Continuous Distillation Column Design

Procedure for Continuous Distillation Column Design

Distillation is used to separate components in a feed mixture based upon their relative boiling points. A simple, continuous distillation column can make the separation between two components into two product streams. In multi-component systems, the two main components to be separated are designated as the light and heavy keys. The light key is the more volatile component in greater purity in the top product stream, and the heavy key is the less volatile component in greater purity in the bottom product stream.

Vapor-Liquid Equilibrium

The starting point upon which all column design is based is to accurately determine the relative volatility of the key components to be separated. Using a mass and energy balance simulation program. The user must set up the basis of the simulation by selecting an appropriate fluid package and the components present in the feed. Activity coefficients, estimated by the program or provided by the user, are used to relate non-ideal component interactions.

Column Operating Objectives

The first step in column design is specifying the column operating objectives. These are defined by a primary product composition and an optimal recovery of the product from the waste, recycle or less important by-product stream. These specifications should be in terms of the heavy key impurity in the top stream and the light key impurity in the bottom stream.

Operating Pressure

Once the top and bottom stream compositions are specified, the dew point of the top stream and the boiling point of the bottom stream may be determined at various pressures. An operating pressure should be selected that allows acceptable temperature differences between available utilities because the overhead vapor must be condensed and the bottom liquid reboiled.

When possible, atmospheric or pressure operation of the column is preferred in order to avoid requiring a vacuum system. However, another consideration is component heat sensitivity, which may require lower pressure operation to avoid fouling, product discoloration or decomposition. Often the relative volatility is also improved at lower pressures.

R/Dmin & Nmin and Feed Stage Estimation

Using the simulation program, shortcut procedures based upon total reflux operation allow the minimum reflux ratio (R/Dmin) and minimum number of ideal separation stages (Nmin) to be determined. Using an actual reflux ratio of 1.2 times the minimum reflux ratio will allow an optimal number of stages to be estimated as well as an appropriate feed stage.

Rigorous simulation of the distillation at a given feed rate and composition may now be accomplished by specifying the following: top and bottom product compositions, number of stages, feed stage, and top and bottom pressure.

Parametric cases of this simulation should be used to verify the estimated number of stages and feed location. Add and subtract stages from both the stripping and rectifying section of the column. Do this until the required reflux ratio becomes approximately 1.2 times the minimum reflux ratio, or the trade off between utility usage and the number of stages appears optimal for the specific column. As more total stages are used, the required reboiler duty will decrease until there are diminishing returns.

Diameter and Height of the Column

At this point, the distillation process is well defined, leaving the column diameter and height to be determined. The chosen design case from the simulation program provides the internal liquid and vapor flows and their physical properties for every stage of the column. The column diameter is chosen to provide an acceptable superficial vapor velocity, or “Fs factor”. This is defined as vapor velocity (ft/sec) times square root of vapor density (lb/ft3), and liquid loading defined as volumetric flow rate (gal/min), divided by the cross sectional area of the column (ft2). The column internals can be chosen as either trays or packing. Trayed columns must avoid flooding, weeping and downcomer backup. Packed columns must avoid flooding, minimum surface wetting and mal-distribution.

Project managers should understand and determine these five key design elements for the projects success. Cost, chemical interactions and equipment needs change in a non-linear fashion, as increased output is required. Qualified engineers should consider these critical steps for distillation column design.

Graph for Pinch Point Analysis

Pinch Point Analysis

Pinch Point Analysis is a systematic process design methodology consisting of a number of concepts and techniques that ensure an optimal use of energy. The Pinch is characterized by a minimum temperature difference between hot and cold streams and designates the location where the heat recovery is the most constraint.

The fundamental computational tool is the Problem Table algorithm. This tool allows the identifications of the Pinch, as well as of targets for hot and cold utilities.

The net heat flow across Pinch is zero. Consequently, the system can be split into two stand-alone subsystems, above and below the Pinch. Above the Pinch there is need only for hot utility, while below the Pinch only cold utility is necessary. For given ΔTmin the hot and cold utility consumption identified so far becomes Minimum Energy Requirements (MER). No design can achieve MER if there is a cross-pinch heat transfer.

The partition of the original problem in subsystems may introduce redundancy in the number of heat exchangers. When the capital cost is high, it might be necessary to remove the Pinch constraint in order to reduce the number of units. The operation will be paid by supplementary energetic consumption, which has to be optimized against the reduction in capital costs.

The result is that heat recovery problem becomes an optimization of both energy and capital costs, constraint by a minimum temperature approach in designing the heat exchangers. Stream selection and data extraction are essential in Pinch Analysis for effective heat integration.

