- 25 October 2007 -

Back to basics: Bulk chemical industry separations

The manufacturing operations of the bulk chemicals sector use filters for processing and product recovery. Ken Sutherland looks at these in detail.

The manufacture of bulk (sometimes called heavy, in the sense of large-scale) chemicals and petrochemicals is one of three components of the total chemicals industry (the others being fine chemicals manufacture, including pharmaceuticals and medicinals, and biochemicals production). It is concerned, as its name implies, with the production of chemicals on a large scale, from natural resources, including the fossil fuels coal, oil and natural gas. It can be conveniently divided into three large components:

. bulk inorganic chemicals, including industrial gases, acids, alkalis, and fertilisers;

. bulk organic chemicals and petrochemicals; and

. man-made fibres (from cellulosic and petrochemical sources).

There is a somewhat indistinct boundary between bulk chemicals production and mineral processing technologies. Thus the production of pure salt by the extraction of underground rock salt could be regarded as either - but in fact is classed as a mineral production process, as are all processes for the extraction of raw materials from their natural sources, for use in the chemicals sector.

Process filtration

Attention was drawn in the earlier article to the possible division of filtration applications into two broad types: the one being simple, almost certainly involved only in clarification of a gas or a liquid, and with a easily defined task, while the other is intended to deal with a possibly complex task, frequently involved in solids recovery, and probably having a very detailed performance specification. The first group, of essentially service functions, and so termed utility applications, are usually independent of the nature of the end-use sector in which they are found, and dependent only on the nature of the service that they provide, such as steam supply or hydraulic power services.

The second group of separation equipment types are used for a more or less important process function, which is intrinsic (if not unique) to the manufacturing process, and without which the process would not work. These process applications are found widely throughout the bulk chemicals sector, usually very specific to its industrial processes, and with important variations in design and construction imposed on them by the process operating temperatures and pressures, by the corrosiveness of the liquid or abrasiveness of the solids in the system, and by the individual process needs, such as filter cake dryness or filtrate clarity.

Apart, obviously, from the processing and production of fuel and industrial gases, the process filter or sedimenter in the bulk chemicals industry is usually a solids-liquid separation device, with performance characteristics dictated by downstream needs. It is these types of equipment that are the subject of this article.

Products and raw materials

The key bulk chemicals are the inorganic acids (sulphuric, nitric and hydrochloric), phosphates, chlorine and caustic soda, plus fertilizers, and the organic intermediate materials for petrochemicals (olefins, methanol, and aromatics). A reasonably complete list of bulk chemicals and their sources includes:

. the processing of coal, petroleum and natural gas to give fuel products, organic chemical intermediates and petrochemicals, leading to man-made fibres;

. the splitting of air into its main constituents and the recovery of minor components, including carbon dioxide processing;

. the processing of clays into ceramic compounds and refractories;

. the manufacture of cements, calcium and magnesium compounds from lime and gypsum;

. glass manufacture from sand;

. salt processing, chlor-alkali industry, sodium and potassium compounds;

. other electrolytic and electrothermal processes;

. the processing of phosphate rock to phosphates and phosphoric acid;

. the manufacture of sulphuric acid and other sulphur compounds from natural and by-product sulphur;

. the manufacture of ammonia from atmospheric nitrogen, nitric acid and other nitrogen compounds; and

. the manufacture of hydrochloric acid and other halogen compounds, alumina and a host of other compounds.

It is to be noted that the first component of the above list encompasses the whole of organic chemical processing, while the remainder make up inorganic products (which is another way of saying that organic chemistry concerns one element, carbon, from the Periodic Table, while inorganic chemistry concerns the other hundred or so). In all of these chemical production processes, the majority of them carried out in the liquid phase, the process use of filtration (and, to a lesser extent, sedimentation) is widely found, as a solid recovery step, or in purifying ingredients, intermediates or final products, and in the recycling or treatment of waste streams.

Bulk solids recovery

Although there are many filters and sedimenters in important use in the bulk chemicals industry for clarification purposes (for any liquid: reactant, product or waste), the key characteristic of the process filter is likely to be its ability to separate a suspended solid from its mother liquor as completely as possible, and with as little damage as possible to the separated solid (where this is the desired product - although it is just as likely for the liquor to be, or to contain, the product). The suspended solid may have been produced by precipitation from a mixture of liquid reactants, or as a quantity of exhausted solid from which a valuable component has been removed by reaction or leaching, or as a mass of crystals separated from solution by evaporation, or even as a surplus sludge from a biological waste treatment process. Whatever the source, the separation stage must recover the suspended solids (or harvest them, as the term sometimes is) as completely as possible, at the same time clarifying the mother liquor as thoroughly as possible (or else there will be a need for a second, clarifying filter).

