Separation processes

Title: Separation Processes

Authors: Nick Pinkerton, Karen Schmidt, and James Xamplas

Date Presented: February 9, 2014 /Date Revised: February 1, 2014

Introduction

Essentially all chemical processes require the presence of a separation stage. Most chemical plants comprise of a reactor surrounded by many separators. Separators have a countless number of jobs inside of a chemical plant. A separator can process raw materials prior to the reaction, remove incondensable gases, remove undesired side products, purify a product stream, recycle materials back into the process, and many other jobs that are essential to the process.

Chemical engineers must understand the science of separation and the variety of ways that separation can take place. There are many ways to perform a separation some of these including: distillation, absorption, stripping, and extraction. The science of separation revolves around the presence of two phases that are in contact and equilibrium [1].

Figure 1. Separation methods by property

Theory

Vapor-Liquid Equilibrium

Separation processes are based on the theory of vapor-liquid equilibrium. This theory states that streams leaving a stage in a separation process are in equilibrium with one another. The idea of equilibrium revolves around the idea that when there is vapor and liquid in contact with one another they are in constantly vaporizing and condensing. Different components in the mixture will condense and vaporize at different rates. There are three types of equilibrium conditions that can be subdivided into thermal, mechanical and chemical potential categories. These separate equilibrium states are given as:

$T_{liquid}=T_{vapor}$

$p_{liquid}=p_{vapor}$

$chemicalpotential_{liquid}=chemicalpotential_{vapor}$

Distillation

Flash Distillation

Flash Distillation is one of the simpler separation processes to be employed in a chemical plant. The main premise of flash distillation is that a portion of a liquid feed stream vaporizes in a flash chamber or a vapor feed condenses. Vapor-liquid equilibrium will cause the vapor phase and the liquid phase to have different compositions. The more volatile component of the mixture will compose of a larger portion of the vapor. This simple separation is easy to manufacture but does not result in large degrees of separation.

Flash distillation requires a feed stream that is pressurized and heated and then passed through a valve into a flash drum. The large pressure drop across the valve will result in a partial vaporization of the fluid. Vapor will be removed overhead from the flash drum while the remaining liquid will collect at the bottom of the drum and be removed. Most flash drums will contain an entrainment eliminator which is a screen that prevents liquid from being carried into the vapor effluent. Figure one shows a simple overview of the flash distillation process. As shown, there is a heater that flows into a let down valve where the two phase flow begins. Variables y and x are the mole fractions of the more volatile component in the vapor and liquid effluents respectively.

Figure 2. Flash Distillation Flow Diagram

Column Distillation

Distillation columns are the most widely used separation technique used in the chemical industry, accounting for approximately 90% of all separations [1]. Distillations in columns consist of multiple trays that each act at their own equilibrium conditions. Large columns are able to perform complete separations of binary mixtures as well as more complex multi-component mixtures.

Stages

Columns are separated into stages by the presence of trays. These trays allow for vapor-liquid contact and equilibrium to occur. Typically, the more stages in a column, the larger separation that can be achieved. There are many different types of trays that can be used in a column.

Sieve Trays

The simplest tray type is the sieve tray which is a sheet of metal with holes punched into it to allow vapor flow. Sieve trays can have different hole patterns and sizes that will affect the tray efficiency and flow rates.

Bubble-Cap Trays

Bubble-Cap trays consist of a weir around each hole in the tray which is covered with a cap that has holes or slots to allow vapor passage. Entrainment is about three times larger than a sieve tray. Bubble capped trays require larger tray spacing than sieve tray design. Bubble-cap tray have been known to have problems with coking, polymer formation, or high fouling mixtures. Recently, very few new bubble-cap columns are being built due to the expense and marginal benefits. However, engineers will likely encounter bubble-cap columns still currently in operation.

Flow Patterns

Cross flow columns are the most common pattern for distillation columns. For liquid flows between 50 and 500 Gal/min, a cross flow column is appropriate. When liquid flow is increased above 500 Gal/min, an engineer should consider designing a double pass or multi-pass column. This will reduce the liquid gradient on the tray and reduce the downcomer loading [1].

Column Sizing

Column height will be dependent on the amount of trays required and the spacing between the trays. Normally, tray spacing of 0.15 m to 1 m is used. For columns, above 1 meter in diameter, 0.5 m can be used as an initial estimate.

Column diameter is influenced by the vapor flow rate in the column. The trays can not have excess liquid entrainment or high pressure drops; therefore, vapor velocity in the column must be maintained at a reasonable level.

An equation based on the Souders and Brown equation can be used as an estimate for the max allowable superficial vapor velocity.

$u_{v}=(-0.171l_{t}^{2}+0.27l_{t}-0.047){\frac {\rho _{L}-\rho _{v}}{\rho _{v}}}^{1}/2$

Absorption

An alternative to distillation for separating solutes from gas streams is absorption. The gas mixture comes into contact with a liquid solvent that readily absorbs the undesirable components from the gas stream, purifying the gas stream. This separation process is determined by the inputs of the liquid flow rate, temperature, and pressure. The absorption factor, which can be determined mathematically, determines how readily a component will absorb in the liquid phase. Higher absorption factors result in higher absorptivity into the liquid and a decrease in the number of trays required for separation, however a diminishing return occurs after the absorbing factor is greater than 2.0. An absorption factor of 1.4 is most commonly used.

Other Separation Processes

Extraction

Liquid-liquid extraction is a process for components with overlapping boiling points and azeotropes. The process requires a solvent such that some of the components of the mixture are soluble, and then the components will be separated based on this solubility in the liquid. This process can operate at moderate temperatures and pressures, so is not very energy intensive. However, a distillation column is required to extract the solvent for recycle. More recently, supercritical fluids have replaced liquid solvents in some processes for L/L extraction, due to the solute’s ability to more rapidly diffuse through them. The issue with these fluids, however, is that they must be operated at extremely high pressures and temperatures, increasing both capital and operating expenses of the process.

References

1. Wankat, P.C. (2012). Separation Process Engineering. Upper Saddle River: Prentice-Hall.
2. Towler, G.P. and Sinnot, R. (2012). Chemical Engineering Design: Principles, Practice and Economics of Plant and Process Design.Elsevier.
3. Biegler, L.T., Grossmann, L.E., and Westerberg, A.W. (1997). Systematic Methods of Chemical Process Design. Upper Saddle River: Prentice-Hall.
4. Peters, M.S. and Timmerhaus, K.D. (2003). Plant Design and Economics for Chemical Engineers, 5th Edition. New York: McGraw-Hill.
5. Seider, W.D., Seader, J.D., and Lewin, D.R. (2004). Process Design Principles: Synthesis, Analysis, and Evaluation. New York: Wiley.
6. Turton, R.T., Bailie, R.C., Whiting, W.B., and Shaewitz, J.A. (2003). Analysis, Synthesis, and Design of Chemical Processes Upper Saddle River: Prentice-Hall.