Weight and volume basis

Every model in AVEVA Unified Supply Chain has a measurement basis, either weight- or volume-based, which can be selected during the initial model creation. The model basis defines whether results are displayed using weight-based or volume-based units of measure.

Example: Within a supply chain model it might be chosen to track the yields of the products from the crude distillation unit in terms of the volume of each fraction. If 100,000 bbl/day of crude oil is processed and a crude has 40% by-volume residue, then 40,000 bbl/day of atmospheric residue is produced. These are both volume-based measurements, so the distillation model for the CDU is volume-based.
Alternatively, the supply chain model might be weight-based, so if the refinery processes 15,000 tonnes/day of crude and the crude is 45% by-weight residue, then 6750 tonnes/day of atmospheric residue would be produced.

The choice of whether models are weight- or volume-based depends on the normal measurement methods within your organization. In AVEVA Unified Supply Chain it does not matter whether a model is weight- or volume-based, and it is possible to mix weight- and volume-based components within a model. When switching measurement basis for streams and products, it is important to ensure that accurate density values are available for all involved streams.

Properties are blended according to their blending basis. It may be necessary to perform a weight/volume flow conversion using the stream density should a property have a different blending basis than the fundamental stream flow. For instance, if an output is in weight yield but you wish to blend Reid Vapour Pressure, the flow in volume basis must be calculated requiring the stream density.

Note: Conversions between weight and volume basis require stream density at 15°C. It is good practice to include this on all streams.
Weight/volume conversions are not limited to flow basis, for example t/d to bbl/d. They may also be required for property conversions, such as aro (v) to aro (w). Therefore streams where these conversions occur should also have density @ 15°C entered.

Flow basis in Base Delta models

Base + Delta models can also be weight- or volume-based; that is, the yields of the products originating in the process unit can be calculated in terms of the weight amounts or volume amounts. When creating Base + Delta models, the yield outputs of the unit should match the flow basis for the Base + Delta model.

The flow basis in Base + Delta models does not have to match the flow basis in the containing supply chain models. For example, it is possible to define a weight-based supply chain model and then use a volume-based kerosene hydrotreater model. However, when mixing weight- and volume-based models, care should be taken to ensure that all streams carry density as well as yield information.

Material expansion in volume-based models

Many process units work by breaking large hydrocarbons into smaller hydrocarbons to increase their value. For example, an FCC uses a catalyst to promote the cracking of low-value vacuum gas oils into high-value gasoline and distillate. This means that during the cracking process these single large molecules are broken into several smaller ones. As a result, the total volume of the products will be greater than the volume of material fed into the process unit. For example, if 40,000 bbl/day of vacuum gas oil is fed into an FCC unit, the total volume of the products might be 55,000 bbl/day.

This phenomenon occurs in many different process units, including cracking units and crude distillation units. In AVEVA Unified Supply Chain the yields are tracked accurately, as all streams have density associated with them as well as flow, allowing for easy conversion between weight and volume basis.

AVEVA Unified Supply Chain handles the volume expansion of crude oil and its products during cutting and will also handle volume expansion in cuts which are wide enough for the phenomenon to occur. If you are using a volume-based model, you may see many diagnostic errors showing that the yields of the process units do not correctly balance. These can be ignored.

When reviewing yields from a crude distillation units in terms of volume per cent, the percentage of product obtained from the tower will be greater than 100%. This is due to the volume expansion effect described above, which in the case of crude oil is caused by smaller hydrocarbon molecules dissolving within larger molecules. During distillation this effect is removed, as the small and large molecules are separated, and so the volume of crude effectively increases. In this case the volume of products is relative to the amount of crude fed to the tower, and as the volume of product is greater than the volume of feed, then the total volume of product is greater than 100%.

This effect is not observed for weight-based yields, as mass must be conserved throughout the tower.

Mass conservation in weight-based models

The summed mass of all the products of a process unit must be equal to mass of feed to that unit. If any process models do not conserve mass, AVEVA Unified Supply Chain issues a warning after simulation or optimization. Any process units with a mass imbalance should be investigated.

You can include a loss term in process models to allow for material mass which is lost in the process unit. For example, within a unit some material may be combusted or deposited within the unit itself and so not be accounted for in the final product mass. This material can be entered as a loss to ensure mass conservation across the unit.

Weight-volume conversions for purchases and sales

When a sale is priced in volume units, it occurs in volume terms. For example, pricing gasoline in $/m3 ensures that the sale occurs in volume terms. Where a material is priced in one basis unit and sold in another, a density conversion is required. For example, a material may be priced per tonne, but sold per cubic meter. To calculate the correct product value, it is therefore necessary to convert the amount (in tonnes) into volume. During optimization the actual product density will be used as part of this calculation.

However, for consistency with pricing data it is often necessary to use a reference density for the price transformation. This is a standard density used to do a conversion from a weight-based price to a volume-based effective price, after which the sale occurs in volume basis. For example, if you entered a price of 1000 $/tonne for diesel and a reference density of 0.83 g/cc, the actual price of diesel used during optimization would be 830 $/m3. Reference densities are only used to transform weight-based prices into volume-based prices.

Similarly, when a weight-priced purchase has a reference density entered, an effective price is calculated for the material. The effective price of the material is equal to the reference density / actual density * actual price. As the material is an import, the feedstock density is fixed and therefore the purchase still occurs in the original basis, but the purchase will instead occur at the effective price rather than the entered price.

Tip: Reference density can be entered on the Purchases and Sales pages, and can also be associated with traded materials in price sets.

When a sale is occurring in volume terms it may be possible to increase the volume of material by reducing the density. This behavior is captured automatically during optimization in AVEVA Unified Supply Chain. Where a volume-based sale has a limit and this limit is in weight-based terms, the sale still occurs in volume terms and the amount is converted using the blended material density. For example, where gasoline is priced in $/m3 and limited to a maximum of 10 kt/d, the sale occurs in volume terms and the maximum constraint is evaluated using the blended stream density.

Imagine a single blended gasoline product which can be made from five different components.

The properties of these components are shown in the Feedstock Properties grid.

These components each have their own prices. Some of these components also have maxima in terms of purchase amount.

A gasoline sale may occur in weight basis, and in this case the amount is fixed at 10 kt/d.

In another case a reference density is entered. This has the effect of transforming the weight-based price into a volume-based price (using the reference density) and selling the material in volume terms (using the actual blended density). The reference density used is 0.75 g/cc, which has the effect of converting the price of 1500 $/tonne into the price of 1125 $/m3 (1500 * 0.75).

In yet another case a volume-based price is entered. As the price is in volume, the sale occurs in volume. However, since the sale is constrained in weight basis, it is necessary to use the actual blended density to determine the mass amount of material. To calculate the volume price, the weight-based price (1500 $/tonne) is converted using a reference density of 0.75 g/cc. Thus this case is identical to previous reference density case.

When running the cases side by side, it can be seen that when using volume-based pricing there is a greater volume of material produced. This is achieved by reducing the density slightly through the use of lower density components (mostly alkylate).