The actual separation of suspended solids from mother liquor can usually be achieved fairly easily - by selection of the right filter medium, for example, with sufficient filtration area as dictated by the cake forming characteristics of the solids. What is often the more difficult task is to extract the separated solids from the working zone of the filter, a difficulty that is usually the greater for continuous filters rather than batch operated ones, for pressure operated filters rather than for vacuum filters, and for large units rather than small ones. These measures of relative difficulty can usually be seen from the complexity of the mechanical design of the filter - the more difficult the solids removal task, the more complex the filter construction.

Bulk chemical filtration characteristics

The bulk chemicals industry has so many different separation applications that it employs almost every type of filtration, sedimentation and centrifugation equipment, often in very large unit sizes. Importantly, it has some of the most severe operating conditions of all industries. Whilst most other industrial sectors deal with fluids that are relatively innocuous as far as separation process equipment goes - moderate operating temperatures and pressures, with relatively low viscosity and non-corrosive liquids - this is most definitely not the case in the bulk chemicals sector. Here, filtration and separation equipment frequently have to be designed to withstand quite high temperatures and pressures, and, especially, to cope with highly corrosive liquids and toxic materials. For this reason, the bulk chemicals sector features equipment made in some of the more exotic materials of construction.

Many of the bulk chemical production processes are large enough such that continuous operation is called for, necessitating either large continuous filters, or the use of several batch filters operating in parallel, so that one filter can be taken off-line in turn for its collected solids to be unloaded. For the continuous filters, reliability of operation is obviously important, with all ancillary features, such as cake heel treatment and filter medium washing, included in the operating function.

Process filter types

The range of filters and other separation equipment used for process filter applications, where solid recovery from liquid suspension is important, is shown in the table. Most practitioners would immediately name one or more of the decanter, peeler and pusher centrifuges, and rotary vacuum, horizontal belt and plate and frame press filters for this purpose - the much wider range that is actually employed for process filtration can be seen from the quoted list.

A wider range of filters is, of course, in use for process purposes when liquid reactant or product clarity is included. For this application, the bulk chemicals industry is a big user of bag and cartridge filters, and increasingly of membrane filters, to achieve the increasing demands from customers for liquid products with improved clarity or reduced levels of contamination.

In the sedimentation range, the gravity driven equipment is mainly used for raw material processing, and the centrifugal types for product treatment. Imperforate basket centrifuges are batch machines, used for relatively low throughputs, but hydrocyclones, and disc and decanter centrifuges, provide mainly continuous flow (semi-continuous for disc separators), with a high degree of efficiency. These types are often used for particle size grading, in a system which recycles fines and oversize materials.

The batch vacuum filters are generally restricted to fairly small-scale processes, but the same is not true for the continuous vacuum range, where the rotary vacuum drum and horizontal belt filters have for long been, and continue to be, the mainstay of the applications for which they are best suited. These uses tend to be for the separation of relatively easily settled solids (so that cake can form easily and quickly), where the formed cake needs to be thoroughly washed.

The range of centrifugal filters also works best with relatively easily filtering solids, which often means crystals following an evaporator or plastic granules following wet precipitation. Here care needs to be taken in equipment selection to ensure that the crystals or granules are not damaged in their flow through the harsh mechanical system that is a filtration centrifuge.

The end of the long history of valuable service by the plate and frame filter press, with its being replaced by more modern filters, is frequently foretold, but even now there are few signs of this occurring, as filter and filter media manufacturer keep improving its performance. Its prime advantages of easy adaptability (by varying the number of plates) and high degree of dewatering of the cake can easily offset its disadvantage of batch operation (usually because of the use of several presses in parallel).

The different filter types mentioned in the table all have important uses in the bulk chemicals sector, sometimes a unique application (such as the tilting pan filter in phosphoric acid production or the tube press for kaolin), but very often as one of a team of separation devices - in the phosphate rock/phosphoric acid industry there are desliming hydrocyclones, stationary and moving screens, spiral concentrators and flotation cells all in the beneficiation process for the phosphate rock, screens widely used in superphosphate production, with recycle sulphuric acid filters as well as the tipping pan filter in phosphoric acid production.

Some other interesting applications within the bulk chemicals industry include screens and plate and frame presses in ceramics preparation for table ware manufacture; rotary vacuum filters and centrifugal filter in soda ash production; centrifugal filters in ammonium sulphate production as a by-product of coal processing; wet classifiers, thickeners and plate and frame filters in cement processing; and pressure filters in silicon carbide refining. The filter and sedimenter are indeed important components of the bulk chemicals sector.

Contact:
Ken Sutherland
Email: ken.suth@ntlworld.com