designingvacuumtower(vdu)

Designing Distillation Columns for Vacuum Service

Karl Kolmetz

Sulzer Chemtech, Singapore [email protected]

Andrew W. Sloley VECO USA, Inc.

[email protected]

Timothy M. Zygula Houston, Texas

[email protected]

Peter W. Faessler Sulzer Chemtech Singapore [email protected]

Wai Kiong Ng

Sulzer Chemtech Singapore [email protected]

K. Senthil


Tau Yee Lim


Prepared for

The 11th India Oil and Gas Symposium

and International Exhibition

6-7 September 2004 Grand Hyatt

Mumbai, India

Kolmetz.Com is a chemical engineering web site that publishestechnical articles on distillation, process optimization, operations training, personal improvement, process unit safety and environmental concerns.

Abstract Many hydrocarbon separation applications can be improved by vacuum distillation. The relative volatility of many hydrocarbon binary pairs of components improves with lower pressure, improving the separation efficiency. At lower pressures hydrocarbon components vaporize at lower temperatures, reducing the degradation of products by condensation or polymerization. Typically applications of distillation columns in vacuum service include refinery vacuum columns, ethyl-benzene / styrene distillation, mono- / di – / tri - / ethanolamine distillation, and oleo chemicals. A review of each of the applications can provide an insight to the overall picture of designing columns for vacuum service. Introduction – Advantages of Vacuum Distillation Distillation is the separation of key components by the difference in their relative volatility, or boiling points. It can also be called fractional distillation or fractionation. Distillation is favored over other separation techniques such as crystallization or membranes when;

1. The relative volatility is greater that 1.2, 2. Products are thermally stable, 3. Large rates are desired, 4. No corrosion, precipitation or explosion issues are present.

Close boiling mixtures may require many stages to separate the key components. One tool to reduce the number of stages required is to utilize vacuum distillation. Vacuum distillation increases the relative volatility of key components in many applications. For vapor and liquid equilibrium a K- value is defined for each species i by,

Ki = Yi / Xi where Y is the mole fraction in the vapor phase and X is the mole fraction in the liquid phase. (1) For vapor liquid separation operations, an index of the relative ease of separation for two chemical species i and j is given by the relative volatility alpha defined as the ratio of their K values

alpha ij = Ki / Kj = Pi / Pj Pi and Pj are the vapor pressures of components i and j at a given temperature.

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The number of theoretical stages required to separate two species to a desired degree is strongly dependent on the value of this index. The greater the departure of the relative volatility from a value of one, the fewer the equilibrium stages required for a desired degree of separation. The alpha value of many binary pairs can be improved by lowering the pressure of the system leading to separation efficiencies.

1.0 0.8 0.6 0.4 0.2 0.0

α = 1

α = 2

α = 3

α = 5

Y

0.0 0.2 0.4 0.6 0.8 1.0 X

Knowing the relative volatility for a system is also useful in determining the amount of separation possible. A relative volatility of 1.0 indicates that both components are equally volatile and no separation takes place via normal distillation. When the relative volatility is low, less than 1.05, separation becomes difficult because a large number of stages are required. The higher the relative volatility, the more separable are the two components; this connotes fewer stages in a distillation column in order to effect the same separation between the overhead and bottoms products. Lower pressures increase relative volatilities in most systems.

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The alpha values for ethyl-benzene and meta-di-ethyl-benzene, which are in styrene distillation, are 2.85 at a pressure of one bar, and 2.50 at a pressure of three bar. Higher pressure for this separation requires more theoretical stages at constant reflux.

One of the few cases where lower pressure does not help distillation is 1,1 – Di Phenyl Ethane, and 1,2,3,4 – Tetra Ethyl Benzene, as shown in the attached graph. (9)

A second advantage of vacuum distillation is the reduced temperature requirement at lower pressures. For many systems the products degrade or polymerize at elevated temperatures. Vacuum Distillation can improved by

1. Prevention of product degradation or polymer formation because of reduced pressure leading to lower tower bottoms temperatures,

2. Reduction of product degradation or polymer formation because of reduced mean residence time especially in packing applications,

3. Increases in capacity, yield, and purity. A third advantage of vacuum distillation is the reduced capital cost, at the expense of slightly more operating cost. Utilizing vacuum distillation can reduce the height and diameter, and thus the capital cost of a column.

