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The Effect of Operating Conditions on Corrosion in HF Alkylation Units

Part 2

By Jonathan D. Dobis at The Equity Engineering Group, Inc., Dana G. Williams at Marathon Ashland Petroleum LLC, and David L. Bryan, Jr. at Marathon Ashland Petroleum LLC. This article appears in the July/August 2004 issue of Inspectioneering Journal
This article is part 2 of a 2-part series.
Part 1Part 2

 

The following is the final part of a 2 part series. Part 1 covered the following considerations that are critical in establishing effective reliability and inspection programs for these complex units:

  • HF Alkylation Process Description
  • An extensive explanation of how HF alky unit process conditions affect corrosion for the various sections of these types of units

The abstract, introduction, explanation of terms, references, figures and a typical unit PFD are re- iterated in part 2 as reference materials.

Abstract

Corrosion and fouling in HF Alkylation Units are closely linked to feed quality and operating conditions. This paper outlines the relationship between key operating parameters and corrosion that has been used to develop a set of guidelines to define an operating envelope. These guidelines have been used to benchmark operating units and to help maintenance and inspection groups understand how corrosion is directly affected by operating parameters. An example where this methodology has been used to troubleshoot operating problems is included. A web-based data collection system has been used as a tool to build a database of actual operating conditions found in the unit, and the corresponding problems or damage observed.

Keywords

HF Alkylation Unit, HFA, hydrofluoric acid, acid strength, reliability, corrosion, stress corrosion cracking, RBI, risk, Isostripper, Depropanizer, alky, regenerator, ASO, feed contaminants, fitness for service, FFS, API RP 751, API RP 571

Introduction

The methodology described in this paper was developed to help maintenance and inspection groups gain a better understanding of the manner in which corrosion is directly affected by operating parameters. Although much of this information is available to process licensees, it is generally not disseminated among the various disciplines in the refinery in a manner which can be clearly understood and applied.

Significant damage can occur when operating parameters drift outside recommended limits, or when licensor-recommended operating procedures are not followed. Except where noted, these issues apply to both UOP and Phillips licensed processes (1).

There has been considerable interest over the past several years in reducing the risk of operating HF Alkylation (HFA) Units1, but little information has been published on the effects of operating conditions. Most of the recent efforts have focused on ancillary issues including galvanic couples and the effect of residual elements in steel 2-7, and materials of construction guidelines 8-11, usually based on laboratory test data and/or exposures in anhydrous acid.

A noteworthy reference is the detailed work published by H. Helle12. This book contains significant information on corrosion and the effects of operating parameters, some of which is reflected in the discussions in this paper. Other relevant articles describing experience with HFA units identify the types of corrosion problems13-16; areas of vulnerability; and the use of hydrogen probes to monitor hydrogen activity 17.

Explanation of Terms

Several terms that are unique to HF alkylation units and that are used throughout this paper are defined below.

Acid Concentration is the concentration of hydrofluoric acid based on its water content. For example, a solution of 98 wt% HF concentration indicates that there is 2 wt% water in the acid.

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