Causes of Column Malfunctions

This chapter first reviews the common causes of column malfunctions and shows where these causes fit in the book. It then looks at the 

Close to 300 case histories of malfunctioning columns were extracted from the literature and abstracted in Chap. 20. Table 1.1 classifies the malfunctions described in these case histories according to their principal causes. If one assumes that these case histories make up a representative sample (note exclusions and limitations in Chap. 20), then the analysis below has statistical significance. Accordingly, Table 1.1 can provide a useful guide to the factors most likely to cause column malfunctions and can direct troubleshooters toward the most likely problem areas. 

TABLE 1.1 Causes of Column Malfunctions (Based on the Analysis of Case Histories In Chap. 20) 

Cause Number of reported cases Percent of reported cases Reference chapter 

The above statements must not be interpreted to suggest that operation personnel and troubleshooters need not be familiar with the primary design. Quite the contrary. A good troubleshooter must have a solid understanding of primary design because it provides the foundation of our distillation know-how. However, the above statements do suggest that in general, when a troubleshooter examines the primary 

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Take out a good set of readings on the column and its auxiliaries, including laboratory analyses. Misleading information supplied by instrvunents, samples, and analyses is a common cause of column malfunctions. Always mistrust or suspect instrument or lab-... 

Common HPLC problems are caused by component malfunctions (pump, degasser, injector, detector, data system, column), and faulty preparation of the mobile phase or sample preparation. Problems can be categorized into several areas ... 

Case study 4 shows several examples of problems caused by equipment malfunctions and their subsequent diagnosis and solution. The first one involved a situation of poor retention time reproducibility of a gradient assay. It involved the analysis of a complex natural product, using a narrowbore column (2-mm i.d.) at 0.5 mL/min. System suitability test showed retention times to be erratic and could vary by 1-2 minutes without any obvious trends. Flow rate accuracy was found to be acceptable, however, the compositional accuracy test failed (see Chapter 9 on HPLC calibration). The tentative diagnosis was that of a malfunctioning of the proportioning valve. After its replacement, the retention time precision performance was re-established. 

Primary design problems, foaming, installation mishaps, relief problems, and tray and downcomer layout problems make up the rest of the column malfunctions. Familiarity with these problems is useful to troubleshooters and operation personnel, but only one incident out of every four is likely to be caused by one of these factors. All these topics, except the primary design, are relatively narrow and are treated accordingly in this book. 

The prime consideration for instrument connections is to avoid hydraulic interference in the column or impulse line, which would lead to erroneous measurements or instrument malfunction. False information supplied by instruments has been the cause of premature flooding, column damage, and poor separation in many columns. This chapter examines the preferred practices, reviews common pitfalls, and supplies guidelines for avoiding pitfalls with column instrument connections. 

Nevertheless, reboilers and condensers are an integral part of a distillation system. As shown in Chap. 1, problems with reboilers and condensers account for a sizable fraction of distillation malfunctions. Distillation supervisors or troubleshooters cannot perform their duties effectively unless acquainted with the pitfalls in the operation of the column heat exchangers. There have been many instances in which the cause of an apparent poor column performance problem was traced back to the reboiler or condenser. 

If changes at constant column dimensions - and constant packing density — it is an indication of a flow change within the system. The flow change is either caused by a malfunctioning pump or a leakage in the system. 

Bed collapse may occur if the column has been shocked mechanically, for example, by an accidental drop on a hard floor. If a colunm has accidentally dried out, it can collapse on restart. Inuniscible solvents or the precipitation of a constituent of the sample or the mobile phase can cause a shift of the bed due to localized high-pressure drops. collapse can be caused by a continued pulsation of a malfunctioning pump, or by storing the column in the wrong solvent. For example, the bed of silica-based cyanopropyl packinp can collapse in solvents of intermediate polarity, such as acetonitrile or THF,... 

Pressure too low Lower than expected system pressure is caused by leaks (piston seal, column connections, injector), pump malfunctions (lost prime, air bubbles in pump head, vapor lock, faulty check valves,broken piston), or inadequate solvent supply (empty solvent reservoir, plugged solvent sinker, bent solvent lines, or wrong solvent mixture). Problem diagnostics can be made by visual inspection for leaks and by monitoring the pressure reading of the pump. 

Incorrect bottom (or reboiler) temperature indication This can be due to a faulty thermocouple, but a more likely cause is fouling of the thermowell. Ironically, bottom or reboiler thermocouple fouling tends to occur in services that are most vulnerable to a malfunction of this instrument, i.e., when heat-sensitive materials are distilled. A fouled thermowell will read low this in turn will enhance heat input into the column bottom, either automatically or by operator action. The greater heat input will accelerate thermal degradation in one peroxide service incident (97), it caused overheating and an explosion. 

A bottom temperature indicator measuring liquid temperature may read vapor temperature (which may be considerably lower) when the level drops. The consequences are similar to those of a fouled thermocouple. A malfunctioning level indicator can thus lead to a dangerously misleading, but apparently consistent, indication of both level and bottom temperature (Fig. 13.8). In one case (275), this led to overheating and an exothermic reaction at the column base, which in turn caused residue discharge from a column vent. 

A malfunctioning control system causes instability. The instability can adversely affect product purity, column capacity, economy, and ease of operation. Instabilities are often transmitted to dovrastream or upstream units, or can amplify small disturbances. In extreme cases, an instability can also lead to column damage or safety hazards. 

The bladder functions as a low-pressure reservoir, filling at the rate of 2 mL/min until approximately 360-400 mL is reached, and the intravesical pressure increases. This pressure activates proprioceptive receptors in the bladder wall to signal the sacral spinal cord, thus triggering detrusor contraction. Sensory stimulation occurs at the micturation center in the brainstem that coordinates urethral sphincter relaxation as the detrusor muscle contracts. Higher controls in the frontal lobe can block this sensory message until conscious direction permits a voluntary void. Medical insults to the spinal column, peripheral sensory nerves, and cerebral cortex will cause malfunction in the voiding pattern (7). 

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