Inclined Plate Separators
Froth Treatment is a gravity separation process by which fine solids and water are removed from bitumen froth produced in Extraction through the addition of a light hydrocarbon. In a naphtha-based Froth Treatment process (NFT), inclined plate separators (IPS) are used as settling vessels, where fine solids and water are allowed to settle to the bottom, producing a relatively clean diluted bitumen product.
IPSs used in the oil sands are based on traditional lamella clarifiers used in the water treatment industry. IPSs use a nest of inclined plates which reduce the settling distance, allowing for more efficient separation of the solids within a smaller footprint.
DESIGN & THEORY
Diluted froth enters the IPS, flows downward through the inlet chamber and rises up through the inclined plates. The solids settle on the plates and "slide" to the underflow. Clarified bitumen/naphtha mixture flows upwards and into the overflow launders through a series of orifices and weirs. Overflow launders are typically arranged as a series of parallel troughs within the IPS.
Any air in the bitumen froth would be liberated into the vapour space. A natural gas or nitrogen blanket is required to maintain pressure in the IPS, which is connected to a Vapour Recovery Unit (VRU).
Since IPSs are gravity separation vessels, they require a very quiescent environment which minimizes turbulence. Unfortunately, any emulsions that form when the froth and naphtha are mixed cannot be separated within the IPS, regardless of retention time or surface area. Therefore, the IPS underflow typically contains a significant amount of trapped bitumen and naphtha which requires further processing.
However, IPSs can produce a good quality overflow, given enough stages and provided the naphtha to bitumen (N:B) ratio is high enough. In most cases, the overflow from the first stage IPS is the final product produced from the NFT facility. IPS overflow typically contains 1 to 2% water and up to 1% fine solids.
The most common supplier of IPSs in the oil sands is Parkson Corporation.
INSTRUMENTATION AND CONTROL
IPSs are primarily operated on flow control. Flowmeters are typically installed at the inlet and each outlet. Depending on the quality of the bitumen froth, the underflow flowrate is set at a fixed value, controlled either by an underflow pump or control valve.
The most important operating variable for the IPS is the inlet flow and the amount of naphtha added (or N:B ratio). Separation improves at higher N:B ratios. However, excessive naphtha addition reduces residence time and may result in too much naphtha reporting to the underflow.
If the bitumen froth quality is poor (or has a very high water/solids content), it may be necessary to reduce flow into the IPS in order to produce a good quality overflow. A refractometer can provide an on-line measurement of the N:B ratio.
Product quality is typically monitored using water cut meters and/or autosamplers which are installed on the product overflow line. Note that in most NFT circuits, only the first stage IPS typically produces a final bitumen/naphtha product.
Temperature within the IPS is normally kept constant. The IPS operating temperature is controlled by adjusting the temperature of the froth and/or the naphtha using steam.
The interface or level of the rag layer can be measured with a segmented capacitance probe, nuclear density profiler and/or differential pressure measurement. A low interface is not desirable since this can cause an excessive amount of bitumen/naphtha to be pulled into the underflow. If the interface is too high, solids carry-over to the overflow can be more pronounced. The interface is adjusted through an underflow control valve or pump.
Inclined plate separators are now used at every NFT facility, specifically Syncrude Mildred Lake, Suncor Millennium and CNRL Horizon.