The key computational assumption in Pinch Point Analysis is constant CP on the interval where the streams are matched. If not, stream segmentation is necessary

The counter-current heat flow of the streams selected for integration may be represented by means of Composite Curves (CC). Another diagram, Grand Composite Curve (GCC) allows the visualization of the excess heat between hot and cold streams against temperature intervals. This feature helps the selection and placement of utilities, as well as the identification of the potential process/process matches.

The synthesis of a Heat Exchanger Network consists of three main activities:

  • Set a reference basis for energy integration, namely:

-Minimum Energy Requirements (MER)

-Utility selection and their placement

-Number of units and heat exchange area

-Cost of energy and hardware at MER

  • Synthesis of heat exchanger network (HEN) for minimum energy requirements and maximum heat recovery. Determine matches in subsystems and generate alternatives.
  • Network optimization. Reduce redundant elements, as small heat exchangers, or small split streams. Find the trade-off between utility consumption, heat exchange area and number of units. Consider constraints

The improvement of design can be realized by Appropriate Placement and Plus/Minus principle. Appropriate Placement defines the optimal location of individual units against the Pinch. It applies to heat engines, heat pumps, distillation columns, evaporators, furnaces, and to any other unit operation that can be represented in terms of heat sources and sinks.

The Plus/Minus principle helps to detect major flow sheet modifications that can improve significantly the energy recovery. Navigating between Appropriate Placement, Plus/Minus Principle and Targeting allows the designer to formulate near-optimum targets for the heat exchanger network, without ever sizing heat exchangers.

Pinch Point principle has been extended to operations involving mass exchange. Saving water can be treated systematically by Water Pinch methodology. Similarly, Hydrogen Pinch can efficiently handle the inventory of hydrogen in refineries. Other applications of industrial interest have been developed in the field of waste and emissions minimization. The systematic methods in handling the integration of mass-exchange operations are still in development. In this area the methods based on optimization techniques are very promising.

RO/DI Water Systems

RO/DI Water Systems

RO/DI stands for Reverse Osmosis and Deionization. The product is a multi-stage water filter, which takes in ordinary tap water and produces highly purified water.

Tap water often contains impurities that can cause problems. These may include phosphates, nitrates, chlorine, and various heavy metals. Excessive phosphate and nitrate levels can cause an algae bloom. Copper is often present in tap water due to leaching from pipes and is highly toxic to invertebrates. An RO/DI filter removes practically all of these impurities.

There are typically four stages in a RO/DI filter:

  • Sediment filter
  • Carbon block
  • Reverse osmosis membrane
  • Deionization resin

If there are less than four stages, something was left out. If there are more, something was duplicated.

The sediment filter, typically a foam block, removes particles from the water. Its purpose is to prevent clogging of the carbon block and RO membrane. Good sediment filters will remove particles down to one micron or smaller.

The carbon, typically a block of powdered activated carbon, filters out smaller particles, adsorbs some dissolved compounds, and deactivates chlorine. The latter is the most important part: free chlorine in the water will destroy the RO membrane.

The RO membrane is a semi-permeable thin film. Water under pressure is forced through it. Molecules larger/heavier than water (which is very small/light) penetrate the membrane less easily and tend to be left behind.

The DI resin exchanges the remaining ions, removing them from the solution.

There are three types of RO membrane on the market:

  • Cellulose Triacetate (CTA)
  • Thin Film Composite (TFC)
  • Poly-Vinyl Chloride (PVC)

The difference between the three concerns how they are affected by chlorine: CTA membranes require chlorine in the water to prevent them from rotting. TFC membranes are damaged by chlorine and must be protected from it. PVC membranes are impervious to both chlorine and bacteria.

Reverse osmosis typically removes 90-98% of all the impurities of significance to the aquarist. If that is good enough for your needs, then you don’t need the DI stage. The use of RO by itself is certainly better than plain tap water and, in many cases, is perfectly adequate.

RO by itself might not be adequate if your tap water contains something that you want to reduce by more than 90-98%.

A DI stage by itself, without the other filter stages, will produce water that is pretty much free of dissolved solids. However, DI resin is fairly expensive and will last only about 1/20th as long when used without additional filtration. If you’re only going to buy either a RO or a DI, it would be best to choose the RO, unless you only need small amounts of purified water.

Duplicating stages can extend their life and improve their efficiency. For example, if you have two DI stages in series, one can be replaced when it’s exhausted without producing any impure water. If you have both a 5-micron sediment filter and a 1-micron filter, they will take longer to clog up. If there are two carbon stages, there will be less chlorine attacking the TFC membrane. Whether the extra stages are worth the extra money is largely a matter of circumstance and opinion.

RO/DI capacities are measured in gallons per day (GPD), and typically fall within the 25-100 GPD range. The main difference between these units is the size of the RO membrane. Other differences are (a) the flow restrictor that determines how much waste water is produced, (b) the water gets less contact time in the carbon and DI stages in high-GPD units than low-GPD units, and (c) units larger than 35 GPD typically have welded-together membranes.

