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Design Of A New Liquid Liquid Hydrocyclone Geometry Engineering Essay

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Published: Mon, 5 Dec 2016

Abstract

A Liquid to Liquid hydrocyclone is a static machine that use centrifugal force and apply it on the liquid mixture which will make the separation of heavy and between the mixture components (light and heavy components) of this liquid.

A Liquid to Liquid hydrocyclone will normally consist of the three parts

Cylindrical section

Conical base

The angle

The key difference between the centrifuges and Liquid to Liquid Hydrocyclones that the Hydrocyclones are passive separators where it capable to apply the modest amounts of centrifugal force, but the centrifuges are called dynamic separators which are generally able to concern more centrifugal force than The Hydrocyclones. Another difference between hydroclones and centrifuges devices is the cost where the Centrifuges are expensive machines because its often need sophisticated control but the Hydrocyclones doesn’t contain moving parts and it usually doesn’t contain controls systems and because of this it at all so they are lesser cost devices

There are any types of a hydrocyclone where it could be used to separate solids from liquids or to separate liquids of unlike density.

This project aims to Make and generate new design for liquid/liquid hydrocyclone to use it in the process of separation of light dispersed phases to overcome all disadvantageous of the old designs of similar systems the features and benefits of this design are to include a compact design with high efficiency with construction materials that provide superior corrosion and erosion resistance for longer design life.

CHAPTER 1:

INTRODUCTION

Introduction

A Liquid to Liquid hydrocyclone is a static machine that use centrifugal force and apply it on the liquid mixture which will make the separation of heavy and between the mixture components (light and heavy components) of this liquid.

A Liquid to Liquid hydrocyclone will normally consist of the below three parts (see figure 1)

Cylindrical section

Conical base

The angle

DSeriesCyclone3 2

Figure : Diagrams of a Hydrocyclones

The key difference between the centrifuges and Liquid to Liquid Hydrocyclones that the Hydrocyclones are passive separators where it capable to apply the modest amounts of centrifugal force, but the centrifuges are called dynamic separators which are usually able to concern more centrifugal force than The Hydrocyclones. Another difference between hydroclones and centrifuges devices is the cost where the Centrifuges are expensive machines because its often need sophisticated control but the Hydrocyclones doesn’t contain moving parts and it usually doesn’t contain controls systems and because of this it at all so they are lesser cost devices

There are any types of a hydrocyclone where it could be used to separate solids from liquids or to separate liquids mixture of unlike density.

The hydrocyclone is used in various applications in many industries, from degritting sewage sludge to removing oil droplets from produced water. The governing principles are difficult to quantify because of the complexity of the fluid dynamics with multiple phases in highly swirling flows. The majority of applications are in the processing of mineral ores however, and experience has helped develop a basis for predicting the hydrocyclone classification performance in these duties. The factors that affect performance, both process and hydrocyclone design, will be covered in this paper. The focus will be on providing information that an engineer who is designing a hydrocylone system will find useful.

A cutaway of a hydrocyclone is shown in Figure 2. The slurry enters the area of the hydrocyclone called the inlet head from the inlet feed pipe. The slurry is introduced next to the wall of the cylindrical inlet, which induces a swirling action.

Figure : Hydrocyclone Cutaway [1] 

This action helps develop the inertial forces that enable the classification of particles within the hydrocyclone. The slurry is further accelerated in the conical sections of the separator. The swirling action produces a low-pressure vortex in the center of the hydrocyclone where the finer, lower-mass particles migrate. The relatively light particles are removed with the overflow stream by an upward swirling flow through the vortex finder. The heavier particles are removed with an underflow stream by a downward swirling flow through the apex region of the hydrocyclone classifier.

