Chemical Engineering

January 25, 2012


Distillation

Distillation: How to Specify & Install Cartridge Trays

Ruth R. Sands, DuPont Engineering Research & Technology

    Cartridge trays, defined below, can be the perfect hardware solution for distillation columns that are less than 3 feet (0.9m) in diameter and have a unique need for trays. Packing is usually the preferred choice for small-diameter columns, but there are several circumstances in which trays are advantageous. Examples include fouling systems, certain foaming systems, high-pressure applications and systems that require high efficiency and a large turndown. All of these can be accommodated with various tray designs installed as cartridge units. Furthermore, cartridge trays can be installed much faster than traditional ring-supported trays. [more]

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Distillation

Dry Tray Pressure Drop Of Sieve Trays

Daniel R. Summers Sulzer Chemtech USA, Inc.

A new correlation that matches most commercial trays The sieve tray has been in the distillation market place for many decades. It has been used extensively in distillation columns worldwide as a highly efficient vapor-liquid-contacting device. Many people have examined dry tray pressure-drop data for sieve tray decks and there have been several attempts to correlate these data. But the error of such correlations is high. Figure 1.  A plot of sieve tray orifice coefficient at 12% open area and high Reynolds Number for three sources Figure 2. [more]

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Distillation

Reboiler Circuits For Trayed Columns

Design Practices Committee Fractionation Research Inc.

The reboiler generally supplies most of the energy required to effect component separation. If too much heat is supplied, the tower may flood; conversely, if too little heat is available, separation performance may decrease via poor reflux ratio (pinching), excessive weeping or poor tray action. Proper design of reboiling systems involves [more]

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Distillation

Estimating Equilibrium Phase Distribution Ratios

S.K.Mukherjee, Hindustan Petroleum Corp.

    The equilibrium phase distribution ratio (K value) is necessary for designing and assessing the performance of distillation columns. This paper proposes a simplified approach to calculate the K value that is suitable for preliminary design work and for checking the performance of existing columns. The article also explains the first principles for calculation of K values from the fugacity of real gases and liquids. To the best of the author's knowledge, the proposed method has not before been available in published literature. Concentration-dependant K values can be calculated from empirical correlations involving lengthy calculations. Alternatively, these values can be obtained from equations of state - using commercial software, as hand calculations are unwieldy. [more]

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Distillation

Feature Report: Column Instrumentation Basics

Ruth R. Sands, DuPont Engineering Research & Technology

An understanding of instrumentation is valuable in evaluating and troubleshooting column performance Instrumentation is critical to understanding and troubleshooting all processes. Very few engineers specialize in this field, and many learn about instrumentation through experience, myth and rumor. A good understanding of the various types of instrumentation used on columns is a valuable tool for engineers when evaluating column performance, starting up new towers or troubleshooting any type of problem. This article gives an overview of the common types of instruments used for pressure, differential pressure, level, temperature and flow. A discussion of their accuracy, common installation problems and troubleshooting examples are also included. The purpose of this article is to provide some basic information regarding the common types of instrumentation found on distillation towers so that process engineers and designers can do their jobs more effectively. [more]

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Distillation

Causes and Prevention of Packing Fires

Design Practices Committee Fractionation Research Inc.

    Most columns shut down without fires. Yet most refineries and chemical plants at some time or another have experienced spontaneous or hot work ignitions in at least one column. When this occurs inside a column during or after shutdown, the results can be devastating and dangerous. This paper concentrates on the fires related to packing in columns.   Many refineries and chemical plants have installed packing in columns as original equipment or conversion from trays. Fires within packed beds have been reported in various services [1, 4]. [more]

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Distillation

Inspecting Fractionation Towers

Juan Manuel Sanchez, Álvaro Valverde, Carlos Di Marco and Eduardo Carosio, YPF S.A.

