An environmentally efficient solution for starch cooking

In paper and board production, starch is used at various stages for drainage control, strength enhancement, dust prevention, and pigment binding. Starch achieves desired properties like viscosity and solids content during cooking. For decades, the conventional method of starch cooking has remained unchanged. The last major innovation was enzymatic conversion. Valmet has delivered over seventy conventional processes since 2004, producing stable quality starch. However, sustainability trends and targets to reduce energy and raw material usage have driven the rethinking and improvement of the starch preparation process, leading to a new cooking process and equipment.

Challenges with conventional cooking process

The conventional starch cooking process often starts and stops based on storage tank limits, leading to constant start and stop sequences and requires frequent system flushing. This not only wastes time, significant amount of water, and energy, but also generates effluent and dilutes the starch-water transition. The load put on water treatment is also high.

Quality consistency is compromised due to these start-stop sequences and process conditions on site as where the temperature is most important. The temperature is affecting starch viscosity, strength, and bacterial growth. The duration of the process start-up and transition period finally determines the amount of wasted starch or the amount of poor end product quality due to substandard chemical properties of the starch.

The process also wastes energy by venting exhaust steam after the post-cooking and steam separator. Additionally poses safety and environmental risks, including hot surfaces, dusting of starch powder, and potential explosions.

Mills often aim for capacity flexibility to accommodate multiple production lines or wide product ranges. However, conventional systems are dimensioned for specific consumption levels, leading to more start-stop sequences during low consumption periods and potential bottlenecks during high consumption. Adjustments often result in non-optimal process dimensioning, causing flow and quality issues.

Introducing Zero Effluent cooking

Zero Effluent cooking process, designed to address conventional cooking challenges, consists of two compact units, slurry- and cooking unit. The same units can be used to manufacture surface starch as well as binder starch for coating. Capacity is now a range since one process is capable cooking from 20 to 120 Tons / day of dry solids starch. By adding multiple units into the system, even higher capacities can be reached and in suitable conditions, the system can supply starch for more than one production line.

The compact design optimizes the on-site footprint and resource efficiency. The process includes innovations that reduce emissions through savings in raw materials, energy, and water, and lessens wastewater management load.

Re-engineering the entire process

The re-engineered process addresses even minor details for optimization. Slurry tank was redesigned with lamella construction dividing the tank into separate compartments where starch powder is applied only in the sealed space in order to eliminate dusting. Starch slurry return flow is guided along the tank walls to prevent any powder accumulation leading to starch lumps in the dissolving. Air escaping from the tank is led out through a vibrating dust filter. With optimized tank design and level control, the amount of residual slurry is minimized close to zero in case the process needs to be stopped.

The compact design cooking unit consists of converting tank, post-cooking pipe and heat recovery tank which are built in the main frames of the unit. Main steam manifold is located between the tanks, and all this is covered with 100 mm thick insulation panels in order to minimize heat losses in the process. Valves, piping, instrumentation and automation cabinet is integrated to the unit outside of the insulation and are fully preassembled before shipping to customer site.

The post-cooking pipe was completely redesigned for uniform and homogeneous flow. Flow analysis of the conventional post-cooking pipe pointed out the uneven velocity distribution. Some areas inside the pipe had velocity over 1,0 m/s while the mean velocity was 0,55 m/s meaning that some of the starch may go through the pipe almost twice as fast while at some parts of pipe starch hardly moves.

Heat recovery is now part of the process, collecting excess cooking energy via condensation for hot water preparation, eliminating the need for an external system. This hot water is used for starch dilution, Sizer washing, rod lubrication, internal system flushing for the system, and other applications. A heat exchanger may be needed for applications intolerant to minor starch residue.

With capacity of a 120 Ton/day dry solids starch powder 1000 Ton CO2 emission is compensated by heat recovery giving also monetary return. When considered steam price of 20 EUR/ton, the annual saving is calculated at roughly 100 000 EUR.

Key innovation - patented circulation loops

The key innovation in the Valmet starch cooking unit ZE is the patented circulation loops. The circulations are slurry circulation and post-cooking circulation. The loops can operate independently for extended periods without disruption, maintaining optimal temperatures and flows, and ensuring consistent starch quality. The slurry circulation is constantly running at same maximum capacity and controlled with the mass-balance principle. The feed to the cooking process is regulated by a control valve, and the slurry is supplied based on the actual starch consumption, making the process truly continuous. Sedimentation is prevented by turbulent flow. Flow stays turbulent throughout the entire capacity range and remains turbulent when cooking is stopped.

During a consumption stoppage, the post-cooking circulation maintains a constant flow, prevents starch retrogradation. The starch temperature is adjusted with a steam injector, the change of solids content can then be compensated on end dilution. This temperature control is the key in keeping the starch strength properties at optimal level and even enables re-cooking of starch. There is no long circulation line to production tank, instead the final dilution is done in the unit with one-way lines to production tanks. 

These continuous circulations eliminate the need to stop the cooking process due to production line stoppages, reducing wasted starch, water, and water treatment load. The circulations can run without consumption of starch over 24 hours or even several days depending on the conditions and additives used.