Feature

Process Intensification Using State-of-the-Art in Separation Technology

Posted on 08 February, 2010 | Tags: Innovative Technologies

With the recent acquisition of the separations technology business of KnitMesh and the existing alliance with Shell Global Solutions, Sulzer Chemtech has expanded its technology within the gas-liquid and liquid-liquid separation business continuously.

-Dr Abhilash Menon &  Daniel Egger

Be it the separation of liquid droplets from a simple gas stream loaded with mist or from a gas-liquid stream under high pressure, the selection of the right technology is essential to achieve the desired performance and specification. It is very important to understand and apply the physics properly within various applications, for e.g. a vane-pack may work properly under normal atmospheric conditions, but in a high pressure hydrocarbon system, the physical properties of the system completely changes, thus rendering conventional mist elimination devices practically useless at times. Also, in many de-bottleneck and revamp cases within the gas, petrochemical and chemical industry, one comes across situations where capacity and separation performance are limited by the type of separation device chosen. This calls for innovative technologies, say for instance, separation of liquid droplets from the gas-liquid mixture by imparting higher centrifugal forces. In many situations, an appropriate combination of technologies yields the optimum techno-economic solution for the case at hand.

Liquid Entrainment
Liquid entrainment in a process gas stream can be formed by either dynamic processes such as contact between gas and liquid phases in a mass transfer operation; or thermal processes such as condensation. Sulzer mist eliminators provide an effective solution to liquid entrainment problems in many types of equipment including scrubbing, absorption, stripping or distillation columns, evaporators, falling film condensers, knock-out vessels, 3 phase separators, desalination plants, refrigeration plants, gas dehydration plants and compression systems.  For best performance, it is important to achieve uniform gas flow distribution and maximum effective area, and the most appropriate positioning is critical, in terms of disengagement distances from vessel inlet, outlet and other vessel internals.
For equipment based on direct and/or inertial interception, gas stream velocity affects all three principles involved in separation (impingement, coalescence and drainage). Flooding, or re-entrainment of liquid, can occur if the flow of gas prevents drainage, and the effective area of the mist eliminator is therefore established by determining an appropriate superficial velocity for the equipment. The overall performance of the mist eliminator is then a balance between efficiency and pressure drop.

where
ν  = maximum superficial gas velocity
ρι = liquid density
ρν = gas density
K = a constant which is specific
to the separation equipment and is a function of process parameters such as:  liquid loading, gas and liquid viscosity, gas pressure, surface tension
Derating factors are often applied to allow a safety margin for exceptional conditions such as liquid slugs and gas surges, and the K-value can be optimized to suit specific process conditions, and challenging physical properties such as low surface tension systems. The selection of K-value is therefore critical. A summary of the relative performance characteristics for mist elimination is listed in Table 1.

Sulzer KnitMeshTM WireMesh Mist Eliminator
Sulzer KnitMesh mist eliminators have an excellent track record as a low cost, highly versatile and efficient method of removing liquid entrainment from gas streams. They are produced as a bed of knitted mesh which presents a tortuous path and large surface area to the droplets entrained in the gas stream. Separation is achieved by impingement on, and capture by, the filaments of the mesh where the droplets coalesce and drain (see Figure 1). Installation can be made in a variety of ways but gas flow is usually either vertically upwards, with the liquid draining countercurrent to gas flow, or horizontal, with the liquid draining in a direction perpendicular to the gas flow. They provide excellent separation efficiency down to droplet sizes as small as 2 μm, and with a pressure drop typically less than 2.5mbar.

Sulzer KnitMesh VKR Mist EliminatorTM
The Sulzer KnitMesh VKR Mist Eliminators provide high performance wherever liquid entrainment must be removed from a vertically flowing gas stream. Mist eliminators with Sulzer KnitMesh VKR technology employ the mechanism of a Von Karman Roll (VKR) around a bluff body (see Figure 2) to obtain high vapour and liquid handling capacity. A vortex develops in a low pressure zone downstream of the channel that has been attached to the bottom of the mesh pad. Collected droplets deposit in the trough and form a flowing liquid stream there. The high capacity channels of the VKR Mist Eliminators shield the collected liquid and then drain it in steady streams from two sides of the mist eliminator at the column wall, where gas velocity is so low that re-entrainment is limited. This simple enhancement to a standard mesh pad improves mist elimination because the higher velocities increase the droplets' inertial impaction with the wires or filaments. In addition, the strategically placed collecting channels provide higher operational gas and liquid flow capacity (see Figure 3). 

