Although drying of plastic pellets is a relatively simple process, in some cases pellets simply cannot be dried completely.
Factors that affect drying effect are:

Heat is key to opening synergy between water molecules and hygroscopic polymer. Above a certain temperature, attraction between water molecules and polymer chains is greatly reduced, and water vapor is carried away by dry air.

In dryer, moist air is first removed so that it has a very low residual moisture (dew point). Then, reduce its relative humidity by heating air. At this time, vapor pressure of dry air is lower. By heating, water molecules inside particles get rid of binding force and diffuse to air around particles.

In air surrounding particle, it takes a certain amount of time for absorption of heat and diffusion of water molecules to the surface of particle. Therefore, resin suppliers should specify time it takes for a material to dry effectively at proper temperature and dew point.

Hot drying air transfers heat to particles in drying silo, removes moisture from surface of particles, and returns moisture to dryer. Therefore, there must be enough air flow to heat resin to drying temperature and maintain this temperature for a certain period of time.
When problem of poor drying occurs, problem should be found from following three aspects:

 

When inspecting dryer, pay special attention to air filter and hoses. A clogged filter or a squashed hose can reduce airflow and affect dryer operation;
Damaged filters can contaminate desiccant, inhibiting its ability to absorb moisture; broken hoses can introduce humid ambient air into drying airflow, causing premature desiccant absorption and high dew points; poorly insulated hoses and drying bins can also affect drying temperatures.

In drying air circuit, drying temperature should be detected at inlet of silo in order to compensate for heat loss of dryer in hose. Low air temperature at silo inlet may be due to improper adjustment of controller and lack of insulation, or a faulty heater element, heater contactor, thermocouple, or controller. In addition, it is important to monitor drying temperature throughout drying process and observe temperature fluctuations when desiccant is changed.
If material is not properly dried after coming out of dryer, it should be checked that drying silo has enough space to provide sufficient and effective drying time. Effective drying time refers to time particles are actually exposed to appropriate drying temperature and dew point. If particles do not stay in silo for an insufficient amount of time, proper drying will not be achieved. Therefore, attention should be paid to size and shape of granular or crushed material, which will affect bulk density and residence time of dry material.
A kinked hose or a clogged filter can restrict airflow and affect dryer performance. Therefore, it is impossible to tell if airflow is adequate if dryer is inspected and no such problems are found. Here, there is a quick, simple and accurate method to detect whether air flow of dryer is sufficient, which is to measure vertical temperature curve of material in drying silo.
Assume that drying time recommended by material supplier is 4h, and processing capacity is 100lb/h (1lb=0.4536Kg). To determine if dryer airflow is sufficient, measure temperature profile in drying silo, paying particular attention to temperature at 4h (400lb). If temperature at 400 lb level in the drying bin reaches set point, then airflow is considered adequate.
If only material at 1h, 2h or 3h is fully heated in drying silo, it means that air flow cannot complete heating and drying of material at predetermined yield. Insufficient heating may indicate that drying silo is too small for this production rate, or that airflow is restricted due to conditions such as clogged filters or damaged hoses. Excessive air volume can also cause problems, not only waste energy, but also lead to high return air temperature, destroying performance of desiccant.
Return air filter prevents filamentary material from contaminating desiccant and affecting its hygroscopic properties. These filters must be kept clean to ensure adequate airflow.
When drying air comes out of top of dryer, most of heat has been released. Most dryers can work efficiently when desiccant temperature is in the range of 120oF to 150oF. If return air overheats desiccant, it reduces its ability to absorb moisture from drying air.
Always check return air temperature of dryer. When return air temperature is high, it may indicate that dryer is too large for this production rate, or temperature of material entering drying silo is high, for example, PET has crystallized before drying, or just some materials (such as PET ) at a drying temperature higher than normal temperature range. In order to prevent return air temperature from becoming higher, as long as a heat exchanger is installed on return air path, it can be ensured that desiccant can effectively remove moisture in drying air.

 

Desiccant has a limited ability to absorb moisture, so moisture it adsorbs must be removed by regeneration. Process is: when ambient air is drawn in, it enters blower through a filter and is then fed into a set of heaters. Heated air is passed through desiccant bed. When temperature of desiccant increases, adsorbed moisture is released. When hot air is saturated with water vapor, it is discharged into atmosphere. High temperature regeneration desiccant must be cooled before returning to drying loop to restore moisture absorption function of desiccant.
Dew point readings can help identify problems, so dry air dew point should be monitored throughout drying process. Dew point reading during normal operation of dryer should be a straight line in the range of 20oF to 50oF, of course, small fluctuations caused by changing desiccant are normal. If dryer is functioning properly, dew point at dry air inlet should be at least 30oF lower than dew point at return air outlet.
On the other hand, after desiccant was replaced, dew point peaked immediately, indicating that desiccant was not sufficiently cooled before it was put in, so that it could not absorb moisture well, and dew point of desiccant would drop to normal standard after cooling. Improper cooling of desiccant can cause temperature spikes, and sudden temperature changes can reduce desiccant's ability to dry heat-sensitive materials such as ionomers, amorphous polyesters, and some nylon grades.
If dew point reading is normal after desiccant bed is replaced, but dew point rises rapidly before desiccant's drying cycle ends, ambient air may have entered closed air path, causing desiccant to absorb moisture prematurely. Another possibility is that desiccant is not fully regenerated or is contaminated. If dew point readings are close to return air dew point readings, regeneration gas circuit has failed completely or desiccant is severely contaminated.