Within the upstream oil and gas processing industry, cyclonic type separation devices are being used in various applications, one of which is as an inlet device for separators and scrubbers. Within this role, many cyclonic devices function very well, but in some applications, serious problems have occurred.
There are three main functions of an inlet device in the oil and gas industry. The first is for reducing incoming stream momentum. This ensures that the final distribution of the phases entering the vessel will be optimized, thus maximizing the degree of separation obtainable within the vessel.
With inlet cyclonics, this is achieved in two ways:
- By separating liquids and vapor into two discrete outlets downstream by means of a secondary device. The latter further decreases momentum and optimizes gas/liquid, and liquid/liquid separation.
- The prevention of liquid shattering and ensuring separation of the bulk liquids. This is closely linked to the reduction in incoming stream momentum. If the momentum is rapidly decreased by use of, for example, simple baffle plates or inverted dished heads, there is a high probability that shattering of the incoming bulk liquids will result - similar to water splashing onto the bottom of an empty container. When shattering occurs, droplets with a smaller diameter are created which are much harder to separate.
Pipeline systems
Pipeline systems, depending upon flow regime, contain a varying proportion of mist to total liquids. With this in mind, the surfaces of the inlet device should not affect or increase the proportion of mist to liquid. The smoothly contoured surfaces and high centrifugal forces within the body of the inlet cyclones make this possible.
The third major function of an inlet device is for the prevention of foaming. This is of concern, especially when the hydrocarbon fluid comprises oil, as opposed to condensate. Foam can be produced both within the upstream piping and the vessel itself. Foaming occurs as a result of gas "bubbling" through liquid and splashing on a solid surface, such as the simple baffle plates and inverted dished plates.
The best way to handle incoming foam is to apply a shear force in order to "break it down." This is achieved by combining smoothly contoured surfaces with ability to apply a controlled shear to the incoming fluids. It is important however, that the shear force applied should not be so high that the entrained droplets are reduced in size. It is also important that a proportion of the bulk liquid does not turn into droplets by re-entrainment at the gas/liquid interface.
Foam generation in the vessel can be minimized by maximizing the gas from the liquid separation within the inlet device. Possible problems with inlet cyclones are caused by a number of factors, including pressure imbalance and the shearing of the dispersed liquid.
Pressure imbalance
By splitting the gas and liquid streams, different pressure drops across the cyclone outlets can occur. The result is that, if incorrectly designed and controlled, gas may exit through the liquid outlet (gas blow-by) or liquid may exit through the gas outlet (liquid carryover).
A computational fluid dynamics (CFD) analysis of liquid carryover caused by a high liquid outlet pressure drop (see figure) shows that the effect of this mechanism can be to put a considerable amount of liquid into the gas space of the vessel, leading to a possible overloading of the down stream mist eliminator. In addition, since the liquid is entrained in the gas, the droplets may shatter, thus effectively reversing the benefits of the cyclonic inlet device.
The adverse affects are visible in that the de-foaming characteristics of the cyclonic device are destroyed by bubbling gas through the liquid. However, the elimination of gas blow-by increasing the liquid outlet pressure can create another problem due to the shearing of the dispersed liquid phase.
In order to maintain the liquid levels inside the cyclone and prevent gas blow-by, a pressure build-up region is located near the liquid outlet. Rapid dissipation of energy at the liquid outlet can sometimes lead to the shearing of the dispersed phases (water droplets in the oil, or oil droplets in the water), thus substantially reducing the downstream separation of these phases.
Flow analysis
CDS Engineering has developed an analytical model, based upon classical theory, which mimics flows within the body of the cyclonic device. This enables a prediction of both velocity and pressure profiles within the cyclone. The development of the model was complicated, due to the rotating flow field, the presence of a two/three-phase feed, and the fact that the gas has to reverse direction in order to exit the cyclone body. Although the model is still being enhanced, the results from both the CFD models and the laboratory experiments, which are being performed both under atmospheric pressure with air and water and under high pressure with natural gas up to 40 bar, were found to be in almost complete agreement.
With this analytical model, it is now possible to accurately predict the pressure balance over the complete cyclone geometry, ensuring that the pressure drops can be controlled, thus alleviating the detrimental effects of gas blow by or gross liquid carryover. The development and use of this model allows cyclones to be used in various complex applications, where "unknowns" commonly associated with cyclonic devices had previously prevented further development.
US Gulf case study
High injection rates of chemical defoamers had been required to operate the production and test separators on Shell's Mars tension leg platform in the Gulf of Mexico. In an effort to reduce the chemical consumption, different means of mechanically breaking the foam were investigated in the test separator - gas outlet axial flow de-misting cyclones and inlet cyclones.
For two different wells, the chemical rate was reduced by 20% and 80%, respectively, when the existing vane packs were replaced by axial flow de-misting cyclones. On the basis of the successful test separator results, the four main production separators were retrofitted with new internals. Chemical reduction has been on the order of 50%. Other operational problems due to foaming - such as:
- Poor level control that led to platform shutdowns
- Liquid carryover in the gas outlet that led to flooding of downstream scrubbers and compressors
- Gas carry-under in the liquid outlet that led to increased compression requirements were reduced.