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Refinery Vacuum Columns General Overview

Refinery vacuum columns utilize vacuum distillation due to increase the distillate recovery which avoiding the thermal degradation of the atmospheric tower bottoms, which is the vacuum tower feed. The goal of a refinery vacuum column is produce light vacuum gas oil (LVGO) and heavy vacuum gas oil (HVGO) which can be catalytically converted to gasoline in a Fluidized Catalytic Cracking Unit (FCCU) or hydrotreater. The LVGO and HVGO should contain low amounts of asphaltenes and metals. Typical ranges for asphaltenes would be 0.02 to 0.20-wt % and 1 to 10 ppm for metals. The atmospheric tower bottom begins thermal cracking to produce coke at the

vacuum tower transfer line. The design of the transfer line and vapor inlet distributor is very important in the design of refinery vacuum distillation. The rate of thermal cracking is directly proportional to time and increases exponentially with temperature. Both residence time and temperature must be minimized to avoid coke deposits. Typical limits in the transfer line are 425 C (800 F) and about 412 C (775 F) in the flash zone. Condensation is a reaction where two or more small molecules combine to from large stable structure molecules. Extreme condensation is the formation of coke at high temperature and long residence times. Coke forms when hydrogen atoms are removed from the hydrocarbon radicals until the extreme of leaving only a layer of elemental carbon or coke. These condensation products leave the gas phase and settle as a layer of hard coke that is difficult to remove. Steam multi stage vacuum ejectors reduce the pressure at the top of the tower with condensers, and may have as many as three stages of vacuum. The top of the tower is typically 20 mbar of pressure and the tower internals are designed to reduce the overall tower pressure drop and liquid volume, thus reducing the transfer line pressure, and by equilibrium, the temperature. Vacuum Service Design Guidelines A tower design is normally made in two steps, a process design followed by a mechanical design. The purpose of the process design is to calculate the required stream flows and number of required theoretical stages. The purpose of the mechanical design is to select the tower internals, column diameter and height. Additional items to be reviewed include thermal stability, chemical stability, corrosion, and safety requirements.

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Older refinery vacuum tower designs utilized trays to make the required separation. Each tower internal has advantages and disadvantages. In low-pressure systems, packing has been shown to be more efficient, but proper application is important for fouling services. In high-pressure distillation applications trays have been shown to be more efficient than packing. For absorption where equilibrium is not the limiting factor, packing however can be utilized in high-pressure applications. Trays have few advantages in refinery vacuum service. Their disadvantages include lower stage efficiency, higher mean residence time, and higher-pressure drop. A normal designed tray will have 5-mbar pressure drop per stage. The advantages of packing in a refinery vacuum service are;

1. the higher stage efficiency, 2. reduced mean residence time, 3. smaller residence time distribution, 4. and lower pressure drop.

These advantages can increase yield of LVGO and HVGO without the addition of asphaltenes or metals. Currently most vacuum towers have grids in the wash section and packing in the HVGO and LVGO sections.

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The challenging part of the Vacuum Column is the wash bed, which removes the entrained heavy key from the light and intermediate keys. The entrained heavy key can lead to many fouling phenomena as highlighted by this example.