Figure : Hydrocyclone, Tangential Velocity Distribution [2] 

Figure : Hydrocyclone, Tangential Velocity Distribution [3] 

Figure : Hydrocyclone, Tangential Velocity Distribution [4] 

Figures 3 and 4 shows the mean axial and tangential components of the velocity at different cross-sections in the upper portion of a 250-mm diameter hydrocyclone (Petty et al., 2002). These single-phase numerical calculations were developed using the Reynolds averaged Navier-Stokes (RANS) equation, and standard transport equations for the Reynolds stress (RSM model) and the turbulence dissipation. The simulation imposes a backpressure on the overflow and underflow streams to avoid the air core. The Reynolds number based on the effective diameter of the feed entry and the volumetric flow rate of the feed stream is about 200,000. Figure 5 shows the pressure distribution predicted by the simulation.

The results, which are qualitatively similar to experiments by Kelsall (1952) and to multi-phase flow calculations reported by Devulapalli and Rajamani (1994), predict a Rankine vortex flow with a maximum tangential velocity near the radius of the vortex finder (see Figure 4). This feature distinguishes hydrocyclone flows from other swirling flows encountered in centrifugal separators. As illustrated by Figure 5, the swirling action of the flow field causes a lower pressure to develop in the core of the hydrocyclone. It is noteworthy that the Computational Fluid Dynamics (CFD) simulation captures the important qualitative flow features of a hydrocyclone classifier.

Applications of Liquid to Liquid Hydrocyclone:

In pulp and paper mills.

In the sector of water treatment industry.

In petroleum industry sector to separate oil from water or water from oil.

In Food industries.

In chemicals industries.

Basic Parameters for Standard Cyclone

The definition of a “standard cyclone” is that cyclone which has the proper geometrical relationship between the cyclone diameter, inlet area, vortex finder, apex orifice, and sufficient length providing retention time to properly classify particles. As with the involutes type design, the graphs and mathematical relationships shown for proper selection and sizing of cyclones apply to the “standard cyclone” geometry. The main parameter is the cyclone diameter. This is the inside diameter of the cylindrical feed chamber. The next parameter is the area of the inlet nozzle at the point of entry into the feed chamber. This is normally a rectangular orifice, with the larger dimension parallel to the cyclone axis. The basic area of the inlet nozzle approximates 0.05 times the cyclone diameter squared. The next important parameter is the vortex finder. 3.jpg

Figure : Hydrocyclone Cutaway [5] 

The primary function of the vortex finder is to control both the separation and the flow leaving the cyclone. Also, the vortex finder is sufficiently extended below the feed entrance to prevent short circuiting of material directly into the overflow. The size of the vortex finder equals 0.35 times the cyclone diameter. The cylindrical section is the next basic part of the cyclone and is located between the feed chamber and the conical section. It is the same diameter as the feed chamber and its function is to lengthen the cyclone and increase the retention time. For the basic cyclone, its length should be 100% of the cyclone diameter. The next section is the conical section, typically referred to as the cone section. The included angle of the cone section is normally between 100 and 200 and, similar to the cylinder section, provides retention time.

Figure : Involuted Feed vs. Tangential Feed [6] 

The termination of the cone section is the apex orifice and the critical dimension is the inside diameter at the discharge point. The size of this orifice is determined by the application involved and must be large enough to permit the solids that have been classified to underflow to exit the cyclone without plugging. The normal minimum orifice size would be 10% of the cyclone diameter and can be as large as 35%. Below the apex is normally a splash skirt to help contain the underflow slurry.

Construction of the Liquid to Liquid Hydrocyclone:

A typical Liquid to Liquid hydrocyclone made of a conically shaped vessel this vessel open at its apex or underflow this is fixed to a cylindrical section which has feed inlet at the tangent. The top of the cylindrical section is congested with a plate which exceed through the axial mounted pipe of overflow and the pipe is lengthen into the body of the hydrocyclone by small removable section known as the vortex finder the function of this vortex finder to prevent the short-circuiting feeding directly into the overflow. The bottom of the vortex finder is protruding below the feed chamber. The feed chamber and the cones are lined inside with the rubber or synthetic linings due to the abrasive nature of most metallurgical slurries.

The lined material is made from hard rubber such as neoprene or urethane and the apex is fixed with a concentric hardwearing synthetic rubber (See Figure 8).

