Blending Plant Detailed Scheduling

Typical Industries for the scenario:

  • blending of lubricants from base oils and additives
  • blending of wax-related products from base components and additives
  • blending of liquid products

Modern blending plants are often designed as highly flexible service units that process base components from various sources in order to create required blended bulk products. In addition to base components that are produced on site in the semi-continuous processing units, they often use base components provided by other production sites, purchased from other vendors or even provided by customers who produce base components but lack the capacity or knowledge for blending and filling.

This means that in addition to base components stored in the output tank farm of the refinery, base components may also enter the blending plant by just-in-time unloading of tank trucks or by transfer from containers, e.g. by a drum decanting unit. This sourcing situation may frequently change for a given range of target products due to adaptation of the provision situation to volatile pricing and availability of base components.

A similar flexibility can be found in additives used in the blending. These can be provided in storage tanks within the blending plant or enter the blending plant by just-in-time unloading of tank trucks or by transfer from containers (e.g. by a drum decanting unit). Quite often, additives need to be prepared prior to use (e.g. heated or pre-mixed with a base component).

A blending plant has a very high "inner complexity" which proves challenging for manual detailed scheduling. However, at the same it has significant potential for gaining efficiency by better scheduling. From the supply chain planning point of view, there is a focus on allocation of the blend orders to the storage tank with consideration of tank farm capacity, the availability of base components and additives, and the coordination with subsequent container filling or bulk loading processes.

"Blending and filling planning" departments are responsible for scheduling the blending, filling and tank truck loading over a short-term planning horizon – based on sales orders and strategic monthly planning figures – with a high rate of flexibility due to volatile sales requirements.

Their business processes include scheduling of storage tank allocations for bulk product storage tank farms, intermediate blend tanks and devices, input and output pipelines as well as interior pipelines and other devices under consideration of limited tank capacities and feasibility of a blend process.

ORSOFT Manufacturing Workbench supports these business processes with the combination of standard APS and MES functions and specific functions that take the oil industries' special requirements into account.

Blending Process and Technology

Blending plants may combine a variety of blending technologies and pre-mixes. In the first stage, all base components are pumped into an intermediate tank that works like a batch blender and quality inspection is done on the resulting "base blend". Once the base blend has been approved, additives can be added. Liquid or low-viscosity additives or additive pre-mixes provided in storage tanks can be injected in a simultaneous metered blend system where they are combined with the base blend and either transferred to the final bulk storage tank or to a mixer tank. Solid or high-viscosity additives in containers need to be pre-heated, transferred from their containers (e.g. by a drum decanting unit) and pumped into a mixer tank using a fraction of the base blend. The mixer tank acts as the final batch blender. Once the blend is completed, it can be pumped into the final bulk storage tank where the finished product is again subject to quality inspection. The storage tank is the source for subsequent container filling or bulk loading processes.

In certain stages, the blending process may be interrupted prior to the next operation, thus allowing the consideration of non-working times and operation of the blending plant under various work schedules as well as giving a flexible time buffer for scheduling of the operations. For example, base blends can remain in an intermediate tank or final blends can remain in a mixer tank.

Any transport of input components or intermediate materials within the blending plant is done by use of piggable pipes, which may become bottlenecks and thus need detailed scheduling. Certain pipes may only be used with certain product groups, and there may be limited connectivity at a given time due to e.g. groups of tanks using the same input or output pipe. Intermediate tanks may have ranges of different sizes and different characteristics that make them suitable only for certain product groups (or even for certain input component characteristics).

Thus, a modern blending plant can be characterized as a true "multi-purpose batch plant" – a category known to require one of the most challenging scheduling models in scientific research for chemical systems engineering and that does not allow for efficient automated solutions in most cases.

Blend Order Creation and Scheduling

Blend orders are created and maintained in an ERP system with a target product, quantity and requirement date.

The challenge for the planner is to manually allocate a blend order to a free – and suitable – storage tank while the previous blend process operations should be scheduled in a technologically feasible way without needing manual interaction.

In the scheduling process it may be necessary to consider all detailed equipment of the blending plant, such as pipelines, drum decanting units, simultaneous metered blenders, intermediate tanks and mixers.

Maintaining all these details in an ERP system is often not desirable or even impossible as it would significantly complicate the ERP system in terms of master data management as well as potentially in terms of production execution (the latter is usually already handled in a process control system in full detail).