Distillation columns are often workhorses in the chemical process industries (CPI), and at times, it is necessary to physically inspect the interior of a column. This article demonstrates what an inspection can reveal, by sharing the experience of actual fractionation-tower inspections in a petroleum refinery. The points mentioned are intended to provide guidance to future tower inspectors, so that they can focus on extremely important and relevant points that will directly impact the operation of distillation columns. [more]

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Distillation

Optimization of Complex Distillation Columns

G.P. Rangaiah, Chan Tsun Yue, Johnny and K. Hidajat, National U. of Singapore

    Distillation, based on differences in the volatility of components, is the most common unit operation for separating homogeneous mixtures. In simple distillation columns, a single feed stream is separated into two product streams. Complex columns are those that involve either two feed streams or three product streams - a side-stream in addition to the top and bottom products. These complex systems are used to separate multiple mixtures in the oil and gas, petrochemical and other chemical process industries (CPI). This article offers a study and concluding guidelines on how to minimize costs by optimizing complex distillation units.   Column optimization, in this article, refers to minimizing the sum of both annualized capital and operating costs. [more]

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Distillation

Minimize Trapped Components in Distillation Columns

Soundar Ramchandran , Solutia, Inc.

    Minor components trapped in distillation columns can react to form solids that cause major problems. The conventional responses to this situation focus upon dealing with the solids after they have become formed. A better strategy consists of preventing or minimizing the solids formation in the first place, by changing the column operating conditions.   Understanding the situation The solids-formation problem arises during distillation when minor components react with each other under the process conditions inside the column, forming polymers or other heavy molecules that are not completely soluble in the liquid phase in certain sections of the column. The presence of these solids often leads to unpredictable vapor-liquid contact, in turn adversely affecting product quality and increasing the plant’s downtime and maintenance expenses. In many cases, engineers approach the problem by aiming to modify trays or other column internals to deal with solids more easily, and/or by injecting additives to either inhibit the reactive species or disperse the solids. [more]

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Distillation

Distillation Back Mixing: Impact on Batch Yields

Steven D. Kohler, Ellen C. Murphy, Louis P. Silveri and Manuel R. Florez; Renessenz LLC and
Laxminarasimhan Padmanabhan; Aspen Technology,

Manufacturers of specialty chemicals are routinely required to reduce costs and increase the production of their high-value products. Specifically, competitive pressures from the developing world, increases in raw material costs and volatility in energy costs are putting significant pressure on manufacturers to minimize processing costs and maximize product yields. The economic impact of optimization efforts can be magnified for temperature-sensitive products due to a decrease in their overall recovery rate with an increase in batch-distillation cycle time. Process-improvement activities related to existing batch distillation columns have traditionally focused on maximizing the separation efficiency, debottlenecking operations, and developing optimal batch distillation strategies. This is typically accomplished by retrofitting equipment (such as column internals, heat exchangers and vacuum systems), following better operating strategies, and improving existing control systems (for instance, using batch-sequencing systems and procedures). In the process analysis phase, less attention is devoted to identifying and correcting poorly designed reflux and product- handling systems. [more]

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Distillation

Reboiler Condensate Drums: Silencing Hydraulic Hammer

Henry Kister,  Fluor Corp.;  Antonio João Dias Prestes and Luiza Milbroth Jorge, Braskem S.A.

Condensate pots are commonly used on the condensing sides of some reboilers, such as those that are heated by steam or refrigerant vapor. These pots provide a liquid seal that prevents uncondensed vapor from blowing out of the reboiler into the condensate system. A break in this seal can result in heat transfer losses. With steam reboilers, blowing vapor into the condensate header may also lead to hammering. Hammering may also be caused by poor design of the condensate pot.

Hammering is the problem described here. Braskem’s deethanizer reboiler-condensate drum experienced hydraulic hammering when the reboiler operated at low steam loads and when the column loads were reduced quickly. The condensate-drum level indicator was unreliable and insensitive to the variations in level-control-valve opening. Condensate-drum level control was poor, and the control valve had to be operated on manual.

[more]

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