Figure : Construction of the Hydrocyclone

Working Principle of Liquid to Liquid Hydrocyclone:

The Liquid to Liquid hydrocyclone generally is a closed vessel that designed to make conversion for the incoming velocity of the liquid into rotary motion. This is achieved by direct inflow tangentially near to the top of a vertical cylinder where this will spins the entire contents of the cylinder and creating centrifugal force in the liquid so that the Heavy Liquid will go Outward toward the cylinder wall, where they farm and a spiral down the wall to a port in the bottom of the ship and in the light of Liquid will move toward the axis of the hydrocyclone (see figure 9) where they will move toward the outlet which is exist at the top of the vessel.

1

Figure : Working principle of the Liquid to Liquid Hydrocyclone

CHAPTER 2:

SIZING AND SELECTION OF HYDROCYCLONES

Performance

In determining the proper size and number of cyclones required for a given application, two main objectives must be considered. The first is the classification or separation that is required, and the second is the volume of feed slurry to be handled. Before determining whether these objectives can be achieved, it is necessary to establish a base condition as follows:

1. Feed liquid – water at 20O C.

2. Feed solids – spherical particles of 2.65 sp gr.

3. Feed concentration – less than 1% solids by volume

4. Pressure drop – 69 kPa (10psi).

5. Cyclone geometry – “standard cyclone” as described above

Classification

Historically, classification has been defined as the particle size of which 1% to 3% reports to the cyclone overflow with coarser particles reporting to the cyclone underflow. Recent investigations have defined classification as the particle size of which 50% reports to the overflow and 50% to the underflow, or the so-called D50C point. Figure 10 shows the typical relationship between particle diameter and the percent recovered to underflow. The portion of the curve near the 50% recovery level is quite steep and lends itself readily to determining an accurate particle diameter. Examination of the recovery curve near the 97% to 99% recovery level shows that the curve is nearly horizontal and a small differential could change the micron diameter considerably.

Figure 11 also shows that the actual recovery curve does not decrease below a certain level. This indicates that a certain amount of material is always recovered to the underflow and bypasses classification. If a comparison is made between the minimum recovery levels of solids to the liquid that is recovered, they are found to be equal. Therefore it is assumed that a percent of all size fractions reports directly to the underflow as bypassed solids in equal proportion to the liquid split. Then each size fraction of the actual recovery curve is adjusted by an amount equal to the liquid recovery to produce the “corrected recovery” curve shown in Figure 10. As the D50C point changes from one application to another, the recovery curves shift, along the horizontal axis.

Figure : Particle Diameter VS. Particle Recovery [7] 

In order to determine a single graph which represents the corrected recovery curve, the particle size of each size fraction is divided by the D50C value and a “reduced recovery” curve can be plotted, as shown in Figure 11. Investigations have shown that this curve remains constant over a wide range of cyclone diameters and operating conditions when applied to a slurry containing solids of a single specific gravity and a typical or normal size distribution such as those encountered in most grinding circuits. Equation 1 gives a mathematical relationship which can be used to calculate the reduced recovery. This recovery, along with the bypassed solids, is used to predict the complete size distribution for the underflow product.

Where

Rr = Recovery to underflow on corrected basis.

X = Particle diameter /D50C particle diameter.

Figure : Reduce Recovery [8] 

In designing comminution circuits the objective is to produce an overflow from the cyclone which has a certain size distribution, normally defined as a given percent passing a specified micron size. An empirical relationship shown in Table 1 is used to relate the overflow size distribution to the D50C required producing the specified separation. The relationship of this table is for typical or average grinding size distributions and may vary slightly

Depending upon the grinding characteristics of the ore itself. The separation a cyclone can achieve can be approximated using Equation 2. The D50C (base) for a given diameter cyclone is multiplied times a series of correction factors designated by C1, C2, and C3.

Table : Relationship of D50C to Overflow Size Distribution

Required Overflow Size Distribution

(percent Passing) of Specified Micron Size

Multiplies (To be Multiplied Times Micron Size)

98.8

0.54

95.0

0.3

90.0

0.91

80.0

1.25

70.0

1.67

60.0

2.08

50.0

2.78

Example: Produce an overflow of 80% passing 149 microns (100 meshes).