The industry average wash bed run length approaches 5 years. The average life of a wash bed is based on a variety of factors, which include;

1. Design of inlet feed system to reduce entrainment 2. Design of inlet transfer line to reduce velocity and resulting entrainment 3. Design of furnace to prevent high flux areas which cause cracking and

coking 4. Design of collector tray, this can be sloped to reduce residence time 5. Rate of wash bed oil flow 6. Amount of vacuum – the high vapour velocities caused by deep cut

vacuum designs will result in more velocity gradients, leading to more entrainment with the same internal tower diameter

Design Of Feed Inlet System to Reduce Entrainment Computational fluid dynamics (CFD), or the use of computers to solve fluid flow problems has advanced enormously in the last ten years. The commercially available CFD packages have led the development in software that have enabled numerous fluid flow applications to become commonplace. These tools are now used extensively throughout the fluid machinery industry. Three dimensional flow analyses through the utilization of CFD have led

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substantial improvements into the performance of specific fluid machinery components through a better understanding and control of the complex flow phenomena involved. The key advantage of CFD analysis is that it can be used as a numerical test bed that can minimize the required prototype testing and other associated costs for product development. It also offers substantial support to design optimization of existing configurations as shown in below examples (Elder, et. al., 2003) (8). One of the keys to successful design of vacuum systems is the feed inlet system. Currently there are several options available. A CFD study was conducted by Wehrli et. al., (4), to evaluate the current systems. Optimum operation of packed separation columns require even distribution of liquid films and gas flow. While the role of proper liquid distribution was never disputed, the vapor initial distribution has been secondary. Its importance becomes evident as large column diameters and packings with lower pressure drop are considered. The purpose of the vapor feed system is to introduce process gas or vapor, coming for example, from the reboiler into the column and to distribute the vapor evenly over the whole cross section. To achieve this, the vapor velocity needs to be reduced over a short distance. At the same time, the inlet should not unduly block the column cross-section or lead to excessive pressure drops. For economic reasons, a minimum distance between the nozzle and the packing is desired. While major challenges within the CFD field, like simulation of multiphase flows, await better understanding and models of general applicability, many problems of lower complexity can now be addressed. The goal of the CFD study was to determine the uniformity of the vapor velocity profile right under the lower packing or tray edge. This is the computational domain and boundary conditions of the CFD Study. The velocities used in this study were within normal design ranges, F factors of less than 20.

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Standard inlet This is a standard radial inlet without a vapor feed inlet device. Typical guidelines would be to have the first tray three to five time the inlet pipe diameter above the top of the inlet piping.

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The CFD study results show very high velocities (red) along the opposite wall from the feed inlet, at a point just below the packing inlet. This would be the expected result from a standard inlet. The vapor flow in the packing would be non-uniform and resulted in reduced packing efficiency.

The streamlines show the similar expected phenomena.

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Orifice Baffle This is a typical orifice baffle vapor feed inlet device. It is used in many applications. It has a specially designed channel baffle with lateral opening and a central orifice.

The orifice baffle design shows high velocities along the opposite wall and adjacent sidewalls. Once again the vapor distribution is less than optimal, but slightly better than the first case. The packing efficiency would again be reduced.

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The streamlines show even more detail of the maldistribution.

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Vapor Horn This is a vapor horn feed inlet device that uses the advantage of a tangential feed entry and wall effects to reduce the vapor feed velocity. This device is usually applied with flashing streams or high velocity vapor feed carrying a large disperse liquid fraction. Utilizing this device can reduce the height requirement to the first tray.

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This is a representation of the vapor distribution at the vapor horn.

This is a representation of the vapor horn profile just below the packing. It has no high velocity red components below the packing inlet, and will lead to increased packing efficiency.

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The streamlines show improved velocity gradients and a vapor horn should be utilized when possible on a tangential tower entry.

This is an example of a vacuum tower CFD with the chimney tray included in the simulation.

