Page 201 - Gas Purification 5E

188 Gas Purification

Thermal Reclaiming, 255
Ion Exchange for Amine Solution Purification, 264
Electrodialysis for Amine Solution Purification, 264

NONACIDIC-GAS ENTRAINMENT IN SOLUTION, 265

REFERENCES, 269

INTRODUCTION

One of the reasons that alkanolamine processes have become the predominant choice for
both refinery gas treating and natural gas purification is their comparative freedom from
operating difficulties. Nevertheless, several factors can result in undue expense and cause
difficulty in the operation of alkanolamine units. Chief among these, from an economic
standpoint, are corrosion and amine loss. Other operating difficulties which occasionally
limit the capacity of a plant for gas purification include foaming and plugging of equipment.
In many cases, operation can be significantly improved by daily monitoring of key plant
operating variables and by proper control and design of the treating plant.
AMINE PLANT CORROSION

Background
The most serious operating problem encountered with alkanolamine gas purification
plants is corrosion, and, as would be expected, this problem has been given the widest atten-
tion. Several theories have been advanced to explain corrosion mechanisms, patents describ-
ing measures to eliminate or deviate corrosion have been issued, and numerous papers have
been published. Based on this appreciable amount of information and experience, the corro-
sion phenomena observed in a large number of plants operating under a wide variety of con-
ditions can be reasonably explained, and certain guidelines can be established to minimize
corrosion.
Amine systems are subject to corrosion by carbon dioxide and hydrogen sulfide in the
vapor phase, in the amine solution, and in the regenerator reflux, and by amine degradation
products in the amine solution. In refineries, amine systems often suffer from corrosion by
several agents not generally found in natural and synthesis gases, namely ammonia, hydro-
gen cyanide, oxygen, and organic acids, some of which tend to accumulate in certain parts of
the refinery amine system.
Amine units must be designed to overcome these special problems. This chapter reviews
both the causes of corrosion problems and possible solutions. It describes the locations with-
in amine units where the various agents cause corrosion, discusses the corrosion mechanisms
in these places, and reviews the design practices and preventive measures required to miti-
gate corrosion. These measures include control of velocities and impingement, process con-