Sep-2007

Control of foaming in amine systems

Amine solutions foam. Solution foaming causes plant upsets. It is the disengagement or break time that differentiates normal, desirable froth from undesirable foam.

Stephen A von Phul, D-Foam
Arthur L Cummings, MPR Services

Article Summary

Plant upsets due to foaming require immediate mitigating actions. A variety of solution contaminants and operating conditions can cause froth to stabilise into foam. Mitigating actions include adding chemical antifoaming agents, altering operating conditions and removing foam causative agents. A series of typical amine foaming incidents, causes and plant responses are presented. The upsets and responses are explained in terms of root causative agents. This paper explains how removing the contaminants that stabilise the froth into foam is the only efficient way to control amine system foaming.

Any liquid will produce froth or foam if gas is introduced into the liquid faster than the two phases disengage. The disengagement or break time differentiates froths from foams in amine treating processes. Free liquid captured between the gas bubbles begins to drain as the bubbles rise past the bulk gas-liquid interface. The free liquid will drain from around the gas bubble until the gas pressure inside the bubble is greater than the liquid wall’s surface or interfacial surface tension. At this point, the bubble will either break or combine with other surrounding bubbles (coarsening). The more stable or elastic the bubble wall, the more resistant the bubble will be to breaking. If bubbles are being formed faster than the existing ones are breaking, they will accumulate as foam.

When the formation and break time are equal, a stable foam column is formed. Anything chemical or environmental that affects free liquid drainage or bubble wall elasticity will directly affect the resulting foam’s break time.

Amine process towers with trayed internals are especially prone to foaming upsets because they are designed to produce froth by violently bubbling gas through the treating solution contained on the trays. The bubble walls in the froth act as mass transfer area for the removal of the gas-borne contaminants. The froth bubbles break quickly enough for the treating liquid to pass down to the next tray in the column before the gas flow is restricted. If anything increases the gas/liquid disengagement time, the froth will remain in the vapour space of the tray and the gas flow will be restricted. This hydraulic restriction is detected as a differential pressure above and below the froth in the vapour space. This condition is described as solution foaming. A variety of solution contaminants and operating conditions can cause froth to stabilise into foam.

Since foaming in amine plants causes upsets, mitigation actions have been employed to avoid its formation. The first mitigation action is normally the addition of chemical antifoam. Antifoams are intended to facilitate gas and liquid disengagement by weakening the cell structure of the bubbles. Antifoams have no positive contaminant removal properties. The injection of these chemicals into recirculating amine solutions is common and even regarded as necessary for normal plant operation.

This paper describes a series typical amine foaming incidents, causes and plant responses; all with respect to the effects on the root causative agents. We also present a new strategy for actually addressing amine solution foaming: causative agent removal.

Controlling amine plant foaming
1. An experienced control room operator noticed that the differential pressure across one of the columns began to fluctuate. The operator’s first thought was solution foaming so a request was radioed to the unit’s outside operator to inject a chemical antifoaming agent into the recirculating amine solution. The system differential pressure dropped to below the normal operating level within minutes. The operator watched the differential pressure slowly rise back to normal and stabilise over a few hours. The AFA (antifoaming agent) injection was noted in the daily log and the incident forgotten.

This common scenario is played out in amine plants every day. A foaming upset was averted. Notice that the surfactant concentration remained unchanged with the addition of antifoaming chemicals. Adding antifoam to the system broke the foam, but had no effect on the surfactants dissolved in the solution. At this point, the foaming tendency of the solution is in equilibrium with the solution’s antifoam concentration. If the foaming tendency of the solution should increase even slightly, the solution will foam again.

2. A few hours later, the operator noticed the differential pressure beginning to rise again. There are a variety of contaminants and operating conditions that can cause the solution to start foaming again, since there is an existing concentration of surfactants in the solution. Injecting more antifoam did not have an immediate effect, so the operator decided to shoot the plant again and have the mechanical filters changed out. Antifoam was becoming less effective. (See “Why does antifoam appear to become less effective over time?”)

Changing filters affects solution foaming both positively and negatively. First, solids increase the solution’s tendency to foam by inhibiting liquid from draining from the foam structure. The foam stays wet longer. Antifoam droplets cannot incorporate into the foam bubbles’ walls unless they are squeezed between two bubbles that have drained. In other words, antifoam cannot work if the foam stays wet. (See “How does froth turn into foam?”) Mechanical filters can also make antifoams less effective by coalescing the small AFA droplets into larger ones, or removing it completely from the flow stream.

In this case, the surfactant concentration of the solution did not increase before the foaming upset started. Solids affected the foaming tendency of the solution by increasing the stability of the froth, but also by disarming the AFA through agglomeration. Small solids suspended in the solution will stick to the AFA bubbles. The antifoam becomes increasingly less effective as solid particles are agglomerated with it. These AFA solids agglomerates also prematurely plug mechanical filters and activated carbon beds.