In a traditional batch-blending process, the final product composition is created by combining different intermediate products (held in storage tanks) in a blend tank. The objective is to create final products that meet customer specifications. However, in many process manufacturing applications, tankless, inline blending may provide a better solution, particularly in grassroots process plants or for expansion projects in existing plants.

Inline blending involves continuous mixing of two or more intermediate products using flowmeters and control valves, to obtain a final product of strictly defined proportions. In theory, inline blending could enable process plants to save money by reducing the blend time, the need for excessive storage capacities and mix tanks, maintenance manpower and costly quality giveaway. Although not as well-established as batch blending, inline blending has also been around for years.

Blending systems. There are two basic types of inline blending systems: controlled-rate and flow-responsive systems. In controlled-rate systems, the flowrate is set by a blend controller and either manually or automatically controlled. The flowrate of the feed streams is maintained as the desired ratio of the component in the end product. Flow-responsive systems utilize the main feed stream as a constant to which all other intermediate stocks are blended at the desired ratios.

Fig. 1. Typical inline blending system.   Source: Jiskoot Quality Systems.

Product and process optimization.

Blending involves numerous issues. What finished products are presently in demand by the market? What intermediate stocks and additives are required to make those finished products, and are they available? And, of course, which product will net the biggest profit? This represents a challenging optimization problem; it requires close coordination between marketing and operations groups.

Inline blending can help simplify this problem to a certain degree. Rather than holding several intermediate products in storage tanks where they are blended one at a time in a mixing tank, analyzed, re-blended (as needed), touched up and reanalyzed prior to delivery to the customer, inline blending allows the product to be analyzed continuously as it is being blended (enabling corrections to be made online as needed) and loaded directly to a truck, rail or tanker ship for delivery to the customer.

To achieve this, an inline blending system is typically comprised of two or more feed streams, each fitted with a strainer, flowmeter and control valve. As the feed streams are combined, the turbulence created is generally not enough to mix the components properly, often requiring an inline mixer to be utilized in the process. Once the intermediate products have been blended, an inline analyzer with a set trim point (such as density or viscosity) ensures that the final product meets minimum quality specifications. The analyzer and blend controller monitor the flow, ratio and trim of each stream continuously. Some inline blending unit suppliers also provide sampling features that take samples at set intervals throughout the blending process.

By reducing the need for mixing tanks, inline final product blending eliminates a time-consuming step to the process and can help reduce capital costs for the tanks themselves and labor costs associated with maintaining the tanks. Inline blending can also help increase flexibility and enable products to be blended on demand, rather than being stored onsite in anticipation of delivery.

Reduced quality giveaway.

In addition to reducing the time of the blend process and eliminating the need for separate blend tanks, inline blending also greatly reduces the risk of quality giveaway. As the intermediate products are combined at predetermined ratios and flowrates, and continuously analyzed throughout the process, production of a final product that over-conforms to the specifications of the desired product is minimized.

Typically, when batch blending, operators are overly cautious with their blends to ensure that their final product meets the customers’ product specifications. This can result in costly quality giveaway. When performed properly, inline blending allows the plant to tighten its control on the blending process and to more closely match the required specifications.

To account and compensate for stratification, tank heel or other process disturbances that may cause stream starvation, a trim strategy can be applied to help further ensure the product quality. A trim strategy throughout the process reduces variance in product quality, enhances product homogeneity and helps produce products as close to the desired specifications as possible. A trim strategy is a crucial aspect of the process, since it can help eliminate the need for re-blending or “touching up” the product after the fact. HP

The author
	Kevin Crisafulli has over 10 years’ experience in software and manufacturing industries. He joined ARC in 2006 and holds a BS degree in marketing from Nichols College.