overall tray efficiency, however, is required at 20 psig. Well known published data, newly developed information, and recently uncovered little known experimental results from the past were used in the evaluation.
After consideration of some background information immediately below, this paper makes use of the aforementioned data, as necessary, along with widely accepted thermodynamic principles to deduce a technical basis for the wastewater stripper design.