As a result of the membrane welding and the reduced carbon contact time, RO membranes larger than 35 GPD produce water that is slightly less pure. This primarily affects the life of the DI resin.

Most aquarists won’t use more than 25 GPD averaged over time. If you have a decent size storage container, that size should be adequate. A higher GPD rating comes in handy, however, when filling a large tank for the first time or in emergencies when you need a lot of water in a hurry.

The advertised GPD values assume ideal conditions, notably optimum water pressure and temperature. The purity of your tap water also affects it. In other words, your mileage will vary.

An RO filter has two outputs: purified water and wastewater. A well-designed unit will have about 4X as much wastewater as purified water. The idea is that the impurities that don’t go through the membrane get flushed out with the wastewater.

There is nothing particularly wrong with the wastewater except for a slightly elevated dissolved solid content. It may actually be cleaner than your tap water because of the sediment and carbon filters. Feel free to water your plants with it.

What is BIM?

The Handbook of BIM (Eastman, Teicholz, Sacks & Liston 2011) defines, “With BIM (Building Information Modeling) technology, one or more accurate virtual models of a building are constructed digitally. They support design through its phases, allowing better analysis and control than manual processes. When completed, these computer-generated models contain precise geometry and data needed to support the construction, fabrication, and procurement activities through which the building is realized.”

BIM or Building Information Modeling is a process for creating and managing information on a construction project across the project lifecycle. One of the key outputs of this process is the Building Information Model, the digital description of every aspect of the built asset. This model draws on information assembled collaboratively and updated at key stages of a project. Creating a digital Building Information Model enables those who interact with the building to optimize their actions, resulting in a greater whole life value for the asset.

B is for Building.

The key point to remember here is that “building” doesn’t mean “a building.” BIM can be used for so much more than designing a structure with four walls and a roof. This preconceived notion of “building” comes from its roots—in an etymological sense, it quite literally means “house.”

In order to get the true gist of BIM, however, it helps to think of the word “building” in terms of the verb “to build.”

BIM is a process that involves the act of building something together, whether it relates to architecture, infrastructure, civil engineering, landscaping or other large-scale projects.

I is for Information.

And that information is embedded into every aspect of your project. This is what makes BIM “smart.”

Every project comes with a staggering amount of information, from prices to performance ratings and predicted lifetimes. It tells your project’s life story long before the ground is ever broken and it will help track potential issues throughout your project’s lifetime. BIM is a way to bring all of these details into one place so it’s easy to keep track of everything.

M is for Modeling.

In BIM, every project is built twice—once in a virtual environment to make sure that everything is just right and once in a real environment to bring the project to life.

This step is the overview of every other aspect of the building and its information. It provides the measure or standard for the building project—an analogy or smaller-scale representation of the final appearance and effect. It will continue to model this representation throughout the building’s lifespan.

This model can become a tool for the building owner’s reference long after construction is completed, helping to inform maintenance and other decisions. It’s also the step that will help to sell a concept while condensing all of those other layers of information that show the building’s every detail.

How can BIM help you?

BIM brings together all of the information about every component of a building, in one place. BIM makes it possible for anyone to access that information for any purpose, e.g. to integrate different aspects of the design more effectively. In this way, the risk of mistakes or discrepancies is reduced, and abortive costs minimized.

BIM data can be used to illustrate the entire building life-cycle, from cradle to cradle, from inception and design to demolition and materials reuse. Spaces, systems, products and sequences can be shown in relative scale to each other and, in turn, relative to the entire project. And by signalling conflict detection BIM prevents errors creeping in at the various stages of development/ construction.

What is a BIM object?

A BIM object is a combination of many things

  • Information content that defines a product
  • Product properties, such as thermal performance
  • Geometry representing the product’s physical characteristics
  • Visualisation data giving the object a recognisable appearance
  • Functional data enables the object to be positioned and behave in the same manner as the product itself.

What is the future of BIM?

The future of the construction industry is digital, and BIM is the future of design and long term facility management; it is government led and driven by technology and clear processes; and it is implementing change across all industries. As hardware, software and cloud applications herald greater capability to handle increasing amounts of raw data and information, use of BIM will become even more pronounced than it is in current projects.

BIM is both a best-practice process and 3D modeling software. By using it, designers can create a shared building project with integrated information in a format that models both the structure and the entire timeline of the project from inception to eventual demolition.

It enables architects and engineers alike to work on a single project from anywhere in the world. It condenses a plethora of information about every detail into a workable format. It facilitates testing and analysis during the design phase to find the best answer to a problem.

It makes for easier design, simpler coordination between team members and easier structure maintenance across the entire built environment—and this is just the beginning.