Laboratory Distillation

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Process Engineering Guide: GBHE-PEG-MAS-602

Laboratory Distillation Information contained in this publication or as otherwise supplied to Users is believed to be accurate and correct at time of going to press, and is given in good faith, but it is for the User to satisfy itself of the suitability of the information for its own particular purpose. GBHE gives no warranty as to the fitness of this information for any particular purpose and any implied warranty or condition (statutory or otherwise) is excluded except to the extent that exclusion is prevented by law. GBHE accepts no liability resulting from reliance on this information. Freedom under Patent, Copyright and Designs cannot be assumed.



Process Engineering Guide: Laboratory Distillation CONTENTS SECTION 0 INTRODUCTION/PURPOSE 2 1 SCOPE 2 2 FIELD OF APPLICATION 2 3 DEFINITIONS 2 4 TYPES OF LABORATORY DISTILLATION COLUMNS 2 5 OLDERSHAW COLUMNS 7 5.1 Experimental Equipment 4 5.2 Start Up Procedure 9 5.3 Shut Down Procedure 9 6 PACKED COLUMNS 9

6.1 Experimental Equipment 10 7 SCALE-UP FROM LABORATORY COLUMNS 10 7.1 Scale-Up from Oldershaw Columns 10 7.2 Scale-Up from Packed Columns 11 7.3 Design of Experiments 11 8 GENERAL POINTS TO NOTE 12



9 SAFETY STANDARDS FOR LABORATORY DISTILLATIONS 13

10 REFERENCES 13 APPENDICES A SAFETY GUIDELINES FOR LABORATORY DISTILLATIONS 14 TABLES 1 OLDERSHAW COLUMN SECTION 3 2 SCHEMATIC OUTLINE OF CONTINUOUS OLDERSHAW

COLUMN UNIT 4 3 FEED SECTION 5 4 SWINGING BUCKET HEAD 6 5 PUMPED REFLUX SYSTEM 7 6 THERMO-SYPHON REBOILER 8 DOCUMENTS REFERRED TO IN THIS PROCESS ENGINEERING GUIDE 16



0 INTRODUCTION/PURPOSE Recent advances in computational techniques and software often allow a rapid assessment of distillation requirements. If the system being studied is close to ideal, for example, a mixture of hydrocarbons, such an approach may suffice for final design purposes. However, for non-ideal systems or for the development of a new process or product where vapor-liquid equilibria data are scant or absent a different approach should be adopted. In these circumstances it is necessary to simulate the separation by means of a laboratory distillation. A laboratory distillation can also prove useful in: (a) providing material for product evaluation studies; (b) gathering information on suitable materials of construction; (c) observing the build-up of minor impurities in the system; (d) determining the height equivalent to a theoretical plate for a given

packing/system. 1 SCOPE This Process Engineering Guide outlines the general procedures to be followed in carrying out laboratory distillations. Practical hints are included, based on the hard earned experience of practitioners in this field. 2 FIELD OF APPLICATION This Guide applies to the process engineering community in GBH Enterprises worldwide. 3 DEFINITIONS For the purposes of this Guide no specific definitions apply.



4 TYPES OF LABORATORY DISTILLATION COLUMNS Two main types of column are used to simulate distillation operations: (a) sieve tray columns (Oldershaw columns); (b) packed columns (random or structured packing). Laboratory distillation columns are usually fabricated in glass and are limited to operation at atmospheric pressure or below. 5 OLDERSHAW COLUMNS Glass Oldershaw columns provide a useful means of simulating full scale operation of trayed columns. A 50 mm diameter column is recommended. Smaller diameter columns are available but suffer from the difficulty of assessing wall effects. A typical feed rate to a 50 mm diameter column would be 1 l/h, though this is obviously dependent on boil up rate and/or operating pressure. The plate efficiencies of Oldershaw columns can be readily measured using standard test mixtures (Ref. 1) and subsequently correlated with plate efficiencies in plant columns. For most systems it is adequate to assume a 50% Oldershaw plate efficiency. The Oldershaw column is essentially a small scale sieve, or perforated tray column containing circular downcomers (see Figure 1). Sections are available containing 5, 10, 15, 20, 30 plates. Ground glass joints make it possible to stack these sections and also connect the overheads, feed and reboiler systems as required. Metal Oldershaw columns are available for operation at pressures above atmospheric.