Sulzer MellachevronTM Mist Eliminator
Sulzer Mellachevron vane mist eliminators are high capacity inertial separators constructed as banks of parallel, chevron profiles which cause the gas to change direction several times from inlet to outlet. Momentum forces entrained liquid droplets to impinge on the vane surfaces where they form a liquid film and drain. The Sulzer Mellachevron range is divided into a number of categories depending on direction of gas flow and the complexity of the vane profile. Simple Sulzer Mellachevron profiles separate liquid by impingement, coalescence and drainage on the vane surface with no disengagement of the liquid from the gas stream. They are particularly suitable for applications with a significant risk of fouling due to solid particles or high viscosity liquids in the feed but have relatively low gas handling capacity. More sophisticated designs provide special separation channels to allow disengagement of liquid and drainage from the vane surface. This increases the capacity of the separator and gas load factors of up to 0.45 m/s are possible. This makes them an excellent choice when equipment size is critical, for example, in offshore applications or for de-bottlenecking existing equipment.

Combined systems
Increasing gas capacities and higher performance requirements in mass transfer equipment are   challenging the capabilities of conventional mist eliminator equipment. To solve this, Sulzer Chemtech offers combined systems which optimize the benefits of individual types of equipment and improve overall performance. For example (Figure 5), KnitMesh mist eliminators can be used in combination with Sulzer Mellachevron vane packs or Shell Swirltubes to produce very high separation efficiencies at high gas loadings. By using the KnitMesh mist eliminator as a pre-conditioner for the Mellachevron, it is operated above its normal entrainment or flooding point and consequently liquid is stripped away from the downstream surface. The liquid dispersion re-entrained from the mesh mist eliminator has a larger mean diameter and is suitable for subsequent separation by secondary, high capacity equipment. Additional benefits of combined systems include:

• the ability to design the equipment inorder to provide very high turndown capabilities
• at low gas velocities, where high capacity separators tend to be ineffective, the mesh pre-conditioners behave as conventional mist eliminators.

Shell SwirltubeTM
Axial flow cyclonic mist eliminators have been widely adopted in hydrocarbon systems where high capacity is important.  They are often used in high pressure applications where the performance of vane packs tends to deteriorate and where mesh mist eliminators are also undesirable because of their limited capacity - and therefore large vessel sizes.  The Shell Swirltube (Figure 6) is, in essence, a stainless steel tube with a swirler at the inlet and longitudinal slits in the tube wall. Liquid is separated by impaction of droplets and on the tube wall by the centrifugal forces induced by the swirling gas flow. Re-entrainment of this liquid is prevented by draining the film via the slits to the liquid collection chamber outside the tube. To ensure the proper functioning of the Swirltube, it is essential that some gas is also bled through these slits. This gas leaves the liquid collection chamber via the secondary outlets at the top of the Swirldeck assembly. Performance can be improved by using a secondary KnitMesh mist eliminator to separate entrainment from the gas leaving the secondary outlets. The main fraction of the gas leaves the Swirltube via the primary gas outlet at the top. Drain pipes guide the liquid, collected in the space between the tubes and on the upper cover of the Swirldeck, to below the liquid level. Scaling-up of a separator equipped with a Swirldeck is done simply by increasing the number of Swirltubes proportional to the gas flow in the separator.

Shell High Capacity Separators
Shell proprietary high-capacity gas/liquid separators, such as Shell SVS™, SMS™,  SMSM™ and SMMSM™ separators, combine the best features of the separation products from Sulzer Chemtech's portfolio. Shell gas/liquid separators combine Schoepentoeter(S), KnitMesh (M) and Swirldeck (S) separation technologies in a single vertical vessel drum. Selection of separation technologies is function of the dispersed liquid phase concentration, droplet size and the required separation efficiency. Their typical application window is for separation of liquid/gas or three-phase liquid/ liquid/gas mixtures. When a high capacity and high separation efficiency is required, SMS(M) technology is the lowest weight and volume solution for phase separation offshore and onshore. It gives up to 2.5 times more capacity compared to a conventional KnitMesh mist eliminator without vessel replacement. Shell SMS, SMSM and SMMSM gas/liquid separators (Figure 7) are named after the configuration of the different internals used for each type of separator:

• Schoepentoeter (S) - used as feed inlet device for vapor distribution with bulk liquid removal
• KnitMesh (M) - which acts as Coalescer and separator, depending on the gas flow rate
• Double Primary KnitMesh (MM) - specially developed for applications with two immiscible liquid phases in the feed to the separators (such as glycol/condensate in Dew-Point Separators)
• Swirldeck (S) - comprising multiple Swirltubes

A second KnitMesh (M) is used downstream of the Swirldeck in SMSM gas/liquid separators for demisting secondary gas. In fouling or waxy service, the KnitMesh can be replaced by a Sulzer Mellachevron vane pack (SVS systems).

Daniel Egger Sulzer Chemtech AG Switzerland

 

 

Dr. Abhilash Menon Sulzer Chemtech AG Switzerland