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Ethyl Benzene/Styrene Distillation

General Overview

The purpose of an ethyl benzene (EB) / styrene splitter is to separate ethyl benzene from styrene. The distillate EB is recycled to Styrene reactors and the bottom product Styrene Monomer (SM) is sent to the Styrene Finishing column for heavy key removal. The EB impurity in the SM should be in the range of 100 ~ 500 ppm. EB/SM Splitters are operated under vacuum due to the polymerization potential of styrene at elevated temperature. Polymers are undesirable in the monomer distillation column and can lead to plugging of distributors or packing and unit outages. The rate of polymerization is directly

proportional to time and increases exponentially with temperature. Both residence time and temperate must be minimized to reduce polymerization deposits. Current guideline is to keep the tower bottoms temperature below 120°C Generally steam ejector systems are used to maintain vacuum at the top of the tower. The typical column top pressure is 100 to 400 mbar and the internals are carefully designed to reduce the tower overall pressure drop, minimize liquid hold up; reduce the bottom temperature and residence time. Some producers are increasing the tower pressure due to improvements in inhibitor formulations. This can increase capacity and improve heat recovery.

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Ethyl Benzene/Styrene Splitter Service Design Guidelines During the early days of styrene manufacturing, trays were used as column internals for the separation. The trayed columns have a high-pressure drop and high bottoms temperature. Currently the industry standard is to use high-capacity structured packing as column internals for EB/SM splitters. The EB/SM splitter may require 80 to 100 theoretical stages depending on the purity requirements. The columns may have 5 to 7 beds of structured packing and require very good quality liquid distributors and collectors.

Table of Number of Theoretical Stages Verses Reflux Ratio at Different Pressures

NTS versus Reflux-Ratio

0

20

40

60

80

100

120

0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30

Reflux Ratio

NTS

P=150mbar

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NTS Versus Reflux-Ratio

0

20

40

60

80

100

120

0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 32 34

Reflux-Ratio

NTS

P=400mbar

CASE

Top Pressure (mbar)

NTS

Reboiler (M.W)

Condenser (M.W)

Reflux-Ratio

1 150 71 38.43 39.47 8.1726 2 400 79 42.45 42.94 8.1726

A special design for overhead condenser and reboiler may be used to reduce the overall column pressure drop. Some of the advantages of using structured packing in these applications are as follows: 1. Minimum polymer formation because of reduced pressure drop leading

to low tower bottoms temperatures. The pressure drop for high capacity structured packing may be in the range of 1 - 4 mmHg per meter.

2. Low inhibitor consumption because of low bottom temperature and minimum liquid hold-up, which results in significant savings in the costs of operation.

3. Smaller residence time distribution of the liquid phase is achieved with high capacity structured packing as compared to tray columns. It also reduces dead pockets and thermal degradation, which usually gives rise to further acceleration of polymer formation.

4. Increase in capacity, yield, and purity when compared to tray column of same size.

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5. Energy saving by increasing the number of theoretical stages when the columns are revamped from trays or conventional packing to high capacity structured packing.

Ethanolamine Distillation Ethanolamine Distillation General Overview The reaction of ethylene oxide with ammonia renders a mixture of mono-, di-, and tri-ethanol amines. The maximum production of mono-ethanolamine from the reactor is 70%. Beyond this maximum restriction on mono-ethanolamine, the plant may be designed for a wide range in product distribution. This means that the plant has very high flexibility and production and may be adapted to changing market demands.

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Ethanolamine Distillation General Overview Column top operating pressure for the di- and tri- ethylamine distillation is typically 1 to 2 mbar and column bottom operating pressure is in the range of 10 to 12 mbar. To achieve this low-pressure drop and still retain high separation efficiency at typically very low specific liquid loads, wire gauze structured packing is specified. Liquid loads can be as low as 0.2 m3/m2hr. Low pressure drop gauze packings in distillation columns create the lowest possible operating temperature, preventing deterioration of product quality, while reducing column shell diameter. The high separation efficiency is leading to;

1. Low energy consumption through reduced reflux rates 2. High product purity, reduced column height 3. No organic wastes – from the products of polymerization.

Special design of the top condenser provides extremely low-pressure drop of vacuum distillation. Falling film reboilers permit use of low steam temperature, avoiding product quality deterioration and losses

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Palm Oil Refining Palm Oil Refining General Overview An important step in palm oil purification is physical refining, which is the removal of free fatty acids present in the vegetable oil. This separation needs to be completed at low temperatures to reduce the degradation of the final products. The following factors are very important in the design of columns for this service.