FIGURE 1 OLDERSHAW COLUMN SECTION



5.1 Experimental Equipment A schematic outline of a typical Oldershaw column system to simulate a continuous operation is given in Figure 2. FIGURE 2 SCHEMATIC OUTLINE OF CONTINUOUS OLDERSHAW

COLUMN UNIT



5.1.1 Column The required number of trays are obtained by stacking sections of Oldershaw columns. These are best connected by means of ground glass joints. B50 joints are suitable for 50 mm diameter columns, smaller joints restrict the diameter of the column and hence throughput, i.e. induce flooding. PTFE sleeves should be used for sealing the joints rather than conventional greases. The use of one full sleeve plus the top half of another sleeve at each joint was found to be an efficient way of preventing leakages. Clamped ball joints are not recommended. They give rise to problems with leakages and introduce a restriction to the column diameter. The column and its associated overheads and reboiler systems should be securely clamped without exerting undue stress on any part of the equipment. A plumb line should be used to ensure that the column is vertical. It is normal practice to leak test the apparatus prior to lagging the column. An important consideration in the operation of laboratory distillation columns is the minimization of heat losses. Heat loss will give rise to internal reflux and result in improved separation performance. This is a potential disaster area if not recognized. Scale-up from laboratory data will give an under-designed plant column. Oldershaw columns can be purchased with Dewar jackets attached for insulation. Their use is not advised as there have been several instances of the jackets losing their vacuum. The preferred course is to thoroughly lag the column and ancillary equipment. The column should be lagged with alternate layers of metal foil and fibre glass lagging (about 25 mm thickness), four layers of each material. It is standard procedure to include within the lagging, although not in direct contact with the metal foil, wrappings of heat-by-the-yard tape (requires a 50 V supply). This is an essential feature for columns containing, approximately, more than 40 trays or when distilling mixtures at temperatures above 150° C. Window flaps should be cut into the lagging at suitable intervals so that column loading can be observed. A typical liquid loading in a 50 mm diameter column would be 2 to 5 ml per plate. The column can be assembled with one or more feed entry sections (see Figure 3). The glass columns as purchased will contain inlets for thermowells to accommodate thermocouples. Similar provision should be made to measure feed, still head and reboiler temperatures. If heat by-the-yard tape is used provision has to be made to measure temperatures within the lagging.



It is normal procedure to keep a continuous record of temperatures throughout the system, this is generally achieved by the use of a multi-point temperature recorder. FIGURE 3 FEED SECTION

5.1.2 Overheads System Two types of overhead system are in common use, these are Swinging Bucket Head and Pumped Reflux. 5.1.2.1 Swinging Bucket Head A schematic outline of a swinging bucket head is given in Figure 4. This is a liquid dividing head, the separation between distillate and reflux is made on the condensate from the overhead condenser. The bucket is actuated by a solenoid which is turned on and off by an electrical timer. The reflux timers in normal operation can be set for a wide range of reflux ratios. A one to three second off time gives smooth column operation with accurate reflux control.



