Examination Of The Importance Of A Packed Bed Construction Essay
|✅ Paper Type: Free Essay||✅ Subject: Construction|
|✅ Wordcount: 5347 words||✅ Published: 1st Jan 2015|
In chemical processing, a packed bed is a hollow tube, pipe, or other vessel that is filled with a packing material. The packing can be randomly filled with small objects like Raschig rings or else it can be a specifically designed structured packing.
1.1 Scope of Work
The purpose of a packed bed is typically to improve contact between two phases in a chemical or similar process. Packed beds can be used in a chemical reactor, a distillation process, or a scrubber, but packed beds have also been used to store heat in chemical plants. In this case, hot gases are allowed to escape through a vessel that is packed with a refractory material until the packing is hot. Air or other cool gas is then fed back to the plant through the hot bed, thereby pre-heating the air or gas feed. (Perry 1984)
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In industry, a packed column is a type of packed bed used to perform separation processes, such as absorption, stripping, and distillation. A packed column is a pressure vessel that has a packed section. The column can be filled with random dumped packing or structured packing sections, which are arranged or stacked. In the column, liquids tend to wet the surface of the packing and the vapors pass across this wetted surface, where mass transfer takes place. Packing material can be used instead of trays to improve separation in distillation columns. Packing offers the advantage of a lower pressure drop across the column (when compared to plates or trays), which is beneficial while operating under vacuum. Differently shaped packing materials have different surface areas and void space between the packing. Both of these factors affect packing performance.
Another factor in performance, in addition to the packing shape and surface area, is the liquid and vapor distribution that enters the packed bed. The number of theoretical stages required to make a given separation is calculated using a specific vapor to liquid ratio. If the liquid and vapor are not evenly distributed across the superficial tower area as it enters the packed bed, the liquid to vapor ratio will not be correct and the required separation will not be achieved. The packing will appear to not be working properly. The height equivalent to a theoretical plate (HETP) will be greater than expected. The problem is not the packing itself but the mal-distribution of the fluids entering the packed bed. These columns can contain liquid distributors and redistributors which help to distribute the liquid evenly over a section of packing, increasing the efficiency of the mass transfer. The design of the liquid distributors used to introduce the feed and reflux to a packed bed is critical to making the packing perform at maximum efficiency. (Seader 2006)
Packed columns have a continuous vapor-equilibrium curve, unlike conventional tray distillation in which every tray represents a separate point of vapor-liquid equilibrium. However, when modeling packed columns it is useful to compute a number of theoretical plates to denote the separation efficiency of the packed column with respect to more traditional trays. In design, the number of necessary theoretical equilibrium stages is first determined and then the packing height equivalent to a theoretical equilibrium stage, known as the height equivalent to a theoretical plate (HETP), is also determined. The total packing height required is the number theoretical stages multiplied by the HETP. (Columns 1989)
Columns used in certain types of chromatography consisting of a tube filled with packing material can also be called packed columns and their structure has similarities to packed beds.
Packed bed reactors can be used in chemical reaction. These reactors are tubular and are filled with solid catalyst particles, most often used to catalyze gas reactions. The chemical reaction takes place on the surface of the catalyst. The advantage of using a packed bed reactor is the higher conversion per weight of catalyst than other catalytic reactors. The reaction rate is based on the amount of the solid catalyst rather than the volume of the reactor. (Seader 2006)
Through the project the packed bed absorber will be studies and experiments will be conducted in order to verify the efficiency of the machine in terms of reducing the alcalinity of waste cement water.
The main purpose of the thesis project is to:
To be able to efficiently treat cement factory and construction site water wastes and neutralize cement waste water from a highly basic state, such that its disposal is ecofriendly.
Exploring the fundamentals of acid base titration in order to find out experimental results, absorption rate and concentration of effluents.
To obtain most effective and highest order of mass transfer in the most economical method using simple solutes and substances.
Figure 1: Schematic of a Packed Bed Absorber
Figure 1: Schematic of a Packed Bed Absorber
1.2 Industrial Application
Packed columns are commonly employed to carry out absorption and scrubbing operations. They provide a medium in which counter-current two-phase flow occurs, and in which a relatively large gas-fluid interface per unit volume of column volume exists. Because of the nature of the packing material (often a ceramic) a packed column can operate using strongly corrosive fluids. Packed columns are often more economical to build and operate than their plate or bubble-cap column counterparts, although pressure drops can be high, which requires larger gas blowers with high energy consumption.
