Cryogenics often is associated with food processing. However, cryogenics offers unique processing advantages in cryogenic size reduction and recycling, solvent recovery systems and reaction cooling.
For many, the concept of cryogenics may conjure up images of an excited lecturer shattering a rubber ball or crunching flowers after they have been dipped into a bucket of liquid nitrogen. But, to many engineers, cryogenics is a real subject, and it is up to them to discover new ways of using this potentially lucrative technology.
What does cryogenics mean?
Its dictionary definition is:
cryogenics: n study of extremely low temperatures: a branch of physics that studies the causes, effects and utilization of extremely low temperatures
In industrial applications, cryogenics commonly is associated with food processing. However, cryogenics offers unique processing advantages in three other areas: size reduction and recycling; solvent recovery systems that reduce emissions to meet increasingly stringent legislation; and reaction cooling in the synthesis of organics.
Cryogenic Size Reduction
Historically, cryogenic recycling was thought to be too expensive except in certain applications. Today, however, manufacturers are demanding material of less than 200 Km in size and that the production process be economical. Cryogenic recycling systems are designed to minimize the amount of liquid nitrogen required by employing correct system sizing, minimizing losses, and recirculating and fully utilizing cold nitrogen gas. Process economics are variable and depend on the raw material, throughput per hour, product size and equipment size.
One material that may be processed cryogenically is rubber. Manufacturing with fine ground powder (100 Km) may allow manufacturers to use higher amounts of recycled rubber in formulations and still maintain a high specification of physical properties. Grinding rubber at ambient temperatures produces highly temperature-stressed particles (500 Km) with a large specific surface area. Sometimes, this surface area is required because it will allow reasonably good mechanical fixing in compounds.
Cryogenically recycled fine ground rubber powders actually have more surface area, and as they are smaller particles, they are not as thermally stressed. Consequently, a large number of particles fit into the spaces in the rubber compound, and the nonstressed surfaces are able to chemically interact with virgin vulcanized particles.
Not all applications require fine ground rubber; sometimes, 200 or 300 Km rubber powder can be reused successfully. In those applications, producing larger particle sizes decreases the nitrogen cost associated with cryogenic processing.
Companies spend a lot of time and money attempting to control the release of volatile organic compounds (VOCs) to the atmosphere. VOCs can cause ground-level ozone, smog and chlorinated compounds, some of which are toxic to human health. Emissions can be reduced in two ways: change the way the process is operated or treat the resulting end-of-pipe problem with VOC abatement technology. Often, a combination of approaches is necessary to reach the increasingly stringent solvent emission regulations being imposed.
The most common methods of removing VOCs are adsorption, absorption, condensation and incineration. To choose the best method for VOC removal, a number of criteria must be considered:
Process flow rate, temperature and pressure.
Type, quantity and concentration of VOCs.
Process operating cycle.
Technology's track record.
Capital and operating costs.
Condensation of some organics can be achieved with simple refrigeration systems, down to a temperature of about -40oF (-40oC). However, most VOCs require much lower temperatures, so the use of cryogenic coolants becomes necessary. As the temperature limit of this technology is that of the coolant -- liquid nitrogen, with a boiling point of -320oF (-196oC) -- operating temperatures as low as -292oF (-180oC) are possible.
Condensation using liquid nitrogen has been overlooked in the past due to perceived high operating costs. But now, with ever more stringent legislation, the potential benefits of liquid nitrogen are becoming more attractive. With cryogenic systems, temperatures can be adjusted when tighter limits are required. If the nitrogen can be used again elsewhere in the plant, the effective operating costs are reduced.
Although in principle nearly all VOCs can be removed with cryogenics, it is most economical when flow rates are below 5,000 normal cubic meters per hour (Nm3/hr) or vapor concentrations are above 5 g per normal cubic meter (g/Nm3).
In the United States, amendments to the Clean Air Act have made control of VOC emissions an absolute necessity. Similar regulations such as the Environmental Protection Act in the United Kingdom are enforced in other parts of the world. Germany's TA Luft standard concentration of 20 mg/m3 is the strictest of these regulations.
In the pharmaceutical and fine chemical industries, the purity of chemical compounds is paramount. Any technology that can increase reaction stereo selectivity is advantageous. Organic synthesis at low temperature is one method that can help to achieve these aims; low temperature re-agents such as n-butyl lithium or Wittig reagents can be used. At cold temperatures, these reagents produce intermediates that after further processing lead to products with greater regularity and better selectivity.
However, n-butyl lithium presents the engineer with a technical challenge. Although it is helpful in the production of optically pure isomers, at room temperature, n-butyl lithium is an unstable compound and requires effective cooling control. Synthesis must take place at very low temperatures to ensure that the reagent's stability is maintained and that the exotherm, which can be released by the addition of such organometallic compounds, is strictly controlled.
Typically, three main cooling methods can be used to keep temperatures sufficiently low and control an exothermic reaction: mechanical refrigeration, sublimation of carbon dioxide and evaporation of liquid nitrogen. Simple mechanical refrigeration that works at the limit of its lower temperature range is expensive.
Sublimation of CO2 can reach temperatures as low as -108°F (-78°C) at atmospheric pressure, so CO2 is a viable option for controlling some cold chemistry reactions. However, liquid nitrogen can achieve much lower temperatures than this.
Most commonly in chemical plants, nitrogen is used as an inert blanketing gas. If the evaporated liquid nitrogen from the cooling process can be recovered and used elsewhere in the plant, the system's running costs can be decreased. In this application, nitrogen cryogenic systems can provide improved reaction yields and selectivity, reduced unwanted by-products and relatively low capital costs. Also, nitrogen is a clean, dry, inert, nontoxic, nonpolluting environmentally friendly fluid.
Three primary options for cooling reaction vessels with liquid nitrogen exist: direct, semidirect and indirect. Each method has advantages and disadvantages.
Direct injection of liquid nitrogen achieves maximum efficiency and is inexpensive to install, but solvent entrainment, foaming and localized freezing can occur. Semi-direct heat transfer takes place either via a coil inside the reactor or in a reactor cooling jacket. The main benefits of this method include accurate temperature control, the ability to reuse the nitrogen and simplicity. Drawbacks include reduced efficiency, the demand on reactor volume and the expense of the cryogenic construction materials. Indirect heat transfer occurs in systems where liquid nitrogen is exchanged with a suitable heat transfer fluid in an external heat exchanger. This approach has the most system flexibility, provides accurate temperature control and accommodates large heat loads.