The reflux ratio should be checked by operators at least once per day with the aid of a stop watch. Time is well spent initially in setting up the swinging bucket/solenoid arrangement to give trouble free operation. Note: Vapor dividing heads are in use, i.e. separation is made on the vapor rising from the column. Again a solenoid/timer arrangement is employed. Vapor by-passing can occur with such devices and is difficult to detect. They are not therefore recommended for general use. FIGURE 4 SWINGING BUCKET HEAD



5.1.2.2 Pumped Reflux A typical pumped reflux system is outlined in Figure 5. The total condensate is collected in a vessel, the reflux drum, and reflux pumped back to the column at a pre-determined rate. Provision can be made to preheat the reflux returned to the column. For operation at atmospheric pressure the distillate overflows an internal weir in the reflux drum and is collected under gravity flow in a graduated vessel, usually a separating funnel. The pumped reflux system can be modified to cope with a two liquid phase condensate. Alternatively vapor dividers are used. Glass condensers with water as coolant are used for the majority of laboratory distillations. The condenser provides an open vent for the system. If the overheads contain volatile components (approximately, boiling point less than 40° C) water may be replaced with a coolant circulating system. Additionally it is normal practice to provide for catching any uncondensed material by passing any vapor escaping from the condenser through 1 or 2 cold catchpots. The contents of the latter should be measured to obtain a true mass balance. Distillation involving high melting point components such as phenol (m pt 43° C) require that the cold water to the condenser is replaced by a warm water supply.



FIGURE 5 PUMPED REFLUX SYSTEM

5.1.3 Bottoms System A thermo-syphon reboiler is in common use for continuous distillations (see schematic outline in Figure 6).



Heat is supplied by means of two 1 kW cartridge heaters. For ease of operation and certainly for extended operations it is desirable to vary the heat input to the reboiler as a function of the temperature at some point in the column. In this mode about 90% of the heat required is supplied via one cartridge heater at a fixed rate. The remainder is supplied via the other heater at a variable rate determined by a temperature controller which is actuated by the system temperature. The system residence time may be adjusted by changing the liquid volume of the reboiler, i.e. by adding glass beads or the like to the reboiler. The bottoms stream is cooled by passing through a cooler and subsequently pumped to a graduated collection vessel, normally a separating funnel. Under steady state conditions the liquid level in the reboiler will remain constant. Unless a constant voltage regulator is installed still operation requires operator vigilance in the early morning/late afternoon periods when voltage surges usually occur. To facilitate disassembly the reboiler should be mounted on a laboratory jack. The jack is positioned in a steel tray of sufficient depth to hold the liquid inventory of the total system. Note: Iso-mantles are normally used as the heating source in batch distillation simulations. Their use in continuous distillation units is limited because of heat requirements.



FIGURE 6 THERMO-SYPHON REBOILER

5.1.4 Feed and Product Systems Feed is normally pumped from a drum, filtered and fed into one of two graduated feed vessels, normally 2 liter separating funnels. Feed to the still is withdrawn from the other separating funnel so that flow rates can be readily measured. A 100 ml burette is attached to the feed vessel by means of a 3 way tap so that spot rate checks can be made.



The feed to the still is pumped via a calibrated metering pump (piston, gear or peristaltic type are all suitable) to a feed preheater and the feed entry point on the column. Feed may be preheated by passing through an insulated, electrically wound tube; flow through a coil situated in a heating medium; passing through a "condenser" connected to a constant boiling liquid. Distillate is collected under gravity flow into a graduated separating funnel. The bottoms stream is removed via a cooler and calibrated pump into a graduated vessel. The pump rate is set to give a constant liquid level in the reboiler under steady state operating conditions. For extended operations it is advantageous to double up on pumps (or pump heads). 5.2 Start Up Procedure Close operator attention is required at start up so that pump rates and heat inputs can be changed as needed. Liquid is charged to the reboiler and heat introduced until liquid is refluxing in the column. Feed is then pumped to the column at the desired rate. The preheater is adjusted to give the required feed temperature. The reflux timer is switched on and the boil up rate adjusted by means of the heat to the reboiler to give the required distillate take-off rate. Material is pumped from the reboiler to maintain a constant liquid level. Heat from the heat-by-the-yard within the lagging is adjusted to match column internal and external temperatures as closely as possible. The still is operated until flow rates and temperatures reach steady state conditions. It is necessary to allow about six volume changes of the key components in the system before steady state is reached and analysis of products becomes meaningful. It may take considerably longer if the quantity of less volatile material in the feed is low and a high concentration of these materials in the reboiler is required. 5.3 Shut Down Procedure The normal shut down procedure is to switch off the heat supplies followed by the pumps.



