Mar-2010

An evaluation of general “rules of thumb” in amine sweetening unit design and operation

Many “rules of thumb” are widely used in the design and operation of amine sweetening units. These rules have been developed over the years and most engineers accept them even though many have little familiarity with how important they may or may not be.

Luke Addington and Chris Ness
Bryan Research and Engineering, Inc

Article Summary

Few ask why we have these rules, how absolute they are, and whether the rules have any flexibility. In this paper, several of these rules are described and evaluated for their usefulness and necessity using parametric studies with a steady-state process simulator. The rules evaluated include the 5°C temperature approach in the absorber, the 0.12 kg/L specification for reboiler steam, the 99°C lean/rich exchanger outlet temperature, and the regenerator pressure/reboiler temperature guideline. Although these four rules of thumb are excellent starting points, none of them represent optimum conditions for all cases and, depending on the situation, violating these rules could offer considerable advantages to process efficiency. Every situation is different and requires a thorough investigation as to whether changes to these set points are beneficial and whether these benefits offset any additional risks.

Introduction
Amine sweetening units have been used in gas processing for nearly 80 years to remove H2S and CO2 from sour gas streams.1 Development first began with TEA and later moved to more advantageous amines such as MEA and DEA. During the last 20 years, MDEA has become a more popular solvent, especially when used for selective removal of H2S over CO2. Speciality blends of MDEA with the addition of various additives to enhance performance have also become commonplace in industry.

Over the years of development, various guidelines, rules of thumb and design practices have evolved. Many designers take these rules for what they are, seldom stopping to ask how applicable they are to today’s designs. Some guidelines have been in place longer than some solvents, such as MDEA, have been in general service. With the considerable differences between past and present solvent performance, the question must be raised by the designer whether these guidelines are appropriate.

While there is a plethora of design rules, a few specific ones come to mind and should be reviewed. The need to have a 5°C (10°F) temperature approach between the gas and solvent feeds, for example, has existed for a number of years. Likewise, the 0.12 kg/l (1 lb/gal) steam ratio rule, the 99°C (210°F) lean/rich exchanger outlet temperature guideline (on the rich side), and the required reboiler pressure/temperature have all been used for years, regardless of the amine type or the situation.

If there is one rule of thumb that is always true in gas processing, it is that every situation is different. In this study, the above-listed guidelines are investigated for their efficacy and usefulness. While deviating from well-established operating points poses some risk, a thorough analysis can provide the designer or unit operator some understanding of those risks and possible ways to mitigate them. An accurate, robust simulation tool can assist in this process. For this particular study, ProMax with TSWEET and PROSIM was used, a simulator whose accuracy has been well established in industry_.^2,3,4

5°C (10°F) temperature approach rule
During operation and design of amine contactors, it has been advised for some time to maintain a minimum temperature approach of 5°C (10°F). The temperature approach is defined as the temperature differential between the incoming acid gas and the lean amine feed. The reason for this guideline is to prevent the condensation of hydrocarbons in the contactor and avoid the subsequent problems that a second liquid phase causes an amine plant.¹

However, it is well known that for most cases decreasing the temperature of the absorber can increase its performance. This is especially true for primary and secondary amines with little kinetic involvement. Even for MDEA there are potential performance gains for the absorption of H2S, whose kinetics are quite fast, at the expense of CO2 whose absorption would be lowered due to the temperature effect on the kinetic rate.5 Many times it is impractical or undesirable to lower the absorber temperatures by cooling the sour gas feed, so one option is to lower the temperature of the lean amine as much as possible. However, this could require violating the 5°C (10°F) approach rule of thumb.

While the potential gains from cooling the absorber are obvious, the possible detrimental effects are equally as obvious. Liquid hydrocarbons can create many problems in the amine plant itself, such as foaming, shortening the life of carbon filters and loss of product.6,7,8 Increased hydrocarbons in the amine solution can also be problematic for downstream units such as Claus plants.

The question for the designer or operator is then when does a hydrocarbon phase form, if at all? Also, while the absorption of H2S is increased with lowering temperature, so too is the absorption of hydrocarbons. Does the good outweigh the bad?

To study these questions, four representative streams were selected from various locations in Central/Eastern Europe. These gases, shown in Table 1, are from various sources, operate at various pressures, and have quite different compositions and hydrocarbon dew points.

Each of these streams is fed to an amine absorber and the lean amine temperature is varied from 10 to 50°C. Two things should be noted. First, all four streams are fed to the absorber at 40°C and are saturated with water. Since condensing a non-aqueous phase is of concern, the hydrocarbon dew point is what is most important and is shown in Table 1. Second, some practical limitations for the system must not be ignored. For example, temperatures below 25°C for an MDEA solution cause a dramatic increase in the solution viscosity, making operation of the contactor exceedingly difficult. However, since most lean amine streams are cooled with aerial coolers, it is rarely practical to reduce the lean amine temperatures to extremely low temperatures in most climates. These temperatures were selected simply to emphasise the trends.