The packed bed absorber in this experiment is to be used eventually to remove ammonia from a waste air stream. Before running ammonia, however, it is necessary to determine what water and air flow rates can be accommodated, to measure the pressure drop across the bed so that the air blower can be specified, and to test a noninvasive method for measuring the amount of water held up on the packing, so that incipient flooding can be detected. (Seader 2006)
An air stream from a drier contains roughly 0.10% by weight (1000 ppm) ammonia. Environmental regulations prohibit discharge of the ammonia into the air, and thus a small packed bed scrubber has been set up to pilot the ammonia removal process. Your job is to run the scrubber at various air flow and water flow rates, and collect data from which can be calculated the height of a transfer unit for the packing used in the scrubber. The results will be passed to a design group which will design a full-scale scrubber, based on the air flow rate from the drier and the maximum allowable ammonia content of the effluent air
Chromatography is the collective term for a set of laboratory techniques for the separation of mixtures. It involves passing a mixture dissolved in a “mobile phase” through a stationary phase, which separates the analyte to be measured from other molecules in the mixture based on differential partitioning between the mobile and stationary phases. Subtle differences in a compound’s partition coefficient result in differential retention on the stationary phase and thus changing the separation. (Laurence M. Harwood 13 June 1989)
Chromatography may be preparative or analytical. The purpose of preparative chromatography is to separate the components of a mixture for further use (and is thus a form of purification). Analytical chromatography is done normally with smaller amounts of material and is for measuring the relative proportions of analytes in a mixture. The two are not mutually exclusive.
1.2.2 Liquid Chromatography
Liquid Chromatography is normally the main area of dealing when involved with the absorption of liquid solution
Liquid chromatography (LC) is a separation technique in which the mobile phase is a liquid. Liquid chromatography can be carried out either in a column or a plane. Present day liquid chromatography that generally utilizes very small packing particles and a relatively high pressure is referred to as high performance liquid chromatography (HPLC). (Rigas n.d.)
In the HPLC technique, the sample is forced through a column that is packed with irregularly or spherically shaped particles, a porous monolithic layer (stationary phase) or a porous membrane by a liquid (mobile phase) at high pressure. HPLC is historically divided into two different sub-classes based on the polarity of the mobile and stationary phases. Methods in which the stationary phase is more polar than the mobile phase (e.g. toluene as the mobile phase, silica as the stationary phase) are termed normal phase liquid chromatography (NPLC) and the opposite (e.g. water-methanol mixture as the mobile phase and C18 = octadecylsilyl as the stationary phase) is termed reversed phase liquid chromatography (RPLC). Ironically the “normal phase” has fewer applications and RPLC is therefore used considerably more. (Laurence M. Harwood 13 June 1989)
Specific techniques which come under this broad heading are listed below. It should also be noted that the following techniques can also be considered fast protein liquid chromatography if no pressure is used to drive the mobile phase through the stationary phase. See also Aqueous Normal Phase Chromatography.
Figure 2: Sophisticated Gas Chromatography System (Source: http://visualsonline.cancer.gov/details.cfm?imageid=2020)
CHAPTER 2: PROJECT DETAILS
There are various forms of construction site wastes which can harm the environment and cause extensive damage to both life and nature. Mainly, as it is well known that water pollution is the main cause of diseases and deaths and more than 14000 people die due to water pollution every single day. It is of utmost importance to at least minimize some forms of water pollution.
2.1 Construction Site Water Wastes
It is common practice in the ready-mixed concrete industry to thoroughly clean the inside of a concrete trucks drum at the end of each day using approximately 150-300 gallons of water. According to the Water Quality Act (part 116), truck wash water is a hazardous substance ( it contains caustic soda and potash) and its disposal is regulated by the Environmental Protection Agency (EPA). In addition, a high pH makes truck wash water hazardous under EPA definition of corrosively. These regulations require accurate and precise record keeping of each waste water disposal, retention of the records for three years, and submission of the records to EPA. The current practices for the disposal of concrete wash water include dumping at the job site, dumping at a landfill, or dumping into a concrete wash water pit at the ready-mix plant. (pHasing Out Unscrupulous Wastewater Disposal 2007).