6 PACKED COLUMNS There are two main types of packing used in laboratory columns: (a) random packings (e.g. Dixon or wire-mesh rings, glass helices); (b) structured packings (e.g. Sulzer laboratory scale structured packing). Structured packings have, in general, replaced the knitted wire-mesh packings introduced into the column in the form of a spiral roll i.e. as supplied by Knitmesh or Goodloe. The latter are not recommended since it is difficult to guarantee a uniform packing density. In consequence, hydraulic capacity and efficiency in the column may be variable and unpredictable. Wire-mesh rings are an efficient laboratory column random packing. Reliable data for a particular separation case can only be ascertained by carrying out tests. However, as a typical example, the HETP for a 50 mm column packed with 4 × 4 mm wire-mesh rings will vary from about 20 to 40 mm as the boil up rate is increased from 2 to 15 l/h. Note: Packing size should always be less than one tenth of the column diameter. The Sulzer laboratory scale structured packing is a recent introduction, developed specifically for use in columns of small internal diameter. HETP is similar to that for Dixon rings but is claimed to be practically independent of the mixture to be distilled. Higher throughputs can be achieved. Note: There is some doubt as to the use of structured packings in plant columns operated at high pressure, approximately, greater than 15 bar. 6.1 Experimental Equipment The overheads, bottoms, feed and product systems are as described in Clause 5, as are the start up and shut down procedures.



6.1.1 Packing Columns Careful attention should be given to the suppliers brochure as to the means of packing a laboratory column. A packing support grid will be required at the bottom of the column. Structured packings (or knitted mesh packings) should not be forced into a column or distorted by pushing down. Advice should be sought from the supplier on the best means of liquid distribution onto the packing. Again with structured packings thermowells should be located between beds of packing and not forced into the packing. 7 SCALE-UP FROM LABORATORY COLUMNS Experiments using laboratory scale distillation equipment should be designed to provide information which can be scaled-up to commercial operation. 7.1 Scale-Up from Oldershaw Columns Scale-up from data collected using a laboratory Oldershaw column is relatively straight forward. For the majority of systems it is adequate to assume a 50% plate efficiency when using a 50 mm diameter Oldershaw column. The efficiency of Oldershaw columns does vary with different types of system. Systems in which surface tension increases from top to bottom of the column generally give higher efficiencies in Oldershaw columns; they are also prone to give foaming problems in trayed columns. Plant columns generally have a higher plate efficiency than 50%. A one to one scale-up from Oldershaw plate to a commercial trayed column should be conservative i.e. the required separation should be easily achieved on the plant scale.



7.2 Scale-Up from Packed Columns Scale-up from data collected using a packed laboratory column is more difficult for two reasons: (a) the laboratory scale packing is itself inherently more efficient than the

commercial packing and, ideally, comparative data on a standard test mixture are required;

(b) large columns (greater than 0.5 m diameter) suffer some loss of efficiency

from imperfect liquid distribution. The HETP of an efficient laboratory column packing (wire mesh rings or Dixon packing) is dependent on the shape and size of the packing and also on the operating conditions and the properties of the mixture to be separated. The HETP of Sulzer laboratory scale structured packing is claimed to be practically independent of the mixture to be distilled. In-house experts and packing suppliers should be approached prior to starting experimental work to discuss the relevance of any data produced opposite the expected commercial size column, i.e. scale-up considerations. 7.3 Design of Experiments The efficiency of Oldershaw columns is well established. Experimental work with such systems usually involves building a replica of the full scale column and operating this in the continuous mode. Few problems are experienced with the design of experiments and subsequent scale-up. Packed columns should be approached more carefully. This is particularly true if tests are carried out under total reflux conditions, probably using a test facility provided by the manufacturer containing less packed height than would be envisaged for the full scale unit. In such circumstances the design of experiments is of particular importance. The test process is that of batch distillation under total reflux conditions. Using the available vapor-liquid equilibrium data and relevant computer program the equilibrium y-x data can be tabulated at total reflux, for example:



