Choosing Heat Exchangers for Viscous Wastes & Sludges

Heat exchangers are an essential bit of kit for a wide range of applications in companies across the wastewater, anaerobic digestion and waste treatment sectors, including heating, pasteurisation and evaporation. There are numerous types of them, from straightforward plate heat exchangers to corrugated tube-in-tube and scraped surface designs. 

Since there is wide variation in the composition of the sludge, digestate, manure and wastewater streams, it is thus vital to select the suitable heat exchanger for the specific material and process, not only for efficient processing and high product quality but also for optimized operation and energy efficiency.

The high viscosity prevalent in the majority of organic waste streams means that they take more energy to transport and pump. They also clog heat exchangers and pipes, thereby reducing heat transfer efficiency and general performance.

What is viscosity?

In assessing different types of sludge, it is informative to establish what viscosity is. It is usually best described as a measure of the tendency of a fluid to resist flow when force is applied to it. More precisely it relates force applied to make one layer move relative to another (shear stress) to the speed of movement of the layers (shear rate). 

It is quantified in units called centipoise (cP), where 1 cP equals 1 mPa·s (millipascal seconds).

As would be expected, digester and sludge viscosities rise with increased solids content as well as change widely with temperature. As an example, a sewage sludge with 2% solids might have a viscosity of about 5 cP, while a similar material with 10% solids might be up to 50 cP. 

As these materials are so distinct, these figures are estimates. Liquid manures can range from potentially 20 cP at 2.5% total solids (TS) to 500 cP at 12% TS.

But in the presence of fluctuating shear stress and temperatures, the viscosity of a particular substance may change. Hence, the majority of fluids fall into either a Newtonian or non-Newtonian category. Newtonian fluids have uniform viscosity regardless of shear rate or temperature change, water being the most frequent example. 

Non-Newtonian fluids (which can be further subdivided into five types) have viscosities that are a function of the applied shear rate.

In practice this will result in that when handling non-Newtonian materials (i.e. most slurries and sludges), each phase of the operation, from heating and pumping through to cooling and travel through pipework, will be able to influence the viscosity of the waste stream and thus the design and operational performance requirements.

Corrugated tube heat exchangers: Design & selection

Selection of the most suitable type of heat exchanger, coupled with system design, facilitates fouling prevention and efficiency retention. HRS-designed and made corrugated tube heat exchangers facilitate more efficient processing of viscous products. Corrugated design minimizes fouling by enhancing mixing and turbulence, which results in better thermal performance when in service and increased clean-out intervals. 

Moreover, corrugated tube exchangers consume less power for pumping than the smooth tube varieties because they are aerodynamically streamlined and have lower pressure drop. This contributes to longer service life and lower maintenance cost compared to other exchangers.

If the sludge possesses relatively low viscosity but it contains suspended solids, then a double-tube (tube-in-tube) heat exchanger such as the HRS DTI Series is appropriate. 

Having two tubes mounted in tandem, the large tube is employed for the product and the possibility of blockage or fouling by particulates within the sludge is minimized. 

Energy recovery from lower viscosity sludges is better accomplished with the HRS DTIR Series; this is equipped with a clean-out inner tube that makes it easy to clean and inspect.

Scraped surface heat exchangers for the most challenging applications

Even corrugated tubes may prove to be insufficient for very viscous streams to prevent fouling or to promote flow. Scraped surface heat exchangers (SSHEs) are the ideal solution in such cases, especially for evaporation.

For most applications, HRS Unicus Series is used. This uses a reciprocating movement to agitate the fluid and continuously scrape the heat transfer surface. 

The hydraulic system is independent of the scrapers themselves, hence the scraper speed can be optimized. This makes the Unicus particularly well-suited to concentrating brines, manures, food wastes and wastewater streams.

There is a special version of the Unicus specifically dedicated to evaporation applications. In evaporation operations fouling and heat transfer degradation are common issues in conventional evaporators. 

The scraping action of the Unicus keeps the surface clean as well as with high heat transfer, and thus has the ability to concentrate waste streams to above the ability of a number of conventional processes. 

This renders it appropriate for volume reduction of environmental wastes. Unicus evaporators can be employed in multi-effect configurations or combined with mechanical vapour recompression. These also give concentration under vacuum conditions, and the continuous scraping gives continuous utilization with minimal downtime.

Special sludge and digestate concentration systems

In addition to the Unicus, HRS also offers the Digestate Concentration System (DCS) as a special system to remove up to 80 % of water content from sludge and digestate, leaving material with about 20 % dry solids.

The DCS superheats the digestate under vacuum to allow evaporation and thereby achieve significant volume reduction while at the same time concentrating nutrients. At the first stage, liquid digestate is preheated using plant available heat, in most instances waste heat from a CHP engine at ~85 °C. 

No supplementary water or external energy is required. The hot digestate is then pumped into a cyclone separator: the rotating air stream causes the heavier solid particles to exit the airflow and accumulate at the bottom, from which they are withdrawn.

The vapour generated during the first cycle (usually around 70 °C) is then reused as the heating medium for the second effect; the vapour generated (around 60 °C) is used to preheat the third effect, etc. The number of the effects (typically not more than four) is a function of target solids content and availability of waste heat. 

After the last effect the steam is condensed to liquid which may be recycled to dilute feedstock that enters the digester, a closed loop. When coupled with an on-site CHP plant, the DCS can be material and energetically self-sufficient: no external inputs of water or energy are needed, nothing is wasted, and all products are recycled.

For more information or to speak with our environmental systems experts today in your local area.

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