The availability of landfill sites for the disposal of truck wash water has been drastically reduced for the past ten years. In 198 1, there were approximately 50,000 such sites in the United States; today, there are only about 5,000. In response to this reduction, most ready-mix batch plants have developed a variety of operational configurations to manage their own wash water. The alternatives include settling ponds, storm water detention/retention facilities and water reuse systems. Recognizing that a typical batch plant generate an average of 20 gallons of wash water discharge per cubic yard of ready-mixed production and that the average concrete production rate for a batch plant is 250 cubic yards per day, the proper disposition of the wash water presents an important issue. If this wash water can be reused, it has been estimated that the volume per cubic yard of production that will require special disposal handling can be reduced to 5 gallons. Few research studies have been conducted related to the problem of handling wastes produced in the RMC operations. This paper summaries the information available regarding the handling of waste water in RMC operations. Major issues discussed include, source, quality and quantity of waste water; waste water control systems; suitability of waste water for making concrete; and chemical stabilizing agents.
2.2 Source of Waste Water
The waste water from the RMC operations is usually generated from truck wash systems, washing of central mixing plant, storm water runoff from the ready-mix plant yard, waste water generated from water sprayed dust control and conveyor wash down. The quantity of waste water generated daily vary due to the unpredictable amount generated from storm water runoff. However, it has been estimated that when a 10 cubic yard ready-mix truck delivers concrete, 1 to 4 percent or approximately 600 lb of concrete adhere to the inside of the drum and mixing blades. At the end of the workday, the ready-mix truck has to be cleaned of all the residue concrete. Removing the residual cementations material adhering to the drum and blades of a single truck unit can require approximately 150 to 300 gallons of water. (Mbwambo 1996)
2.3 Quality of Waste Water
The quality of waste water from the RMC operations is derived from the source of the water itself. Wash water discharge from truck wash contains cementitious materials and chemical admixture residue. Storm water runoff may also contain cementitious materials and other impurities washed from the plant yard. Due to the high content of dissolved limestone solids the wash water is caustic and has a high pH value ranging between 11 and 12. In general the waste water contains dissolved solids which include: sulfates and hydroxides from cement, chlorides from the use of calcium chloride as an admixture, oil and grease from the equipment, and small quantities of other chemicals associated with hydration of Portland Cement and derivatives from chemical admixtures . The most common derivatives of chemical admixtures are: ethanolamine, diethanolamine, formaldehyde, K-naphthalene sulfonate, and benzene sulfonic acid. (Mbwambo 1996)
2.4 The pH Problem
The pH of concrete wash water is incredibly high – typically 12 to 13.5 on a scale which runs from zero (very acidic) to 14 (highly alkaline). The water is extremely corrosive, highly toxic to fish and other aquatic wildlife and, with prolonged skin contact, capable of causing second degree burns. The only commonly-used commodity with a higher pH is domestic oven cleaner. Therefore, high pH water’s potential to cause environmental harm is often overlooked or misunderstood.
Figure 3 A Cement Ready Mix Truck (pHasing Out Unscrupulous Wastewater Disposal 2007)
A recent Environmental Audit on a large commercial building project revealed that washing truck mixer discharge chutes alone generates about 20 liters of high pH wash-water and up to five liters of waste concrete per truck. Over the entire construction phase of the project this amounted to 75m3 of potentially environmentally-damaging wash-water. Yet dilution is definitely not the solution to pollution from concrete wash-water. Unfortunately, pH is a logarithmic scale and so the release of even a small quantity of concrete wash-water can significantly increase the pH of a receiving watercourse. (pHasing Out Unscrupulous Wastewater Disposal 2007)
For example, the release of 75m3 of pH 13 wash-water would theoretically raise the pH of 750,000m3 of neutral river water to pH 9 – the limit commonly adopted for discharge to the environment.
This is equivalent to polluting 300 Olympic-sized swimming pools. Consequently, the uncontrolled release of even a small quantity of high pH wash-water can have a devastating effect on the environment.
