troubleshooting in distillation columns

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Process Engineering Guide: Troubleshooting in Distillation Columns

CONTENTS SECTION 0 INTRODUCTION/PURPOSE 2 1 SCOPE 2 2 FIELD OF APPLICATION 2 3 DEFINITIONS 2 4 FLOW DIAGRAM FOR TROUBLESHOOTING 2 5 GENERAL APPRAISAL OF PROBLEM 4 5.1 Is the Problem Real? 4 5.2 What Is the Magnitude of the Problem? 4 5.3 Is it the Column or the Associated Equipment which is

Causing the Problem? 4

6 PROBLEMS IN THE COLUMN 4

6.1 Capacity Problems 4 6.2 Efficiency Problems 5 7 PROBLEMS OUTSIDE THE COLUMN 6 7.1 Effect of Other Units on Column Performance 6 7.2 Column Control System 6 7.3 Improper Operating Conditions 6 7.4 Auxiliary Equipment 7



8 USEFUL BACKGROUND READING 7 9 BIBLIOGRAPHY 8 FIGURES 1 FLOW DIAGRAM FOR TROUBLESHOOTING 3 2 DETERMINATION OF COLUMN CAPACITY 5 DOCUMENTS REFERRED TO IN THIS PROCESS ENGINEERING GUIDE 9



0 INTRODUCTION/PURPOSE The old adage - prevention is better than cure - applies to the design of all process equipment. The majority of distillation column designs meet their defined requirements, the designer having assessed and made suitable allowances for uncertainties in equilibria data or tray/packing efficiency. Occasionally a new column will not give the required performance because of a mistake made at the design stage - maybe as a result of poor distribution in a packed tower. More often problems in distillation columns are associated with changed circumstances - changed operating conditions or changed product specifications - or in the extreme a changed duty to effect a different separation. Whatever gave rise to the problem the distillation column is not achieving the required separation and/or capacity. This is the realm of the troubleshooter. Troubleshooting is not an exact science. A diversity of approaches can be successfully adopted - one such approach is outlined in this guide. 1 SCOPE This Guide offers advice on how to approach troubleshooting in distillation columns and refers readers to useful published information. 2 FIELD OF APPLICATION This Guide applies to GBHE process engineering community worldwide. 3 DEFINITIONS For the purposes of this Guide, no special definitions apply. With the exception of terms used as proper nouns or titles, those terms with initial capital letters which appear in this document and are not defined above are defined in the Glossary.



4 FLOW DIAGRAM FOR TROUBLESHOOTING A general guideline in the form of a flow diagram for approaching troubleshooting problems is given in Figure 1 [Ref 1]. Such a diagram cannot cover all problem areas but it does serve as a useful starting point. However the most important attribute the chemical engineer can bring to a plant problem is the ability to think and apply logic. Do not be rushed into taking ill considered actions. Discount the inevitable suggestion that this is one case that does not follow universally proven laws. FIGURE 1 FLOW DIAGRAM FOR TROUBLESHOOTING



FIGURE 1 FLOW DIAGRAM FOR TROUBLESHOOTING



5 GENERAL APPRAISAL OF PROBLEM The loss of production resulting from poor column performance produces an atmosphere which demands that actions have to be taken. However actions should not be taken just to demonstrate that things are being done. Accept the flak, the problem is more likely to be quickly resolved by following a systematic procedure. All aspects should be investigated before attempting to determine the solution. 5.1 Is the Problem Real? The first action is to establish that there really is a problem. Collect and analyze all the data available. Check that instruments, metering devices, analyses etc are all measuring what they should be i.e. instruments have not been transposed, chart ranges are correct, agreed analytical procedures are being followed. Carry out a component and an overall mass balance and an overall heat balance. The former can point to analyzer error, the latter to incorrect flow rate measurement. 5.2 What Is the Magnitude of the Problem? A realistic estimate of the loss of profit associated with poor column performance helps decide the effort which should be expended to solve the problem. The cost of lost production also helps in setting technical targets. A less than perfect technical solution may be appropriate to minimize production losses in the short term. The long term solution may involve considerable capital outlay and/or have to await a planned plant shut down. 5.3 Is it the Column or the Associated Equipment which is Causing the

Problem? Although a distillation column may be said to be not giving the desired performance it is important to establish if the problems are with the column itself or a result of other factors outside the column. This distinction may be difficult to define because of limited and often inconsistent data. The troubleshooter must gather all the information available (charts, log books, design information) and listen to and assess the views of plant operators.



