In the manufacture of chemicals, various reactions are commonplace, such as esterification (used in fragrances, pharmaceuticals, agrochemicals, plasticisers, coalescents, surfactants, etc), transesterification (used in similar applications), quaternisation (surfactants, phase transfer, water treatment, etc) and endcapping (lubricants, heat fluids, refrigerants, etc). Esterification is the well-known equilibrium reaction of acids and alcohols to form esters, with varying means being applied to push the equilibrium to the product. This process is one of the most practiced technologies in the chemical industry. Many manufacturers routinely produce esters, some of which require difficult separation of products and raw materials. While it is a relatively simple reaction to perform in the laboratory, plants that manufacture on a large industrial scale need to be more precisely engineered. They need to take into account many parameters, such as:

  • Heat flow;
  • Fluid flow;
  • Physical state of materials to be transferred;
  • Venting;
  • Scrubbing;
  • Materials of construction;
  • Viscosity;
  • Environmental protection;
  • Containment;
  • Materials flow during processing (gravity feeds, etc).
So, even though esterification is a fairly trivial process, engineers have many more considerations during plant design and scale up. This also includes more strategic issues such as whether to run batch (hence the build of a multi-purpose plant), semi-continuous or continuous plant. Each of these options has its own merits, but continuous processes have low inventory of materials during reaction and are good for hazardous reactions or those that need good heat flow.

Esterification chemistry

At Chemoxy International, most of our operating units at both of our sites are suitable for esterification chemistry, and the best unit for manufacturing a product will depend on the quantity of the esters needed and the separation technology required. This could result in either a batch process or a continuous process and we also able to combine units to offer the optimum set-up for efficiency in both reaction and distillation. The reason these are not depicted as equilibrium reactions below is that the Chemoxy advantage allows us to effectively obtain quantitative yields based on our technology. [caption id="attachment_36237" align="aligncenter" width="300"]Chemoxy CLICK TO ENLARGE[/caption]   At Chemoxy, we can use a range of catalysts, including inorganic acids, organic acids and Lewis acids, among others. We also have the specialist equipment to enable the process to efficiently remove the water of reaction quickly from various process streams, be it from the top, middle or bottom of the distillation column. The simplest and most frequently used distillation configuration is the batch distillation. The batch still consists of a pot (or reboiler), rectifying column, a condenser, some means of splitting off a portion of the condensed vapour (distillate) as reflux, and one or more receivers. The pot is filled with liquid mixture and heated. Vapour flows upwards in the rectifying column and condenses at the top. Usually, the entire condensate is initially returned to the column as reflux. This contacting of vapour and liquid considerably improves the separation. The first condensate is the lights or fores, and it generally contains undesirable components, but can contain materials for recycling. There are other fractions, one of which will be the desired product. The last condensate is the heel and it can be one of the raw materials that just happens to have a higher boiling point than the other constituents, or it may also contain, or be undesirable. [caption id="attachment_36238" align="aligncenter" width="262"]Chemoxy 1 CLICK TO ENLARGE[/caption] The lights and heel may be thrown out, or added to the next batch depending upon the analysis, according to the practice of the distiller. Owing to the differing vapour pressures of the distillate, there will be a change in the overhead distillation with time, as early on in the batch distillation, the distillate will contain a high concentration of the component with the higher relative volatility. As the supply of the material is limited and lighter components are removed, the relative fraction of heavier components will increase as the distillation progresses. Continuous distillation is an ongoing separation in which a mixture is continuously (without interruption) fed into the process and separated fractions are removed as output streams.

Distillation process

As described above, distillation is the separation or partial separation of a liquid feed mixture into components or fractions by selective boiling (or evaporation) and condensation. The process produces at least two output fractions. These fractions include at least one volatile distillate fraction, which has boiled and been separately captured as a vapour condensed to a liquid, and practically always a bottoms (or residual) fraction, which is the least volatile residue that has not been separately captured as a condensed vapour. The principle for continuous distillation is the same as for batch distillation: when a liquid mixture is heated so that it boils, the composition of the vapour above the liquid differs from the liquid composition. If this vapour is then separated and condensed into a liquid, it becomes richer in the lower boiling point component(s) of the original mixture. This is what happens in a continuous distillation column. A mixture is heated up, and routed into the distillation column. On entering the column, the feed starts flowing down but part of it, the component(s) with lower boiling point(s), vaporises and rises. However, as it rises, it cools and while part of it continues up as vapour, some of it (enriched in the less volatile component) begins to descend again. The diagram below depicts a simple continuous fractional distillation tower for separating a feed stream into two fractions, an overhead distillate product and a bottoms product. The ‘lightest’ products (those with the lowest boiling point or highest volatility) exit from the top of the columns and the ‘heaviest’ products (the bottoms, those with the highest boiling point) exit from the bottom of the column. [caption id="attachment_36239" align="aligncenter" width="201"]CLICK TO ENLARGE CLICK TO ENLARGE[/caption] The overhead stream may be cooled and condensed using a water-cooled or air-cooled condenser. The bottoms reboiler may be a steam-heated or hot oil-heated heat exchanger, or even a gas or oil-fired furnace. In a continuous distillation, the system is kept in a steady state or approximate steady state. Steady state means that quantities related to the process do not change as time passes during operation. Such constant quantities include feed input rate, output stream rates, heating and cooling rates, reflux ratio, and temperatures, pressures, and compositions at every point (location). Unless the process is disturbed due to changes in feed, heating, ambient temperature, or condensing, steady state is normally maintained. This is the main attraction of continuous distillation: apart from the minimum amount of (easily monitored) surveillance, if the feed rate and feed composition are kept constant, product rate and quality are also constant. Even when a variation in conditions occurs, modern process control methods are commonly able to gradually return the continuous process to another steady state. The system can be a little more complex, as is the case at Chemoxy International (see diagram below). [caption id="attachment_36240" align="aligncenter" width="184"]CLICK TO ENLARGE CLICK TO ENLARGE[/caption]

Continous distillation unit

The system can be set up to take off more than one fraction, so that in the case of esterification, perhaps there are the following streams:
  • Alcohol stream (recycled);
  • Water stream (for disposal);
  • Fores;
  • Product;
  • Acid.
Since a continuous distillation unit is fed constantly with a feed mixture and not filled all at once like a batch distillation, a continuous distillation unit does not need a sizeable distillation pot, vessel, or reservoir for a batch fill. Instead, the mixture can be fed directly into the column, where the actual separation occurs. The height of the feed point along the column can vary on the situation and is designed so as to provide optimal results. Distillation can be both simple and complex. Utilising chemical understanding, mass balance, energy balance, and other design parameters can lead to a very efficient process which, for high volume products, generally leads to plants designed for a single product where all parameters are optimised. It is, however, commonplace for multi-purpose plants to be used on smaller volume products, quite often leading to inefficiencies and higher costs. Here at Chemoxy International, our operations are geared towards hundreds to thousands of mt products that require sophisticated separation technology. It is all about choosing the process that best suits your operation so that you can maximise efficiency. Brian Tarbit is the global technician/business development manager at Chemoxy International Ltd, one of Europe’s largest independent chemical contract manufacturers. The company specialises in custom processing and manufacturing a variety of environmentally friendly solvents for paints and cleaning products.