Multiplier from Table 1 at 80% passing = 1.25.

Micron size for application = 149 microns (100 mesh).

D50C required = 1.25×149 = 186 microns for application.

This D50C (base) is the micron size that a “standard cyclone” can achieve operating under the base conditions and is given in Figure 12 or calculated from Equation 3. For example, a 25.4 cm (10 in.) diameter cyclone has a base D50C point of 24 microns.

Figure : Cyclone Diameter V.s D50 (For “Typical” Cyclones) [9] 

Where D = Cyclone diameter in cm. The first correction (C1) is for the influence of the concentration of solids contained in the feed slurry. The graphical representation of this correction is shown in Figure 13 and can be calculated using Equation 4. Figure 13 indicates that the level of percent solids is extremely important in determining the proper separation, as the higher the concentration the coarser the separation. It should be pointed out that this correction is a relative measure of slurry viscosity and is affected by such things as the size of particles present as well as particle shape. For example, a feed that contains a large amount of clay would tend to shift this curve to the left and result in a coarser separation, whereas the absence of fines would shift the curve to the right and result in a finer separation. Many other variables such as liquid viscosity also affect this correction.

Where

C1 = Correction for the influence of cyclone feed concentration.

V = Percent solids by volume of cyclone feed.

Figure : Correction for Feed Concentration [10] 

The second correction is for the influence of pressure drop across the cyclone as measured by taking the difference between the supply or feed pressure and with overflow pressure. Pressure drop is a measure of the energy being utilized in the cyclone to achieve the separation. It is recommended that pressure drops, whenever possible, be designed in the 40 to 70 kPa (5 to 10 psi) range to minimize energy requirements as well as reduce wear rates. This is especially true for coarse separations usually associated with primary or secondary grinding circuits. The correction for pressure drop is shown in Figure 14 and can be calculated from Equation 5. As indicated, a higher pressure drop would result in a finer separation and lower pressure drop in a coarser separation.

Where

C2 = Correction for influence of pressure drop.

∆P = Pressure drop in kPa.

Figure : Correction for Pressure Drop [11] 

The next correction is for the effect that specific gravity of the solids and liquid have on the separation. Since the cyclone does not actually achieve a size separation but rather a mass separation, the specific gravity of the particle is extremely important in determining the separation. It is especially meaningful in applications where the mineral has a higher specific gravity than the gangue material which allows better liberation of mineral particles at a coarser overall separation size. It has been found that Stoke’s law can be applied to determine particle diameters which would produce the same terminal settling velocity for a particle of known specific gravity in a liquid of known specific gravity as compared to a particle of 2.65 specific gravity in water. This relationship is shown in Figure 15 and can be calculated using Equation 6.

Where

C3 = Correction for influence of specific gravity

GS = Specific gravity of solids

GL = Specific gravity of liquid (normally 1.0)

Figure : Correction for Solids Specific Gravity (in water) [12] 

The cyclone diameter, along with the three corrections of percent solids, pressure drop, and specific gravity, are the main variables necessary for preliminary sizing and selection of cyclones. Other variables, such as the vortex finder and inlet size, also have an effect on separation. For example, a larger vortex finder size would tend to coarsen the separation, whereas a smaller size would tend to achieve a finer separation. Due to this fact, most cyclones have a replaceable vortex finder with different sizes available. Vortex finder diameters vary from a minimum of about 25% of the cyclone diameter to a maximum of about 45%. The inlet area also shows the same effect as the vortex finder, but not as pronounced. The apex size also has an effect on separation but the effect is minor unless the apex is too small and becomes a physical constraint, forcing material into the overflow. Cyclone retention time is also a minor factor influencing cyclone performance. Within limits, increased retention time would help achieve a finer separation; whereas reduced retention time would coarsen the separation. The retention time of the cyclone can be altered by either changing the length of the cylindrical section or by changing the cone angle.

