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Solids Handling in Flow

Overview

Suspended solids (intentional or otherwise) are a common feature of many chemical processes. Examples include reactions with catalysts, live cells, slurry feeds, impurities and precipitated products. The absence of low throughput reactors which can handle slurries under flow conditions has severely limited the progress of flow technology in this field.

Handling solids under flow conditions requires reactors with simple internal geometry and channel diameters which are large enough to prevent bridging. Good mixing is also essential to maintain uniform solids/liquid dispersion.

The design characteristics of the Coflore reactor are well suited to solids handling. It uses free moving agitators which generate mixing when the reactor body is subject to rapid transverse movement. Good mixing combined with simple cell geometry and comparatively large inter-stage channels provide an effective means of keeping slurries flowing.

Solid particles are present in many chemical process operations. In a continuous reactor these materials need to flow through the system without blocking or settling. Standard methods for handling homogeneous fluids in flow rarely work when solids are present. This gives the perception that solids are particularly difficult to handle in flow. A small number of materials are inherently unsuited to flow (sticky materials or very large solids) however these are rare. In most cases continuous processes with solids can be handled for prolonged periods without difficulty. This however is contingent upon using the right equipment and set up.

Solids will flow without difficulty when kept in suspension by mixing. Passive mixing relies on high axial velocities to generate turbulent mixing. This generally limits passively mixed reactors to processes where the residence time is short (typically less than 20 seconds). Active mixing keeps solids suspended independently of residence time. Coflore® reactors use active mixing and can therefore handle mixtures of solids, liquids and gases for reaction times lasting from seconds to hours.

Flow control is essential as this is used to regulate the residence time. Control elements such as pumps, valves and mass flow meters however are generally unsuitable where solids are present and these devices should be located in solids free lines.

Solids Bridging in the Process Lines

Bridging occurs when stationary particles in the system trap more particles to form a bridge across the face of the channel. This rapidly leads to a complete blockage. The primary means for preventing bridging relies on adequate channel sizing. A channel with a diameter which is 10-20 times greater than the particle diameter is not susceptible to bridging. This minimum diameter rule applies to all line elements including valves, pumps and instruments. Small pumps, flow control valves and reduced bore instruments such as mass flow meters are generally unsuitable where solids are present.​

Solids Settling in the Process Lines

In horizontal transfer lines, mixing is required to keep solids suspended and moving. For this high axial velocity with turbulent flow is required. The same approach is used for rising lines but the minimum velocity is generally higher. Stagnant pockets where solids will settle out can be eliminated by keeping the diameter of the channels uniform and using long radius bends. The volumetric flow rate is dictated by the required residence time in the reactor and the diameter of transfer lines need to be a sufficient to avoid bridging. In many cases the resulting fluid velocity is insufficient to keep solids suspended in transfer lines into and out of the reactor. These lines need to be minimum length and mounted in a vertical or near vertical position so that solids flow is gravity assisted.

The Coflore® ACR uses ten actively mixed reaction cells in a vertical cascade. Active mixing keeps the solids mobile and moving through the system. Short horizontal channels link the cells. Active movement of the reactor block keeps solids moving between stages. The ACR is generally suitable for solids of up to 100 microns in diameter.

The Coflore® RTR operates as a single-tube, ten-stage, actively-mixed, self-baffling continuous flow reactor.  The RTR is suitable for solids in the order of hundreds of microns in diameter.

The Coflore® ATR is comprised of up up to eight actively-mixed reactor tubes. Active mixing keeps solids suspended and moving through the reactor. Short vertical channels link the tubes to give gravity assisted flow in the transfer lines. The ATR is suitable for solids in the order of hundreds of microns in diameter.

Solids Dosing in the Coflore Flow Reactor Range

SXV

Solids Handling Valve

SXV: for efficient addition of slurries into the ATR

  • Pump 1 is used to control the flow rate of reactant A

  • Pump 2 controls total process flow through pressure drop.

  • Stirred Head Tank pumps slurry through the recycle loop under turbulent flow conditions, maintaining a uniform distribution of solids.

  • Solids Collection Tank collects and removes solids from the product stream

  • SXV feeds slurry into the reactor

  • Gas is fed at the bottom of the reactor

MHR

Multi-Phase Handling Rig

  • Pump 1 is used to control the flow rate of reactant A

  • Pump 2 controls total process flow through pressure drop.

  • Stirred Head Tank pumps slurry through the recycle loop under turbulent flow conditions, maintaining a uniform distribution of solids.

  • Solids Collection Tank collects and removes solids from the product stream

  • SXV feeds slurry into the reactor

  • Gas is fed at the bottom of the reactor

Solids-Handling in Flow. A Case Study

Hydrogenation

AM Technology, Cheshire, UK
robert.field@amt.uk
+44 1928 237420