diff --git a/ensysmod/core/file_upload.py b/ensysmod/core/file_upload.py
index 6a06448..a758ddd 100644
--- a/ensysmod/core/file_upload.py
+++ b/ensysmod/core/file_upload.py
@@ -1,4 +1,5 @@
 import json
+from io import BytesIO
 from typing import Any
 from zipfile import ZipFile
 
@@ -260,8 +261,8 @@ def process_excel_file(
 def read_excel_file(file: UploadFile | tuple[ZipFile, str]) -> tuple[str, pd.DataFrame]:
     if isinstance(file, UploadFile):
         file_path = file.filename if file.filename is not None else ""
-        content = file.file.read()
-        df: pd.DataFrame = pd.read_excel(content, engine="openpyxl")
+        with BytesIO(file.file.read()) as content:
+            df: pd.DataFrame = pd.read_excel(content, engine="openpyxl")
     elif isinstance(file, tuple):
         zip_archive, file_path = file
         with zip_archive.open(file_path) as content:
diff --git a/ensysmod/core/fine_esm.py b/ensysmod/core/fine_esm.py
index c501d64..5c13784 100644
--- a/ensysmod/core/fine_esm.py
+++ b/ensysmod/core/fine_esm.py
@@ -1,3 +1,4 @@
+from contextlib import chdir
 from pathlib import Path
 from tempfile import TemporaryDirectory
 from typing import Any
@@ -28,7 +29,7 @@
     EnergyStorage,
     EnergyTransmission,
 )
-from ensysmod.utils.utils import chdir, create_temp_file, df_or_s
+from ensysmod.utils.utils import create_temp_file, df_or_s
 
 
 def generate_esm_from_model(db: Session, model: EnergyModel) -> EnergySystemModel:
diff --git a/ensysmod/core/security.py b/ensysmod/core/security.py
index b60310f..c09ad69 100644
--- a/ensysmod/core/security.py
+++ b/ensysmod/core/security.py
@@ -1,4 +1,4 @@
-from datetime import datetime, timedelta, timezone
+from datetime import UTC, datetime, timedelta
 from typing import Any
 
 from jose import jwt
@@ -12,10 +12,10 @@
 
 
 def create_access_token(subject: str | Any, expires_delta: timedelta | None = None) -> str:
-    if expires_delta:
-        expire = datetime.now(tz=timezone.utc) + expires_delta
+    if expires_delta is not None:
+        expire = datetime.now(tz=UTC) + expires_delta
     else:
-        expire = datetime.now(tz=timezone.utc) + timedelta(minutes=settings.ACCESS_TOKEN_EXPIRE_MINUTES)
+        expire = datetime.now(tz=UTC) + timedelta(minutes=settings.ACCESS_TOKEN_EXPIRE_MINUTES)
     to_encode = {"exp": expire, "sub": str(subject)}
     return jwt.encode(to_encode, settings.SECRET_KEY, algorithm=ALGORITHM)
 
diff --git a/ensysmod/crud/base_depends_excel.py b/ensysmod/crud/base_depends_excel.py
index 5381b64..a98a9be 100644
--- a/ensysmod/crud/base_depends_excel.py
+++ b/ensysmod/crud/base_depends_excel.py
@@ -70,9 +70,9 @@ def get_dataframe(self, db: Session, *, component_id: int) -> pd.DataFrame:
             dataset_id = crud.energy_component.get(db, id=component_id).ref_dataset
             region_names = list(db.execute(select(Region.name).where(Region.ref_dataset == dataset_id)).scalars().all())
 
-            dataframe = pd.DataFrame(0, index=region_names, columns=region_names)
+            dataframe = pd.DataFrame(0, index=region_names, columns=region_names, dtype=float)
             for d in data:
-                dataframe[d.region_to.name][d.region.name] = getattr(d, self.data_column)
+                dataframe.loc[d.region.name, d.region_to.name] = getattr(d, self.data_column)
             return dataframe
 
         # otherwise return dataframe with 1 row and regions as columns
diff --git a/ensysmod/utils/utils.py b/ensysmod/utils/utils.py
index 30dd13b..c3818a3 100644
--- a/ensysmod/utils/utils.py
+++ b/ensysmod/utils/utils.py
@@ -1,6 +1,4 @@
 import os
-from collections.abc import Generator
-from contextlib import contextmanager
 from pathlib import Path
 from tempfile import mkstemp
 
@@ -21,19 +19,6 @@ def remove_file(file_path: Path) -> None:
     file_path.unlink()
 
 
-@contextmanager
-def chdir(path: Path | str) -> Generator[None, None, None]:
-    # In Python 3.11 use contextlib.chdir().
-    current_dir = Path.cwd()
-    try:
-        os.chdir(path)
-        yield
-    finally:
-        os.chdir(current_dir)
-
-
 def df_or_s(df: pd.DataFrame) -> pd.DataFrame | pd.Series:
-    # Convert a dataframe to a series if it has only one row.
-    if df.shape[0] == 1:
-        return df.squeeze(axis=0)
-    return df
+    """Convert a dataframe to a series if it has only one row."""
+    return df.squeeze(axis=0) if df.shape[0] == 1 else df