2.5 Possible Solution
The proposal of this project prospects the introduction of the packed bed reactor in the process of waste water treatment. It is well known that the pH level of the waste water shows that the liquid is highly basic and this water if reacted with gasses will absorb in order to turn neutral up to a limit that it can be safely disposed of. The scope of this project will deal with the absorption of these basic fluids with CO2 gas which will help convert the liquid from basic to neutral.
CHAPTER 3: TESTING OF WASTE WATER
Titration is a common laboratory method of quantitative chemical analysis that is used to determine the unknown concentration of a known reactant. Because volume measurements play a key role in titration, it is also known as volumetric analysis.
A reagent, called the titrant or titrator, of a known concentration (a standard solution) and volume is used to react with a solution of the analyte or titrand, whose concentration is not known. Using a calibrated burette or chemistry pipetting syringe to add the titrant, it is possible to determine the exact amount that has been consumed when the endpoint is reached. The endpoint is the point at which the titration is complete, as determined by an indicator (see below). This is ideally the same volume as the equivalence point-the volume of added titrant at which the number of moles of titrant is equal to the number of moles of analyte, or some multiple thereof (as in polyprotic acids). In the classic strong acid-strong base titration, the endpoint of a titration is the point at which the pH of the reactant is just about equal to 7, and often when the solution takes on a persisting solid color as in the pink of phenolphthalein indicator (Rosenfeld 1999)
A typical titration begins with a beaker or Erlenmeyer flask containing a precise volume of the reactant and a small amount of indicator, placed underneath a burette or buretting syringe containing the reagent. By controlling the amount of reagent added to the reactant, it is possible to detect the point at which the indicator changes color. As long as the indicator has been chosen correctly, this should also be the point where the reactant and reagent neutralize each other, and, by reading the scale on the burette, the volume of reagent can be measured.
As the concentration of the reagent is known, the number of moles of reagent can be calculated (since Molarity = number of moles / volume (L). Then, from the chemical equation involving the two substances, the number of moles present in the reactant can be found. Finally, by dividing the number of moles of reactant by its volume, the concentration is calculated. (Rosenfeld 1999)
Color on Acidic Side
Range of Color Change
Color on Basic Side
0.0 – 1.6
3.0 – 4.6
3.1 – 4.4
4.4 – 6.2
5.0 – 8.0
6.0 – 7.6
8.3 – 10.0
10.1 – 12.0
Table 1: Data on Indicators
CHAPTER 4: LITERATURE SURVEY
Structured packing’s were developed in early 1960s, and in the 1970s they came into wide use in various separation processes as contact devices of industrial columnar apparatuses for implementing heat and mass exchange processes, such as absorption, fractionation, extraction, purification and drying of gas, etc.
We propose to define structured packing’s as regular packing’s that are comprised of packets assembled from flat or corrugated sheets forming a spatial multichannel structure. Sheet packing components made of metal foil, network, and polymer, ceramic and other materials may lie in the apparatus as a pack of sheets, may be twisted into spirals and cylinders, and assembled into honeycombed or cellular structures. Coaxial channels thus formed, depending on the shape and relative dispositions of individual sheets; have various configurations ranging from simple (round or polygonal cross section) to complex spatial packing periodically changing with height. Thus, the proposed term “structured packing’s” encompasses a whole multitude of film-type heat-exchanging contact devices (for instance, sprinklers) for cooling towers.
4.0 Introduction on Packing Materials:
Most popular in the industry are structured packing’s of the type that are well adaptable to process conditions and geometric dimensions of the apparatuses. Such packing’s are distinguished by equally high and stable indices of separation of the components of mixtures in a wide range of diameters of the mass-exchange columns (80 mm-20 m). They are usable in almost all process conditions regardless of pressure in the apparatus, gas flow velocity, and liquid load per unit cross sectional area of the apparatus. (D.K. 1980)
Various packing heights of the absorber component were tested to determine the optimal performance of the combined unit. And the various research areas include.
Improved understanding of physical phenomena of the packed bed column.
Better sampling of output mass transfer values
Analytical and flow-visualization methods.
Better predictive modelling.
The Packing of the packed bed column must be selected on various parameters, specially to suit the requirements and the application of the packed bed column. Even though the main objectives of any packing is to maximize the efficiency of mass transfer which can be attained in the following ways:
To maximize the specific surface area
To spread surface area uniformly.