There are numerous other variables which also have an effect of separation; however, these variables are relatively minor and may be neglected for the preliminary sizing and selection of cyclones.

Flow Rate

The second main objective which must be considered is to provide adequate cyclone capacity for the application. The volume of feed slurry that a given cyclone can handle is related to the pressure drop across the cyclone. The relationship between flow rate and pressure drop for several different sizes of standard cyclones is shown in Figure 16. As shown, the flow rate increases as the pressure drop increases. In order to utilize this graph, the pressure drop used for calculating the separation is used to determine the flow rate for the cyclone diameter which was also used for determining the separation. The flow rate is then divided into the total flow for a specific application to determine the number of units necessary. Since the flow rate given in Figure 16 is for water rather than slurry, it should be mentioned that slurry normally increases the capacity of a cyclone over that shown for water; however, for preliminary estimates this factor can be neglected. This will result in the number of cyclones calculated being slightly higher than those actually needed. Approximately 20% to 25% standby cyclones are recommended for operational as well as maintenance flexibility. The vortex finder size and inlet area of a cyclone also have an effect on the volumetric flow rate that a given cyclone can handle. Larger vortex finders or inlet areas would increase the capacity, whereas smaller vortex finders or inlet areas would decrease the capacity. The shaded area in Figure 17 corresponding to each standard cyclone gives the approximate range of capacity for each cyclone.

Figure : Pressure Drop V.s Volumetric Flow rate [13] 

Shown that an underflow density of 50% to 53% solids by volume is typical for primary grinding circuits, whereas an underflow density of 40% to 45% solids by volume is normal for regrind circuits. Therefore, an underflow density can be assumed which establishes the total flow rate that must report through each cyclone apex. Figure 17 shows the approximate flow rate for a given diameter apex orifice.

Figure : Apex Capacity Diameter VS. Flow rate [14] 

Operational and Design Considerations

One of the most important considerations is to insure that cyclones are installed properly. A detailed list of “Do’s and Don’ts” is given in a later chapter.

Feed Piping and Distribution

A most important consideration for a given cyclone system is proper delivery of the slurry to the cyclone or cyclones. It has been found that a pipe size which produces a line velocity of 200 to 300cm/sec (7 to 10 ft/sec) is high enough to prevent particles from settling, even in horizontal sections, but low enough to minimize wear. Normally for a single cyclone installation the inlet pipe size of the manufacturer’s recommendation produces a velocity in this area. If the slurry is to be distributed to a number of cyclones operating in parallel, extreme care should be given to the design of the distribution system, and a radial type of manifold is recommended. This is a system where the cyclones are fed from a central circular chamber. When properly designed the central chamber becomes a mixing area and the line velocity should be lowered to approximately 60 to 90 cm/sec (2 to 3 ft/sec). This will help insure that each cyclone is fed with the same slurry concentration as well as the same particle size distribution and also will reduce wear rates. Using the radial manifold also makes it easier to install standby cyclones. Should an inline type manifold be utilized, the cyclones do not receive good distribution. It is typical that the high mass particles or coarser particles tend to pass the first cyclones and report to the final cyclone. This results in the last cyclone receiving a higher feed concentration of coarser particles, which accelerates the wear of the last cyclone as well as produces a coarser separation due to the higher feed density. Also, the last cyclone, once shut off, becomes difficult to restart because the solids will tend to pack into the feed pipe. For applications where the separation is not critical or one in which the feed concentration is extremely low, an inline manifold is acceptable and is much less expensive than the radial type.

Pressure Drop Calculation

As mentioned earlier, the pressure drop across a cyclone is measured by taking the difference between the feed pressure and the overflow pressure. If the overflow is discharged at near atmospheric pressure as recommended, the feed pressure is the same as the pressure drop. Cyclone selection provides the pressure drop required, and for pump calculations this must be converted to meters of slurry which can then be added to the static and friction heads to determine the total dynamic head for the pump. Equation 7 is used for conversion of pressure drop to meters of slurry.

Where

M = Meters, slurry.

∆P = Pressure drop, kPa.

