Filtration

Maximize Plant Throughput by Optimizing Wash Frequency

Fouling and scale buildup is the bane of any processing plant. It restricts capacity, reduces heat transfer, increases cleaning requirements, fuel consumption, and total costs.

In some high fouling industries pumps and pipelines can require cleaning and washing on extremely high frequencies, and the ability to identify when these become a ‘bottleneck’ can increase total plant throughput.

The type of equipment which can foul and require cleaning is vast, and can include anything and everything:

But how can you identify when a piece of equipment’s throughput or performance is degrading?

Every piece of equipment is different, but the key identifier is the same - historic key performance indicator degradation. The Key Performance Indicator (KPI) should be identified for every critical piece of equipment, but can be looked at for individual pipelines as well.

For example, take a heat exchanger with the purpose of recovering as much heat as possible (that is, it is not controlling to a set point):

In terms of fouling and wash requirements, the KPI would be total heat transfer (MW). If the heat transfer is continually reducing over time then the economic breakeven point can be used.

Another example is a simple pump. Looking at the pumping efficiency, or another common identifier is pumping throughput at maximum speed. If 6 months ago a pump was able to achieve 300 kL/hr at 100% speed, but is now only able to achieve 250 kL/hr, then the possibility of a wash should be considered (as well as pumping maintenance). This could be a result of solids buildup on either the pump suction or discharge or both. Looking at line pressures is often enough to identify the location of any restrictions.

Another tip to identify solid buildups within pipes where the fluid is hot is a thermal imaging camera - the buildup will act as an insulation and will result in a cold spot, further proof of washing requirements.

All different pieces of equipment have different KPIs which can be used to identify performance degradation. This degradation can then be tracked and measured to estimate the rate of scale build up, or fouling rate, leading to an improvement in planning and wash frequency.

“Success is simple. Do what’s right, the right way, at the right time”

Arnold Glasow

Washing frequency is often one of the simplest ways to maximize plant performance, throughput, and efficiency. The rule of thumb is to complete the wash before the restriction becomes a restriction.

Filtration Fundamentals - Optimizing and Improving Filter Performance

Filters are generally used to separate the liquid and solids from within a slurry. Their impact on downstream processes are critical and their performance can have a large impact on their entire sites operability. Filters are very well understood, and the ability to optimize them for improved performance is well documented. Although every filter is slightly different they all follow the same principles. The process of optimization should always be done in a systematic manner.

Source: CDEGlobal

Filtration may be defined as the separation of solids from liquids by passing a suspension through a permeable medium which retains the particles.

L. Svarovsky, Solid-Liquid Separation

1. Viscosity

The viscosity of the slurry/filtrate is a key consideration as it will have a significant impact on the total filtration rate. The lower the viscosity the easier the liquid flows, and the less time it takes to be removed from the solid cake. The viscosity of any solution can be controlled in a variety of ways, such as changing the temperature or the concentration. Specialty chemicals, also known as drainage aids, are also available which can change the properties of the solution to improve filterability.

When considering your ability to alter the viscosity to improve the filtration rate a number of questions must be asked:

  • Will the increased energy consumption required to increase the temperature be economically justified by the increase in filter performance?
  • Will the change in filtrate properties have an impact on downstream processes?
  • What will the impact of diluting the solution be?

2. Specific Cake Resistance

The actual resistance of the cake is critical and one of the most important factors. It can be measured relatively easily through lab experimentation, so any changes can be clearly observed when experimenting or in site trials. The resistance caused by the cake can also viewed as the voidage between individual particles.

  • Particle size
  • Particle shape
  • Cake thickness
  • Compressibility

There are also several methods which can be used to reduce the specific cake resistance which leads to improved filterability:

  • Increase the particle size to increase the voidage
  • Use of filter aids - additional larger particles added to the slurry which are used to increase the voidage
  • Reducing the cake thickness - can be a result of reducing the online time or filter speed

3. Medium Resistance

The medium is the physical barrier which separates the solid cake from the liquid filtrate and is one of the easier filter attributes the change. The weave size is chosen to satisfy a combination of filtration rate and filtrate solids. There are lots of circumstances which can change the resistance of the filter medium, including:

  • Cloth blinding
  • Tears and rips

Filter cloths blind when the particles block up the pores, resulting in decreased filter performance. The impact of blinding can be reduced by washing or re-clothing, which can be optimized by identifying the relationship between operating time, particle size, and the blinding rate.

Tears and rips in the medium also have a significant impact because they allow the solids to bypass the filter through the path of least resistance. Changing to a more durable cloth can often be economically justifiable in filter performance increases.

4. Driving Force - Pressure/Vacuum

The appliable pressure on a filter is limited by the maximum operating pressure of the mechanical components or the maximum pressure available from the pumping systems. Additional or larger pumps may not be possible if the filter cannot handle it. Increasing the number of vacuum pumps is often a costly process when considering the change in filtration rate.

Filters are a great unit to work with and operate because they are one of the few systems where their performance can often be visually observed as well as through lab samples and online monitoring.