To promote uniform distribution of vapor & liquid throughout the bed
To freely drain any liquid so that stagnant liquid pockets are minimized
To maximize the wetting of packing surface
To maximize the void space per unit column volume.
To minimize friction (good Aerodynamic characteristics).
To ensure uniform resistance to vapour & liquid flow throughout the bed.
To permit easy disengagement of vapour from liquid.
To maximize resistance to mechanical deformation under the weight of bed.
To minimize cost.
To maximize the fouling resistance.
To minimize the liquid holdup.
To minimize damage during abnormal operation.
4.1 Types of Random Packing:
There are various types of random packing materials available in the market with specialized industries dealing with the manufacture of these packing materials. With the growing demand technology in the designing of packing materials has also advanced with generations of packing materials such as:
First generation packing (Raschig rings, Lessing Rings, Berl saddles)
Second generation packing(intalox saddles, pall rings, Hy-pak )
Third generation packing(CMR, Chempak, Nutter rings, Hckp, fleximax, hiflow rings, Lanpac, Impac etc)
Other miscellaneous random packing (Dinapac, super Torus VSP, Interpack, Tellerette, maspac, Levapac etc) (C.G. 1989)
4.2 Rasching Rings:
Raschig rings are pieces of tube (approximately equal in length and diameter) used in large numbers as a packed bed within columns for distillations and other chemical engineering processes. They are usually ceramic or metal and provide a large surface area within the volume of the column for interaction between liquid and gas or vapour. Raschig rings made from borosilicate glass are sometimes employed in the handling of nuclear materials, where they are used inside vessels and tanks containing solutions of fissile material, for example solutions of enriched uranyl nitrate, acting as neutron absorbers and preventing a potential criticality accident. 
First generation random packing.
made from metals like carbon steel or very high alloys such as Monel 400 or Hastelloy C276.
Special Carbon or Graphite made are used in specific applications
Resistant to most acids, alkalis and solvents at temperatures as high as 1500C
Good erosion and thermal shock resistance.
Figure 4: Raschig Rings (CARBON PRODUCTS (INDIA) 2006)
4.3 Pall Rings
Second-generation, random packing.
Primarily made of 304SS and 316L SS metal alloys for quick replacement in kind from stock materials.
Carbon Steel and specialty alloys, such as Monel 400 and Hastelloy C276, made are used for specific applications
Pall Rings are available in various sizes such as (mm) 16, 25, 38, and 50.
Figure 5: Pall Rings (CARBON PRODUCTS (INDIA) 2006)
4.4 Saddle Rings
Saddle Rings were originally introduced to the industry under the product name IMTP® Intalox Metal Tower Packing, a registered trademark of Koch-Glitsch, LP. In terms of performance they are:
In terms of performance, i.e., low-pressure drop and high efficiency.
Used in both high pressure as well as vacuum towers.
Large effective interfacial area,
High mechanical strength
And lower cost due to less metal than previous generations of random packing. (D.K. 1980)
Available in various sizes, which give different combinations of efficiency and pressure drop
Figure 6: Saddle Rings (CARBON PRODUCTS (INDIA) 2006)
4.5 High Performance Random Packing
The random packing with high performance have high void fraction & well distributed surface area which increases the efficiency. They also have a more open shape which improves liquid spreading, low pressure drop and high capacity with high specific heat transfer coefficient. The main specifications of these high performance random packing are:
High strength to weight ratio.
High vapor capacity and low pressure drop
High liquid handling capacity
Open side facing vapor flow, reduces friction.
High capacity and low pressure drop
Figure 7: High Performance Random Packing’s (CARBON PRODUCTS (INDIA) 2006)
4.6 Nutter Ring
Nutter rings were mainly developed for efficiency enhanced by lateral liquid spreading and surface film renewal along with superior surface utilization in mass and heat transfer, allowing shorter packed bed heights. It is also a high performance random packing verified by tests conducted at Fractionation Research Institute which provides mechanical structure with maximum randomness with minimal nesting. (Random Packing n.d.)
4.7 Katapakâ„¢-SP Specifications:
Corrugated sheet spreads in a series of parallel planes.