1. High Vacuum 2. Low Pressure Drop 3. Low Bottoms Temperature 4. Minimum hold up 5. Short residence time

The free fatty acids are removed using stripping steam at 250 °C. The column top pressure is around 2 - 3 mmHg. The columns with structured packing require much lower stripping steam than the columns equipped with trays.

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Fatty Acids Fatty acids are saturated and unsaturated aliphatic carboxylic acids with carbon chain lengths in the range of C6 up to C24. An example of a fatty acid is palmitic acid

CH3 – (CH2)14 – COOH

Splitting oils and fats produces the fatty acids. Glycerine is also produced as by product. The split fatty acid is a mixture of fatty acids ranging from C6 to C18 depending on the type of oil or fat. The pure fatty acid is used as an important raw material in the manufacture of soaps, washing powder and other personal care products. So it is important to purify the fatty acid to as high a purity as possible. Knowledge of the chemical and physical properties of the fatty acid is one of the basic prerequisites for the design of distillation column for this service. The odor and color specification is very important for fatty acid. The boiling points of fatty acids are very high at atmospheric pressure. Therefore, it is necessary to distill the fatty acids at high vacuum to reduce the vapor pressure otherwise the product will degrade at high temperatures. The general schematic flow for the purification of fatty acid is as follows:

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Glycerol Glycerol (C2H8O3), 1,2,3 – propanetriol, commonly known as glycerin, is the simplest triol. It is obtained as a by-product during the conversion of fats and oils to fatty acids and fatty acid methyl esters. As glycerin is a thermally very sensitive product, low column bottoms temperatures are absolutely essential in order to achieve good product qualities. Glycerin is utilized in the cosmetic and pharmacy applications and has very stringent specifications. Low pressure drop, high efficiency sheet metal structured metal and wire gauze packing is the best combination to insure liquid film distribution of the packing surface. The liquid film distribution is the key to achieving high efficiency. Due to the presence of water in Glycerol Fractionation, the wet-ability of the sheet metal structured packing is reduced, leading to reduced stage efficiency. The capillary action of the wire gauze packing to spread the flow evenly through the wire gauze at low liquid loads, even with the presence of water, improves the separation efficiency and thus gives it an advantage in this application. Wire gauze packing is utilized in the middle and bottom section with the structured sheet metal utilized in the top section.

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At liquid loads of less than 10 m3/m2hr, the liquid flows within the wire gauze (internal flow). For higher loads the liquid flows on the exterior surface of the wire gauze (external flow) (10)

Capillarity Action in Wire Gauze Packing

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Conclusions This paper presents four aspects of vacuum distillation. First was a review of vacuum distillation principles. Second was a general overview of vacuum distillation applications. Third was a review of tower feed inlet devices, which is very important in vacuum distillation. And finally, in several application sections, a comparison of trays and structure packing was presented, with the advantages and disadvantages of each mentioned. There are many advantages to Vacuum Distillation including;

1. The alpha values of binary pairs are generally improved leading to improved separations.

2. The temperature requirement for distillation is reduced at lower

pressures. For many systems the products degrade or polymerize at elevated temperatures. a. Prevention of product degradation or polymer formation because

of reduced pressure leading to lower tower bottoms temperatures.

b. Reduction of product degradation or polymer formation because of reduced mean residence time especially in packing applications

c. Increased in capacity, yield, and purity 3. The reduced capital cost, at the expense of slightly more operating

cost. Utilizing vacuum distillation can reduce the height and diameter, and thus the capital cost of a column.

Acknowledgments The authors are grateful to Sulzer Chemtech for the use of pictures and CFD results in this paper. We also extend our special appreciations to Dr. M. Wehrli (Sulzer Chemtech, Switzerland), and Mr. D. Summers (Sulzer Chemtech, USA) for their invaluable comments and review.

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http://www.cbu.edu/~rprice/lectures/distill.html

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