Crude oil blending is accomplished by two methods: on-line blending and tank blending.
On-line Blending
In this method two or more components are injected from separate pipelines and are mixed in a single line. Ensuring adequate mixing is a necessity and often requires some type of in-line static mixer or mechanical mixing device. The use of piping elements alone may not provide adequate mixing. The efficiency of this method will depend upon the resulting stream’s Reynolds number, the type and number of piping elements, and the time allowed for mixing. With either method, the use of an injection quill for the smaller of the two streams will assist in mixing.
Often only the smaller stream’s flow rate is varied whereas the larger is kept constant. The ratio of the two streams depends upon a controlling parameter that is monitored downstream of the common injection point. Such monitoring can be performed automatically using on-line analytical equipment or manually by collection of samples. The manual collection method can provide a semi-automatic operation at best. The injection rate of the smaller stream is based upon the sample analysis. An improvement in this application would be a ration control system where the flow rates of both streams are measured and the analysis from the manual sample determines the setpoint for the ratio. This is beneficial where the flow rate of the larger stream tends to vary.
Tank Blending Components are added in a common tank and the concentration of the two is based upon a recipe approach. Achieving homogenous blending is difficult and depends upon the ratios of the two components, similarity in physical properties, size of tank, number of tank mixers and mixing time.
Preference is to add both components simultaneously as the tank is filled rather than filling with one at a time. If the only available option is to fill with one at a time, the component with the lowest concentration is added first.
There are a large variety of tank mixer configurations that can be customized for specific applications. Tank manufacturers and vendors may have simulation and sizing programs that can be used to select the proper mixer for an application.
Obtaining representative samples from tanks can be difficult at best and often impossible which is another reason why tank blending uses a recipe style operation. The upside of a recipe approach is lower capital cost requirements; quite often, existing equipment can be used. The downside is potentially less homogeneity and less certainly in meeting proper specification, as there is no process feedback until the blending is complete.
Transportation
The efficient movement of crude oil is dependent upon it’s physical properties such as density, temperature and viscosity. For any given size of pipeline, a crude oil with a lower viscosity and density will have lower horsepower requirements and higher throughput capabilities than a crude oil with a higher viscosity and density. Fluid density has a direct impact on the horsepower required whereas the viscosity will impact the pipe capacity by affecting the pressure drop along the pipeline.
Pipelines must operate below a MOP or maximum operating pressure that they were designed and licensed for. If the pressure drop in a given line increases due to increases in viscosity, the flow rate must be reduced to 184 maintain pressures below the MOP. The alternative is to increase the pipeline line size which has both capital and maintenance costs associated with it.
Blending a lighter crude oil or diluent with the crude oil that is to be transported can reduce the resulting blended crude’s viscosity and density. As a rule of thumb, the lighter the crude oil’s density, the lower the viscosity. This rule also applies to most diluents but there are some exceptions and therefore not all diluents are equal in the sense.