6 PROBLEMS IN THE COLUMN Problems in the column relate to loss of capacity or efficiency. In some cases, usually if the column performance has suddenly deteriorated rather than declined gradually, the root cause of the problem can be easily identified. In other cases there may be a number of possible causes and the information collected will have to be sifted and analyzed to support or eliminate possibilities. Is the column being asked to do a duty beyond what it was designed for? Is it being operated in a manner very different from the design intent? 6.1 Capacity Problems The maximum hydraulic capacity of a column is the highest throughput at which it will operate without flooding. The capacity is a function of the vapor and liquid rates, column geometry and system properties. At the flood point liquid accumulates on trays or in packing because of excessive vapor flow, i.e. pressure drop, or maybe mechanical restriction. Flooding in a column could be indicated by: (a) surges of liquid overhead; (b) erratic or high pressure drop across the column; (c) fluctuating level in column bottom; (d) a high temperature profile; (e) falling base level or reduced bottoms flow. Flooding usually continues until the operator takes action. The primary tool for investigating capacity problems is a differential pressure instrument, or preferably instruments, to measure pressure drop across various tray or packing sections. Static pressure gauges with pressure drop readings obtained by difference should not be considered, as they will rarely be accurate enough. The actual column capacity can be determined by plotting the pressure drop against, say, the total overhead vapor rate, as outlined in Figure 2 below.



FIGURE 2 DETERMINATION OF COLUMN CAPACITY

The use of radioisotope scanning is a useful method of determining the cause of flooding, e.g. downcomer back-up. 6.2 Efficiency Problems Poor efficiency usually results from inadequate liquid and vapor contact or insufficient disengagement of liquid from vapor. The key indicators of efficiency problems are:

(a) off specification product; (b) excessive reflux requirement; (c) high boil-up rate. Poor efficiency can also be confused with analytical errors or improper operating conditions - these causes should be eliminated first.



If low efficiency is established and if the column is operating stably with no hydraulic problems there is little chance of increasing efficiency in a trayed column without recourse to the redesign of a more efficient contacting device. The same holds for packed columns if the distributor design and installation is good. 7 PROBLEMS OUTSIDE THE COLUMN There are many ways in which problems outside the column can effect column performance. A few instances are given below. 7.1 Effect of Other Units on Column Performance It is difficult to evaluate the performance of a column if upstream and sometimes downstream units are not operating steadily. For example, pressure changes may limit pump capacity or there may be surges of low boiling components in the feed to the column which can lead to problems in the condenser or vacuum system. Even with other units operating steadily the performance may not be as required. This can be as a result of changes in feed composition, rate or conditions compared to those for which the column was originally designed. Such changes should be carefully evaluated. For example a cold rather than a flashing feed can upset the heat balance of the column. Also a feed rate above design may not be compatible with reboiler/condenser capabilities in effecting the required separation. 7.2 Column Control System The control systems employed can cause, or contribute to, operational problems in distillation columns. A few typical examples are given below: (a) A temperature sensor for composition control should be situated at a point

in the column that will provide sensitivity to composition changes - failure to do so can lead to sluggish control and products being off specification for unacceptable periods of time.

(b) The wrong setting of a temperature sensor/control can lead to the situation

where a required tops or bottoms composition cannot be achieved.



(c) An incorrectly calibrated instrument for gauging the base level in a reboiler can initiate flooding. If the instrument was calibrated for a liquid of higher density than is practically the case, the actual level would be higher than the indicated level - maybe even into the column.