Packing for reactive distillation and trickle-bed reactors
High separation efficiency and high reaction capacity
Figure 8: Katapakâ„¢-SP (CARBON PRODUCTS (INDIA) 2006)
Universal packing type with surface area of 250m2 /m3
Suitable for a wide range of applications for low to very high liquid loads/ vacuum to gauge pressure.
Available made of wide pallet of stainless steel, alloys and thermoplastics
Figure 9: Mellapakâ„¢ (CARBON PRODUCTS (INDIA) 2006)
4.9 Borosilicate Glass Raschig Rings:
Borosilicate glass is a type of glass with the main glass-forming constituents silica and boron oxide. Borosilicate glasses are known for having very low coefficients of thermal expansion (~5 Ã- 10âˆ’6 /°C at 20°C), making them resistant to thermal shock, more so than any other common glass. Such glass is less subject to thermal stress and is commonly used for the construction of reagent bottles. (D.K.Fedrich 1980)
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Raschig rings used as a safety mechanism, to avoid critical reactions in solutions containing radioactive materials, are usually made of borosilicate glass. Since boron is the active neutron absorbing ingredient, it is important to determine the boron content in the Raschig rings at any given time. A method has been developed to determine rapidly the boron content of borosilicate glasses. Ion exchange and potentiometric measurement are used to determine boron as the tetrafluoroborate ion. The precision of the method is ±2.0 mV. The average difference between values of a wet chemical analysis and those of the potentiometric method is 7.7%.
4.10 Comparison of Structured packing against Random:
The comparison is made for the following crieteria:
Specific surface area vs. packing factor
HETP vs. sp. Surface area
Liquid holdup vs. liquid flow rate
Figure 10: Specific surface area vs. packing factor (Sung Yong Cho n.d.)
Pressure drop per theoretical stage
Figure 11: Specific surface area vs. packing factor
Figure 12: HETP vs. Sp. Surface Area (Sung Yong Cho n.d.)
Figure 13: Pressure Drop Per Theoretical Stage (Sung Yong Cho n.d.)
Figure 14: Liquid Holdup Vs. Liquid Flow Rate (Sung Yong Cho n.d.)
4.11 Use of Packed Bed Absorber
The packed bed absorber is the perfect device that can be used for the treating of the waste cement water. The solute can be run through the packing while the CO2 gas can help in the absorption and neutralization process. The literature survey will consist of an elaborate study of four different types of structured packing and their performances which can be used in this process.
The Gempak 2AT packings of the German company Glitsch consist of vertically oriented thin metal plates whose disposition creates a labyrinth for vapor-liquid contact and improve mixing. Even flows of the liquid film run down these plates and come in contact with their opposite sides, and the vaporous flow moves upward between the plates without obstruction. Unique topography of the packing surface facilitates mixing of the liquid films on both sides of the plates. The vapor and liquid mix uniformly throughout the cross section of the column on account of 90° turn of the Gempak packing packets arranged sequentially across the column height. Besides high efficiency, Gempak packing ensures lower pressure differential, greater power, and longer service life compared to the preceding packing’s. (Dmitrieva 2005)
The MELLAPAK packing consists of structure-forming elements, which fill the entire section of the column. Neighboring elements are oriented at an angle of 90 ~ with respect to one another. Each element consists of parallel folded corrugated metallic sheets. The corrugations are applied at a fixed angle ff to the vertical axis, and are directed toward the opposite sides of the column on adjacent sheets. The area of the specific geometric surface a i of the packing can be varied over a broad range by varying the geometric characteristics of the corrugation. Of the 12 different types of MELLAPAK packings currently produced, four types (MELLAPAK I25.Y, 250.Y, 350.Y, and 500.Y) will be described in detail. The conventional notation of the type of packing includes two geometric indicators: Y represents the value of the angle ff = 45 ~ and the number the specific geometric surface of the packing in m2/m 3 (thus, a i = 250 m2/m 3 in the case of the MELLAPAK 250.Y packing). (Meier 1994)
Figure 15 Mellapack 500Y
220.127.116.11 Performance of Mellapak
The performance characteristics of the four types of MELLAPAK packings (125.Y-500.Y) were determined in an experimental column with an internal diameter of 1 m on test machines manufactured by the “Zultzer” Compa W in Winterthur. This column and the method of investigation were conducted in the complete-reflux-recovery mode. The pressure in the upper part of th
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