Thus by inspection the experimental value of y is approximately at N + 1 = 11, and that of x at N + 1 = 5. Therefore the plate equivalence of the column is 11 - 5 = 6 theoretical plates. To obtain the best accuracy, conditions should be chosen so that the sum of x and y is unity, i.e. x and y are equally disposed about the mole fraction 0.5. To satisfy this condition it is necessary that the charge to the boiler should have a higher concentration of the more volatile component than the desired value of x under total reflux because some of the volatile component will be in the hold-up in the column. The appropriate equation to calculate the desired initial mole fraction, xc is given by:



If, as mentioned earlier, the test facility contains less equivalent height of packing than is envisaged for the full scale column consideration should be given to carrying out experiments with more than one initial starting composition. The compositions are chosen to evaluate performance of packing at approximately the top, middle and bottom sections of the commercial column. As always due consideration should be given to the reliability of the vapor-liquid equilibria data. For ideal systems the HETP can be confidently predicted from experimental runs. For non-ideal or complex systems with poor or estimated vapor-liquid equilibria data the apparent HETP should be carefully assessed. If the HETP for a given packing is much lower than that normally expected the equilibria data were probably not correct and should be checked. 8 GENERAL POINTS TO NOTE Carrying out laboratory distillation simulations can be a rewarding and a frustrating exercise. Over the years a number of general points to note have emerged. (a) Corrosion test pieces can be placed at convenient points in the column

system. Make provision for them prior to assembling the unit. (b) There are various types of Oldershaw column (number of holes per plate,

higher weirs for extractive distillation operations etc). Make sure that the column sections to be stacked are compatible.

(c) Check the mass balance frequently, what goes in should come out

otherwise a leak may have developed. Similarly at steady state conditions a loss of liquid level in the reboiler can be the result of a leak in the system.

(d) The absence of froth or foam in a laboratory column is no guarantee that

this will be the case in a plant column. Downcomer areas can be significantly different.

(e) Take care in pressure extrapolation. Laboratory columns are usually only

operated up to atmospheric pressure. Vapor-liquid equilibria characteristics can be quite different for a plant column operating at a few bar pressure.



(f) In distillations carried out under vacuum the feed and product pumps need to be thoroughly primed before the vacuum is switched on.

9 SAFETY STANDARDS FOR LABORATORY DISTILLATIONS The application of suitable safety standards for carrying out laboratory distillations is imperative. Safety guidelines for laboratory distillations are shown in Appendix A. The Guidance for Safety Standards for Research and Technology Sites SHOULD be studied carefully and any relevant action taken before experimental work is started. 10 REFERENCES 1 E F G Herington, Testing Distillation Columns, International Union of Pure

and Applied Chemistry, p.2421, Vol.51, Pergamon Press Ltd, (1979). 2 R K Badhwar, Department Paper 67/51, "Distillation : Overall Efficiency of

Oldershaw Columns, 1/3/67".



APPENDIX A SAFETY GUIDELINES FOR LABORATORY DISTILLATIONS The technique of distillation is commonly used in chemical laboratories. The purpose of the following guidelines is to set out the basic safety principles upon which instructions for distillation operation should be based. A.1 PRINCIPLES The following should be incorporated into any instructions relating to distillation procedure: (a) Equipment should be appropriate for intended use. (b) Equipment should be assembled carefully and securely. (c) Equipment should be appropriately screened and vented. (d) Containment in the event of spillage should be adequate. (e) Heating and cooling systems should be appropriate to the type of

distillation. (f) Controls for heating and cooling should be remote from the equipment. (g) Where appropriate raw materials should be tested for peroxides which

should be destroyed prior to distillation. (h) Appropriate emergency provisions should be made prior to commencing

any distillation. A.1.1 Consideration In order to comply with the above principles, consideration should be given to the following: (a) Glassware should be examined for any defect.