The possibilities are manifold, sorting them out requires careful checking of the calculated versus apparent conditions at as many points as possible and analyzing any apparent differences. 7.3 Improper Operating Conditions It is common practice to start-up plant at reduced design rates. If care is not taken, for example operation at a reflux ratio higher than design, this can lead to problems with columns operating below their minimum operating points. Another frequent situation is that conditions are set which are incompatible with the objectives. For example, setting a specific overhead product rate that is insufficient to remove the required amount of the more volatile component(s) as distillate. With a set amount of distillate removal a similar situation can exist if no account is taken of a small change in feed composition. 7.4 Auxiliary Equipment Auxiliary equipment may cause problems that seem to originate in the column itself. It is easy to apportion a deterioration in column performance above a certain feed rate as a fall off in tray efficiency rather than a reboiler or condenser limitation. However if the energy flow does not remain proportional to the feed rate a capacity limitation in auxiliary equipment should be suspected. A possible short cut to pinpointing capacity limitations in auxiliary devices is to ascertain if valves are consistently running wide open. This may be an indication of control valve limitations, or of reboiler, condenser or pump inadequacy. Numerous other factors may give the appearance of faulty column design: (a) corrosion effects; (b) a drift in instrument calibration; (c) degradation of materials;



(d) accumulation of trace impurities in recycle streams; (e) leaking heat exchangers etc. 8 USEFUL BACKGROUND READING As mentioned earlier, troubleshooting is a complex and diverse topic. Many papers have been written on the subject. Four recent publications summarized below, provide useful additional material. (a) Harrison, France, Troubleshooting Distillation Columns [Ref 2] This is a four part series dealing with trouble shooting in operational distillation columns. Part 1 Technique and Tools The basic troubleshooting technique - identify the problems, determine the cause and recommend a remedy - is discussed. This is followed by an outline of the tools available to the troubleshooter - isotope scanning, video-camera viewing, shut down inspection. Part 2 Packed Columns General characteristics of packed columns and types of packing are discussed. The importance of liquid distribution and the design of distribution opposite good column performance is emphasized. Part 3 Trayed Columns Different types of flooding are discussed together with the most common tray types (sieve, valve and bubble-cap). A short treatment of common causes of inefficiency is presented.



Part 4 Auxiliary Equipment Heat exchangers, vacuum pumps or ejectors, process pumps and instrumentation are recognized as critical components of distillation columns. Reboilers, in particular, are considered in detail. (b) Tray Distillation Columns [Ref 3] This booklet gives a useful overview as to planning tests (including pre-start up), methods of measurement and sampling and computation and interpretation of results. For detailed information on a specific topic reference is made to other articles in the literature. There is a clearly presented worked example illustrating how mass and enthalpy balances can be checked and column capacity and efficiency calculated. (c) Packed Distillation Columns [Ref 4] This booklet for the performance evaluation of packed distillation columns draws heavily on that for tray distillation columns [Ref 3], suitably modified for the different internals. (d) Henry Z Kister, Distillation Operation [Ref 5] An excellent book dedicated to the distillation troubleshooter. Past experiences with distillation columns are considered and used to suggest means of preventing similar incidents in the future. The whole gamut of distillation technology is covered - trayed and packed columns, column commissioning and field testing, control philosophy, case histories of columns that did not work etc. An invaluable source for the troubleshooter.



9 BIBLIOGRAPHY Ref Source [1] McLaren, Upchurch, Guide to Trouble Free Distillation,

Chem. Eng, 139, 1 June, 1970. [2] Harrison, France, 'Troubleshooting Distillation Columns', Chemical

Engineering, March-June, 1989. Also available as Bulletin 389 from Glitsch Ltd., 1990.

[3] Tray Distillation Columns: A Guide to Performance Evaluation.

AIChE Equipment Testing Procedure, 1987, 2nd edition. [4] Packed Columns: A Guide to Performance Evaluation.

AIChE Equipment Testing Procedure, 1990, 2nd edition. [5] Henry Z Kister, Distillation Operation, McGraw-Hill Publishing Company,

1990.