Using solvents in any manufacturing process requires solvent charging, removal and storage. Each stage creates solvent vapours in addition to those that are generated when the solvents are physically used in the manufacturing process. Recovering these solvent vapours is critical for process operational efficiency, plant cost savings, operator health & safety and environmental legislation compliance.
There are many industrial applications which involve the processing of solvents. Such applications include; distillation, drying, evaporation/crystallisation, solvent/vapour recovery and filtration and equipment includes reactors, mixers and dryers. In the Pharmaceutical sector, for example, solvents which are used in the reactors are typically displaced through a vent system to a solvent recovery system and in larger plants, multiple reactor systems often to vent to a common solvent recovery unit.
Some vent systems are “one-pass” systems whereby the vapours are exhausted from the reactor, pass through the recovery unit, are cooled, and expelled to the atmosphere (where applicable). In the reactors, mixing, coating, and drying/granulating operations, a more efficient and economical method of solvent recovery is to use a “closed-loop” system.
A closed-loop system is best suited for batch processing systems, but can also be adapted to a continuous feed system. The closed-loop system has the advantage of having no emissions during operation, which in effect gives a 100% recovery of the solvent. This is very commercially attractive to the manufacturing plant and much safer for operator welfare.
The most common closed-loop system employs a vacuum pump for handling solvents. There are several types of vacuum pumps which could be employed including Liquid Ring, Dry Pumps, Rotary Lobe, Claw and Screw pumps. These systems tend to not require internal lubrication so allows solvent vapors to be sucked through the pumps without jeopardising the lubrication. By contrast, oil sealed pumps such as rotary vane or piston that do require internal lubrication typically have problems on such duties.
Both Liquid Ring and Dry Pumps have external bearings which are isolated from the process fluid. In addition the Dry Pump also has oil lubricated timing gears to maintain the two parallel shafts rotating in the correct phase to avoid contact. Given the variety of options, the user needs to understand the potential issues of each.
For example, screw pumps have a high potential wear rate as they have to operate with very close radial clearances. Stainless steel rotary parts contacting stainless steel stationary parts creates galling issues which can lead to high heat generation and premature pump failure.
Some pump manufacturers help to overcome potential galling contact issues by manufacturing their pumps from ductile cast iron materials. Unfortunately, this opens up another set of issues given condensation, as encountered in solvent recovery applications, is disastrous for pumps which are made of materials which can corrode. Furthermore, as many pumps are horizontally orientated, this condensation can not escape. This results in high corrosion attach of ductile iron material leading to loss of radial clearances and premature pump failure.
Wherever possible, the best approach is a self-draining dry pump design, manufactured from non-galling materials which is mounted in a vertical orientation. This ensures that there is no stagnation of the product media and less chance of condensation between counter-rotating components. As such, one design that has won favour with rotating equipment engineers around the world, is the ADVV - Automated Dry Vertical Vacuum pump from Stanpumps (part of the Standard Group of Companies). The Stanpumps ADVV creates an ideal operating environment for 100%, closed loop solvent extraction and recovery, as found in Reactor, Mixing and Drying applications. Click here to find out more.
Reactors are extensively used across many industry sectors and specifically pharmaceutical, chemical, food and fertiliser processing plants. They are designed to safely contain chemical reactions and/or thermal changes of ingredient substances in a pressurised environment, acting in a similar way to a pressure cooker in a domestic household kitchen.
If the substances used in the vessel are highly potent, toxic and of a particularly hazardous nature with aggressive chemical reaction affects, glass lined reactors are typically used rather than stainless steel reactors. A glass lined reactor has a hard wearing enamel surface coating on the wetted surfaces which provides high chemical resistance to the vessel – ideal for reactor applications.
After the reaction, the media inside the vessel is then discharged in a number of ways to a variety of subsequent operations including condensing, separation, distillation, evaporation and weighing/packaging. Ultimately the media is sent to a finishing process such as a tablet forming and packing machine or dispensed into bottles and containers for human consumption. In addition to obvious health issues, if the media is not reacted correctly inside the vessel then large amounts of time and money can be wasted by the subsequent process operations. Therefore, in-cycle reactor checking of the media to ensure the reactions are to the correct quality is the secret of a successful process.
Plant designers endeavour to ensure that the substances contained within the vessel react to the correct quality standard and at the highest efficiency. As with any process optimisation the first priority is to ensure safety and quality standards are not compromised. Then, plant designers must aim to produce the highest yield of product while spending the least amount of money to purchase and sustainably operate the reactors.
‘Sustainably operate’ is a key phrase. Above all the normal operating expenses such as energy input, energy removal, raw material costs and labour, is the issue of sustainable stakeholder safety is of paramount importance. Reactor risk management is all about expecting the unexpected and minimising human process intervention helps to mitigate risk and keep operations safe.
Probably the highest risk in the reactor process is taking a process media sample at elevated temperatures. This is major cause of concern for Pharmaceutical plants especially when taking the sample from a Reactor where the reaction is hazardous as found on practically all glass lined reactor applications. Spill over and exposure of sampling liquid is a perennial cause of batch rejection, reprocessing and various quality-related issues ultimately leading to personnel risk and lost profit.
Forward thinking plants are now standardising on reactor sampling systems which are designed to provide automatic vacuum-operated safe sampling.
In India, Standard Glass Lining Technology (SGLT) has sales and service branches located across the country as well overseas in Europe and the UK and are leading the way in this field with their innovative automated reactor sampling device. This device is offered as a value-adding option on their glass lined reactor supply, to ensure they provide their clients with best available practice solutions.
In operation, a pre-established sample quantity is directly pulled into the sampling container by vacuum in a closed-loop manner. At all times, the operator is safe, isolated from the media and in full control. The innovative design is optical sensor powered and is offered with a clear sampling pot so that the operator can see exactly what is happening to the reacted substances at all stages of the reaction process. Furthermore, Distributed Control System (DCS) operation is also possible with the auto sampler and unlike conventional systems, cleaning of the sampling pot is easy, fast and risk-free.
Given the potentially hazardous nature of the substances inside stainless steel, exotic alloy and glass lined reactors, SGLT’s safe auto sampler is now fitted as a site-wide standard by many leading international pharmaceutical manufacturing companies for both new build plants/extensions as well as retro fits to all existing their reactors.
Click here for a downloadable version of this article that appeared in the Chemical Weekly magazine in September 2016