The hallenges of Flexible Sourcing and Sizing

Due to the volatile sourcing situation, there may be frequent changes in the source storage location for the input materials of the blend orders. Even for the exact same target product in another week, the final decision on which exact base component input sources to use is quite often dependent on quality inspection results and made by the lab even after the blend order has been scheduled. Reflecting these frequent changes in the master bills of material is highly undesirable – it would be necessary to modify the bills of material for all target products if the sourcing situation for one of their components changed.

The specific sourcing situation of a blend order, even if the bill of material remains unchanged in terms of material numbers and input quantities, influences the possible allocation path the blend order may take through the plant; for example, using pig pipelines from the base component tank farm, drum decanting units, simultaneous metered blenders and mixers. This flexibility is difficult to model in a typical ERP master routing or master recipe. It would be necessary to do it for each target product and modify it for all target products if the sourcing situation for one of their components changes.

Order Rooting
Order Rooting

Furthermore, blend orders for the same target product can have different batch sizes, which results in different durations of the individual order steps. Durations for some steps may depend on throughputs or pump speed relative to the quantity that is transferred through the device, and the quantity may be only a fraction of the total batch quantity. It may be difficult to model these durations and their quantity-dependent calculation in a typical ERP master routing or master recipe.

All of these challenges are best to be resolved by the introduction of an APS system with a highly detailed yet material-neutral blend recipe model in combination with rules-based scheduling.

Blend Recipe Model and Blend Order Model

ORSOFT Manufacturing Workbench introduces blend master recipes and blend orders which allow a very detailed modeling of the blend process. Unlike how it is typically done in classic ERP systems, master recipes do not contain any reference to materials, neither as input components nor as target products.

In addition, the material master is extended by some fields to maintain certain characteristics for the target products and sourcing information for the input materials.

This way, even for a blend plant with 500 different target products and 1500 different input components, a few dozen highly complex blend master recipes may be sufficient. While it may take some time to create these recipes and model all technological restriction and flexibility into them – especially for a new plant – their structure will hardly ever need to be adapted later. Only the duration timing of steps may need adjustment with production reality from time to time in order to improve accuracy of the schedule.

The names of existing blend recipes in ORSOFT Manufacturing Workbench must correspond to the names of master routings in the ERP. Furthermore, the ERP master routings must use control keys that correspond to some control keys within the blend recipes.

If a blend order is created in the ERP, it gets its own ERP order bill of material and ERP order routing- based on ERP master data.

This complete ERP order is loaded by ORSOFT Manufacturing Workbench and a blend process order is created. In this process, the ERP data is matched with the detailed blend recipe and the blend process order is "enriched" with the detail operations and steps of the blend recipe. Only then is the information on target product and input components, which are taken from the ERP order, injected into the detailed order steps.

Rules-based Preparation of the Blend Order Prior to Scheduling

ORSOFT Manufacturing Workbench can be customized with a pre-scheduling rules evaluator that analyzes the blend process order and executes various rules-based decision-making processes and calculations on it the moment a user selects a blend process order and drops it at a target tank at a desired time within a Gantt chart. The rules-based evaluator prepares the blend process order so that it is suitable for the standard scheduling algorithms provided by ORSOFT Manufacturing Workbench.

Dynamic Selection for Intermediate Tanks and Blend Plant Devices

In addition to the pre-scheduling rules evaluator, it is also possible to define expressions for the dynamic selection of candidates for the resources of blend order steps with dependency of target material properties, input component properties and decisions made during scheduling other steps of the same order. These expressions are evaluated by the standard scheduling algorithm during each trial.

Detailed Scheduling of Blend Orders

A blending scheduling workframe within the ORSOFT Manufacturing Workbench – using Gantt charts for devices and pipes, combined histograms and Gantt charts for storage tanks and detailed schedule editors – supports the planner in maintaining blending schedules and monitoring resulting allocations and stock levels at the finished bulk products tank farm.

Blending Scheduling Workframe
Blending Scheduling Workframe

The screenshot (click to enlarge) shows how the blending scheduling workframe helps optimize blend schedules, utilize resources to their maximum capacity and observe all restrictions (tank limits etc.) at the same time. The Gantt chart on the left hand side shows the allocation of pipes, intermediate tanks and blending equipment for a blend order within the blending plant in conjunction with other orders on the same as well as similar devices. The upper right windows shows a chart of similar bulk storage tanks – suitable for a certain range of target products and batch sizes with a combined visualization of allocation and stock levels. This chart displays not only how the allocation of the blend order will increase the stock level, but also how the allocation of filling orders or tank truck load allocations will decrease the stock level. The lower right window shows a list of scheduled blend orders. It is possible to display a details editor for a single blend order.