G = Sp gr of slurry.

As stated, it is recommended that both the overflow and underflow products be discharged at atmospheric pressure. Should the overflow be discharged against a positive head, some of the fluid which normally reports to the overflow is forced to report to the underflow. This does not have a major effect on classification but does increase the amount of bypass solids and reduces underflow density. Should the overflow be discharged at a point lower than the feed entrance, a possible siphon can be established which would cause a breakdown in classification and could bring larger particles into the overflow. A large siphon effect could actually dislodge a worn liner which in turn would plug the overflow piping. Siphons can be prevented by installing a vent pipe on the overflow piping of each cyclone. The underflow should also be discharged at or near atmospheric pressure. Should the underflow be discharged at a negative pressure, the effect would be similar to a positive pressure at the cyclone overflow. If the underflow is discharged against a positive pressure, the amount of flow is reduced and a larger apex must be selected in order to insure that the

SUMP/PUMP Design

Another chapter covers the selection and sizing of slurry pumps and should be consulted for more detailed information concerning sump/pump design. Specifically regarding cyclone applications, the feed slurry being delivered to a cyclone should be as steady as possible with regard to both volumetric flow rate and slurry density. Unsteady feed conditions such as severe pump surging or extreme variations in slurry density are very detrimental to good cyclone performance. In general, a sump/pump system for a cyclone application should have a sump with as much depth as possible and a minimum cross-sectional area consistent with the pump manufacturer’s recommended retention time. A sump of this design will normally eliminate pump surging by allowing small variations in sump level well above the minimum pump suction level. The small cross-sectional area will reduce the buildup of solids in the bottom of the sump and help prevent large sections of the settled solids to slough into the pump suction and plug either the cyclone feed line or the cyclone apex. Therefore, a tall sump with a small crosssectional area provides much smoother operation.

Apex Discharge Pattern

An Important part of cyclone operation is being able to observe the type of pattern that the cyclone apex is producing. An apex operating at atmospheric pressure should produce a cone shaped discharge with an angle of 20O to 30O and a hollow center. If the cyclone consistently produces a high angle cone spray, the apex orifice should be reduced in size to maximize the slurry density being discharged. On the other hand, should the cone spray be void of the hollow center and resemble a “rope”, then the apex is too small and oversize material may be reporting to the cyclone overflow. In this case, a larger apex orifice should be installed.

CHAPTER 3:

DESIGN VARIABLES, HYDROCYCLONE GEOMETRY

Hydrocyclone Inlet Design

Hydrocyclones designed prior to 1950 featured outer wall tangential feed entry and 12-15 mm thick rubber liners. This design was not adequate for fine separations or for abrasive slurry applications. Most hydrocyclone manufacturers have redesigned their inlets to include some form of involutes, ramped or scrolled feed style and all of these provide a measured advantage in hydrocyclone performance compared to earlier tangential designs. Figure 18 illustrates the various types of hydrocyclone feed entries. The inlet opening or cross-sectional area of the orifice feeding into the cylindrical section of the inlet has an effect on capacity as well as D50, and most hydrocyclone models have several options to increase or decrease this area based on the desired flow rates and cutpoint. In general, the larger inlet area, the higher the hydrocyclone capacity and the larger predicted D50.

Cylinder Section

Typically hydrocyclones have a cylinder section length equal to the hydrocyclone diameter. This can be a separate section or integral to the inlet head. Figure 19 illustrates a hydrocyclone without a cylinder section plus hydrocyclones with a single and double cylinder. While the longer cylinder section provided greater residence time and thus more capacity, it also reduces the tangential velocity. The added cylinder length results in minimal improvement in hydrocyclone separation and will increase hydrocyclone capacity at the same pressure by 8-10%. Larger 660-840mm diameter hydrocyclones typically have shorter cylinder sections.

Figure : Hydrocyclone Inlet Styles [15] 

Figure : Hydrocyclone Cylinder Length [16] 

Cone Section

Figure 20 illustrates the different hydrocyclone cone angles that are used in different applications. The 20-degree cones ha


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