Round bottom flasks (or small heavy walled separating funnels with a high vacuum tap) should be used as receivers for distillations under reduced pressure.



Sample points should be easily accessible without removing safety screens. If necessary platform steps with ”hand holds” should be provided to enable samples to be taken or thermometers read.

(b) The apparatus should be securely clamped without exerting undue stress

on any part of the equipment.

Glass joints should be secured so that no leakage occurs.

Always use boiling chips, a stirrer or an inert gas bleed to prevent ”bumping”.

Ensure water lines are securely fixed to the condenser and tap.

A flow monitor should be fitted to the water lines from the condenser or the line fixed so that the water flow is visible, e.g. by fixing the exit above the tun dish.

Plastic and rubber tubing should be replaced every 3 months if running for extended periods.

(c) The distillation assembly should have all-round screening. material of high

toxicity or low TLV should be distilled in a fume cupboard or an extracted cubicle.

If necessary the atmosphere in and around the structure should be monitored regularly.

(d) The equipment should stand in a tray capable of containing the volume of

liquid being distilled. (e) Use electrical heating, water, oil, or a fluidized sand bath as the source of

heat - never a bunsen burner.

Avoid overheating the still bottoms at the end of a distillation.

Do not use a water cooled condenser if the freezing point of the distillate is close to or greater than the temperature of the cooling water. If necessary use an air condenser.

Systems should be vented at all times.



(f) For distillation under reduced pressure the vacuum pump should be situated outside the completely shielded structure.

There should be a catchpot or cold trap between the still and the vacuum pump.

(g) Do not distil ethers or other materials which may potentially contain

peroxides until the peroxides have been removed by the addition of concentrated ferrous sulfate solution or by another suitable method.

(h) When distilling flammable materials a suitable fire extinguisher should be

placed to hand. (j) Suitable antidote solutions and/or shower facilities should be available for

treatment of splashes from, for example, phenolic material. (k) Eye protection shall be worn at all times when in a designated area.

Notices are displayed in these areas showing the type of protection required.

A.2 KEY DEFINITIONS A.2.1 Distillation Distillation involves the separation of the components of a liquid mixture by partial vaporization of the mixture and separate recovery of vapor and residue. A.2.2 Personal Responsibilities Personal responsibilities are as defined in the Health and Safety at Work etc. Act 1974 and all initiators of this type of operation have a clear responsibility to research the proposed work and carry it out with due regard for the safety of themselves and others. Individual Managers are responsible for ensuring that their staff comply with safe systems of work and for ensuring the safe guarding of new and existing equipment used by their staff. Safety is the personal responsibility of all Line Managers and includes safe design, operation of experiments, safety clothing and equipment, and response to Experimental Officer in Charge instructions.



A.3 DOCUMENTATION REQUIREMENTS A.3.1 Recording Experiments All experimental work shall be recorded in the standard Laboratory Notebooks. Data which are too bulky to be attached to the Notebook should be kept in a separate folder which is registered. A.4 OPERATING INSTRUCTIONS Each experimental program should have an introduction explaining the background and aims of the work. Experimental requirements should be clearly defined. Reference should be made by specific number to any COSHH Assessment. Potential hazards governing the experiments should be outlined. Special precautions and requirements for personal protection should be given. DOCUMENTS REFERRED TO IN THIS PROCESS ENGINEERING GUIDE This Process Engineering Guide makes reference to the following documents: PROCESS ENGINEERING GUIDES NA STATUTORY ACTS AND REGULATIONS Health and Safety at Work etc. Act 1974 (referred to in Appendix A) SI 917 Highly Flammable Liquids and Liquefied Petroleum Gases Regulations 1972 (referred to in Appendix A) SI 1681 The Protection of Eyes Regulations 1974 (referred to in Appendix A) SI 1657 The Control of Substances Hazardous to Health (COSHH) Regulations 1988 (referred to in Appendix A).

Laboratory Distillation

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