Composition

In AVEVA Unified Supply Chain the different components in each pipe are tracked separately where possible. This process is known as pooling. At any point the materials within that pipe can be separated or depooled and routed independently. By tracking materials separately, AVEVA Unified Supply Chain can more accurately calculate the properties of blended streams.

For example, a crude unit is typically fed with a blend of several different crudes. The amount of each crude in the CDU feed pipe and the properties of each of these crudes are tracked separately and used to calculate the overall properties of the pipe contents. This information is preserved through the distillation unit, so that the kerosene originating in the CDU is actually comprised of the individual kerosene components of the constituent crudes, blended together according to the ratio of the crudes in the CDU feed and the yield of kerosene from each crude.

Property × yield predictions

When calculating composition it is often necessary to use property × yield predictions. Property × yield predictions are used to predict property values that are dependent on the relative amount of yield of the stream containing that property. This is useful where the amount of material in a feed needs to be conserved across the process unit, and are particularly relevant for gas compositions, where the proportion of a gas in a stream will change with respect to the amount of that stream produced.

Alkylation units are a typical example of process unit models that contain property × yield structure.

Sulfolane process

In the sulfolane process, a solvent extraction is used to concentrate benzene in a particular phase, allowing its removal from a feed stream. This may be used, for example, to remove benzene from reformate before gasoline blending. The amount of benzene is conserved, but the concentration of benzene in the output products is relative to the amount of each output product, which may change depending on other conditions within the process unit. Thus, to calculate the concentration of benzene in the output products, it is necessary to know both the amount of benzene in each output product and the amount of each output product.

That is, the predicted value for the property is subsequently divided by the yield of its containing stream expressed in that stream's units of measure.

When a component is distributed between the different output streams in different proportions, another term is necessary. For example, in the sulfolane unit if the benzene were not completely separated into the extract stream, but a small proportion still remained in the product, then this distribution could be represented by another factor in the calculation to describe the proportion of the input benzene which ends up in each stream.

Separating light ends

Imagine a fractionation unit that separates hexane, benzene and toluene. All the hexane goes to the light product and all the toluene goes to the heavy product. The benzene is split equally between the two products. The base delta model for the unit would look as follows:

To calculate the composition of the final product streams, it is necessary to use property × yield predictions. Imagine the unit was fed with 1000 kg of feed, 50% hexane and 50% benzene. The light stream would end up with 750 kg of material, whilst the heavy stream would have 250 kg of material. The heavy stream would be pure benzene, as all hexane always goes to the light product. Therefore, given the feed is 0.5 proportion benzene and 50% of this goes to heavy, 0.5 × 0.5 = 0.25 units of material would be produced. Again, given that 0.5 proportion of the benzene goes to the heavy stream and 0.5 proportion of the feed is benzene, the final benzene ratio of the heavy product is feed ratio × split ratio / yield = (0.5 × 0.5)/0.25 = 1.

The light stream would be composed of 500 kg of hexane and 250 kg of benzene. Given the feed is 0.5 proportion hexane and 0.5 proportion benzene, this follows from the benzene split ratio. As such, the final composition is 66.6% hexane and 33.3% benzene. This can be calculated again using a property × yield prediction where the hexane content is (feed ratio × split ratio) / yield = (0.5 × 1)/0.75 = 0.66. The benzene ratio is (feed ratio × split ratio) / yield = (0.5 × 0.5)/0.75 = 33.3%.

The density of each product stream will be the composition multiplied by the component density. For the light stream we know that the composition is 2/3 hexane and 1/3 benzene, therefore we expect the density to be 2/3 × 0.7 + 1/3 × 0.8 (if hexane had a density of 0.7 g/cc and benzene a density of 0.8 g/cc). This gives us an average density of 0.7333. However, as the benzene is split across two products, then its density is also split across these products. So we must multiply the density of each component by its split ratio. Thus the stream density is the sum of (feed ratio × split ratio) / yield × (density × split ratio).